Abscisic Acid Signaling and Involvement of Mitogen Activated Protein Kinases and Calcium‐Dependent Protein Kinases During Plant Abiotic Stress

Abiotic stresses, such as drought, high salinity, heat, cold, freezing, limited nutrient availability, heavy metals, and hypoxia, are severe environmental stresses that impair productivity in crop systems. The phytohormone abscisic acid (ABA) belongs to the class of isoprenoids (terpenoids) involved primarily in plant adaptation to various environmental stress conditions. ABA induces the SnRK2 family of protein kinases (plant-specific Ser/Thr kinases) to mediate a number of responses. The mitogen activated protein kinase (MAPK) cascades have also been shown to be implicated in ABA signaling. Treating plants with ABA induces transcriptional regulation, protein accumulation and stability, and kinase activity of several components of the MAPK signaling pathway in many plant species, suggesting an important function of MAPK pathways in ABA signaling. The available evidence suggests that MAPK cascades are involved in several ABA responses, including antioxidant defense, guard cell signaling, and seed germination. The MAPK and immediate upstream activators, MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK), constitute a functionally interlinked MAPK cascade. Activation of the MAPK can facilitate its translocation to the nucleus where it can phosphorylate and activate transcription factors, thereby modulating gene expression. Thus, MAPKs constitute one of the most important classes of proteins that can link perception of stimuli with several cellular and adaptive responses. Fast changes in the concentration of free Ca2+ ions, so called Ca2+ signals, are among the first responses to many stress situations. Ca2+ signals mainly activate calcium dependent protein kinases (CDPKs), which also participate in downstream signaling cascades. CDPKs together with MAPKs are involved in regulating the cellular metabolism to changing environments. The present chapter will highlight some of the recent findings on receptor-mediated ABA perception and ABA-inducible CDPK and MAPK signaling cascades, with respect to potential crosstalk and the several classes of MAPK and CDPK reported from different plant species in mediating abiotic stress responses.