Metabolic and developmental control of cytosolic glutamine synthetase genes in soybean

At least two genes encoding cytosolic glutamine synthetase (GS) are actively transcribed in soybean (Glycine max. L. cv. Prize) plants. The expression of the first gene, GS15, is regulated by the availability of ammonia provided externally to non‐inoculated roots or formed as the result of nitrogen fixation in nodulated plants. Moreover, in addition to transcription in roots and root nodules, GS15 is also actively transcribed in other organs such as anthers and pulvini. Separate promoter elements are able to control either the organ‐ or ammonia‐regulated expression of GS15 independently. Another cytosolic GS gene, GS21, has also been identified and is specifically expressed in nodules. The differential expression of GS15 and GS21 suggests that two independent mechanisms, one metabolic, the other developmental, may control ammonia assimilation in nodulated plants. This recent finding is discussed and compared with other results concerning the regulation of glutamine synthetase activity in legume and non‐legume plant species.