5 Must-Have Features in a jackfruit peptide exporter

This review provides an overview of recent research and advances in biology, diagnostics, pathogenomics, and potential disease management strategies of P. stewartii subsp. stewartii &#;s infection to improve the quality of local jackfruits. It is crucial to ensure the high quality and quantity of local jackfruit supplies to meet the increasing demand of domestic and international market needs.

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A public review on jackfruit-bronzing was published by [ 23 ], focusing on the diagnostic approaches forsubsp.in Malaysia, particularly its pathogenicity and genetic diversity, highlights polymerase chain reaction (PCR) amplification assay and multilocus sequencing analysis (MLSA). Despite little information on the commercialization value of jackfruit, limited information exists on the bacterial pathogen&#;s virulence factors and genome sequence. There is merely one complete genome sequence ofsubsp.strain DC283 (GenBank Accession No: CP) is available in public databases.

In Malaysia, jackfruit-bronzing was first reported in the plantation areas of Selangor and Pahang [ 20 ]. Indicated by yellowish to reddish discoloration on the affected fruit pulp and rags, the disease urgently threatens jackfruit industries. It has reduced the quality of fresh jackfruit and further afflicts jackfruit marketing economically [ 21 22 ]. Recently, a persistent onslaught of jackfruit-bronzing has significantly constrained jackfruit production, resulting in a massive loss of yield and further distracting the jackfruit growth sector.

Previously, as far as we knew, jackfruit had few disease problems; those we knew of were caused by bacterial and fungal infections and some nematode species.subsp.andare two prevalent bacterial pathogens that cause jackfruit-bronzing and fruit rot diseases [ 14 ]. However, most reported cases of issues with jackfruit in the past were fungal-causing diseases such as leaf spots, dieback, fruit rot, and pink disease caused byandsp. The diseases are commonly spread in orchards with poor air circulation or during wet seasons with high relative humidity and temperatures [ 15 16 ]. In India and China, the most common species infecting and parasitizing jackfruit trees wereand root-knot nematodes 19 ]. These plant pathogens pose a significant threat to the plant productivity, yields, and long-term viability of plantations. Uncontrolled epidemic diseases resulting from such pathogens in the future might restrict the agriculture growth sector at domestic and international levels.

Jackfruit was listed under the National Key Economic Areas (NKEAs) as one of the economic-driven fruit crops of Malaysia, where its cultivation has been upgraded to a large scale [ 10 ]. The production value per metric ton was RM 1.25 in , which increased considerably to RM 1.60, RM 1.81, and RM 1.92 in the following consecutive years. In and , the production value showed zero increments and remained static at RM 2.09. The harvested area decreased at that time from to Ha. It was then that outbreaks of jackfruit-bronzing were first reported in the country. Nearly 10,000 ha of jackfruit plantations were affected by the disease symptoms, with a 50&#;80% disease incidence. The production value of jackfruit then decreased to RM 1.95 and RM 1.88 in and , respectively [ 11 12 ]. While this was happening, jackfruit production in the Philippines declined, falling from 47.09 metric tons in to 40.2 metric tons in [ 13 ].

Jackfruit (Lam) falls into the classorderand belongs to the family 2 ]. It is a non-seasonal fruit and a close relative of) and breadfruit (). It is the largest tree-borne fruit, at almost 1 m (39 in) long, and can weigh up to 60 kg (132 lb). For decades, it has been grown for food, fuel, timber, medicinal extracts, and as a source of income for rural people [ 3 4 ]. Jackfruit is indigenous to the Western Ghats of southern India. Today, it is widely cultivated throughout the South and Southeast Asian region, the Caribbean, Latin America, and parts of Africa [ 5 7 ]. The latest WorldAtlas has recorded India as the world&#;s top jackfruit producer, with 1.4 million tons produced, followed by Bangladesh, which claims jackfruit to be its national fruit, with 926 tons produced. Thailand and Indonesia are two other top jackfruit producers, with 392 and 340 metric tons, respectively ( Figure 1 ) [ 8 ]. The jackfruit prices appear to have converged (below USD 1.00 per kg), except in areas of the world that are difficult to access, such as Australia (USD 2.91) and North America (USD 1.86) [ 9 ].

Typical fruit bronzing is a skin disorder of irregularly shaped patches marked by purplish or bronze-colored skin [ 28 30 ]. Unlike typical fruit bronzing, jackfruit-bronzing is associated with internal symptoms and can only be observed whenever the fruit is cut open. However, the plant pathologists indeed shared and highlighted the similar significant characteristics of these disease symptoms. Jackfruit-bronzing symptoms indicate rusty and yellowish-orange to reddish discoloration of the affected pulps and rags ( Figure 3 ). The symptoms primarily exist in sweetened jackfruit variety and higher brix composition, specifically the Tekam Yellow (J33) variety [ 2 ]. Once infected, the color of the internal parts is deviated from their original and is no longer attractive with the presence of bronzing specks all over the jackfruits pulps and rags.

This disease was reported in Malaysia three years later, where the bronzing symptoms appeared in the jackfruit plantations of Pahang state. This discovery ultimately answered the mystery of the rotting, rust, and rot-like symptoms previously discovered in plantation areas of Pahang and Negeri Sembilan state in [ 25 ]. The identified pathogen retained similar morphological and biochemical characteristics described by the Philippines research group. Species-specific PCR amplification confirmedsubsp.as the causal agent of jackfruit-bronzing disease in Malaysia [ 20 27 ]. The disease later spread across North America when the fruit bronzing symptoms were observed in Nayarit jackfruit orchards, Mexico. The so-called &#;Mexican bacterial isolates&#; matched the causal pathogen found earlier in the Philippines and Malaysia [ 22 ].

Jackfruit-bronzing disease was first reported in the Philippines when the internal parts of the jackfruit changed to yellowish-orange and reddish discoloration. As a result, the quality of jackfruit dropped and disrupted the farmers-consumers supply chain. A research group [ 21 ] discovered a bacterium known assubsp.act as the disease&#;s causal agent. Interestingly, this bacterium was the same pathogen causing Stewart&#;s wilt on corn plants and localized lesions symptoms in pineapple ( Figure 2 ).

The life cycle ofsubsp.associated with jackfruit-bronzing is yet to be discovered, and there is little information available on the etiology of this disease. However, jackfruit seeds may be a potential vector of the disease, seeing the propagation by direct seeding is still being practiced worldwide, apart from grafting and cuttings techniques [ 48 ]. Shoot borers, bark borers, mealy bugs, and scale insects are the prevalent insect pests linked to most jackfruit diseases [ 49 ]. A current review by [ 50 ] reported that trunk borer, shoot, and fruit borer as major common insect pests attacking jackfruit. In contrast, bud weevil, spittle bugs, bark-eating caterpillars, and aphids are considered minor insect pests of jackfruit.

subsp.originated in the USA and is widely disseminated to Africa, North, Central and South America, Asia, and Europe [ 46 ]. A recent pest survey card by [ 47 ] reinforcedsubsp.as a Union Quarantine Pest and urged specific measures to prevent the introduction of this bacterium. At present, the corn flea beetleacts as the primary vector for the pathogen. At the same time, the European Union considered maize seeds the main pathway for introducing and spreading Stewart&#;s wilt disease.

Another subspecies,subsp.is a known pathogen of rot on pineapplerot and leaf spot on foxtail milletas well as pearl millet. In addition, it is virulent against various plant hosts. For example, it is known as a new causative agent of rice () BLB disease [ 42 43 ], center rot of onion 44 ], and leaf blight wilt on a flowering plant species of, the 45 ]. Figure 4 illustrates the host plants and diseases caused byand its two subspecies;subsp.andsubsp.

Two subspecies were proposed underwhen [ 36 ] managed to differentiatesubsp.fromsubsp.. In opposite tosubsp.subsp.cannot produce indole, utilize citrate, grow on cis-aconitate, and form acid from carbohydrates.subsp.recognizes maize (), sweet corn (subsp.), teosinte (subsp.andsubsp.) as its main hosts [ 37 ]. Instead, the bacterium is known for causing Stewart&#;s wilt or Stewart&#;s bacterial wilt of maize and sweet corn, an economically significant disease affecting corn industries in Canada and the USA in the late s [ 38 39 ]. A decade after, Stewart&#;s wilt disease was detected in Argentina [ 40 ] and Bogor district in Indonesia [ 41 ].

The past five years&#; studies showedandappeared as the top three&#;s worldwide phytopathogenicspp. inflicting damage and harming economically important crops and ornamental and flowering plant species.has been identified as the causal agent of leaf blotch on Sudan grass and Stewart&#;s wilt onin California and South Korea [ 31 32 ]. Recent publications associatedwith bacterial leaf blight (BLB) of rice in Thailand and the West African countries of Benin and Togo [ 33 35 ].

Essentially, the genusis linked to the MLSA scheme to clarify the problematic taxonomic situation within the family Enterobacteriaceae due to some indistinguishable paraphyletic genera from 16S rRNA gene phylogeny. As a result, the former genera were amended, and different species were reclassified into new genera. Allstrains were involved in the massive analysis by referring to the DNA groups II, IV, and V [ 66 ]. A huge amendment was made to the genusby which new species were included and former species were reclassified.andwere transferred to the genus 67 ]. The scheme also brought in theto the genusas 68 ].

A recent report on the genetic diversity ofsubsp.isolated from jackfruit-bronzing samples was published by Abidin et al. [ 26 ], following the protocol described by Brady et al. [ 64 ]. Results were interpreted in two phylogenetic trees based on (i) single and (ii) concatenatedandgenes. Although phylogenetic trees based on a single gene scored maximum similarities (99% to 100%) between subspeciesand, the concatenated genes-based phylogenetic tree is capable of pointing out the closer relation withsubsp., although thesubsp.were in the same cluster. In a simple explanation, thesubsp.strains from diseased jackfruits were polyphyletic in the 16S rRNA gene tree, thus forming a distinct monophyletic cluster in the MLSA analysis [ 65 ].

Apart from PCR, barcoding is another molecular method to identifysubsp.EPPO recommended MLSA for a higher resolution of phylogenetic relationships of species within a genus or family [ 59 ]. It is an up-to-date method in prokaryotic taxonomy utilizing housekeeping genes or partial sequences of(encodes DNA gyrase subunit B),(encodes RNA polymerase beta subunit),(encodes ATP synthase F1, β-subunit), and(encodes translation initiation factor IF-2) genes to build phylogenetic trees and deduce phylogenies [ 61 ]. A minimum of four genes are required in MLSA since a single gene would introduce bias and not reflect the general phylogenetic relationships other than the evolution of that single gene. Furthermore, MLSA deals precisely with internal fragments of four protein-coding genes at once (concatenated), reflecting the genuine phylogenetic relationship of bacterial taxa [ 63 ].

The most commonly used primers are (fD1; rD1) [ 62 ], (ES16; ESIG2c), (, and 54 ] that target 16S rRNA and 16S-23S rRNA/internal transcribed spacer (ITS) partial gene sequences,and, correspondingly ( Table 2 ). Theandgenes are chosen as targets as they appear unique tosubsp.DNA samples of infected jackfruit parts and ooze [ 21 ] can be used as templates, and the PCR program is set according to the manual procedure described by Coplin et al. [ 54 ]. After separation on an electrophoresed agarose gel (1%), amplification is observed at respective ~1.5 kb, ~0.92 kb, ~1.1 kb, and ~0.9 kb size amplicons. BLASTn and phylogenetic analyses will be conducted to see their similarities withsubsp.reference strains in the GenBank databases.

So far, rapid and sensitive PCR amplification targeting a single gene is generally used againstsubsp.-specific primers. The most-targeted gene regions for this bacterium areand, responsible for pathogenicity and virulence. Thegene cluster comprises 12 genes (cpsA to cpsM) that encode proteins to synthesize capsular polysaccharide (CPS) stewartan built up from seven monosaccharides repeating unit counting glucose, galactose, and glucuronic acid. On the other hand, thegene encodes proteins for a type-III secretion system (T3SS) required for water-soaked lesions and general pathogenicity [ 54 ].

The molecular methods are one of the primary identification tests forsubsp. 59 ]. Besides serological tests and fatty acid profiling assay, the molecular approaches are applicable for phylogenetic and diversity analyses of prokaryotic taxa. The methods utilized two alternatives, either (i) hybridization of DNA&#;DNA homology to determine the relatedness of two microorganisms or (ii) sequencing of 16S ribosomal RNA (rRNA) to identify the bacterial species [ 58 61 ]. In addition, molecular methods can be further directed into PCR and barcoding.

Hypersensitivity reaction (HR) is a quick and useful determinative test to discriminate plant pathogens from saprophytes by their capabilities to produce an HR in the leaf mesophyll tissue [ 60 ]. Essentially, a pathogenicity test should be carried out to confirm the infection by 59 ]. The test can be performed on healthy detached or attached jackfruit or jackfruit pulps [ 21 27 ]. The 10CFU/mL bacterial inoculum is injected into the sterile and dried pulps and incubated in a controlled sterile chamber. Observation and examination for bronzing symptoms development are monitored daily for 2-weeks post-inoculation. The bacterial pathogen is reisolated and verified to identify the morphology, biochemical, and microscopic properties to confirm Koch&#;s postulates. Although similar type symptoms were perceived in artificially inoculated jackfruits, the bronzing symptoms are relatively brighter than the naturally infected ones. It is most likely due to the different periods, considering the artificially inoculated fruits are cut open when it is still at an early stage. Disease incubation is longer in naturally infected jackfruits since the infection occurs earlier and longer than 2-weeks [ 21 ]. Therefore, different jackfruit varieties could be used to discover the most resistant and sensitive varieties to bronzing disease. The most recent publication investigated the pathogenic variability ofsubsp.infection against three different jackfruit varieties in Malaysia; Tekam Yellow (J33), Hong (J34), and Subang Chap Boy (J39) [ 24 ].

Following the colony mentioned above and morphological identification, Gram-staining was performed to differentiate thespecies under a compound light microscope with oil immersion lenses for a 100x magnification. Biochemical tests were carried out to determine the biochemical properties of the bacterium. Table 1 represents selected tests and outcomes for identifying the jackfruit-bronzing bacterium. Biochemical tests show that bacteria use carbon sources to obtain energy and sustain life. Therefore, the test outcomes on which carbon sources react allow for a probabilistic assessment [ 58 ].

Phenotypic characterization is a popular method due to low-price reagents and affordable equipment. This method allows the identification of the bacterial species from the genus up to the species level through the colony and cell morphology, Gram reaction, and metabolic and growth characteristics. Initially, a pure culture should be grown on media with various salt concentrations to identify the jackfruit-bronzing bacterium. Media such as 523, yeast dextrose carbonate, PA 20 [ 31 53 ], casamino acids peptone glucose [ 54 ], Luria-Bertani [ 22 55 ], andgenus-specific agar [ 56 ] are examples of media that serve for the purpose. Meanwhile, the King&#;s B supplemented with nutrient-broth yeast extract agar or yeast peptone glucose agar has been applied as the selective media for the growth ofsubsp.at 25 to 28 °C [ 57 ].

CPS and O-antigen are critical components of cell walls and have been recognized as significant pathogenic factors in pathogenic bacteria [ 140 ]. CPS is best known for pathogenesis, yet it is intricate in promoting bacteria adherence to host organisms, facilitating biofilm formation, and conferring resistance to host innate immunity [ 141 ]. The O-antigen is the outer surface of lipopolysaccharide (LPS) and is built up together with lipid A (endotoxin) and core oligosaccharide (shown in Figure 9 ). There is abundant literature defining the association of the O-antigen with bacterial virulence in humans, animals, and plants [ 142 146 ].

A set of polysaccharides export lipoprotein (Wza), low molecular weight protein-tyrosine-phosphatase (Wzb), and tyrosine-protein kinase (Wzc) were later discovered. The Wza and Wzb typically span the periplasmic space and promote the export of EPS polymer. The Wzb alone helps support the oligomerization of dephosphorylated Wzc with its phosphatase activity [ 137 139 ]. The remaining two proteins found insubsp.SQT1 was a putative CPS transport protein (YegH) and a putative uncharacterized protein (YmcB).

EPS biosynthesis is similar to the Wzy-dependent pathway of O-antigens and groups 1 and 4 of CPS [ 133 134 ]. None of the O-antigen flippase, Wzx (related to MurJ of peptidoglycan assembly), or oligosaccharide repeat unit polymerase (Wzy) existed in thesubsp.SQT1 genome exports the LPS across the plasma membrane to the periplasmic space. Interestingly, the detection of OM lipoprotein carrier protein (LolA), OM protein (YaeT), and four lipoproteins (YfgL, YfiO, NlpB and SmpA) suggested the involvement of the Lol system in transporting LPS to the OM ( Figure S1 ) [ 135 136 ].

CPS and EPS gene clusters suggest the active production of EPS stewartan fromsubsp.SQT1. Gram-negative bacteria are naturally covered in a surface-bound polysaccharide layer or capsule for protection against recognition by plant defense mechanisms, bind water to moisten the bacteria, and retain nutrients and ions released from damaged plant cells. These circumstances create a favorable environment for bacterial multiplication, aids the bacterial dissemination through plant tissues, and develop the characteristic symptoms of infection [ 126 ]. The CPS is released and free from the cell surface molecules as free EPS in certain circumstances, thus explaining the slime-forming or mucoid trait of many bacterial species ( Figure 9 ) [ 127 128 ]. EPS is the primary pathogenicity factor forsubsp. 129 ] and plays a vital role in bacterial survival and persistence, particularly in cell aggregation, cell adhesion, biofilm formation, and protection from hostile environments [ 128 131 ]. These acidic complex EPSs mask pathogens&#; recognition through plant defense reactions and promote bacterial growth and movement 132 ].

Following ABC transporter and T6SS are the type-II, -IV, and -V secretion systems. The T2SSs and T5SSs secrete proteins in two steps: (i) Sec/Tat secretion pathways and (ii) second secretion pathway. Meanwhile, T4SS is a Sec/Tat-independent and transport substrate across both bacterial membranes in a single step, resembling the T6SS [ 125 ].

andare the close relative of thesubsp.to illustrate the macromolecular machines of T6SS [ 88 ] in virulence mechanism and interbacterial competition [ 91 ]. During disease interactions ofwith its plant host, T6SSs have altered metabolic and motility processes, and most importantly, they impacted the disease&#;s progression [ 123 ]. While in, a compromised virulence phenotype was observed due to the incapability of the T6SS mutant to cause disease in onionplants [ 124 ].

The T6SS complex also revealed the presence ofencoding a protein with an FHA (forkhead-associated) domain, which is known to have an affinity for phosphothreonine [ 121 122 ]. Several studies have found a link betweenand HCP1 secretion and its phosphorylation is associated with PpkA and PppA. The Fha phosphorylation may initiate a signal transduction cascade that leads to T6SS assembly and function [ 120 ]. However, not every T6SS gene cluster possesses the PpkA/PppA/Fha1 complex at the post-translational level, with some having only part of it and others having none.

(caseinolytic peptidase B) and(pyoverdine chromophore 109) are other T6SSs spotted in the draft genome sequence. The Clp family belongs to the AAA+ superfamily of ATPases [ 114 ]. They form ring-shaped oligomers necessary for their ATPase activities and mode of action, particularly for macromolecular structural disruption [ 115 116 ]. A homolog ofis considered one of the core components of T6SS and acts as the T6SS motor [ 117 118 ]. Meanwhile, uncharacterized Pvc109 protein is similar to VCA109 and functions as a base plate assembly protein [ 119 ]. Significant gene clusterencoding Ser/Thr phosphatases was also detected. Theis located next to theon the contig 6, butencoding Ser/Thr kinases was not found. Bothandwere encoded in the first cluster, HIS-I (Hcp1 secretion island I) of. Yet in a second cluster (HIS-II),andgenes were discovered to have similar encoding functions asand. The PpkA and PppA work antagonistically in regulating Hcp1 secretion [ 120 ].

Another significant gene,(intracellular multiplication F), was identified in the T6SSs ofsubsp.SQT1. Thewas previously reported on the inner membrane ofand located downstream of(defect in organelle trafficking U) gene [ 89 106 ]. Many studies suggested that bothandmay work together and interact in assisting the assembly and stability of a functionalcomplex [ 107 110 ] and are even required for virulence [ 111 112 ]. These genes are the only T6SS components with transmembrane domains that render a membrane channel traversing the bacterial cell envelope [ 89 102 ]. Thewas not observed in the draft genome sequence; however, twohomologs were found;(impaired in nodulation K) and(virulence associate secretion F), each encoded byandclusters. The outer membrane (OM) ImpK protein-coding gene is among eight genes encoding proteins with unknown functions (ImpA, ImpB, ImpC, ImpD, ImpF, ImpH, ImpI, ImpJ), and three other genes encode for avirulence locus ImpE protein, ImpG protein, and phosphatase ImpM protein. Theare homologous toand encode related proteins [ 113 ]. ImpG protein had homology with VasA, while uncharacterized proteins ImpH, ImpI, and ImpJ are homologous to VasB, VasC, and VasE. Agene encoding type-VI secretion lipoprotein is also present, but not the(sigma-54 dependent transcriptional regulator),(type VI secretion protein) and(type-VI secretion-related protein). Thesystem is required for secreting proteins lacking an N-terminal signal peptide and has no effects on proteins with signal peptides such as chitinase and neuraminidase [ 89 104 ].

Based on subsystem feature counts on the RAST seed viewer, thesubsp.SQT1 consists of 18 of 27 T6SS genes ( Figure 8 ) and half encodes proteins with unknown functions. VgrG (valine-glycine-repeats G) appears to be this genome&#;s most plausible secreted protein. T6SS secreted the VgrG protein inand Hcp (hemolysin co-regulated protein) [ 104 ]. Hcp structurally creates hexameric rings with a central channel surrounding the VgrG tube. The VgrG punctures the outer cell membrane, allowing the Hcp tube to extrude and consequently breach the host cell membrane to transport the effectors/toxins into target cells. These proteins are notably essential components of the system and substrates of the T6SS [ 105 ].

T6SS is another central protein secretion system in the &#;membrane transport&#; category despite ABC transporters. T6SS is a complex structure composed of 13 to 15 proteins conserved in all T6SS clusters across species of Gram-negative bacteria [ 88 91 ]. T6SS is critical in delivering toxins into eukaryotic and prokaryotic cells, either host or competitors [ 89 91 ], and is always referred to as nanomachine. Horizontal gene transfer is the leading way of T6SS traits acquisition and is not the standard duplication [ 92 ]. Many studies have reported the implication of T6SS in virulence by modifying the cytoskeleton of the eukaryotic host and in broad, violent, and purposeful inter-bacterial competition to prevent the growth of rival cells [ 93 95 ]. Bacterial pathogens mediate virulence into neighboring bacteria to compete for a specific host niche [ 96 100 ], particularly the space and resources [ 101 ]. These toxins are crucial for significant fitness during host colonization as they generate immunity proteins to avoid self-intoxication or being targeted by sister cells [ 95 102 ]. The secretion system is distributed intoT6SS-I, T6SS-II, and T6SS-III, which are directly involved in respective cell subversion/pathogenesis, virulence, and bacterial competition [ 103 ].

The soft rotandencode approximately 80 and 160 ABC transporters per genome [ 84 ], while thesubsp.SQT1 encodes an average of 46 ABC transporters. The genera Pectobacterium and Pantoea are members of the order Enterobacterales, which is well-known for producing agriculturally harmful phytopathogens ( Figure 6 ). Dipeptide permease () is the major peptide transporter detected, followed by oligopeptide permease (). Branched-chain amino acid () and alkyl phosphonate () are the least peptide transporters in the genome. The peptide transport systems are commonly used to import peptides for nutrient sources and get cellular function signals [ 81 ]. The detection of dipeptide permease () and oligopeptide permease () systems is comparable with two hetero-oligomeric oligopeptide transporters found in 85 ], yetusesas the core peptide transport system.

Secretion of proteins across phospholipid membranes is one of the essential component strategies for many bacterial pathogens to invade susceptible hosts and promote virulence. They primarily draw attachment to eukaryotic cells, scavenge resources in an environmental niche, intoxicate target cells and disrupt their functions. The largest and most diversified superfamily, ABC transporters, play a pivotal role in bacterial pathogenicity and cell survival for thesubsp.SQT1. The ABC transport system comprises three proteins transporting substrates across cellular membranes using ATP binding and hydrolysis [ 80 82 ]. Two main transporter subgroups (importer and exporters) mediate the uptake of nutrients into the cell and extrude toxins and drugs. The third subgroup involves diverse cell functions [ 83 ]. Rather than saprophytes or animal pathogens, ABC transporter homologs are abundant in phytobacterial pathogens, strengthening their importance for pathogenesis.

Based on pathogenicity and virulence of thesubsp.DC283 strain, the T3SS of &#;membrane transport&#; and exopolysaccharides (EPS) of &#;cell wall and capsule&#; categories are the key pathogenicity factors contributing to Stewart&#;s wilt disease developing symptoms [ 70 ], with an essential type-III effector protein from the AvrE family, WtsE. Other virulence factors, such as quorum sensing, biofilms, endoglucanase, cell-wall-degrading enzymes, and transcription factors, are necessary for successful colonization and infection of host tissue [ 71 73 ]. The T3SS is associated with the water-soaked formation during the wilt phase, while EPS is produced during the leaf blight phase and causes wilt seedlings, dry and necrotic leave lesions, and stunting and pitch rot mature corn plants [ 74 75 ]. The structure of EPS stewartan from the corn pathogen bacterium was published in -ties [ 76 77 ], with high-molecular-weight (~1.4 MDa) heteropolysaccharide and over repeating units [ 78 ]. Furthermore, a fragile yet robust flagellar was one of the imperative characteristics of a bacterium that mediates its surface-based motility, which is essential for community development [ 70 79 ]. In comparison, ATP-binding cassette (ABC) transporter, type-VI secretion system (T6SS), and EPS are the main pathogenicity tools used by thesubsp.SQT1 to deliver virulence factor into the host cells.

For this review, two available reports of (i) the complete genome sequence ofsubsp.DC283 (NCBI Ref. No: CP.1) [ 55 ] and (ii) draft genome sequence ofsubsp.SQT1 (NCBI Ref. No: PRJEB) [ 69 ] were compared and further discussed ( Table 3 ). Thesubsp.DC283 was isolated from the corn plant of the USA, which is associated with Stewarts&#; wilt disease. The genome size is 5,314,092 bp with 53.8% GC content, coding sequences (CDSs), and 100 RNAs. Meanwhile,subsp.strain SQT1 was isolated from Malaysian jackfruit suffering from a bronzing disease. The draft genome sequence is relatively shorter (4,783,993 bp long) with 67 contigs and contains less G+C content (53.7%), CDSs (), and RNAs (71). Rapid annotation using subsystem technology (RAST) has identified that 52% of genes fall into 27 active variants of subsystems. This review expanded the subsystem categories to the secondary metabolism to discover specific genes or gene clusters of pathogenicity and virulence of the jackfruit-bronzing pathogen.

Both insecticide and antibiotic treatments have been applied to controlsubsp.on corn crop mainly in the US and some parts of Indonesia. Clothianidin, imidacloprid, and thiamethoxam are systemic insecticides reported to lower systemic infection by up to 85% [ 179 182 ]. Unlike insecticides used in-furrow at planting or applied as foliar treatments, antibiotics have only been studied. Antibiotic treatment can be applied to the short-lived jackfruit seeds for 30 days. To defend jackfruits from infection before the bacterium is transmitted [ 183 185 ]. Xinzhimeisu (the mixture of Streptomycin and Terramycin), Wuyijunsuvar.and Zhongshengjunsu (antibiotic 120) are several antibiotics that used to treat the corn seeds fromsubsp.infection [ 186 ]. Antibiotics-seeds soaking technique at the optimum temperature of 40 °C to 47 °C for 90 min can destroy the bacterium pathogen and stimulate the seed germination [ 187 ].

Chemical control using copper-based bactericides is one of the imperative mitigation strategies for the jackfruit-bronzing disease proposed by the DOA of Malaysia [ 25 ]. Copper is essential for normal plant development and growth [ 165 166 ]. However, it is harmful to cells at high doses as it could disrupt the enzyme active sites, interfere with the energy transport system, and compromise the integrity of cell membranes [ 167 ]. The copper sprays must be applied evenly to the jackfruit surface before the disease infections. The copper starts functioning by water on the jackfruit&#;s surface, forming exudates that weaken the acids and lower the pH. The solubility increases the dissolving and releasing of copper ions. When it comes into contact with bacteria, the ions make entry into the cell walls and disrupt the cellular enzyme activity. This bactericide spray is more effective yet less toxic by applying it frequently at low rates rather than infrequent applications at high rates. The fruit temperatures are low, humidity is low, and the fruit is not wet [ 168 ]. A relative ofsubsp.has shown sensitivity towards copper compounds and was relatively effective with copper bactericides to control the center rot disease of onion 170 ]. In particular, the copper prophylactic sprays have shown excellent efficacy in preventing plant diseases caused byspp., such as canker and bacterial spot of citrus [ 171 ], the bacterial blight of walnut) [ 172 ], leaf blight of onion () [ 173 ], the bacterial spot in pepper) and bell pepper () [ 174 175 ], bacterial spot and bacterial speck of tomato (L.) [ 176 ] and many more. In Nepal, the copper prophylactic spray has been effective against greening and canker of citrus, black rot of crucifers (), and bacterial leaf spot of pumpkin ()diseases [ 177 ]. On the other hand, long-term usage of copper may result in copper-resistant bacterial strains, making disease management more difficult. Once the targeted plant pathogen acquires resistance genes, the frequency of resistant strains in the pathogen population rapidly increases, and subsequent applications become less effective in disease management [ 178 ].

Intercropping may be enforced to combat the risk of jackfruit-bronzing and weeds and insects [ 156 157 ]. Comparatively, intercrops are more effective in utilizing light, water, and nutrients, making a lesser amount available to weeds. In addition, intercrops reduce the number of susceptible hosts by acting as a physical barrier to the susceptible or host plants [ 158 159 ]. In this way, the diversity of pests and diseases is enlarged, thus plummeting the speed of pest adaption. Jackfruit and eggplant perhaps could be grown for the intercropping system [ 160 ] or jackfruit and pineapple [ 161 ], or jackfruit, pineapple, and aroid plant [ 162 ]. People in Bangladesh are used to the agroforestry cropping system, whereby more than one crop is planted in proximity for productivity, yield stability, and profit. The concept of multispecies systems has been practiced for decades [ 163 ]. The systems may include annual and perennial crops on a gradient of complexity from 2 species to more. Intercropping offers benefits such as upgraded soil health, decreased non-point source pollution by lowering nitrogen losses, excellent overall production, enhanced pest and disease control, enhanced ecological services, increased economic profitability, and improved ecological services.

The biological control method is one of the sound and effective means to control the growth ofsubsp.. No biological agents are available to inhibit or mitigate bacterial infection [ 151 ]. Forty years back, there was an effort to isolate a bacteriophage ofsubsp.fromand characterize it according to the host range. However, it has not been developed adequately enough to be used [ 155 ]. Only eight of the 13 pathogen strains were tested, partially completed phage characterization. It was hypothesized that virulent bacteriophages might efficiently eliminatesubsp.inside its beetle vector under field settings, but no further assertions or additional discussion was uncovered from the research effort.

Besides the disease-free seeds, jackfruit-resistant varieties are another imperative prevention strategy that commercial jackfruit growers may apply. In North America, planting resistant varieties (C123) has effectively controlled Stewart&#;s wilt disease, which is caused by the same causal pathogen [ 152 ]. The disease resistance gene is inherited and shown in four inbred lines of dent corn by quantitative and qualitative analyses [ 153 ]. In Malaysia, there are a total of 15 jackfruit varieties; J2, J27, J28, J29, J30, J31 (NS1), J32 (Mantin), J33 (Tekam Yellow), J34 (Hong), J35 (Crystal Jackfruit 1), J36 (Crystal Jackfruit 6), J37 (Mastura), J38 (Subang Lao Zhang), J39 (Subang Chap Boy) and J40 (CJ3) registered for the national crop list [ 154 ]. A recent publication has reported the jackfruit variety J39 was the most resistant to the bronzing disease. Meanwhile, variety J34 was most susceptible to the disease [ 24 ] with sweet pulps and nearly 15% brix composition. Choosing the jackfruits with much less flavor variety and a low brix may help thwart the incidence of jackfruit-bronzing disease composition [ 2 23 ].

The disease may potentially be transmitted by infected plant material and infected seeds. A disease-free seed from a certified source is the best choice to prevent the spread of the bronzing disease. The certified seed is generally handled under procedures acceptable to the Department of Agriculture (DOA) and Forestry to maintain good genetic purity and identity [ 150 ]. The disease-free seed needs to be produced in areas where the disease is absent not to introduce the disease. If the seed is infected following the visual inspections for symptoms, an ELISA-based seed health test could be performed for confirmation. At the same time, regular inspections should be carried out on seed parent plants and the jackfruit growing field [ 151 ].

Additionally, a shallow waterway could be built around the infected tree to drain superficial liquid or water-containing bacterial inoculum to limit bacterial dispersal. If the bacterial is spread to the secondary host trees, the male inflorescence&#;s de-budding process should be performed in an instance. Finally, infected wrapping bags should be disinfected and removed from the plantation areas. While in disease-free areas, it is a must to avoid the entry of the causal pathogen by practicing reasonable sanitation procedures and using clean and sanitized planting tools and materials [ 149 ].

Jackfruit-bronzing attacks the fruit&#;s internal part, and the visual symptom can only be seen when the fruit is cut open. Continuous monitoring of the incidence could be practiced in commercial growing areas by checking on the male inflorescence and internal fruit symptoms by inspecting the peduncle [ 25 ]. Direct monitoring, such as eradicating the infected trees, is necessary and should be at the forefront of management control. It is recommended that jackfruits be inspected at regular intervals of two weeks from each tree in the jackfruit plantation. Considering environment factors, clone types, tree age, and entry point of the pathogen, one month or longer monitoring intervals may practically apply. In certain areas where bacterial diseases are already present, immediate control should be taken in limiting any access into and out of the infected plantation, whether humans, animals, or any tools and equipment [ 148 ]. Destroying diseased fruits, disinfecting and sanitizing the suspected trees and their surroundings, pruning the low branches, and restricting the number of fruits are such practices that should be implemented. All actions must be done cautiously to avoid any wounds or injuries, particularly to the developing jackfruits.

The peak incidences of jackfruit-bronzing were reported after or during the rainy season when the humidity ratio is relatively high. Healthy jackfruit trees are more tolerant to this disease, while stressed trees, mostly of nutrition imbalances, soil types, terrain conditions, and injury, are more vulnerable [ 25 ]. Therefore, adequate fertilization levels should be well maintained, concerning calcium and potassium, but not nitrogen and phosphorus. Excess soil moisture should be avoided to prevent the susceptibility of jackfruit trees to bronzing disease [ 147 ]. Cultural practices and chemical control are powerful mitigation strategies that could be integrated into managing the jackfruit-bronzing disease ( Figure 10 ).

7. Conclusions and Future Prospects

Jackfruit-bronzing disease is a recent trending threat to the jackfruit industries in Southeast Asia. Uncontrolled spread may weaken the enthusiasm of jackfruit growers and diminish the jackfruit plantation areas. In Malaysia, the DOA has reported that the planted areas of jackfruit cultivation significantly reduced from to Ha from to , reducing the harvested area from to Ha. In , the plantation areas were reduced to Ha and maintained the same hectares in . In line with the increasing population, sustainable jackfruit production needs to be secured to meet domestic and foreign demands. Moreover, further research is required to understand the pathogen&#;s nature and pathogenicity better.

Single-plex PCR targeting a single gene is the primary technique to identify

P. stewartii

subsp.

stewartii

from symptomatic jackfruit either by using the universal 16S rRNA and 16S rRNA ITS primers or housekeeping genes (e.g.,

gyrB

,

atpD

,

infB

, and

rpoB)

that are conserved in the genus and unique to each lineage, or subspecies-specific primers (e.g.,

hrp

and

cps

) in narrowing down to the subspecies level. The amplified band sizes are expected at 0.9 kb to 1.5 kb on the electrophoresed gel. The diagnostic procedure proceeds with DNA sequencing and gene sequence analysis (BLASTn) before depositing into the GenBank. To gain information on the

P. stewartii

subsp.

stewartii&#;s

evolutionary relationships, phylogenetic analysis is performed to analyze the genetic differences. More recently, multiplex PCR, loop-mediated isothermal amplification (LAMP), and quantitative PCR have been applied for multi-detection and the random detection of target sequences for large-scale studies to balance the needs, cost, and handling time.

Insights into the pathogen genome have shown that CPS and EPS as

P. stewartii

subsp.

stewartii

Contact us to discuss your requirements of jackfruit peptide exporter. Our experienced sales team can help you identify the options that best suit your needs.

&#;s core pathogenicity factors in disease infection. Besides, the

Dpp and Opp

have been discovered as the significant peptide of ABC transporters in virulence. Hence, there is a possibility that these two may not be the primary peptide transporters for the pathogen, the

TPP

, or any other mode of transportation that requires more evidence from in-depth experimental trials. The genomes of wild-type and mutant strains of

P. stewartii

subsp.

stewartii

SQT1 must also be investigated in host plants, as the precise T6SS effectors are currently unknown. This genomic information is vital to determine the sources and patterns of transmission during specific disease outbreaks and to revolutionize infectious disease epidemiology research. At the same time, it helps to understand the correlation between taxonomy and mechanisms of virulence imposed by this pathogen and scrutinize new environmentally-friendly control strategies further to combat the jackfruit-bronzing disease globally.

Associated Data

Data Availability Statement

Data sharing is not applicable.

Abstract

Mango (Mangifera indica L.), known as the king of fruits, has an attractive taste and fragrance and high nutritional value. Mango is commercially important in India, where ~55% of the global crop is produced. The fruit has three main parts: pulp, peel, and kernel. The pulp is the most-consumed part, while the peel and kernel are usually discarded. Mango pulp is a source of a variety of reducing sugars, amino acids, aromatic compounds, and functional compounds, such as pectin, vitamins, anthocyanins, and polyphenols. Mango processing generates peels and kernels as bio-wastes, though they also have nutraceutical significance. Functional compounds in the peel, including protocatechuic acids, mangiferin and β-carotene are known for their antimicrobial, anti-diabetic, anti-inflammatory, and anti-carcinogenic properties. The mango kernel has higher antioxidant and polyphenolic contents than the pulp and peel and is used for oil extraction; it&#;s possible usage in combination with corn and wheat flour in preparing nutraceuticals is being increasingly emphasized. This review aims to provide nutraceutical and pharmacological information on all three parts of mango to help understand the defense mechanisms of its functional constituents, and the appropriate use of mangoes to enhance our nutrition and health.

Keywords:

tropical fruit, nutraceutical composition, mango byproducts, health benefits

1. Introduction

Mango has been cultivated in India for more than years. Among the other tropical fruits, mango is the most popular dessert worldwide. Mango belongs to the Anacardiaceae family, which includes a number of deadly poisonous plants. It has a delicious taste (delightfully blended sweetness and acidity) and aroma, and high nutritional value. The production of mango continues to rank it as the predominant tropical fruit in the 21st century [1]. The global mango production reached 51 million tons in . Most of the mango crop is produced in Asia, especially India, where production has reached 22 million tons per year, remaining the top exporter in the world [1,2]. Due to the optimal geographical and climatic conditions, the state of Andhra Pradesh is ideal for fruit and vegetable cultivation. Mango, banana, papaya, coconut, pomegranate, sweet orange, grape, and cashew are the principal fruit crops cultivated in Andhra Pradesh. Mango is cultivated on about 0.37 million hectares in Andhra Pradesh, with a production of 4.69 million tons, which is the highest level of production in the entire country (a 20% share). Due to the rising international popularity of fresh and processed mango products, in &#;, India shipped about 36,000 tons of mangoes and 129,000 tons of pulp overseas [3]. Mango is cultivated in 85 countries worldwide; Asian countries, including India, China, Thailand, and Indonesia provide for 80% of the total world production ( a).

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Mango is economically important, being the third-largest agricultural product in India. The cultivation of mango in India covers around 2.26 million acres, which is 40% of the total area used for Indian fruit production ( b). Over 30 varieties of mangoes are commercially available in India. The huge diversity in Indian mango cultivation is due to the large number of cultivars (the country is home to approximately varieties, only a few of which are commercially cultivated) and wild varieties. Apart from Mangifera indica, many other species such as M. khasiana, M. andamanica, M. camptosperma and M. sylvatica have been reported in India [4].

Fruit plays an important role in the world&#;s economy and food security from various perspectives [5]. However, economic success or failure of commercial mango production primarily relies on local weather and climate changes. A recent study indicated that the frequency and severity of extreme weather events have affected mango production in Central Queensland, Australia [6]. Although mango trees are adapted to dry weather, weather factors like floods, rainfall, humidity, and temperature can influence tree growth, flowering, fruit growth, the color and size of fruit, and farmer income [6,7,8]. Temperature has a dominant influence on the appearance, quality, and taste of mango fruit. The increasing temperatures are offering opportunities for mango production in new areas. The mangoes grown in the northern states of India and Thailand require longer to mature than those in the central and southern states [7]. High temperature has positive effects on mango fruit growth and maturation. The estimated duration of mango fruit development decreased by 12&#;16 days in Australia due to the rise in winter temperature (1.5 °C) over the last 45 years [9]. In addition, high-temperature-induced stress can trigger the synthesis of secondary metabolites, which are popular due to their nutritional and medicinal values [10].

Mango contains a blend of sugar (16&#;18% w/v) and acids and high amounts of antioxidants (ascorbic acid) and polyphenols (carotene, as vitamin A). The principal carbohydrates that are used are different in green unripe and matured ripe mango [4]. Starch is the principal carbohydrate in green mango; during maturation, it converts to reducing sugars (sucrose, glucose, and fructose). Along with these carbohydrates, small quantities of cellulose, hemicellulose, and pectin are present in ripe mango [4]. Unripe mango tastes sour because of the presence of different acids, such as citric acid, malic acid, oxalic acid, succinic, and other organic acids, whereas the sweet taste of the ripened fruit is due to the blending of reducing sugar and the main acid source, malic acid [11]. High concentrations of β-carotene and other phytochemicals in mangoes can prevent leukemia and progression of prostate, breast, and colon cancers [12,13,14,15,16]. Mangoes can be differentiated in to three parts: pulp (mesocarp), peel (epicarp), and seed kernel (endocarp), as presented in .

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2. Mango Pulp

Pulp is the main and directly consumable part of the mango. Mango pulp accounts for 50s to 60% of the weight of the total fruit, and is used to prepare various products like juice, jam, puree, and nectar. Pulp is the source of many nutritional and functional compounds. Mango pulp is used in dairy and beverage industries as a flavoring agent and in neonatal food formulations. The market for mango-related products is steadily increasing, with an annual growth of 5% [1,3]. The Totapuri cultivar is used extensively for pulp production because of its high pulp-yielding rate. During the processing, 60% to 75% of pulp is produced and the remaining 25% to 40% is generated as a waste byproduct [17]. The pulp-processing units usually collect mangoes from the designated market yards. Around 10&#;12 tons of processing byproducts (peel and stones) are generated per day from a plant that processes 40 tons of Totapuri mangoes [18,19,20]. The factories dump this processing residue in open fields, the accumulation of which may cause environmental issues if not managed. This residue is usually used by surrounding people or farmers as feed for their cattle [21,22]. The World Health Organization (WHO) recommends the daily consumption of 400 g of fruits and vegetables, as their nutrients could prevent chronic diseases, such as heart diseases, cancer, diabetes, and obesity [23]. Therefore, demand is increasing for mango as a functional food. Bioactive compounds provide color to the fruits and vegetables; therefore, more quantities of colorful plant products should be consumed to promote health [24].

2.1. Nutritional Composition

The nutritional composition of mango pulp mainly depends on the type/variety of the mango, the locality and climatic conditions of its production region, and the maturity of the fruit [25,26,27,28]. The nutritional value of mango is shown in . Mango contains a variety of macro- and micronutrients. In terms of macronutrients, the mango pulp contains carbohydrates (16&#;18%), proteins, amino acids, lipids, organic acids, as well as dietary fiber [29]. The pulp is also a good source of micronutrients, including trace elements such as calcium, phosphorus, iron, and vitamins (vitamins C and A). Consumption of mango pulp provides high energy: 60&#;190 Kcal from 100 g of fresh pulp. Along with the above essential nutritional elements, mango pulp contains 75&#;85% water ( ).

Table 1

Compound (Per 100 g)PulpPeelSeed KernelWater (g)83..59.1Energy (kcal)60 327Carbohydrate, by difference (g)14..218.2Protein (g)0.823.66.61Total lipid (fat) (g)0.382.29.4Sugars, total (g) Total dietary fiber (g)1.640&#;72.52.8 Minerals (mg) Calcium (Ca)Iron (Fe)0..611.9Magnesium (Mg)Phosphorus (P)14-140Potassium (K)Sodium (Na)Zinc (Zn)0.091.741.1Copper (Cu)0.04&#;0..4-Selenium (Se)0&#;0.6-- Vitamins Vitamin C (total ascorbic acid, mg)36.418&#;Thiamin (mg)0.028 0.08Riboflavin (mg)0.038 0.13Niacin (mg)0.669 0.19Pantothenic acid (mg)0.119 0.12Folate, dietary folate equivalents (μg)43 -Vitamin A, retinol activity equivalents (μg)-Vitamin E (α-tocopherol, mg)0.90.25&#;0.591.3Vitamin A (IU)-15Vitamin K (phylloquinone, μg)4.2 59Vitamin B12 0.12 Organic acids Citric acid (%)0.7 Mallic acid (%)0.5 Polyphenols Cyanidin (mg)0.1 Catechin (mg)1.7 Kaempferol (mg)0.13.6 Myricetin (mg)0.1 Proanthocyanidin dimers (mg)1.8 Proanthocyanidin trimers (mg)1.4 Proanthocyanidin 4&#;6 mers (mg)7.2 Gallic acid0.69 23&#;838Ellagic acid 3&#;156Coumarin 12.7Caffeic acid 7.7Vanillin 202Cinnamic acid 11.2Ferulic acid 10.4Mangiferin (mg) .2Mangiferin gallate 321 Isomangiferin 13.4 Isomangiferin gallate 82 Quercetin2.26.5 Rhamnetin 3-0 galactoside/glucoside 9.4 Tannins 20.7 Flavonoids (catechin equivalent/100 g) 0.9&#;9.219.91&#;75.35 Anthocyanins 360&#;565 Cyaniding 22.1 Pelargonidins 22.73 Delphinidins 18.02 Malvidins 5.26 Petunidins 21.6 Peonidins 24.42 Carotenoids (μg) β-carotene α-carotene9 β-cryptoxanthin Lycopene3 Lutein and zeaxanthin Open in a separate window

2.1.1. Macronutrients

Mango contains different types of carbohydrates based on the stage of maturity. Ripe mango is a rich a source of sugars (fructose, glucose, sucrose), whereas unripe mango is a source of starch and pectin. During the ripening process, starch is converted to fructose and glucose. The increase in mono- and disaccharide concentrations upon maturation is observed in many varieties, including Alphonso [30,31], Deshahari [32], and Tommy Atkins [33]. Pectin is a gelling sugar and is important for preserving the firmness of the fruit; its levels decrease during the ripening, making the flesh sweeter and smoother [26,34]. Sucrose, fructose, and glucose (in decreasing order of their concentration) are the principal sugars present in mature and ripe mango [26,30,35].

Compared to carbohydrates, the protein content in mango fruit is low (0.5&#;5.5%) ( ). Depending on the region of cultivation, mangoes have different protein contents. For example, the highest protein content is found in mangoes produced in Peru, and the lowest protein is found in those from Columbia. The content of constituent amino acids varies with the maturity level, region, and species of the fruit [36]. The usually occurring amino acids in the pulp are leucine (6.9 g), lysine (4.3 g), methionine (1.2 g), threonine (3.4 g), valine (5.8 g), arginine (7.3 g), glutamic acid (18.2 g), glycine (4.0 g) and proline (3.5 g) per 100 g protein [35].

2.1.2. Minerals and Vitamins (Micronutrients)

Vitamins and minerals are important micronutrients present in mango pulp. According to the United States Department of Agriculture (USDA) database, mango pulp comprises four different types of water- and fat-soluble vitamins. The concentrations of vitamins A and C are higher compared to those of vitamins B, E, and K ( ) [37]. Similar to the sugar content, the organic acid type and content changes as the fruit matures, being also dependent on the locality and species. The quantity of vitamin C ranges from 98 mg to 18 g/kg; thus, consumption of mango on regular basis could fulfill the recommended dietary intake of vitamins C and A [23]. Vitamin C, an antioxidant and immune booster, is required for collagen regeneration, scurvy prevention, and iron absorption [38]. The USDA [39] reported 36.4 mg/100 g of vitamin C, on average, in mango pulp ( ). Vitamin A and its metabolites have attracted interest due to their antioxidant activity, vision benefits, immunity, and beneficial effects against cancer and cardiovascular diseases [40]. The concentration of vitamin A ranges from &#; IU, and the consumption of one fresh fruit (250&#;300 g) provide 10&#;12% the recommended daily amount (RDA) of retinol. Mango consumption is thus one of the best ways to prevent vitamin A deficiency [41,42]. Compared to vitamin A, vitamin E and K concentrations are lower in mango pulp. Their amounts also increase as the fruit ripens. Conversely, Indian varieties like Deshahari possess low levels of vitamin E in mature fruit [43]. Fresh mango pulp (100 g) contains roughly 1.3 mg of α-tocopherol, an active form of vitamin E. [44]. An inverse relationship exists between the contents of vitamin C and vitamin E in mango fruit [45]. The vitamin B complex is important for both plant and human metabolism and nutrition. The mango pulp contains all the B complex vitamins except biotin. The B complex vitamin content in mango changes as the fruit matures, ranging from 1.5&#;2.5 mg/100 g of fresh fruit pulp [26]. Mango pulp is a good source of elemental minerals that are essential for a variety of biochemical reactions. The consumption of mango provides amounts of many micro- and macrominerals such as calcium, sodium, copper, iron, phosphorus, manganese, magnesium, zinc, boron (0.6&#;10.6 mg/kg), and selenium ( ).

2.1.3. Lipids and Fatty Acids

The lipid and fatty acid proportion in mango pulp is lower than the protein content and much lower than the carbohydrates. The fatty acid composition of pulp was studied in many mango varieties, and triglycerols were found to be the major constituent along with minor levels of mono- and diglycerols. The total content ranges from 0.8% to 1.36% [46]. Mango pulp contains essential fatty acids, the concentrations of which increase during ripening and stabilize finally at around 1% [47]. The fatty acid concentration is used as an index of the maturity of the mango, with a palmitic&#;palmitoleic acid ratio of one indicating the fully ripened stage [47,48].

2.1.4. Organic Acids

Mango pulp contains various organic acids including citric acid, malic acid, oxalic acid, succinic acid, ascorbic acid, and tartaric acid. Organic acids are generally weak acids with low molecular weight. The presence of organic acids provides fruits their characteristic tastes and flavors, playing an important role in their organoleptic quality [37]. Among others, citric acid, malic acid, and succinic acid are the major organic acids present in various types of Indian mangoes [49].

2.2. Phytochemical Composition

The mango pulp is known to have excellent bioactive compounds like carotenoids (pro-vitamin A, IU/100 g), phenolic acids, polysaccharides, sterols, and alkaloids. Mango is included in the TRAMIL list (a research project on medicinal plant resource in the Caribbean) as it is used by the indigenous people to treat diarrhea, fever, gastritis, and ulcers [50].

2.2.1. Phenolic Compounds

Phenolic compounds are the important secondary metabolite and can be categorized into phenolic acids and polyphenols. These are mainly present in combination with sugar moieties, linked to one or more phenolic groups, or can occur as ester or methyl-ester derivatives. Mango is one of the rich sources of these phenolic compounds.

Phenolic Acids

Phenolic acids are the important secondary metabolites that help to protect against various diseases and play pivotal role in management of human health. Pulp contains mainly hydroxycinnamic- and hydroxybenzoic-derivative phenolic acids, which are present in free form or conjugated with glucose or quinic acid or both. Gallic, syringic, vanillic, and protocatechuic acids are the main constituents of the hydroxybenzoic acid group, and p-coumaric, ferulic, chlorogenic, and caffeic acids are the main constituents of the hydroxycinnamic acid group [51]. The phenolic acid type and concentrations vary with mango variety, growing location, and maturity stage. In the pulp of most mango varieties, the ferulic acid has the highest concentration (33.75 mg), followed by chlorogenic (0.96&#;6.20 mg), gallic (0.93&#;2.98 mg), vanillic (0.57&#;1.63 mg), protocatechuic (0.77 mg), and caffeic (0.25&#;0.10 mg) acids per 100 g of fresh fruit weight [28]. Conversely, chlorogenic acid is the main constituent (90%) in Ataulfo mango pulp, followed by gallic (4%), vanillic (30%), and protocatechuic (56%) acids at the final ripe stage.

Hu et al. [52] identified 34 different phenolic acid derivatives, such as gallotannins and quercetin, and, for the first time, identified rosmarinic acid in mango pulp using UPLC-ESI-QTOFMS. Ramirez et al. [53] identified mangiferin, homomangiferin, and dimethyl mangiferin in the pulp of Tommy Atkins and Pica varieties. Mangiferin is a yellow crystal that chemically belongs to the xanthone family. Mangiferin is known for its pharmacological activities, including anticancer, antimicrobial, anti-atherosclerotic, antiallergenic, anti-inflammatory, analgesic, and immunomodulatory activities [26,54,55,56,57].

2.2.2. Pigments

The maturation stage of the fruit can be determined by changes in the color and texture of the fruit peel and flesh. Raw mango is generally green in color, changing to yellow or orange when fully ripe. The color change depends on the type of cultivar, and can also be an indicator of the quality of the fruit. There are several pigments, including chlorophylls, carotenoids, and flavonoids, that are responsible for the change in the color and metabolism of the mango fruit [58].

Chlorophylls

The green color of mango is due to the presence of chlorophyll. Two types of chlorophylls are found in mango fruit: chlorophyll a provides a blue-green color and chlorophyll b provides a yellow-green; these are present in a 3:1 ratio [59]. The chlorophyll content decreases as the fruit ripens, as the thylakoids start to collapse in the chloroplasts. Chlorophyll content decrease increases the carotenoid concentration in the pulp and peel of the fruit, and the color changes from green to golden yellow, red, or orange, depending on the variety. The chlorophyll content can be decreased by ethylene and by up-regulation of de novo synthesis of the chlorophyllase enzyme [60].

Carotenoids

Mango is one of the best sources of carotenoids; carotenoids are mostly responsible for the peel and flesh colors: yellow, orange, or red. Carotenoids, which exist in chromoplasts, are usually covered by chlorophyll and non-photosynthetic plant tissues [60]. Carotenoids present in the mango belong to two main groups: hydrocarbon carotenoids or carotenes (α-, β-, and γ-carotene) and xanthophylls or oxygenated derivatives (auraxanthin, antheraxanthin, neoxanthin, lutein, violaxanthin, cryptoxanthin and zeaxanthin). There are 25 different carotenoids that have been identified in the pulp and peel of mango. Among them, all-trans-β-carotene is the most abundant (around 60% of the total carotenoid content) followed by the all-trans and 9-cis-violaxanthin [25,35,56,61].

The carotenoid content usually changes depending on the fruit maturity stage and the local environment. Ellong et al. [27] reported four different varieties of carotenoids in the Bassignac variety, and the highest content (almost two-fold) was noted in fully ripened mango (4.138 mg/100 g) compared to unripe mango. According to the Nutrient Database of the USDA [35], the Tommy Atkins mango contain 0.64 mg β-carotene, 0.009 mg α-carotene, 0.01 mg β-cryptoxanthin and lutein, and 0.023 mg zeaxanthin per 100 g. Manthey and Perkins-Veazie [62] suggested that the variation in carotenoid content mainly depends on the type of cultivar, not on the location of production.

2.2.3. Flavonoids and Flavanols

The important phytochemicals with antioxidant and anti-inflammatory activities, including ascatechins, quercetin, anthocyanins, kaempferol, rhamnetin, and tannic acid, belong to a class of flavonoids. A high proportion of quercetin and its glycosides (46.6 mg/kg) and lower quantities of kaempferol, rhamnetin, myricetin, and fistien are present in the fresh mango pulp [42,56]. Among the condensed tannins and pro-anthocyanins, catechin presents in higher concentrations (1.72 ± 1.57 mg per 100 g fresh weight (FW)) compared to epicatechin (0.15 ± 0.0 mg per 100 g FW). In addition to the above pro-anthocyanin monomers, mango pulp contains dimers, trimers and tetra-hexamer compounds [58,63]. In addition to the above-mentioned condensed tannins, mango pulp contains hydrolysable tannins, gallotannins, and a wide array of their derivatives in small quantities (2 mg/100 g) [64]. The chemical structures of major bioactive molecules that are present in three parts of mango fruit are depicted in .

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2.2.4. Phytosterols

Mango pulp is known to have low amounts of lipids and fatty acids, but the mango seed is a rich source of lipids. Vilela et al. [65] analyzed the lipid profiles in twelve M. indica L. cultivars grown on Madeira Island using GC-MS, and reported similar quantity and quality compositions of free and glycosylated sterols (44.8&#;70.7%) and fatty acids (22.6&#;41.9%) in the total lipophilic component. Vilela et al. [65] further identified lower quantities of long-chain aliphatic alcohols and α-tocopherol. These findings revealed that the consumption of 100 g of fresh mango can significantly benefit health by providing 9.5&#;38.2 mg of phytosterols (free and glycosylated) and 0.7&#;3.9 mg of ω-3 and ω-6 fatty acids.

3. Mango Peel

Mango fruit processing generates peel and kernel as the two main byproducts. Approximately 15&#;20% of the peel is not commercially used and causes pollution in landfills. However, mango peel consists of various valuable phytochemicals, including carotenoids, polyphenols, and other bioactive compounds, which have been reported to promote human health [66,67]. Owing to the high fiber content, mango peel has been used in a variety of food supplements to enhance their functional properties. Abbasi et al. [28] and Thokas et al. [68] have reported the presence of various bioactive constituents and dietary fiber in mango peel, which possesses antioxidant and free-radical-scavenging properties.

3.1. Nutritional Composition

The mango peel composition mainly depends on the maturity stage, locality, variety, and climatic conditions in its production region. As shown in , mango peel contains a variety of macronutrients (total carbohydrates (20&#;30%), protein, amino acids, lipids, organic acids as well as dietary fiber) and micronutrients. Dietary fiber is an important functional nutrient and its concentration in different mango varieties, ranging between 16% and 28% soluble and 29% and 50% insoluble fiber [69]. The content of vitamin C ranges from 188&#; µg/g, varying widely in different cultivars. Ajila et al. [69] reported 188&#;392 µg/g of vitamin C in both Badami and Raspuri varieties, and higher amounts in the ripened peel compared to the raw peel ( ). The presence of vitamin E (205&#;509 µg/g) in mango peel led to its use in the preparation of skin care products. The concentration of vitamin E is also higher in ripened mango peel than in raw mango peel ( ). The mango peel contains significantly higher levels than pulp of the following minerals: Ca > K > Mg > Na > Fe > Mn > Zn > Cu [70].

3.2. Phytochemical Composition

3.2.1. Polyphenols

Mango peel has a higher polyphenol content than mango pulp at all growth stages of the fruit [55,71]. Many reports have been published on the polyphenolic content of the mango peel for various varieties available, and the variance mainly depends on the maturity stage, locality, variety, and climatic conditions in its production region. The polyphenol content in the peel ranges from 55&#;110 mg/gm dry weight, and higher levels are found in the ripe than in the unripe peel [72]. Two more important phytochemicals, quercetin 3-O-galactoside and mangiferin, are also present in the peel. It was estimated that mango peel has 1.69 g of mangiferin/kg dry weight, and this content is temperature-dependent. At high temperatures, mangiferin concentration decreases as its derivatives content increases [54]. Due to this transformation process, xanthones from the benzophenone derivative form, which is helpful in the formation of xanthone C-glycosides in the mango peel [64,67] ( ). For the Uba cultivar from Brazil, the polyphenol content is 6&#;8% of dry weight, and the amounts of flavonoids and xanthones in the peel are 4.6 and 7.3 times greater than in the pulp [55]. Of the six xanthone derivatives identified in mango peel, mangiferin (C2-β-D-glucopyranosyl-1,3,6,7-tetrahydroxyxanthone), a C-glucosyl xanthine, has many pharmacological activities. Mangiferin is a glucosyl xanthone with a characteristic isomeric form (mangiferin + isomangiferin + homo-mangiferin), and was found to be in higher concentrations in mango peel than in pulp and seed [42]. Mangiferin is a predominant bioactive compound in mango tree bark and is found in the fruits, roots, and leaves as well. The amount of mangiferin and its derivatives is higher in the peel than in the pulp (22.15 and 9.68 mg/100 g FW, respectively) and in Pica compared to Tommy Atkins mangoes (4.24 and 3.25 mg/100 g FW, respectively) [53]. Higher concentrations of mangiferin were reported from peels of Uba and Tommy Atkins mangoes (12.4 and 2.9 mg/kg dry weight, respectively), which are cultivated in Brazil [55].

In the peels of Tommy Atkins cultivar, 18 gallotannins were identified; the total concentration was 1.4 mg/g dry weight, expressed as gallic acid equivalents (GAE). The identified gallotannins are galloyl-glucose and quercetin derivatives. The major quercetins are quercetin-3-O-galactoside, quercetin-3-O-xyloside, quercetin-3-O-glucoside, quercetin-3-O-arabinofuranoside, and quercetin-3-O-arabinopyranoside [53,54,73]. The highest concentration of phenolic compounds (66.02 mg/100 g FW) is present in the Pica cultivar from Chile. Seven phenolic acid derivatives, three procyanidin dimers, and four xanthenes (homomangiferin, mangiferin, and mangiferin gallate), in a total of 13 compounds, were identified in the Pica peel [53].

3.2.2. Carotenoids

Carotenoids are fat-soluble pigments that create the different fruit colors, such as yellow, orange, and red. Similar to pulp, mango peel contains high concentrations of carotenoids in the form of β-carotene, which provides vitamin A [55]. Unlike the pulp carotenoids, peel carotenoids have been less studied [74]. Ranganath et al. [75] analyzed the carotenoid compositions of different-colored mango peel at different phases of ripening, and identified eight carotenoids in 12 selected cultivars of various colors. They further identified content variation with respect to the color of the fruit. In all cultivars, β-carotene, cis-β-carotene, and violaxanthin isomers presented as the principal compounds. The highest content (31.18 µg/g FW) was observed in yellow-colored Arka Anmol, and the lowest content (0.74 µg/g) in Janardhan Pasand. The highest β-carotene concentration (13.01 µg/g FW) was reported in yellow-colored Arka Anmol, along with its presence in all the tested mango cultivars. The concentration of carotenoids usually increases during ripening and is high in the yellow-colored stage.

Anthocyanins are water-soluble pigments; their presence provides red, blue, and purple colors to the fruits. These are presently used as biocolorants instead of synthetic colors [76]. Anthocyanins are known for their beneficial effects in the prevention of various diseases, including cancer, diabetes, and neuronal and cardiovascular diseases, thereby promoting human health [77,78,79,80]. Different ranges of anthocyanin concentrations have been reported by different authors. For example, total anthocyanin content ranges from &#; µg/g dry weight (DW) in the fully ripe stage and from &#; µg/g in the raw and unripe stages [72]. Berardini et al. [54] reported very low anthocyanin content (0.21&#;3.71 µg/g DW) in some red-colored mangoes (Tommy Atkins). Ranganath et al. [75] observed the highest anthocyanin content in red-colored mango peels (228.2 µg/100 g FW) compared to yellow- and green-colored mango peels. The major anthocyanins observed in different-colored peels of the various cultivars of mangoes include cyanidin, pelargonidin, delphinidin, malvidin, petunidin, and peonidin.

4. Mango Seed Kernel (MSK)

Consumption of fresh fruit by individuals and large-scale processing by the pulp industries, results in a significant quantity of mango seeds generated as a byproduct [22,81]. The MSK accounts for approximately 35&#;55% of the weight of the fresh fruit depending on the variety [82]. Landfilling of mango byproducts causes environmental problems because they do not decompose quickly. However, use of mango seed for the extraction of oil and phytochemicals may be economically profitable and environmentally safe. The mango seed is comprised of kernel (68%), shell (29%) and testa (3%) [83,84]. Although enough information is available on the nutritional composition of mango seed kernel, the composition varies mainly due to varietal and geographical differences. The composition of the MSK is unique and comparable with other oil-yielding seeds such as cocoa butter, shia, illipe, kokum, and sal butter [85,86,87].

4.1. Nutritional Composition

Like mango pulp, the seed is rich in nutrients, and has been used for developing various value-added products. The MSK contains high amounts of carbohydrates, protein, lipids, and several minerals [56,88,89].

4.1.1. Carbohydrates

Mango seeds contain higher concentrations of carbohydrates: the 58&#;80% starch yields 21% of pure starch. The quality of mango seed starch is the same as tapioca starch [90] ( ). The quality and composition of carbohydrates mainly depend on the fruit variety and region of cultivation. Lakshminarayana et al. [91] analyzed 43 varieties cultivated all around India, and found a large variation in the concentrations of carbohydrates, proteins, and lipids. Sandhu and Lim [92] observed differences in the starch content of Chausa (75.6%) and Kuppi (80.0%) varieties.

4.1.2. Proteins and Amino Acids

The MSK contains 6&#;13% protein by DW and content, mainly depending on variety. Though the protein content is low in the seed, it is nutritious protein because of its essential amino acid composition, including leucine (6.9 g), isoleucine (4.4 g), methionine (1.2 g), phenylalanine (3.4 g), lysine (4.3 g), threonine (3.4 g), tyrosine (2.7 g), and valine (5.8 g) per 100 g protein [93]. These essential amino acids, except methionine, occur in higher levels in the MSK than the Food and Agriculture Organization (FAO)-referenced protein: the essential amino acids content in MSK is 70% higher compared to the standard proteins [94] ( ). The indigestibility and toxic nature of MSK flour is due to the presence of higher concentrations of tannins. Nevertheless, the protein present in MSK is high quality due to its high essential amino acid content and protein quality index [93,95].

4.1.3. Lipids and Fatty Acids

Lipids are highly nutritious, have high energy values, and are well-known for their functional properties. Mango seed kernel, containing 8.15% to 13.16% of oil, is an appealing lipid source because of its nutritive and health beneficial properties [96]. The physical and chemical properties of the lipids and fatty acids present in the MSK are listed in . The MSK fat characteristics are comparable with those of vegetable butter [86], with a predominant presence of stearic and oleic acids. In some varieties, the unsaturated fatty acid content, especially linoleic acid, is twice and even three times higher than that of saturated fatty acids [94]. The high stearic and oleic acid contents in the total fatty acid profile of the MSK is attributed to its higher stability than polyunsaturated fatty acids (PUFAs)-containing oils. Hence, the MSK oil is suitable and safe for daily cooking [85,96]. Furthermore, the antioxidant potency of 1% mango seed crude oil extract is comparable with that of 200 ppm butylated hydroxytoluene (BHT) [97]. The good quality of edible oil in MSK is comparable to soybean and cotton seed oil. The total phenolic content and induction period of MSK oil is greater than several commercial vegetable oils [98]. The yield of MSK oil can be increased by increasing temperature, time and volume of extraction solvent [84]. However, the safety and suitability of MSK oil is depends on the extraction procedure and quality of the MSK.

Table 2

Characteristic/CompositionContent Oil (%)11.5Free fatty acid (FFA%)0.22Moisture (%)0.21Iodine Value 54.6Refractive Index 1.457Melting Point (°C) 35.2Saponification 193Unsaponifiable Matter (%)1.68Peroxide Value (meq O2/kg)0.65Color R2.8 + 30YC16:0 7.43C18:0 37.5C18:1 45.59C18:2 5.48C18:3 0.40C20:02.48C22:0 0.45C24:0 0.40Campesterol 0.07%Stigma Sterol 10.66β-Sitosterol 58.63Δ5-Avenasterol 10.19Δ7-Stigmasterol 4.34Δ7-Avenasterol 19.10Open in a separate window

The fatty acids obtained from the MSK have many similarities with cocoa butter in terms of properties, such as solid fat content, triglycerides, crystallization, and melting point; therefore, it used to replace cocoa butter. Premium-grade mango seed fat and oils (extracted by modern technologies like supercritical fluids) have premium characteristics and, along with palm stearin, can be used in the preparation of temperature-resistant chocolates in tropical countries [87].

4.1.4. Minerals and Vitamins

The MSK contains high levels of minerals, specifically potassium (368 mg/100 g), calcium (170 mg/100 g), and phosphorus and magnesium (210 mg/100 g). The MSK also contains vitamins C and E (antioxidant vitamins) and other essential vitamins, including K, B1, B2, B3, B5, B6, B9, and B12, in various concentrations ranging from 0.1 to 1 mg per 100 g. In addition, about 15 IU of vitamin A can be found in the MSK. The MSK is one of the rich sources of vitamin B12 (0.12 mg/100 g), which is higher than the recommended daily intake of the vitamin (2&#;3 µg); therefore, MSK can be used to prevent vitamin B12 deficiency in vegetarians ( ) [70,102].

4.2. Phytochemical Composition

Similar to the pulp and the peel, the MSK is also considered a prospective source of polyphenols with potent antioxidant activity. Ahmed et al. [93] estimated that MSK extract contains about 112 mg GAE/100 g of total polyphenols, and the identified constituents were tannins (20.7 mg), gallic acid (6.0 mg), coumarins (12.6 mg), caffeic acid (7.7 mg), vanillin (20.2 mg), mangiferin (4.2 mg), ferulic acid (10.4 mg), cinnamic acid (11.2 mg), and unidentified compounds (7.1 mg/100 g) ( ). Soong and Barlow [103] found that the antioxidant activity of the MSK is greater than that of other fruit seeds, namely jackfruit, longan, and avocado. These reports suggest that the polyphenolic content of the MSK contributes to its potent antioxidant activity. Approximately 75% of the total tannins are in hydrolyzable form, which need to be processed before using in food and feed preparations to reduce their toxic effect in vivo. A large difference in the quantity of gallic acid (23 to 838 mg/100 g) and ellagic acid (3 to 156 mg/100 g GAE) was reported in the MSK. The high compositional variation was attributed to the differences in extraction methods ( ) [103]. Use of ellagitannins from natural sources was found to be more effective than using purified ellagic acid. The total phenolic content (TPC) of mango seed oil ranges from 9&#;10 TPC mg/g. Kittiphoom and Sutasinee [104] studied the composition of various phenolic compounds in MSK using the HPLC technique. For the first time, they reported hesperidin as the major compound. The MSK was further characterized using the number of functionally important phenolic compounds, phenolic acids, and antioxidant minerals (selenium, zinc, copper, manganese, and potassium). Among them, quercetin, mangiferin, isomangiferin, homomangiferin, kaempferol, and anthocyanins were found to be the phenolic compounds; gallic acid, caffeic, protocatechuic, coumaric, ferulic, and ellagic acids were reported to be the phenolic acids [88].

In the MSK, the total flavonoid content was estimated as about ± 120 mg catechin equivalent (CE)/100 g seed [105]. The reported flavonoids content was higher than those reported in previous investigations, at about 10&#; mg/100 g [55,69,106,107]. With the rich presence of functional compounds, the MSK shows potential for the preparation of functional foods with health benefits.

6. Conclusions

Mango is the main fruit produced in India, accounting for almost 55% of the global production [3]. Mango is one of the best sources of nutrient, such as carbohydrates, proteins, and fatty acids. Various parts of mango contain several bioactive phytochemical compounds, namely polyphenols, carotenoids, flavonoids, tannins, and vitamins. The amounts of bioactive and phytochemicals are different in the epicarp, pericarp, and mesocarp. In the recent years, mango cultivation and research has gained popularity due to their potent antioxidant and anti-inflammatory properties. Mango pulp is processed into a number of high-value products, like juice, puree, and jam, with the peel and kernel as byproducts. Mango peel is known to contain pectin, dietary fiber, vitamins, carotenoids, and phenolic compounds, having health-promoting effects. Mango seed contains notable amounts of starch, essential amino acids, and oil. Mango seed oil is rich in oleic and stearic acids, and contains different phytochemicals. Mango seed has been used in the production of mango butter and seed flour, which are used in functional foods. The proper use of mango peel and seed (raw materials) in food and feed preparation not only improves the economy, but also reduces the environmental impacts. The bioactive compounds found in the three parts (pulp, peel and seed) of M. indica, such as mangiferin, gallic acid, catechin, quercetin, β-carotene, shikimic acid, and kaempferol, have been reported to have antioxidant effects. These compounds are also well-known for their anticancer, anti-diabetic, anti-inflammatory, skin-protecting, neuron-protecting, antimicrobial, and anti-aging effects. However, more pharmacokinetics, pharmacodynamics, and clinical trials are required to assess the toxicity or the safe dose of these molecules. Nevertheless, mango is considered safe and even beneficial for cellular functioning, and regular consumption of fresh mango fruit and/or use of its byproducts could promote overall health.

Author Contributions

V.R.L. and M.K. conceptualized and prepared the original draft; W.Y. and Y.-J.W. reviewed, edited the manuscript, and supervised the study. All authors have read and agreed to the published version of the manuscript.

Funding

We thank Shen Ma for the support from the grant (18YJA), Humanities and Social Science Research Fund, Ministry of Education, China.

Institutional Review Board Statement

Not applicable for this review article.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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