Ca2+, The Miracle Molecule in Plant Hormone Signaling During Abiotic Stress

Abstract

Ca2+ is a miracle messenger in cellular signaling for all eukaryotes. Ca2+ signatures modulate response to hormone signaling, various biotic and abiotic stresses, germination, cell division, cell expansion, pollen tube growth, and fertilization. These Ca2+ signatures are perceived, decoded, and transduced to a downstream response toolkit comprising a complex group of Ca2+ binding proteins that function as Ca2+ sensors. Ca2+-dependent protein kinase (CPK), Ca2+/calmodulin-dependent protein kinase (CCaMK), calmodulin (CAM), CAM-like proteins, calcineurin B-like proteins (CBL), their interacting kinases (CIPK), and other functional proteins; namely Ca2+ATPase, Ca+/H+ antiporter, calreticulin, and CAMTA are directly or indirectly regulated by Ca2+. Phytohormones are well known for being an integral part of abiotic stress signaling, where more work has been done on the involvement of ABA, auxin, cytokinin, and ethylene. However, lately brassinosteroids, salicylic acid, jasmonic acid, and strignolactones have also been discovered as playing important roles in abiotic stress signaling and responses. Ca2+ mediated signaling of hormones like ABA has been implicated as indispensible in stomatal closure during abiotic stress signaling. Influx of Ca2+ increases into the guard cells minutes after ABA treatment, activating a number of downstream channels, including CDPK and CBL-CIPKs, contributing to the efflux of ions from the guard cells causing stomatal closure. Here, we review possible signaling pathways that could possibly be addressing the connection between Ca2+ and hormone signaling during abiotic stress.