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Burkholderia phymatum is a soil bacterium able to develop a nitrogen-fixing symbiosis with species of the legume genus Mimosa, and is frequently found associated specifically with Mimosa pudica. The type strain of the species, STM 815(T), was isolated from a root nodule in French Guiana in 2000. The strain is an aerobic, motile, non-spore forming, Gram-negative rod, and is a highly competitive strain for nodulation compared to other Mimosa symbionts, as it also nodulates a broad range of other legume genera and species. The 8,676,562 bp genome is composed of two chromosomes (3,479,187 and 2,697,374 bp), a megaplasmid (1,904,893 bp) and a plasmid hosting the symbiotic functions (595,108 bp).

The arbuscular mycorrhizal (AM) fungi are ubiquitous biotrophic root endophytes that form a significant link in macronutrient cycles. The AM fungi occupy two niches simultaneously, the intraradical niche, from which they gain carbon, and the bulk soil. We predict that AM community structure will vary through time, depending upon season and plant identity. In this study, we compared the AM fungal community colonising eight species of Acer L. (maple) grown in an arboretum. DNA from root samples was analysed using terminal restriction fragment length polymorphism (TRFLP) of AM fungal sequences. Three time points were sampled during the growing season, and ordination analysis showed this was the most important factor discriminating the TRFLP profiles. Analysis of each month separately showed that the host species was a significant factor in all samplings, but that the response of each species through time was different. This study demonstrates the dynamic nature of AM communities, and the importance of sampling habitats throughout the season to generate a complete picture of the AM community. Key words: maple, TRFLP, arbuscular mycorrhizas, communities, seasonality. Les champignons mycorhiziens a arbuscules (MA) sont des endophytes biotrophes ubiquistes des racines qui forment un lien significatif dans les cycles des macronutriments. Les champignons MA occupent simultanement deux niches, la niche intra-radicalaire, a partir de laquelle ils acquierent le carbone, et le sol en vrac. Nous predisons que la structure de la communaute de MA variera en fonction du temps, dependamment de la saison et de l'identite de la plante. Dans cette etude, nous comparons la communaute de champignons MA colonisant huit especes d'erables (Acer L.) cultives en arboretum. L'ADN d'echantillons de racines a ete analyse par polymorphisme de longueur de fragments de restriction terminaux (TRFLP) des sequences de MA fongiques. Trois periodes de temps ont ete echantillonnees durant la saison de croissance, et une analyse d'ordination a montre que cela constituait le facteur le plus important permettant de discriminer les profils en TRFLP. Une analyse de chacun des mois a montre separement que l'espece d'hote constituait un facteur significatif dans tous les echantillonnages, mais que la reponse de chaque espece etait differente en fonction du temps. Cette etude demontre la nature dynamique des communautes de MA et l'importance des habitats d'echantillonnage au cours de la saison, afin de dresser une image complete de la communaute de MA. [Traduit par la Redaction] Mots-cles: erable, TRFLP, mycorhize a arbuscules, communautes, saisonnalite., Introduction The arbuscular mycorrhizal (AM) symbiosis is the most widespread and abundant of all terrestrial mutualistic interactions. The symbiosis is thought to be generalist, in that those fungal taxa that [...]

Summary: Competitive and facilitative interactions influence bacterial community composition, diversity and functioning. However, the role of genetic diversity for determining interactions between coexisting strains of the same, or closely related, species remains poorly understood. Here, we investigated the type (facilitative/inhibitory) and potential underlying mechanisms of pairwise interactions between 24 genetically diverse bacterial strains belonging to three genospecies (gsA,C,E) of the Rhizobium leguminosarum species complex. Interactions were determined indirectly, based on secreted compounds in cell‐free supernatants, and directly, as growth inhibition in cocultures. We found supernatants mediated both facilitative and inhibitory interactions that varied greatly between strains and genospecies. Overall, gsE strains indirectly suppressed growth of gsA strains, while their own growth was facilitated by other genospecies' supernatants. Similar genospecies‐level patterns were observed in direct competition, where gsA showed the highest susceptibility and gsE the highest inhibition capacity. At the genetic level, increased gsA susceptibility was associated with a non‐random distribution of quorum sensing and secondary metabolite genes across genospecies. Together, our results suggest that genetic variation is associated with facilitative and competitive interactions, which could be important ecological mechanisms explaining R. leguminosarum diversity. [ABSTRACT FROM AUTHOR]

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2007.01239.x Byline: THORUNN HELGASON (*), JAMES W. MERRYWEATHER, J. PETER W. YOUNG, ALASTAIR H. FITTER Keywords: arbuscular mycorrhizal fungi; Ajuga reptans; host range; Glomeromycota; phosphorous; SSUrDNA Abstract: Summary The composition of communities of arbuscular mycorrhizal (AM) fungi can have a large effect on the performance of their plant hosts. The role of individual fungal species in shaping this response is as yet unresolved. We have used the fungicide benomyl to alter the community of AM fungi in undisturbed monoliths of soil in a natural community. Changes in the community were characterized by root colonization (%RLC), cloning, sequencing and tRFLP of a partial SSUrDNA fragment. Eleven plant species were sufficiently abundant in the monoliths to be examined. In the highly mycorrhiza-dependent perennial herb Ajuga reptans, phosphate concentration was significantly reduced after benomyl treatment over a full growing season. The other plant species showed low colonization and no significant difference in phosphate concentration after benomyl treatment. Although colonization in A. reptans was reduced, many mycorrhizal fungi survived in the roots. Some became more abundant following fungicide treatment, suggesting competitive release. Fungi that increased were generalists that have been identified in field samples from published studies colonizing a wide range of plant species. Those that declined were specialists with a narrow host range; five types had not been recorded previously in field samples. AM fungi in this study differed greatly in their response to perturbation, independent of the identity of the host plant. If such functional diversity is widespread, then elucidating the part played by AM fungal diversity in regulating plant community structure will be key to our understanding and management of ecosystems. Article History: Received 18 September 2006; revision accepted 23 February 2007 Handling Editor: Marcel van der Heijden Article note: (*) Correspondence: Thorunn Helgason (fax +44 0 1904 328505; e-mail th7@york.ac.uk).

Research has been conducted on Sinorhizobium meliloti. The isolation of this bacterium from nodules and soil has been carried out and the results indicate the presence of recombination but disequilibrium between its megaplasmids and chromosomes.

Research has been conducted on the Astragalus sinicus nodulation genes. The cloning of these genes by functional complementation of Sinorhizobium meliloti nod mutants is discussed.

RFLP, PCR-RFLP and plasmid profile analysis were used to investigate the distribution of four types of related plasmid replication genes (repC), in field populations of Rhizobium leguminosarum. The results indicated the compatibility of the four repC classes. The transfer of a Tn5-labelled plasmid with a group IV repC sequence, into field isolates, showed that group IV sequences were incompatible with each other, although compatible with other repC groups.

Rhizobial isolates from Acacia senegal, Prosopis chilensis and related leguminous trees growing in Africa and Latin America were investigated to study the diversity and phylogeny of nodA and nifH genes. Gene sequence analysis revealed that all the strains had similar host ranges and belonged to the genera Sinorhizobium and Mesorhizobium. Similar symbiotic gene types were found in different chromosomal backgrounds suggesting that transfer of symbiotic genes has occurred across species boundaries.

Most strains of Rhizobium leguminosarum have gene repC-like sequences that may serve as suitable PCR primer sites. Thirty nine out of forty one strains tested, and several Sinorhizobium strains amplify repC fragment sequences of 750bp. There is a possibility of repC groups corresponding to Rhizobium plasmid incompatibility groups. RepC sequences from R. leguminosarum show great genetic diversity.