Molecular genetics of the sinorhizobium meliloti - alfalfa symbiosis

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Sinorhizobium meliloti is a bacterium that forms N2-fixing root nodules on alfalfa. My research program has dealt with understanding various aspects of the symbiosis between S. meliloti and alfalfa and also extends to studying S. meliloti as a model soil microorganism. We have investigated the pathway of carbon metabolism employed by the bacteria within root nodules and at the genome level, we have analyzed the biological role(s) of the 1,683-kilobase pSymB megaplasmid. That work led to our involvement in the sequencing of the S. meliloti genome and more recently on the development of genomic approaches to the analysis of the it's genome. We focused our recent NSERC research towards the analysis of the repABC gene cluster which is required for stable replication of pSymB and have also extended our work on phosphate transport in S. meliloti towards the identification of many genes whose expression is regulated by PhoB in response to phosphate (Pi) limitation i.e. the Pho-regulon. Here we propose to continue our analysis of Pi-regulated metabolism in S. meliloti: i) by investigating the physiological bases for their induction/repression and ii) by investigating whether these genes are subject to Pi-responsive regulation in diverse organisms (those whose genomes are sequenced). This work will result in the recognition of a core gene set that responds to Pi-limitation and by inference a biological response to Pi-limitation. This represents fundamental information on a key cellular component that is abundant in cells but often limits growth in soil environments. We will investigate carbon metabolism in the bacteria within nodules and also apply sophisticated genomic approaches towards addressing what genes are induced in the soil and rhizosphere surrounding plant roots. This information will help in understanding how bacteria (including pathogens) survive in soil environments and how the agriculturally and economically important nitrogen fixation process can be improved.