Glutamine-synthetase isoforms appearing in sunflower cotyledons during germination

Ion-exchange chromatography has been used to separate the isoforms of glutamine synthetase (GS; EC 6.3.1.2) appearing in sunflower (Helianthus annuus L. cv. Peredovic) cotyledons during seedling growth under different light and nitrogen conditions. Both in dry and imbibed seeds, only a single form of GS (GSs) was detected. Upon seed germination, the GSs isoform was gradually replaced by cytosolic (GS1) and plastidic (GS2) isoforms. Light and nitrate decreased the levels of GS1. In contrast, the appearance of GS2 was greatly stimulated by light. Nitrate also had a positive effect, particularly in the light. Light and nitrate acted synergistically on the appearance of GS2. The GS2:GS1 ratio in cotyledons of 9-d-old seedlings ranged from about 2, in darkness and nitrate-deprivation conditions, to 16 under light and nitrate application. The possible physiological roles of the distinct GS isoforms appearing in the epigeal cotyledons of sunflower during germination, and their differential regulation by light and nitrate, are discussed.