Psyma of Sinorhizobium Meliloti: Nitrogen Fixation and More

Abstract

pSymA, the smaller of the two megaplasmids of Sinorhizobium meliloti, contains a large number of the genes known to be needed for the development and productivity of the nitrogen-fixing symbiosis with legumes. However, pSymA contains no genes that are essential for cell viability and is, therefore, squarely in the category of accessory elements. Analysis of the genes contained in pSymA suggests many roles that the plasmid may play beyond symbiosis or to supplement the symbiotic interaction. But there are a large number of genes whose functions are either unknown or are difficult to specify precisely enough so that a specific role for the genes can be assigned. In this review, we attempt to set a context for further functional analysis of pSymA by discussing the content of this plasmid and the tools that can be brought to bear on its analysis. Sinorhizobium meliloti is a nitrogen-fixing bacterial symbiont of several important forage crops, which include Medicago species, like alfalfa and the model legume M. truncatula, Melilotus species, like sweet clover, and Trigonella species, like fenugreek. The bacteria infect the roots of these plants and induce formation of an organ called a nodule, within which they are able to fix enough atmospheric N2 to satisfy the host plant's fixed nitrogen requirements. This type of nitrogen-fixing symbiosis is ecologically important by providing a large fraction of the nitrogen available to natural ecosystems. It is also agronomically significant both by supporting optimal growth of many crop plants and by serving as an important nutrient input in many systems of sustainable agriculture. In addition, as an intimate cellular symbiont that acts by manipulating fundamental plant processes,