Brassinosteroids (BRs) are a group of steroid hormones that play essential roles in plant growth and development. However, the roles of BRs in plant defense to pathogens are mainly studied in Arabidopsis, and there are few studies reporting the effects of BRs on plant resistance to Botrytis cinerea, an important pathogen in horticulture. Here, we found that high levels of BRs increased B. cinerea resistance in tomato (Solanum lycopersicum). Enhancing the expression of DWARF (DWF), a rate-limiting gene in BRs synthesis, in tomato resulted in a decrease in lesion area in detached leaf assay, and attenuated the leaf damages in the intact plants assay, as compared to wild type. Application of 24-epibrassinolide (EBL) at a saturation level also induced the B. cinerea resistance, with the upregulation of protease inhibitor encoding genes. Silencing of BAK1, which encodes the coreceptor of BR receptor BRI1 and the critical regulator of innate immunity, could not suppress BR-induced B. cinerea resistance. Interestingly, the bioactive gibberellins (GAs) were strongly decreased, while the expression of GAI, the tomato homologue of DELLA which suppresses GA signaling, was increased in DWF-OE plants. Silencing of GAI strongly compromised BR-induced B. cinerea resistance. The results indicated that increasing the BRs levels by either overexpression of DWF or application of high dose of EBL enhanced tomato resistance to B. cinerea. BRs levels exceeding the optimum may inhibit GA synthesis and signaling, and thus enhance the defense response.
