Crown Gall Disease and Prospects for Genetic Manipulation of Plants

Multilocus sequence-based analysis delineates a clonal population of Agrobacterium (Rhizobium) radiobacter (Agrobacterium tumefaciens) of human origin

The genus Agrobacterium includes plant-associated bacteria and opportunistic human pathogens. Taxonomy and nomenclature within the genus remain controversial. In particular, isolates of human origin were all affiliated with the species Agrobacterium (Rhizobium) radiobacter, while phytopathogenic strains were designated under the synonym denomination Agrobacterium tumefaciens. In order to study the relative distribution of Agrobacterium strains according to their origins, we performed a multilocus sequence-based analysis (MLSA) on a large collection of 89 clinical and environmental strains from various origins. We proposed an MLSA scheme based on the partial sequence of 7 housekeeping genes (atpD, zwf, trpE, groEL, dnaK, glnA, and rpoB) present on the circular chromosome of A. tumefaciens C58. Multilocus phylogeny revealed that 88% of the clinical strains belong to genovar A7, which formed a homogeneous population with linkage disequilibrium, suggesting a low rate of recombination. Comparison of genomic fingerprints obtained by pulsed-field gel electrophoresis (PFGE) showed that the strains of genovar A7 were epidemiologically unrelated. We present genetic evidence that genovar A7 may constitute a human-associated population distinct from the environmental population. Also, phenotypic characteristics, such as culture at 42°C, agree with this statement. This human-associated population might represent a potential novel species in the genus Agrobacterium.

Cytokinin gene fused with a strong promoter enhances shoot organogenesis and zeatin levels in transformed plant cells

The isopentenyltransferase (ipt) gene associated with cytokinin biosynthesis in plants was cloned from a tumor-inducing plasmid carried by Agrobacterium tumefaciens and placed under the control of promoters of differing activities, the cauliflower mosaic virus 35S promoter and the nopaline synthase promoter. These promoter-gene constructs were introduced into wounded Nicotiana stems, leaf pieces, and cucumber seedlings by A. tumefaciens infection. Shoots were observed in the infection site on all responding genotypes of Nicotiana plants infected with the 35S promoter construct (35S-ipt), whereas only 41% responded similarly to infection with the unmodified gene. Furthermore, shoots were observed 19 days after infection with the 35S-ipt gene but not until 28 to 45 days with the unaltered ipt gene. Shoots were more numerous (>40) on galls incited by 35S-ipt and were up to 6 times taller than shoots induced by the native gene. On Cucumis (cucumber), shoots were observed only on galls incited by the 35S-ipt construct. These galls were on the average 7.5 times larger than those incited by the nopaline synthase promoter construct (NOS-ipt) or the unmodified ipt gene. Zeatin and zeatinriboside concentrations averaged 23 times greater in the 35S-ipt transformed shoots than in ones transformed with the native ipt gene. These results suggest that a more active promoter on the ipt gene can enhance or change the morphogenic potential of transformed plant cells by increasing their endogenous cytokinin levels.

Leucinopine, a characteristic compound of some crown-gall tumors

An unusual compound has been found in crowngall tumors induced by those Agrobacterium tumefaciens strains that utilize neither octopine nor nopaline. The compound has been isolated and shown by proton and carbon-13 NMR spectroscopy and by synthesis to be N(2)-(1,3-dicarboxypropyl)-L-leucine, which also exists in a cyclized (i.e., lactam) form. This compound, which we name "leucinopine," was not detected in octopine tumors, nopaline tumors, or the tumors induced by A. tumefaciens strains 181 and EU6.

Compilation and analysis of eukaryotic POL II promoter sequences

A representative set of 168 eukaryotic POL II promoters has been compiled from the EMBL library and subjected to computer signal search analysis. Application of this technique to E. coli promoters as a control ensemble revealed the well known consensus sequences at -35 and -10 which indicates that the methods are adequate to approach problems of this kind. The results obtained from the eukaryotic promoter set can be summarized as follows: Common sequence features are confined to a region between -50 and +10 relative to the transcriptional initiation site. The only well conserved consensus sequence is TATAAA, centered at -28. A weak motif, CA followed preferentially by pyrimidines, surrounds the cap-site. Two pentanucleotides which have been shown by experiments to stimulate transcription of certain genes, GGGCG and CCAAT, are moderately over-represented in the upstream region (between -129 and -50). However, they occur at highly variable distances from the initiation site.

Eight DNA insertion events of Agrobacterium tumefaciens Ti-plasmids in isogenic sunflower genomes are all distinct

We investigated whether the same or different T-DNA insertions occur every time Agrobacterium tumefaciens, the octopine type strain pTi 15955 strr, infects genetically identical sunflower plants. Eight newly established crown gall tissue culture lines were analyzed for their T-DNA content. Our data showed that all isogenic crown gall callus DNA produced distinct hybridization patterns. These eight patterns were also different from three standard lines included for comparison. In addition, all the tumor lines analyzed produced octopine, albeit in different quantities, and five produced agropine and mannopine. We concluded, that each A. tumefaciens crown gall tissue line derived from isogenic sunflower plants contained a distinct insertion pattern of T-DNA. Possible causes and reasons for this diversity will be discussed.

Simple binary vectors for DNA transfer to plant cells

Cosmid binary vectors for the introduction of DNA into plant cells have been constructed. These vectors are derived from the replicon of the broad host range plasmid pRK2 and contain the T-DNA border regions between which have been placed a chimaeric gene conferring resistance to kanamycin in plant cells. Appropriate restriction endonuclease targets have also been placed between the border regions. These binary vectors, in conjunction with appropriate Agrobacterium strains, are capable of delivering DNA to plant cells in cocultivation experiments with very high efficiency. The transformation frequency is shown to be somewhat dependent on the replicon used. re]19850121 rv]19850506 ac]19850513.

Identification and genetic analysis of an Agrobacterium tumefaciens chromosomal virulence region

A genetic analysis of Agrobacterium tumefaciens chromosomal functions required for virulence was undertaken. Large Tn5-containing cosmid clones were isolated from DNA of avirulent A. tumefaciens mutants having chromosomal Tn5 insertions and exhibiting defective attachment to plant cells. The clones from several different mutants each contained overlapping segments of a 30-kilobase A. tumefaciens chromosomal region, which were physically mapped. All chromosomal Tn5 insertions leading to the avirulent, attachment-defective phenotype were localized within an 11-kilobase portion of this chromosomal virulence region. Transposon Tn3::HoHo1 (Tn3 containing lacZ) was used to simultaneously mutagenize and create lac fusions within the virulence region. This analysis demonstrated the presence of two distinct chromosomal virulence loci, which were 1.5 and 5 kilobases long; transposon insertions into these loci led to avirulence and defective attachment. The beta-galactosidase activity associated with various Tn3::HoHo1-created lac fusions indicated that the loci are transcribed in opposite directions, and complementation studies suggested that each locus consists of a single transcriptional unit. A cosmid clone of the chromosomal virulence region containing a lac fusion in the extreme 3' portion of the 5-kilobase locus was used to demonstrate that expression of this region is dependent on the presence of sequences in the 5' portion of the locus, confirming its operon-like nature.

Specific attachment of Agrobacterium tumefaciens to bamboo cells in suspension cultures

Agrobacterium tumefaciens was tested for its ability to attach to tissue culture cells of bamboo, a monocotyledonous plant. Phase-contrast microscopy and kinetic experiments with radiolabeled bacteria showed that attachment to bamboo cells was indistinguishable from attachment to cells of dicotyledonous plants. Bacterial mutants defective in attachment to dicotyledonous plants showed similar behavior with bamboo, and extensive washing of the bamboo cells had no effect on the number of bacteria which attached.

Phaseolin gene from bean is expressed after transfer to sunflower via tumor-inducing plasmid vectors

Sequences coding for the bean seed protein phaseolin were inserted into transferred DNA regions of tumor-inducing plasmids. Constructions were devised in which the coding region of phaseolin was fused in the correct reading frame with the coding region of octopine synthase and placed under the transcriptional control of the octopine synthase promoter. Other plasmids were prepared to permit expression of the phaseolin-encoding sequences from the flanking phaseolin promoter region. The RNA transcribed in sunflower cells transformed with these constructions was characterized by hybridization procedures, SI nuclease mapping, and by translation in vitro of extracted RNA. These tests showed that the genomic intervening sequences were correctly excised. Immunoreactive phaseolin polypeptides were detected by enzyme-linked immunosorbent assay and by antibody hybridization to electrophoretically separated protein extracts of sunflower tissues isolated from crown gall tumors and of transformed sunflower cells grown in tissue culture. These results demonstrate the expression of a plant gene after transfer to a taxonomically distinct botanical family.

Prospects in plant genetic engineering

The functional expression of a novel gene in a genetically engineered plant has not yet been reported. One major barrier in movement toward this goal is our limited understanding of the molecular bases of gene expression. Attempts to establish genetic engineering as a practical facet of plant breeding are also complicated by the fact that genes for most important plant characteristics have not yet been identified. However, the benefits to be gained from all aspects of plant improvement are stimulating research into both the development of plant transformation technology and the isolation and characterization of genes responsible for valuable traits. As scientists develop greater knowledge of plant molecular genetics, we can expect to see practical applications in such diverse areas as improvement of plant nutritional quality, decreases in fertilization requirements, and increases in resistance to environmental stresses and pathogens.

Development of DNA-mediated transformation systems for plants

The genetic engineering of plants by DNA-mediated gene transfer requires that efficient transformation systems be developed. Considerable progress has been made in manipulating the Ti plasmid of Agrobacterium tumefaciens as a vehicle for delivery of foreign genes into protoplasts of dicotyle-donous plants. Part of the Ti plasmid, the T-DNA, can be incorporated into the genome of the host cell; the T-DNA can carry a foreign DNA sequence which co-integrates with it; under normal conditions, the tumorigenic-causing portion of the T-DNA can be inactivated so that transformed protoplasts can be regenerated and T-DNA with an inserted foreign gene can be stably maintained during regeneration, meiosis and gamete formation. A foreign gene has yet to be expressed in regenerated plants although a T-DNA gene for opine synthesis can function in regenerates. Developing a more ubiquitous transformation system for monocotyledons is further from fruition. Based on transformation systems for simple eukaryotic organisms, it is reasonable to expect that a DNA vector which is capable of amplifying a novel plant gene and which contains both a drug resistance marker to facilitate the selection of transformed plant protoplasts and a species-specific autonomously replicating sequence to ensure the stable maintenance of the input gene in the recipient cell can be constructed.