Occupation of peanut root nodules by oportunistic Gammaproteobacteria.

IBAÑEZ, F., ANGELINI, J., TAURIAN, T., TONELLI, ML, FABRA, A.

Ciudad de Rosario, Santa Fe, Argentina

Congreso; V Congreso de la Sociedad Argentina de Microbiología General (SAMIGE); 2008

Occupation of peanut root nodules by opportunistic Gammaproteobacteria Fernando Ibáñez, Jorge Angelini, Tania Taurian, María Laura Tonelli, Adriana Fabra. Dpto. de Ciencias Naturales, FCEFQyN, Universidad Nacional de Río Cuarto.            Peanut (Arachis hypogaea L.) is a widespread leguminous plant of great agricultural and economic significance that can be used as food crop or as source of several food products. Argentina is one of the main peanut producer countries in the world and the 94% of its production is concentrated in Córdoba province.          Bacteria that nodulate peanut in natural environments from all over the world have been classified as Bradyrhizobium (Arachis) sp., but species names have not been defined yet. To assess the diversity of bacteria associated to A. hypogaea nodules in the peanut producing area of the province of Córdoba, we have characterized a collection of isolates obtained from surface-sterilized root nodules. Besides to the slow-growing Bradyrhizobium spp., fast-growing bacteria were also recovered from the nodules. The 16S rDNA sequences of seven fast-growing strains were obtained and the phylogenetic analysis revealed that these isolates belong to the Phylum Proteobacteria, Subclass Gammaproteobacteria, and include Pseudomonas spp., Enterobacter spp. and Klebsiella spp. These strains were unable to induce nodule formation in Arachis hypogaea L. plants, but enhanced plant yield upon inoculation. Nodulation nodC gene could neither be amplified by PCR nor detected by Southern blotting in any of the isolates. Analysis of nitrogen fixation genes showed that nifH gene was detected for the Klebsiella-like isolates. Furthermore, these strains were able to grow in a nitrogen-free culture medium. These results suggest that the increase in the shoot dry weight of plants inoculated with these strains could be related with their ability to fix nitrogen inside the plant tissues.          Simultaneous and delayed co-inoculation of the Gammaproteobacteria strains and Bradyrhizobium sp. SEMIA 6144 conducted in peanut plants revealed that the fast-growing strains were able to colonize nodules even after their formation.          The results obtained in this work are supporting the idea that the Gammaproteobacteria recovered from peanut nodules are best described as opportunistic bacteria and not genuine peanut symbionts, being necessary the cooperation of host-endophyte-rhizobia for nodule occupation.   This work was supported by CONICET, ANPCyT-SECyT.