Regulation of fruit ripening by the brassinosteroid biosynthetic gene SlCYP90B3 via an ethylene-dependent pathway in tomato

The essential role of ethylene in fruit ripening has been thoroughly studied. However, the involvement of brassinosteroids (BRs) in the regulation of fruit ripening and their relationship with the ethylene pathway are poorly understood. In the current study, we found that BRs were actively synthesized during tomato fruit ripening. We then generated transgenic lines overexpressing or silencing SlCYP90B3, which encodes a cytochrome P450 monooxygenase that catalyzes the rate-limiting step of BR synthesis. The expression level of SlCYP90B3 was positively related to the contents of bioactive BRs as well as the ripening process in tomato fruit, including enhanced softening and increased soluble sugar and flavor volatile contents. Both carotenoid accumulation and ethylene production were strongly correlated with the expression level of SlCYP90B3, corroborated by the altered expression of carotenoid biosynthetic genes as well as ethylene pathway genes in transgenic tomato fruits. However, the application of the ethylene perception inhibitor 1-methycyclopropene (1-MCP) abolished the promotion effect of SlCYP90B3 overexpression on carotenoid accumulation. Taken together, these results increase our understanding of the involvement of SlCYP90B3 in bioactive BR biosynthesis as well as fruit ripening in tomato, thus making SlCYP90B3 a target gene for improvement of visual, nutritional and flavor qualities of tomato fruits with no yield penalty.