Medicago truncatula supernodulation mutants sunn4 and lss show enhanced seed yield and seed nitrogen allocation from nitrogen fixation under low nitrogen availability

Background and Aims: Symbiotic nitrogen fixation in legumes has an energetic cost, which the host controls through inhibition of nodulation in response to externally available nitrogen. Autoregulation of nodulation (AON) mutants supernodulate, even in the presence of sufficient nitrogen. Here, we tested whether three AON mutants of Medicago truncatula, sunn4, rdn1-1 and lss, display fitness costs due to supernodulation, and to what extent this is dependent on the supply of nitrogen. Methods: We grew wild type (WT) and AON mutants under controlled conditions with a supply of either 0.5 or 2 mM of ¹⁵N-labelled nitrate until seed maturation, and determined biomass, seed yield, tissue N content, and N derived from nitrogen fixation. Plants were either inoculated with Sinorhizobium meliloti or left uninoculated. Results: The sunn4 and lss, but not rdn1-1 mutant showed significantly higher seed yield than the WT plants in the low (0.5 mM) nitrate treatment, but only if they were inoculated with rhizobia. The mutants allocated significantly higher proportion of N derived from nitrogen fixation to seeds. At the higher (2 mM) nitrate concentration, mutants and WT showed similar biomass and N derived from nitrogen fixation. Conclusions: We conclude that, despite the higher potential energetic cost of supernodulation, nodulated sunn4 and lss AON mutants can have a fitness advantage (higher yield and biomass) under low N due to higher nitrogen fixation over the whole growth period, and that they allocate more fixed N and more biomass into their seeds than WT plants, at least under controlled conditions.