Tracing the evolution of the arbuscular mycorrhizal symbiosis in the plant lineage

The Arbuscular Mycorrhizal (AM) symbiosis is formed by ~80% of land plants with a specific group of soil fungi, the AM fungi. Through this symbiosis, plants obtain nutrients that they otherwise would not be able to access. Based on data from fossils and extant plants, it has been predicted that the AM symbiosis evolved in early land plants. Research in the past two decades utilising angiosperm model plant species has identified several plant genes that regulate the AM symbiosis. These studies have also revealed that these symbiosis genes are highly conserved in the angiosperms but whether this conservation extends to the non-flowering plants has not been explored. In the present study, a comprehensive phylogenetic analysis of the symbiosis genes was conducted, using genomic and transcriptomic data from the non-flowering plant lineages, to gain insights into the evolution of these genes in plants. The results of this analysis indicate that these genes evolved in a stepwise fashion. While some genes appeared in the algal ancestors of the charophytes, others appeared in the early land plant ancestors of liverworts. To further study the AM symbiosis in the non-flowering plants, key methods and genomic resources were established for the liverwort Marchantia paleacea to enable its use as a model plant for the study of the evolution of the AM symbiosis. Using the resources established, phylogenomic comparisons were conducted between M. paleacea and a related liverwort species, M. polymorpha, that is predicted to have lost the ability to engage in the AM symbiosis. These analyses revealed that the homologs of angiosperm symbiosis genes are required for functional symbioses in the liverworts. The findings presented here provide insights into the processes that contributed to the evolution and maintenance of this ancestral symbiosis in the plant lineage