Role of Calcium in Conferring Abiotic Stress Tolerance

Abiotic and biotic stresses are major environmental factors reducing crop productivity and causing losses worth billions of dollars annually. Understanding the mechanisms of biotic and abiotic stress using physiological, biochemical and advanced molecular biology-based techniques will help in developing crop cultivars resistant to abiotic and biotic stresses. Over the past two decades, it has been established that calcium (Ca²⁺) is an essential macronutrient and plays a vital role in plant tolerance against biotic and abiotic stresses. Addition of Ca²⁺ fertilizers to the growth medium or its application as seed priming and as a foliar spray has already been shown to enhance stress tolerance in plants. Besides some fundamental roles of Ca²⁺ in plant growth and development, it has been recognized as a principal secondary-messenger molecule regulating diverse physiological processes in plants in a stressful environment. Biotic and abiotic stresses enhance the cytosolic Ca^2+, which then binds with Ca²⁺ sensor proteins such as Ca calmodulin (CaM), Ca-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), CBL interacting protein kinase (CIPKs) and mediates downstream responses for stress tolerance. Activation of downstream responses for inducing stress tolerance in plants depends on changes in cytosolic Ca²⁺ concentration, which depends on duration and amplitude. However, increase in cytosolic Ca²⁺ concentration for a longer duration causes the production of toxic compounds which damage proteins and membrane lipids. To maintain ion homeostasis, plants efficiently export Ca²⁺ outside the cell or to the cellular organelles via ATP-dependent cation exchangers. Here we reviewed the recent updates on the physiological roles of Ca²⁺ in plant development and stress tolerance. Moreover, uptake and interaction with other mineral nutrients have also been discussed. In addition, the emerging role of Ca²⁺ signaling in mediating stress tolerance, which needs further research, is also reviewed. Finally, we discussed at length up to what extent Ca²⁺ fertilization or exogenous application mitigate the adverse effects of abiotic stresses.