The transcription factor BoMYC2 negatively regulates seed size by activating cytokinin dehydrogenase BoCKX5 in Broccoli.

Broccoli is an economically significant vegetable with high nutritional and medicinal value. Seed size/weight is one of the important agronomic traits that determine crop yield, which is regulated by multiple plant hormones. However, limited information is known about the regulation of seed size control in broccoli. Here, we report that the basic helix-loop-helix transcription factor BoMYC2 exerts a negative regulatory influence on seed-related traits in broccoli. The overexpression of BoMYC2 in Arabidopsis and broccoli led to notably decreased seed size and seed weight, likely by repressing cell proliferation of embryonic. Besides, overexpression of BoMYC2 increased the fatty acid accumulation while reducing the protein and soluble sugar levels in stably transgenic broccoli seeds, affecting seed storage compounds composition and seed size. Based on DNA affinity purification sequencing, BoMYC2 targeted the cytokinin oxidase gene BoCKX5 by binding to the G-box motif in its promoter regions. BoMYC2 activated the expression of BoCKX5 to modulate cytokinin metabolism. The expression levels of cytokinin-related genes and jasmonic acid-related genes were further analyzed in transgenic broccoli seeds, implying that BoMYC2 leads to higher jasmonic acid and lower cytokinin hormone content, which might affect the cell proliferation in seeds. Furthermore, cytokinin levels were significantly lower in BoMYC2-overexpressing broccoli than in wild-type plants. Collectively, these findings reveal a BoMYC2-BoCKX5 regulatory module that controls seed size/weight, enriching our understanding of the internal mechanism in seed size regulation and providing promising targets for high-yield broccoli breeding.