mms21-1 roots show a stem-cell-death phenotype and elevated levels of DSBs. A, Root stem cells and their early descendants stained with PI in mms21-1 roots at 1 DAG, indicating cell death. B, PI staining of the wild type (WT; top) and mms21-1 (bottom) after treatment with 15 μm cisplatin for 0 h (left), 24 h (center), and 48 h (right). Bar = 20 μm. C, qRT-PCR analysis of the expression of DSB-inducible genes in wild-type and mms21-1 roots. Data are means ± sd of three biological repeats. Asterisks indicate significant differences compared with control plants (P < 0 0.005; Student’s t test). D, Representative examples of nuclei seen in the comet assay from wild-type (top) and mms21-1 (bottom) roots. E, DNA damage as measured by the percentage of DNA in the tail of nuclei in comet assays for wild-type and mms21-1 roots. The mean value of more than 200 nuclei is shown with error bars indicating sd. The asterisk indicates a statistically significant difference according to Student’s t test (P < 0 0.01).