The B-box protein BBX13/COL15 suppresses photoperiodic flowering by attenuating the action of CONSTANS in Arabidopsis

The optimal timing of transition from vegetative to floral reproductive phase is critical for plant productivity and agricultural yields. Light plays a decisive role in regulating this transition. The B-box (BBX) family of transcription factors regulates several light-mediated developmental processes in plants, including flowering. Here, we identify a previously uncharacterized group II member of the BBX family, BBX13/COL15, as a negative regulator of flowering under long-day conditions. BBX13 is primarily expressed in the leaf vasculature, buds, and flowers. Its spatial expression pattern is similar to the major flowering time regulators CO and FT. bbx13 mutants flower early, while BBX13-overexpressors exhibit delayed flowering under long days. Genetic analyses showed that BBX13 acts upstream to CO and FT in the flowering pathway and negatively regulates their expression. We found that BBX13 physically interacts with CO and inhibits the CO-mediated transcriptional activation of FT. In addition, BBX13 directly binds to the CORE2 motif on the FT promoter, a site where CO also binds. Furthermore, our ChIP data indicates that BBX13 reduces the in vivo binding of CO on the FT promoter. Through luciferase assay, we found that BBX13 inhibits the CO-mediated transcriptional activation of FT. All these data together suggest that BBX13/COL15 represses flowering in Arabidopsis by attenuating CO activity. Summary statementBBX13, a previously uncharacterized protein in the B-Box family of transcription factors, negatively regulates flowering time in Arabidopsis thaliana. BBX13 physically and genetically interacts with CO and inhibits the CO-mediated transcriptional activation of FT.