Frankia and Actinorhizal Symbiosis

Abstract

The actinobacterial member Frankia has a special significance as nitrogen fixing partner in symbiosis with some non-leguminous plants. Frankia seems to have entered into symbiotic association with many diverse dicotyledonous plants belonging to eight families independently several times during the course of evolution of this important symbiosis. Since this association confers a special ability to colonize soils low in nitrogen, actinorhizal plants are able to act as pioneers in the regeneration of Jhoom fallows, land slide affected areas, mine spoils, etc. They are also useful as wind breaks and as sand dune stabilizers. The accessibility of the complete genome sequences for three Frankia strains has provided new evidences pertaining to its evolution and structure, interactions between actinorhizal plants and their diversity. The establishment of the structure of NifH proteins from Frankia provided significant insights into the structure-function relationships. Analysis of the nif genes and whole genomes of Frankia using nucleotide triplet based phylogeny highlighted the roles of lateral gene transfer and gene duplications.