BnUC1 Is a Key Regulator of Epidermal Wax Biosynthesis and Lipid Transport in Brassica napus

The bHLH (basic helix-loop-helix) transcription factor AtCFLAP2 regulates epidermal wax accumulation, but the underlying molecular mechanism remains unknown. We obtained <i>BnUC1^mut</i> (<i>BnaA05g18250D</i> homologous to <i>AtCFLAP2</i>) from a <i>Brassica napus</i> mutant with up-curling leaves (<i>Bnuc1</i>) and epidermal wax deficiency via map-based cloning. BnUC1^mut contains a point mutation (N200S) in the conserved dimerization domain. Overexpressing <i>BnUC1^mut</i> in ZS11 (Zhongshuang11) significantly decreased the leaf epidermal wax content, resulting in up-curled and glossy leaves. In contrast, knocking out <i>BnUC1^mut</i> in ZS11-NIL (Zhongshuang11-near-isogenic line) restored the normal leaf phenotype (i.e., flat) and significantly increased the leaf epidermal wax content. The point mutation weakens the ability of BnUC1^mut to bind to the promoters of VLCFA (very-long-chain fatty acids) synthesis-related genes, including <i>KCS</i> (<i>β</i>-ketoacyl coenzyme synthase) and <i>LACS</i> (long-chain acyl CoA synthetase), as well as lipid transport-related genes, including <i>LTP</i> (non-specific lipid transfer protein). The resulting sharp decrease in the transcription of genes affecting VLCFA biosynthesis and lipid transport disrupts the normal accumulation of leaf epidermal wax. Thus, BnUC1 influences epidermal wax formation by regulating the expression of <i>LTP</i> and genes associated with VLCFA biosynthesis. Our findings provide a foundation for future investigations on the mechanism mediating plant epidermal wax accumulation.