BBX32a and BBX32b Regulate Flowering Time in Soybean Under Long-Day Conditions

Photoperiod regulates flowering time and maturity in soybean, thereby determining yield performance and latitudinal adaptation. However, the molecular network through which photoperiod regulates flowering remains incompletely elucidated. Here, we identify two BBX family transcription factors, BBX32a and BBX32b, that act as positively regulators flowering under long-day (LD) conditions in soybean. We demonstrate that BBX32a and BBX32b can form both homologous and heterologous dimers. The bbx32a and bbx32b mutants exhibit significantly delayed flowering compared to wild-type W82. However, the bbx32a bbx32b double mutants flower at a similar time to the single mutants, suggesting that the BBX32a-BBX32b heterodimer plays a central role in regulating soybean flowering. E3 and E4 upregulate the transcription of BBX32a and BBX32b, which repress E1 transcription to promote flowering under LD conditions. Genetic evidence demonstrates that BBX32a and BBX32b regulate flowering time, completely dependent on functional E3, E4 and E1 family genes. Four haplotypes of BBX32a were identified in 1295 soybean accessions; BBX32a^Hap3 exhibits significantly reduced nuclear accumulation relative to BBX32a^Hap1. The BBX32a^Hap1 allele is predominantly fixed in cultivated soybeans, whereas BBX32a^Hap2 and BBX32a^Hap3 alleles remain largely unexploited. Collectively, our findings identify novel genetic targets for developing novel soybean cultivars adapted to high-latitude regions, thereby maximising yield potential.