Cloning and nucleotide sequence of the tzs gene from Agrobacterium tumefaciens strain T37

Rhizobium rhizogenes infection in threatened Indian orchid Dendrobium ovatum mobilises 'Moscatilin' to enhance plant defensins

The present study illustrates the transformation ability of two wild-type bacterial strains of Rhizobium rhizogenes (MTCC 532 and MTCC 2364) on the embryogenic callus and callus-derived plantlets of a threatened Indian orchid, Dendrobium ovatum. Co-culture of the bacterium with the explants gave marginal hairy root phenotype that failed to multiply in the culture medium. Some primary and secondary metabolites were subdued in infected explants. Moscatilin, the stilbenoid active principle in D. ovatum, was found below the detection limit. The presence of two metabolites viz., Laudanosine, a benzyltetrahydroisoquinoline alkaloid and Lyciumin B, a cyclic peptide, were detected exclusively in the infected explants. The subjugated amino acids and phenolics in the infected plantlets were routed to produce phytoanticipins, and phenanthrenes, strengthening the defence mechanism in infected tissues. This research implies that the plant's defence mechanism activation could have prevented the extensive hairy root formation in the explants, even though nodulations and phenotype transitions were witnessed. Moscatilin has a structural resemblance with Resveratrol, a phytoalexin that combats bacterial and fungal pathogens. The study favours the possibility of Moscatlin being a precursor for phenanthrene compounds, thereby serving as a 'phytoanticipin' during the infection phase.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03180-9.

Biochemical and Structural Aspects of Cytokinin Biosynthesis and Degradation in Bacteria

It has been known for quite some time that cytokinins, hormones typical of plants, are also produced and metabolized in bacteria. Most bacteria can only form the tRNA-bound cytokinins, but there are examples of plant-associated bacteria, both pathogenic and beneficial, that actively synthesize cytokinins to interact with their host. Similar to plants, bacteria produce diverse cytokinin metabolites, employing corresponding metabolic pathways. The identification of genes encoding the enzymes involved in cytokinin biosynthesis and metabolism facilitated their detailed characterization based on both classical enzyme assays and structural approaches. This review summarizes the present knowledge on key enzymes involved in cytokinin biosynthesis, modifications, and degradation in bacteria, and discusses their catalytic properties in relation to the presence of specific amino acid residues and protein structure.

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.

Isopentenyltransferase-1 (IPT1) knockout in Physcomitrella together with phylogenetic analyses of IPTs provide insights into evolution of plant cytokinin biosynthesis

The moss Physcomitrella patens is part of an early divergent clade of land plants utilizing the plant hormone cytokinin for growth control. The rate-limiting step of cytokinin biosynthesis is mediated by isopentenyltransferases (IPTs), found in land plants either as adenylate-IPTs or as tRNA-IPTs. Although a dominant part of cytokinins in flowering plants are synthesized by adenylate-IPTs, the Physcomitrella genome only encodes homologues of tRNA-IPTs. This study therefore looked into the question of whether cytokinins in moss derive from tRNA exclusively. Targeted gene knockout of ipt1 (d|ipt1) along with localization studies revealed that the chloroplast-bound IPT1 was almost exclusively responsible for the A37 prenylation of tRNA in Physcomitrella. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based cytokinin profiling demonstrated that the total amount of all free cytokinins in tissue was almost unaffected. However, the knockout plants showed increased levels of the N (6) -isopentenyladenine (iP)- and trans-zeatin (tZ)-type cytokinins, considered to provide active forms, while cis-zeatin (cZ)-type cytokinins were reduced. The data provide evidence for an additional and unexpected tRNA-independent cytokinin biosynthetic pathway in moss. Comprehensive phylogenetic analysis indicates a diversification of tRNA-IPT-like genes in bryophytes probably related to additional functions.

Plant responses to Agrobacterium tumefaciens and crown gall development

Agrobacterium tumefaciens causes crown gall disease on various plant species by introducing its T-DNA into the genome. Therefore, Agrobacterium has been extensively studied both as a pathogen and an important biotechnological tool. The infection process involves the transfer of T-DNA and virulence proteins into the plant cell. At that time the gene expression patterns of host plants differ depending on the Agrobacterium strain, plant species and cell-type used. Later on, integration of the T-DNA into the plant host genome, expression of the encoded oncogenes, and increase in phytohormone levels induce a fundamental reprogramming of the transformed cells. This results in their proliferation and finally formation of plant tumors. The process of reprogramming is accompanied by altered gene expression, morphology and metabolism. In addition to changes in the transcriptome and metabolome, further genome-wide ("omic") approaches have recently deepened our understanding of the genetic and epigenetic basis of crown gall tumor formation. This review summarizes the current knowledge about plant responses in the course of tumor development. Special emphasis is placed on the connection between epigenetic, transcriptomic, metabolomic, and morphological changes in the developing tumor. These changes not only result in abnormally proliferating host cells with a heterotrophic and transport-dependent metabolism, but also cause differentiation and serve as mechanisms to balance pathogen defense and adapt to abiotic stress conditions, thereby allowing the coexistence of the crown gall and host plant.

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.

Early nodulin genes are induced in alfalfa root outgrowths elicited by auxin transport inhibitors

Rhizobium nod genes are essential for root hair deformation and cortical cell division, early stages in the development of nitrogen-fixing root nodules. Nod(-) mutants are unable to initiate nodules on legume roots. We observed that N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid, compounds known to function as auxin transport inhibitors, induced nodule-like structures on alfalfa roots. The nodule-like structures (pseudonodules) were white, devoid of bacteria, and resembled nodules elicited by Rhizobium meliloti exopolysaccharide (exo) mutants at both the histological and molecular level. Two nodulin genes, ENOD2 and Nms-30, were expressed. RNA isolated from the nodule-like structures hybridized to pGmENOD2, a soybean early nodulin cDNA clone. RNA isolated from roots did not hybridize. We determined by in vitro translations of total RNA that the alfalfa nodulin transcript Nms-30 was also expressed in the nodule-like structures. The late expressed nodulin genes, such as the leghemoglobin genes, were not transcribed. Because N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid induce the development of nodules on alfalfa roots, we suggest that the auxin transport inhibitors mimic the activity of compound(s) made upon the induction of the Rhizobium nod genes.

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 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.

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.

Identification of the Agrobacterium tumefaciens C58 T-DNA genes e and f and their impact on crown gall tumour formation

DNA sequence analysis of the 4.4 kilobases (kb) Eco RI fragment 14 from T-DNA of Agrobacterium tumefaciens C58 revealed three open reading frames. One of them (945 bp) was supposed to encode the transcript e, the function of which has not been identified to date. Furthermore, a so far undescribed open reading frame (1035 bp) was identified, located in the centre of the Eco RI fragment 14 and termed gene f. The third open reading frame encoded the carboxy-terminal part of the agrocinopine synthase (Acs). The gene e-encoded protein showed significant homologies to the gene products of the Agrobacterium rhizogenes rolB gene and the Agrobacterium tumefaciens gene 5. Both gene products are supposed to regulate the plant's reaction on auxin. Depending on the plant species tested, Agrobacterium strains carrying mutations in gene e induced only small or almost no detectable crown gall tumours. According to these mutational studies and the protein homologies observed, the gene e product is suggested to be involved in tumour formation. Infection of several plant species with Agrobacterium carrying a mutated gene f, as well as expression of the gene f in transgenic tobacco plants did not lead to visible morphological changes. Therefore, in contrast to gene e, the gene f seems not to be essential for tumour formation. In order to study whether gene f is an active gene, its expression in agrobacteria and plants was monitored by translational lacZ fusion. In planta, the putative gene f-promoter mediates a tissue-specific expression pattern. Although gene f was expressed in free-living agrobacteria as well as in transgenic plants, the function of the f locus remained unclear. DNA homology studies with the f gene region revealed a mosaic-like DNA structure, indicating that this locus might be the result of genetic exchanges between different Agrobacterium strains during evolution.

Synthesis of phytohormones by plant-associated bacteria

The plant hormones, auxins and cytokinins, are involved in several stages of plant growth and development such as cell elongation, cell division, tissue differentiation, and apical dominance. The biosynthesis and the underlying mechanism of auxins and cytokinins action are subjects of intense investigation. Not only plants but also microorganisms can synthesize auxins and cytokinins. The role of phytohormone biosynthesis by microorganisms is not fully elucidated: in several cases of pathogenic fungi and bacteria these compounds are involved in pathogenesis on plants; auxin and cytokinin production may also be involved in root growth stimulation by beneficial bacteria and associative symbiosis. The genetic mechanism of auxin biosynthesis and regulation by Pseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, and Azospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis. The pathogenic bacteria Pseudomonas and Agrobacterium produce indole-3-acetic acid via the indole-3-acetamide pathway, for which the genes are plasmid borne. However, they do possess also the indole-3-pyruvic acid pathway, which is chromosomally encoded. In addition, they have genes that can conjugate free auxins or hydrolyze conjugated forms of auxins and cytokinins. In Agrobacterium there are also several genes, located near the auxin and cytokinin biosynthetic genes, that are involved in the regulation of auxins and cytokinins sensibility of the transformed plant tissue. Symbiotic bacteria Rhizobium and Bradyrhizobium synthesize indole-3-acetic acid via indole-3-pyruvic acid; also the genetic determinants for the indole-3-acetamide pathway have been detected, but their activity has not been demonstrated. In the plant growth-promoting bacterium Azospirillum, as in Agrobacterium and Pseudomonas, both the indole-3-pyruvic acid and the indole-3-acetamide pathways are present, although in Azospirillum the indole-3-pyruvic acid pathway is of major significance. In addition, biochemical evidence for a tryptophan-independent indole-3-acetic acid pathway in Azospirillum has been presented.

Morphogenetic Rescue of Rhizobium meliloti Nodulation Mutants by trans-Zeatin Secretion

The development of nitrogen-fixing nodules is induced on the roots of legume host plants by Rhizobium bacteria. We employed a novel strategy to probe the underlying mechanism of nodule morphogenesis in alfalfa roots using pTZS, a broad host range plasmid carrying a constitutive trans-zeatin secretion (tzs) gene from Agrobacterium tumefaciens T37. This plasmid suppressed the Nod- phenotype of Rhizobium nodulation mutants such that mutants harboring pTZS stimulated the formation of nodulelike structures. Alfalfa roots formed more or fewer of these nodules according to both the nitrogen content of the environment and the position along the root at which the pTZS+ bacteria were applied, which parallels the physiological and developmental regulation of true Rhizobium nodule formation. This plasmid also conferred on Escherichia coli cells the ability to induce root cortical cell mitoses. Both the pattern of induced cell divisions and the spatially restricted expression of an alfalfa nodule-specific marker gene (MsENOD2) in pTZS-induced nodules support the conclusion that localized cytokinin production produces a phenocopy of nodule morphogenesis.

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.

Genetic transformation of Chrysanthemum using wild type Agrobacterium strains; strain and cultivar specificity

To develop an Agrobacterium mediated transformation protocol for chrysanthemum we studied the transformation efficiency of commonly used A.tumefaciens strains on 14 genotypes by comparing tumour size and frequency. One genotype was analyzed in detail using 14 strains of both A.tumefaciens and A.rhizogenes. Only a few genotype/strain combinations resulted in significant tumour formation. Especially 0-type strains were highly efficient. An 0-type strain was used to transfer genes for neomycine phosphotransferase (NPT II) and ß-glucuronidase (GUS) to a susceptible cultivar. Transfer of the GUS gene was confirmed by using the Polymerase Chain Reaction (PCR).

A functional analysis of T-DNA gene 6b: the fine tuning of cytokinin effects on shoot development

The physiological function in planta of T-DNA gene 6b was studied under various experimental conditions. For this purpose the coding region of gene 6b was cloned behind the 1'-promoter of the TR-DNA to enhance expression of the gene product in transformed plant cells. Expression of the recombinant gene in leaf discs of Nicotiana tabacum altered the capacity for shoot formation of the discs, induced by exogenous (i.e. BAP in the growth medium or agrobacterial trans-zeatin produced under control of gene tzs) or endogenous cytokinins (i.e. isopentenyladenosine produced under control of T-DNA gene 4). The data obtained indicate a reduction of cytokinin activity within the plant cells by the product of T-DNA gene 6b.

Structure of the Caulobacter crescentus trpFBA operon

The DNA sequences of the Caulobacter crescentus trpF, trpB, and trpA genes were determined, along with 500 base pairs (bp) of 5'-flanking sequence and 320 bp of 3'-flanking sequence. An open reading frame, designated usg, occurs upstream of trpF and encodes a polypeptide of 89 amino acids which seems to be expressed in a coupled transcription-translation system. Interestingly, the usg polypeptide is not homologous to any known tryptophan biosynthetic enzyme. S1 nuclease mapping of in vivo transcripts indicated that usg, trpF, trpB, and trpA are arranged into a single operon, with the transcription initiation site located 30 bp upstream from the start of usg. Sequences centered at -30 and -6 bp upstream from the transcription initiation site are somewhat homologous to the Escherichia coli promoter consensus sequence and are homologous to sequences found upstream of genes from several organisms which are evolutionarily related to C. crescentus. Furthermore, the trpFBA operon promoter sequence lacks homology to promoter sequences identified for certain developmentally regulated C. crescentus genes. The structures of the C. crescentus usg, trpF, trpB, and trpA genes were further analyzed in terms of codon usage, G+C content, and genetic signals and were related to genetic signals previously identified in C. crescentus and other bacteria. Taken together, these results are relevant to the analysis of gene expression in C. crescentus and the study of trp gene structure and regulation.

Expression of an Agrobacterium Ti plasmid gene involved in cytokinin biosynthesis is regulated by virulence loci and induced by plant phenolic compounds

The nopaline-type Ti plasmid T37 of Agrobacterium tumefaciens carries two distinct genes that encode enzymes involved in cytokinin biosynthesis. In this report, we show that the level of expression of one of these genes was increased dramatically by culture conditions that increased the expression of Ti plasmid virulence genes, including coculture with plant cells or treatment with acetosyringone, a plant phenolic compound. When this nopaline-type Ti plasmid gene was introduced into Agrobacterium strains containing an octopine-type Ti plasmid, similar induction of expression by culture conditions was observed, and analysis of virulence region mutants demonstrated that this induction was under the control of the virA and virG regulatory loci. We further show that induction was strongly pH dependent in octopine strains but, under the conditions examined, pH independent in nopaline strains.

Regulation of the vir genes of Agrobacterium tumefaciens plasmid pTiC58

The virulence (vir) region of pTiC58 was screened for promoter activities by using gene fusions to a promoterless lux operon in the broad-host-range vector pUCD615. Active vir fragments contained the strongly acetosyringone-inducible promoters of virB, virC, virD, and virE and the weakly inducible promoters of virA and virG. Identical induction patterns were obtained with freshly sliced carrot disks, suggesting that an inducer is released after plant tissue is wounded. Optimal conditions for vir gene induction were pH 5.7 for 50 microM acetosyringone or sinapic acid. The induction of virB and virE by acetosyringone was strictly dependent on intact virA and virG loci. An increase in the copy number of virG resulted in a proportional, acetosyringone-independent increase in vir gene expression, and a further increase occurred only if an inducing compound and virA were present.

Cytokinin production by Agrobacterium and Pseudomonas spp

The production of cytokinins by plant-associated bacteria was examined by radioimmunoassay. Strains producing trans-zeatin were identified in the genera Agrobacterium and Pseudomonas. Agrobacterium tumefaciens strains containing nopaline tumor-inducing plasmids, A. tumefaciens Lippia isolates, and Agrobacterium rhizogenes strains produced trans-zeatin in culture at 0.5 to 44 micrograms/liter. Pseudomonas solanacearum and Pseudomonas syringae pv. savastanoi produced trans-zeatin at levels of up to 1 mg/liter. In vitro cytokinin biosynthetic activity was measured for representative strains and was found to correlate with trans-zeatin production. The genetic locus for trans-zeatin secretion (tzs) was cloned from four strains: A. tumefaciens T37, A. rhizogenes A4, P. solanacearum K60, and P. syringae pv. savastanoi 1006. Southern blot analysis showed substantial homology of the Agrobacterium tzs genes to each other but not to the two Pseudomonas genes.

Secretion of Zeatin, Ribosylzeatin, and Ribosyl-1'' -Methylzeatin by Pseudomonas savastanoi: Plasmid-Coded Cytokinin Biosynthesis

Cytokinin production by strains of the phytopathogenic bacterium Pseudomonas syringae pv savastanoi was measured by immunoaffinity chromatography of the culture medium on immobilized anti-cytokinin antibodies, followed by high performance liquid chromatography, radioimmunoassay and mass spectrometry. P. savastanoi strain PB213-2 secretes zeatin (80 nanograms per milliliter) and ribosylzeatin (80 nanograms per milliliter). Even higher levels of zeatin (400 nanograms per milliliter) are produced by the olive-specific strain EW1006, which also produces 180 nanograms per milliliter of the recently identified cytokinin, ribosyl-1'' -methylzeatin. The amounts secreted were approximately 1000 times greater than those secreted by Agrobacterium tumefaciens (DA Regier, RO Morris 1982 Biochem Biophys Res Commun 104: 1560-1566). Examination of cytokinin production by plasmid deletion mutants of PB213-2 and EW1006 indicated that cytokinin biosynthesis was specified, at least in part, by plasmid-borne genes. A fragment of the 105 kilobase pair plasmid from EW1006 was cloned into Escherichia coli where its expression resulted in dimethylallyl transferase activity and the secretion of zeatin.

A new cell division operon in Escherichia coli

At 76 min on the E. coli genetic map there is a cluster of genes affecting essential cellular functions, including the heat shock response and cell division. A combination of in-vivo and in-vitro genetic analysis of cell division mutants suggests that the cell division gene fts E is the second gene in a 3 gene operon. A cold-sensitive mutant, defective in the third gene, is also unable to divide at the restrictive temperature, and we designate this new cell division gene fts X. Another cell division gene, fts S, is very close to, but distinct from, the 3 genes of the operon. The fts E product is a 24.5 Kd polypeptide which shows strong homology with a small group of proteins involved in transport. Both the fts E product and the protein coded by the first gene (fts Y) in the operon have a sequence motif found in a wide range of heterogeneous proteins, including the Ras proteins of yeast. This common domain is indicative of a nucleotide-binding site.

Nucleotide sequence of an insertion sequence (IS) element identified in the T-DNA region of a spontaneous variant of the Ti-plasmid pTiT37

We have identified and determined the nucleotide sequence of an IS element (IS136) of Agrobacterium tumefaciens. This is the first IS element isolated and sequenced from a nopaline type Ti-plasmid. Our IS element has 32/30 bp inverted repeats with 6 mismatches, is 1,313 bp long and generates 9 bp direct repeats upon integration. IS136 has 3 main open reading frames (ORF's). Only ORF1 (159 codons) is preceded by sequences that are proposed to serve functional roles in transcriptional and translational initiation. No DNA sequence homology was found between IS136 and IS66, an IS element isolated from an octopine type Ti-plasmid.

Evidence for a cellular gene with potential oncogenic activity in plants

During tumor inception in crown gall disease, a portion of the tumor-inducing (Ti) plasmid, the transferred DNA (T-DNA), is integrated into the genome of the plant cell. Autonomous growth of the transformants requires expression of genes in the tmr and tms regions of the T-DNA, which code for enzymes concerned with biosynthesis of the plant growth hormones cytokinin and auxin, respectively. We show that a mutation of the Habituated leaf gene, Hl, of tobacco can compensate for a defective tmr locus in expression of the tumor phenotype. This provides evidence that a specific host-cell gene has an oncogenic function similar to tmr in the T-DNA.

Nucleotide sequence and expression of a Pseudomonas savastanoi cytokinin biosynthetic gene: homology with Agrobacterium tumefaciens tmr and tzs loci

The nucleotide sequence of a Pseudomonas trans-zeatin producing gene (ptz) from the pCK1 plasmid of Pseudomonas syringae pv. savastanoi strain 1006 has been determined. This gene confers upon E. coli the ability to synthesize and secrete several cytokinins including trans-zeatin, iso-pentenyladenine and their respective N9-ribosyl derivatives. Sequence analysis indicates an open reading frame encoding a protein of 234 amino acids with a molecular weight of 26,816. Significant sequence homology is found between ptz and both the tzs and tmr genes from Agrobacterium tumefaciens. The results suggest a close relationship between the cytokinin biosynthetic pathways in P. savastanoi and A. tumefaciens.