Functions And Cross-talks Between Heat Shock Protein 70 And H₂O₂ In Abscisic Acid-Induced Antioxidant Defenses Systems Of Maize Plants

HSP70 is induced in plants exposed to heat, drought, the application of exogenous ABA or H₂O₂, and ABA invoved in the enhancement of antioxidant defense in maize seedlings exposed to the combination of heat and drought. However, it is uncertain whether HSP70 provides protection against ROS-induced cytotoxicity by upregulating cellular antioxidant defense enzyme, as the case in animal cells. We investigated the interaction between heat shock protein 70 and the abscisic acid -induced enzymatic response of maize to drought and heat combination.First, The increased activity of enzymes, such as superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), induced by drought was obviously less than those by heat or drought and heat combination except some individual enzyme (e.g. CAT in leaves, GR in roots), but only APX in leaves induced by drought and heat combination was significantly higher than that by heat.Second, HSP70 synthesis and H₂O₂ production was all prominently increased by drought, heat or their combination. In leaves, the increase induced by drought and heat combination was the highest, followed by heat and by drought. Moreover, pretreatment with ABA inhibitor, HSP70 inhibitor and H₂O₂ scavenger, significantly arrested the increase of antioxidant enzymes under stress conditions, and ABA inhibitor and H₂O₂ scavenger obviously suppressed HSP70 synthesis. However, Pretreatment with HSP70 inhibitor slightly heightened H₂O₂ accumulation caused by several stress treatments.Finally, pretreatment with 100μM ABA significantly enhanced the activity of enzymes, HSP70 expression and H₂O₂ production in comparison with vp5 maize plants without pretreatment under stresses.These results indicated that the damage caused by the combination stress is most severe and ABA-induced H₂O₂ production enhances the HSP70 synthesis, which up-regulates the activities of antioxidant enzymes, resulting in the suppression of cellular ROS levels. Therefore, HSP70 may play a role in ABA-induced antioxidant defense of maize to drought and heat combination.