Recombination in the plant genome and its application in biotechnology

Genetic variation within and between species is based on recombination of D NA molecules. Recombination also plays a very important role in the repair of damaged DNA. Clarity about the mechanism by which recombination occurs i s of profound interest not only to understand how this process assures the maintenance of genome integrity and at the same time is the driving force o f evolution, but also for its application in biotechnology. The isolation o f genes involved in recombination and the elucidation of the role of many o f the corresponding gene products in Escherichia coli and Saccharomyces cer evisiae has formed the basis for comparative analysis in other, more comple x eukaryotic systems. The identification of homologous genes from different organisms, including plants, suggests a conservation of the general mechan isms of recombination. Transgenes introduced in an organism may be incorporated in the genome by e ither homologous or nonhomologous recombination (end joining). The preferre d pathway differs strongly between organisms. In plants there is a preferen ce for random integration of the introduced DNA by nonhomologous recombinat ion, which might lead to the accidental inactivation of important genes and to variable and unpredictable expression of the transgene itself. Therefor e, there is an urgent need for the development and improvement of technique s for the directed integration of transgenes at specific locations in the g enome. The integration of transgenes by homologous recombination would allo w specific modification or disruption of endogenous genes, providing a tool for more detailed analysis of gene function. In combination with the recen t introduction of site-specific recombination systems from E. coil or yeast into plants, this may lead to the development of versatile systems for mod ification of the plant genome.