| Summary: | Magnesium (Mg<sup>2+</sup>) is an essential nutrient in all organisms. However, high levels of Mg<sup>2+</sup> in the environment are toxic to plants. In this study, we identified the vacuolar-type H<sup>+</sup>-pyrophosphatase, AVP1, as a critical enzyme for optimal plant growth under high-Mg conditions. The <i>Arabidopsis</i> <i>avp1</i> mutants displayed severe growth retardation, as compared to the wild-type plants upon excessive Mg<sup>2+</sup>. Unexpectedly, the <i>avp1</i> mutant plants retained similar Mg content to wild-type plants under either normal or high Mg conditions, suggesting that AVP1 may not directly contribute to Mg<sup>2+</sup> homeostasis in plant cells. Further analyses confirmed that the <i>avp1</i> mutant plants contained a higher pyrophosphate (PPi) content than wild type, coupled with impaired vacuolar H<sup>+</sup>-pyrophosphatase activity. Interestingly, expression of the <i>Saccharomyces cerevisiae</i> cytosolic inorganic pyrophosphatase1 gene <i>IPP1</i>, which facilitates PPi hydrolysis but not proton translocation into vacuole, rescued the growth defects of <i>avp1</i> mutants under high-Mg conditions. These results provide evidence that high-Mg sensitivity in <i>avp1</i> mutants possibly resulted from elevated level of cytosolic PPi. Moreover, genetic analysis indicated that mutation of <i>AVP1</i> was additive to the defects in <i>mgt6</i> and <i>cbl2 cbl3</i> mutants that are previously known to be impaired in Mg<sup>2+</sup> homeostasis. Taken together, our results suggest AVP1 is required for cellular PPi homeostasis that in turn contributes to high-Mg tolerance in plant cells.
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