The BBX6-2–NF-YC4–JAZ1 complex mediates <i>SOC1</i> activation to control jasmonate-responsive flowering time in <i>Brassica juncea</i>

Jasmonate (JA) accumulation and signaling play key roles in regulating plant growth and flower development. The optimal timing of flowering is critical for the quality and yield of Brassica juncea. B-box (BBX) factors play roles in plant floral transition and stress response processes. However, whether BBX6-2 responds to JA signaling to regulate flowering time in B. juncea remains unclear. Here, we characterized the biological function of BjuBBX6-2 in response to JA in regulating the flowering time of B. juncea. Subcellular localization and transcriptional activation activity assays showed that BjuBBX6-2 localizes in the nucleus and exhibits transcriptional activation activity. Spraying BjuBBX6-2-overexpressing or BjuBBX6-2-silenced B. juncea plants with 50 μmol/L methyl JA significantly accelerated or delayed their flowering time, respectively. We demonstrated that BjuBBX6-2 interacts with the flowering factor Nuclear Factor Y, Subunit C4 (BjuNF-YC4), which interacts with Nuclear Factor Y, Subunit B2/3 (BjuNF-YB2/3). The BjuBBX6-2-BjuNF-YC4-BjuNF-YB2/3 multiple-protein complex bound to the promoter of the downstream flowering integrator gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (BjuSOC1) and promoted its expression. JASMONATE ZIM-DOMAIN 1 (BjuJAZ1), a key factor in the JA signaling pathway, interacted with BjuBBX6-2 to inhibit the activation of BjuSOC1 by BjuBBX6-2. In summary, BjuBBX6-2 cooperates with BjuNF-YC4 and BjuJAZ1 in response to JA signaling to participate in the flowering regulation of B. juncea. These findings highlight a previously uncharacterized mechanism of JA signaling-mediated flowering time regulation via interactions between BjuBBX6-2 and the integrator gene BjuSOC1, providing prospects for breeding enhanced B. juncea cultivars.