A tumorous growth that develops on the roots of leguminous and certain other plants in response to infection by symbiotic microorganisms. In legumes the symbiont is always a bacterium of the genus Rhizobium. In nonleguminous plants with root nodules the symbiont appears to be either a member of the Plasmodiophorales, e.g. in the roots of bog myrtle (Myrica gale) and alder (Alnus glutinosa), or a blue-green alga, e.g. in the roots of some Gunnera species. In legumes, following invasion of the root tissues, the bacteria induce a localized proliferation of the host tissues. Like the induction of crown gall (see tumour inducing principle ), the ability of Rhizobium to induce nodules appears to be controlled by a plasmid. The plasmid also controls the host specificity of different Rhizobium strains. * Nitrogen-fixation is carried out by the bacteria, which assume a characteristic shape and size within the host cells (see bacteroid). |
