Abstract
The design of vectors for plant transformation has recently progressed to the development of intragenic systems. This involves identifying plant-derived DNA sequences similar to important vector components. The most useful approach involves adjoining two fragments from plant genomes to form sequences that have the functional equivalence of vector elements such as: T-DNA borders for Agrobacterium-mediated transformation, bacterial origins of replication and bacterial selectable elements. Such DNA fragments have been identified from a wide range of plant species, suggesting that intragenic vectors can be constructed from the genome of any plant species. Intragenic vectors provide a mechanism for the well-defined genetic improvement of plants with the entire DNA destined for transfer originating from within the gene pool already available to plant breeders. In this manner, genes can be introgressed into elite cultivars in a single step without linkage drag and, most importantly, without the incorporation of foreign DNA. The resulting genetically modified (GM) plants are non-transgenic, although they are derived using the tools of molecular biology and plant transformation. The genetic makeup of the resulting plants is equivalent to minor rearrangements or micro-translocations that could theoretically arise through natural or induced rearrangements of the endogenous genome. Given the public concerns over the deployment of GM crops in agriculture, especially for food crops, intragenic vector systems offer a more socially acceptable and responsible way forward for genetic engineering. It may especially help us to alleviate some of the ethical issues associated with the transfer of DNA across wide taxonomic boundaries. Plants derived from the use of intragenic vectors raise important issues concerning the definition, regulation and testing of GM plants. This review critically evaluates the progress toward the development and use of intragenic vectors and the implications of their use for the genetic improvement of crops.