Metabolism And Regulation Of Volatile Esters In Peach Fruit

Volatile esters are important aroma components that contribute to fruity note of ripe peach fruit.The biosynthesis of volatile esters is catalyzed by alcohol acyltransferase(AAT),and hydrolysis is catalyzed by carboxylesterase(CXE).Identifying of genes and transcription factors associated with the formation and hydrolysis of volatile esters is of great significance for revealing mechanism of aroma quality and then to improve fruit flavor quality.In the present study,peach fruit is used to investigate the metabolism and regulatory mechanism of volatile esters.The main results are as follows:1.PpAAT1 was associated with biosynthesis of volatile esters in peach fruit.According to peach genome and phylogenetic tree analysis,28 candidate AATs related to ester synthesis were identified.Based on RNA-seq,PpAAT1 was the member with the highest transcript abundance in ripe peach fruit,whose expression pattern was significantly positively correlated with ester content.Recombinant protein PpAAT1 could catalyze the synthesis of esters in vitro.Based on the heterologous transgenic of tobacco and tomato fruits and the homologous transient overexpression of peach,increased expression of PpAAT1 resulted in significant increase of esters in planta.2.Transcription factor PpNAC1 is associated with biosynthesis of volatile esters by activating target gene PpAAT1 expression.RNA-seq revealed that there were 23 transcription factors whose expression patterens were positively correlated with PpAAT1.Tobacco double luciferase assay showed that PpNAC1 significantly activated PpAAT1 promoter activity by approximate 14 times.EMSA confirmed that PpNAC1 directly binds to two NAC binding sites of the PpAAT1 promoter.Homologous transient overexpression of PpNAC1 significantly promoted the expression of target gene PpAAT1,leading to a significant increase in ester content.Further studies in tomato showed that NAC transcription factor NOR activated SlAAT1 expression by direct binding to SlAAT1 promoter.Tomato fruit nor mutant produced lower SlAAT1 expression and lower ester content in relative to wild type.Further studies showed that peach PpNAC2 could also binds to the promoter of PpAAT1 and activates gene expression.BiFC showed that PpNAC1 and PpNAC2 could form homodimer or heterodimer.These results indicate that NAC transcription factors regulate content of volatile esters in fruit.3.Ethylene,MeJA and UV-B postharvest treatment regulate the synthesis of esters in peach fruit.Exogenous ethylene induced PpNAC1 expression,promoted accumulation of PpAAT1 transcripts and increased esters content.MeJA treatment maintained high expression levels of PpNAC1 and PpNAC2,and further maintained the transcripts level of PpAAT1 and esters content during postharvest cold storage;UV-B irradiation significantly inhibited PpAAT1 expression and ester accumulation.4.PpCXE1 was associated with hydrolysis of volatile esters in peach fruit.A total of 33 peach CXE members were identified by GXSXG motif of CXEs.Correlation analysis between ester content and gene expression showed that PpCXE1 expression negatively correlated with volatile esters content in different varieties of ripe peach fruit.Similar reverse pattern between CXE expression and ester content were also observed for peach fruits treated with MeJA and UV-B.Recombinant protein activity in vitro showed that PpCXE1 hydrolyzed volatile esters.Homologous transient overexpression in peach fruit and heterologous stable overexpression PpCXE1 in tomato fruit resulted in significant reduced content of esters.Subcellular localization showed that both PpCXE1 and PpAAT1 were presented in cytoplasm.Results mentioned above suggested that PpCXE1 and PpAAT1 were involved in metabolism of peach volatile esters.