Effects of blue-light irradiation on abscisic acid signaling and sugar translocation in Vitis labruscana L.H. Bailey grapevines

The effects of blue-light irradiation on abscisic acid (ABA) signaling, sugar metabolism and translocation, and photoreceptors and gene expressions were investigated to clarify the mechanism by which blue-LED irradiation increases sugar concentrations in grape berries (Vitis labruscana L.). Blue light-emitting diode (LED) irradiation increased the portion of ¹³C-photosynthates in the grapevine clusters that were fed ¹³CO₂; compared to the portion in the cluster in the untreated control. Fructose and glucose concentrations and the expressions of VvSWEET10, VvSUC11, and VvSUS4 in blue LED-irradiated berries were increased. The blue LED-irradiated berries’ sucrose concentrations were significantly lower than the untreated control at 14 days after treatment. We speculated that the blue LED-treated berries’ decreased sucrose was associated with the increased Sugars Will Eventually be Exported Transporter (VvSWEET10), sucrose transporter (VvSUC11), and sucrose synthase (VvSUS4) expressions and promoted the translocation of ¹³C-photosynthates from the leaves that were fed ¹³CO₂. Blue-LED irradiation increased the expressions of SNF1-related protein kinases (VvSnRK2.6) and ABA responding element binding transcription factor (VvABF1), while decreasing the expression of protein phosphateses 2C9 (VvPP2C9) genes, which are related to ABA signaling. Blue-LED irradiation increased the expressions of cryptochrome (VvCRYa) and phototropin (VvPHOT2), which are photoreceptor genes. The application of the pyrabactin resistance-like (PYL)-PP2C ABA receptor interaction antagonist AS6 did not affect endogenous ABA concentrations in the grape berries, but it decreased sucrose concentrations at harvest. The application of ABA did not affect sucrose, glucose, or fructose concentrations or the expressions of VvSnRK2.6 and VvPP2C9. The application of nordihydroguaiaretic acid (NDGA, an inhibitor of 9-cis-epoxycarotenoid dioxygenase activity in ABA biosynthesis) did not affect sugar concentrations at harvest. These results suggest that upregulation of photoreceptor gene expressions and ABA signaling are associated with sugar concentrations in grape berries.