Reactive Oxygen Species Are Involved in Brassinosteroid-Induced Stress Tolerance in Cucumber

Brassinosteroids (BRs) induce plant tolerance to a wide spectrum of stresses. To study how BR induces stress tolerance, we manipulated the BR levels in cucumber (Cucumis sativus) through a chemical genetics approach and found that BR levels were positively correlated with the tolerance to photo-oxidative and cold stresses and resistance to Cucumber mosaic virus. We also showed that BR treatment enhanced NADPH oxidase activity and elevated H(2)O(2) levels in apoplast. H(2)O(2) levels were elevated as early as 3 h and returned to basal levels 3 d after BR treatment. BR-induced H(2)O(2) accumulation was accompanied by increased tolerance to oxidative stress. Inhibition of NADPH oxidase and chemical scavenging of H(2)O(2) reduced BR-induced oxidative and cold tolerance and defense gene expression. BR treatment induced expression of both regulatory genes, such as RBOH, MAPK1, and MAPK3, and genes involved in defense and antioxidant responses. These results strongly suggest that elevated H(2)O(2) levels resulting from enhanced NADPH oxidase activity are involved in the BR-induced stress tolerance.