ORIGINAL ARTICLE Phenotypic and genotypic characteristics of Acacia senegal (L.) Willd. root-nodulating bacteria isolated from soils in the dryland part of Senegal D. Fall 1,2 , D. Diouf 1,2 , M. Ourarhi 3 , A. Faye 2 , H. Abdelmounen 3 , M. Neyra 4 , S.N. Sylla 1,2 and M. Missbah El Idrissi 3 1 De ´ partement de Biologie Ve ´ ge ´ tale, Universite ´ Cheikh Anta Diop, Dakar, Senegal 2 Laboratoire Commun de Microbiologie IRD ⁄ ISRA ⁄ UCAD, Dakar, Senegal 3 Laboratoire de Biologie des Plantes et des Microorganismes, De ´ partement de Biologie, Faculte ´ des Sciences, Universite ´ Mohammed I, Oujda, Morocco 4 IRD ⁄ UMR 113 IRD ⁄ CIRAD ⁄ AGRO-M ⁄ UM2, USC INRA 1242 Laboratoire des Symbioses Tropicales et Me ´ diterrane ´ ennes, Campus International de Baillarguet, Montpellier Cedex, France Introduction The use of nitrogen fixing trees has been shown to be a promising alternative to fertilizer use for resource-poor farmers in the arid regions of Africa (Dakora and Keya 1997). In sub-Saharan Africa, Acacia species are widely used in reafforestation processes (Midgley and Bond 2001). Acacia senegal (L.) Willd. has considerable potential in agroforestry systems, fuelwood production, forage and medicinal products. It has long been used by farmers in the arid and semi-arid zones of Africa for gum production (Von Maydell 1986). This tree contributes to soil conserva- tion and the enhancement of soil fertility in agroforestry systems. In this respect, more attention is given to A. senegal due to their inherent ability to fix atmospheric nitrogen; which eventually returns to the soil through N-rich litter-fall, root and nodule turnover. Recently, Faye et al. (2006) showed that inoculation of mature trees of A. senegal, in the field during the rainy season had been effective and significantly improved the yield of Keywords Acacia senegal, diversity, ecology, genotyping, molecular genetic, stress response symbiosis. Correspondence Diegane Diouf, De ´ partement de Biologie Ve ´ ge ´ tale, Universite ´ Cheikh Anta Diop, BP 5005, Dakar, Senegal. E-mail: diegane.diouf@ird.sn 2007 ⁄ 1868: received 19 November 2007, revised 21 March 2008 and accepted 1 April 2008 doi:10.1111/j.1472-765X.2008.02389.x Abstract Aims: We characterized phenotypically and genotypically root-nodulating bac- teria associated with Acacia senegal (L.) Willd. isolated from the soils surround- ing A. senegal trees in the dry land area of Senegal. Methods and Results: The phenotypical and genotypical characterizations we carried out showed a high diversity of A. senegal root-nodulating bacteria. Phe- notypic patterns showed adaptations of the rhizobial strains to many environ- mental stresses such as heat, drought, and salinity. Twelve molecular groups were distinguished by profiles obtained using polymerase chain reac- tion ⁄ restriction fragment length polymorphism techniques from intergenic spacer region rDNA. The highest genetic diversity was found around the A. senegal rhizosphere. Therefore, A. senegal seemed to have a positive influ- ence on occurrence and genotypical diversity of rhizobial populations. Rhizo- bial isolates obtained in this study belonged phylogenetically to the genera Mesorhizobium and Rhizobium. Conclusions: Our results provided information about the genetic diversity of the rhizobial strains associated with A. senegal and suggested the adaptability of natural rhizobial populations to major ecological environmental stress within these soil environments. Significance and Impact of the Study: These results suggested a potential selec- tion of compatible and well adapted strains under stress conditions as inocu- lants for successful A. senegal growth in arid lands. Letters in Applied Microbiology ISSN 0266-8254 ª 2008 The Authors Journal compilation ª 2008 The Society for Applied Microbiology, Letters in Applied Microbiology 1