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

Stem mustard (<i>Brassica juncea</i> var. tumida <i>Tsen</i> 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 <i>F-box-containing flavin binding proteins</i> (<i>ZTL/FKF1/LKP2</i>), <i>phytochrome</i> (<i>PHY</i>), <i>cryptochrome</i> (<i>CRY</i>), <i>phototropin</i> (<i>PHOT</i>) and <i>UV RESISTANCE LOCUS 8</i> (<i>UVR8</i>) were identified in <i>Brassica</i> species. RNA-seq analysis revealed their expression patterns during organogenesis in <i>B. juncea</i>. Seven candidate PHRs were validated by qRT-PCR in <i>B. juncea</i> early-bolting ('YA-1') and late-bolting ('ZT-1') cultivars. <i>Agrobacterium</i>-mediated <i>BjuFKF1_1</i> overexpression (OE) lines resulted in significantly earlier flowering under field conditions. Histochemical GUS staining indicated that <i>BjuFKF1_1</i> was expressed in seedlings, leaves, flower buds and siliques. Transcript analysis revealed that the expression level of <i>BjuFKF1_1</i> 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 (<i>BjuCOL5</i>, <i>BjuSKP1</i>, <i>BjuCOL3</i>, <i>BjuAP2</i>, <i>BjuAP2-1</i> and <i>BjuLKP2</i>) were up-regulated in flower buds, whereas <i>BjuCOL</i> and <i>BjuPP2C52</i> were down-regulated in flowers. Developmental stage analysis revealed the up-regulation of five (<i>BjuAP2</i>, <i>BjuCOL3</i>, <i>BjuCOL5</i>, <i>BjuAP2-1</i> and <i>BjuLKP2</i>) and four (<i>BjuCOL</i>, <i>BjuCOL5</i>, <i>BjuAP2</i> and <i>BjuLKP2</i>) interaction protein-encoding genes during the reproductive stage under white and blue light, respectively. These findings elucidate the role of <i>BjuFKF1_1</i> in flowering regulation and provide molecular targets for <i>B. juncea</i> bolting-resistant variety breeding.