Appropriate Drought Training Induces Optimal Drought Tolerance by Inducing Stepwise H2O2 Homeostasis in Soybean.

Soybean is considered one of the most drought-sensitive crops, and ROS homeostasis can regulate drought tolerance in these plants. Understanding the mechanism of H2O2 homeostasis and its regulatory effect on drought stress is important for improving drought tolerance in soybean. We used different concentrations of polyethylene glycol (PEG) solutions to simulate the progression from weak drought stress (0.2%, 0.5%, and 1% PEG) to strong drought stress (5% PEG). We investigated the responses of the soybean plant phenotype, ROS level, injury severity, antioxidant system, etc., to different weak drought stresses and subsequent strong drought stresses. The results show that drought-treated plants accumulated H2O2 for signaling and exhibited drought tolerance under the following stronger drought stress, among which the 0.5% PEG treatment had the greatest effect. Under the optimal treatment, there was qualitatively describable H2O2 homeostasis, characterized by a consistent increasing amplitude in H2O2 content compared with CK. The H2O2 signal formed under the optimum treatment induced the capacity of the antioxidant system to remove excess H2O2 to form a primary H2O2 homeostasis. The primary H2O2 homeostasis further induced senior H2O2 homeostasis under the following strong drought and maximized the improvement of drought tolerance. These findings might suggest that gradual drought training could result in stepwise H2O2 homeostasis to continuously improve drought tolerance.

α-naphthaleneacetic acid positively regulates soybean seed germination and seedling establishment by increasing antioxidant capacity, triacylglycerol mobilization and sucrose transport under drought stress.

Drought stress is an important constraint for the germination of soybean (Glycine max [L.] Merr.) seeds and seedling establishment. A pot experiment was conducted to determine the effects of priming soybean seeds with 5 µM α-naphthaleneacetic acid (NAA) and the mechanism responsible for the induced tolerance of drought stress (soil relative water content of 55%). NAA priming inhibited drought-induced oxidative damage in seeds, and further analysis indicated that it induced an early spike in hydrogen peroxide content by the upregulation of abscisic acid-dependent GmRbohC2, resulting in an enhancement of antioxidant capacity. Moreover, NAA priming also improved the hydrolysis of triacylglycerol (TAG) to sucrose in stressed cotyledons by causing a 2- to 5-fold increase in the transcript levels of GmSDP1, GmACX2, GmMFP2, GmICL, GmMLS, GmGLI1, GmPCK1, GmFBPase1, GmSPS1 and GmSPS2. Consistently, it upregulated the expression levels of GmSUT1, GmCWINV1 and GmMST2 under drought stress, thus enhancing the transport of sucrose from cotyledons to embryonic axes, providing carbon skeletons and energy for axis growth. The seed germination percentage increased by 208.1% at 21 h after sowing, and seedling establishment percentage increased by 47.8% at 14 days after sowing. Collectively, the positive effects of NAA priming on seed germination and seedling establishment can be attributed to enhanced antioxidant ability in seeds, TAG mobilization in cotyledons and sucrose transport from cotyledons to embryonic axes under drought stress.

[Effects of exogenous α-naphthaleneacetic aid on the antioxidation system in soybean leaves subjected to long-term drought stress during flowering].

Two different drought tolerance soybean ( Glycine max) varieties (Nannong 99-6 and Kefeng 1) were used to study the effects of α-naphthaleneacetic acid (NAA) on the antioxidation system under long-term drought stress after flowering with pot experiment, which lasted for 110 days at Pailou Experiment Station, Nanjing Agricultural University, in 2012. The results showed that long-term stress decreased the shoot dry mass significantly, however, increased the level of reactive oxygen species (ROS) and malondialdehyde (MDA) content. It also obviously increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) , ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), glutathione reductase (GR) and glutathione peroxidase (GPX). The contents of ascorbic acid (AsA) and glutathione (GSH), and the ratios of AsA/DHA (dehydroascorbic acid) and GSH/GSSG (L-glutathione oxidized) were obviously enhanced. Kefeng 1 showed a higher antioxidation ability than Nannong 99-6, and could consequently maintain lower ROS and MDA levels. NAA distinctly enhanced the activities of APX, POD, CAT, MDHAR, GPX, and ratios of AsA/DHA and GSH/GSSG, while decreased the levels of ROS and MDA. The AsA content and dehydroascorbate reductase (DHAR) activity were significantly increased in Kefeng 1.