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

Generation of Marker- and/or Backbone-Free Transgenic Wheat Plants via Agrobacterium-Mediated Transformation

Horizontal transfer of antibiotic resistance genes to animals and vertical transfer of herbicide resistance genes to the weedy relatives are perceived as major biosafety concerns in genetically modified (GM) crops. In this study, five novel vectors which used gusA and bar as a reporter gene and a selection marker gene, respectively, were constructed based on the pCLEAN dual binary vector system. Among these vectors, 1G7B and 5G7B carried two T-DNAs located on two respective plasmids with 5G7B possessing an additional virGwt gene. 5LBTG154 and 5TGTB154 carried two T-DNAs in the target plasmid with either one or double right borders, and 5BTG154 carried the selectable marker gene on the backbone outside of the T-DNA left border in the target plasmid. In addition, 5BTG154, 5LBTG154, and 5TGTB154 used pAL154 as a helper plasmid which contains Komari fragment to facilitate transformation. These five dual binary vector combinations were transformed into Agrobacterium strain AGL1 and used to transform durum wheat cv Stewart 63. Evaluation of the co-transformation efficiencies, the frequencies of marker-free transgenic plants, and integration of backbone sequences in the obtained transgenic lines indicated that two vectors (5G7B and 5TGTB154) were more efficient in generating marker-free transgenic wheat plants with no or minimal integration of backbone sequences in the wheat genome. The vector series developed in this study for generation of marker- and/or backbone-free transgenic wheat plants via Agrobacterium-mediated transformation will be useful to facilitate the creation of "clean" GM wheat containing only the foreign genes of agronomic importance.

Insights into recognition of the T-DNA border repeats as termination sites for T-strand synthesis by Agrobacterium tumefaciens

The recognition of the T-DNA left border (LB) repeat is affected by its surrounding sequences. Here, the LB regions were further characterized by molecular analysis of transgenic plants, obtained after Agrobacterium tumefaciens-mediated transformation with T-DNA vectors that had been modified in this LB region. At least the 24-bp LB repeat by itself was insufficient to terminate the T-strand synthesis. Addition of the natural inner and/or outer border regions to at least the LB repeat, even when present at a distance, enhanced the correct recognition of the LB repeat, reducing the number of plants containing vector backbone sequences. In tandem occurrence of both the octopine and nopaline LB regions with their repeats terminated the T-strand synthesis most efficiently at the LB, yielding a reproducibly high number of plants containing only the T-DNA. Furthermore, T-strand synthesis did not terminate efficiently at the right border (RB) repeat, which might indicate that signals in the outer RB region inhibit the termination of T-strand synthesis at the RB repeat.

Transfer of non-T-DNA portions of the Agrobacterium tumefaciens Ti plasmid pTiA6 from the left terminus of TL-DNA

We introduced a plant selection marker, nptII, to the left of border A in the Agrobacterium Ti plasmid pTiA6. Infection of tobacco leaf discs with the modified Agrobacterium strain gave rise to kanamycin-resistant calli which grew in a hormone-dependent manner. Southern hybridization analysis of DNA isolated from four transformants indicated initiation of DNA transfer at or near border A and absence of T-DNA sequences. These results demonstrate that DNA transfer events starting at a left border on a native Ti plasmid and moving away from the T-DNA region occur and that they can be detected by designing a suitable selection strategy.

Analysis of TR-DNA/plant junctions in the genome of a Convolvulus arvensis clone transformed by Agrobacterium rhizogenes strain A4

A Charon 4A phage library, containing insert DNA isolated from a morning glory (Convolvulus arvensis) plant genetically transformed by Ri T-DNA from Agrobacterium rhizogenes strain A4, was used to isolate a lambda clone that contains part of the Ri TL-DNA and the complete TR-DNA. The two Ri T-DNAs were recovered adjacent to each other in a tail-to-tail configuration (i.e. with the TR-DNA inverted with respect to the TL-DNA). Comparison of nucleotide sequences from this lambda clone with the corresponding sequences from the Ri plasmid allowed us to determine the location of the T-DNA/plant junction for the right end of the TL-DNA and the left and right ends of the TR-DNA. We located, near each of these borders, a 24 bp sequence that is similar to the 24 bp consensus sequence found near the pTi T-DNA extremities. In addition, sequences similar to the "core" overdrive sequence from pTi are located near each right border. Hybridization and nucleotide sequence analysis of the DNA adjacent to the TL/TR junction shows that no plant DNA is located between the TL and TR-DNAs and suggests that the plant DNA adjacent to the end of the TR-DNA may have been rearranged during the integration into the plant genome.

Function of heterologous and pseudo border repeats in T region transfer via the octopine virulence system of Agrobacterium tumefaciens

The successful transfer of the Ti plasmid T region to the plant cell is mediated by its 24 bp border repeats. Processing of the T-region prior to transfer to the plant cell is started at the right border repeat and is stimulated by a transfer enhancer sequence called "overdrive". Left and right border repeats differ somewhat in nucleotide sequence; moreover, the repeats of different Ti and Ri plasmids are slightly different. Our data indicate that these differences do not have a significant influence on border activity. However, the overdrive sequence is essential for the efficient transfer of a T region via an octopine transfer system. Our data suggest that an overdrive sequence must also be present next to the right border repeats of the nopaline Ti plasmid and the agropine of octopine and nopaline Ti plasmids express some differences in T-DNA processing activities. of cotopine and nopaline Ti plasmids express some differences in T-DNA processing activities.Furthermore, we demonstrate that certain pseudo border repeats, sequences that resemble the native 24 bp border repeat and naturally occur within the octopine Ti plasmid T-region, are able to mediate T region transfer to the plant cell, albeit with much reduced efficiency as compared to wild-type border repeats.

Right-border sequences enable the left border of an Agrobacterium tumefaciens nopaline Ti-plasmid to produce single-stranded DNA

The T-region of nopaline-type Ti-plasmids (the portion of the plasmid that is transferred to plant cells) of Agrobacterium tumefaciens is delimited by 23-25 bp direct repeats. They are nicked by the products of the virD locus and the presence of these nicked sites is correlated with the synthesis of single-stranded T-region copies. Despite previous indications to the contrary, we show that the pTiT37 T-region left border is capable of producing single-stranded DNA with high efficiency and that its ability to do so is totally dependent on right border-proximal cis-acting sequences, most probably overdrive, located several kilobases from the border. The absence of overdrive does not affect the single-strand nicking activity of the virD product but only the production of single-stranded copies from the nicked substrate.

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

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