Characterization of an Acid Tolerant Rhizobium spp. Population-Low Diversity as a Consequence of Adaptation to Acid Soil?

Abstract

The isolation of alfalfa-nodulating bacteria from acid soils in Argentina resulted in 12 distinct acid tolerant Rhizobium strains able to grow in synthetic media adjusted to pH 5.2. In this work we analysed the biodiversity and the symbiotic behaviour of the acid tolerant Rhizobium population. All strains nodulated Medicago sativa and Medicago truncatula, nevertheless the acetylene reduction assay revealed that all acid tolerant strains had only a poor nitrogen fixation capacity. Light and electron microscopy of M. sativa nodules induced by the acid tolerant rhizobia showed that only a few plant cells were occupied by bacteroids. In comparison to nodules induced by the S. meliloti strain 2011 it is conspicuous that several bacteriods are combined in a vesicle-like structure. Furthermore we could show that the acid tolerant rhizobia are able to nodulate common bean (Phaseolus vulgaris). Using ERIC-fingerprinting, plasmid pattern analysis and SDS-PAGE of Lipopolysaccharides no differences between the acid tolerant Rhizobium isolates could be detected, although they were isolated in different parts of Argentina. In further genetical analysis we could show that the acid tolerant strains from Argentina hybridise to the nodH gene of S. meliloti and that all possess at least two copies of a ISRm 2011–1 homologous sequence in the same position. This results indicate a very low level of diversity among this rhizobial strains. The ERIC, plasmid, LPS and ISRm2011–1 patterns obtained with the acid tolerant strains and the nucleotide sequence of a part of the 16S rDNA were identical to that of Rhizobium spp. strain Or 191 which was isolated in acid soils in Oregon (Eardly et al., 1992