ABA signaling and metabolism on anthocyanin and sugar syntheses in grapes

We performed several experiments to clarify the effects of light wavelength, abscisic acid (ABA) signaling and ABA metabolism on the synthesis of anthocyanin and sugar in grapes. In experiments using either red or blue light-emitting diodes (LED), anthocyanin concentrations were higher in blue- than in red-LED-treated berries, although red-LED treatment increased anthocyanin concentrations compared to the untreated control. On the other hand, red-LED-treated berries showed higher levels of endogenous ABA, its metabolite phaseic acid (PA), and the expressions of 9-cis-epoxycarotenoid dioxygenase (VvNCED) gene, encoding an ABA precursor and ABA 8’-hydroxylase (VvCYP707A) gene, which is downstream enzyme catabolize ABA to PA. NDGA (nordihydroguaiaretic acid) (an inhibitor of NCED activity), which is an ABA biosynthesis inhibitor, and AS-6 (an antagonist of PYL-PP2C receptor interaction), which is an ABA signaling inhibitor treatment, decreased anthocyanin concentrations in grape berries. Light also influenced ABA biosynthesis and ABA signal transduction. Blue LED irradiation decreased protein phosphatase 2C (VvPP2C) gene expression levels; i.e., it increased ABA signal transduction. Application of ABA and abscinazole (Abz), which is the specific inhibitor of CYP707A, affected anthocyanin biosynthesis genes in different ways. ABA application upregulated MYB113-like, bZIP42-like, and UGT85A2-like genes, which are anthocyanin biosynthesis-relating genes, while Abz application induced bZIP42-like genes only. Induction of DNA methylation of the MYB113-like gene was found in ABA application only. The results suggest that exogenous ABA and endogenous ABA influence grape coloration through different pathways. The NDGA treatment significantly decreased the translocation of ¹³C-photosynthates in ¹³C-fed leaves to fruit clusters. The sucrose concentrations in berry skins were higher in the untreated control than in those with NDGA leaf treatment. The transcription levels of sucrose synthase 4 (VvSUS4) and sucrose transporter 4 (VvSUT4) in the untreated control skins were higher than those in the NDGA-treatment skins. NDGA treatment on leaves led to the suppression of endogenous ABA levels and the expression of the ABA signal transduction gene VvABF2 in skins. The results suggest that NDGA treatment on leaves inhibited the accumulation of both ABA and genes related to ABA signal transduction in berry skins and triggered the inhibition of anthocyanin and the accumulation of sucrose. These results clarify important details of the complex system by which light, ABA signaling and ABA metabolism regulate anthocyanin and sugar syntheses in grapes.