| Arbuscular mycorrhiza is a mutually beneficial symbiosis between the majority ofland plants and fungi from the subphylum Glomeromycotina resulting in the transfer ofphosphates and other nutrients from fungi to plants in exchange for photosynthates.Throughout the interaction, arbuscular mycorrhizal (AM) fungi form several inter- andintracellular structures within plant roots, including arbuscules inside the cells of the innerroot cortex, which are the site of nutrient exchange. Both plants and AM fungi releasesignaling molecules, reviewed in Chapter 1, that initiate signaling pathways leading to thecellular changes necessary for symbiosis. Two major types of AM fungal signaling moleculesare short-chain chitooligosaccharides and lipochitooligosaccharides, which are both glucanoligomers. Additionally, AM fungal walls contain chitin and other glucan molecules that mustcome into close contact with plant cell membranes. Plant LysM receptors bind to chitin andother glucans at the cell membrane and trigger a ligand-specific signaling response, either fordefense or symbiosis. Thus, I hypothesize that certain plant LysM proteins are important forAM symbiosis. Throughout this work, I utilized Agrobacterium rhizogenes-mediated roottransformation of Medicago truncatula for RNA silencing, described in Chapter 2, and otherexpression and localization studies. In Chapter 3 I targeted LysM receptor-like kinases thatmay participate in relaying fungal signals via their intracellular kinases. In Chapter 4 Icharacterized small Medicago LysM proteins that surround arbuscules and may be importantfor both arbuscule development and degeneration. Finally, in Chapter 5 I characterized asmall effector-like LysM protein from the AM fungus, Rhizophagus irregularis, which isunder positive selection and can bind to chitin and chitosan. Despite their specific expression,intriguing localization, and subtle effects on colonization as a result of mutation or overexpression,none of the Medicago LysM proteins proved essential for AM symbiosis, likelydue to redundancy within their gene families. Deletion of multiple genes at once still does notdisrupt the interaction. Such robustness is not surprising in light of 300 million years of asymbiotic relationship growing ever more stable throughout evolution. |
