Differential expression of the two cytosolic glutamine synthetase genes invarious organs of Medicago truncatula

In order to clarify the physiological roles of the cytosolic forms of gluta mine synthetase (GS) in Medicago truncatula, we have performed a detailed a nalysis of the expression of the two functional cytosolic GS genes, MtGSa a nd MtGSb in several organs of the plant. Transcriptional fusions were made between the 2.6 or 3.1 kbp 5' upstream regions of MtGSa or MtGSb, respectiv ely, and the reporter gene gusA encoding beta -glucuronidase and introduced into the homologous transgenic system. MtGSa and MtGSb were found to be di fferentially expressed in most of the organs, both temporally and spatially . The presence of GS proteins at the sites where the promoters were active was confirmed by immunocytochemistry, providing the means to correlate gene expression with the protein products. These studies have shown that the pu tative MtGSa and MtGSb promoter fragments were sufficient to drive GUS expr ession in all the tissues and cell types where cytosolic GS proteins were l ocated. This result indicates that the cis acting regulatory elements respo nsible for conferring the contrasting expression patterns are located withi n the region upstream of the coding sequences. MtGSa was preferentially exp ressed in the vascular tissues of almost all the organs examined, whereas M tGSb was preferentially expressed in the root cortex and in leaf pulvini. T he location and high abundance of GS in the vascular tissues of almost all the organs analysed suggest that the enzyme encoded by MtGSa plays an impor tant role in the production of nitrogen transport compounds. The enzyme syn thesised by MtGSb appears to have more ubiquitous functions for ammonium as similation and detoxification in a variety of organs. (C) 2000 Elsevier Sci ence Ireland Ltd. All rights reserved.