HOMOLOGOUS RECOMBINATION IN PLANTS

In plants three different approaches have been used to study homologou s DNA recombination; extrachromosomal recombination (ECR) between tran sfected DNA molecules, intrachromosomal recombination (ICR) between re peated genes integrated into and resident at the genome and recombinat ion between introduced DNA and homologous sequences in the genome (gen e targeting). ECR is efficient (10(-1) to 10(-3)) and occurs mainly du ring a limited time period early after transfection. It proceeds predo minantly via nonconservative single-strand annealing. ICR, which in mo st 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) ce lls. ICR takes place throughout the whole life-cycle of a plant, in al l organs and at different developmental stages. As there exists no pre determined germline in plants, somatic recombination events can be tra nsferred to the next generation. Recombination frequencies are enhance d by DNA damage. Gene targeting, like ICR, occurs at low rates in plan t cells. Almost nothing is known about the enzymes involved in homolog ous recombination in plants.