Proteins conferred by the virulence-specifying plasmid of Agrobacterium tumefaciens C-58

Effect of Plasmid pSa and of Auxin on Attachment of Agrobacterium tumefaciens to Carrot Cells

When the plasmid pSa is introduced into Agrobacterium tumefaciens, its presence results in the suppression of bacterial virulence. A. tumefaciens(pSa) cells are virulent on Bryophyllum diagremontiana only when inoculated with auxin. A. tumefaciens(pSa) cells also bind to plant cells only in the presence of auxin. The effect of auxin is on the bacteria rather than on the plant cells, since the bacteria require auxin to bind to heat-killed carrot cells. Bacteria containing pSa and grown in the absence of auxin showed a lag in binding to carrot cells in auxin-containing medium. This lag was not seen during the binding of wild-type strains. Tetracycline inhibited the binding of A. tumefaciens(pSa) in auxin-containing medium, suggesting that bacterial protein synthesis is required for the auxin effect. No difference was seen in the size or ability to inhibit bacterial binding of lipopolysaccharides from bacteria containing or lacking pSa and grown with or without auxin. A. tumefaciens(pSa) cells grown in the absence of auxin lacked surface polypeptide(s) found in bacteria grown in the presence of auxin and in the wild-type bacteria, which do not contain pSa. Thus, the presence of certain polypeptides appears to be associated with the ability of the bacteria to bind to plant cells.

Characterization of three Agrobacterium tumefaciens avirulent mutants with chromosomal mutations that affect induction of vir genes

Three Agrobacterium tumefaciens mutants with chromosomal mutations that affect bacterial virulence were isolated by transposon mutagenesis. Two of the mutants were avirulent on all hosts tested. The third mutant, Ivr-211, was a host range mutant which was avirulent on Bryophyllum diagremontiana, Nicotiana tabacum, N. debneyi, N. glauca, and Daucus carota but was virulent on Zinnia elegans and Lycopersicon esculentum (tomato). That the mutant phenotype was due to the transposon insertion was determined by cloning the DNA containing the transposon insertion and using the cloned DNA to replace the wild-type DNA in the parent bacterial strain by marker exchange. The transposon insertions in the three mutants mapped at three widely separated locations on the bacterial chromosome. The effects of the mutations on various steps in tumor formation were examined. All three mutants showed no alteration in binding to carrot cells. However, none of the mutants showed any induction of vir genes by acetosyringone under conditions in which the parent strain showed vir gene induction. When the mutant bacteria were examined for changes in surface components, it was found that all three of the mutants showed a similar alteration in lipopolysaccharide (LPS). LPS from the mutants was larger in size and more heavily saccharide substituted than LPS from the parent strain. Two of the mutants showed no detectable alteration in outer membrane and periplasmic space proteins. The third mutant, Ivr-225, was missing a 79-kDa surface peptide. The reason(s) for the failure of vir gene induction in these mutants and its relationship, if any, to the observed alteration in LPS are unknown.

Characterization of nonattaching mutants of Agrobacterium tumefaciens

The first step in tumor formation by Agrobacterium tumefaciens is the site-specific binding of the bacteria to plant host cells. Transposon mutants of the bacteria which fail to attach to carrot suspension culture cells were isolated. These mutants showed no significant attachment to carrot cells with either microscopic or viable cell count assays of bacterial binding. The nonattaching mutants were all avirulent. When revertants of the mutants were obtained by enriching for bacteria which do bind to carrot cells, the bacteria were found to have regained the ability to bind to carrot cells and virulence simultaneously. These results suggest that the ability of the bacteria to bind to plant cells is required for virulence. Like the parent strain, all of the nonattaching mutants synthesized cellulose, but unlike the parent strain, they failed to aggregate carrot suspension culture cells. The transposon Tn5, which was used to obtain the mutants, was located on a 12-kilobase EcoRI fragment of the bacterial chromosomal DNA in all of the nonattaching mutants from strain C58. That the mutant phenotype was due to the Tn5 insertion was shown by cloning the Tn5-containing DNA fragment from the mutant bacteria and using it to replace the wild-type fragment in the parent strain by marker exchange. The resulting bacteria had the same mutant phenotype as the original Tn5 mutants; they did not attach to carrot cells, they did not cause the aggregation of carrot cells, and they were avirulent. No difference was seen between the parent strain and the nonattaching mutants in hydrophobicity, motility, flagella, fimbriae, beta-2-glucan content, size of lipopolysaccharide, or ability of the lipopolysaccharide to inhibit bacterial attachment to tissue culture cells. Differences were seen between the parent strain and the nonattaching mutants in the polypeptides removed from the bacteria during the preparation of spheroplasts. Three of the mutants were lacking a polypeptide of about 34 kilodaltons (kDa). One mutant was lacking the 34-kDa polypeptide and another polypeptide of about 38 kDa. The fifth mutant was lacking a polypeptide slightly smaller than the 34-kDa polypeptide missing in the other four mutants. These missing polypeptides all reappeared in the revertants of the mutants. Thus, bacterial binding to plant cells appears to require the presence of these polypeptides.

Integration and partial excision of a cryptic plasmid in Pseudomonas syringae pv. phaseolicola

A virulent strain of Pseudomonas syringae pv. phaseolicola, a pathogen of the common bean Phaseolus vulgaris (L.), was shown to harbor a 98-megadalton cryptic plasmid, pMC7105. After exposure of this strain to the plasmid-curing agent mitomycin C, a colony was isolated which had no detectable extrachromosomal DNA. Hybridization of labeled pMC7105 probe to nitrocellulose filters containing Southern-blotted BamHI cleavage products of cellular DNA revealed that pMC7105 was integrated into the chromosome rather than cured from this strain. Imprecise excision of pMC7105 resulted in the formation of three smaller plasmids of 34, 50, and 58 megadaltons. BamHI and EcoRI fingerprint analyses revealed that these plasmids were excised from a common region of pMC7105. The BamHI fragments of pMC7105 which were not present in the excision plasmids remained integrated and could be detected by hybridization of pMC7105 probe to Southern-blotted cellular DNA from these strains. Certain chromosomal fragments also had homology with the pMC7105 probe. The excision plasmids were stably maintained and neither integration nor excision altered the pathogenicity of these strains.

Diversity among B6 strains of Agrobacterium tumefaciens

A total of 20 laboratory substrains of Agrobacterium tumefaciens strain B6 were compared with respect to six characteristics, including 3-ketolactose production, lysogeny, octopine catabolism, tumorigenic host range, and plasmid content. Within this group of strains diversity was found for all characteristics except 3-ketolactose production. Six substrains were lysogenized with an omega-type phage, whereas one substrain appeared neither sensitive to nor lysogenized with this bacteriophage. All but two substrains catabolized octopine and induced tumors on carrot disks. These 18 substrains harbor deoxyribonucleic acid sequences homologous to pTiB6-806. The two substrains unable to catabolize octopine were nontumorigenic and lacked detectable Ti plasmid sequences. Of the 20 substrains, 13 also contained sequences homologous to the cryptic plasmid pAtB6-806; 2 of the 18 substrains tumorigenic on carrots failed to induce tumors on Kalanchoe leaves. Their inability to induced tumors on this host, could not be correlated with lysogeny, with the presence or absence of pAtB6-806, or with the very large cryptic plasmid recently described. The Ti plasmids from these two strains were indistinguishable from pTiB6-806 by restriction enzyme analysis and could genetically convert a cured A. tumefaciens strain to tumorigenicity on both plant species. The results with these two strains suggest that parameters of tumorigenicity, such as host range, may be controlled by the bacterial chromosome.