BjuFKF1_1, a Plant-Specific LOV Blue Light Receptor Gene, Positively Regulates Flowering in Brassica juncea

Stem mustard (Brassica juncea var. tumida Tsen et Lee) is an important economic vegetable in China. Premature bolting induced by temperature fluctuations has become a major cultivation constraint. Photoreceptors (PHRs) serve as critical photosensor proteins that interpret light signals and regulate physiological responses in plants. In this study, five core PHR families, namely F-box-containing flavin binding proteins (ZTL/FKF1/LKP2), phytochrome (PHY), cryptochrome (CRY), phototropin (PHOT) and UV RESISTANCE LOCUS 8 (UVR8) were identified in Brassica species. RNA-seq analysis revealed their expression patterns during organogenesis in B. juncea. Seven candidate PHRs were validated by qRT-PCR in B. juncea early-bolting ('YA-1') and late-bolting ('ZT-1') cultivars. Agrobacterium-mediated BjuFKF1_1 overexpression (OE) lines resulted in significantly earlier flowering under field conditions. Histochemical GUS staining indicated that BjuFKF1_1 was expressed in seedlings, leaves, flower buds and siliques. Transcript analysis revealed that the expression level of BjuFKF1_1 was up-regulated in all tissues at both the vegetative and reproductive stages, whereas the expression of BjuFKF1_1 interacting protein-encoding genes were down-regulated in flowers. Under blue light, genes encoding interacting proteins (BjuCOL5, BjuSKP1, BjuCOL3, BjuAP2, BjuAP2-1 and BjuLKP2) were up-regulated in flower buds, whereas BjuCOL and BjuPP2C52 were down-regulated in flowers. Developmental stage analysis revealed the up-regulation of five (BjuAP2, BjuCOL3, BjuCOL5, BjuAP2-1 and BjuLKP2) and four (BjuCOL, BjuCOL5, BjuAP2 and BjuLKP2) interaction protein-encoding genes during the reproductive stage under white and blue light, respectively. These findings elucidate the role of BjuFKF1_1 in flowering regulation and provide molecular targets for B. juncea bolting-resistant variety breeding.