Routes of pyruvate synthesis in phosphorus-deficient lupin roots and nodules

Here, nodulated lupins (Lupinus angustifolius (cv Wonga)) were hydroponically grown at low phosphate (LP) or adequate phosphate (HP). Routes of pyruvate synthesis were assessed in phosphorus (P)-starved roots and nodules, because P-starvation can enhance metabolism of phosphoenolpyruvate (PEP) via the nonadenylate-requiring PEP carboxylase (PEPc) route. Since nodules and roots may not experience the same degree of P stress, it was postulated that decreases in metabolic inorganic phosphorus (Pi) of either organ, should favour more pyruvate being synthesized from PEPc-derived malate. Compared with HP roots, the LP roots had a 50% decline in Pi concentrations and 55% higher ADP : ATP ratios. However, LP nodules maintained constant Pi levels and unchanged ADP : ATP ratios, relative to HP nodules. The LP roots had greater PEP metabolism via PEPc and synthesized more pyruvate from PEPc-derived malate. In nodules, P supply did not influence PEPc activities or levels of malate-derived pyruvate. These results indicate that nodules were more efficient than roots in maintaining optimal metabolic Pi and adenylate levels during LP supply. This caused an increase in PEPc-derived pyruvate synthesis in LP roots, but not in LP nodules.