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), O-1(2), O-2(-), 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.