Department of Microbiology and Immunology
University of Otago
Genes, symbiosis islands and legume root nodules
Rhizobia infecting a
plant root hair.
Professor Ronson’s research involves several different aspects of the symbiotic interaction between soil bacteria called rhizobia and leguminous plants that leads to biological nitrogen fixation. The symbiosis involves a fascinating interaction between bacterium and plant in which the two partners engage in a complex molecular conversation that results in the development of a new plant organ, the root nodule. The bacteria infect the developing nodule and, once inside the plant cells, differentiate to a form able to reduce molecular nitrogen to ammonia that the plant partner assimilates and uses for growth. The symbiosis is of fundamental agricultural and ecological importance – for example, nitrogen fixation by white clover-rhizobium symbiosis underlies the sustainability of New Zealand’s pastoral agriculture.
The role of horizontal gene transfer in the evolution and ecological adaptation of rhizobia has been a major focus of research by Professor Ronson’s group at the University of Otago. The studies had their origin in an AgResearch field trial initiated in 1986, when Lotus corniculatus seeds coated with a single Mesorhizobium loti inoculant strain were planted in a tussock grassland site in the Lammermoor range in Otago. No indigenous rhizobia capable of forming root nodules on Lotus were present in the soil and uninoculated plants died of nitrogen starvation. However, rhizobia isolated 7 years later from nodules off Lotus plants at the site were found to be genetically diverse; this surprising result was explained when it was found that the diverse rhizobia shared the same chromosomally-located symbiotic DNA. Subsequent work showed that the "new" M. loti strains arose by transfer of a 502-kb "symbiosis island" from the original inoculant strain to non-symbiotic mesorhizobia present in the soil, thus evolving a symbiont in a single quantum leap. The island integrates at a specific site on the chromosome and becomes part of the genome but, when the environment is right, it is able to excise from the chromosome and resume conjugal transfer to other rhizosphere bacteria.
The above results have interesting parallels to studies of the evolution of bacterial pathogens. The analysis of complete microbial genomes has emphasised the role of horizontal gene transfer in determining genomic diversity and fostering biochemical innovation. It is now clear that the major events underlying the emergence of bacterial pathogens are the acquisition of virulence gene clusters located on genomic islands. The structure of the symbiosis island emphasises the remarkable similarities in the evolutionary strategies adopted by symbionts and pathogens in their quest to interact with eukaryotic hosts. Most genomic islands found in pathogens have been rendered immobile over evolutionary time and the symbiosis island is one of few mobile genomic islands currently identified. Current research on the mechanism and factors influencing transfer of the symbiosis island is revealing information directly relevant to unraveling the evolution of both symbionts and pathogens.
