Green light for gene targeting in plants.

P recision engineering of eukaryotic organisms requires efficient techniques for homology-based replacement of an endogenous gene by an introduced gene, a process termed gene targeting (GT). Because foreign DNA is preferentially integrated by nonhomologous recombination in plants and animals, special strategies are needed to increase the frequency and facilitate the detection of GT. Developing these strategies has proved particularly difficult in plants, for which GT efficiencies are on the order of 10 3 to 10 4 targeted events per transformed plant (1, 2). This low frequency seems to be the result of a natural barrier to integration of homologous sequences in higher plants. By contrast, efficient GT has been demonstrated in lower plants, such as the moss Physcomitrella patens (3). The time appears ripe for a breakthrough in this field, and one has now been reported in this issue of PNAS. Avraham Levy and his group (4) describe a jump of between one and two orders of magnitude in GT frequency in Arabidopsis thaliana plants overexpressing the yeast chromatin remodeling protein Rad54. Their innovative approach was facilitated by the development of an ingenious assay that uses fluorescent seeds to identify targeted insertions.