The Importance of Sorting Calcium in Plant Cells: Uncovering the Roles of A Sarcoplasmic/Endoplasmic Reticulum-Like Calcium ATPase
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The spatial and temporal dynamics of intracellular Ca2+ in response to environmental and hormonal cues underscore the importance of Ca2+ transport during plant growth and development. The Arabidopsis thaliana genome predicts multiple genes encoding Ca2+ transporters, though the biological roles of most are unknown. Here I determine the function of AtECA3 which represents the first plant P-type 2A ATPase resembling mammalian sarcoplasmic/endoplasmic reticulum Ca ATPase (SERCA). AtECA3 (At1g10130) expressed in a yeast mutant lacking its endogenous Ca2+ pumps functionally substitutes for the defective Ca2+-ATPases. AtECA3-dependent yeast growth is blocked by thapsigargin, a specific SERCA inhibitor. The results suggest that AtECA3 is a cation pump with specificity for Ca2+ and Mn2+, and that AtECA3 enhances yeast growth on Ca2+-depleted medium or on medium with high Mn2+ by sequestrating Ca2+ or Mn2+, respectively, into endomembrane compartments. AtECA3 is expressed in pollen grains as revealed by promoter::GUS analyses and a green fluorescence protein (GFP)-tagged to AtECA3 labels endomembranes at the pollen tip. In vitro tube growth of wild-type pollen is enhanced by 10 mM Ca2+, and inhibited by thapsigargin, suggesting that AtECA3 supports tube elongation by sorting intracellular Ca2+ to appropriate compartments. This idea is supported by genetic evidence, where three T-DNA insertional mutants show 33% reduction in pollen tube length. This defect lowers sperm transmission shown as segregation distortion and decreased seed set. AtECA3 is also expressed in vascular tissues of young roots and leaves, and the GFP-tagged protein colocalizes with two Golgi markers. Three millimolar Ca2+ stimulate root growth of wild-type but not of mutants, indicating that Ca2+ accumulation in Golgi lumen is critical for growth. Root growth of eca3-4, but not of wild-type, is hypersensitive to 50 &#956;M Mn2+. Thus loading Mn2+ into Golgi lumen by AtECA3 supports root growth. Intriguingly, mutant roots show 80% increase in apoplastic peroxidase suggesting that secretory activities became deregulated. In conclusion, I provide molecular evidence for a Golgi Ca2+/Mn2+ pump in plants. Ca2+ and Mn2+ accumulation into Golgi/secretory compartments by AtECA3 and perhaps cation release from these stores affect secretory activities critical for root growth, pollen tube elongation and male fertility.