Brassinolides Enhance Seed Kernel Development via CoSMT2-Mediated 24-Ethylsterol Accumulation in Camellia oleifera

Brassinolide (BL), the sixth major plant hormone, is synthesized from phytosterols. At present, research on the relationship among BL, phytosterols, and C. oleifera fruit development remains limited. In this study, we found that 0.5 mg/L of BL promoted the growth of seed kernel cells and increased the growth of ‘Huashuo’ seeds. BL treatment further promoted seed development and increased phytosterol content. 24-Ethylsterol, particularly stigmasterol, demonstrated a strong correlation with seed development. Furthermore, CoSMT2 was identified as a regulator of 24-ethylsterol synthesis. In addition, the CoMYB86 gene was found to be associated with CoSMT2. The seeds of Arabidopsis thaliana overexpressing CoSMT2 were fuller than those of the wild type, with BL and phytosterol contents significantly increased. The ratio of 24-ethylsterol to 24-methylsterol also increased significantly, from 1.49 to 2.93. These results provide new insights into understanding the effects of BL on C. oleifera fruit development and offer a new perspective on the balance between phytosterols and brassinolide.