Investigating the link between microbiome composition and nitrogen fixation efficiency in the model legume Medicago truncatula

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"Plants and microbes interact with one another in a multitude of ways, ranging from pathogenic to mutualistic. One of the most important plant-microbe interactions in agriculture is the legume-rhizobia co-symbiosis, a mutualistic relationship where the host provides photosynthates in the form of organic acids like succinate whilst the rhizobia provide fixated atmospheric nitrogen in the form of ammonia. Whilst the presence of rhizobia often increases the biomass of their host, not all rhizobia species are as efficient at producing fixated nitrogen and certain species and strains of rhizobia can be defined as high- or low-efficient symbionts. Rhizobia are certainly not the only group of bacteria that can influence plant phenotype and health. Soil contains a multitude of bacterial species that exist in the rhizosphere or even endosphere of plants, including legumes, that produce metabolites that impact plant physiology. This forms a feedback cycle where the compounds plants secrete into the soil impact the local microbial community which in turn influence the host. Recent work that forms the basis of this project has found that high-efficiency symbionts can influence the microbial community in the endosphere of the model legume Medicago truncatula. In this case, inoculation led to the reshaping of the microbiome - decreasing overall species diversity but increasing the diversity and abundance of bacteria belonging to the genus Actinobacteria. This shows that the process of nodulation can influence the microbiome of the host at at least one stage in this symbiotic interaction. This project will first aim to ask if the bacteria whose abundance changes as a result of rhizobia inoculation influence host biomass upon addition to the soil or root environment. The bacteria will be grouped based on their associated abundance increase with either high-efficiency rhizobia inoculation or low-efficiency/mock inoculation. Subsequent experiments will attempt to characterise the mechanisms behind why these changes in community composition and possible host biomass occur. Firstly, this will be carried out by investigating bacterial localisation within the host system using bacterial labelling with fluorophores. Secondly, 'omics characterisation will be used to investigate what processes are occurring on a molecular level that led to altered community dynamics as a result of nodulation. As part of this, there will be an attempt to generate a microbial library of the microbiome of Medicago truncatula which does not currently exist and would provide an important output for longer term study of the interaction between plant microbial communities, host plants and rhizobial symbionts. "