RESUMEN

Central southern Mexico contains highly diverse legumes. In this study, nodule-associated bacteria (NAB) were isolated from wild legume nodules and from nodules on Phaseolus vulgaris plants used as a plant-trap in soils from the same areas as the wild legumes. The bacteria were identified through the 16S rRNA gene sequence analysis, tested for plant growth-promoting (PGP) activities and the production of antimicrobial compounds, and analyzed for potential nodulation by amplifying the nodC gene. Several genera with PGP activity were isolated from legume nodules, including Achromobacter, Acinetobacter, Bacillus, Brevibacillus, Brevibacterium, Burkholderia, Cupriavidus, Dyella, Ensifer, Enterobacter, Herbaspirillum, Kosakonia, Labrys, Microbacterium, Moraxella, Paraburkholderia, Pseudomonas, Rhizobium, Stenotrophomonas; and Aeromonas, Marinococcus Pseudarthrobacter and Pseudoxanthomonas were found in plant legume nodules for the first time. Pseudomonas was the most common bacteria, and Mimosa pudica was colonized by the largest number of genera (6 different genera). A Burkholderia strain from the Burkholderia cepacia complex and a firmicutes strain harbor the nodC gene, identifying them as potential novel nodulating bacteria and showing that most of the strains isolated in this study were NAB. The most frequent PGP activity identified among the strains isolated from wild legumes was IAA synthesis. Two bacteria, Stenotrophomonas sp. and Rhizobium sp., synthesized more than 250 µg/ml, which is more than the level of synthesis reported in this study for Azospirillum brasilense Sp7 (59.77 µg/ml). Nitrogen fixation and antimicrobial compound production were not common, but the production of siderophores was frequently found among all the strains. This study shows that diverse NAB with PGP activity are very common in the legume nodules from central southern Mexico.

Asunto(s)

Bacterias/clasificación , Mimosa/microbiología , Phaseolus/microbiología , Nódulos de las Raíces de las Plantas/microbiología , Bacterias/aislamiento & purificación , Ácidos Indolacéticos/metabolismo , México , Phaseolus/crecimiento & desarrollo , Filogenia , ARN Ribosómico 16S/genética , Simbiosis

RESUMEN

Calicotome villosa is a spontaneous Mediterranean legume that can be a good candidate as pioneer plants to limit regression of vegetation cover and loss of biodiversity in Tunisian arid soils. In order to grow legumes in such soils, pairing rhizobia and nodule associated bacteria (NAB) might provide numerous advantages. In this work, cultivable biodiversity of rhizobial symbionts and NAB in nodules of C. villosa plants growing in five arid regions of south Tunisia was characterized. Phylogenetic analysis using 16S rDNA gene, dnak, recA and nodD sequences separated nodule-forming bacteria in six clades associated to genera Ensifer, Neorhizobium, Phyllobacterium and Rhizobium. Among NAB, the strain Variovorax sp. CT7.15 was selected due to its capacity to solubilise phosphate and, more interestingly, its high level of aminocyclopropane-1-carboxylate deaminase (ACC deaminase) activity. C. villosa plants were inoculated with representative rhizobia of each phylogenetic group and co-inoculated with the same rhizobia and strain CT7.15. Compared with single rhizobia inoculation, co-inoculation significantly improved plant growth and nodulation, ameliorated plant physiological state and increased nitrogen content in the plants, independently of the rhizobia used. These results support the benefits of pairing rhizobia and selected NAB to promote legume growth in arid or degraded soils.