Discovering the phenotypic variation within Rhizobium leguminosarum and determining the best strains for soil inocula

Biological nitrogen fixation (BNF) with rhizobia is an environmentally friendly, sustainable alternative to chemical fertilisers. Despite many legumes having the ability to form relationships with a wide range of indigenous soil bacteria, the efficiency of the symbiosis is altered greatly by the combination of partners. Additionally, choosing favourable rhizobia strains for use as soil inocula requires consideration of both the climate and soil conditions. Traditionally, the efficiency of the symbiosis is tested for its versatility under a variety of abiotic and biotic stressors with plant biomass measurements. Consequently, I will measure the ability of free-living rhizobia to survive and grow in a number of conditions associated with the rhizosphere.

Rhizobium leguminosarum bv. trifolii (Rlt) forms a symbiotic relationship with Trifolium repens (white clover). 192 strains of Rlt were used in the study, isolated in the UK, France and Denmark (both conventional and organic farms) and grouped into genospecies (gsA, gsB, gsC, gsD and gsE) based on their core genome. The strains were cultured onto modified liquid Tryptone-Yeast (TY) media and bacterial density (OD600) was measured at various timepoints post inoculation. When ranking the strains based on their highest bacterial density across experiments, the UK strains were the most consistently high scoring but were significantly impacted by a decrease in nutrient availability. Therefore, based on the experiments conducted, the gsA strains would be the best candidates for soil inoculation across the broadest range of conditions.