Reduced resource competition from the stem when brassinosteroid biosynthesis is inhibited enhances curd yield in cauliflower

Stems play diverse roles across different plant species and developmental stages. In cauliflower, both stem growth and resource allocation to the curd remain under-investigated, so in this study we examined stem and curd growth dynamics to identify potential connections between these processes. We found that the stem dry weight and length increased from budding to the stage of commercial maturity, whilst stem dry matter accumulation during this period was negatively correlated with stem length at budding and also with curd yield, implying that the stem acts as a sink organ and potentially competes with the curd. Module trait analysis of transcriptomic data revealed a strong correlation between brassinosteroid (BR)-related pathways and stem length and sink capacity. Treatment of seedlings and plants at the rapid elongation stage with the BR biosynthesis inhibitor propiconazole (PCZ) reduced stem dry matter accumulation and length, confirming that BR positively regulates these traits. Notably, treatment with 1000 μM PCZ increased curd yields in field-grown cauliflower and broccoli, and was associated with higher sucrose, cellulose, and dry matter accumulation in the curds, and enhanced the harvest index. Thus, our study shows that cauliflower yield can be increased by optimizing resource distribution between the stem and curd.