Calcium and ZmCCaMK are involved in brassinosteroid-induced antioxidant defense in maize leaves.

Brassinosteroids (BRs) have been shown to enhance stress tolerance by inducing antioxidant defense systems. However, the mechanisms of BR-induced antioxidant defense in plants remain to be determined. In this study, the role of calcium (Ca2+) and maize calcium/calmodulindependent protein kinase (CCaMK), ZmCCaMK, in BRinduced antioxidant defense, and the relationship between ZmCCaMK and Ca2+ in BR signaling were investigated. BR treatment led to a significant increase in cytosolic Ca2+ concentration in protoplasts from maize mesophyll, and Ca2 + was shown to be required for BR-induced antioxidant defense. Treatment with BR induced increases in gene expression and enzyme activity of ZmCCaMK in maize leaves. Transient overexpression and silencing of ZmCCa MK in maize protoplasts demonstrated that ZmCCaMK was required for BR-induced antioxidant defense. The requirement for CCaMK was further investigated using a loss-of-function mutant of OsCCaMK, the orthologous gene of ZmCCaMK in rice. Consistent with the findings in maize, BR treatment could not induce antioxidant defense in the rice OsCCAMK mutant. Furthermore, Ca2+ was required for BR-induced gene expression and activation of ZmCCaMK, while ZmCCaMK was shown to enhance the BR-induced increase in cytosolic Ca2+ concentration. Moreover, our results also showed that ZmCCaMK and H2O2 influenced each other. These results indicate that Ca2+ works together with ZmCCaMK in BR-induced antioxidant defense, and there are two positive feedback loops between Ca2+ or H2O2 and ZmCCaMK in BR signaling in maize. [ABSTRACT FROM AUTHOR]