Effect of Calcium and Calmodulin on Intrinsic Heat Tolerance in Relation to Antioxidant Systems in Maize Seedlings

Abstract

Pretreatment of maize (Zea mays L.) seeds with CaCl₂ solution significantly raised calcium and calmodulin levels and enhanced intrinsic heat tolerance in maize seedlings. This enhancement is specific to Ca²⁺ and cannot be substituted by Mg²⁺. In contrast, treatments with the Ca²⁺ chelator EGTA or plasma membrane Ca²⁺ channel blockers La³⁺ or verapamil lowered the intrinsic heat tolerance. Concurrent treatment with Ca²⁺ and Ca²⁺ -channel blockers (La³⁺, verapamil) also decreased the Ca²⁺ -enhanced heat tolerance. In addition, calmodulin antagonists chlorpromazine or W7 treatment eliminated the Ca²⁺ -enhanced heat tolerance, whilst W5, a less active analogue of W7, had little effect on the Ca²⁺ -enhanced heat tolerance. Measurement of antioxidant enzyme activities and lipid peroxidation showed that heat stress induced an oxidative stress in maize seedlings. External Ca²⁺ treatment enabled the seedlings to keep relatively higher activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) and lower levels of lipid peroxidation than Ca²⁺ -deficient treatments; on the contrary, EGTA treatment led to more rapid loss of SOD, CAT and APX activities and higher levels of lipid peroxidation in the seedlings under heat stress. In addition, concurrent Ca²⁺ and W7 treatment weakened the effects of Ca²⁺ treatment on SOD, CAT and APX activities and caused more severe lipid peroxidation. These results suggested that external Ca²⁺ can enhance the intrinsic heat tolerance of maize seedlings. This enhancement requires the entry of external Ca²⁺ into cells across plasma membranes and the mediation of intracellular calmodulin, and is associated with the increase of antioxidant system activity.