In plants three different approaches have been used to study homologous DNA recombination; extrachromosomal recombination (ECR) between transfected DNA molecules, intrachromosomal recombination (ICR) between repeated genes integrated into and resident at the genome and recombination between introduced DNA and homologous sequences in the genome (gene targeting). ECR is efficient (10(-1) to 10(-3)) and occurs mainly during a limited time period early after transfection. It proceeds predominantly via nonconservative single-strand annealing. ICR, which in most cases is described best by the double-strand break repair model of recombination, occurs at frequencies of one event in 10(5) to 10(7) cells. ICR takes place throughout the whole life-cycle of a plant, in all organs and at different developmental stages. As there exists no predetermined germline in plants, somatic recombination events can be transferred to the next generation. Recombination frequencies are enhanced by DNA damage. Gene targeting, like ICR, occurs at low rates in plant cells. Almost nothing is known about the enzymes involved in homologous recombination in plants.
