Effect of Exogenous 2,4-Epibrassinolide (EBR) on Color Change in Tomato Fruit

Fruit ripening and color change form a complex physiological and biochemical process involving the accumulation and breakdown of a series of metabolites. Brassinolide plays an important role in the regulation of fruit ripening. In this study, the effects of exogenous EBR (2,4-epibrassinolide) and BRZ (Brassinazole, an inhibitor of BR biosynthesis) on fruit color change were investigated using ‘Micro-Tom’ tomatoes (Solanum lycopersicum L.) as an experimental material. The experiment was set up with five treatments: CK (distilled water + 0.01% Tween-80) and T1–T4 (0.05, 0.1, 0.15, 0.2 mg/L EBR). In addition, a BRZ-treated group (4 μmol/L BRZ + 0.01% Tween-80) was set up in a follow-up experiment. The results showed that different concentrations of EBR treatments significantly increased the carotenoid and lycopene contents and decreased the chlorophyll contents in fruits compared with CK, with the T3 treatment (0.15 mg/L EBR) showing the most significant effect. Simultaneously, EBR induced the expression of the carotenoid metabolism genes SlGGPPS, SlPSY, SlPDS and SlZDS and promoted carotenoid accumulation. On the 20th day, compared with the CK and BRZ treatments, chlorophyll a and chlorophyll b contents were significantly reduced by 20.06% and 46.03% respectively; the expression of the chlorophyll degradation-related genes SlNYC, SlSGR1, SlPPH, and SlPAO was upregulated under a 0.15 mg/L EBR treatment, accelerating chlorophyll degradation. Furthermore, the EBR treatment reduced fruit brightness (L*) and increased fruit red saturation (a*), while yellow saturation (b*) showed an increasing and then decreasing trend; on the 20th day, compared with CK and BRZ, the red saturation of the EBR treatment group increased by 125.57% and 67.37% respectively, while the brightness decreased significantly by 24.28% and 23.83% respectively. In conclusion, exogenous application of 0.15 mg/L EBR significantly accelerated fruit ripening and color transformation by promoting the accumulation of carotenoids and the degradation of chlorophyll.