Increased glutamine synthetase activity and changes in amino acid pools inleaves treated with 5-aminoimidazole-4-carboxiamide ribonucleoside (AICAR)

Feeding 5-aminoimidazole-4-carboxiamide ribonucleoside (AICAR) through the petiole of detached young barley leaves rapidly increased activities of NAD H-nitrate reductase (NR) and glutamine synthetase (GS) in leaf extracts and at least partly prevented the usual slow decrease of these enzyme activiti es during prolonged illumination. Further, AICAR caused drastic changes in amino acid levels: glutamine and serine levels were increased whereas gluta mate and glycine were decreased, probably indicating a higher GS activity a nd more rapid conversion of glycine into serine. The latter may be responsi ble for the higher ammonium contents found in AICAR treated leaves. We tent atively suggest that GS (located in the chloroplast) and glycine decarboxyl ase (located in the mitochondria) are regulated in a manner similar to NR. This is discussed in the light of recent reports that 14-3-3 isoforms exist in chloroplasts and that GS binds to 14-3-3s in vitro.