Nodulating strains of Rhizobium loti arise through chromosomal symbiotic gene transfer in the environment.

Abstract

Rhizobia were isolated from nodules off a stand ofLotus corniculatus established with a single inoculant strain, ICMP3153, 7 years earlier in an area devoid of naturalized Rhizobium loti. The isolates showed diversity in growth rate, Spe I fingerprint of genomic DNA, and hybridization pattern to genomic DNA probes. The 19%o of isolates that grew at the same rate as strain ICMP3153 were the only isolates that had the same fingerprint as strain ICMP3153. Sequencing of part of the 16S rRNA gene of several diverse isolates confirmed that they were not derived from the inoculant strain. Nevertheless, all non-ICMP3153 strains gave EcoRI and Spe I hybridization patterns identical to ICMP3153 when hybridized to nodulation gene cosmids. Hybridization of digests generated by the very rare cutting enzyme Swa I revealed that the symbiotic DNA region (at least 105 kb) was chromosomally integrated in the strains. The results suggest that the diverse strains arose by transfer of chromosomal symbiotic genes from ICMP3153 to nonsymbiotic rhizobia in the environment.

An experimental and modelling exploration of the host-sanction hypothesis in legume–rhizobia mutualism

“…About 20 % of total nodules can be co-occupied by different rhizobial strains in artificial inoculations (Rolfe and Gresshoff, 1980). Another potential complication is the horizontal transmission of symbiotic plasmids, turning non-nodulating strains into nodulating rhizobia, that is frequent between different strains of rhizobia (Sullivan et al, 1995). This genetic exchange can also be easily added to our model.…”

Diversity of rhizobia associated with Amorpha fruticosa isolated from Chinese soils and description of Mesorhizobium amorphae sp. nov.

“…The introduction of these specific rhizobia may occur either by management through inoculation, or by seed-borne bacteria as described for common bean seeds (Pkrez-Ramirez et al, 1998). Occasionally, remnants of a rhizobial population which have lost symbiotic determinants may become recipients of genetic information from closely related inoculated strains, acquiring the ability for symbiosis with an introduced legume host as was reported for Lotus corniculatus in New Zealand (Sullivan et al, 1995(Sullivan et al, , 1996. The possibility also exists that exotic legumes are nodulated by rhizobia which are members of the local soil population.…”

Plant nodulation inducers enhance horizontal gene transfer of Azorhizobium caulinodans symbiosis island

“…The symbiosis islands of the Mesorhizobium, Azorhizobium, and Bradyhizobium species are mainly located on the chromosome (9-11). For example, the rhizobium strain Mesorhizobium loti R7A has a 502-kb ICE called ICEMlSym R7A that contains genes required for nitrogen-fixing symbiosis with Lotus corniculatus (12). The ICEMlSym R7A is inserted downstream of a phe-tRNA gene in the M. loti chromosome and can be transferred to nonsymbiotic mesorhizobia both in the laboratory and in the field, converting these strains into symbionts of L. corniculatus (12,13).…”