Repair of genomic double-strand breaks in somatic plant cells by one-sided invasion of homologous sequences

Double-strand breaks (DSB) strongly enhance homologous recombination in the genome of somatic plant cells. To characterize the mechanism of homologous DSB repair in detail, Agrobacterium-mediated targeting experiments were performed in tobacco, with repair constructs being homologous to either one or both ends of a transiently induced DSB. The recombination frequencies obtained were at least two orders of magnitude higher than without DSB induction and differed between the two T-DNA by only one-third. Thus, occurrence of homologies at both ends of the break is no prerequisite for recombination. The results demonstrate that most, if not all, reactions are initiated by one-sided invasion of a homologous sequence. This indicates that the classical double-strand break repair (DSBR) model originally proposed for meiotic recombination might be less appropriate for the description of homologous DSB repair in somatic plant cells than an integrative model based on a combination of the synthesis-dependent strand annealing (SDSA) model and of a modified version of the one-sided invasion (OSI) model of recombination. Direct consequences of this finding for the performance of gene targeting experiments, as well as possible implications for the stability of highly repetitive plant genomes, are discussed.