Figure 4
Activity of GS2 and chlorophyll synthesis.
(a) When the transport of chloroplastic ammonia to the cytosol was not considered, then the activity of GS2 was found to be essential (marked in red). Since Rubisco oxygenase activity was not involved, only 0.5 flux unit of mitochondrial ammonia was released into the chloroplast. The chloroplastic ammonia released during porphobilinogen (PBG) to hydroxymethylbilane (HMB) conversion for one unit of chlorophyll synthesis is 4.0 unit.The total chloroplastic ammonia (4.5 unit) was refixed in GS2. (b) If the chloroplastic ammonia was allowed to transport from chloroplast to the cytosol , then, GS2 was found to be inactive. (c) Under combined activity of Rubisco carboxylase/oxygenase at 3:1 (photorespiration is also considered), higher amount of ammonia is released from the mitochondria into the chloroplast. GS2 mutants are unable to fix the ammonia in the chloroplast. Due to increased concentration of ammonia within the chloroplast and its slow diffusion from chloroplast to cytosol, the chloroplastic ammonia accumulates within the chloroplast which in turn makes PBG to HMB conversion unfavourable and consequently affects chlorophyll synthesis pathway and therefore causes chlorosis. Metabolites. Gln, glutamine; Glu, glutamate; PBG, porphobilinogen; HMB, hydroxymethylbilane; chl a, chlorophyll a. Enzymes. GDC, glycine decarboxylase; GS1, glutamine synthetase 1; GS2, glutamine synthetase 2.
