Transcriptional responses to arbuscular mycorrhizal symbiosis development are conserved in the liverwort Marchantia paleacea
The mutualistic arbuscular mycorrhizal (AM) symbiosis arose in land plants more than 450 million years ago. This symbiosis is still widely found across  major land plant lineages, including bryophytes. Despite its broad taxonomic distribution, little is known about the molecular components underpinning symbiosis outside of flowering plants. Here, we demonstrate that a broad AM genetic programme is conserved amongst land plants. In this study, we characterised the dynamic response of the liverwort Marchantia paleacea to Rhizophagus irregularis colonization by time-resolved transcriptomics across three stages of symbiosis. Comparative analysis of transcriptional responses to symbiosis in the liverwort M. paleacea and the legume Medicago truncatula further revealed evolutionarily conserved expression patterns for genes underpinning pre-symbiotic signalling, intracellular colonization and nutrient exchange. This study demonstrates that the genetic machinery regulating key aspects of symbiosis in plant hosts is largely conserved and coregulated across land plants. Overall design: To understand the molecular changes associated with development of AM symbiosis in Marchantia paleacea, we characterised the transcriptional response of the liverwort at five, eight- and eleven-weeks post-inoculation (WPI) with the AM fungus Rhizophagus irregularis. For every time point, transcript levels of genes in mock-inoculated (control) thalli were compared to colonised M. paleacea thalli.