Characterization of nonattaching mutants of Agrobacterium tumefaciens

Attachment of Agrobacterium tumefaciens to carrot cells and Arabidopsis wound sites is correlated with the presence of a cell-associated, acidic polysaccharide

An early step in crown gall tumor formation involves the attachment of Agrobacterium tumefaciens to host plant cells. A. tumefaciens C58::A205 (C58 attR) is a Tn3HoHo1 insertion mutant that was found to be avirulent on Bryophyllum daigremontiana and unable to attach to carrot suspension cells. The mutation mapped to an open reading frame encoding a putative protein of 247 amino acids which has significant homology to transacetylases from many bacteria. Biochemical analysis of polysaccharide extracts from wild-type strain C58 and the C58::A205 mutant showed that the latter was deficient in the production of a cell-associated polysaccharide. Anion-exchange chromatography followed by 1H nuclear magnetic resonance and gas chromatography-mass spectrometry analyses showed that the polysaccharide produced by strain C58 was an acetylated, acidic polysaccharide and that the polysaccharide preparation contained three sugars: glucose, glucosamine, and an unidentified deoxy-sugar. Application of the polysaccharide preparation from strain C58 to carrot suspension cells prior to inoculation with the bacteria effectively inhibited attachment of the bacteria to the carrot cells, whereas an identical preparation from strain C58::A205 had no inhibitory effect and did not contain the acidic polysaccharide. Similarly, preincubation of Arabidopsis thaliana root segments with the polysaccharide prevented attachment of strain C58 to that plant. This indicates that the acidic polysaccharide may play a role in the attachment of A. tumefaciens to host soma plant cells.

Differences in susceptibility of Arabidopsis ecotypes to crown gall disease may result from a deficiency in T-DNA integration

We show that among ecotypes of Arabidopsis, there is considerable variation in their susceptibility to crown gall disease. Differences in susceptibility are heritable and, in one ecotype, segregate as a single major contributing locus. In several ecotypes, recalcitrance to tumorigenesis results from decreased binding of Agrobacterium to inoculated root explants. The recalcitrance of another ecotype occurs at a late step in T-DNA transfer. Transient expression of a T-DNA-encoded beta-glucuronidase gusA gene is efficient, but the ecotype is deficient in crown gall tumorigenesis, transformation to kanamycin resistance, and stable GUS expression. This ecotype is also more sensitive to gamma radiation than is a susceptible ecotype. DNA gel blot analysis showed that after infection by Agrobacterium, less T-DNA was integrated into the genome of the recalcitrant ecotype than was integrated into the genome of a highly susceptible ecotype.

Requirement for genes with homology to ABC transport systems for attachment and virulence of Agrobacterium tumefaciens

Transposon mutants of Agrobacterium tumefaciens which were avirulent and unable to attach to plant cells were isolated and described previously. A clone from a library of Agrobacterium tumefaciens DNA which was able to complement these chromosomal att mutants was identified. Tn3HoHo1 insertions in this clone were made and used to replace the wild-type genes in the bacterial chromosome by marker exchange. The resulting mutants were avirulent and showed either no or very much reduced attachment to carrot suspension culture cells. We sequenced a 10-kb region of this clone and found a putative operon containing nine open reading frames (ORFs) (attA1A2BCDEFGH). The second and third ORFs (attA2 and attB) showed homology to genes encoding the membrane-spanning proteins (potB and potH; potC and potI) of periplasmic binding protein-dependent (ABC) transport systems from gram-negative bacteria. The homology was strongest to proteins involved in the transport of spermidine and putrescine. The first and fifth ORFs (attA1 and attE) showed homology to the genes encoding ATP-binding proteins of these systems including potA, potG, and cysT from Escherichia coli; occP from A. tumefaciens; cysA from Synechococcus spp.; and ORF-C from an operon involved in the attachment of Campylobacte jejuni. The ability of mutants in these att genes to bind to host cells was restored by addition of conditioned medium during incubation of the bacteria with host cells.

Inhibition of VirB-mediated transfer of diverse substrates from Agrobacterium tumefaciens by the IncQ plasmid RSF1010

The transfer of DNA from Agrobacterium tumefaciens into a plant cell requires the activities of several virulence (vir) genes that reside on the tumor-inducing (Ti) plasmid. The putative transferred intermediate is a single-stranded DNA (T strand), covalently attached to the VirD2 protein and coated with the single-stranded DNA-binding protein, VirE2. The movement of this intermediate out of Agrobacterium cells and into plant cells requires the expression of the virB operon, which encodes 11 proteins that localize to the membrane system. Our earlier studies showed that the IncQ broad-host-range plasmid RSF1010, which can be transferred from Agrobacterium cells to plant cells, inhibits the transfer of T-DNA from pTiA6 in a fashion that is reversed by overexpression of virB9, virB10, and virB11. Here, we examined the specificity of this inhibition by following the transfer of other T-DNA molecules. By using extracellular complementation assays, the effects of RSF1010 on movement of either VirE2 or an uncoated T strand from A. tumefaciens were also monitored. The RSF1010 derivative plasmid pJW323 drastically inhibited the capacity of strains to serve as VirE2 donors but only partially inhibited T-strand transfer from virE2 mutants. Further, we show that all the virB genes tested are required for the movement of VirE2 and the uncoated T strand as assayed by extracellular complementation. Our results are consistent with a model in which the RSF1010 plasmid, or intermediates from it, compete with the T strand and VirE2 for a common transport site.

The octopine-type Ti plasmid pTiA6 of Agrobacterium tumefaciens contains a gene homologous to the chromosomal virulence gene acvB

Although the majority of genes required for the transfer of T-DNA from Agrobacterium tumefaciens to plant nuclei are located on the Ti plasmid, some chromosomal genes, including the recently described acvB gene, are also required. We show that AcvB shows 50% identity with the product of an open reading frame, designated virJ, that is found between the virA and virB genes in the octopine-type Ti plasmid pTiA6. This reading frame is not found in the nopaline-type Ti plasmid pTiC58. acvB is required for tumorigenesis by a strain carrying a nopaline-type Ti plasmid, and virJ complements this nontumorigenic phenotype, indicating that the products of these genes have similar functions. A virJ-phoA fusion expressed enzymatically active alkaline phosphatase, indicating that VirJ is at least partially exported. virJ is induced in a VirA/VirG-dependent fashion by the vir gene inducer acetosyringone. Primer extension analysis and subcloning of the virJ-phoA fusion indicate that the acetosyringone-inducible promoter lies directly upstream of the virJ structural gene. Although the roles of the two homologous genes in tumorigenesis remain to be elucidated, strains lacking acvB and virJ (i) are proficient for induction of the vir regulon, (ii) are able to transfer their Ti plasmids by conjugation, and (iii) are resistant to plant wound extracts. Finally, mutations in these genes cannot be complemented extracellularly.

Rhicadhesin-mediated attachment and virulence of an Agrobacterium tumefaciens chvB mutant can be restored by growth in a highly osmotic medium

Cyclic beta-1,2-glucan is considered to play a role in osmoadaptation of members of the family Rhizobiaceae in hypotonic media. Agrobacterium tumefaciens chvB mutants, lacking beta-1,2-glucan, exhibit a pleiotropic phenotype, including nonmotility, attachment deficiency, and avirulence. Here we report that by growth of chvB mutant cells in tryptone-yeast extract medium supplemented with 7 mM CaCl2 and 100 mM NaCl, the mutant cells become motile, attach to pea root hair tips, and are virulent on Kalanchoë leaves. Moreover, whereas chvB mutants grown in tryptone-yeast extract medium containing 7 mM CaCl2 do not produce active rhicadhesin, addition of 100 mM NaCl to this medium resulted in restoration of rhicadhesin activity. The presence of CaCl2 appeared to be required for attachment, virulence, and activity of rhicadhesin. The results support a role for cyclic beta-1,2-glucan in osmoadaptation and strengthen the notion that rhicadhesin is required for attachment and virulence of A. tumefaciens.

Restoration of attachment, virulence and nodulation of Agrobacterium tumefaciens chvB mutants by rhicadhesin

In contrast to wild-type Agrobacterium tumefaciens strains, beta-1,2-glucan-deficient chvB mutants were found to be unable to attach to pea root hair tips. The mutants appeared to produce rhicadhesin, the protein that mediates the first step in attachment of Rhizobiaceae cells to plant root hairs, but the protein was inactive. Both attachment to root hairs and virulence of the chvB mutants could be restored by treatment of the plants with active rhicadhesin, whereas treatment of plants with beta-1,2-glucan had no effect on attachment or virulence. Moreover, nodulation ability of a chvB mutant carrying a Sym plasmid could be restored by pretreatment of the host plant with rhicadhesin. Apparently the attachment-minus and avirulence phenotype of chvB mutants is caused by lack of active rhicadhesin, rather than directly being caused by a deficiency in beta-1,2-glucan synthesis. The results strongly suggest that rhicadhesin is essential for attachment and virulence of A. tumefaciens cells. They also indicate that the mechanisms of binding of Agrobacterium and Rhizobium bacteria to plant target cells are similar, despite differences between these target cells.

A chromosomally encoded two-component sensory transduction system is required for virulence of Agrobacterium tumefaciens

TnphoA mutagenesis of Agrobacterium tumefaciens identified new extracytoplasmic protein-encoding virulence loci. Mutations in these loci conferred increased sensitivity to detergents and several antibiotics. Clones carrying these loci were isolated from an A. tumefaciens cosmid library by complementation of the detergent sensitivities of the mutants. The locus on one complementing clone was delineated by Tn5 and TnphoA mutagenesis. DNA sequence analysis of the delineated region revealed that this locus is made up of two transcriptional units, chvG and chvI, which were predicted, on the basis of amino acid sequence homology, to encode the members of a two-component sensory transduction system. The membrane-spanning sensor, a histidine protein kinase, was designated ChvG, and the response regulator, presumably a transcriptional activator, was designated ChvI. Surprisingly, ChvG was also predicted to contain a Walker type A consensus nucleotide binding site, which is unusual for sensor histidine protein kinases. Site-specific insertion mutations in either chvG or chvI abolished tumor formation ability, as well as the ability to grow on complex media. Neither the genes which are regulated nor the inducing signal is known yet for this system.

Attachment of Agrobacteria to Grape Cells

The presence of the Ti plasmid favorably influences the attachment of agrobacteria to grape callus cells, especially during the early stages of a 2-h incubation. Agrobacterium strains attached to a similar extent to both the crown gall-resistant cultivar (Catawba), Vitis labruscana , and the crown gall-susceptible cultivar (Chancellor), Vitis sp. Attachment of the virulent strain to grape callus cells is blocked by the avirulent strain HLB-2 in both the tissue culture cell suspension and the seedling root systems.

Isolation and characterization of a new chromosomal virulence gene of Agrobacterium tumefaciens

A mutant (strain B119) of Agrobacterium tumefaciens with a chromosomal mutation was isolated by transposon (Tn5) mutagenesis. The mutant exhibited growth rates on L agar and minimal medium (AB) plates similar to those of the parent strain (strain A208 harboring a nopaline-type Ti plasmid). The mutant was avirulent on all host plants tested: Daucus carota, Cucumis sativus, and Kalanchoe diagremontiana. The mutant was not impaired in attachment ability to carrot cells. The mutant had one insertion of Tn5 in its chromosome. The avirulent phenotype of B119 was shown to be due to the Tn5 insertion in the chromosome by the marker exchange technique. A wild-type target chromosomal segment (3.0 kb) which included the site of mutation was cloned and sequenced. Two open reading frames, ORF-1 (468 bp) and ORF-2 (995 bp), were identified in the 3.0-kb DNA segment. Tn5 was inserted in the middle of ORF-2 (acvB gene). Introduction of the acvB gene into the mutant B119 strain complemented the avirulent phenotype of the strain. Homology search found no genes homologous to acvB, although it had some similarity to the open reading frame downstream of the virA gene on the Ti plasmid. Thus, the acvB gene identified in this study seems to be a new chromosomal virulence gene of A. tumefaciens.

Involvement of a vitronectin-like protein in attachment of Agrobacterium tumefaciens to carrot suspension culture cells

Infections of dicotyledonous plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. Attachment of the bacteria to plant host cells is required for tumor formation. Human vitronectin and antivitronectin antibodies both inhibited the binding of A. tumefaciens to carrot cells. Wild-type bacteria are able to bind radioactive vitronectin; nonattaching mutants showed a reduction in the ability to bind vitronectin. The binding of biotype 1 A. tumefaciens to carrot cells or to radioactive vitronectin was not affected by high ionic strength. Detergent extraction of carrot cells removed the receptor to which the bacteria bind. The extract was found to contain a vitronectin-like protein. These results suggest that A. tumefaciens utilizes a vitronectin-like protein on the plant cell surface as the receptor for its initial attachment to host cells.

Two-way chemical signaling in Agrobacterium-plant interactions

The discovery in 1977 that Agrobacterium species can transfer a discrete segment of oncogenic DNA (T-DNA) to the genome of host plant cells has stimulated an intense interest in the molecular biology underlying these plant-microbe associations. This attention in turn has resulted in a series of insights about the biology of these organisms that continue to accumulate at an ever-increasing rate. This excitement was due in part to the notion that this unprecedented interkingdom DNA transfer could be exploited to create transgenic plants containing foreign genes of scientific or commercial importance. In the course of these discoveries, Agrobacterium became one of the best available models for studying the molecular interactions between bacteria and higher organisms. One extensively studied aspect of this association concerns the exchange of chemical signals between Agrobacterium spp. and host plants. Agrobacterium spp. can recognize no fewer than five classes of low-molecular-weight compounds released from plants, and other classes probably await discovery. The most widely studied of these are phenolic compounds, which stimulate the transcription of the genes needed for infection. Other compounds include specific monosaccharides and acidic environments which potentiate vir gene induction, acidic polysaccharides which induce one or more chromosomal genes, and a family of compounds called opines which are released from tumorous plant cells to the bacteria as nutrient sources. Agrobacterium spp. in return release a variety of chemical compounds to plants. The best understood is the transferred DNA itself, which contains genes that in various ways upset the balance of phytohormones, ultimately causing neoplastic cell proliferation. In addition to transferring DNA, some Agrobacterium strains directly secrete phytohormones. Finally, at least some strains release a pectinase, which degrades a component of plant cell walls.

Mutation of the miaA gene of Agrobacterium tumefaciens results in reduced vir gene expression

vir regulon expression in Agrobacterium tumefaciens involves both chromosome- and Ti-plasmid-encoded gene products. We have isolated and characterized a new chromosomal gene that when mutated results in a 2- to 10-fold reduction in the induced expression of vir genes by acetosyringone. This reduced expression occurs in AB minimal medium (pH 5.5) containing either sucrose or glucose and containing phosphate at high or low concentrations. The locus was cloned and used to complement A. tumefaciens strains harboring Tn5 insertions in the gene. Sequence analysis of this locus revealed an open reading frame with strong homology to the miaA locus of Escherichia coli and the mod5 locus of Saccharomyces cerevisiae. These genes encode tRNA: isopentenyltransferase enzymes responsible for the specific modification of the A-37 residue in UNN codon tRNA species. The function of the homologous gene in A. tumefaciens was proven by genetic complementation of E. coli miaA mutant strains. tRNA undermodification in A. tumefaciens miaA mutant strains may reduce vir gene expression by causing a reduced translation efficiency. A slight reduction in the virulence of these mutant Agrobacterium strains on red potato plants, but not on tobacco, tomato, kalanchoe, or sunflower plants, was observed.

Transformation of Wall Deficient Cultured Tobacco Protoplasts by Agrobacterium tumefaciens

Attachment of virulent Agrobacterium tumefaciens to plant cells is required for transformation. To further study the components of the plant cell wall that may be involved in the attachment process, tobacco (Nicotiana tabacum L.) protoplasts were cultured in the presence of 2,6 dichlorobenzonitrile (DB), an inhibitor of cellulose biosynthesis, and then assayed for their ability to be transformed by Agrobacterium. The DB treated protoplasts were deficient in wall production. Nevertheless, they were transformable at high frequency by wild type Agrobacterium strains but not by mutant strains that lack the ability to bind to normal, walled cells. Small quantities of calcofluor white positive material present on DB treated cells were correlated with their competence to be transformed. Further, the plant:bacterial association that leads to transformation is shown to become stable within 5 hours after bacterial co-cultivation with either control or DB treated 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.

Agrobacterium rhizogenes mutants that fail to bind to plant cells

Transposon insertion mutants of Agrobacterium rhizogenes were screened to obtain mutant bacteria that failed to bind to carrot suspension culture cells. A light microscope binding assay was used. The bacterial isolates that were reduced in binding to carrot cells were all avirulent on Bryophyllum diagremontiana leaves and on carrot root disks. The mutants did not appear to be altered in cellulose production. The composition of the medium affected the ability of the parent and mutant bacteria to bind to carrot cells. The parent strain bound to carrot cells in greatest numbers in low-ionic-strength media such as 4% sucrose but still showed significant binding in Murashige-Skoog tissue culture medium. All of the mutants showed reduced binding in 4% sucrose after 2 h of incubation with carrot cells. One mutant was delayed in binding in 4% sucrose. This mutant and one other mutant also showed reduced binding to carrot cells in Murashige-Skoog medium. To determine whether the Tn5 insertion was responsible for the mutant phenotype, DNA containing the Tn5 insertion was cloned from the mutant bacteria and used to introduce Tn5 into the parent strain in the same location as in the original mutant by marker exchange. The resulting transconjugants had the same avirulent, nonattaching phenotype as the original mutants, suggesting that the mutant phenotype was due to the Tn5 insertion. The cloned DNA containing the Tn5 insertion was also tested for homology to DNA of known genes that affect attachment of Agrobacterium tumefaciens to plant cells by DNA hybridization. No homology to chv, att, or pscA clones was observed. In addition, cloned chv, att, and pscA genes from A. tumefaciens were unable to complement the attachment-minus A. rhizogenes mutants. Thus, the A. rhizogenes nonattaching mutants appear to be different from the previously described A. tumefaciens mutants.

Inhibition by Agrobacterium tumefaciens and Pseudomonas savastanoi of development of the hypersensitive response elicited by Pseudomonas syringae pv. phaseolicola

Injection into tobacco leaves of biotype 1 Agrobacterium tumefaciens or of Pseudomonas savastanoi inhibited the development of a visible hypersensitive response to the subsequent injection at the same site of Pseudomonas syringae pv. phaseolicola. This interference with the hypersensitive response was not seen with injection of bacterial growth medium or Escherichia coli cells. Live A. tumefaciens cells were required for the inhibitory effect. Various mutants and strains of A. tumefaciens were examined to determine the genes involved. Known chromosomal mutations generally had no effect on the ability of A. tumefaciens to inhibit the hypersensitive response, except for chvB mutants which showed a reduced (but still significant) inhibition of the hypersensitive response. Ti plasmid genes appeared to be required for the inhibition of the hypersensitive response. The bacteria did not need to be virulent in order to inhibit the hypersensitive response. Deletion of the vir region from pTi had no effect on the inhibition. However, the T region of the Ti plasmid was required for inhibition. Studies of transposon mutants suggested that the tms but not tmr or ocs genes were required. These genes were not acting after transfer to plant cells since they were effective in strains lacking vir genes and thus unable to transfer DNA to plant cells. The results suggest that the expression of the tms genes in the bacteria may inhibit the development of the hypersensitive response by the plant. An examination of the genes required in P. savastanoi for the inhibition of the hypersensitive response suggested that bacterial production of auxin was also required for the inhibition of the hypersensitive response by these bacteria.

Identification and Plant Interaction of a Phyllobacterium sp., a Predominant Rhizobacterium of Young Sugar Beet Plants

The second most abundant bacterium on the root surface of young sugar beet plants was identified as a Phyllobacterium sp. ( Rhizobiaceae ) based on a comparison of the results of 39 conventional identification tests, 167 API tests, 30 antibiotic susceptibility tests, and sodium dodecyl sulfate-polyacrylamide gel electrophoretic fingerprints of total cellular proteins with type strains of Phyllobacterium myrsinacearum and Phyllobacterium rubiacearum. It was found on 198 of 1,100 investigated plants between the 2nd and 10th leaf stage on three different fields in Belgium and one field in Spain. Densities ranged from 2 × 10 4 to 2 × 10 8 CFU/g of root. Five isolates exerted a broad-spectrum in vitro antifungal activity. DNA-DNA hybridizations showed that Phyllobacterium sp. does not contain DNA sequences that are homologous with the attachment genes chvA, chvB , the transferred-DNA (T-DNA) hormone genes iaaH and ipt from Agrobacterium tumefaciens, iaaM from A. tumefaciens and Pseudomonas savastanoi , or the nitrogenase genes nifHDK from Klebsiella pneumoniae. Phyllobacterium sp. produces indolylacetic acid in in vitro cultures and induces auxinlike effects when cocultivated with callus tissue of tobacco. When Phyllobacterium sp. was transformed with a Ti plasmid derivative, it gained the capacity to induce tumors on Kalanchoe daigremontiana. The potential role of Phyllobacterium sp. in this newly recognized niche is discussed.

Cyclic diguanylic acid and cellulose synthesis in Agrobacterium tumefaciens

The occurrence of the novel regulatory nucleotide bis(3',5')-cyclic diguanylic acid (c-di-GMP) and its relation to cellulose biogenesis in the plant pathogen Agrobacterium tumefaciens was studied. c-di-GMP was detected in acid extracts of 32P-labeled cells grown in various media, and an enzyme responsible for its formation from GTP was found to be present in cell-free preparations. Cellulose synthesis in vivo was quantitatively assessed with [14C]glucose as a tracer. The organism produced cellulose during growth in the absence of plant cells, and this capacity was retained in resting cells. Synthesis of a cellulosic product from UDP-glucose in vitro with membrane preparations was markedly stimulated by c-di-GMP and its precursor GTP and was further enhanced by Ca2+. The calcium effect was attributed to inhibition of a c-di-GMP-degrading enzyme shown to be present in the cellulose synthase-containing membranes.

Construction of an Agrobacterium tumefaciens C58 recA mutant

Clones encoding the recA gene of Agrobacterium tumefaciens C58 were isolated from a cosmid bank by complementation of an Escherichia coli recA mutation. Subcloning and mutagenesis with the lacZ fusion transposon Tn3HoHo1 located the Agrobacterium recA gene to a 1.3-kilobase segment of DNA. beta-Galactosidase expression from the fusions established the direction in which the gene was transcribed. The gene restored homologous recombination as well as DNA repair functions in E. coli recA mutants. Similar complementation of DNA repair functions was observed in the UV-induced Rec- Agrobacterium mutant, LBA4301. The Agrobacterium recA gene was disrupted by insertion of a cassette encoding resistance to erythromycin, and the mutated gene was marker exchanged into the chromosome of strain NT-1. The resulting strain, called UIA143, was sensitive to UV irradiation and methanesulfonic acid methyl ester and unable to carry out homologous recombination functions. The mutation was stable and had no effect on other genetic properties of the Agrobacterium strain, including transformability and proficiency as a conjugal donor or recipient. Furthermore, strain UIA143 became tumorigenic upon introduction of a Ti plasmid, indicating that tumor induction is independent of recA functions. Sequence homology was detected between the recA genes of strain C58 and E. coli as well as with DNA isolated from agrobacteria representing the three major biochemically differentiated biovars of this genus. In some cases, biovar-specific restriction fragment length polymorphisms were apparent at the recA locus.

Alternative Environmental Roles for Cellulose Produced by Acetobacter xylinum

The cellulose-producing bacterium Acetobacter xylinum has been considered a strict aerobe, and it has been suggested that the function of cellulose is to hold cells in an aerobic environment. In this study, we showed that A. xylinum is capable of growing microaerophilically. Cellulose pellicles provided significant protection to A. xylinum cells from the killing effects of UV light. In experiments measuring colonization by A. xylinum , molds, and other bacteria on pieces of apple, cellulose pellicles enhanced colonization of A. xylinum on the substrate and provided protection from competitors which use the same substrate as a source of nutrients. Cellulose pellicles produced by A. xylinum may have multiple functions in the growth and survival of the organism in nature.

Variation in Binding and Virulence of Agrobacterium tumefaciens Chromosomal Virulence (chv) Mutant Bacteria on Different Plant Species

Chromosomal virulence (chv) mutants of Agrobacterium tumefaciens have been reported to be deficient in binding to cells of zinnia, tobacco, and bamboo. The mutants are nonpathogenic on stems of Kalanchoë, sunflower, tomato, Jerusalem artichoke, and tobacco, but they cause tumors on tubers of Solanum tuberosum. We used a root cap cell binding assay to test ability of cells from individual plants of 13 different plant species to bind parent or chv mutant bacteria. The same plants were then inoculated to test for disease response. Cells from nine of the plant species were grossly deficient in their abilities to bind mutant bacteria, and the plants inoculated with mutant bacteria failed to form tumors. In contrast, root cap cells as well as root hairs and root surfaces of S. tuberosum, S. okadae, and S. hougasii bound chv mutant bacteria as well as wild type. Nevertheless, S. tuberosum roots inoculated with mutant bacteria did not develop tumors. Although S. okadae plants inoculated with mutant bacteria formed a few tumors, and S. hougasii developed as many tumors in response to chv mutants as in response to the parent strain, the tumors induced by mutant bacteria were smaller.

Specificity of signal molecules in the activation of Agrobacterium virulence gene expression

The activation of the Agrobacterium virulence system is known to be induced by certain phenolic compounds. We have tested the vir-inducing ability of fifty compounds, by using a virB-lacZ gene fusion, and analysed the relationship between structure and activity of these compounds. In this way we have identified several new vir-inducers: coniferylalcohol, 3,5-dimethoxy-4-hydroxybenzene, homovanillic acid, ferulic acid, 3-ethoxy-4-hydroxybenzaldehyde and guaiacol, all of which are compounds with strong or moderate activity and four compounds with weak vir-inducing activity. In view of the specificity of vir-inducers, our data extended observations of others and enabled us to define the specific structural features of a vir-inducer molecule. In addition we show here that induction of the octopine Ti vir-genes is (i) optimal at 29 degrees C and totally abolished at 37 degrees C, and (ii) strongly inhibited at low concentrations of sodium chloride. The implications for plant transformation are discussed.

Use of a Root Tumorigenesis Assay to Detect Genotypic Variation in Susceptibility of Thirty-four Cultivars of Pisum sativum to Crown Gall

We developed a quantitative assay to measure tumorigenesis on roots and root crowns, the natural sites of Agrobacterium tumefaciens infection. Efficiency of tumor formation and tumor weight on seedlings of Pisum sativum ;Little Marvel' were directly proportional to the logarithm of inoculum concentration. Depth of wounding prior to inoculation also significantly influenced tumor weight but not efficiency. Mean weight of tumors that developed in response to inoculation with strain B6 varied significantly among 34 different commercial cultivars. Tumors on the most susceptible cultivar, Target, were more than tenfold heavier than those formed on the least susceptible cultivar, Sweet Snap. Efficiency of tumorigenesis on ;Sweet Snap' was also relatively low: only 64% of inoculated seedlings developed tumors compared with 89 to 100% efficiencies for all other cultivars.

Role for [corrected] Agrobacterium tumefaciens ChvA protein in export of beta-1,2-glucan

Functional chvA and chvB genes are required for attachment of Agrobacterium tumefaciens to plant cells, an early step in crown gall tumor formation. Strains defective in these loci do not secrete normal amounts of cyclic beta-1,2-glucan. Whereas chvB is required for beta-1,2-glucan synthesis, the role of chvA in glucan synthesis or export has not been clearly defined. We found that cultures of chvA mutants contained as much neutral beta-1,2-glucan in the cell pellets as did the wild type, with no detectable accumulation of glucan in the culture supernatant. The cytoplasm of chvA mutant cells contained over three times more soluble beta-1,2-glucan than did the cytoplasm of the wild-type parent. Unlike the wild type, chvA mutants contained no detectable periplasmic glucan. The amino acid sequence of chvA is highly homologous to the sequences of bacterial and eucaryotic export proteins, as observed previously in the case of ndvA, a rhizobial homolog of chvA. Strong sequence homology within this family of export proteins is concentrated in the carboxy-terminal portions of the proteins, but placement of consensus ATP-binding sites, internal signal sequences, and hydrophobic domains are conserved over their entire lengths. These data suggest a model for beta-1,2-glucan synthesis in A. tumefaciens in which glucan is synthesized inside the inner membrane with the participation of ChvB and transported across the inner membrane with the participation of ChvA.

Two chromosomal loci involved in production of exopolysaccharide in Agrobacterium tumefaciens

The chromosomal locus pscA (exoC) of Agrobacterium tumefaciens LBA4301 has been cloned by complementation of the avirulent and exopolysaccharide (EPS)-deficient mutant LBA4301 pscA. We have also identified a new locus, termed psdA (polysaccharide depression) and located 16 kilobases from pscA in the A. tumefaciens chromosome, that negatively affects EPS production when it is present in more than one copy in A. tumefaciens LBA4301. Subcloning, transposon mutagenesis, and transcriptional analysis have been conducted for both loci and indicate that pscA and psdA are transcribed in the same orientation. Acidic-EPS assays showed that psdA depresses succinoglycan production and that its negative effect increases with the copy number of the gene. Virulence tests of psdA transconjugants on Datura stramonium showed no visible alteration in virulence, while LBA4301 pscA was totally avirulent.

Transcriptional regulation of the virA and virG genes of Agrobacterium tumefaciens

We have used transcriptional and translational fusions between various vir gene promoters and the lacZ gene to study the regulation of vir genes. Like other vir promoters, the virA promoter was induced by acetosyringone in a virA virG-dependent fashion. In addition to being induced by acetosyringone, the virG promoter was partially induced by acidic growth conditions and by starvation for inorganic phosphate. These two conditions appeared to act synergistically. The response to low pH and to phosphate starvation occurred in the absence of the Ti plasmid and must therefore have been mediated by chromosomal genes. Two transposon-generated mutations were obtained which attenuated induction by low pH. One of these transposons was cloned along with flanking DNA; the flanking DNA was sequenced (858 base pairs total), and the predicted amino acid sequence showed homology with a family of proteins including the Rhizobium leguminosarum nodI gene, many of whose members bind ATP and have been implicated in active transport systems. These results are discussed as possible explanations for previous observations that the induction of the octopine vir regulon (i) occurs only in acidic media and (ii) shows hyperbolic kinetics after a long lag phase.

Analysis of the complete nucleotide sequence of the Agrobacterium tumefaciens virB operon

The complete nucleotide sequence of the virB locus, from the octopine Ti plasmid of Agrobacterium tumefaciens strain 15955, has been determined. In the large virB-operon (9600 nucleotides) we have identified eleven open reading frames, designated virB1 to virB11. From DNA sequence analysis it is proposed that nearly all VirB products, i.e. VirB1 to VirB9, are secreted or membrane associated proteins. Interestingly, both a membrane protein (VirB4) and a potential cytoplasmic protein (VirB11) contain the consensus amino acid sequence of ATP-binding proteins. In view of the conjugative T-DNA transfer model, the VirB proteins are suggested to act at the bacterial surface and there play an important role in directing T-DNA transfer to plant cells.

Virulence genes, borders, and overdrive generate single-stranded T-DNA molecules from the A6 Ti plasmid of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers the T-DNA portion of its Ti plasmid to the nuclear genome of plant cells. Upon cocultivation of A. tumefaciens A348 with regenerating tobacco leaf protoplasts, six distinct single-stranded T-DNA molecules (T strands) were generated in addition to double-stranded T-DNA border cleavages which we have previously reported (K. Veluthambi, R.K. Jayaswal, and S.B. Gelvin, Proc. Natl. Acad. Sci. USA 84:1881-1885, 1987). The T region of an octopine-type Ti plasmid has four border repeats delimiting three T-DNA regions defined as T left (TL), T center (TC), and T right (TR). The six T strands generated upon induction corresponded to the TL, TC, TR, TL + TC, TC + TR, and TL + TC + TR regions, suggesting that the initiation and termination of T-strand synthesis can occur at each of the four borders. Most TL + TC + TR T-strand molecules corresponded to the top T-DNA strand, whereas the other five T strands corresponded to the bottom T-DNA strand. Generation of T strands required the virA, virG, and virD operons. Extra copies of vir genes, harbored on cosmids within derivatives of A. tumefaciens A348, enhanced production of T strands. The presence of right and left border repeats in their native orientation is important for the generation of full-length T strands. When a right border repeat was placed in the opposite orientation, single-stranded T-DNA molecules that corresponded to the top strand were generated. Deletion of overdrive, a sequence that flanks right border repeats and functions as a T-DNA transmission enhancer, reduced the level of T-strand generation. Induction of A. tumefaciens cells by regenerating tobacco protoplasts increased the copy number of the Ti plasmid relative to the bacterial chromosome.

Mapping of Agrobacterium tumefaciens chromosomal genes affecting cellulose synthesis and bacterial attachment to host cells

Six chromosomal transposon mutations in Agrobacterium tumefaciens which result in an inability to synthesize cellulose fibrils were mapped to the vicinity of trp-2 and met-6. Mutations which result in an inability to attach to plant cells, attC43 and attC69, also mapped in this region, as did one Tn5 mutation which caused overproduction of cellulose. Another cellulose overproduction mutation mapped at a distance and was closely linked to ilv-13. The results suggest that there is a region of the A. tumefaciens chromosome located near met-6 which is concerned with the interaction of the bacterium with the host cell surface.

Dual control of Agrobacterium tumefaciens Ti plasmid virulence genes

The virulence genes of nopaline (pTiC58) and octopine (pTiA6NC) Ti plasmids are similarly affected by the Agrobacterium tumefaciens ros mutation. Of six vir region complementation groups (virA, virB, virG, virC, virD, and virE) examined by using fusions to reporter genes, the promoters of only two (virC and virD) responded to the ros mutation. For each promoter that was affected by ros, the level of expression of its associated genes was substantially elevated in the mutant. This increase was not influenced by Ti plasmid-encoded factors, and the mutation did not interfere with the induction of pTiC58 vir genes by phenolic compounds via the VirA/VirG regulatory control mechanism. The effects of the ros mutation and acetosyringone were cumulative for all vir promoters examined. The pleiotropic characteristics of the ros mutant include the complete absence of the major acidic capsular polysaccharide.