[Evolution of nitrogen-fixing symbioses based on the migration of bacteria from mycorrhizal fungi and soil into the plant tissues].

Abstract

The hypothesis is suggested on the emergence of N2-fixing plant symbionts from soil diazotrophs and from the satellites of Glomeromycota fungi forming arbuscular mycorrhizae (AM). This universal form of plant-microbe symbiosis possibly appeared from integration of ancestral land plants (rhyniophytes, psylophytes) and microbial consortia composed of AM-fungi assimilating soil phosphates and bacteria fixing atmospheric CO2 and/or N2. Releasing of these bacteria from AM-fungal hyphae into the plant tissues elicited the selection of genotypes capable of the fungi-independent multiplication in planta, as well as the fixation in bacterial genomes of the genes for synthesis of chitin-like signal factors stimulating the development of symbiotic structures. An early stage of this evolution might been represented by formation of N2-fixing syncyanoses, the late stage - by formation of nodular symbioses of dicots from Eurosid I clade with rhizobia (α- and β-proteobacteria) and with actinobacteria Frankia. Emergence of these symbioses was possibly based on the migration of soil and endophytic bacteria into the storage organs (modified stems or lateral roots), where the optimal conditions were established not only for N2 fixation but also for the evolution of bacteria towards an increased symbiotic activity. This evolution resulted in the emergence of primary rhizobia (Bradyrhizobium, Burkholderia) which acted as the donors of sym-genes for a broad spectrum of microbes transformed into the secondary rhizobia (Rhizobium, Sinorhizobium). The succeeding evolution of nodular symbioses was directed at an increased efficiency of symbiotrophic nitrogen nutrition in host plants following two scenarios: (i) “expensive”, based on the increase of N2- fixing activity via transformation of bacteria into non-reproducible bacteroids; (ii) “economic”, based on acquiring the determinate nodule structure and ureide nitrogen assimilation.