Genetic Basis Of Aroma/Flavor Regulation In Papaya Fruit

Maintaining quality throughout the supply chain is usually a key objective of fruit physiologists and postharvest technologists.Fruit flavor,aroma,softening,color,and sugar-acid blend are very important qualitative characteristics that are critical to consumer acceptance and the success of fruit on market.However,very little has been studied on unraveling regulatory genetic networks during papaya fruit ripening and whether involved genes were under domestication during human selection.In our study,we used next-generation sequencing to investigate the gene expression changes between cultivated(ZhongBai)and non-improved(AU9)papaya fruits at different fruit developmental stages.A total of 4607 unigenes were identified to be differentially expressed genes(DEGs)between cultivated(ZhongBai)and non-improved AU9 papaya fruits and among three different development stages.More DEGs were shared between the two genotypes at green and ripe stages than the ones at color breaking stage.We further annotated the DEGs using by BLASTp analysis(E-value threshold of 1e-6)against the reference Vitis vinifera Genoscope 12X(Grapevine)and a total of 3639 DEGs(79%)were successfully annotated.to Vitis vinifera genome.Majority of the annotated DEGs occurred between ZB_RP and ZB_GRN while the lowest was between AU9_RP and AU9 CB.More DEGs were annotated in ZhongBai stages compared to their corresponding stages in AU9.Gene Ontology(GO)enrichment analysis revealed that binding,oxidoreductase activities,transcription factors and primary metabolism were significantly enriched.In addition,there was enormous expression of genes related to nucleotide,nucleic acid binding,translation initiation as well as ribosome which might indicate control of ripening at both transcription as well translation level.Further analyses of the DEGs via KEGG pathway enrichment revealed biosynthesis of secondary metabolites,glycolysis,fatty acid degradation and phenylpropanoid pathway as significant determinant of flavour in AU9 and ZhongBai cultivar.Terpenoids,flavonoids,carotenoids were all identified in our study with ZhongBai having more than AU9 while sulphur containing amino acid were expressed more in AU9 than ZhongBai.Fatty acid degradation and phenylpropanoid pathway are sources of volatile precursors for various flavors in fruit and papaya as well.Glycolysis on the other hand plays a vital role in providing carbon skeleton of the upstream of various volatile compounds.These highlight the complexity behind fruit flavor and aroma where products from one pathway acts as precursor in the other.Fatty acid degradation and amino acid metabolism through phenylpropanoid contributed to AU9 and ZhongBai volatiles while glycolysis has a bigger role in replenishing precursors responsible for volatiles in both fatty acid and amino acid metabolism.Our study has shed more light on biosynthetic pathways for flavor compounds at different ripening stages and also in wild and cultivated carica papaya but further research is needed to confirm if there is a relationship between flavor volatiles and domestication of papaya.