Fig. 8

Schematic representation of metabolic processes of N-remobilisation during dark-induced proteolysis on carbohydrate depletion. Extended darkness results in carbohydrate depletion and low energy status that triggers proteolysis and amino acid catabolism with glutamate as primary amino acid. GDH deaminates glutamate to oxoglutarate and free NH4+ that is captured by cytosolic glutamine-synthetase (GS) to produce glutamine and glutamate. Glutamine and aspartate are used by asparagine synthetase to generate glutamate and asparagine. Both, asparagine and glutamine are exported to sink tissues as a nitrogen resource. Arginine accumulates in the urea cycle to prevent NH4+ accumulation to toxic levels. Enzymes: GDH – Glutamate-dehydrogenase, GS – Glutamine-synthetase, Asn – Asparagine-synthetase, GOT – Glutamic-oxaloacetic transaminase = AspAT – Aspartate-aminotransferase, ASS – Argininosuccinate-synthetase, ASL – Argininosuccinate-lyase, ARGAH – Arginine-amidohydrolase, OCT – Ornithin-carbamoyl-transferase, CPS – Carbamoylphosphat-synthetase; Metabolites: α-KG – α-Ketoglutarate, OAA – Oxaloacetic acid, CP – Carbamoylphosphate, CT – Carbamate; ● – Transport protein in cell membrane, black arrow – dark exposure, blue arrow – no dark