AMMONIA REGULATED EXPRESSION OF A SOYBEAN GENE ENCODING CYTOSOLIC GLUTAMINE-SYNTHETASE IS NOT CONSERVED IN 2 HETEROLOGOUS PLANT-SYSTEMS

A 3.5 kb promoter fragment fused to the reporter beta-glucuronidase ge ne (gus) had previously been shown to contain separate regulatory elem ents controlling the ammonia-stimulated and organ-specific transcripti on of GS15, a soybean gene encoding root and root nodule cytosolic glu tamine synthetase (GS). In order to determine if the regulatory elemen ts conferring the ammonia-regulated and the organ-specific expression are conserved in different plant species, a GS15 promoter-gus (pGS15GU S) construct was introduced via Agrobacterium-mediated transformation both in tobacco and alfalfa. Histochemical localisation of GUS activit y revealed that, in both heterologous systems, the cytosolic GS gene w as expressed in anthers, theca and pollen at a late stage of flower de velopment. 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 t he reporter gene either in tobacco or alfalfa. In mutant nodules of al falfa formed by two different Fix(-) strains of Rhizobium meliloti bot h GUS staining and GUS activity were similar to the Rhizobium wild-typ e infected nodules indicating that GS15 expression in alfalfa root nod ules does not depend on the production of ammonia coming from symbioti cally fixed nitrogen. The results are discussed in relation to the pos sible role of cytosolic GS in different organs of legumes and other pl ant species. (C) 1997 Elsevier Science Ireland Ltd.