VvBBX32 responds to high temperatures and mediates anthocyanin biosynthesis in 'Merlot' grape (Vitis vinifera L.) berries

Abstract

Heat waves are becoming increasingly frequent and intense during the growing season in global viticultural regions. The physiological performance and productivity of grapevines (Vitis vinifera L.) are negatively affected by excessive heat stress, especially under protected cultivation. The synthesis of anthocyanins in grapevine berries is significantly vulnerable to heat stress, resulting in substantial economic losses. However, little research has been conducted to elucidate the molecular mechanism of the suppressed coloration of grape berries. In this study, five-year-old potted 'Merlot' grapevines were exposed to heat stress (48/28 °C day/night) and normal ambient temperature (25/18 °C day/night) before veraison. The fruit quality indexes and the transcriptome of grape berries under different temperatures at different stages were investigated. The BBX family is crucial in modulating anthocyanin accumulation and in responding to heat stress. VvBBX32 was identified by the correlation analysis of differentially expressed genes (DEGs) and anthocyanin structural genes, suggesting its response to high temperatures. The transient overexpression of VvBBX32 revealed its function in promoting anthocyanin in grape berries. Moreover, we demonstrated that VvBBX8 and VvBBX11 physically interact with the VvBBX32 protein. Their transient overexpression in grape berries suggested that the two genes were positive regulators of anthocyanin synthesis. Furthermore, VvBBX8, VvBBX11, and VvBBX32 can activate BRASSINAZOLE-RESISTANT 1 (VvBZR1) by directly binding to its promoter. Poor pigmentation under heat stress was successfully rescued with the application of exogenous 24-epibrassinolide (EBR). Finally, these findings indicated the existence of a BBXs-BR signaling pathway that promotes anthocyanin synthesis in grape berries under heat stress, providing insight into the molecular regulation of anthocyanin biosynthesis through the integration of BR signaling under elevated temperatures.

Keywords: Anthocyanin; BBX32; Brassinosteroid signals; High temperatures.