Fig. 3.
Hypothetical glutamate feed depicting the leaf N hub in stable state C2 species. (A) Glutamine synthetase (GS1) and glutamate dehydrogenase (GDH) isoenzymes are located in both the cytosol and companion cells. These mechanisms could work to detoxify NH3 produced following glycine decarboxylation, with possible signals from H2O2 and a potential increase of vein density compared with C3 relatives facilitating rapid removal of NH3 and distribution of nitrogen into non-toxic amino acids. This scenario may work independently or co-dependently with amino acid synthesis (as shown in B), in addition to 2-oxoglutarate (2-OG) and γ-aminobutyric acid (GABA) feeds into the TCA pathway (glutamate–2-OG interconversion may operate in cyclic cooperation with GS/GOGAT according to Stitt et al. (2002). Rapid nitrogen transport through the vein to sink tissue may be via glutamate, ornithine, arginine, and citrulline, by cytosolic relocation of metabolites and enzymes.