Early Events in Plant Abiotic Stress Signaling: Interplay Between Calcium, Reactive Oxygen Species and Phytohormones

Abstract

Upon exposure to abiotic stresses, plants activate early stress-signaling mechanisms within a few seconds to a few hours to counter the stress responses and bring tolerance. The most versatile signaling molecules involved during the early events of abiotic stress signaling are Ca²⁺ (calcium ion) and reactive oxygen species (ROS), ¹O₂, O₂⁻, and H₂O₂. Initially, apoplastic Ca²⁺ activates plasma membrane-bound NADPH oxidase and generates H₂O₂, which acts as a second messenger and further leads to the activation of downstream signaling processes. Subsequently, H₂O₂ activates calcium-dependent protein kinase (CDPK) and mitogen activated protein kinase (MAPK) pathways, leading to stress tolerance through downstream signaling cascades. In addition, fast influx of Ca²⁺ from the apoplast to the cytosol further activates cytosolic CDPKs and respiratory burst oxidase D and regulates Ca²⁺ and ROS signaling. Sub-cellular organelles further produce ROS and Ca²⁺ to bring stress tolerance. Excessive ROS produced during these processes are quenched by ROS scavenging enzymes, whereas excessive Ca²⁺ is neutralized by the action of the calcium binding proteins CDPKs, CaMs, CMLs, and CBLs. The phytohormone ABA further regulates the production of H₂O₂, thus maintaining the positive feedback system for ROS production and stress tolerance. Additionally, CBL proteins modulate H₂O₂ production in the presence of NADPH oxidase via interaction with CIPK, thus maintaining a positive feedback mechanism in stress tolerance. Similarly, CaM proteins bind with MAPK and regulate stress tolerance by activating the MAPK cascade.