Brassinosteroids (BRs) and ABA co-ordinately regulate water deficit tolerance in maize leaves. ZmMAP65‐1a, a maize microtubule-associated protein (MAP) which plays an essential role in BR-induced antioxidant defense, has been characterized previously. However, the interactions among BR, ABA and ZmMAP65‐1a in water deficit tolerance remain unexplored. In this study, we demonstrated that ABA was required for BR-induced antioxidant defense via ZmMAP65‐1a by using biochemical blocking and ABA biosynthetic mutants. The expression of ZmMAP65‐1a in maize leaves and mesophyll protoplasts could be increased under polyethylene glycol- (PEG) stimulated water deficit and ABA treatments. Furthermore, the importance of ABA in the early pathway of BR-induced water deficit tolerance was demonstrated by limiting ABA availability. Blocking ABA biosynthesis biochemically or by a null mutation inhibited the downstream gene expression of ZmMAP65‐1a and the activity of ZmMAPK5 in the pathway. It also affected the activities of BR-induced antioxidant defense-related enzymes, namely ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), superoxide dismutase (SOD) and NADPH oxidase. In addition, combining results from transiently overexpressed or silenced ZmMAP65‐1a in mesophyll protoplasts, we discovered that ZmMAP65‐1a mediated the ABA-induced gene expression and activities of APX and SOD. Surprisingly, silencing of ZmMAP65‐1a in mesophyll protoplasts did not alter the gene expression of ZmCCaMK and vice versa in response to ABA. Taken together, our data indicate that water deficit-induced ABA is a key mediator in BR-induced antioxidant defense via ZmMAP65‐1a in maize.
Via Christophe Jacquet