Ammonia regulated expression of a soybean gene encoding cytosolic glutamine synthetase is not conserved in two heterologous plant systems

A 3.5 kb promoter fragment fused to the reporter beta-glucuronidase gene (gus) had previously been shown to contain separate regulatory elements controlling the ammonia-stimulated and organ-specific transcription of GS15, a soybean gene encoding root and root nodule cytosolic glutamine synthetase (GS). In order to determine if the regulatory elements conferring the ammonia-regulated and the organ-specific expression are conserved in different plant species, a GS15 promoter-gus (pGS15GUS) construct was introduced via Agrobacterium-mediated transformation both in tobacco and alfalfa. Histochemical localisation of GUS activity revealed that, in both heterologous systems, the cytosolic GS gene was expressed in anthers, theca and pollen at a late stage of flower development. Strong GUS staining was also visible in transgenic alfalfa pulvini and petioles. pGS15GUS was also found to be expressed in roots, however, treatment with ammonia did not increase the expression of the reporter gene either in tobacco or alfalfa. In mutant nodules of alfalfa formed by two different Fix(-) strains of Rhizobium meliloti both GUS staining and GUS activity were similar to the Rhizobium wild-type infected nodules indicating that GS15 expression in alfalfa root nodules does not depend on the production of ammonia coming from symbiotically fixed nitrogen. The results are discussed in relation to the possible role of cytosolic GS in different organs of legumes and other plant species. (C) 1997 Elsevier Science Ireland Ltd.