An endoplasmic reticulum-bound Ca2+/Mn2+ pump, ECA1, supports plant growth and confers tolerance to Mn2+ stress

Plants can grow in soils containing highly variable amounts of mineral nutrients, like Ca2+ and Mn2+, though the mechanisms of adaptation are poorly understood. Here, we report the first genetic study to determine in vivo functions of a Ca2+ pump in plants. Homozygous mutants of Arabidopsis harboring a T-DNA disruption in ECA1 showed a 4-fold reduction in endoplasmic reticulum-type calcium pump activity. Surprisingly, the phenotype of mutant plants was indistinguishable from wild type when grown on standard nutrient medium containing 1.5 mM Ca2+ and 50 muM Mn2+. However, mutants grew poorly on medium with low Ca2+ (0.2 mM) or high Mn2+ (0.5 mM). On high Mn2+, the mutants failed to elongate their root hairs, suggesting impairment in tip growth processes. Expression of the wild-type gene (CAMV35S::ECA1) reversed these conditional phenotypes. The activity of ECA1 was examined by expression in a yeast (Saccharomyces cerevisiae) mutant, K616, which harbors a deletion of its endogenous calcium pumps. In vitro assays demonstrated that Ca2+, Mn2+, and Zn2+ stimulated formation of a phosphoenzyme intermediate, consistent with the translocation of these ions by the pump. ECA1 provided increased tolerance of yeast mutant to toxic levels of Mn2+ (1 mM) and Zn2+(3 mM), consistent with removal of these ions from the cytoplasm. These results show that despite the potential redundancy of multiple Ca2+ pumps and Ca2+/H+ antiporters in Arabidopsis, pumping of Ca2+ and Mn2+ by ECA1 into the endoplasmic reticulum is required to support plant growth under conditions of Ca2+ deficiency or Mn2+ toxicity.