BjuBBX6-1 interacts with BjuNF-YB2/3 to regulate flowering time and drought tolerance in Brassica juncea

An inappropriate flowering time and drought stress resistance in Brassica juncea (mustard) will decrease its yield and quality. B-box (BBX) transcription factors play an important role in the regulation of flowering and drought tolerance, but the biological functions and molecular mechanisms of the BBX family—especially BjuBBX6—remain largely unknown in Brassica juncea. In this study, we cloned the BjuBBX6-1 gene from Brassica juncea and found that it was expressed mainly in flowers and leaves. BjuBBX6-1 was localized in the nucleus and had transcriptional activation activity. Overexpressing BjuBBX6-1 caused earlier flowering compared with the wild type, while BjuBBX6-1 silencing in RNAi lines led to later flowering. BjuBBX6-1 interacted with flowering factors BjuNF-YB2 and BjuNF-YB3 and subsequently bound to the promoters of its downstream genes to promote BjuABI2 expression. However, it repressed BjuFLC and BjuGLK expression, resulting in earlier flowering. Under drought stress, overexpressing BjuBBX6-1 plants reduced chlorophyll content and antioxidant enzyme activities, and increased malondialdehyde (MDA) content, while silencing BjuBBX6-1 plants exhibited the opposite trend. The results indicate that BjuBBX6-1 negatively regulates drought tolerance in Brassica juncea. This study lays a theoretical foundation for in-depth research on the molecular mechanism of the BBX family in flowering regulation and drought tolerance breeding of Brassica juncea.