Study On The Mechanism Of Flavonoids In Photoprotection Of Apple

When excessive light energy was accumulated,the photosynthetic apparatus of plants faces the danger of photooxidation.In the long-term evolution process,plants have formed a series of strategies to defend against photodamage,which is called ‘photoprotection’.Because some flavonoid compounds have light filtering and reactive oxygen species scavenging effects,researchers generally believe that they play a role in the defense against photodamage.However,there is a wide variety of flavonoid compounds,it is still unclear whether all flavonoid compounds play the role in photoprotection.Even anthocyanins as an important subgroup of flavonoids,their role in photoprotection still exists dispute.Therefore,in this study,three different flavonoids content altered of apple transgenic materials with Md PGT2 overexpression,silence of Md FLS and Md MYB10 overexpression were used to explore the roles and mechanisms of flavonoids in photoprotection under strong light stress.The key research findings are as follows:1.Flavonoids synthetic pathway is regulated by strong sunlight stress.Long-term lowlight-adaptation ’GL-3’ apple seedlings were suddenly exposed to sunlight on sunny days,transcriptome assay showed that structural genes of flavonoid metabolic pathways were significantly up-regulated.Combined with flavonoid compound content analysis,it was found that dihydrochalcone compounds did not respond to sunlight,Quercetin-3-O-galactoside and quercetin-3-O-glucoside of flavonol increased significantly in response to sunlight,but quercetin-3-O-xyloside,quercetin-3-O-arabinoside and quercetin-3-O-rhamnoside not.The synthesis of anthocyanins increased significantly after sunlight exposure.In addition,visible light did not change flavonoids content in leaves.2.Dihydrochalcone has no photoprotective function.Overexpression of Md PGT2 in ’GL-3’ resulted in the accumulation of a large amount of trilobatin.The transgenic plants was used to explore the role of dihydrochalcone content in photoprotection.The increase of dihydrochalcone content did not affect photosynthesis,reflectance spectra,stomatal phenotype,chlorophyll content,xanthophyll cycle,and enzymatic and non-enzymatic antioxidant systems in apple plants.The maximum quantum efficiency of PSII(Fv/Fm)did not change before and after intensive light treatment in lab and field conditions between Md PGT2 overexpressing transgenic plants and wild-type.In addition,the differential expression gene enrichment analysis by transcriptome sequencing showed that compared with the wild-type plants,the accumulation of dihydrochalcone did not affect the expression of photosynthetic related genes,but mainly changed the protein processing in the endoplasmic reticulum pathway.3.The accumulation of flavonol has no effect on photoprotection.Compared with wild type ‘GL-3’,transgenic plants with silenced Md FLS had significantly decreased flavonol content.The pavement cells and stomata of the transgenic leaves became larger,which were consistent with many hormone signal related genes belong to differential expression gene.However,there were no significant difference between wild type and transgenic plants in terms of photosynthesis,reflectance spectra,chlorophyll content,xanthophyll cycle,and enzymatic and non-enzymatic antioxidant systems.The value of Fv/Fm were similar between transgenic and wild type leaves under strong light treatment in lab and field conditions.4.Anthocyanins play a complex role in photoprotection.Overexpression of Md MYB10 resulted in excessive accumulation of anthocyanins in leaves of ’Galaxy Gala’.The transgenic red leaves increased blue-green light absorption,but decreased the ratio of chlorophyll a/b,photosynthetic rate,the activities of key enzymes involved in the Calvin cycle,and the accumulation of photosynthates such as sorbitol and sucrose.So,the transgenic plants had shade plants characteristics.Transcriptome analysis showed that photosynthation-related genes were not enriched in the differential expression genes,but anthocyanin accumulation affected the carbohydrate and nitrogen metabolism compared with wild-type.The content of compounds in the carbon and nitrogen metabolic pathways was generally reduced,except that the content of glutamine and asparagine was higher in the transgenic red leaves through metabolomic analysis.In addition,compared with that in the wild type,the xanthophyll cycle capacity and the antioxidant systems remained unchanged while the contents of ascorbate and glutathione were lower in the transgenic red leaves.Compared with the wild type,the photoinhibition was less severe in transgenic red leaves under white,green or blue high light stress,but was more severe under strong red light stress.The value of Fv/Fm was similar in wild type and transgenic red leaves under UV-B and UV-A stress.Under strong sunlight stress in the field conditions,the photoinhibition degree was not alleviated in the transgenic red leaves,but became less severe in the transgenic red bark.In general,the accumulation of anthocyanin may alleviate the light absorption by plants,then lower the irradiation intensity to chloroplast,and protect plants from strong light stress.However,on the other side,the accumulation of anthocyanin may lower the photosynthesis,the major pathway to consume the absorbed light energy,and played a negative role in photoprotection.In conclusion,flavonoid biosynthesis pathway responded to strong sunlight stress in apple leaves.The synthesis of dihydrochalcone did not respond to strong sunlight exposure and elevated levels of dihydrochalcone had no effect on the photoprotection of apple leaves.The flavonol synthesis in apple leaves responded to strong sunlight stress,but the levels of flavonol content did not play a role in photoprotection.Anthocyanin synthesis in apple leaves responded to strong sunlight stress and participated in photoprotection,but its ultimate contribution to the photoprotection depended on the dynamic balance between the positive effect of anthocyanins in reducing light intensity through light filtering and the negative effect of anthocyanins in reducing photosynthetic capacity.