Interpreting the Plastid Carbon, Nitrogen, and Energy Status. A Role for PII?

Plants take up inorganic nitrogen (N) primarily as nitrate but also to a lesser extent as ammonium. Nitrate is reduced to nitrite in the cytoplasm by the highly regulated enzyme nitrate reductase. Nitrite is transported into the chloroplast and further reduced by nitrite reductase to ammonium. Ammonium is assimilated into the amino acid pool by the coordi- nate action of Gln synthetase (GS) and Glu synthase (GOGAT), also known as the GS/GOGAT cycle. Glu dehydrogenase (GDH) resides in the chloroplast and mitochondria. It is thought to function primarily in a catabolic role, and likely only plays a capacity in the assimilation of inorganic N when ammonium con- centrations are very high (Melo-Oliveira et al., 1996). The reactions catalyzed by these three enzymes are shown below.