Abstract Rhizobia are one of the best-studied symbionts of leguminous plants, with high efficiency in biological nitrogen fixation and able to supply up to 90% of the plant nitrogen needs. Among legumes from tropical dry forests, Clitoria brachystegia Beth. (Fabaceae) is endemic to this ecosystem and its bacterial symbionts have never been reported. This study aimed to establish the diversity and nitrogen fixation potential of rhizobia isolated from C. brachystegia root nodules found in tropical dry forest remnants. The use of molecular techniques like BOX-PCR and 16S ribosomal gene sequencing, allowed the identification of 24 Rhizobium and 22 Bradyrhizobium strains which showed differential geographic distribution with most Rhizobium being recovered from Ecuador and all Bradyrhizobium from Peru. Diversity varied between sampling sites but no significant relationships were found with soil characteristics or climate conditions. Several Rhizobium and Bradyrhizobium strains showed good nitrogen fixation potential and thus may represent valuable resources in future conservation efforts of C. brachystegia, an endangered plant species.

Resumen Los rizobios son los simbiontes de plantas leguminosas mejor estudiados, con alta eficiencia en fijación biológica de nitrógeno y capaces de abastecer hasta 90% de las necesidades de este elemento en dichas plantas. Entre las leguminosas de bosques secos tropicales, Clitoria brachystegia Benth. (Fabaceae) se encuentra declarada endémica de este ecosistema y sus bacterias simbiontes nunca se habían reportado. Esta investigación pretende establecer la diversidad y capacidad de fijación de nitrógeno de los rizobios encontrados en los nódulos de C. brachystegia en remanentes de bosque seco tropical. El empleo de técnicas moleculares como BOX-PCR y secuenciación del gen ribosomal 16S, permitió identificar 24 aislados de Rhizobium y 22 de Bradyrhizobium que mostraron una distribución geográfica diferenciada, recuperándose la mayoría de Rhizobium de Ecuador y todos los Bradyrhizobium de Perú. La diversidad de los rizobios entre localidades mostró diferencias, pero no se evidenció una relación significativa con las características de los suelos o el clima. Varias de las cepas de Rhizobium y Bradyrhizobium mostraron un buen potencial como fijadoras de nitrógeno y por lo tanto pueden considerarse como un recurso útil en futuros esfuerzos de conservación de C. brachystegia, una planta considerada en peligro de extinción.

Bacillus spp. are well known rhizosphere residents of many crops and usually show plant growth promoting (PGP) activities that include biocontrol capacity against some phytopatogenic fungi. Potato crops in the Andean Highlands of Peru face many nutritional and phytophatogenic problems that have a significant impact on production. In this context is important to investigate the natural presence of these microorganisms in the potato rhizosphere and propose a selective screening to find promising PGP strains. In this study, sixty three Bacillus strains isolated from the rhizosphere of native potato varieties growing in the Andean highlands of Peru were screened for in vitro antagonism against Rhizoctonia solani and Fusarium solani. A high prevalence (68%) of antagonists against R. solani was found. Ninety one percent of those strains also inhibited the growth of F. solani. The antagonistic strains were also tested for other plant growth promotion activities. Eighty one percent produced some level of the auxin indole-3-acetic acid, and 58% solubilized tricalcium phosphate. Phylogenetic analysis revealed that the majority of the strains belonged to the B. amyloliquefaciens species, while strains Bac17M11, Bac20M1 and Bac20M2 may correspond to a putative new Bacillus species. The results suggested that the rhizosphere of native potatoes growing in their natural habitat in the Andes is a rich source of Bacillus fungal antagonists, which have a potential to be used in the future as PGP inoculants to improve potato crop.