Characterization of Rhizobium sp. (Cicer arietinum L.) by immunofluorescence, immunodiffusion, and intrinsic antibiotic resistance

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Abstract
Twenty-nine cultures of Rhizobium spp. for chickpea (Cicer arietinum L.) from various culture collections were characterized by serological techniques and by intrinsic antibiotic resistance (IAR). Fluorescent antibodies (FAs) against the somatic antigens of eight of the inoculum-quality strains grouped all cultures into at least four distinct somatic serogroups. Serogroup I (Nitragin 27A8) contained the majority of the strains. Serogroups II (Nigragin 27A3) and III (TAL-480) each crossreacted with one strain, while serogroup IV (USDA 3HOa9) consisted only of the homologous strain. Nine of the chickpea strains and 55 other cultures from among strains of R. leguminosarum, R. lupini, R. japonicum, and Rhizobium spp. (Vigna, Sesbania, and Stylosanthes) did not react with any of the four specific FAs. Immunofluorescence – cross adsorption and immunodiffusion analyses of cultures within serogroup I revealed that four of the strains most commonly recommended for inoculum use were serologically identical. The intrinsic antibiotic resistance patterns of these four cultures were also identical. Serogroup II was shown to contain two distinct serotypes (Nitragen 27A3 and 27A16), while the two strains in serogroup III (TAL 480 and 622) were found to be the same. Although intrinsic antibiotic resistance could differentiate among the strains, some cultures that were completely different by serological means were found to group in the same IAR categories. Additionally, some cultures that were serologically identical were found to have different IARs. Our data on antigenic uniqueness of the chickpea rhizobia and their lack of serological crossreaction with any other Rhizobium species (especially with those in R. leguminosarum, where they had previously been classified), as well as the results of other workers on their nodulation specificity for Cicer, led support to the proposal that C. arietinum and its root-nodule bacteria constitute a separate cross-inoculation group.