Inducible expression of cytokinin biosynthesis in Agrobacterium tumefaciens by plant phenolics

BnKAT2 Positively Regulates the Main Inflorescence Length and Silique Number in Brassica napus by Regulating the Auxin and Cytokinin Signaling Pathways

Brassica napus is the dominant oil crop cultivated in China for its high quality and high yield. The length of the main inflorescence and the number of siliques produced are important traits contributing to rapeseed yield. Therefore, studying genes related to main inflorescence and silique number is beneficial to increase rapeseed yield. Herein, we focused on the effects of BnKAT2 on the main inflorescence length and silique number in B. napus. We explored the mechanism of BnKAT2 increasing the effective length of main inflorescence and the number of siliques through bioinformatics analysis, transgenic technology, and transcriptome sequencing analysis. The full BnKAT2(BnaA01g09060D) sequence is 3674 bp, while its open reading frame is 2055 bp, and the encoded protein comprises 684 amino acids. BnKAT2 is predicted to possess two structural domains, namely KHA and _CNMP-binding domains. The overexpression of BnKAT2 effectively increased the length of the main inflorescence and the number of siliques in B. napus, as well as in transgenic Arabidopsis thaliana. The type-A Arabidopsis response regulator (A-ARR), negative regulators of the cytokinin, are downregulated in the BnKAT2-overexpressing lines. The Aux/IAA, key genes in auxin signaling pathways, are downregulated in the BnKAT2-overexpressing lines. These results indicate that BnKAT2 might regulate the effective length of the main inflorescence and the number of siliques through the auxin and cytokinin signaling pathways. Our study provides a new potential function gene responsible for improvement of main inflorescence length and silique number, as well as a candidate gene for developing markers used in MAS (marker-assisted selection) breeding to improve rapeseed yield.

The Cytokinin Complex Associated With Rhodococcus fascians: Which Compounds Are Critical for Virulence?

Virulent strains of Rhodococcus fascians cause a range of disease symptoms, many of which can be mimicked by application of cytokinin. Both virulent and avirulent strains produce a complex of cytokinins, most of which can be derived from tRNA degradation. To test the three current hypotheses regarding the involvement of cytokinins as virulence determinants, we used PCR to detect specific genes, previously associated with a linear virulence plasmid, including two methyl transferase genes (mt1 and mt2) and fas4 (dimethyl transferase), of multiple strains of R. fascians. We inoculated Pisum sativum (pea) seeds with virulent and avirulent strains of R. fascians, monitored the plants over time and compared these to mock-inoculated controls. We used RT-qPCR to monitor the expression of mt1, mt2, and fas4 in inoculated tissues and LC-MS/MS to obtain a comprehensive picture of the cytokinin complement of inoculated cotyledons, roots and shoots over time. The presence and expression of mt1 and mt2 was associated with those strains of R. fascians classed as virulent, and not those classed as avirulent. Expression of mt1, mt2, and fas4 peaked at 9 days post-inoculation (dpi) in cotyledons and at 15 dpi in shoots and roots developed from seeds inoculated with virulent strain 602. Pea plants inoculated with virulent and avirulent strains of R. fascians both contained cytokinins likely to have been derived from tRNA turnover including the 2-methylthio cytokinins and cis-zeatin-derivatives. Along with the isopentenyladenine-type cytokinins, the levels of these compounds did not correlate with virulence. Only the novel 1- and 2-methylated isopentenyladenine cytokinins were uniquely associated with infection by the virulent strains and are, therefore, the likely causative factors of the disease symptoms.

Agrobacterium-mediated plant transformation: biology and applications

Plant genetic transformation heavily relies on the bacterial pathogen Agrobacterium tumefaciens as a powerful tool to deliver genes of interest into a host plant. Inside the plant nucleus, the transferred DNA is capable of integrating into the plant genome for inheritance to the next generation (i.e. stable transformation). Alternatively, the foreign DNA can transiently remain in the nucleus without integrating into the genome but still be transcribed to produce desirable gene products (i.e. transient transformation). From the discovery of A. tumefaciens to its wide application in plant biotechnology, numerous aspects of the interaction between A. tumefaciens and plants have been elucidated. This article aims to provide a comprehensive review of the biology and the applications of Agrobacterium-mediated plant transformation, which may be useful for both microbiologists and plant biologists who desire a better understanding of plant transformation, protein expression in plants, and plant-microbe interaction.

Constitutive expression of the tzs gene from Agrobacterium tumefaciens virG mutant strains is responsible for improved transgenic plant regeneration in cotton meristem transformation

KEY MESSAGE : virG mutant strains of a nopaline type of Agrobacterium tumefaciens increase the transformation frequency in cotton meristem transformation. Constitutive cytokinin expression from the tzs gene in the virG mutant strains is responsible for the improvement. Strains of Agrobacterium tumefaciens were tested for their ability to improve cotton meristem transformation frequency. Two disarmed A. tumefaciens nopaline strains with either a virGN54D constitutively active mutation or virGI77V hypersensitive induction mutation significantly increased the transformation frequency in a cotton meristem transformation system. The virG mutant strains resulted in greener explants after three days of co-culture in the presence of light, which could be attributed to a cytokinin effect of the mutants. A tzs knockout strain of virGI77V mutant showed more elongated, less green explants and decreased cotton transformation frequency, as compared to a wild type parental strain, suggesting that expression of the tzs gene is required for transformation frequency improvement in cotton meristem transformation. In vitro cytokinin levels in culture media were tenfold higher in the virGN54D strain, and approximately 30-fold higher in the virGI77V strain, in the absence of acetosyringone induction, compared to the wild type strain. The cytokinin level in the virGN54D strain is further increased upon acetosyringone induction, while the cytokinin level in the virGI77V mutant is decreased by induction, suggesting that different tzs gene expression regulation mechanisms are present in the two virG mutant strains. Based on these data, we suggest that the increased cytokinin levels play a major role in increasing Agrobacterium attachment and stimulating localized division of the attached plant cells.

Agrobacterium-derived cytokinin influences plastid morphology and starch accumulation in Nicotiana benthamiana during transient assays

BACKGROUND

Agrobacterium tumefaciens-based transient assays have become a common tool for answering questions related to protein localization and gene expression in a cellular context. The use of these assays assumes that the transiently transformed cells are observed under relatively authentic physiological conditions and maintain 'normal' sub-cellular behaviour. Although this premise is widely accepted, the question of whether cellular organization and organelle morphology is altered in Agrobacterium-infiltrated cells has not been examined in detail. The first indications of an altered sub-cellular environment came from our observation that a common laboratory strain, GV3101(pMP90), caused a drastic increase in stromule frequency. Stromules, or 'stroma-filled-tubules' emanate from the surface of plastids and are sensitive to a variety of biotic and abiotic stresses. Starting from this observation, the goal of our experiments was to further characterize the changes to the cell resulting from short-term bacterial infestation, and to identify the factor responsible for eliciting these changes.

RESULTS

Using a protocol typical of transient assays we evaluated the impact of GV3101(pMP90) infiltration on chloroplast behaviour and morphology in Nicotiana benthamiana. Our experiments confirmed that GV3101(pMP90) consistently induces stromules and alters plastid position relative to the nucleus. These effects were found to be the result of strain-dependant secretion of cytokinin and its accumulation in the plant tissue. Bacterial production of the hormone was found to be dependant on the presence of a trans-zeatin synthase gene (tzs) located on the Ti plasmid of GV3101(pMP90). Bacteria-derived cytokinins were also correlated with changes to both soluble sugar level and starch accumulation.

CONCLUSION

Although we have chosen to focus on how transient Agrobacterium infestation alters plastid based parameters, these changes to the morphology and position of a single organelle, combined with the measured increases in sugar and starch content, suggest global changes to cell physiology. This indicates that cells visualized during transient assays may not be as 'normal' as was previously assumed. Our results suggest that the impact of the bacteria can be minimized by choosing Agrobacterium strains devoid of the tzs gene, but that the alterations to sub-cellular organization and cell carbohydrate status cannot be completely avoided using this strategy.

Cytokinins secreted by Agrobacterium promote transformation by repressing a plant myb transcription factor

Agrobacterium-mediated transformation is the most widely used technique for generating transgenic plants. However, many crops remain recalcitrant. We found that an Arabidopsis myb family transcription factor (MTF1) inhibited plant transformation susceptibility. Mutating MTF1 increased attachment of several Agrobacterium strains to roots and increased both stable and transient transformation in both susceptible and transformation-resistant Arabidopsis ecotypes. Cytokinins from Agrobacterium tumefaciens decreased the expression of MTF1 through activation of the cytokinin response regulator ARR3. Mutating AHK3 and AHK4, genes that encode cytokinin-responsive kinases, increased the expression of MTF1 and impaired plant transformation. Mutant mtf1 plants also had increased expression of AT14A, which encodes a putative transmembrane receptor for cell adhesion molecules. Plants overexpressing AT14A exhibited increased susceptibility to transformation, whereas at14a mutant plants exhibited decreased attachment of bacteria to roots and decreased transformation, suggesting that AT14A may serve as an anchor point for Agrobacteria. Thus, by promoting bacterial attachment and transformation of resistant plants and increasing such processes in susceptible plants, treating roots with cytokinins may help engineer crops with improved features or yield.

Biological activity of the tzs gene of nopaline Agrobacterium tumefaciens GV3101 in plant regeneration and genetic transformation

Agrobacterium tumefaciens has been widely used in plant genetic transformation. Hormone-encoding genes residing in the T-DNA region have been removed, resulting in disarmed Agrobacterium strains that are used in various transformation experiments. Nopaline Agrobacterium strains, however, carry another hormone gene, trans-zeatin synthesizing (tzs), that codes for trans-zeatin in the virulence region of the tumor-inducing plasmids. We investigated the activity and function of the tzs gene of a nopaline Agrobacterium sp. strain GV3101 in plant in vitro regeneration. Leaf explants of tobacco and Nicotiana benthamiana co-cultured with strain GV3101 exhibited active shoot regeneration in media without added plant growth regulators. On medium without plant growth regulators, transgenic shoots were also induced from explants co-cultured with GV3101 containing a binary vector. Enzyme-linked immunosorbent assay showed that cell-free extracts of Agrobacterium sp. strain GV3101 culture contained the trans-zeatin at 860 ng/liter. Polymerase chain reaction using tzs-specific primers showed that the tzs gene was present in strain GV3101 but not in other Agrobacterium strains. The study showed that the tzs gene in GV3101 was actively expressed, and that trans-zeatin produced in the Agrobacterium strain can promote plant shoot regeneration.

The Tzs protein and exogenous cytokinin affect virulence gene expression and bacterial growth of Agrobacterium tumefaciens

The soil phytopathogen Agrobacterium tumefaciens causes crown gall disease in a wide range of plant species. The neoplastic growth at the infection sites is caused by transferring, integrating, and expressing transfer DNA (T-DNA) from A. tumefaciens into plant cells. A trans-zeatin synthesizing (tzs) gene is located in the nopaline-type tumor-inducing plasmid and causes trans-zeatin production in A. tumefaciens. Similar to known virulence (Vir) proteins that are induced by the vir gene inducer acetosyringone (AS) at acidic pH 5.5, Tzs protein is highly induced by AS under this growth condition but also constitutively expressed and moderately upregulated by AS at neutral pH 7.0. We found that the promoter activities and protein levels of several AS-induced vir genes increased in the tzs deletion mutant, a mutant with decreased tumorigenesis and transient transformation efficiencies, in Arabidopsis roots. During AS induction and infection of Arabidopsis roots, the tzs deletion mutant conferred impaired growth, which could be rescued by genetic complementation and supplementing exogenous cytokinin. Exogenous cytokinin also repressed vir promoter activities and Vir protein accumulation in both the wild-type and tzs mutant bacteria with AS induction. Thus, the tzs gene or its product, cytokinin, may be involved in regulating AS-induced vir gene expression and, therefore, affect bacterial growth and virulence during A. tumefaciens infection.

Genetic engineering of cytokinin metabolism: prospective way to improve agricultural traits of crop plants

Cytokinins (CKs) are ubiquitous phytohormones that participate in development, morphogenesis and many physiological processes throughout plant kingdom. In higher plants, mutants and transgenic cells and tissues with altered activity of CK metabolic enzymes or perception machinery, have highlighted their crucial involvement in different agriculturally important traits, such as productivity, increased tolerance to various stresses and overall plant morphology. Furthermore, recent precise metabolomic analyses have elucidated the specific occurrence and distinct functions of different CK types in various plant species. Thus, smooth manipulation of active CK levels in a spatial and temporal way could be a very potent tool for plant biotechnology in the future. This review summarises recent advances in cytokinin research ranging from transgenic alteration of CK biosynthetic, degradation and glucosylation activities and CK perception to detailed elucidation of molecular processes, in which CKs work as a trigger in model plants. The first attempts to improve the quality of crop plants, focused on cereals are discussed, together with proposed mechanism of action of the responses involved.

Agrobacterium-produced and exogenous cytokinin-modulated Agrobacterium-mediated plant transformation

Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called 'crown gall', via the transfer and integration of transferred DNA (T-DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)-induced tumour-inducing (Ti) plasmid gene, tzs (trans-zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline-type A. tumefaciens strains. The loss of Tzs protein expression and trans-zeatin secretions by the tzs frameshift (tzs-fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs-fs mutant with a wild-type tzs gene restored wild-type levels of trans-zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium-produced cytokinin. Interestingly, although the tzs-fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans-zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens.

Structural insight into the reaction mechanism and evolution of cytokinin biosynthesis

The phytohormone cytokinin regulates plant growth and development. This hormone is also synthesized by some phytopathogenic bacteria, such as Agrobacterium tumefaciens, and is as a key factor in the formation of plant tumors. The rate-limiting step of cytokinin biosynthesis is catalyzed by adenosine phosphate-isopentenyltransferase (IPT). Agrobacterium IPT has a unique substrate specificity that enables it to increase trans-zeatin production by recruiting a metabolic intermediate of the host plant's biosynthetic pathway. Here, we show the crystal structures of Tzs, an IPT from A. tumefaciens, complexed with AMP and a prenyl-donor analogue, dimethylallyl S-thiodiphosphate. The structures reveal that the carbon-nitrogen-based prenylation proceeds by the SN2-reaction mechanism. Site-directed mutagenesis was used to determine the amino acid residues, Asp-173 and His-214, which are responsible for differences in prenyl-donor substrate specificity between plant and bacterial IPTs. IPT and the p loop-containing nucleoside triphosphate hydrolases likely evolved from a common ancestral protein. Despite structural similarities, IPT has evolved a distinct role in which the p loop transfers a prenyl moiety in cytokinin biosynthesis.

The type IV secretion system component VirB5 binds to the trans-zeatin biosynthetic enzyme Tzs and enables its translocation to the cell surface of Agrobacterium tumefaciens

VirB5 is a minor component of the extracellular T pilus determined by the Agrobacterium tumefaciens type IV secretion system. To identify proteins that interact with VirB5 during the pilus assembly process, we purified VirB5 as a recombinant fusion protein and, by using a gel overlay assay, we detected a 26-kDa interacting protein in Agrobacterium cell lysates. The VirB5-binding protein was purified from A. tumefaciens and identified as the cytokinin biosynthetic enzyme Tzs. The VirB5-Tzs interaction was confirmed using pulldown assays with purified proteins and the yeast two-hybrid system. An analysis of the subcellular localization in A. tumefaciens showed that Tzs was present in the soluble as well as the membrane fraction. Tzs was extracted from the membranes with the mild detergent dodecyl-beta-D-maltoside in complexes of different molecular masses, and this association was strongly reduced in the absence of VirB5. Using immunoelectron microscopy, we also detected Tzs on the Agrobacterium cell surface. A functional type IV secretion system was required for efficient translocation to the surface, but Tzs was not secreted into the cell supernatant. The fact that Tzs localizes on the cell surface suggests that it may contribute to the interaction of Agrobacterium with plants.

Proteomic analysis ofAgrobacterium tumefaciens response to thevir gene inducer acetosyringone

Agrobacterium tumefaciens causes crown gall disease in a wide range of plants by transforming plants through the transfer and integration of its transferred DNA (T-DNA) into the host genome. In the present study, we used two-dimensional gel electrophoresis to examine the protein expression profiles of A. tumefaciens in response to the phenolic compound acetosyringone (AS), a known plant-released virulence (vir) gene inducer. Using mass spectrometry, we identified 11 proteins consisting of 9 known AS-induced Vir proteins and 2 newly discovered AS-induced proteins, an unknown protein Y4mC (Atu6162) and a small heat shock protein HspL (Atu3887). Further expression analysis revealed that the AS-induced expression of Y4mC and HspL is regulated by the VirA/VirG two-component system. This report presents the first proteomics study successfully identifying both known and new AS-induced proteins that are implicated in Agrobacterium virulence.

Cytokinins: activity, biosynthesis, and translocation

Cytokinins (CKs) play a crucial role in various phases of plant growth and development, but the basic molecular mechanisms of their biosynthesis and signal transduction only recently became clear. The progress was achieved by identifying a series of key genes encoding enzymes and proteins controlling critical steps in biosynthesis, translocation, and signaling. Basic schemes for CK homeostasis and root/shoot communication at the whole-plant level can now be devised. This review summarizes recent findings on the relationship between CK structural variation and activity, distinct features in CK biosynthesis between higher plants and Agrobacterium infected plants, CK translocation at whole-plant and cellular levels, and CKs as signaling molecules for nutrient status via root-shoot communication.

Tissue localization of cytokinin dehydrogenase in maize: possible involvement of quinone species generated from plant phenolics by other enzymatic systems in the catalytic reaction

The degradation of cytokinins in plants is controlled by the flavoprotein cytokinin dehydrogenase (EC 1.5.99.12). Cytokinin dehydrogenase from maize showed the ability to use oxidation products of guaiacol, 4-methylcatechol, acetosyringone and several other compounds as electron acceptors. These results led us to explore the cability for indirect production of suitable electron acceptors by different quinone-generating enzymes. The results reported here revealed that the electron acceptors may be generated in vivo from plant phenolics by other enzymatic systems such as peroxidase and tyrosinase/laccase/catechol oxidase. Histochemical localization of cytokinin dehydrogenase by activity staining and immunochemistry using optical and confocal microscopy showed that cytokinin dehydrogenase is most abundant in the aleurone layer of maize kernels and in phloem cells of the seedling shoots. Cytokinin dehydrogenase was confirmed to be present in the apoplast of cells. Co-staining of enzyme activity for laccase, an enzyme poised to function on the cell wall in the apoplast, in those tissues suggests a possible cooperation of the enzymes in cytokinin degradation. Additionally, the presence of precursors for electron acceptors of cytokinin dehydrogenase was detected in phloem exudates collected from maize seedlings, suggestive of an enzymatic capacity to control cytokinin flux through the vasculature. A putative metabolic connection between cytokinin degradation and conversion of plant phenolics by oxidases was proposed.

Cytokinin biosynthesis and regulation

Most natural cytokinins (CKs) are adenine derivatives that carry an isoprene-derived side chain at the N6-terminus. Structural variation at the isoprenoid side chain alters their biological activity and stability. The first step of de novo synthesis of CKs is catalyzed by adenosine phosphate-isopentenyltransferase (IPT), which produces isopentenyladenine nucleotide. In higher plants, trans-zeatin (tZ), a major CK, is formed by subsequent hydroxylation, which is catalyzed by a cytochrome P450 monooxygenase (P450), CYP735A1 or CYP735A2. Biochemical characterization of IPTs revealed that the substrate specificities differ between Agrobacterium and higher plants. Agrobacterium IPTs have the ability to produce tZ-type species directly by use of hydroxymethylbutenyl diphosphate as the side chain donor. Analyses of expression patterns of genes for CK metabolic enzymes suggest that CK biosynthesis and homeostasis are finely controlled by internal and external environmental factors such as phytohormones and inorganic nitrogen sources. This regulatory system appears important in linking nutrient signals and morphogenetic responses.

Reexamining the role of the accessory plasmid pAtC58 in the virulence of Agrobacterium tumefaciens strain C58

Isogenic strains of Agrobacterium tumefaciens carrying pTiC58, pAtC58, or both were constructed and assayed semiquantitatively and quantitatively for virulence and vir gene expression to study the effect of the large 542-kb accessory plasmid, pAtC58, on virulence. Earlier studies indicate that the att (attachment) genes of A. tumefaciens are crucial in the ability of this soil phytopathogen to infect susceptible host plants. Mutations in many att genes, notably attR and attD, rendered the strain avirulent. These genes are located on pAtC58. Previous work also has shown that derivatives of the wild-type strain C58 cured of pAtC58 are virulent as determined by qualitative virulence assays and, hence, pAtC58 was described as nonessential for virulence. We show here that the absence of pAtC58 in pTiC58-containing strains results in reduced virulence but that disruption of the attR gene does not result in avirulence or a reduction in virulence. Our studies indicate that pAtC58 has a positive effect on vir gene induction as revealed by immunoblot analysis of Vir proteins and expression of a PvirB::lacZ fusion.

The Tzs protein from Agrobacterium tumefaciens C58 produces zeatin riboside 5'-phosphate from 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate and AMP

The plant pathogen Agrobacterium tumefaciens produces cytokinins upon induction of the virulence genes by secondary metabolites from wounded plants, and these hormones are believed to stimulate the infection process. To study the biosynthetic pathway, the tzs gene, encoding the Tzs (trans-zeatin-synthesizing) protein from A. tumefaciens, was cloned and the protein was overproduced and purified. Analysis of its reactivity with radioactively labeled substrate demonstrated conversion of 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate, a product of the deoxyxylulose phosphate pathway, with AMP to zeatin riboside 5'-phosphate. This suggests that A. tumefaciens uses an alternative pathway of cytokinin biosynthesis, which had previously been hypothesized to operate in plants.

Leafy gall formation by Rhodococcus fascians

Rhodococcus fascians infects a wide range of plants, initiating the formation of leafy galls that consist of centers of shoot amplification and shoot growth inhibition. R. fascians is an epiphyte but it also can establish endophytic populations. Bacterial signals involved in symptom development initiate de novo cell division and shoot meristem formation in differentiated tissues. The R. fascians signals exert activities that are distinct from mere cytokinin effects, and the evidence points to a process that adopted cytokinin biosynthetic enzymes to form derivatives with unique activity. Genes implicated in leafy gall formation are located on a linear plasmid and are subject to a highly controlling, complex regulatory network, integrating autoregulatory compounds and environmental signals. Leafy galls are considered as centers with specific metabolic features, a niche where populations of R. fascians experience a selective advantage. Such "metabolic habitat modification" might be universal for gall-inducing bacteria.

The right end of the vir region of an octopine-type Ti plasmid contains four new members of the vir regulon that are not essential for pathogenesis

We sequenced the virD-virE, virE-virF, and virF-T-DNA intergenic regions of an octopine Ti plasmid. Four newly described genes were induced by the vir gene inducer acetosyringone, two of which are conserved in the nopaline-type Ti plasmid pTiC58. One gene resembles a family of phosphatase genes. Each of these genes is dispensable for tumorigenesis.

The genes involved in cytokinin biosynthesis in Erwinia herbicola pv. gypsophilae: characterization and role in gall formation

A locus conferring cytokinin production was previously isolated from the gall-forming bacterium Erwinia herbicola pv. gypsophilae. This locus resided in a cluster with the genes specifying indole-3-acetic acid production on the pathogenicity-associated plasmid pPATH (A. Lichter, S. Manulis, O. Sagee, Y. Gafni, J. Gray, R. Meilen, R. O. Morris, and I. Barash, Mol. Plant Microbe Interact., 8:114-121, 1995). Sequence analysis of this locus indicated the presence of a cytokinin biosynthesis gene (etz) homologous to other described cytokinin biosynthesis genes. A unique open reading frame (pre-etz) encoding 169 amino acids preceded etz and together with etz formed a region with a distinctive low G+C content. Northern (RNA) analysis indicated the presence of an etz-specific transcript of 1 kb and a common transcript for pre-etz and etz of 1.4 kb. The level of the 1-kb transcript was high in the late logarithmic phase and very low in the stationary phase. In contrast, the level of the 1.4-kb transcript was lower than that of the 1-kb transcript in the late logarithmic phase and predominant in the stationary phase. A marker exchange mutant of etz which did not produce cytokinins exhibited a reduction in gall size on Gypsophila cuttings and almost abolished disease symptoms in a whole-plant assay. Complementation of this marker exchange mutant with the intact etz gene on a multicopy plasmid resulted in overproduction of cytokinins and larger plant galls from which small shoots emerged. Insertional mutation in pre-etz resulted in a sharp decrease in both the level of the etz-specific transcript and cytokinin production. A frameshift mutation in pre-etz caused a similar reduction in the cytokinin level. A marker exchange mutation in pre-etz caused a reduction of symptoms but to lower degree than the etz mutation. In the former mutant, cytokinin production and pathogenicity could not be restored by complementation. Furthermore, attempts to complement the etz marker exchange mutant with a plasmid containing an intact etz gene and a frameshift mutation in the pre-etz gene were unsuccessful. These results suggest that the mutations in pre-etz were trans dominant.

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.

virF, the host-range-determining virulence gene of Agrobacterium tumefaciens, affects T-DNA transfer to Zea mays

The monocotyledonous plant Zea mays does not develop tumors after inoculation with Agrobacterium tumefaciens and is thus defined as nonhost. Agroinfection, Agrobacterium-mediated delivery of maize streak virus, demonstrates that transferred DNA (T-DNA) transfer to the plant does occur. Nopaline-type Agrobacterium strains such as C58 are efficient in the transfer process whereas the octopine-type strain A6 is unable to transfer T-DNA to maize. This phenotypic difference maps to the tumor-inducing (Ti) plasmid but not to the T-DNA. Steps preceding T-DNA transfer, such as attachment and induction of the virulence genes, were shown to take place in the octopine strain. The nopaline-plasmid-specific locus tzs and the octopine-plasmid-specific locus pinF (virH) are not involved in the strain specificity. However, mutations in the virF locus rendered the octopine strain agroinfectious on maize, whereas such virF-defective octopine strains, when complemented by virF on a plasmid, completely lost their agroinfectivity. We propose that VirF, known to increase the host range of the bacteria in other systems, acts as an inhibitor of T-DNA transfer to maize.

The regulatory VirG protein specifically binds to a cis-acting regulatory sequence involved in transcriptional activation of Agrobacterium tumefaciens virulence genes

Virulence genes of Agrobacterium tumefaciens are induced in parallel in the presence of plant phenolic compounds such as acetosyringone and the two regulatory vir genes virA and virG. In this study we identified a cis-acting regulatory sequence in the 5'-noncoding region of the virE operon that is essential for this activation. To do this, we constructed a series of deletion mutants by using exonuclease Bal 31. Western blot (immunoblot) analysis showed that the 70 base pairs upstream of the transcriptional start site were sufficient for full virE gene induction. A conserved dodecadeoxynucleotide sequence (vir box), which was previously identified in the nontranscribed sequences of all vir genes, was located at 5' end of the minimum required promoter sequence. Deletion of this vir box only completely abolished induction of the virE gene. This demonstrates that the vir box functions as an upstream regulatory sequence. To study the role of the VirG protein in the activation process, we overproduced the native-sized VirG protein in Escherichia coli by fusing the lacZ' start codon ATG with the second virG codon AAA using site-directed mutagenesis. The VirG protein was purified and renatured from E. coli and was shown to bind to a specific sequence in two vir gene promoters. Footprinting analysis of the virE and virB promoters identified the 12-base-pair vir box as the VirG-binding core sequence.