The Role of Rhizobium Plasmids in Host Specificity
There is no such thing as a non-nodulating field isolate of Rhizobium. It is a contradiction in terms because Rhizobium is defined by its ability to induce root nodules on a legume host. Indeed, the species to which a particular Rhizobium field isolate is assigned depends exclusively on the genus of the host legume from which it was originally isolated. Thus R. japonicum would have been isolated from a soybean root nodule and R. meliloti from an alfalfa root nodule. This form of classification works well provided that there is very little overlap between the host-ranges of the various Rhizobium species (Wilson, 1944). However it would appear to be unsatisfactory in three major areas. Firstly, the large and heterogeneous ‘cowpea miscellany’ is a collection of Rhizobium isolates with broad and diverse host ranges. Secondly, it is doubtful whether nodulation per se (rather than the formation of nitrogen-fixing nodules) can be regarded as an adequate criterion for species assignment, because it is known for example that R. leguminosarum strain 300, which forms effective pitrogen-fixing nodules on peas, will also form nodules on Phaseolus and Trifolium (Hepper and Lee, 1979; J. E. Beringer, personal communication; J. L. Beynon, unpublished results) although on these last two hosts the induced nodules fail to fix nitrogen. Thirdly, and perhaps more fundamentally, we shall argue in the ensuing presentation that strains of R. leguminos arum, R. trifolii and R. phase-oli are almost indistinguishable by all taxonomic criteria (including chromosome homology) except host-range itself, and that host-range characteristics represent a plasmid-borne trait that can be transferred from strain to strain across the ‘species barrier’.
KeywordsBacteriocin Production Rhizobium Leguminosarum Strain 128C53 Large Plasmid Rhizobium Species
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