Currently the term “transgenic organism” is used when referring to an organism which harbors additional genetic information as the result of a genetic engineering step, namely the transfer of purified or cloned DNA. For plants there are two very different approaches for obtaining such DNA transfer. First there is the so called “natural” way of DNA transfer. This method exploits the conjugation-like DNA transfer which can occur when some soil bacteria such as Agrobacterium tumefaciens colonize plants (for recent reviews, see Zambryski, 1988; Gheysen et al.,1989). Many gene vectors have been constructed based on this transfer mechanism and these have allowed the engineering of the first transgenic plants expressing selectable marker genes (Herrera-Estrella et al., 1983). Agrobacterium-mediated gene transfer has also been the method of choice for introducing new economically important traits into plants such as insect resistance (Vaeck et al., 1987), virus resistance (Abel et al., 1986; Nelson et al., 1988) and also for constructing plants with engineered seed proteins which can be the starting material for producing peptides of importance to mammalian physiology (Vandekerckhove et al., 1989). An appreciated advantage of the Agrobacterium system is the fact that the majority of the transformed plants obtained after selection harbour one or two copies of a well defined DNA sequence. However, several important crops such as most leguminous plants and all of the Graminae remain recalcitrant to this type of DNA transfer. Some results have been obtained with such plants by employing the other DNA transfer system which is equivalent to the in vitro DNA uptake methods used with other organisms. To introduce the DNA, polyethyleneglycol (PEG), electroporation or micro injection can be used, but the recipient cell has to be a protoplast capable of regenerating (Lazzeri and Lörz, 1988; Gasser and Fraley, 1989). This severely limits the usefulness of this approach. Nevertheless the method has allowed a breakthrough in the transformation of rice (Shimamoto et al., 1989). Recently promising results have been obtained with a spectacular new mechanical method, the particle gun (Klein et al., 1987; McCabe et al., 1988; Sanford, 1989). This “ballistic” approach can probably be used with any plant species. It should allow the transformation of meristematic cells, hence enhancing the chance of obtaining transgenic plants from those species or cultivars which as yet cannot be taken through a cell culture step.
KeywordsSugar Toxicity Sucrose Fermentation Manganese
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