Abstract
Magnesium (Mg2+) is an essential nutrient in all organisms. However, high levels of Mg2+ 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 Mg2+. 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 Mg2+ 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 Mg2+ homeostasis. Taken together, our results suggest AVP1 is required for cellular PPi homeostasis that in turn contributes to high-Mg tolerance in plant cells.
Highlights
Inorganic pyrophosphate (PPi) is an intermediate compound generated by a wide range of metabolic processes, including biosynthesis of various macromolecules such as proteins, DNA, RNA, and polysaccharides [1]
By analyzing the avp1-4 mgt6 double mutant and avp1-4 cbl2 cbl3 triple mutant, we showed that the role of AVP1 in high-Mg tolerance was independent of previously reported MGT6 or CBL/CBL-interacting protein kinases (CIPK)-mediated pathway
Compared with wild-type plants (Col-0), avp1-4 mutants exhibited no obvious phenotypic changes during the life cycle including vegetative and reproductive periods (Figure S1e), which is quite different from avp1-1 [43], because pleiotropic phenotypes observed in avp1-1 are caused by mutation in the GNOM (At1g13980) gene [22]
Summary
Inorganic pyrophosphate (PPi) is an intermediate compound generated by a wide range of metabolic processes, including biosynthesis of various macromolecules such as proteins, DNA, RNA, and polysaccharides [1]. Being a high-energy phosphate compound, PPi can serve as a phosphate donor and energy source, but it can, at high levels, become inhibitory to cellular metabolism [2,3,4]. To maintain an optimal PPi level in the cytoplasm, timely degradation of excessive PPi is carried out by two major types of enzymes: soluble inorganic pyrophosphatases (sPPases) and proton-translocating membrane-bound pyrophosphatases (H+-PPases) [1,5,6]. The importance of maintaining an optimal cellular PPi level has been demonstrated in several different organisms. Inorganic pyrophosphatase is indispensable for cell viability because loss of its function results in cell cycle arrest and autophagic cell death associated with impaired NAD+ depletion [8,9]
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