Investigating the link between rhizobial strain, microbiome composition and plant phenotype in Medicago truncatula and Parasponia andersonii

Plant-microbe interactions range widely, from pathogenic to mutualistic. The legume-rhizobia co-symbiosis is vital in agriculture and is a relationship where the host provides carbon compounds and the rhizobia provide nitrogen. Plant phenotype can also be influenced by soil or endospheric bacteria which produce metabolites that impact plant physiology. This forms a feedback cycle where plant compounds secreted into the soil impact the local microbial community, which in turn influence the host.
However, rhizobia are not equally efficient at producing fixed nitrogen for their hosts. Recently our lab has found that high-efficiency symbionts can influence the other microbes that colonise Medicago truncatula. Inoculation of rhizobia leads to microbiome reshaping, decreasing overall species diversity but increasing the diversity and abundance of bacteria belonging to the genus Actinobacteria in particular. This shows that nodulation may influence the microbiome of the host.
This project will first ask if the bacteria whose abundance changes because of rhizobia inoculation influence host biomass upon addition to the root environment. Subsequent experiments will attempt to characterise the mechanisms behind why these changes in community composition and host biomass occur. Bacterial localisation within the host system will be investigated using bacterial labelling with fluorophores. Sequencing of bacterial strains will also provide genetic information on how they may be influencing host phenotype.
Parasponia is the only non-legume genus to form symbiotic associations with rhizobia bacteria, thus it forms an interesting comparison point in studying nodulation and its origins. This project will also attempt to characterise how the non-legume Parasponia andersonii responds to rhizobia strains of different nitrogen fixation efficiency. This will be carried out by assessing nodule morphology and studying Parasponia gene expression responses to rhizobia.