Plant-microbe symbioses as an evolutionary continuum

In spite of enormous taxonomic, structural and functional diversity of plant-microbe interactions, they are characterized by a historical succession which allows us to consider different forms of symbioses as the components of an evolutionary continuum. Their ancestral form is represented by arbuscular mycorrhiza (AM) which originated at the outset of terrestrial flora evolution and constituted a key factor for the land colonization by plants. In the course of AM evolution the plant acquired a basal set of genes for regulating the performance of microbes which colonize the root tissues. Later, these genes were repeatedly reorganized to meet the involvement of novel mutualistic symbionts (N-2-fixing bacteria, ectomycorrhizal fungi, endophytes and epiphytes) and pathogens into the symbiotic interactions. Form the microbial side, the evolutionary succession of mutualism and antagonism is restricted to the defensive symbioses formed by plants with the ergot fungi, Clavibacter, Bacillus and Pseudomonas bacteria. Involvement of the similar systems for symbiotic interactions may be related to convergent evolution in the distant microorganisms (adaptation to the conservative host defense/regulatory factors), to molecular mimicry (imitation of the mechanisms of interaction used by the more ancient symbionts) or to the horizontal gene transfer. The hypotheses of the successive substitution of symbionts is suggested to address the relationships between AM and N-2-fixing nodular symbioses in dicotyledons plants. AM formation is considered as a source of preadaptations responsible for the substitution of glomalean fungi which occupied the plant symbiotic compartments by the actinomycetes Frankia (in Rosid I plants) which were exchanged for the more competitive root nodule bacteria (in legumes). The development of nutritional symbioses with microbes is considered as an ancestral function of plant roots which were later supplemented or substituted with the function of assimilating the soil nutrients.