Genome size and sensitivity to DNA damage by X-rays-plant comets tell the story

Among several factors affecting radiation sensitivity, genome size has received limited attention during the last 50 years since research at Brookhaven National Laboratory (USA) and other locations demonstrated substantial differences in radiation sensitivities, e.g. between tree species with large (e.g. conifers such as pines) versus small (e.g. dicots such as oaks) genome sizes. Taking advantage of the wide range of genome sizes among species, we investigated radiation sensitivity which we define in this study as DNA damage (break frequency) measured with the alkaline comet assay in isolated nuclei exposed to X-rays. As a starting point, we considered two possible explanations for the high radiation sensitivity of plants with large genome sizes: (i) inherently higher sensitivity of larger genomes and/or (ii) impaired DNA repair. In terms of genome size effects, experiments exposing isolated nuclei from six different plant species to X-rays, varying in genome sizes from 2.6 to 19.2 Gbp, showed that larger genomes are more sensitive to DNA damage by a relationship approximating the cube-root of the nuclear volume; e.g. a 10-fold increase in genome size increases sensitivity by about 2-fold. With regard to DNA repair, two conifer species, Sawara cypress (Chamaecyparis pisifera, 8.9 Gbp genome size) and Scots pine (Pinus sylvestris, 20 Gbp genome size), both effectively repaired DNA damage within 50 and 70 min, respectively, after acute X-ray exposures. Both species also showed delayed repair of double-strand DNA breaks, as we previously showed with Arabidopsis thaliana and Lolium multiflorum.