H(2)O(2) mediates the crosstalk of brassinosteroid and abscisic acid in tomato responses to heat and oxidative stresses

The production of H(2)O(2) is critical for brassinosteroid (BR)- and abscisic acid (ABA)-induced stress tolerance in plants. In this study, the relationship between BR and ABA in the induction of H(2)O(2) production and their roles in response to heat and paraquat (PQ) oxidative stresses were studied in tomato. Both BR and ABA induced increases in RBOH1 gene expression, NADPH oxidase activity, apoplastic H(2)O(2) accumulation, and heat and PQ stress tolerance in wild-type plants. BR could only induced transient increases in these responses in the ABA biosynthetic mutant notabilis (not), whereas ABA induced strong and prolonged increases in these responses in the BR biosynthetic mutant d (^im) compared with wild-type plants. ABA levels were reduced in the BR biosynthetic mutant but could be elevated by exogenous BR. Silencing of RBOH1 compromised BR-induced apoplastic H(2)O(2) production, ABA accumulation, and PQ stress responses; however, ABA-induced PQ stress responses were largely unchanged in the RBOH1-silenced plants. BR induces stress tolerance involving a positive feedback mechanism in which BR induces a rapid and transient H(2)O(2) production by NADPH oxidase. The process in turn triggers increased ABA biosynthesis, leading to further increases in H(2)O(2) production and prolonged stress tolerance. ABA induces H(2)O(2) production in both the apoplastic and chloroplastic compartments.