Leaves of maize (Zea mays L.) seedlings were supplied with different concentrations of abscisic acid (ABA). Its effects on the levels of superoxide radical (O₂^–), hydrogen peroxide (H₂O₂) and the content of catalytic Fe, the activities of several antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), the contents of several non-enzymatic antioxidants such as ascorbate (ASC), reduced glutathione (GSH), α-tocopherol (α-TOC) and carotenoid (CAR), and the degrees of the oxidative damage to the membrane lipids and proteins were examined. Treatment with 10 and 100 µM ABA significantly increased the levels of O₂^– and H₂O₂, followed by an increase in activities of SOD, CAT, APX and GR, and the contents of ASC, GSH, α-TOC and CAR in a dose- and time-dependent pattern in leaves of maize seedlings. An oxidative damage expressed as lipid peroxidation, protein oxidation, and plasma membrane leakage did not occur except for a slight increase with 100 µM ABA treatment for 24 h. Treatment with 1,000 µM ABA led to a more abundant generation of O₂^– and H₂O₂ and a significant increase in the content of catalytic Fe, which is critical for H₂O₂-dependent hydroxyl radical production. The activities of these antioxidative enzymes and the contents of α-TOC and CAR were still maintained at a higher level, but no longer further enhanced when compared with the treatment of 100 µM ABA. The contents of ASC and GSH had no changes in leaves treated with 1,000 µM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 µM) induced an antioxidative defence response against oxidative damage, but a high concentration of ABA (1,000 µM) induced an excessive generation of AOS and led to an oxidative damage in plant cells.

The interrelationship among water‐stress‐induced abscisic acid (ABA) accumulation, the generation of reactive oxygen species (ROS), and the activities of several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) was investigated in leaves of detached maize (Zea mays L.) plants exposed to –0.7 MPa water stress induced by polyethylene glycol (PEG 6000). Time‐course analyses of ABA content, the production of ROS, and the activities of antioxidant enzymes in water‐stressed leaves showed that a significant increase in the content of ABA preceded that of ROS, which was followed by a marked increase in the activities of these antioxidant enzymes. Pretreatment with an ABA biosynthesis inhibitor, tungstate, significantly suppressed the accumulation of ABA, and also reduced the increased generation of ROS and the up‐regulation of these antioxidant enzymes in water‐stressed leaves. A mild oxidative stress induced by paraquat, which generates O₂^– and then H₂O₂, resulted in a significant enhancement in the activities of antioxidant enzymes in non‐water‐stressed leaves. Pretreatment with some ROS scavengers, such as Tiron and dimethylthiourea (DMTU), and an inhibitor of NAD(P)H oxidase, diphenyleneiodonium (DPI), almost completely arrested the increase in ROS and the activities of these antioxidant enzymes induced by water stress or ABA treatment. These data suggest that water stress‐induced ABA accumulation triggers the increased generation of ROS, which, in turn, leads to the up‐regulation of the antioxidant defence system.

• The role of nitric oxide (NO) and the relationship between NO, hydrogen peroxide (H₂O₂) and mitogen-activated protein kinase (MAPK) in abscisic acid (ABA)-induced antioxidant defense in leaves of maize (Zea mays) plants were investigated. • Both ABA and H₂O₂ induced increases in the generation of NO in mesophyll cells of maize leaves, and H₂O₂ was required for the ABA-induced generation of NO. Pretreatment with NO scavenger and nitric oxide synthase (NOS) inhibitor substantially reduced the ABA-induced production of NO, and partly blocked the activation of a 46 kDa MAPK and the expression and the activities of several antioxidant enzymes induced by ABA. Treatment with the NO donor sodium nitroprusside (SNP) also induced the activation of the MAPK, and enhanced the antioxidant defense systems. • Conversely, SNP treatment did not induce the production of H₂O₂, and pretreatments with NO scavenger and NOS inhibitor did not affect ABA-induced H₂O₂ production. • Our results suggest that ABA-induced H₂O₂ production mediates NO generation, which, in turn, activates MAPK and results in the upregulation in the expression and the activities of antioxidant enzymes in ABA signaling.

The role of mitogen-activated protein kinase (MAPK) in abscisic acid (ABA)-induced antioxidant defense was investigated in leaves of maize (Zea mays) plants. Treatments with ABA or H₂O₂ induced the activation of a 46-kD MAPK and enhanced the expression of the antioxidant genes CAT1, cAPX, and GR1 and the total activities of the antioxidant enzymes catalase, ascorbate peroxidase, glutathione reductase, and superoxide dismutase. Such enhancements were blocked by pretreatment with several MAPK kinase inhibitors and reactive oxygen species inhibitors or scavengers. Pretreatment with MAPK kinase inhibitors also substantially arrested the ABA-induced H₂O₂ production after 2 h of ABA treatment, but did not affect the levels of H₂O₂ within 1 h of ABA treatment. Pretreatment with several inhibitors of protein tyrosine phosphatase, which is believed to be a negative regulator of MAPK, only slightly prevented the ABA-induced H₂O₂ production, but did not affect the ABA-induced MAPK activation and ABA-enhanced antioxidant defense systems. These results clearly suggest that MAPK but not protein tyrosine phosphatase is involved in the ABA-induced antioxidant defense, and a cross talk between H₂O₂ production and MAPK activation plays a pivotal role in the ABA signaling. ABA-induced H₂O₂ production activates MAPK, which in turn induces the expression and the activities of antioxidant enzymes. The activation of MAPK also enhances the H₂O₂ production, forming a positive feedback loop.

C2H2-type zinc finger proteins (ZFPs) have been shown to play important roles in the responses of plants to oxidative and abiotic stresses, and different members of this family might have different roles during stresses. Here a novel abscisic acid (ABA)- and hydrogen peroxide (H₂O₂)-responsive C2H2-type ZFP gene, ZFP36, is identified in rice. The analyses of ZFP36-overexpressing and silenced transgenic rice plants showed that ZFP36 is involved in ABA-induced up-regulation of the expression and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). Overexpression of ZFP36 in rice plants was found to elevate the activities of antioxidant enzymes and to enhance the tolerance of rice plants to water stress and oxidative stress. In contrast, an RNA interference (RNAi) mutant of ZFP36 had lower activities of antioxidant enzymes and was more sensitive to water stress and oxidative stress. ABA-induced H₂O₂ production and ABA-activated mitogen-activated protein kinases (MAPKs) were shown to regulate the expression of ZFP36 in ABA signalling. On the other hand, ZFP36 also regulated the expression of NADPH oxidase genes, the production of H₂O₂, and the expression of OsMPK genes in ABA signalling. These results indicate that ZFP36 is required for ABA-induced antioxidant defence, for the tolerance of rice plants to water stress and oxidative stress, and for the regulation of the cross-talk between NADPH oxidase, H₂O₂, and MAPK in ABA signalling.