Non-oncogenic plant vectors for use in the agrobacterium binary system

Initiation of Plant Virus Infections by Agroinfiltration of Infectious Viral cDNAs (icDNAs)

Agroinfiltration uses Agrobacterium to deliver T-DNA-based gene expression constructs into plants. This chapter focuses on the standard method, specifically from the perspective of plant virus research, and describes a protocol for the initiation of virus infections in plants via infiltration of Agrobacterium strains carrying infectious viral cDNAs (icDNAs). The method outlines the culture and preparation of Agrobacterium for infiltration, the infiltration procedure, optimization of the optical density of the Agrobacterium suspension, and sampling of infected plants post-agroinfiltration. The advantages of the agroinfiltration method compared to traditional mechanical inoculation using sap from infected plants are discussed. The protocol is applicable for different pathosystems, although case-specific optimization of infiltration parameters and sampling is recommended.

Secretion of biologically active human interleukin-2 and interleukin-4 from genetically modified tobacco cells in suspension culture

Biologically active human interleukin-2 (IL-2) and IL-4, key lymphokines involved in immune regulation, were produced and secreted into the medium by genetically modified Nicotiana tabacum cells grown in suspension culture. Secretion through the plasma membrane and cell wall into the medium was facilitated by the natural mammalian leader sequences. IL-2 and IL-4 were detected in the medium at concentrations of 0.10 and 0.18 microgram/mL, respectively, although higher levels were detected within the lymphokine-producing cells (approximately 0.80 microgram/mL for IL-2 and approximately 0.28 microgram/mL for IL-4). By Western blot, IL-4 was found to be secreted as two small polypeptides with molecular masses of approximately 18-20 kDa. The biological activity of IL-2 was determined by cell proliferation of the IL-2-dependent murine CTLL-2 cell line, while that of IL-4 was determined by cell proliferation of the CTLL-2 cell line [CT.h4S] which was stably transfected with the human IL-4 receptor. These findings indicate that plant suspension culture can be used to produce and secrete into the medium a variety of biologically active mammalian proteins that are of clinical and diagnostic relevance.

Light-regulated expression of the Arabidopsis thaliana ferredoxin gene requires sequences upstream and downstream of the transcription initiation site

The effect of light on the expression of the Arabidopsis thaliana ferredoxin gene (fedA) was studied in mature tobacco plants. In light-treated leaves of tobacco plants transformed with a full-length ferredoxin gene, fedA-specific mRNA levels were more than twenty fold higher than in dark-treated controls. This indicates that all components for regulation of the Arabidopsis ferredoxin gene are present in tobacco. To identify light-regulatory elements in the fedA gene, we have tested a set of chimeric genes containing various parts of the fedA gene for light-dependent expression in mature tobacco plants. A fedA promoter-GUS fusion gene was not light-responsive, indicating that the 5'-upstream promoter region is not sufficient for light regulation. Fusion genes in which different transcribed regions of the fedA gene were expressed from the CaMV 35S promoter showed only limited light regulation, if any at all. This indicates that, like the fedA upstream region, the region downstream of the transcription start site is also not sufficient for full light regulation. The combined results suggest that for full light-regulated expression of the fedA gene, both the promoter region and sequences downstream of the transcription start site are required.

Light-regulated expression of the Arabidopsis thaliana ferredoxin A gene involves both transcriptional and post-transcriptional processes

Ferredoxin is part of the photosynthetic apparatus of the chloroplast and is encoded in the nucleus. In Arabidopsis thaliana expression of the ferredoxin A gene is influenced by both the presence of chloroplasts and light. Tobacco plants transformed with a ferredoxin promoter-GUS fusion gene showed a tissue-specific and light-dependent expression pattern. The effect of light on the expression of the fusion gene in transgenic seedlings was only two- to fourfold, which is less pronounced than the 20-fold effect in Arabidopsis itself. Run-on transcription assays with nuclei isolated from Arabidopsis revealed a twofold modulation of transcriptional activity of the ferredoxin A gene under the influence of light. These results suggest the involvement of post-transcriptional processes in light-regulated gene expression. A ferredoxin promoter deletion series ranging from -1205 to -143 was studied. All but the smallest deletion construct (at position -143 relative to the translation start site) showed comparable expression levels in mature leaves, suggesting the presence of a positive regulating element between -269 and -143. The same pattern of tissue specificity was found in all promoter deletions studied. Expression of the fusion genes is high in all chloroplast-containing cells: mesophyll, chlorenchyma, paravascular tissue, epidermal and stomatal guard cells and trichomes. Transgenic seedlings treated with norflurazon, which blocks the development of green chloroplasts, showed a two- to fourfold reduction in GUS expression for all constructs. In Arabidopsis seedlings the effect of norflurazon on the expression of the ferredoxin A was eightfold. This again can be explained by the need for post-transcriptional processes of the regulated gene expression of Arabidopsis ferredoxin A.

Plant endogenous beta-glucuronidase activity: how to avoid interference with the use of the E. coli beta-glucuronidase as a reporter gene in transgenic plants

We have detected a plant beta-glucuronidase activity, present in several tissues and organs of plant species belonging to different families. The fluorimetric beta-glucuronidase assay was used to partially characterize this activity in post-ribosomal supernatants of tobacco leaves. The tobacco activity is very stable at low temperatures, but quickly inactivated above 45 degrees C. It is relatively resistant to proteases and insensitive to -SH group reagents and to ionic conditions. It does not require, nor is it inhibited by, divalent cations. Although these properties are shared by the Escherichia coli beta-glucuronidase, the two activities can be distinguished by: (i) their different sensitivity to the specific inhibitor saccharic acid-1,4-lactone; (ii) their different thermal stability (iii) their different pH optima (5.0 for the plant activity and close to neutral for the bacterial enzyme). Therefore, under appropriate experimental conditions, it should be possible to assay the E. coli beta-glucuronidase in transgenic plants without interference from the endogenous plant activity.

Tissue-specific expression directed by an Arabidopsis thaliana pre-ferredoxin promoter in transgenic tobacco plants

We have isolated and analyzed a pre-ferredoxin gene from Arabidopsis thaliana. This gene encodes a 148 amino acid precursor protein including a chloroplast transit peptide of 52 residues. Southern analysis shows the presence of a single copy of this ferredoxin (Fd) gene in the A. thaliana genome. Its expression is tissue-specific and positively affected by light. Response times, both to dark and light conditions, are remarkably rapid. A chimeric gene consisting of a 1.2 kb Fd promoter fragment fused to the beta-glucuronidase reporter gene was transferred to tobacco. This fusion gene is expressed in a tissue-specific way; it shows high levels of expression in green leaves, as compared to root tissue.

The Agrobacterium tumefaciens T-DNA gene 6(b) is an onc gene

In this article it is shown that the T-DNA of Agrobacterium tumefaciens contains besides the well-known cyt and aux genes another gene with an oncogenic effect in plants. The gene in question is called 6(b) and causes the formation of small tumors in plant species such as Nicotiana glauca and Kalanchoe tubiflora.

Expression and inheritance of kanamycin resistance in a large number of transgenic petunias generated by Agrobacterium-mediated transformation

One hundred and four kanamycin-resistant Petunia "Mitchell" plants were regenerated from leaf discs cocultivated with Agrobacterium tumefaciens strain LBA4404 containing a binary vector pCGN200. Selection for kanamycin resistance was applied during plant regeneration at the initiation of both shoots and roots. The regenerated plants were analysed for expression and inheritance of their kanamycin resistance phenotype. Approximately half of the plants showed normal Mendelian inheritance for one or two kanamycin resistance genes. In one case, the two copies were inserted at closely linked sites on homologous chromosomes, and gave <0.05% kanamycin-sensitive progeny on backcrosses. Six plants had inheritance patterns suggesting that the kanamycin gene had inserted into an essential region of DNA. Forty-five plants showed lower than expected transmission of kanamycin resistance, which was associated with low expression of the resistance phenotype in most cases. Ten plants produced segregation ratios that are not readily interpreted by Mendelian inheritance.

Functional analysis of the Agrobacterium tumefaciens octopine Ti-plasmid left and right T-region border fragments

Border fragments of the octopine Ti-plasmid were tested for their ability to restore tumorigenicity of an avirulent mutant carrying a deleted right border. It was found that neither introduction of left border fragments nor that of small right border fragments at the position of the deletion resulted in a complete restoration of oncogenicity. However, insertion of a larger right border fragment in the deletion mutant gave fully oncogenic strains. In the latter case sequences to the right side of the right border repeat were found to be responsible for a complete restoration of oncogenicity. Also a left border repeat inserted together with this enhancer sequence fully restored the oncogenicity of the deletion mutant. The enhancer-sequence on itself was not able to mediate the transfer of the T-region to the plant cell. Border fragments inserted in inverted orientation in the deletion mutant were able to mediate the transfer of the T-region to the plant cell, but at a reduced frequency.

A set of novel Ti plasmid-derived vectors for the production of transgenic plants

We describe in this paper the construction and use of a set of novel Ti plasmid-derived vectors that can be used to produce transgenic plants. These vectors are based on one of two strategies: 1) double recombination into the wild-type Ti plasmid of genetic information flanked by two T-DNA fragments on a wide-host range plasmid; 2) the binary vector strategy. The vector based on the double recombination principle contains a kanamycin resistance gene for use as a plant selectable marker, a polylinker for the insertion of foreign genes, and a nopaline synthase gene. The vector was constructed such that a disarmed T-DNA results from the double recombination event. The binary vector combines several advantageous features including an origin of replication that is stable in Agrobacterium in the absence of selection, six unique sites for insertion of foreign genes, an intact nopaline synthase gene, and a kanamycin resistance marker for selection of transformed plant cells. All of these vectors have been used to produce tobacco plants transformed with a variety of foreign genes.

Clones from a shooty tobacco crown gall tumor I: deletions, rearrangements and amplifications resulting in irregular T-DNA structures and organizations

Transformed clones from a shooty tobacco crown gall tumor, induced byAgrobacterium tumefaciens strain LBA1501, having a Tn1831 insertion in the auxin locus, were investigated for their T-DNA structure and expression. In addition to clones with the expected phenotype, i.e. phytohormone autonomy, regeneration of non-rooting shoots and octopine synthesis (Aut(+)Reg(+)Ocs(+) 'type I' clones), clones were obtained with an aberrant phenotype. Among these were the Aut(-)Reg(-)Ocs(+) 'type II' clones. Two shooty type I clones and three type II callus clones (all randomly chosen) as well as a rooting shoot regenerated from a type II clone via a high kinetin treatment, all had a T-DNA structure which differed significantly from 'regular' T-DNA structures. No Tn1831 DNA sequences were detected in these clones. The two type I clones were identical: they both contained the same highly truncated T-DNA segments. One TL-DNA segment of approximately 0.7 kb, originating form the left part of the TL-region, was present at one copy per diploid tobacco genome. Another segment with a maximum size of about 7 kb was derived from the right hand part of the TL-region and was present at minimally two copies. Three copies of a truncated TR-DNA segment were detected, probably starting at the right TR-DNA border repeat and ending halfway the regular TR-region. Indications have been obtained that at least some of the T-DNA segments are closely linked, sometimes via intervening plant DNA sequences. The type I clones harbored TL-DNA transcripts 4, 6a/b and 3 as well as TR-DNA transcript 0'. The type II clones harbored three to six highly truncated T-DNA segments, originating from the right part of the TL-region. In addition they had TR-DNA segments, similar to those of the type I clones. On Northern blots TR-DNA transcripts 0' and 1' were detected as well as the TL-DNA transcripts 3 and 6a/b and an 1800 bp hybrid transcript (tr.Y) containing gene 6b sequences. Possible origins of the observed irregularities in T-DNA structures are discussed in relation to fidelity of transformation of plant cells viaAgrobacterium.

Bleomycin resistance: a new dominant selectable marker for plant cell transformation

Plant cells are sensitive to the antibiotic bleomycin, a DNA damaging glycopeptide. A bleomycin resistance determinant, located on transposon Tn5 and functional in bacteria, has been cloned in a plant expression vector and introduced into Nicotiana plumbaginifolia using Agrobacterium tumefaciens. The expression of this determinant in plant cells confers resistance to bleomycin and allows selection of transformed plant cells.