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The
evolution of Rhizobium-legumes symbiosis
Abstract:
The process of biological nitrogen fixation was developed by the
living organisms over two billions years ago. Since that time it
has been undergoing continuous evolution in parallel with the
evolution of living organisms which eventually led to development
of numerous varieties of this process.
Non-symbiotic reduction of N2
performed by free-living Archea and Bacteria is the
oldest form of biological nitrogen fixation. The evolution of
terrestrial seminal plants and their common predisposition for
symbiotic interactions with microorganisms, especially with fungi,
permitted the development of very effective symbiotic systems.
The formation of correctly functioning
symbiotic systems in which diazotrophic bacteria reduce
atmospheric dinitrogen requires the expression of a genetic
information, originating from three independently evolving sources:
the plant genome, the bacterial genome and the nod-nif
region. The plant genome and the nod-nif genes interact
most pronouncedly (phenotypic interactions), whereas the bacterial
genome plays but a secondary role in the creation of symbiotic
systems. The group of bacterial diazotrophic microsymbionts
enlarges dynamically as an outcome of a significant mobility of
the nod-nif region and its dissemination among new
bacterial species by the lateral gene transfer mechanism.
1. Importance
of the biological nitrogen fixation. 2. Symbiotic and
non-symbiotic diazotrophs. 3. Symbiotic nitrogen fixation: an
overview.4. Phylogeny of plant-host species. 5. The
molecular predisposition of plants for nodulation. 6. Phylogeny
of rhizobia. 7. The evolution of enzymatic nitrogenase
complex and Nod factors biosynthesis pathway. 8. nod-nif
genes - a region with its own phylogeny. 9. Dissemination of
the symbiotic genes among the bacterial species. 10. Bacteria
and plants - the evolution of species and their interactions. 11. "Dead
ends" of evolution - non-effective symbioses as a result of
incompatibility between plant host and its microsymbiont. 12. The
perspectives of symbiotic nitrogen fixation. 13. Summary |
