Rhizobium Population Genetics: Enzyme Polymorphism in Rhizobium leguminosarum from Plants and Soil in a Pea Crop

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A population of Rhizobium leguminosarum biovar viceae symbiotic on the roots of a commercial pea (Pisum sativum cv. Maro) crop was sampled by extracting a total of 249 isolates from root nodules on nine plants. Another 104 isolates were obtained by using soil from the same site to inoculate test plants, and a further 86 isolates were similarly obtained from soil 20 m distant within the crop. Each isolate was characterized for mobility variants of the enzymes glucose-6-phosphate dehydrogenase, superoxide dismutase, and β-galactosidase by polyacrylamide gel electrophoresis. All three enzymes were polymorphic, and there was a strong disequilibrium among them. Of the 15 observed combinations of alleles (electrophoretic types [ETs]), 12 were indistinguishable from those previously described for isolates from a site 25 km distant. ET frequencies were significantly different among isolates from nodules on primary roots as opposed to lateral roots. The population on each individual plant was very diverse, but ET frequencies were similar from plant to plant. The ETs nodulating the primary roots were almost, although not perfectly, mixed, since the incidence of the same ETs in adjacent nodules was only about twice that expected by chance. The two samples derived from soil had the same ET frequencies but were significantly different from the field nodule sample, although the level of diversity was similar and there were no new ETs.

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