Cation/H+ exchanger in plants

Channels and transporters are involved in regulation of ion homeostasis. Cation/H + exchanger (CAX) functions are indispensable for normal plant growth and development. CAXs are established to mediate calcium (Ca 2+ ) homeostasis besides other ions. Involvement of Ca 2+ signaling is unequivocal for plants to initiate defense responses against unfavorable conditions, be it biotic or abiotic stresses. CAXs belong to secondary energized ion transporters that are being energized by pH gradient established by proton pump such as H + -ATPase or H + -pyrophosphatase. These are associated in a wide range of cellular and physiological functions in plant system. CAXs are of three types: type I CAXs are similar to Arabidopsis thaliana CAX1 , found in bacteria, fungi, and plants; type II CAXs are having a long N-terminus hydrophilic region, found in Dictyostelium , fungi, and lower vertebrates; and type III CAXs are close to Escherichia coli ChaA, found only in bacteria. In eukaryotes, most of the CAXs are found to be localized at tonoplast which is responsible for sequestration of excess cytosolic Ca 2+ into the vacuole. Due to this, potential plant CAXs have been engineered for nutritional enrichment and phytoremediation to clean up heavy metal contamination in the soil. The diversity in CAX functions depicts their potential role in biochemical and physiological aspects of plant growth and development.