The possibility to generate precise modifications in a genome by gene targeting provides the ultimate genetic tool for functional studies and account for the success of yeast or mouse as model systems. Yet this methodology is not available in most plant and animal systems in which transgenes essentially integrate at random locations. This situation affects the predictability of genetic modifications and dramatically impedes gene therapeutic approaches. Numerous studies conducted in yeasts and animal cells strongly suggest that the ratio of targeted to random integration observed upon transgenesis reflects the relative activity of to distinct DNA double strand break (DSB) repair pathways in the cell: random integration being associated with the non homologous end-joining (NHEJ) pathway and targeted insertion with the homologous recombination (HR) pathway. The molecular mechanisms of both pathways are well described and highly conserved among eukaryotes, but the factors that control the balance between these two pathways are not fully understood, especially upon transgenesis.The moss Physcomitrella patens is the only plant proficient for gene targeting. The complete sequence of its genome has been established recently and this moss is being more and more recognised as a model "green yeast" in plant biology. A collection of P.patens deletion mutants in distinct DNA repair pathways has been generated ans is being characterised in collaboration with the INRA in Versailles. In this project, we shall combine molecular, cellular, biochemical and genetic experimental approaches to identify among the core genes of the DNA DSB repair pathways of P.patens the ones that are specifically involved in GT. The information acquired in this project will provide insight into the relative function of these genes during DSB repair and upon GT in P.patens. Genes identified as displaying a specific action on GT will be introduced in Arabidopsis to test if their overexpression can improve GT efficiencies in other plants.