OsBBX2 Delays Flowering by Repressing Expression Under Long-Day Conditions in Rice.

Members of the B-Box (BBX) family of proteins play crucial roles in the growth and development of rice. Here, we identified a rice BBX protein, Oryza sativa BBX2 (OsBBX2), which exhibits the highest expression in the root. The transcription of follows a diurnal rhythm under photoperiodic conditions, peaking at dawn. Functional analysis revealed that possesses transcriptional repression activity. The was overexpressed in the rice japonica cultivar Longjing 11 (LJ11), in which and were non-functional or exhibited weak functionality. The overexpression of () resulted in a delayed heading date under a long-day (LD) condition, whereas the mutant exhibited flowering patterns similar to the wild type (WT). Additionally, transcripts of , , and were downregulated in the lines under the LD condition. interacted with (), and the double mutant displayed a late flowering phenotype comparable to that of . Furthermore, enhanced the transcriptional repression of through its interaction with , as demonstrated in the protoplast-based assay. Taken together, these findings suggest that the delays flowering by interacting with and co-repressing transcription.