Cell-specific expression in transgenic plants reveals nonoverlapping roles for chloroplast and cytosolic glutamine synthetase.

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Abstract
Chloroplast and cytosolic isoforms of glutamine synthetase (GS; EC 6.3.1.2) are encoded by separate nuclear genes in plants. Here we report that the promoters for chloroplast GS2 and cytosolic GS3A of Pisum sativum confer nonoverlapping, cell-specific expression patterns on the .beta.-glucuronidase (GUS) reporter gene in transgenic tobacco. The promoter for chloroplast GS2 directs GUS expression within photosynthetic cell types (e.g., palisade parenchymal cells of the leaf blade, chlorenchymal cells of the midrib and stem, and photosynthetic cells of tobacco cotyledons). The promoter for chloroplast GS2 retains the ability to confer light-regulated gene expression in the heterologous transgenic tobacco system in a manner analogous to the light-regulated expression of the cognate gene for chloroplast GS2 in pea. These expression patterns reflect the physiological role of the chloroplast GS2 isoform in the assimilation of ammonia generated by nitrite reduction and photorespiration. In contrast, the promoter for cytosolic GS3A directs expression of GUS specifically within the phloem elements in all organs of mature plants. This phloem-specific expression pattern suggests that the cytosolic GS3A isoenzyme functions to generate glutamine for intercellular nitrogen transport. In germinating seedlings, the intense expression of the cytosolic GS3A-GUS transgene in the vasculature of cotyledons reveals a role for cytosolic GS in the mobilization of seed storage reserves. The distinct, cell-specific patterns of expression conferred by the promoters for chloroplast GS2 and cytosolic GS3A indicate that the corresponding CS isoforms perform separate metabolic functions.