PHOSPHINOTHRICIN REVERTS THE AMMONIA-DEPENDENT ENHANCEMENT OF PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITY

The effect of ammonium assimilation on phosphoenolpyruvate carboxylase (PEPCase) activity was investigated in detached leaves from N-limited barley plants. The time-course induction of PEPCase was dependent on the rate of ammonia assimilation and not on ammonia uptake or accumulation. Treatment of N-deprived leaves with phosphinothricin (an inhibitor of glutamine synthetase activity) caused inhibition of ammonia assimilation, resulting in the reversion of ammonia-dependent enhancement of PEPCase. The results indicate that glutamine level controls phosphoenolpyruvate carboxylase activation; consequently, if glutamine synthesis is inhibited, PEPCase activity is not enhanced. Determination of malate content showed that as PEPCase activity increased in response to increasing ammonia assimilation, there was a linear decline in the level of that metabolite. We have also analyzed the in vivo effect of malate on ammonium-dependent activation of PEPCase. High malate uptake partially abolished PEPCase activation, but the induced PEPCase activity seems to be less sensitive to malate than the control one.