Co–Transformation of Unlinked Foreign Genes into Plants by Direct Gene Transfer

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.

A super twin T-DNA vector that allows independent gene expression during Agrobacterium-mediated transformation

In this study, we designed and constructed a super twin T-DNA vector (pTRIDT313-g) containing two independent T-DNA cassettes-one for the selection gene Hyg and the other for the target gene Gus-to produce marker-free transgenic lines. The resulting vector was transformed into tobacco, and polymerase chain reaction (PCR) analysis showed four types of gene combinations in the T1 and T2 generations: Gus only, Hyg only, Gus+Hyg, and untransformed lines. The intermediate region from the T-DNA of the right border of Hyg to the left border of Gus in the Hyg and Gus lines was not amplified. Genome walking confirmed that the Hyg and Gus T-DNA cassettes were independently inserted in different regions of the tobacco genome. Thus, the two T-DNA cassettes were integrated randomly as independent loci into the tobacco genome. The results of reverse transcription-PCR indicated that Hyg could normally be expressed in the roots, stems, and leaves of transgenic lines, and the resistance test showed that all Hyg transgenic lines could grow in the presence of 50mg/L hygromycin. All Gus transgenic lines showed obvious blue coloration in enzyme activity tests, indicating that the Gus gene could be normally expressed in all the lines. Therefore, the super twin T-DNA vector (pTRIDT313-g) exhibits independent integration, heredity, and normal gene function from two T-DNA cassettes. This vector could be a useful and valuable tool in the production of marker-free transgenic lines.

From the concept of totipotency to biofortified cereals

I was a college teacher when opportunity opened a path into academia. A fascination with totipotency channeled me into research on tissue culture. As I was more interested in contributions to food security than in scientific novelty, I turned my attention to the development of genetic modification technology for cereals. From my cell culture experience, I had reasons not to trust Agrobacterium for that purpose, and I developed direct gene transfer instead. In the early 1990s, I became aware of the problem of micronutrient deficiency, particularly vitamin A deficiency in rice-eating populations. Golden Rice, which contains increased amounts of provitamin A, was probably instrumental for the concept of biofortification to take off. I realized that this rice would remain an academic exercise if product development and product registration were not addressed, and this is what I focused on after my retirement. Although progress is slowly being made, had I known what this pursuit would entail, perhaps I would not have started. Hopefully Golden Rice will reach the needy during my lifetime.

Techniques in plant molecular biology--progress and problems

Progress in plant molecular biology has been dependent on efficient methods of introducing foreign DNA into plant cells. Gene transfer into plant cells can be achieved by either direct uptake of DNA or the natural process of gene transfer carried out by the soil bacterium Agrobacterium. Versatile gene-transfer vectors have been developed for use with Agrobacterium and more recently vectors based on the genomes of plant viruses have become available. Using this technology the expression of foreign DNA, the functional analysis of plant DNA sequences, the investigation of the mechanism of viral DNA replication and cell to cell spread, as well as the study of transposition, can be carried out. In addition, the versatility of the gene-transfer vectors is such that they may be used to isolate genes not amenable to isolation using conventional protocols. This review concentrates on these aspects of plant molecular biology and discusses the limitations of the experimental systems that are currently available.

A rapid screen for the detection of specific DNA sequences in plants

We have developed a simple and quick method ("wick blot") for detecting the presence of specific DNA sequences in plants, using radiolabeled DNA probes. The method requires only small amounts of tissue, about 15-25 mg. More than a hundred samples per day can be easily extracted and blotted. It works well on various species and tissues, including leaves, embryos, and callus. The method is ideally suited for screening large numbers of putative transformants, especially populations that have not been screened by prior selection.