Improved drought tolerance by early IAA- and ABA-dependent H2O2 accumulation induced by α-naphthaleneacetic acid in soybean plants

α-Naphthaleneacetic acid (NAA), a synthetic auxin, improves plant stress tolerance. A hydroponic experiment was carried out to investigate how NAA pretreatment could improve drought resistance in soybean seedlings. NAA pretreatment triggered an early increase in hydrogen peroxide (H2O2), indole-3-acetic acid (IAA) and abscisic acid (ABA), a late elevation of net photosynthetic rate (Pn), a late decrease in malondialdehyde (MDA) and electrolyte leakage and a rise in antioxidant enzyme activities during both early and late phases, under drought stress. However, the application of the ABA biosynthesis inhibitor tungstate and the IAA biosynthesis inhibitors l-Amino-oxyphenylpropionic acid and aminoethoxyvinylglycine resulted in an early decrease in H2O2 and an early and late decline in the activities of antioxidant enzymes in NAA-pretreated stressed plants. Moreover, the inhibitors also induced an increase in H2O2, MDA and electrolyte leakage and decrease in Pn in the late phase in stressed leaves pretreated with NAA. Similarly, the application of the NADPH oxidase inhibitor diphenyleneiodonium and H2O2 scavenger dimethylthiourea arrested the early accumulation of H2O2, but not of IAA and ABA in stressed plants pretreated with NAA, and blocked the early and late rise in antioxidant enzyme activities, the late increase in Pn and the late decrease in H2O2, MDA and electrolyte leakage induced by NAA in drought-stressed plants. Our data suggest that NAA application triggers early IAA- and ABA-dependent H2O2 accumulation, which, in turn, results in an enhancement of antioxidant capacity and drought stress tolerance in soybean seedlings.