The role of double-strand break-induced allelic homologous recombination in somatic plant cells.

During meiosis, homologous recombination occurs between allelic sequences. To evaluate the biological significance of such a pathway in somatic cells, we used transgenic tobacco plants with a restriction site for the rare cutting endonuclease I-SceI within a negative selectable marker gene. These plants were crossed with two tobacco lines containing, in allelic position, either a deletion or an insertion within the marker gene that rendered both marker gene and restriction site inactive. After the double-strand break induction, we selected for repair events resulting in a loss of marker gene function. This loss was mostly due to deletions. We were also able to detect double strand break-induced allelic recombination in which the break was repaired by a faithful copying process from the homologue carrying the shortened transgene. The estimated frequency indicates that homologous recombination in somatic cells between allelic sites appears to occur at the same order of magnitude as between ectopic sites, and is thus far too infrequent to act as major repair pathway. As somatic changes can be transferred to the germ line, the prevalence of intrachromatid rearrangements over allelic recombination might be an indirect prerequisite for the enhanced genome plasticity postulated for plants.