Endoplasmic reticulum-localized CCX2 is required for osmotolerance by regulating ER and cytosolic Ca²⁺ dynamics in Arabidopsis.

Ca²⁺ signals in plant cells are important for adaptive responses to environmental stresses. Here, we report that the Arabidopsis CATION/Ca²⁺ EXCHANGER2 (CCX2), encoding a putative cation/Ca²⁺ exchanger that localizes to the endoplasmic reticulum (ER), is strongly induced by salt and osmotic stresses. Compared with the WT, AtCCX2 loss-of-function mutant was less tolerant to osmotic stress and displayed the most noteworthy phenotypes (less root/shoot growth) during salt stress. Conversely, AtCCX2 gain-of-function mutants were more tolerant to osmotic stress. In addition, AtCCX2 partially suppresses the Ca²⁺ sensitivity of K667 yeast triple mutant, characterized by Ca²⁺ uptake deficiency. Remarkably, Cameleon Ca²⁺ sensors revealed that the absence of AtCCX2 activity results in decreased cytosolic and increased ER Ca²⁺ concentrations in comparison with both WT and the gain-of-function mutants. This was observed in both salt and nonsalt osmotic stress conditions. It appears that AtCCX2 is directly involved in the control of Ca²⁺ fluxes between the ER and the cytosol, which plays a key role in the ability of plants to cope with osmotic stresses. To our knowledge, Atccx2 is unique as a plant mutant to show a measured alteration in ER Ca²⁺ concentrations. In this study, we identified the ER-localized AtCCX2 as a pivotal player in the regulation of ER Ca²⁺ dynamics that heavily influence plant growth upon salt and osmotic stress.

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