We studied a collection of 126 rhizobial isolates from eight species of Crotalaria (C. comosa, C. glaucoides, C. goreensis, C. hyssopifolia, C. lathyroides, C. perrottetii, C. podocarpa, and C. retusa) growing in Senegal. Nodulation and nitrogen-fixation tests on nine Crotalaria species revealed two specificity groups within the genus Crotalaria. Group I consists of plants solely nodulated by very specific fast-growing strains. Group II plants are nodulated by slow-growing strains similar to promiscuous Bradyrhizobium spp. strains already reported to nodulate many tropical legumes. SDS-PAGE studies showed that slow-growing strains grouped with Bradyrhizobium while fast-growing strains constituted a homogeneous group distinct from all known rhizobia. Amplified ribosomal DNA restriction analysis (ARDRA) of 10 representative strains of this group using four restriction enzymes showed a single pattern for each enzyme confirming the high homogeneity of group I. The 16S rDNA sequence analysis revealed that this specific group belonged to the genus Methylobacterium, thus constituting a new branch of nodulating bacteria.
We studied a collection of 126 rhizobial isolates from eight species of Crotalaria (C. comosa, C. glaucoides, C. goreensis, C. hyssopifolia, C. lathyroides, C. perrottetii, C. podocarpa, and C. retusa) growing in Senegal. Nodulation and nitrogen-fixation tests on nine Crotalaria species revealed two specificity groups within the genus Crotalaria. Group I consists of plants solely nodulated by very specific fast-growing strains. Group II plants are nodulated by slow-growing strains similar to promiscuous Bradyrhizobium spp. strains already reported to nodulate many tropical legumes. SDS-PAGE studies showed that slow-growing strains grouped with Bradyrhizobium while fast-growing strains constituted a homogeneous group distinct from all known rhizobia. Amplified ribosomal DNA restriction analysis (ARDRA) of 10 representative strains of this group using four restriction enzymes showed a single pattern for each enzyme confirming the high homogeneity of group I. The 16S rDNA sequence analysis revealed that this specific group belonged to the genus Methylobacterium, thus constituting a new branch of nodulating bacteria.
Polymerase Chain Reaction/Restriction Fragment Length Polymorphism (PCR/RFLP) of the InterGenic Spacer (IGS) between rDNA 16S and 23S was used to identify indigenous strains nodulating four clones of Acacia mangium-Acacia auriculiformis hybrids cultivated in non-sterilized sandy soil from Sangalkam (Senegal) under greenhouse conditions. The experiment was for 4 months. The analysis of restriction fragment length polymorphism obtained with MspI and HaeIII restriction enzymes allowed the identification of 15 different IGS Groups with a distribution which significantly differed according to the clone of the hybrid (strains of one clone can belong to three and five different IGS Groups). Three large multi-lobed nodules were obtained on the root system of clone 3.26 within 5 months. Also, the nature of the rhizobia contained in each lobe was determined. The results showed that the lobes of large nodules can be occupied by one or two strains and the nodules analysed were mainly occupied by those belonging to IGS Group 12.
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