Gibberellic Acid Priming Improved Rapeseed Drought Tolerance by Modulating Root Morphology, ROS Homeostasis, and Chloroplast Autophagy

Although gibberellic acid (GA(3)) improves seed germination and seedling growth in stressful conditions, the mechanisms underlying GA(3)-priming improved drought tolerance have not been defined. To investigate the effects of GA(3)-priming on physiology and antioxidant enzymes, we primed rapeseed seeds with polyethylene glycol (PEG-6000)-induced stress and quantified the changes in reactive oxygen species (ROS) homeostasis, root morphology, osmotic adjustment, stomatal morphology, and cell ultrastructure analysis. Drought stress reduced the germination rate and the subsequent growth. GA(3)-priming improved rapeseed germination rate and seedling growth under PEG-6000-simulated drought stress. Our results indicate that the lower germination rate and the inhibition of seedling growth in non-primed seeds under drought conditions were associated with higher production of ROS. However, GA(3)-primed seedlings accumulated higher activities of superoxide dismutase, peroxidase, catalase, ascorbic acid, and glutathione. They thus developed a stronger antioxidant defense system for scavenging the over-accumulated ROS under drought. GA(3)-priming also contributed to osmotic adjustments by promoting the accumulation of proline, soluble sugars, and soluble proteins. Ultimately, GA(3)-priming protected the cell wall from degradation and preserved chloroplast from autophagy. These ameliorative effects of GA(3)-priming under drought stress were further confirmed by significantly improving the rapeseed root morphology in hydroponic experiments. We conclude that GA(3)-priming improves seed germination and promotes productivity by regulating physiological and biochemical responses to drought stress.