GLUTAMINE-SYNTHETASE ISOZYMES IN GERMINATING BARLEY-SEEDS

Glutamine synthetase (GS; EC 6.3.1.2) is a key enzyme of ammonia assim ilation in higher plants. In the present study the subunit composition and localization of GS in germinating barley (Hordeum vulgare) seed h ave been clarified. Analysis of the GS polypeptide composition by immu noblotting revealed two different polypeptides, A and B, with a molecu lar mass of 42 and 40 kDa, respectively. In the scutellum subunit A wa s already present in the ungerminated seed and remained unchanged, whe reas subunit B appeared on day 2 and increased about 5-fold during ger mination. Polypeptide B also appeared later during germination in the aleurone layer, roots and weakly in the etiolated shoots. By immunogol d microscopy, GS was detected in the scutellum and the aleurone layer of barley seeds during germination. Subcellular localization of GS on ultrathin cryosections showed that a cytosolic isozyme was present in the scutellum. Our study confirms that only a cytosolic GS is expresse d in barley seed, and its subunit composition changes during germinati on.