The WRKY28-BRC1 Transcription Factor Module Controls Shoot Branching in Brassica napus

The trade-off between growth and defense is common in plants. We previously demonstrated that BnaA03.WRKY28 weakened resistance strength but promoted shoot branching in Brassica napus (rapeseed). However, the molecular mechanism by which WRKY28 promotes branching formation is still obscure. In this study, we found that BnaA01.BRC1, BnaC01.BRC1, and BnaC03.BRC1 are mainly expressed in the leaf axils and contained W-box cis-acting elements in the promoter regions. BnaA03.WRKY28 directly bound to the promoter regions of these three copies and inhibited their expression. The brc1 mutants, the BnaA01.BRC1, BnaC01.BRC1 BnaA03.BRC1 and BnaC03.BRC1 were simultaneously knocked out, mediated by CRISPR/Cas9, and exhibited excessive branching. The expression level of the ABA biosynthesis encoding gene NCED3 was significantly reduced in the mutant compared to that in the WT. Instead, the expression level of the ABA catabolism encoding gene CYP707A3 was significantly higher than that in WT. These results suggest that the excessive branching of the brc1 mutant may be caused by the release of ABA-mediated bud dormancy. This study provides direct evidence for the potential mechanism of the WRKY28-BRC1 transcription factor module contributing to shoot branching in rapeseed.