Life in soil by the actinorhizal root nodule endophyte Frankia. A review

Frankia is a genus of soil actinomycetes famous for its ability to form N-2-fixing root nodule symbioses with actinorhizal plants. Although Frankia strains display a high diversity in terms of ecological niches in soil, current knowledge about Frankia is dominated by its life as an endophyte in root nodules. Increased use of molecular methods has refined and expanded insights into endophyte-host specificities and Frankia phylogeny. This review has focus on Frankia as a soil organism, including its part of microbial consortia, and how to study Frankia in soil. We highlight the use of nodulation tests and molecular methods to reveal population size and genetic diversity of Frankia in soil and discuss how autoregulation of nodulation and interactions with other soil microorganisms may influence the results. A comprehensive record of published interactions between Frankia and other soil microbes is summarized.