The RcMYB21 transcription factor activates flavonol biosynthesis by targeting RcFLS in Chinese raspberry

Chinese raspberry (Rubus chingii Hu) is a medicinal and edible plant belonging to genus Rubus in traditional Chinese medicine. Flavonoids are key bioactive compounds found in abundance in raspberry. However, the regulatory mechanism underlying flavonoid biosynthesis in raspberry remains poorly understood.

Here, we identified RcMYB21, a novel R2R3-MYB transcription factor derived from R. chingii. RT-qPCR analysis revealed a co-expression pattern between RcMYB21 and the flavonol synthase gene RcFLS1. Subcellular localization and transactivation assays confirmed that RcMYB21 localizes to the nucleus and exhibits transcriptional activation activity, primarily mediated by its C-terminal domain. Further mechanistic studies (via Y1H and dual-luciferase assays) demonstrated that RcMYB21 directly binds to the promoters of key flavonoid biosynthesis genes, thereby activating RcFLS expression. Transient overexpression of RcMYB21 in raspberry leaves significantly upregulated RcFLS, while stable overexpression in tobacco (N. tabacum) resulted in elevated flavonol accumulation and suppressed anthocyanin levels in flowers, consistent with its role as a flavonoid pathway regulator. Notably, RcMYB21 also conferred stress adaptability. Under salt stress, RcMYB21-OE Arabidopsis exhibited enhanced lateral root development and improved tolerance, implicating its dual function in secondary metabolism and abiotic stress response.

We identified the RcMYB21 transcription factor in R. chingii. Our study indicated that the RcMYB21 transcription factor specifically regulates flavonoid biosynthesis and accumulation, and enhances salt stress tolerance through flavonol accumulation. These findings provide significant insights into the molecular mechanisms underlying flavonoid regulation in R. chingii and highlight the dual functional role of RcMYB21 in both secondary metabolism and stress response.