SlSAM3 interacts with SlACS4 to facilitate brassinosteroid signaling-mediated tomato (Solanum lycopersicum) fruit coloring.

Brassinosteroids (BRs) are among the plant hormones that regulate climacteric fruit ripening. However, the precise molecular mechanisms underlying this regulatory process remain poorly characterized. In this study, exogenous 24-epibrassinolide (EBR) prominently promoted tomato fruit coloring and ethylene production, with a notable induction of S-adenosyl-L-methionine synthetase (SlSAM3) expression. Genetic knockout and overexpression of SlSAM3 demonstrated its pivotal role in accelerating tomato fruit coloration. Notably, EBR failed to promote coloration in slsam3 mutants, underscoring that SlSAM3 is required for BR to promote tomato fruit coloration. The results showed that interaction between SlSAM3 and 1-aminocyclopropane-1-carboxylate synthase (SlACS4) is enhanced by EBR. addition, Furthermore, transient silencing and overexpression of SlACS4 in wild-type and slsam3 mutants revealed that SlACS4 is a key contributor to tomato fruit coloration, while SlSAM3 is required for SlACS4 to exert its full promotive effect. Collectively, this study reveals an unique mechanism in which EBR promotes tomato fruit coloration by inducing SlSAM3 expression and facilitating its interaction with SlACS4, highlighting a distinct layer of crosstalk between BR and ethylene signaling pathways.