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