Phenotypic and genetic diversity in nitrate-responsive regulation of symbiotic nitrogen fixation in Medicago truncatula : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Breeding at Massey University, Manawatū, New Zealand. EMBARGOED until 16th April 2026

Abstract

Finding alternatives to synthetic nitrogen fertilisers is important in order to reduce human fossil fuel use. An alternative method is biological nitrogen fixation by legume-rhizobia symbiosis, but the amount of nitrogen fixed by symbiosis is too small to fully replace use of synthetic fertilisers in high-productivity agricultural systems. Supplementing fertiliser is inefficient because symbiotic fixation is downregulated by plants when nitrogen fertilisers are present in the soil. In this study, several Medicago HapMap Project accessions are tested for sensitivity to nitrates. It was shown that nitrate in soil causes the plant host to regulate new infections by rhizobia and rhizobial nitrogen-fixing activity in a partially independent manner. Sensitivity to nitrates varied widely between accessions. Several accessions were identified with minimal inhibition of infections under nitrate conditions, and others were identified that show only moderate inhibition of symbiotic nitrogen fixation activity.--Shortened abstract