Ca²⁺ flux in plant responses to abiotic stress

Calcium (Ca²⁺) plays versatile roles in plant growth and development, as well as in responses to environmental stimuli. Abiotic stressors, including abnormal temperature, drought, salt, heavy metals, and flooding, induce instantaneous and rapid free cytosolic Ca²⁺ ([Ca²⁺]_cyt) elevation, known as Ca²⁺ signatures. Ca²⁺ signatures are stress-specific and regulated by Ca²⁺ flux. Ca²⁺ flux contains Ca²⁺ influx, which initiates Ca²⁺ signatures, and Ca²⁺ efflux, which terminates them. Ca²⁺ flux is achieved through the co-operation of Ca²⁺ channels and pumps. Here, we highlight recent advances in Ca²⁺ flux and Ca²⁺ channel functions in plant responses to abiotic stress. Ca²⁺ influx channels in plants include the hyperosmolarity-gated calcium-permeable channel family of proteins (OSCAs), glutamate receptor-like channels (GLRs), cyclic-nucleotide-gated calcium channels (CNGCs), annexins (ANNs), two-pore channels (TPCs), Piezo channels (PZOs), Mid1-complement activity protein channels (MCAs), and mechanosensitive channels of small conductance (MscS)-like proteins (MSLs). Ca²⁺ efflux channels mainly contain Ca²⁺-ATPases and Ca²⁺ exchangers. Most Ca²⁺ channels have been found to participate in plant responses to single abiotic stress, whereas some are reported to be involved in responses to multiple abiotic stresses. This improved knowledge advances our understanding of Ca²⁺ signaling in plant responses to abiotic stress and offers new strategies for cultivating stress-resilient crops.