Origin and Evolution of Mycorrhizal Symbioses in Land Plants.

Abstract

Most extant vascular plants can enter into mycorrhizal symbioses with fungi. In addition, mycorrhiza-like associations also occur in bryophytes. The evolutionary homology between these two types of associations is still in debate, resulting in the uncertainty of the origin of such important land plant-fungus symbioses. In this thesis, I attempted to address this question by conducting both bioinformatic and molecular studies. Phylogenetic distribution of mycorrhizae across land plants was investigated by carrying out a broad survey and mapping the information onto a reconstructed land plant phylogeny. A shared evolutionary pattern is observed in liverworts and all major groups of vascular plants, i.e., glomeromycetous fungi engaged in symbiosis with early diverging lineages of plants and asco- and basidiomycetous fungi participated in symbiosis with derived lineages. This suggests an early origin of mycorrhizal associations in the common ancestors of all land plants. Several genes involved in mycorrhizal symbiosis have been identified in legumes. In this study, three of them, DMI3/CCaMK, DMI1/Pollux, and IPD3/Cyclops were successfully amplified from other major lineages of land plants, especially bryophytes, using PCR, RT-PCR, and RACE. Phylogenetic analyses indicate vertically inheritance of these genes in land plants. Further, bioinformatic and molecular evolutionary analyses indicated that the genes have well conserved domains and are under purifying selection. These results suggest that the genes have been functionally conserved throughout the history of land plants. Finally, yeast-two-hybrid and cross-species complementation assays were conducted on DMI3/CCaMK. The putative CCaMK proteins from liverworts and hornworts interacted with the Medicago IPD3, as the two proteins from the Medicago did. The genes from two groups of bryophytes also successfully rescued the Medicago dmi3 mutant, producing vesicles and arbuscules – the full phenotype of mycorrhizae. These experimental functional assays provide conclusive evidence that the gene indeed has conserved function in mediating mycorrhizal symbiosis across land plants. In conclusion, both bioinformatic and experimental studies performed in this thesis demonstrated an early origin of mycorrhizae in the common ancestor of all land plants, and further supported the hypothesis that plant – mycorrhizal fungus symbiosis played an instrumental role in the successful colonization of land by plants.