Genetic Diversity of Acacia seyal Del. Rhizobial Populations Indigenous to Senegalese Soils in Relation to Salinity and pH of the Sampling Sites

Abstract

The occurrence and the distribution of rhizobial populations 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 compatible rhizobia associated to A. seyal in various ecogeographic 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.

Distribution and Diversity of Rhizobial Populations Associated with <i>Acacia senegal</i> (L.) Willd. Provenances in Senegalese Arid and Semiarid Regions

“…They agree with previous studies which highlight considerable genetic diversity in the microsymbionts originating from the Sahel (Diouf et al, 2007;Fall et al, 2008;Sarr et al, 2005a). The 42 isolates were shared out into 14 IGS-RFLP profiles: three exclusively found in Dahra, nine specific to Goudiry and two that occurred in both regions.…”

“…Germination, inoculation and plant cultivation were conducted as described by Diouf et al (2007). Infectivity and effectivity of rhizobial isolates were evaluated by counting the number of nodules and from the dry weight of the produced nodules and shoots in relation to control, respectively.…”

“…PCR were performed with 2 µl of bacterial culture, as described above, in 25 µl reaction tube containing 0.625U of Taq polymerase according to Diouf et al (2007). The partial IGS region was amplified using primers Br5; 5'-CTT-GTA-GCT-CAG-TTG-GTT-AG-3' ) and 23S-38; 5'-CCG-GGT-TTC-CCC-ATT-CGG-3' (Normand et al, 1992).…”

“…PCR products were checked according to Diouf et al (2007). Expected bands were excised and DNA was purified using PureLink TM Gel Extraction Kit (Invitrogen, USA) according to the manufacturer's instructions.…”

“…Before inoculation, knowledge of these relationships is crucial for formulation of appropriate A. senegal inoculants for different seed provenances for resilience to soil stresses in various environmental conditions. Indeed, several authors showed that environmental stresses, such as soil acidity or salinity, were detrimental to the survival and diversity of natural rhizobial populations and to their ability to establish symbiotic associations (Bala et al, 2003;Diouf et al, 2007).…”