Vacuolar Proton Pyrophosphatase Is Required for High Magnesium Tolerance in .

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