PPi plays a role in sugar starvation tolerance

H+-PPase acidifies the endo-membrane compartments utilizing the energy of PPi hydrolysis. This acidification creates an electrochemical proton gradient that powers the secondary active transport and allows for vacuolar accumulation of several materials against their concentration gradients. The hydrolysis of the cytosolic PPi is necessary for the forwardness of the PPi-generating reactions. However, information is lacking on the role of PPi in adaptation to sugar starvation and low energy status in plants. Here, several mutants lacking the functional H+-PPases were used to illuminate this role. Three alleles of fugu5 mutants defective in type I H+-PPase exhibited better tolerance to sugar starvation than wild-type plants, when grown on [1/2] and full-strength MS media under photosynthesis-constraining low light intensity. The PPi level in fugu5 mutants was significantly higher than its level in wild type and type II H+-PPase-defective mutants. SnRK1 (Sucrose-non-fermenting1-Related kinase-1) plays a central role in the coordination of the plant transcriptome to the energy signals. SnRK1 senses the energy depletion in plant cells, and controls the expression of genes and phosphorylation of proteins in a way that promotes catabolism, and inhibits anabolism. Sugar starvation significantly induced the SnRK1 phosphorylation activity in wild type and type II H+-PPase lacking mutants. Whereas the activity remained unchanged in sugar-starved fugu5 mutants. This is possibly achieved through activation of PPi-dependent enzymes. Results suggest that the high PPi level in fugu5 mutants might contribute to more efficiently use of low level of ATP under sugar starvation and low light conditions.