BnaGSK3-BnaIDD16 module negatively regulates branch angle in Brassica napus

Brassinosteroids (BRs) play a key role in regulating plant height and leaf angle in plants. However, the mechanism underlying the BR signaling pathway involved in branch angle regulation is poorly understood. Here, we isolated a rapeseed (Brassica napus L.) mutant named dwarf and compact architecture 1 (dca1), which is controlled by a single semi-dominant locus. Map-based cloning revealed that this gain-of-function gene encodes a GSK3/SHAGGY-like kinase (BnaGSK3/DCA1). We demonstrated that BnaIDD16.A09 is a substrate phosphorylated by BnaGSK3/DCA1 and negatively controls branch angles by activating auxin biosynthesis in response to gravitropism. Further, we identified two elite haplotypes from natural germplasm with large variations in the BnaGSK3/DCA1 promoter that exhibited smaller branch angles and slightly shorter height, which have significant potential for improvement of rapeseed plant architecture. Our findings reveal a regulatory mechanism that the BnaGSK3-BnaIDD16 module integrates BR signaling to modulate auxin biosynthesis, thereby controlling branch bending in B. napus.