Prospects for the precise engineering of plant genomes by homologous recombination

The targeting of chromosomal genes via homologous recombination (HR) is an essential tool of reverse genetics as applied for the functional assay of g enes within complex genomes. However, in higher plants, foreign DNA integra tes almost exclusively at random, non-homologous sites. A variety of enviro nmental parameters known to influence levels of HR do not increase targetin g frequencies when combined in gene-targeting experiments. The identificati on of cellular factors that may control the level of chromosomal HR in plan t somatic cells is required. Plant genes encoding proteins similar to those involved in HR in other organisms can be found in the expanding sequence d atabases. Evidence for evolutionary conservation should help to decipher me chanisms of plant HR and possibly detect limiting factors. At present, howe ver, only one genetic locus influencing levels of chromosomal recombination in plants has been well defined. Here we summarise current knowledge of HR acid the status of gene targeting (GT) in plants, focusing on genetic appr oaches to molecular factors regulating HR levels.