No abstract

Plant specificities and deoxyribonucleic acid homologies were studied among 122 strains of Rhizobium. Some strains were assigned to species on the basis of their source of isolation and present nodulation capabilities, but many did not fit into one of the six currently recognized species of the genus Rhizobium. Among those strains assigned to species were many which also nodulated plants outside their species-specific, cross-inoculation group. Conversely, isolates from a wide variety of plants could be designated Rhizobiumphaseoli since they were capable of nodulating Phaseolus vulgaris. Acid production and growth rate on yeastmannitol agar were tested for all strains. Some strains grew rapidly but did not produce an acid reaction; these were grouped with the fast growing acid producers. Deoxyribonucleic acid homology was used to identify four genetic groups of fastgrowing, acid-producing rhizobia. Group 1 included strains of Rhizobium trifolii (except strains obtained from Trifolium lupinaster), Rhizobium leguminosarum, Rhizobium phaseoli (obtained from Phaseolus vulgaris), and two strains obtained from Neptunia gracilis. Group 2 comprised six American strains obtained from crown vetch (Coronilla varia), sainfoin (Onobrychis vicifozia), and Sophora spp. Species status for this group should remain tentative until further strains have been studied. Group 3 corresponded with Rhizobium meliloti as presently defined. Group 4 included fast-growing Lotus rhizobia, two strains obtained from 2 ' . lupinaster, and a wide variety of previously unclassified strains. Nine fastgrowing strains could not be included in any of these groups. The nine slowgrowing, non-acid producing strains included in this study showed 4 0 % homology with DNAs from seven fast-growing reference strains. The relationships between subgroups in group 1 are discussed, and the genetic diversity of strains obtained from Phaseolus vulgaris is examined. It is proposed that fast-growing rhizobia comprise at least four species corresponding with the four genetic groups described.The classification of legume root-nodule bacteria presented in the 8th edition of Bergey's Manual of Determinative Bacteriology (18) places them in one genus, that of Rhizobium. This genus is divided into two groups distinguished by differences in growth rate, carbohydrate utilization, and flagellation. The six species of rhizobia described are distinguished by the leguminous plants they nodulate. The unsatisfactory aspects of this species classification have been reviewed previously (6, 15).Deoxyribonucleic acid (DNA) reassociation has been used to study the relationships between fast-growing, acid-producing rhizobia and agrobacteria (7, 12) and between root-nodule bacteria from legumes indigenous to New Zealand and strains designated as Rhizobium lupini (16). Recently, Jarvis et al. (15) used this technique to determine DNA homologies among 27 strains of Rhizobium trifolii, 4 strains of Rhizobium leguminosarum, and 4 strains of Rhizobium phaseoli. It was proposed that Rhizobium t...

Deoxyribonucleic acid homologies were determined among 27 strains of Rhizobium trifolii, 4 strains of Rhizobium leguminosarum, and 4 strains of Rhizobium phaseoli. Results from related strains indicated that deoxyribonucleic acid homologies correlate with serological relationships and that the ability to form nodules on legume roots can be lost without a detectable change in homology with an independent reference strain. All rhizobia which effectively nodulated Trifolium repens, Trifolium subterraneum, Trifolium ambiguum, and Vicia hirsuta formed one population with an average relatedness of 70% (range, 49 to 94%) and a ATm(e1 of 0.0 to 11.8OC with respect to reference strains capable of nodulating the first two clover species. Two strains from African Trifolium species and two strains from a northern Asiatic species were less closely related. The average relatedness of strains from Phaseolus vulgaris to clover rhizobia was 46% (range, 37 to 50%), and the ATm(e1 was 6.5 to ll.S0C. Taxonomic revisions consistent with these observations are discussed. It is proposed that R. trifolii and R. leguminosarum should be combined under the name which has priority, Rhizobium leguminosarum Frank. Within this species various biovars should be designated according to plant specificity. R. phaseoli should be retained at present as a separate species and examined in more detail. The results are discussed in relation to proposed genetic bases for plant specificity.The bacteria which form nodules on the roots of leguminous plants comprise the genus Rhizobium (22). Rhizobia have long been classified and named according to the specific plant or group of plants which they can nodulate (12,22). The plant hosts comprise a cross-inoculation group and harbor symbionts which nodulate only plants in the group. This classification is unsatisfactory for several reasons, Many legume hosts are not included in the six cross-inoculation groups which define the recognized species. Cross-inoculation groups are not mutually exclusive, and isolation of bacteria which nodulate plants in more than one group is a common event (10,39). Rhizobia readily lose their ability to effectively nodulate specific legumes (5, 23), and then their identity as rhizobia is dependent on knowledge of their earlier nodulating capability. Graham (16) has reviewed the limitations of the classification in Bergey's Manual of Determinative Bacteriology (22). Several groups have suggested that infectivity and effectiveness may be plasmid-borne characters (11,18,21,30), and it seems likely that this w i l l be demonstrated in the near future. Cross-inoculation groupings would then be untenable as a basis for classifying root nodule bacteria.Several alternative classifications have been proposed. Norris (27) examined 717 strains of 42 bacteria from 278 species of legumes and concluded that they could be classified into two groups: those which grew rapidly and formed acid on laboratory media and those which grew slowly and produced a neutral or alkaline reaction. Measurements of...

Rhizobia may be classified as either fast or slow growers, or alternatively into different cross-inoculation groups depending on the host plants they can nodulate. The clover, lucerne, lotus-lupin. and sainfoin cross-inoculation groups are of importance in New Zealand grasslands. Within each group there are numerous strains which differ in many respects, in particular in their ability to fix nitrogen with different host species. The clover and slow-growing lotus rhizobia are now widespread in New Zealand pasture soils but the others are not.

ALTHOUGH LEGUMES are of prime importance in New Zealand pastures, the number of species involved is small, and is restricted almost entirely to the genera Trifolium (the clovers), Medicago (lucerne and other medics), and Lotus. These three genera form nodules with three separate groups of rhizobia (root nodule bacteria) with no effective crossnodulation between them. Also, within any one of these groups of rhizobia, there are numerous strains. These may differ in a number of respects, but in particular they differ in the range of species with which they are effective in fixing nitrogen. Thus, among clover rhizobia, one strain may be effective on white (Trifolium repens L.) and red clovers (T. pratense L.) but not on subterranean clover (T. subterraneum L.), a second strain effective on subterranean, weakly effective on red and ineffective on white clover, and a third strain effective on all three species.