Nitrogen and metabolic regulation of the expression of plastidic glutamine synthetase in alfalfa (Medicago sativa)

We have identified a single putative plastidic glutamine synthetase (GS2), isolated from Medicago sativa (alfalfa) leaf. Analysis of organ/tissue specific expression of the GS2 gene in this study has shown that it is expressed in all green tissues. We show that the alfalfa GS2 gene is also expressed in nitrogen fixing root nodules where its expression is not regulated by fixed nitrogen. Treatment with nitrate (NO3−) results in the induction of GS2 in the roots and leaves of alfalfa, but the signaling mechanism in the two organs is different. In the roots NO3− appears to act as a direct signal for the induction of GS2 whereas in the leaves secondary metabolites of NO3− probably act as the signal. We also demonstrate that 2-oxoglutarate (2-OG), in combination with NO3−, appears to significantly induce GS2 expression, pointing to 2-OG as a potential primary metabolic inducer of alfalfa GS2. Treatment with glutamine or sucrose, in combination with NO3−, also appears to induce GS2 in the roots, but there is a lag in the induction when compared to the 2-OG/NO3− combination. Our interest ultimately lies in dissecting how carbon:nitrogen status modulates the expression of GS at transcriptional and post-transcriptional levels.