We have transferred the hairy root plasmid (pRi) from Agrobacterium rhizogenes into Rhizobium trifolii. The plasmid fully expressed its root-inducing activity in the R. trifolii (pRi) transconjugant. Generally, the R. trijolii (pRi) transconjugant was unable to nodulate clover. However, occasionally a spontaneous mutant(s) of the R. trifolii (pRi) transconjugant arose which was capable of nodulating clover and of inducing root proliferation in a number of plant species. The mutant(s) induced significantly more nodules than the wild-type bacterium, but significantly less Nz was fixed by these nodules.
I N T R O D U C T I O NRecently, we have introduced the Ri plasmid of Agrobacterium rhizogenes into Rhizobium meliloti . The Ri plasmid is responsible for the induction of the hairy root syndrome in dicotyledonous plants (Moore et al., 1979; White & Nester, 1980a, b). In order to accomplish the direct transfer of the Ri plasmid into R. meliloti, a mutant derivative of A. rhizogenes strain TR105 was prepared by Tn5 transposon mutagenesis. This derivative carries a Tn.5 insertion in its Ri plasmid which permits a much higher efficiency of transfer than that obtained with the wild-type Ri plasmid (B. Lam, unpublished). The R. meliloti transconjugants prepared by this procedure were unable to cause root proliferation in any of the dicotyledonous plants that were tested. However, the transconjugants were capable of nodulating alfalfa seedlings to a much greater extent than wild-type R. meliloti. The increased nodulation also resulted in significantly more Nz being fixed than by either the wild-type or antibiotic-resistant mutants of R. meliloti. Furthermore, there were no ultrastructural differences between the nodules produced by the wild-type R. meliloti and those produced by the transconjugants.In this paper we report the construction and characteristics of Ri plasmid transconjugants of Rhizobium trijolii. The transconjugants differed in several ways from those of R . meliloti.
M E T H O D SBacterial strains and culture conditions. The strains of Agrobacterium spp. and R. trifolii used are listed in Table 1. All bacteria were grown on yeast extract/mannitol (YM) medium and the antibiotic concentrations used were as previously reported . All bacteria used to inoculate plants were grown from single colonies.Bacterial mating. Matings between A. rhizogenes and R. trifblii were done on sterile Millipore filters placed on nutrient agar medium. The A . rhizogenes donor (BL31 l), derived from TR105, contains only the Ri plasmid into which there is a Tn5 insertion which permits a higher efficiency of transfer (B. Lam, unpublished; and allows growth on a medium containing kanamycin. The R . trifolii recipient (LH3) is a spontaneous streptomycin-and rifampicin-resistant derivative of the wild-type R. trifolii strain 2067. Back mating of R. trifolii (pRi) transconjugants with Agrobacteriurn rudiobacter NTI was done as described above; however, selection of the A . rudiobacter transconjugants, i.e. LH16, was done on an agrobacte...
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