Research progress on the regulation of crop drought tolerance by exogenous calcium

Drought is one of the primary abiotic stresses limiting agricultural production; therefore, enhancing crop drought tolerance is crucial for ensuring global food security. As an essential plant nutrient and a key second messenger, calcium plays a multi-level regulatory role in crop responses to drought stress. This paper systematically reviews the physiological and molecular mechanisms by which exogenous calcium enhances crop drought tolerance, as well as progress in its application. At the physiological level, exogenous calcium alleviates drought injury by maintaining membrane stability, activating antioxidant enzymes, and regulating stomatal movement; notably, calcium oxalate crystals in mesophyll cells serve unique functions in regulating calcium homeostasis and acting as an emergency carbon source. At the molecular level, calcium signaling network interacts with abscisic acid (ABA) and reactive oxygen species (ROS) signaling pathways to finely regulate the expression of downstream stress-responsive genes and the accumulation of osmolytes. At the rhizosphere level, exogenous calcium not only promotes soil aggregate formation via cation bridging to improve soil water retention capacity, but also reshapes the rhizosphere microbial community by enriching drought-tolerant plant growth-promoting rhizobacteria, such as Bacillus and Streptomyces. These beneficial microorganisms synergistically enhance crop systemic resistance by secreting phytohormones and siderophores. Elucidating the mechanisms underlying calcium-mediated drought tolerance and developing varieties with high endogenous calcium use efficiency through molecular breeding will effectively reduce the costs of agricultural drought management.