Homologous recombination in planta is stimulated in the absence of Rad50

Chromosomal double-strand DNA breaks must be repaired; in the absence of re pair the resulting acentromeric (and telomere-less) fragments may be lost a nd/or the broken DNA ends may recombine causing general chromosomal instabi lity. The Rad50/Mre11/Xrs2 protein complex acts at DNA ends and is implicat ed in both homologous and non-homologous recombination. We have isolated a rad50 mutant of the plant Arabidopsis thaliana and show here that it has a somatic hyper-recombination phenotype in planta. This finding supports the hypothesis of a competition between homologous and illegitimate recombinati on in higher eukaryotes. To our knowledge, this is the first direct in vivo support for the role of this complex in chromosomal recombination in a mul ticellular organism and the first description of a mutation of a known gene leading to hyper-recombination in plants.