Microbial Ecology Genetic Diversity of Acacia seyal Del. Rhizobial Populations Indigenous to Senegalese Soils in Relation to Salinity and pH of the Sampling Sites Diegane Diouf 1,2 , Ramatoulaye Samba-Mbaye 2 , Didier Lesueur 3 , Amadou T. Ba 1 , Bernard Dreyfus 4 , Philippe de Lajudie 4 and Marc Neyra 4 (1) De ´partement de Biologie Ve ´ge ´tale, Universite ´ Cheikh Anta Diop, BP 5005, Dakar, Senegal (2) Laboratoire Commun de Microbiologie IRD/ISRA/UCAD, Centre de Recherche de Bel-Air, BP 1386 - CP 18524, Dakar, Senegal (3) CIRAD-Foret, UPR 80 BETP^ Tropical Soil Biology and Fertility Institute (TSBF), CIAT c/o World Agroforestry Centre (ICRAF), P.O. Box 30677, Nairobi, Kenya (4) IRD, UMR 113 IRD/CIRAD/AGRO-M/UM2, USC INRA 1242 Laboratoire des Symbioses Tropicales et Me ´diterrane ´ennes, Campus International de Baillarguet, TA 10/J 34398, Montpellier Cedex 5, France Received: 1 March 2007 / Accepted: 2 March 2007 / Online publication: 4 April 2007 Abstract The occurrence and the distribution of rhizobial pop- ulations naturally associated to Acacia seyal Del. were characterized in 42 soils from Senegal. The diversity of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism (RFLP) analysis of 16S–23S rDNA, performed on DNA extracted from 138 nodules resulted in 15 clusters. Results indicated the widespread occurrence of compat- ible rhizobia associated to A. seyal in various ecogeo- graphic areas. However, the clustering of rhizobial populations based on intergenic spacer (IGS) RFLP profiles did not reflect their geographic origin. Four genera were discriminated on the basis of 16S rRNA gene sequences of the strains representative for the IGS-RFLP profiles. The majority of rhizobia associated to A. seyal were affiliated to Mesorhizobium and Sinorhizobium 64 and 29%, respectively, of the different IGS-RFLP profiles. Our results demonstrate the coexistence inside the nodule of plant-pathogenic non-N 2 -fixing Agrobacterium and Burkholderia strains, which induced the formation of ineffective nodules, with symbiotic rhizobia. Nodulation was recorded in saline soils and/or at low pH values or in alkaline soils, suggesting adaptability of natural rhizobial populations to major ecological environmental stress and their ability to establish symbiotic associations within these soil environments. These results contribute to the progressing research efforts to uncover the biodiversity of rhizobia and to improve nitrogen fixation in agroforestry systems in sub-Saharan Africa. Introduction As with other woody leguminous plants, Acacia species play an important socioeconomic role [21, 53]. They are widely used in reforestation programs in regions facing extreme environmental stress that are not suitable for many crop plants [1, 16]. Acacia seyal has an affinity for clayey soils along rivers and tolerates temporary flooding. It can also be found on rocky soils, on alluvial soils, and in shallow depressions. In Senegal, A. seyal grows on saline as well as on nonsaline soils. The ability of Acacia species, such as A. seyal Del., to establish in poor soils is due to their aptitude to enter effective symbiosis with compatible legume-nodulating bacteria and vesicular arbuscular mycorhizal fungi that occur naturally in their rhizosphere [1, 14, 16]. Despite the agronomic and ecological importance of A. seyal, the available data on its nodulation pattern are scarce and little attention has been given to the diversity of rhizobial strains that associate with this species [9, 17, 34]. Nitrogen fixation efficiency in legumes could be optimized by inoculating plants with characterized strains of rhizobia [5, 11, 14, 46]. However, environmental stress such as drought, high soil temperature, salinity, and high or low soil pH values adversely affect survival, growth, and the capacity of nitrogen fixation of rhizobial strains [5, 8]. It is well known that tropical soils may contain strains adapted to these extreme environmental stresses [33, 57, 58, 59, 60]. Correspondence to: Diegane Diouf; E-mail: diegane.diouf@ird.sn DOI: 10.1007/s00248-007-9243-0 & Volume 54, 553–566 (2007) & * Springer Science + Business Media, LLC 2007 553