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

Niokhor Bakhoum, Christine Le Roux, Diégane Diouf, Aboubakry Kane, Fatou Ndoye, Dioumacor Fall, Robin Duponnois, Kandioura Noba, Samba Ndao Sylla, Antoine Galiana

Baillarguet Research Center UMR 113 IRD/CIRAD UMR82/AGRO-M/UM2, USC INRA 1242, Laboratory of Tropical and Mediterranean Symbioses (LSTM), Montpellier, France.
Bel-Air Research Center IRD/ISRA/UCAD, Common Microbiology Laboratory (LCM), Dakar, Senegal.
Bel-Air Research Center IRD/ISRA/UCAD, Common Microbiology Laboratory (LCM), Dakar, Senegal; National Centre of Forest Research (CNRF)/Senegalese Institute of Agricultural Research (ISRA), Dakar, Senegal.
Plant Biology Department, Cheikh Anta Diop University of Dakar (UCAD), Dakar, Senegal; Bel-Air Research Center IRD/ISRA/UCAD, Common Microbiology Laboratory (LCM), Dakar, Senegal.

DOI: 10.4236/ojf.2014.42019                

Abstract

Abstract: Distribution and diversity of rhizobial strains associated with Acacia senegal (L.) Willd. in relation to seed provenances in soils from arid (Dahra) and semiarid (Goudiry) zones of Senegal were investigated. PCR-RFLP performed on 16S-23S rDNA intergenic spacer (IGS) of nodule crude extracts revealed a high genetic diversity of rhizobial strains, which was higher in the semiarid region than in the arid region. The distribution of rhizobial populations was influenced by soil physical and chemical characteristics, and by A. senegal provenances as shown by the analysis of correspondence. In contrast, the phenotypic diversity of rhizobial strains was not correlated with the soil origin. The phylogenetic tree (performed by the maximum likelihood algorithm) of IGS 16S-23S sequences showed that most of the rhizobial strains nodulating A. senegal were closely related to Mesorhizobium plurifarium. Our results showed that rhizobial taxa associated with A. senegal were mainly distributed according to soil physical and chemical characteristics, and A. senegal provenances. A large subset of A. senegal root-nodulating bacteria had high diversity that correlated with the most favourable environmental conditions. Understanding the diversity and distribution of rhizobial strains may be exploited in the formulation of A. senegal inoculants for different seed provenances for resilience to soil stresses in various environmental conditions.

 

Keywords

Acacia senegal; Mesorhizobium; Diversity; Symbiosis; Ecology; Phylogeny

Table 1.

Received November 27^th, 2013; revised December 31^st, 2013; accepted January 16^th, 2014

Table 2.

Figure 2.

Phylogenetic (maximum likelihood) tree based on aligned sequences of 16S-23S rDNA intergenic spacer (700 nt) using MEGA5. The accession numbers for the sequences are indicated in parentheses. The sequences generated in this work are shown in bold. Only bootstrap probability values >50% (100 replicates) are indicated at the branching points. Type strains are indicated as (T).

rather than Ensifer (Sinorhizobium) and Rhizobium. Ba et al. (2002) showed that Nod factors of rhizobia isolated from Aca- cia tortilis subsp. raddiana in Africa are identical or very close to Nod factors produced by other fast-growing Acacia nodulat- ing rhizobia i.e. Sinorhizobium terangae ORS1073 and M. plurifarium ORS1001 isolated from A. senegal (Lorquin et al., 1997), strain GRH2 isolated from Acacia cyanophylla (Lopez- Lara et al., 1995) and Rhizobium tropici CFN299 and CIAT899 (Folch-Mallol et al., 1996) isolated from bean. Since, as A. senegal appears to have strict requirements in regard to Nod factors the rhizobia that nodulate it produce (Ba et al., 2002), the species seems to be mainly nodulated by a type of rhizobia according to the geographic position.

Phenotypic Properties of Rhizobial Isolates

Rhizobial strains from both regions were phenotypically he- terogeneous and formed four distinct clusters (Figure 3). The strains tested were linked with a similarity level of about 80%. Majority of strains were able to metabolize all sugars except salicin. In addition, most of the strains tolerated heavy metals at the concentrations tested. However, all strains occurring in clusters A, B and C were sensitive to CuSO₄, HgCl₂ and CdSO₄, Pb-acetate (1000) and FeCl₃∙6H₂O (750). Concerning antibiot- ics sensitivity, rhizobial strains were more sensitive to tetracyc- line. The strains isolated from A. senegal in our study exhibited a high phenotypic diversity. These results are consistent with those of Fall et al. (2008). In contrast to genotypic results based on IGS-RFLP profiles, the phenotypic diversity of rhizobial strains was not correlated to the geographical origin. However, Xavier et al. (1998) observed that the antibiotic sensitivity of rhizobial strains was influenced by climatic and soil conditions, respectively. These results may indicate that there is no rela- tionship between the genotypic and phenotypic diversity of rhizobial strains. Similar results were reported by Elboutahiri et al. (2010) for Sinorhizobium strains nodulating Medicago sati- va in Morocco. Contrary to our results, Fall et al. (2008) found a correlation between phenotypic and genotypic characteristics of A. senegal rhizobial strains from Senegalese dry land areas.

Conclusion

In conclusion, the distribution of root-nodulating bacteria associated with A. senegal was correlated to physical and chemical characteristics of the soils. The root-nodulating bacte- ria associated to A. senegal seemed to be selected by the seed provenance. Genotypic and phenotypic diversity of rhizobia associated with A. senegal is considerable in the Sahalian part of Senegal with an even higher genetic diversity in less drastic environmental conditions. A. senegal appears to be mainly nodulated by members of M. plurifarium strains in Senegal. In the future, specific studies on the symbiotic performances of rhizobial strains in relation to plant provenance and soil type should be performed aiming at selecting both symbionts in relation to environmental conditions.

Acknowledgments

This work was financially supported by ACACIAGUM INCO STREP project (No. 032233). We thank the Service de Coopération d’Action Culturelle of the France Embassy in Senegal for supporting our training courses in France, and Ir

Figure 3.

Dendrogram showing phenotypic relationships between rhizobial strains representative of the 14 IGS-RFLP groups isolated from A. senegal based on similarity. Dendrogram was obtained from 52 phe- notypic traits: 17 carbohydrate sources (ribose, D xylose, L (+)-rham- nose, L (+)-arabinose, D (+)-glucose, D (+)-galactose, saccharose, D (+)-maltose, trehalose, lactose, D (+)-raffinose, mannitol, glycerol, sorbitol, amidon, salicilin and levulose (Diouf et al., 2008)), 11 heavy metals (µg∙ml⁻¹): CuSO₄, 5H₂O (500); HgCl₂ (5); CdSO₄ (50); ZnSO₄ (250); Pb-acetate (1000); FeCl₃, 6H₂O (750); MnCl₂, 4H₂O (1000); MnSO₄ (1000); MgSO₄ (1000); BaCl₂, 2H₂O (1000) and CoSO₄ (150)) and 6 antibiotics: (kanamicin, streptomycin, ampicillin, chlorampheni- col, tetracycline, rifampicin with four concentrations for each 10, 20, 50, 100 µg∙ml⁻¹).

Oumar SADIO for R statistical analysis. The authors are grate- ful to Marie-Cécile Maraval (Cirad/TAC-112/A, Baillarguet, Montpellier) for the English editing.

References

Conflicts of Interest

The authors declare no conflicts of interest.

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