Journal of Biotechnology 91 (2001) 181 – 188 The diversity of rhizobia nodulating beans in Northwest Argentina as a source of more efficient inoculant strains O. Mario Aguilar *, M. Vero ´nica Lo ´ pez, Pablo M. Riccillo Instituto de Bioquı ´mica y Biologı ´a Molecular, Facultad de Ciencias Exactas, Uniersidad Nacional de La Plata, Calles 47 y 115, 1900 La Plata, Argentina Received 24 November 2000; received in revised form 30 March 2001; accepted 4 April 2001 Abstract The common bean (Phaseolus ulgaris L.) is cultivated widely in Central and South America and particularly in the Northwest of Argentina. In order to describe the diversity of the common bean nodulating rhizobial population from the bean producing area in Northwest Argentina (NWA), a collection of about 400 isolates of common beans recovered from nodules and soil samples from NWA were characterized by using nif H-PCR, analysis of genes coding for 16S rRNA and nod C, and REP-fingerprinting, respectively. It was found that species Rhizobium etli is predominant in common bean nodules although a high degree of diversity was found within the species. Other bean nodulating genotypes recovered from soils by using Leucaena sp. as the trapping host was found to have the 16S rDNA alleles of species such as Sinorhizobium fredii, Sinorhizobium saheli, Sinorhizobium teranga, Mesorhizobium loti, and Rhizobium tropici. Some of the bean genotypes that were found to be more efficient in green house experiments were selected and assayed in two successive bean-cropping seasons in the field environment in NWA, and an increase in yields with inoculation was found. The performance of strains isolated from the region indicates potential for exploiting the diversity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Bean rhizobia; Diversity; Inoculation; Yields www.elsevier.com/locate/jbiotec 1. Introduction Common beans (Phaseolus ulgaris L.) are an important crop in South America and in Africa, supplying in some cases up to 20% of the protein intake per person. Beans are grown in a great variety of farming systems, ranging from a highly mechanized cropping in Northwest Argentina (NWA) to the low-input subsistence cultivation in Africa and Central America. Generally, the nitro- gen requirements of legumes can be met either by the inorganic N from the soil or from the chemi- cal fertilizers applied as well as by biological nitrogen fixation (BNF) which results from the symbiotic association with the soil bacteria rhizo- bia. The rhizobial isolates that are able to nodu- late bean roots form a very heterogeneous group. Until now five species have been described; how- ever some other isolates, all of which are also able to nodulate common beans with different degrees * Corresponding author. Tel.: +54-221-4250497. E-mail address: aguilar@nahuel.biol.unlp.edu.ar (O.M. Aguilar). 0168-1656/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0168-1656(01)00336-4