OsBBX2 Delays Flowering by Repressing Hd3a 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 <i>OsBBX2</i> follows a diurnal rhythm under photoperiodic conditions, peaking at dawn. Functional analysis revealed that <i>OsBBX2</i> possesses transcriptional repression activity. The <i>BBX2</i> was overexpressed in the rice japonica cultivar Longjing 11 (LJ11), in which <i>Ghd7</i> and <i>PRR37</i> were non-functional or exhibited weak functionality. The overexpression of <i>OsBBX2</i> (<i>OsBBX2 OE</i>) resulted in a delayed heading date under a long-day (LD) condition, whereas the <i>bbx2</i> mutant exhibited flowering patterns similar to the wild type (WT). Additionally, transcripts of <i>Ehd1</i>, <i>Hd3a</i>, and <i>RFT1</i> were downregulated in the <i>OsBBX2 OE</i> lines under the LD condition. <i>OsBBX2</i> interacted with <i>Hd1</i> (<i>BBX18</i>), and the <i>bbx2 hd1</i> double mutant displayed a late flowering phenotype comparable to that of <i>hd1</i>. Furthermore, <i>OsBBX2</i> enhanced the transcriptional repression of <i>Hd3a</i> through its interaction with <i>Hd1</i>, as demonstrated in the protoplast-based assay. Taken together, these findings suggest that the <i>OsBBX2</i> delays flowering by interacting with <i>Hd1</i> and co-repressing <i>Hd3a</i> transcription.