Role of phosphate in drought stress regulation in developing rice (Oryza sativa L.) seedlings

The investigation focuses on the role of phosphate in influencing the growth, ROS production, oxidative stress markers and antioxidant defence metabolism in rice seedlings subjected to polyethylene glycol-induced drought stress. Since phosphate is an essential element with multifarious physiological functions in stress regulation, the current investigation was designed. The experiments were done in randomised design with four treatments combination including controls and replicated three (n = 3) times. The phosphate was supplemented by increasing the phosphate supply by twofold in the nutrient medium. Our results indicated that phosphate supplementation substantially ameliorated the drought stress in rice seedlings. After 2, 4 and 6 days of drought stress, the reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and superoxide radical (O2·−) contents in both root and shoot increased significantly (p ≤ 0.001) as compared to the respective controls. In phosphate-supplemented seedlings, the ROS content after 2, 4 and 6 days of drought stress was substantially low (p ≤ 0.001) as compared to those without phosphate supplementation. In response to drought, the oxidative stress markers such as malondialdehyde (MDA) and carbonyl proteins contents increased significantly (p ≤ 0.001) in both root and shoot as compared to control and drought-stressed seedlings in the presence of phosphate supplementation. The antioxidant metabolism was also affected by phosphate supplementation during drought stress. The activities of catalase (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and superoxide dismutase (SOD) increased significantly (p ≤ 0.05) in phosphate supplemented rice seedlings under drought stress as compared to non-supplemented seedlings. Proline content in rice shoots was not significantly affected (p ≤ 0.05) by drought either in the presence or absence of phosphate supplementation, and its level was significantly lower (p ≤ 0.001) in roots under phosphate supplementation as compared to the drought-stressed rice seedlings in the absence of supplemented phosphate during the period of stress. In shoot, phosphate supplementation resulted in significantly (p ≤ 0.001) lowering the carbonyl protein content as compared to those without phosphate supplementation, while both the root and shoot malondialdehyde (MDA) showed no significant (p ≥ 0.05) difference between phosphate supplemented and non-supplemented seedlings after 2, 4 and 6 days of stress. As compared to the phosphate supplemented and non-supplemented seedlings, the root carbonyl protein content was not affected significantly till 2 and 4 days of stress, while significantly low (p ≤ 0.001) carbonyl protein content was observed in phosphate supplemented rice seedlings. This observation was concurrent with lower levels of ROS production in phosphate-supplemented seedlings during drought stress. Our results suggested that supplementation of phosphate can effectively modulate drought-induced oxidative stress by influencing antioxidant defence metabolism.