Brassinolides Enhance Seed Kernel Development via <i>CoSMT2</i> -Mediated 24-Ethylsterol Accumulation in <i>Camellia oleifera</i>

Brassinolide (BL), the sixth major plant hormone, is synthesized from phytosterols. At present, research on the relationship among BL, phytosterols, and <i>C. oleifera</i> 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, <i>CoSMT2</i> was identified as a regulator of 24-ethylsterol synthesis. In addition, the <i>CoMYB86</i> gene was found to be associated with <i>CoSMT2</i>. The seeds of <i>Arabidopsis thaliana</i> overexpressing <i>CoSMT2</i> 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 <i>C. oleifera</i> fruit development and offer a new perspective on the balance between phytosterols and brassinolide.