Legumes form a large group of plants that constitute the third largest family of angiosperms, including near 20,000 species and 750 genera. Most of them have the ability to establish symbioses with diazotrophic bacteria, collectively known as rhizobia, which induce root nodules where biological nitrogen fixation takes place, conferring legumes a relevant ecological advantage. This group of bacteria that for many years was thought to be formed by a scarce number of genera and species within alpha proteobacteria, shows nowadays an important genetic diversity including species phylogenetically divergent both in core and symbiotic genes sequences. Together with rhizobia, other endophytic bacteria are present in legume nodules coexisting with rhizobial strains and their ecological role remains unknown in most cases, but they likely have an effect in plant health, plant growth or even in the rhizobia-legume symbiosis. In this review we present an overview of the associations of bacteria with legumes, the current available knowledge on the phylogenetic diversity of both rhizobia and endophytic bacteria inhabiting root nodules, and the symbiotic features used to define symbiovars in rhizobia.

Maize (Zea mays L.) is the staple diet of the native peasants in the Quechua region of the Peruvian Andes who continue growing it in small plots called chacras following ancestral traditions. The abundance and structure of bacterial communities associated with the roots of amilaceous maize has not been studied in Andean chacras. Accordingly, the main objective of this study was to describe the rhizospheric bacterial diversity of amilaceous maize grown either in the presence or the absence of bur clover cultivated in soils from the Quechua maize belt. Three 16S rRNA gene libraries, one corresponding to sequences of bacteria from bulk soil of a chacra maintained under fallow conditions, the second from the rhizosphere of maize-cultivated soils, and the third prepared from rhizospheric soil of maize cultivated in intercropping with bur clover were examined using pyrosequencing tags spanning the V4 and V5 hypervariable regions of the gene. A total of 26031 sequences were found that grouped into 5955 distinct operational taxonomic units which distributed in 309 genera. The numbers of OTUs in the libraries from the maize-cultivated soils were significantly higher than those found in the libraries from bulk soil. One hundred ninety seven genera were found in the bulk soil library and 234 and 203 were in those from the maize and maize/bur clover-cultivated soils. Sixteen out of the 309 genera had a relative abundance higher than 0.5% and the were (in decreasing order of abundance) Gp4, Gp6, Flavobacterium, Subdivision3 genera incertae sedis of the Verrucomicrobia phylum, Gemmatimonas, Dechloromonas, Ohtaekwangia, Rhodoferax, Gaiella, Opitutus, Gp7, Spartobacteria genera incertae sedis, Terrimonas, Gp5, Steroidobacter and Parcubacteria genera incertae sedis. Genera Gp4 and Gp6 of the Acidobacteria, Gemmatimonas and Rhodoferax were the most abundant in bulk soil, whereas Flavobacterium, Dechloromonas and Ohtaekwangia were the main genera in the rhizosphere of maize intercropped with bur clover, and Gp4, Subdivision3 genera incertae sedis of phylum Verrucomicrobia, Gp6 and Rhodoferax were the main genera in the rhizosphere of maize plants. Taken together, our results suggest that bur clover produces specific changes in rhizospheric bacterial diversity of amilaceous maize plants.

Denitrification is defined as the dissimilatory reduction of nitrate or nitrite to nitric oxide (NO), nitrous oxide (N2O), or dinitrogen gas (N2). N2O is a powerful atmospheric greenhouse gas and cause of ozone layer depletion. Legume crops might contribute to N2O production by providing nitrogen-rich residues for decomposition or by associating with rhizobia that are able to denitrify under free-living and symbiotic conditions. However, there are limited direct empirical data concerning N2O production by endosymbiotic bacteria associated with legume crops. Analysis of the Ensifer meliloti 1021 genome sequence revealed the presence of the napEFDABC, nirK, norECBQD and nosRZDFYLX denitrification genes. It was recently reported that this bacterium is able to grow using nitrate respiration when cells are incubated with an initial O2 concentration of 2%; however, these cells were unable to use nitrate respiration when initially incubated anoxically. The involvement of the nap, nirK, nor and nos genes in E. meliloti denitrification has not been reported.