Cloning of the plasma membrane H+-ATPase gene with suppressed expression under Pi-Deficient conditions inSesbania rostrata

For a long time, it had been suggested that the activation of plasma membrane (PM) H+-ATPase in roots was enhanced by Pi-deficiency. However, gene(s) encoding an isoform that is expressed in response to Pi-deficiency had not been isolated. In this study, attempts were made to isolate PM H+-ATPase genes encoding such Pi-deficiency-responsible isoforms from Sesbania rostrata. We cloned three full-length (SrHA1, SrHA4, SrHA5) and three partial (SrHA2, SrHA3, SrHA6) cDNAs encoding PM H+-ATPases from the roots of Pi-deficient S. rostrata. The transcript levels of SrHA4 were high in roots. Unexpectedly, the transcript levels of SrHA4 decreased under Pi-deficient conditions where plants were provided with less than 30–50 µM Pi, or where plants formed many nodules on the stems. Under such conditions, high-affinity Pi-transporters and phosphoenolpyruvate carboxylase (PEPC) genes were highly expressed. The decrease in the transcript levels of SrHA4 corresponded to the increase in the transcript levels of high-affinity Pi-transporters and PEPC genes under Pideficient conditions. This fact suggested that the expression of SrHA4 as well as that of high-affinity Pi-transporters and PEPC genes responded to the decrease in the Pi content in plants.