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 BBX family member, BBX13/COL15, as a negative regulator of flowering under long‐day conditions. <italic>BBX13</italic> is primarily expressed in the leaf vasculature, buds, and flowers, showing a similar spatial expression pattern to the major flowering time regulators <italic>CO</italic> and <italic>FT. bbx13</italic> mutants flower early, while <italic>BBX13</italic>‐overexpressors exhibit delayed flowering under long days. Genetic analyses showed that <italic>BBX13</italic> acts upstream to <italic>CO</italic> and <italic>FT</italic> and negatively regulates their expression. BBX13 physically interacts with CO and inhibits the CO‐mediated transcriptional activation of <italic>FT</italic>. In addition, BBX13 directly binds to the <italic>CORE2</italic> motif on the <italic>FT</italic> promoter, where CO also binds. Chromatin immunoprecipitation data indicates that BBX13 reduces the in vivo binding of CO on the <italic>FT</italic> promoter. Through luciferase assay, we found that BBX13 inhibits the CO‐mediated transcriptional activation of <italic>FT.</italic> Together, these findings suggest that BBX13/COL15 represses flowering in Arabidopsis by attenuating the binding of CO on the <italic>FT</italic> promoter. [ABSTRACT FROM AUTHOR]