Stress responses in both plants and yeast utilize calcium-mediated signaling. A yeast strain, K616, which lacks Ca(2+)-pumps, requires micromolar Ca(2+) for growth. In medium containing 100 muM Ca(2+), K616 can withstand osmotic stress (750 mM sorbitol), and ionic stress (300 mM KCl), but not hypersodic stress (300 mM NaCl). Heterologous expression of the endoplasmic reticulum located Arabidopsis thaliana Ca(2+)-ATPase, ACA2, permits K616 to grow under NaCl stress even in Ca(2+) depleted medium. All stresses tested generated transient elevation of cytosolic Ca(2+) in wild type yeast, K601, whereas NaCl alone induced prolonged elevation of cytosolic Ca(2+) in K616. Both the Ca(2+) transient and survival of cultures subjected to NaCl stress was similar for the ACA2 transformant and K601. However, whereas K601 maintained low cytosolic Na(+) predominantly by pumping it out across the plasma membrane, the transformant sequestered Na(+) in internal organelles. This sequestration requires the presence of an endomembrane Na(+)/H(+)-antiporter, NHX1, which does not play a significant role in salt tolerance of wild type yeast except at acidic pH. Transcript levels of the plasma membrane Na(+) ATPase, ENA1, were strongly induced only in K601, while NHX1 was strongly induced in both K601 and the ACA2 transformant. The CaM kinase inhibitor KN62 significantly reduced the salt tolerance of the ACA2 transformant and the transcriptional induction of NHX1. Thus, the heterologous expression of a plant endomembrane Ca(2+) pump results in the rapid depletion of cytosolic Ca(2+) and the activation of an alternate mechanism for surviving saline stress.
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