Signal Transduction Mechanism Of Anthocyanin Synthesis Regulated By Visible Light In Plum Fruit

Plum belongs to the Prunus genus in the Rosaceae.It is an important temperate fruit tree and has a long history of cultivation all over the world.Plum is the second largest stone fruit after peach.Among them,Chinese plum(Prunus salicina L.)is one of the most commercially valuable varieties.China is the world’s largest producer of plums.It is understood that the proportion of Chinese plums and their hybrids exceeds 70% of the global plum production.Anthocyanins are important secondary metabolites in fruits,which not only participate in a variety of metabolic reactions in plants,but also are widely used in various industries such as foods,health and fitness,and beauty products.The synthesis of anthocyanins in plants was regulated by external environment and internal factors.Among them,hormones and light have a great influence on the synthesis of anthocyanins.There were many studies on the effect of hormones on anthocyanin synthesis,mainly focusing on the regulation of abscisic acid and ethylene.The research on the mechanism of light-regulated anthocyanin synthesis was mostly focused on ultraviolet light,while the research on the mechanism of anthocyanin synthesis under visible light was less.Based on the results of the accumulation of anthocyanins in plum fruits in the dark,this study analyzed the regulatory mechanism of anthocyanin synthesis under visible light.The main results are as follows:1.The synthesis of anthocyanins under visible light is regulated by the photosynthetic electron transfer(PET)in photosynthesis.After plum fruits were treated under visible light and in the dark,the synthesis rate of anthocyanins in the peel under visible light was significantly higher than that of dark-treated fruits.The fruits were treated with photosynthetic inhibitors 3’,4’-dichlorophenyl)-1,1-dimethylurea(DCMU)and methyl viologen(MV),inhibiting the activity of photosystem II(PSII)and photosystem I(PSI),respectively.After treatments,the ATP and NADPH content in the fruit also decreased.The anthocyanin content was far lower than the control,and the expression of anthocyanin synthesis related genes Ps CHS,Ps DFR,Ps ANS and Ps UFGT were significantly down-regulated.The anthocyanin content of fruits after treatment with ATP,NADH and NADPH all increased significantly.It is speculated that ATP and NADPH produced by the photoreaction of photosynthesis under visible light can promote the synthesis of anthocyanins.2.The synthesis of anthocyanins under visible light was regulated by malate dehydrogenase(MDH).Through the evolutionary analysis of MDH family in plants,it was found that MDHs were distinguished by the different organelles they locate,and MDHs of different species in the same organelle are highly conserved.Four MDHs(Psm MDH1,Psm MDH2,Psch MDH,and Pscy MDH)from plums were cloned,and the expression levels of these genes were detected after different plum fruit treatments.The results showed that the expressions of Psm MDH2 and Psch MDH in the fruits treated with DCMU and MV were significantly reduced,indicating that the inhibition of photosynthetic electron transfer will affect the expression of MDHs.After using VIGS technology to silence the MDHs(Psm MDH2,Psch MDH and Pscy MDH)in plums,the anthocyanin content in the fruit was reduced to varying degrees,and the expression of anthocyanin synthesis related genes Ps DFR,Ps ANS and Ps UFGT were also significantly reduced.It shows that MDHs can promote the synthesis of plum fruit anthocyanin under visible light.At the same time,after using the mutant of Arabidopsis MDHs for low temperature treatment,the anthocyanin content in chmdh,mmdh1 and mmdh2 were significantly reduced,and the content of ATP and NADPH also decreased to varying degrees.These results indicate that the excessive reducing power produced by the photosynthetic can be transferred out of the chloroplast by the malate-OAA shuttle,and be used in other organelles to promote the synthesis of anthocyanins.3.The synthesis of anthocyanins under visible light is regulated by ethylene signals.The anthocyanin content of plum fruits treated with ethephon(Eth)and methylcyclopropene(1-MCP)showed opposite results,the anthocyanin content increased significantly after Eth treatment,while the anthocyanin was inhibited after 1-MCP treatment,indicating that Eth can promote the synthesis of anthocyanins under visible light.The anthocyanin content was restored after the co-treatment with DCMU and Eth compared with the DCMU treatment alone,indicating that the regulation of anthocyanin synthesis by photosynthesis was through the effect of ethylene.After treating the ethylene receptor etr1-1 mutant at low temperature,the anthocyanin content was also significantly reduced,and the ATP content in the mutant was significantly reduced,indicating that the ATP content in the cell plays an important role in ethylene synthesis.The Eth and 1-MCP treatment results of Arabidopsis mutants and plum fruits showed that ATP content under visible light affects ethylene,and ethylene can promote the synthesis of anthocyanins.4.Changes in the synthesis of anthocyanins in plum fruits after inhibiting respiration and photosynthesis under visible light.After sealed treatment of plum fruit,the content of anthocyanin in the peel first rose rapidly than the control,then the speed gradually decreased,and finally lower than the control.The sealed treatment inhibited the respiration of the fruit.The result of the change in the anthocyanin content in the meat was similar to that of the dark-treated fruit,which was significantly inhibited.The lack of light in the flesh is equivalent to dark treatment of the fruit,and the respiration was also affected after the seald treatment,resulting in a significant decrease in the anthocyanin content.It shows that both photosynthesis and respiration under visible light are involved in the synthesis of fruit anthocyanins.The anthocyanins did not change significantly in the early stage of bagged fruit after treatment under visible light,but some quercetin derivatives and neochlorogenic acid etc.showed significant differences with the control before exposure to light.It is speculated that the metabolism of anthocyanins in bagged fruits is regulated by another metabolic mechanism,which leads to greater changes in the content of compounds in the phenolic metabolic pathway.It showed that anthocyanin synthesis under visible light was closely related to respiration and photosynthesis,and bagged fruit may have a completely different anthocyanin regulation mechanism.Overall,this study shows that both photosynthesis and respiration are involved in the regulation of plum fruit anthocyanin synthesis under visible light.Visible light regulates the metabolism of malate through MDHs,and the excess reducing power generated by the photosynthetic photoreaction was transferred to the mitochondria through the malate-OAA shuttle,Then it affects the production of ethylene through respiratory electron transfer,and finally regulates the synthesis of anthocyanins through the ethylene signaling.