High frequency transfer of nodulating ability between strains and species of Rhizobium

Abstract

THE most striking and important feature of members of the genus Rhizobium is their ability to nodulate and fix nitrogen in the roots of legumes. There is specificity in this interaction. Legumes belonging to one cross-inoculation group are nodulated only by certain classes of Rhizobium and indeed Rhizobium species are defined in terms of the host legumes that they can nodulate—for example, R. leguminosarum nodulates peas, R. trifolii nodulates clover and R. phaseoli nodulates beans of the genus Phaseolus. The genetic bases of the nodulation process and of host specificity are poorly understood. Many Rhizobium strains harbour plasmids^1–4 and genetic evidence has suggested that such plasmids might carry some of the information required for the development of the symbiosis^5,6. We report here that the ability to nodulate peas can be transferred at high frequency from a strain of R. leguminosarum to a non-nodulating strain of R. leguminosarum and to three other species of Rhizobium that nodulate legumes other than peas. We infer from this that some of the genetic information required for nodulation is plasmid-linked.