Leguminous plants are able to fix nitrogen by establishing a symbiotic relationship with soil dwelling bacteria, called rhizobia. The model plant Medicago truncatula forms a partnership with Sinorhizobium meliloti whereby the plant gains bioavailable nitrogen and in exchange the bacteria gains carbohydrates. This process occurs within nodules, which are structures produced on the roots of the plants within which nitrogen is fixed. M. truncatula incomplete root elongation (MtIRE) was localized to the infection zone, which is zone II of indeterminate nodules. It was shown to encode a signaling kinase so it was anticipated to play a role in nodulation. Mutants of MtIRE in the R108 background, mutagenized with the Tnt1 retrotransposon, were obtained from reverse screen, and were assessed to determine if a disrupted MtIRE gene was the cause of nitrogen fixation defective nodules. Mutant line NF1320, having a mutant phenotype, showed typical Mendelian segregation of 3:1 when backcrossed to R108. Experimental results show that MtIRE gene is not the cause of the mutant phenotype, but was linked to the causative locus. MtIRE co-segregated with the mutant phenotype 83%. Southern blot and the first version of the M. truncatula genome (version 3.5) reported a single MtIRE gene and this was …