Reverse Genetic Study of Four Medicago Truncatula Defence Genes to Elucidate Involvement in Arbuscular Mycorrhizal Symbiosis

Abstract

The arbuscular mycorrhizal (AM) symbiosis is a mutualistic relationship between plants and AM fungi that increases global plant nutrition. Although the benefits of forming the AM symbiosis is significant for around 80% of terrestrial plants, little is known about the molecular pathways involved in facilitating the interaction. In this thesis, we sought to understand the involvement of plant defence-associated genes in regulating the beneficial arbuscular mycorrhizal (AM) symbiosis. Based on GeneChip and RNA sequencing data, we identified four genes in the model legume Medicago truncatula with increased expression during AM symbiosis that are predicted to encode proteins associated with host defence pathways. We hypothesized that these proteins are involved in regulating AM symbiosis, either by: a) mediating the suppression of defence pathways to enable fungal colonization of root cells, or alternatively: b) by restricting the growth of the symbiont to prevent over-proliferation in host tissues. Two of the four genes encode putative Toll/inkerleukin-1 nucleotide-binding leucine rich repeat (TIR-NB-LRR) receptors, a group of intracellular receptors that allow plants to identify non-self-microorganisms (Zhai et al., 2011). One gene encodes a leucine-rich repeat (LRR) receptor kinase, a transmembrane receptor that activates signal transduction pathways upon perception of microbe associated antigens (Torii, 2004). The fourth gene encodes an enhanced disease susceptibility-like (EDS1-like) defence protein that mediates signals involved in resistance (Wang et al., 2014). To screen the involvement of these defence genes of interest, we used a reverse genetics approach to compare the mean root length colonized of mutant knockout Medicago truncatula plants inoculated with Rhizophagus irregularis to wildtype plants. Results showed that the TIR-NB-LRRs had no consistent role in the AM symbiosis, while the the EDS1-like gene and the LRR-kinase displayed promising preliminary results for a potential role in regulating the mutualism