Occupation of peanut root nodules by opportunistic Gammaprotobacteria

IBAÑEZ, F.; ANGELINI, J.; TAURIAN, T.; TONELLI, M.; FABRA, A.

Rosario

Congreso; V Congreso Argentino de Microbiología General; 2008

Peanut (Arachis hypogaea U 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 94% of its production is concentrated in Cordoba 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 L. nodules in the peanut producing area of the province of Cordoba, we have characterized a collection of isolates obtained from surfacesteri lized root nodules. Besides to the slow-growing Bradyrhizobium spp., fast-growing bacteria were also recovered from the nodules. The 16S rDNA sequences of seven fastgrowing strains were obtained and the phylogenetic analysis revea led that these isolates belong to the Phylum Proteobacteria, Subclass Gammaproteobacterra, and include Pseudomonas spp., Enlerobacter 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 lixation genes showed that nifH gene was detected in the Klebsiella-like isolates. Furthermore, these stra ins were able to grow in a nitrogen-free cultl!re 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-inocu lation of the Gammaproteobacteria stra ins 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-endophyterhizobia for nodule occupation. This work was supported by CONICET, ANPCyT, SECyT