Marked modulation by NH < inf > 4 < /inf > < sup > + < /sup > , but limited effects of NO < inf > 3 < /inf > < sup > - < /sup > on phosphoenolpyruvate carboxylase in leaves of Amaranthus hypochondriacus, A C < inf > 4 < /inf > plant

Abstract

Nitrogen nutrition, besides being essential for plant growth, modulates several components of photosynthesis. The most common sources of inorganic nitrogen are ammonium (NH4+) and nitrate (NO 3-). Modulation of phosphoenolpyruvate carboxylase (PEPC) in leaves of Amaranthus hypochondriacus, a C4 plant was studied in response to NH4+ or NO3- ions. Inclusion of mM ranges of NH4+ during assay increased the PEPC activity by almost 2-fold in dark-adapted leaf extracts and decreased the malate sensitivity of enzyme by 20% in illuminated extracts but the PEPC activity was marginally inhibited and there was only marginal change in malate sensitivity. The effect of NH4+ was pronounced, also when fed to leaves. Up on NH4+ feeding in dark the PEPC activity increased by almost 55% and malate sensitivity decreased marginally. The extent of light activation of PEPC was dramatically enhanced by almost 3-fold in presence of NH4+ and malate sensitivity decreased by 40%. However feeding NO3- increased PEPC activity in dark-adapted leaves by 2.5-fold with decrease in malate sensitivity. The effect of NO3- on PEPC was marginal during assay. The phosphorylation level of PEPC was much higher in NH4+ fed leaves than that in control. Our results suggest that NH4+ regulates PEPC of A. hypochondriacus, in two ways. In short-term incubation, i.e., during enzyme assay, NH4+ causes allosteric activation of PEPC. In a long term effect, i.e., feeding, there is a marked increase in phosphorylation level, due to the biosynthesis of PEPC and/or PEPC-PK.