The Roles Of Mitogen-Activated Protein Kinase In Abscisic Acid-Induced Antioxidant Defense In Leaves Of Maize Plants

Although the phytohormone abscisic acid (ABA) regulates many important aspects of plant growth and development, its main function is to regulate plant adaptive responses to various adverse environmental conditions. ABA can cause the generation of nitric oxide (NO) and reactive oxygen species (ROS) in various plant cells or tissues, induce the expression of antioxidant genes, and enhance the capacity of antioxidant defense systems, including enzymatic and non-enzymatic constituents. ABA, NO and ROS can activate mitogen-activated protein kinases (MAPKs), and the activation of MAPKs play important roles in the regulation of physiological status in plant response to various environmental stresses. ROS are an important intermediate component in the ABA-induced antioxidant defense. However, the detailed mechanism about how ABA induces antioxidant defense in plant cells remains to be determined.In the present study, the role of MAPK and the relationship between H₂O₂, MAPK, and NO in the ABA-induced antioxidant defense were investigated in leaves of maize (Zea mays L.) plants. The results as follows:To investigate the effects of ABA on the activation of MAPKs in leaves of maize plants, in-gel kinase assays were performed on protein extracts from the leaves of maize plants treated with ABA, using myelin basic protein (MBP) as a substrate. Treatment with 100 μM ABA led to a significant increase in the activity of a 46 kDa kinase. To investigate whether the MBP kinase can be activated by endogenous ABA, the ABA-deficient maize vp5 mutant was used. Treatment with 10% PEG for 2 h resulted in a significant increase in the activity of the MBP kinase in the wild-type, but only a slight increase in the mutant, indicating that water stress-induced ABA accumulation can activate the MBP kinase. To demonstrate the exogenous and endogenous ABA-activated MBP kinase is a MAPK-like enzyme, protein extracts from control- or ABA-treated leaves were immunoprecipitated with anti-phosphotyrosine monoclonal antibody 4G10, and then subjected to the in-gel kinase assay. Treatment with 100 μM ABA resulted in an increase in immunoprecipitated tyrosine phosphorylated-MBP kinase activity, when compared to the control. Moreover, pretreatment with the widely-used specific MAPKK inhibitors PD98059 and U0126 inhibited the increase in the activity of the MBP kinase induced by ABA. These results obtained from the above clearly suggest that the ABA-activated 46 kDa MBP kinase is a MAPK-like enzyme. However, pretreatment with PAO and 3,4 DP, two specific inhibitors...