Heat shock protein 70 regulates the abscisic acid-induced antioxidant response of maize to combined drought and heat stress

We investigated the interaction between heat shock protein 70 (HSP70) and abscisic acid (ABA)-induced antioxidant response of maize to the combination of drought and heat stress. First, the increased activities of enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), induced by drought were less than those by heat or combined drought and heat stress, except some individual cases (e.g. CAT in leaves, GR in roots). Second, both HSP70 synthesis and H2O2 production increased prominently under drought, heat or their combination stress; the increase in leaves induced by drought and heat combination was the highest, followed by heat and by drought, while the increase in roots had not visible difference. Third, either in leaves or roots, pretreatment with ABA inhibitor, HSP70 inhibitor and H2O2 scavenger, significantly arrested the stress-induced increase of antioxidant enzyme activities, and ABA inhibitor and H2O2 scavenger obviously suppressed HSP70 synthesis, while HSP70 inhibitor slightly heightened H2O2 accumulation. Finally, 100 μM ABA significantly enhanced the activities of antioxidant enzymes, HSP70 expression and H2O2 production under stresses in comparison with ABA-deficient mutant vp5 maize plants without pretreatment. Thus, ABA-induced H2O2 production enhances the HSP70 synthesis and up-regulates the activities of antioxidant enzymes, resulting in the suppression of cellular reactive oxygen species (ROS) levels. Our results suggest that HSP70 may play a crucial role in ABA-induced antioxidant defense of maize to drought and heat combination.