Diversity and specificity of Rhizobium leguminosarum biovar viciae on wild and cultivated legumes

Abstract

The symbiotic partnerships between legumes and their root-nodule bacteria (rhizobia) vary widely in their degree of specificity, but the underlying reasons are not understood. To assess the potential for host-range evolution, we have investigated microheterogeneity among the shared symbionts of a group of related legume species. Host specificity and genetic diversity were characterized for a soil population of Rhizobium leguminosarum biovar viciae (Rlv) sampled using six wild Vicia and Lathyrus species and the crop plants pea (Pisum sativum) and broad bean (Vicia faba). Genetic variation among 625 isolates was assessed by restriction fragment length polymorphism (RFLP) of loci on the chromosome (ribosomal gene spacer) and symbiosis plasmid (nodD region). Broad bean strongly favoured a particular symbiotic genotype that formed a distinct phylogenetic subgroup of Rlv nodulation genotypes but was associated with a range of chromosomal backgrounds. Host range tests of 80 isolates demonstrated that only 34% of isolates were able to nodulate V. faba. By contrast, 89% were able to nodulate all the local wild hosts tested, so high genetic diversity of the rhizobial population cannot be ascribed directly to the diversity of host species at the site. Overall the picture is of a population of symbionts that is diversified by plasmid transfer and shared fairly indiscriminately by local wild legume hosts. The crop species are less promiscuous in their interaction with symbionts than the wild legumes.

The effect of biotic and physical factors on the competitive ability of Rhizobium leguminosarum

“…The effect of the abundance of indigenous rhizobia on the competition of the inoculant strains has been described previously [34,35]. The influence of the host plant on rhizobia competitiveness may be explained by the host favoring particular bacterial genotypes during the recruitment of microsymbionts from the rhizosphere [9,25,26]. However this may only explain differences in the competitiveness of certain strains and not others.…”

“…Vegetative forms of bacteria multiply in the infection threads and saprophytic zones of indeterminate type nodules, and are released into the soil during the decay of nodules [4,5]. Amongst the heterogeneous microbial communities in the soil, rhizobia constitute numerous and very differentiated populations [6][7][8][9][10]. Several factors can influence the number and diversity of indigenous rhizobia, including nutrient availability, soil type, acidity and agricultural management regimes [6,11,12].…”

<i>Rhizobium</i> alters inducible defenses in broad bean, <i>Vicia faba</i>

“…Vicia faba, for example, is one of the most discriminating of the legume hosts towards different strains of R. leguminosarum [5]. Adding additional complexity, symbioses are frequently nested within each other [30], thus changes in one mutualism may indirectly alter the functioning of other mutualisms.…”

“…Fixed nitrogen, the conversion of atmospheric nitrogen to ammonium in plant root nodules, is used for plant growth and reproduction as well as chemical defenses [3,4]. Despite their ubiquity, these symbioses are very host specific; only certain species or strains of bacteria can colonize host plants [5,6]. Broad bean, Vicia faba L., roots are one of the most discriminating mutualists, being colonized only by Rhizobium leguminosarum bv.…”

“…Broad bean, Vicia faba L., roots are one of the most discriminating mutualists, being colonized only by Rhizobium leguminosarum bv. viciae Frank [5,7,8]. This nitrogen-fixing bacteria increases plant size, vigor, and seed set [9].…”