Variation in Rhizobium leguminosarum biovar trifolii Sym Plasmids and the Association with Effectiveness of Nitrogen Fixation

Abstract

A number of strains of Rhizobium leguminosarum biovar trifolii were examined for variation in Sym plasmid size and number, using an R. leguminosarum nifA/nod probe. Size variation was great and, in a large proportion of strains, multiple Sym plasmids were observed. A probe containing a repeated sequence (RtRS) incorporating the nifH promoter, reported to be specifically found on the Sym plasmid of this biovar, was used to determine whether Sym plasmids were of trifolii origin. This probe did not bind to all Sym plasmids identified by the nifA/nod probe and some isolates were suspected of being inter-biovar hybrids. The RtRS probe was also used to test certain isolates for DNA homology to the nifH promoter region. Many strains possessed commonly occurring co-migrating fragments that hybridized to this probe. Beyond this basic relatedness, greater variation in fragment number and size existed, allowing differentiation of seemingly closely related strains. Strains carrying greater amounts of symbiotic DNA in the form of multiple Sym plasmids or a greater number of hybridizing fragments were less effective in fixing nitrogen.