Specificity in the symbiotic association of Lotus corniculatus and Rhizobium loti from natural populations

Abstract

To test whether Rhizobium loti are coadapted to nodulate local plant genotypes, we competed R. loti strains in a common environment with clonally propagated Lotus corniculatus. Both the plants and bacterial strains were originally collected from natural populations in three localities and the R. loti strains used were distinguishable by enzyme electrophoretic markers and differed in geographical origin relative to host plant origin. The proportions of nodules occupied by symbiont strains varied widely and depended on both host plant and symbiont genotype. Nonrandom nodulation patterns resulted primarily from preferential nodulation of host genotypes by the symbiont strain that had been associated with the host in the natural environment. Symbionts nodulating their original hosts were preferentially found in nodules on adventitious tap roots as opposed to the younger, lateral roots (for one host‐symbiont pair) or in large nodules, independent of location on the root system (for a second host‐symbiont pair). The proportion of nodules occupied by a symbiont on novel host genotypes varied, ranging from nearly random expectation to a significant reduction in the proportion of nodules occupied. The analysis of the bacteria recovered from 994 nodules by multilocus enzyme electrophoresis revealed that 952 (95.8%) nodules were occupied by one of the four inoculant strains and 11 (1.1%) were co‐occupied by two inoculant strains. A total of 31 (3.1%) nodules were occupied by strains that did not match the electrophoretic profiles of the original inoculant strains. Based on the comparison of multilocus profiles for 23 enzyme loci, we concluded that these bacteria were foreign strains and not recombinants of the original inoculant strains. Our findings indicate a strong host genotype by strain interaction underlying the outcome of rhizobial competition for nodulation sites and suggest there are distinct mechanisms leading to differential recognition of compatible host and symbiont genotypes.

In Situ Phylogenetic Structure and Diversity of Wild Bradyrhizobium Communities

“…The phylogenetic data in this study support a key conclusion of inoculation studies, which is that certain rhizobial genotypes are more likely than others to adsorb to (34) and ultimately infect (33,39,65) specific host species. The pattern of phylogenetic clustering in the sample of nodule rhizobia indicates that each of the Lotus hosts studied usually forms nodules with a small, more genetically related subset of the population of rhizobia that are available to it in its zone near the root.…”

“…A total of 301 nodules from 34 plants were cultured. Single rhizobial colonies were sampled from each nodule because naturally occurring legume nodules usually contain single rhizobial genotypes (33,56).…”

“…Nodule formation on hosts is guided by reciprocal molecular signaling between bacteria and plant (5,46,58), and successful infection requires a compatible pairing of legume and rhizobial genotypes. While both host and symbiont genotypes can alter the outcome of rhizobial competition for adsorption (34) and nodulation (33,39,65) of legume roots, little is known about how this competition plays out in nature.…”

Direct amplification of nodD from community DNA reveals the genetic diversity of Rhizobium leguminosarum in soil

“…All the nodules were collected from each plant, and a rhizobium culture was established from each nodule by standard methods (Palmer and Young, 2000). To propagate multiple clones of each plant, 5–10 shoots were sterilized as described by Lieven‐Antoniou and Whittam (1997), planted in pots filled with an autoclaved mixture of moist peat and vermiculite (1:3) and grown in a greenhouse, where they remained free of nodules.…”

Distribution of fungal endophyte genotypes in doubly infected host grasses

“…For example, individual strains of the anther smut fungus (Microbotryum [formerly Ustilago] violacea) mutually exclude each other to some extent from flowering shoots and almost completely from flowers of multiply infected Silene plants (Day 1980). Similarly, individual nitrogen‐fixing Rhizobium strains very rarely co‐occupy root nodules (Lieven‐Antoniou & Whittam 1997), probably because each nodule arises through infection by a single bacterium.…”