A forward genetic screen for genes involved in the arbuscular mycorrhizal symbiosis in the model legume Medicago truncatula

The arbuscular mycorrhizal symbiosis (AMS) provides a significant part of nutrient uptake for the majority of plant species. Engineering increased symbiotic potential in crops offers great benefits for agriculture, reducing the demand for fertilisers and increasing resiliency to disease and abiotic stress. We attempted to increase understanding of the AMS, by identifying genes involved in the symbiosis, focusing on the poorly understood parts of the symbiotic process outside of the legume common symbiosis pathway. Prior work had carried out an initial screen of the model legume Medicago truncatula, mutagenised with the retrotransposon tnt1, to obtain lines showing a phenotype of impaired arbuscular mycorrhizal colonisation while retaining normal rhizobial colonisation. This project took candidate lines from that screen, and used morphological and genetic phenotyping to confirm four Medicago lines with defects in different parts of the AMS. We developed a computational pipeline to quickly locate the 30-60 tnt1 insertions in each mutant line with Illumina whole genome sequencing (WGS). We backcrossed the mutants to produce populations segregating for the different insertions. This population was genotyped for the insertions located by WGS. Co-segregation analysis was used to show correlation between tnt1 insertions and the impaired arbuscular mycorrhizal colonisation phenotype in these lines. Finally, we attempted to replicate reports that plant mycorrhizal colonisation phenotypes are dependent on fungal genotype, and question the assumed universality of signalling across this highly generalist symbiosis.