Trick or treat: Mechanisms regulating fungal compatibility in arbuscular mycorrhizal symbiosis

Most land plants (>80%) rely on the symbiosis with arbuscular mycorrhizal (AM) fungi belonging to the Glomeromycotina subphylum to obtain sufficient nutrients from the soil. How AM fungi manage to colonize and cooperate with such an extremely broad range of plants is a key question in AM biology. AM fungi must have evolved efficient ways to deal with the plant immune system and be able to adapt to their different host environments to ensure a beneficial interaction. However, the underlying mechanisms are still largely unknown. In this thesis, I studied cell-type transcriptome of both the fungus and the plant at different stages of the interaction and the fungal responses in diverse host plants, with a focus on effector biology and reciprocal nutrient transfer. My results suggests that AM fungi can adjust their transcriptome and secretome to colonize evolutionary distantly related hosts, namely Medicago truncatula, Nicotiana benthamiana and Allium schoenoprasum. By applying laser microdissection coupled with RNA sequencing in Medicago, I dissected the expression pattern of AM genes particularly AM effectors into different stages, including arbuscules, intraradical mycelium, extraradical mycelium and germinating spores. These approaches collectively resulted in an expression atlas of the fungal transcriptome and secretome and allowed me to pinpoint potential key genes in AM symbiosis. In addition, I performed detailed functional analyses on one of the highest intraradically expressed effector, encoding a LysM domain-containing effector (RiSLM). This showed that RiSLM can bind chitin (oligomers), protect fungal cell walls from plant chitinases and suppress chitin-triggered immune responses, therefore play a positive role in the symbiosis. These results revealed a conserved role for LysM effectors in both pathogenic as well as symbiotic fungal interactions with plants. Although sugars have long been proposed to be transferred from the host to AM fungi, the mechanisms by which these sugars are transported by the plant to the fungus have remained elusive. By analysing the stage-specific transcriptome data, a single Sugars-Will-Eventually-bE-Secreted (SWEET) transporter, called MtSWEET1b, was shown significantly expressed in arbuscule-containing cells of Medicago. MtSWEET1b was shown to transport glucose and to localize to the peri-arbuscular membrane. Overexpression of MtSWEET1b increased fungal colonization levels, while interference with its activity via overexpression of dominant-negative versions caused an early collapse of arbuscules. These results suggest a (redundant) role for MtSWEET1b in the transport of glucose across the peri-arbuscular membrane to maintain arbuscules for a healthy mutually beneficial symbiosis. Taken together, by combining cell-specific transcriptome analyses with molecular functional approaches I was able to shed new light on some of the mechanisms that contribute to the broad host-range of AM fungi in light of effector biology and reciprocal nutrient transfer. The datasets generated and described in this thesis offer a valuable resource to deepen our understanding of the ecologically and agriculturally key AM symbiosis in the future.