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Summary   Arabidopsis thaliana  mutants in FAS1 and FAS2 subunits of chromatin assembly factor 1 ( CAF 1) show progressive loss of 45S  rDNA  copies and telomeres. We hypothesized that homology‐dependent DNA damage repair (HDR) may contribute to the loss of these repeats in  fas  mutants. To test this, we generated double mutants by crossing  fas  mutants with knock‐out mutants in  RAD51B , one of the Rad51 paralogs of  A .  thaliana . Our results show that the absence of RAD51B decreases the rate of  rDNA  loss, confirming the implication of RAD51B‐dependent recombination in  rDNA  loss in the CAF1 mutants. Interestingly, this effect is not observed for telomeric repeat loss, which thus differs from that acting in  rDNA  loss. Involvement of DNA damage repair in  rDNA  dynamics in  fas  mutants is further supported by accumulation of double‐stranded breaks (measured as γ‐H2AX foci) in 45S  rDNA . Occurrence of the foci is not specific for S‐phase, and is ATM‐independent. While the foci in  fas  mutants occur both in the transcribed (intranucleolar) and non‐transcribed (nucleoplasmic) fraction of  rDNA , double  fas rad51b  mutants show a specific increase in the number of the intranucleolar foci. These results suggest that the repair of double‐stranded breaks present in the transcribed  rDNA  region is RAD51B dependent and that this contributes to  rDNA  repeat loss in  fas  mutants, presumably via the single‐stranded annealing recombination pathway. Our results also highlight the importance of proper chromatin assembly in the maintenance of genome stability.

Homology‐dependent repair is involved in 45 S rDNA loss in plant CAF ‐1 mutants