Organ-specific changes in the activity and subunit composition of glutamine-synthetase isoforms of barley (Hordeum vulgare L.) after growth on different levels of NH+ 4

One cytosolic glutamine synthetase (GS, EC 6.3.1.2) isoform (GS 1a) was active in the germinating seeds of barley (Hordeum vulgare L.). A second cytosolic GS isoform (GS 1b) was separated from the leaves as well as the roots of 10-d-old seedlings. The chloroplastic isoform (GS 2) was present and active only in the leaves. The three GS isoforms were active in N-supplied (NH⁺₄ or NO₃^–) as well as in N-free-grown seedlings. This indicates (i) that a supply of nitrogen to the germinating seeds was not necessary for the induction of the GS isoforms and (ii) that no nitrogen-specific isoforms appeared during growth of seedlings with different nitrogen sources. The activity of GS, however, depended on the seedlings' nitrogen source: the specific activity was much higher in the leaves and much lower in the roots of NH⁺₄-grown barley than in the respective organs of NO₃^–-fed or N free-grown plants. With increasing concentrations of NH⁺₄ (supplied hydroponically during growth), the specific activity of GS 1b increased in the leaves, but decreased in the roots. The activity of GS 2 (leaf) also increased with increasing NH⁺₄ supply, whereas GS 1a activity (leaf and root) was not affected. The changes in the activities of GS 1b and GS 2 were correlated with changes in the subunit compositions of the active holoenzymes: growth at increased levels of external NH⁺₄ resulted in an increased abundance of one of the four GS subunits, and of two of the five GS 1b subunits in the leaves. In the roots, however, the abundance of these two GS 1b subunits was decreased under the same growth conditions, indicating an organ-specific difference either in the expression of the genes coding for the respective GS 1b subunits or in the assembly of the GS 1b holoenzymes. Furthermore, growth at different levels of NH⁺₄ resulted in changes in the substrate affinities of the isoforms GS 1b (root and leaf) and GS 2 (leaf), presumably due to the changes in the subunit compositions of the active holoenzymes.Abbreviations FPLC fast protein liquid chromatography - GHA -glutamyl hydroxamate - GS glutamine synthetaseDr. Roger Wallsgrove's (Rothamsted Experimental Station, Harpenden, UK) generous gift of GS antiserum is greatly appreciated.