Australia’s land of opportunity for horizontal gene transfer and symbiotic nitrogen fixation

Legumes are plants that form symbioses with soil bacteria termed rhizobia, which convert (fix) atmospheric nitrogen (N2) into NH3. Symbiotic nitrogen fixation enhances crop and pasture legume growth and in Australia is worth an estimated A$4 billion per annum in equivalent nitrogen fertiliser costs. Rhizobia genes facilitating symbiosis are found in diverse bacterial genera and are frequently located on mobile genetic elements such as plasmids and integrative & conjugative elements. Australian soils have historically lacked rhizobia capable of symbiosis with introduced agricultural legumes, so exotic rhizobia have been introduced as inoculants to establish biological nitrogen fixation. Research reviewed here discusses the widespread dissemination of symbiosis genes from introduced inoculant strains to native Australian soil bacteria and how this has facilitated evolution of new symbiotic rhizobia with varying capabilities in N2 fixation. We discuss how dissecting the molecular genetics of symbiosis gene transfer and constructing high-quality closed rhizobia genomes are providing tools and opportunities to manage symbiosis gene transfer for sustainable agriculture.