The RrHY5-RrNAC2-RrOSC5 Module Promotes UV-B-Induced Sterol Defence in Rosa roxburghii

Rosa roxburghii Tratt. is a high-elevation fruit shrub with strong ultraviolet-B (UV-B) tolerance, and plant sterols are critical for its UV-B adaptation, but their regulatory mechanisms remain unknown. Here, we identified a key oxidosqualene cyclase gene, RrOSC5, which encodes a functional cycloartenol synthase and is dynamically upregulated under UV-B stress. Overexpression of RrOSC5 enhanced sterol accumulation in both native and transgenic hosts, significantly improving their UV-B resistance. We further discovered that the transcription factor RrNAC2 directly binds to a non-canonical AC-rich repeat within the RrOSC5 promoter, activating its transcription. Crucially, heterologous overexpression of RrNAC2 in tobacco conferred the enhanced UV-B tolerance, establishing its role as a core regulator in this pathway. Notably, we found that the master UV-B signalling regulator RrHY5 physically interacts with RrNAC2, thereby integrating the RrNAC2-RrOSC5 sterol module Into the canonical UV-B signalling pathway. Our findings revealed a hierarchical RrHY5-RrNAC2-RrOSC5 cascade governs UV-B-induced sterol defence, wherein RrNAC2 drives protective sterol biosynthesis via RrOSC5, and both components synergistically enhance antioxidant and osmotic adjustment defences. This work provides novel insights into sterol-mediated plant adaptation to UV-B stress for molecular breeding of fruit plants.