Regulatory Mechanism Of UV-B Irradiation On Volatile Compounds Derived From Terpene Pathway In Peach Fruit

Light is an important environment factor that regulate plants growth and development.Effects of UV-B component(280-315 nm)in solar have been causing more concern in decades due to the depletion of the ozone.Terpenoids are the most abundant class of plant secondary metabolites,However,effects of UV-B irradiation on fruit volatiles derived from terpenoid metabolism remains unclear.In the present study,peach fruit is used to investigate the regulatory mechanism of UV-B irradiation on fruit volatiles.The main results are as follows:1.UV-B irradiation significantly affects volatile profiles of peach fruit.UV-B irradiation of peach fruit and leaves for 48 h reduced the flavor-related linalool by 60%,significantly inhibited accumulation of the C-13 norisoprenoids of ?-ionone derived from carotenoids,and induced a 3-fold accumulation of the sesquiterpene(E,E)-?-farnesene.2.PpTPSl and PpTPS3 are involved in the synthesis of linalool in peach.RNA-seq revealed altered levels of mRNAs for two terpene synthases:PpTPSl,a TPS-g subfamily member,decreased by 86%and PpTPS3,a TPS-g subfamily member,decreased by 53%.Heterologous expression in Escherichia coli show that both of them catalyze GPP to produce linalool and transient overexpression in tobacco and peach fruit showed PpTPS1 was localized in plastids and associated with production of linalool,while PpTPS3 was responsible for linalool and nerolidol biosynthesis in the cytoplasm.Combining with the expression levels of PpTPS1 and PpTPS3,we conclude that both PpTPSl and PpTPS3 cooperate with each other for the production of linalool during the peach ripening process.3.PpTPS2 is responsible for the synthesis of(E,E)-?-famesene in peach.RNA-seq revealed that transcript levels of PpTPS2 was increased by approximately 80-fold in response to UV-B irradiation.Phylogenetic tree relationship showed that PpTPS2 belong to TPS-b subfamily.PpTPS2catalyzes the formation of(E,E)-?-famesene in the cytoplasm based on results from in vitro heterologous expression in E.coli and in vivo transient overexpression in tobacco and peach fruit.In addition to fruit,induction of PpTPS2 and(E,E)-?-farnesene caused by UV-B irradiation was also observed in peach leaves.4.PpCCD4 is involved in the production of ?-ionone in peach fruit.The transcript level of PpCCD4 was reduced by 70%,and showed the similar trend of?-ionone content after 48 h of UV-B irradiation.Heterologous expression in Escherichia coli showed PpCCD4 produced ?-ionone toward ?-carotene as substrates.Content of ?-ionone positively correlated with transcript level of PpCCD4 during peach fruit ripening,both of which tended to increase.5.Bagging influenced peach fruit volatile production due to difference in UV-B transmittance.Bagging fruits in the field with black non-woven bags had less UV-B light when compared to white bags.Consequently,peach fruit treated with black bags showed higher transcript levels of PpTPS1 and content of linalool.Consumer test showed higher aroma liking and overall flavor liking of peach fruit treated with black bags.Compared to UV-C,UV-A,UV-420,visible light and a combination of UV-A/B/C treatment,UV-B irradiation resulted in the lowest concentration of linalool.Bags transmitting less UV-B light ameliorated the reduction in the flavour volatile linalool,indicating that flavour components of peach fruits can be modulated by selecting an appropriate source of environmental screening material.