Diversity of indigeneous bradyrhizobia associated with three cowpea cultivars (Vigna unguiculata (L.) Walp.) grown under limited and favorable water conditions in Senegal (West Africa)

The diversity of #Bradyrhizobium# strains nodulating three cowpea (#Vigna unguiculata# L. Walp.) cultivars in favorable and water-limited conditions occuring at flowering was analysed. PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) analysis of 16S-23S rDNA intergenic spacer region (IGS) directly applied on 85 crushed nodules distinguished four genetic profiles, IGS types l, ll, lll and IV. The distribution of these IGS types according to water conditions and cowpea cultivars (B-21, TN 88-63 and Mouride) showed that nodulating strains appeared more diverse in water-limited condition. More than three quarters of prospected nodules presented the IGS type l. They were formed on all three cultivars and in both water conditions. Only a small part of nodules was distributed between the IGS type ll, lll and IV. Nodules showing the IGS types ll and III were found mainly in limited conditions on TN 88-63 and Mouride cultivars, whereas nodules presenting the IGS type IV were collected only from cultivars B-21 and Mouride, in both water conditions. Strains corresponding to the different profiles were isolated. The phylogenetic analysis of 16S rRNA gene sequences showed that they belong to the genus #Bradyrhizobium#. The sequence analysis of 16S-23S rDNA IGS revealed that the strains exhibiting IGS types ll, lll and IV were closely related to some #Faidherbia albida# isolates from Senegal. IGS type ll can be assigned with at least 98% similarity to #Bradyrhizobium# genospecies IV. IGS types III and IV showed more than 96% similarity with genospecies Vll and could belong to the same genospecies. IGS type l, the most frequent, exhibits low IGS similarity with reported sequences in the databases, and could represent a new genospecies. (Résumé d'auteur