Host plant effects on hybrids of Rhizobium leguminosarum biovars viceae and trifolii

1 Soil Microbiology Department, Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ, UK 2 Unit of Molecular Genetics, University of Bristol, Woodland Road, Bristol BS8 1UG, UK ABSTRACT The Rhizobium leguminosarum biovar viceae host-range plasmid pJB5JI was transferred to six R. leguminosarum biovar trifolii strains. Inheritance of pJB5JI enabled the R. leguminosarum biovar trifolii strains to nodulate both peas and clover species. Bacteria were isolated from nodules formed on peas (the ‘correct’ host for R. leguminosarum biovar viceae ) and the hosts for R. leguminosarum biovar trifolii (white, red and subterranean clover). Isolates from peas and white clover appeared to have lost or changed the host-range plasmid conferring the ability to nodulate white clover or peas respectively. Isolates from subterranean and red clover could often nodulate all host plants. These results show that the host exerts some form of functional incompatibility when interacting with hybrid Rhizobium strains and that some hosts are more stringent than others. Present address: Huazhong Agricultural University, Wuhan, Hubei, Peoples Republic of China.