Live-cell chromosome dynamics in Arabidopsis thaliana reveals increased chromatin mobility in response to DNA damage

Double-strand breaks (DSBs) are a particularly deleterious type of DNA damage potentially leading to translocations and genome instability. Homologous recombination (HR) is a conservative repair pathway in which intact homologous sequences are used as a template for repair. How damaged DNA molecules search for homologous sequences in the crowded space of the cell nucleus is, however, still poorly understood, especially in plants. Here, we measured global chromosome and DSB site mobility, in Arabidopsis thaliana, by tracking the motion of specific loci using the lacO/LacI tagging system and two GFP-tagged HR regulators, RAD51 and RAD54. We observed an increase in chromatin mobility upon the induction of DNA damage, specifically at the S/G2 phases of the cell cycle. Importantly, this increase in mobility was lost on sog1-1 mutant, a central transcription factor of the DNA damage response (DDR), indicating that repair mechanisms actively regulate chromatin mobility upon DNA damage. Interestingly, we observed that DSB sites show remarkably high mobility levels at the early HR stage. Subsequently, a drastic decrease of DSB mobility is observed, which seems to be associated to the relocation of DSBs to the nucleus periphery. Altogether, our data suggest that changes in chromatin mobility are triggered in response to DNA damage, and that this may act as a mechanism to enhance the physical search within the nuclear space to locate a homologous template during homology-directed DNA repair.