THE GENETIC DETERMINATION OF HOST RANGE IN THE RHIZOBIACEAE

Abstract

The family Rhizobiaceae includes bacteria which form nitrogen-fixing root nodules on the roots of leguminous plants (the genus Rhizobium ) and bacteria which form tumours on dicotyledenous plants (the genus Agrobacterium). For both genera classification at the species level is determined by plant interactions: strains of Agrobacterium which induce root proliferation (hairy root disease) are included in the species A. rhizogenes , those which form tumours are classified as A. tumefaciens and those which elicit no plant response as A. radiobacter. For Rhizobium , speciation is determined primarily by the range of legume hosts which are nodulated. Strains of Rhizobium and Agrobacterium contain large amounts of extrachromosomal DNA (plasmids). Host recognition and tumour-forming properties are determined by these plasmids, which can be transferred between different species and also between Agrobacterium and Rhizobium. Such transfer experiments have shown that for Agrobacterium and some fast-growing species of Rhizobium the plant-interaction characteristics of strains and hence the criterion for their taxonomie status is plasmid mediated. Whether or not host range properties are readily exchanged between these bacteria in nature needs to be determined.