Appropriate Drought Training Induces Optimal Drought Tolerance by Inducing Stepwise HO 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 HO 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 HO 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 HO homeostasis, characterized by a consistent increasing amplitude in HO content compared with CK. The HO signal formed under the optimum treatment induced the capacity of the antioxidant system to remove excess HO to form a primary HO homeostasis. The primary HO homeostasis further induced senior HO 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 HO homeostasis to continuously improve drought tolerance.