Study On The Regulation Mechanism Of UV-B Treatment Promoting The Enrichment Of Phenols In Fresh-cut Carrots

Due to the mechanical damage of fresh-cut fruits and vegetables,phenolic compounds are often synthesized in damaged and adjacent parts,resulting in the enrichment of polyphenols.For fruits and vegetables that are not prone to browning,more phenolic substances can be induced by cutting,thereby improving their antioxidant efficacy.Previous studies have indicated that the synergistic treatments of mechanical damage and other abiotic stresses can further induce the synthesis of phenols in fruits and vegetables.In this thesis,the effects of UV-B radiation on the synthesis and accumulation of phenolic compounds in fresh-cut carrots during storage were studied.Firstly,the optimal dose of UV-B was determined according to the amount of phenolics in wounded carrots.Secondly,the signaling role of reactive oxygen species（ROS）and ethylene（ET）in UV-B-induced phenolic synthesis in fresh-cut carrots was investigated.Finally,the molecular regulation mechanism of UV-B in promoting the synthesis and accumulation of phenolic compounds in fresh-cut carrots was revealed at the transcriptional level.The main research results are as follows:（1）Fresh-cut carrots were irradiated at 0.25,0.5,1,2,3 and 4 k J m^-2.The irradiated samples were stored at 15℃for 48 h,and the unirradiated samples were used as control.The results showed that all doses of UV-B treatment could increase the total phenol content and antioxidant activity of fresh-cut carrots.UV-B-treated samples with 1 KJ m^-2 underwent a more rapid increase in TSP content,relative to that of other doses.At this dose,HPLC analysis showed that UV-B irradiation mainly induced the increase of chlorogenic acid,caffeic acid,ferulic acid,3,4-dicaffeoylquinic acid,and 3,5-dicaffeoylquinic acid.In addition,UV-B treatment induced the increase of phenylalanine ammonia lyase（PAL）,cinnamic acid 4-hydroxylase（C4H）and 4-coumaroyl-Co A ligase（4CL）activities,which may be an important reason for the synthesis of phenols compounds in fresh-cut carrots.（2）The signaling role of ROS in the synthesis and accumulation of phenolic compounds in fresh-cut carrots after UV-B radiation was studied by analyzing the ROS levels in UV-B treated samples and the samples with ROS inhibitor and elicitor,respectively.The results showed that UV-B treatment induced the production of O₂^·-and the accumulation of H₂O₂ in fresh-cut carrots.In addition,the treatment of diphenyl iodide（DPI）,as an inhibitor of NADPH oxidase,inhibited the production of ROS in UV-B samples,depressed the increase of total phenolic content and antioxidant capacity,and reduced the activity of key enzymes（PAL,C4H,4CL）of phenylpropanoid metabolism.Glucose/glucose oxidase（G/GO）,as a H₂O₂-producing strain,increased the production of H₂O₂ in UV-B samples,promoted the increase of total phenol content and antioxidant capacity,and elevated the activity of phenylpropanoid metabolism related key enzymes（PAL,C4H,4CL）.These results suggest that ROS are positively correlated with the synthesis of phenolic compounds,and ROS play an important signaling role in UV-B-induced synthesis and accumulation of phenolic compounds in fresh-cut carrots.（3）The signaling role of Eth in regulating UV-B-induced biosynthesis and accumulation of phenolic compounds in carrot was investigated by analyzing the Eth level in UV-B treated samples and the samples with Eth inhibitor and elicitor,respectively.In addition,the treatment of 1-MCP,as an inhibitor of Eth,reduced the production of ROS in UV-B samples,inhibited the increase of total phenol content and antioxidant capacity,and depressed the activities of key enzymes（PAL,C4H,4CL）of phenylpropanoid metabolism;Conversely,the treatment of 1-MCP,an inhibitor of Eth,increased the production of endogenous ethylene in the sample,promoted the increase of total phenol content and antioxidant capacity,and accelerated the activities of key enzymes（PAL,C4H,4CL）of phenylpropanoid metabolism.These results suggest that Eth plays an important signaling role in UV-B-induced synthesis and accumulation of phenolic compounds in fresh-cut carrots.（4）After storage at 15℃for 1 h,whole,wounded and UV-B–treated carrots were subjected to whole transcriptome sequencing.The results showed that UV-B irradiation induced the up-regulation of a large number of genes in the phenolic synthesis related metabolic pathway“Phenylpropanoid biosyntheis”in fresh-cut carrots,and also affected the related genes in the metabolic pathways related to ROS and Eth biosynthesis.These results were consistent with the effects of UV-B treatment on the total phenolic content and ROS and Eth in fresh-cut carrots,indicating that UV-B treatment could induce biosynthesis of phenolic compounds in fresh-cut carrots by stimulating the expression of related genes in the phenylpropane,ROS and Eth metabolic pathways.In addition,the results of q RT-PCR were consistent with those of RNA-Seq,indicating that the results of RNA-Seq were reliable.