Elucidation of the role of the vacuolar type proton pyrophosphatase in early stages of plant development

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Following germination, oil-seed plants rely on triacylglycerol (TAG) reserves for growth during a heterotrophic period, where TAG is converted to sucrose (Suc). Also, macromolecular syntheses consume huge amounts of ATP, often hydrolyzed to AMP plus PPi. In Arabidopsis, the vacuolar H+-PPase uses energy from PPi hydrolysis to acidify the vacuole. The fugu5 mutants, lacking the AVP1 H+-PPase, failed to sustain postgerminative heterotrophic growth; recovered upon Suc supply, or specific PPi removal by the cytosolic PPase IPP1. Thus the major function of H+-PPase in seedling development is the removal of inhibitory PPi, rather than H+ pumping. Suc and PPi quantification in WT versus fugu5 indicated that gluconeogenesis is inhibited by elevated level of PPi, but the targets of PPi inhibition remained unclear. Here, profiling of major metabolites that occur during TAG mobilization showed that UDP-Glucose Pyrophosphorylase (UGPase) is specifically inhibited by excess cytosolic PPi in fugu5.