Bradyrhizobium japonicum FN1 produces an inhibitory substance that affects competition for nodule occupancy

Symbiotic efficiency of Rhizobium leguminosarum sv. trifolii strains originating from the subpolar and temperate climate regions

Red clover (Trifolium pratense L.) is a forage legume cultivated worldwide. This plant is capable of establishing a nitrogen-fixing symbiosis with Rhizobium leguminosarum symbiovar trifolii strains. To date, no comparative analysis of the symbiotic properties and heterogeneity of T. pratense microsymbionts derived from two distinct geographic regions has been performed. In this study, the symbiotic properties of strains originating from the subpolar and temperate climate zones in a wide range of temperatures (10-25 °C) have been characterized. Our results indicate that all the studied T. pratense microsymbionts from two geographic regions were highly efficient in host plant nodulation and nitrogen fixation in a wide range of temperatures. However, some differences between the populations and between the strains within the individual population examined were observed. Based on the nodC and nifH sequences, the symbiotic diversity of the strains was estimated. In general, 13 alleles for nodC and for nifH were identified. Moreover, 21 and 61 polymorphic sites in the nodC and nifH sequences were found, respectively, indicating that the latter gene shows higher heterogeneity than the former one. Among the nodC and nifH alleles, three genotypes (I-III) were the most frequent, whereas the other alleles (IV-XIII) proved to be unique for the individual strains. Based on the nodC and nifH allele types, 20 nodC-nifH genotypes were identified. Among them, the most frequent were three genotypes marked as A (6 strains), B (5 strains), and C (3 strains). Type A was exclusively found in the temperate strains, whereas types B and C were identified in the subpolar strains. The remaining 17 genotypes were found in single strains. In conclusion, our data indicate that R. leguminosarum sv. trifolii strains derived from two climatic zones show a high diversity with respect to the symbiotic efficiency and heterogeneity. However, some of the R. leguminosarum sv. trifolii strains exhibit very good symbiotic potential in the wide range of the temperatures tested; hence, they may be used in the future for improvement of legume crop production.

The evolutionary ecology of rhizobia: multiple facets of competition before, during, and after symbiosis with legumes

Rhizobial bacteria have complex lifestyles that involve growth and survival in bulk soil, plant rhizospheres and rhizoplanes, legume infection threads, and mature and senescing legume nodules. In nature, rhizobia coexist and compete with many other rhizobial strains and species to form host associations. We review recent work defining competitive interactions across these environments. We highlight the use of sophisticated measurement tools and sequencing technologies to examine competition mechanisms in planta, and highlight environments (e.g. soil and senescing nodules) where we still know exceedingly little. We argue that moving toward an explicitly ecological framework (types of competition, resources, and genetic differentiation) will clarify the evolutionary ecology of these foundational organisms and open doors for engineering sustainable, beneficial associations with hosts.

Genetic diversity of microsymbionts nodulating Trifolium pratense in subpolar and temperate climate regions

Rhizobia are soil-borne bacteria forming symbiotic associations with legumes and fixing atmospheric dinitrogen. The nitrogen-fixation potential depends on the type of host plants and microsymbionts as well as environmental factors that affect the distribution of rhizobia. In this study, we compared genetic diversity of bacteria isolated from root nodules of Trifolium pratense grown in two geographical regions (Tromsø, Norway and Lublin, Poland) located in distinct climatic (subpolar and temperate) zones. To characterize these isolates genetically, three PCR-based techniques (ERIC, BOX, and RFLP of the 16S-23S rRNA intergenic spacer), 16S rRNA sequencing, and multi-locus sequence analysis of chromosomal house-keeping genes (atpD, recA, rpoB, gyrB, and glnII) were done. Our results indicate that a great majority of the isolates are T. pratense microsymbionts belonging to Rhizobium leguminosarum sv. trifolii. A high diversity among these strains was detected. However, a lower diversity within the population derived from the subpolar region in comparison to that of the temperate region was found. Multi-locus sequence analysis showed that a majority of the strains formed distinct clusters characteristic for the individual climatic regions. The subpolar strains belonged to two (A and B) and the temperate strains to three R. leguminosarum genospecies (B, E, and K), respectively.