Abscisic acid-induced thermotolerance in maize seedlings is mediated by calcium and associated with antioxidant systems

Summary

ABA treatment significantly increased thermotolerance in maize seedlings. Pretreatment of maize seeds with CaCl₂ solution, which raises calcium content of maize seedlings, further enhanced the ABA-induced thermotolerance. In contrast, pretreatment with the Ca²⁺ chelator EGTA, which lowers calcium content of maize seedlings, decreased the ABA-induced thermotolerance. In addition, pretreatment with the plasma membrane Ca²⁺ channel blockers La³⁺ and verapamil, which are expected to inhibit the influx of extracellular Ca²⁺ into cells, also weakened the ABA-induced thermotolerance in maize seedlings. However, the calmodulin antagonists W7 and CPZ had little effect on the ABA-induced thermotolerance. Measurement of activities of the antioxidant enzymes SOD, CAT, APX and GPX and the level of lipid peroxidation (in terms of M DA) indicated that heat stress induced an oxidative stress in maize seedlings. ABA treatment enabled maize seedlings to maintain higher activities of these antioxidant enzymes and a lower level of lipid peroxidation at normal culture temperature and under heat stress. Ca²⁺ pretreatment further enhanced the ABA-induced increase in SOD and APX activities and lowered the heat stress-induced lipid peroxidation in the ABA-treated seedlings, but EGTA pretreatment had a contrary effect. These results suggest that the ABA-induced thermotolerance is mediated by Ca²⁺ and requires the entry of extracellular Ca²⁺ into cells across the plasma membrane; antioxidant enzyme systems take part in the final generation of the ABA-induced thermotolerance.