Genetic structure of a soil population of nonsymbiotic Rhizobium leguminosarum

Abstract

The genetic structure of a population of nonsymbiotic Rhizobium leguminosarum strains was determined by the electrophoretic mobilities of eight metabolic enzymes. Nonsymbiotic strains were isolated from the rhizosphere of bean plants and characterized by growth on differential media and at different temperatures, intrinsic antibiotic resistances, the lack of homology to a nijH probe, and their inability to form nodules on bean roots. All the isolates clustered with R. leguminosarum bv. phaseoli reference strains and did not encompass any other Rhizobium taxa. Their rRNA operon restriction fragment length polymorphisms and the nucleotide sequence of a fragment of the 16S rRNA gene were also found to be identical to those of R. leguminosarum bv. phaseoli reference strains. When complemented with an R. leguminosarum bv. phaseoli symbiotic plasmid (p42d), the nonsymbiotic isolates were able to fix nitrogen in symbiosis with bean roots at levels similar to those of the parental strain. The symbiotic isolates were found at a relative frequency of 1 in 40 nonsymbiotic R. leguminosarum strains.

Evolutionary history shapes patterns of mutualistic benefit in Acacia –rhizobial interactions

“…Such patterns have a potentially simple mechanistic explanation in that several studies have reported the loss of mutualistic effectiveness in rhizobia via rapid evolution. In some cases this has been demonstrated to be associated with either the gain or loss of symbiosis-related genes carried on plasmids (Segovia et al 1991;Nandasena et al 2006;Crook et al 2012). Other clear examples of variation in rhizobial average effectiveness are strains with intermediate average effectiveness (e.g., strain CPI232) that are closely related to strains with high average effectiveness (e.g., strain CPI246).…”

Lifestyle alternatives for rhizobia: mutualism, parasitism, and forgoing symbiosis

“…phaseoli strain with the genetic capacity to fix N 2 in symbiosis is still considered a symbiotic strain when it is between hosts, in contrast to nonsymbiotic R. leguminosarum bv. phaseoli strains, which are unable to infect legumes [3].…”

Direct amplification of rhizobial nodC sequences from soil total DNA and comparison to nodC diversity of root nodule isolates

“…Almost all the work published about rhizobia, including a wide variety of publications about rhizobial diversity, is based on pure cultures obtained in this way. Due to the lack of reliable selection techniques, only a few studies are available analysing rhizobia that were isolated directly from soil and consequently did not undergo any competition or selection process through the host plant [1][2][3][4][5][6][7][8][9]. Some of these reports compared strains directly isolated from soil with strains selected by trap plants [4,8,9].…”