Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca(2+)) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca(2+) communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca(2+)](cyt) as a result of the Ca(2+) influx permitted by membrane-localized Ca(2+) permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM(2+) PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca(2+) permeable cation channels. On the contrary, some Ca(2+) transporting membrane proteins, mainly Ca(2+)-ATPase and Ca(2+)/H(+) exchangers, are involved in Ca(2+) efflux for removal of the excessive [Ca(2+)](cyt) in order to maintain the Ca(2+) homeostasis in cells. The Ca(2+) efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca(2+) influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.