The study estimates the spontaneous mutation rate and spectrum in *Arabidopsis thaliana* by analyzing five mutation accumulation lines that had been maintained for 30 generations. The researchers identified 99 base substitutions and 17 insertions and deletions, yielding a spontaneous mutation rate of 7 × 10−9 base substitutions per site per generation, primarily G:C→A:T transitions. They attribute this biased spectrum to deamination of methylated cytosines and ultraviolet light-induced mutagenesis. The mutation rate is close to the lower bound estimated from the divergence between monocots and dicots, and the rate of mutations affecting fitness is not significantly different from previous estimates. The study also found a higher mutation rate in pericentromeric regions, which may contribute to the higher polymorphism levels near centromeres in *A. thaliana*.The study estimates the spontaneous mutation rate and spectrum in *Arabidopsis thaliana* by analyzing five mutation accumulation lines that had been maintained for 30 generations. The researchers identified 99 base substitutions and 17 insertions and deletions, yielding a spontaneous mutation rate of 7 × 10−9 base substitutions per site per generation, primarily G:C→A:T transitions. They attribute this biased spectrum to deamination of methylated cytosines and ultraviolet light-induced mutagenesis. The mutation rate is close to the lower bound estimated from the divergence between monocots and dicots, and the rate of mutations affecting fitness is not significantly different from previous estimates. The study also found a higher mutation rate in pericentromeric regions, which may contribute to the higher polymorphism levels near centromeres in *A. thaliana*.