Analysis of the expression of the entire M. truncatula GS gene family during nodule development. Nodules were collected at 0, 3, 6, 10, 14, 20 and 40 days after infection. A. Quantification of MtGS1a, MtGS1b, MtGS2a and MtGS2b transcripts by qRT-PCR. GS transcript abundance was normalized to that of the housekeeping gene MtElf1-α. B. Western blot analysis of GS polypeptides using anti-Glnγ antibody recognizing both the cytosolic (GS1) and plastid-located (GS2) polypeptides . 40 day old nodules were dissected into three parts, enriched in meristematic zone (Mer), nitrogen fixation zone (Fix) and senescence zone (Sen). Equal amount of protein (30 μg) were loaded on each lane. C. Quantification of GS activity on soluble protein extracts. Results are representative of at least 3 biological replicates.