Glutamine synthetase of potato (Solanum tuberosum L. cv. Desiree) plants: cell- and organ-specific expression and differential developmental regulation reveal specific roles in nitrogen assimilation and mobilization.

Potato (Solanum tuberosum L. cv. Desiree) glutamine synthetase (GS) (EC 6.3.1.2) gene expression and polypeptide accumulation patterns were analysed in several organs and at several developmental stages. Three GS genes have been identified, one gene encoding plastidic GS (GS2) and two encoding cytosolic GS (GS1) that are differentially expressed in the plant at cellular and organ levels. Specific developmental regulation of different GS genes was also observed. Potato GS seems to be regulated essentially at transcription and/or RNA stability levels. GS2 polypeptides and mRNAs were detected in leaves and their content decreased as leaves senesced. A similar pattern of expression was observed for the GS1 gene Stgs1a. GS1 transcripts and polypeptides were present in all organs analysed and are the only GS detected in non-photosynthetic tissues and in the later leaf senescing stages. The increase in GS1 during leaf senescence mainly reflected polypeptide and transcript accumulation of the GS1-encoding gene Stgs1b. In situ hybridization results point to a cell-specific expression of GS1 genes within the vascular bundles, Stgs1b being expressed in the xylem and phloem parenchyma cells, and Stgs1a being expressed only in the phloem companion cells. This pattern of spatial distribution and differential developmental regulation of different GS1 genes differs from what has been previously described for genes of other Solanaceae with a high degree of similarity with the ones described here and suggests that distinct GS1 isozymes have specific and possibly distinct roles within the same organ. These new findings highlight the physiological importance of different GS1 isoenzymes in plant nitrogen metabolism.