Abstract
We have identified a single putative plastidic glutamine synthetase (GS 2), isolated from Medicago sativa (alfalfa) leaf. Analysis of organ/tissue specific expression of the GS 2 gene in this study has shown that it is expressed in all green tissues. We show that the alfalfa GS 2 gene is also expressed in nitrogen fixing root nodules where its expression is not regulated by fixed nitrogen. Treatment with nitrate (NO 3 −) results in the induction of GS 2 in the roots and leaves of alfalfa, but the signaling mechanism in the two organs is different. In the roots NO 3 − appears to act as a direct signal for the induction of GS 2 whereas in the leaves secondary metabolites of NO 3 − probably act as the signal. We also demonstrate that 2-oxoglutarate (2-OG), in combination with NO 3 −, appears to significantly induce GS 2 expression, pointing to 2-OG as a potential primary metabolic inducer of alfalfa GS 2. Treatment with glutamine or sucrose, in combination with NO 3 −, also appears to induce GS 2 in the roots, but there is a lag in the induction when compared to the 2-OG/NO 3 − combination. Our interest ultimately lies in dissecting how carbon:nitrogen status modulates the expression of GS at transcriptional and post-transcriptional levels.
Published Version
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