REGULATION OF THE SUBUNIT COMPOSITION OF TOMATO PLASTIDIC GLUTAMINE-SYNTHETASE BY LIGHT AND THE NITROGEN-SOURCE

The co-action of light and the N-source in the regulation of the expre ssion of the single-copy gene encoding plastidic glutamine synthetase (GS-2) and of the multigene family encoding cytosolic glutamine synthe tase (GS-1) was investigated in the cotyledons of tomato (Lycopersicon esculentum L.). Light, acting at red/far red or at blue regions of th e spectrum increased the abundance of the GS-2 gene product and induce d a modification of GS-2 subunits, resulting in the appearance of two GS-2 proteins exhibiting different molecular weights. The magnitude of the light stimulation of GS-2 gene expression was independent of the nitrogen source. However, following red- or far-red-light treatment of etiolated tomato cotyledons, two GS-2 proteins were found when nitrat e was the N-source, while only one GS-2 protein was present with ammon ium as the sole nitrogen source. Thus, light of specific wavelengths a nd N-substrates seem to act in concert to regulate GS-2 subunit compos ition. Tomato GS-1 gene expression was unaffected by light. Ammonium p rovided externally increased the level of the tomato GS-1 protein. Irr espective of the N-source or the light quality, the GS-1 subunits were represented by polypeptides of similar molecular weight in tomato cot yledons. However, phosphinothricin-induced inhibition of GS activity r esulted in the appearance of at least one additional GS-1 polypeptide in etiolated or in green tomato cotyledons. In addition, impairment of GS activity in green tomato cotyledons by phosphinothricin was correl ated with an increased level of the GS-1 transcript. Taken together, o ur data suggest a metabolic control of GS-1 gene expression in green t omato cotyledons.