Physiological and transcriptomic analysis of effects of abscisic acid on grape fruit color quality

Objective This study aimed to investigate the effects of abscisic acid （ABA） treatment on color quality of grape fruits based on analysis of physiological indicators and transcriptome sequencing， so as to provide a theoretical basis for quality regulation and high-quality development of grape industry.Method Four table grape cultivars （Yuan Xiangfei， Yuan Jinxiang， Wagamichi and Shine Muscat） were used as materials. At initial veraison stage， the clusters were dipped in 250 mg/L ABA solution （T）， and those clusters dipped in the solution without ABA were taken as control. The control group in the first sampling was labeled CK0； the control groups in the second and the third samplings were labeled CK1 and CK2； the ABA treatments in the second and third samplings were labeled T1 and T2. Fruit longitudinal dia-meter， transverse diameter， berry weight， total soluble solids （TSS）， and contents of anthocyanins， carotenoids， chlorophyll a and chlorophyll b in berry skins of different grape cultivars were measured under different treatments. Transcriptome sequencing of grape skin samples of Yuan Xiangfei cultivar was performed to screen differentially expressed genes （DEGs）. The expression of key genes was validated through Agrobacterium-mediated transient transformation in Kyoho grapes.Result Compared with CK2， T2 treatment significantly increased （P<0.05） the longitudinal diameter， transverse diameter， and berry weight of Yuan Xiangfei cultivar （by 5.8%， 8.3% and 16.0%） and Yuan Jinxiang （by 8.9%， 5.4% and 16.5%）， while only transverse diameter and berry weight were significantly enhanced in Shine Muscat （by 8.2% and 21.7%）. Compared with CK2， total anthocyanin content increased highly significantly （P<0.001，the same below） in grape skins of the Yuan Xiangfei and Wagamichi cultivars under T2 treatment. Among the five anthocyanin components in the grape skins of Yuan Xiangfei and Wagamichi， the highest accumulation was observed in T2， which was significantly higher that than in the other four groups. Compared with CK2， the contents of carotenoid， chlorophyll a and chlorophyll b were decreased significantly in Yuan Jinxiang， while chlorophyll a was reduced in Wagamichi， and both carotenoid and chlorophyll a were significantly decreased in Shine Muscat. Transcriptome analysis identified 5379 DEGs in the grape skin of Yuan Xiangfei. GO functional annotation analysis of DEGs revealed that in the group of T2 vs CK2， biological processes were mainly annotated to temperature and mechanical stimulus responses. KEGG signal pathway enrichment analysis showed that： in the groups of T1 vs CK1 and T2 vs CK2， significant enrichments were observed in stilbenoid， diarylheptanoid， gingerol and flavonoid biosynthesis； compared with CK2， in the Yuan Xiangfei cultivar， the anthocyanin-related DEGs such as the genes of phenylalanine ammonia-lyase （PAL）， cinnamate-4-hydroxylase gene （C4H）， flavanone-3'hydroxylase （F3'H）， flavonol synthase （FLS）， dihydroflavonol 4-reductase （DFR） gene exhibited obvious upregulated expressions. Transient overexpression of Kyoho grape fruits showed that， after overexpression treatment of MYB transcription factor gene （VvMYB24）， the anthocyanin content in grape skin was significantly increased， and chalcone isomerase gene （VvCHI）， VvDFR， grape resveratrol dioxygenase （VvLDOX） also showed a highly significantly increased related expression.Conclusion ABA promotes anthocyanin biosynthesis in grape skins by upregulating expression of anthocyanin biosynthesis-related genes （VvPAL， VvC4H） and transcription factor genes （VvMYBA1， VvMYBA2， VvMYBA24） while inhibiting the expression of the transcription factor （VvMYBC2-L1）.