Determination of nitrogen fixation effectiveness in selected Medicago truncatula isolates by measuring nitrogen isotope incorporation into pheophytin

Effectiveness is a term used to describe the input that a bacterial nitrogen-fixing symbiosis makes to plant nitrogen metabolism. In legumes, effectiveness is considered a polymorphic trait where specific interactions between the plant and symbiotic rhizobia contribute to the success of the interaction. Evaluation of effectiveness using model legumes like Medicago truncatula may open new avenues for genetic studies. In previous work, an isotope dilution mass spectrometry method, which uses the effect of nitrogen fixation on the nitrogen isotope composition of chlorophyll in plants grown on N-15 fertilizer as a measure of effectiveness, was developed for estimating the contribution of symbiotic nitrogen fixation to plant nitrogen content. This N-15-dilution assay was used to evaluate the level of nitrogen fixation effectiveness in three Medicago truncatula lines that have been used as parents in generating recombinant inbred lines. Three Sinorhizobium meliloti strains, USDA 1600, 1021751 and MK506, differ in this measure of effectiveness on three lines of M. truncatula: Jemalong A17, DZA315.16 and F83005.5. Plant-rhizobia combinations grown in two different conditions showed comparable differences in effectiveness.