Deciphering anthocyanin biosynthesis: Shading-induced accumulation in black rice grains and its molecular regulation

Black rice is highly valued for its elevated anthocyanin content, which confers significant nutritional benefits, potent antioxidant activity, and enhanced plant tolerance to abiotic stress. Currently, the frequent occurrence of low solar radiation has emerged as a major environmental constraint on rice production in Southern China. However, the specific effects of low solar radiation stress on anthocyanin content and the underlying mechanism of anthocyanin biosynthesis in black rice remain unclear. To address this, field experiments were conducted in 2022, 2023, and 2024, involving two black rice genotypes (Suixiangheinuo (SXHN) and Taiheixiang 9 (THX9)) and four shading intensities (0, 20, 40, and 60%) applied during the grain filling stage. Results showed that the total anthocyanin content (TAC) under 20 to 60% shading was 22.45–32.26% and 15.80–29.33% higher than that under 0% shading in SXHN and THX9, respectively. Shading stress (20-60%) primarily increased the accumulation of cyanidin-3-O-glucoside and peonidin-3-O-glucoside among the anthocyanin metabolites in black rice. The increase in TAC was likely associated with elevated activities of chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase, dihydroflavonol 4-reductase, and anthocyanidin synthase involved in anthocyanin biosynthesis. Concurrently, shading stress upregulated the expression of structural genes in the anthocyanin biosynthetic pathway and downregulated the anthocyanin degradation–related genes. Furthermore, transcription factors such as MYB, bZIP, C2H2, and GARP were identified as key candidate regulators of anthocyanin biosynthesis under low solar radiation stress. The findings of this study revealed the molecular regulation of shading-induced anthocyanin synthesis, providing guidance for enhancing anthocyanin accumulation in black rice cultivated under low solar radiation conditions in southern China.