Abstract
The purpose of this study was to investigate whether the moss Physcomitrella patens cells are more resistant to ionizing radiation than animal cells. Protoplasts derived from P. patens protonemata were irradiated with γ-rays of 50–1000 gray (Gy). Clonogenicity of the protoplasts decreased in a γ-ray dose-dependent manner. The dose that decreased clonogenicity by half (LD50) was 277 Gy, which indicated that the moss protoplasts were 200-times more radioresistant than human cells. To investigate the mechanism of radioresistance in P. patens, we irradiated protoplasts on ice and initial double-strand break (DSB) yields were measured using the pulsed-field gel electrophoresis assay. Induced DSBs linearly increased dependent on the γ-ray dose and the DSB yield per Gb DNA per Gy was 2.2. The DSB yield in P. patens was half to one-third of those reported in mammals and yeasts, indicating that DSBs are difficult to induce in P. patens. The DSB yield per cell per LD50 dose in P. patens was 311, which is three- to six-times higher than those in mammals and yeasts, implying that P. patens is hyperresistant to DSBs. Physcomitrella patens is indicated to possess unique mechanisms to inhibit DSB induction and provide resistance to high numbers of DSBs.
Highlights
Organisms show diverse radiosensitivity in a broad range [1,2] and are expected to possess various genome maintenance strategies
We investigated the clonogenicity of protoplasts derived from the tobacco BY-2 cell line and determined that tobacco protoplasts were 10-times more resistant to radiation than mammalian cells [5]
277 Gy (Figure 3e), which indicated that the moss protoplasts were 200-times more resistant to ionizing radiation compared with that of human non-cancer cells (1.4 Gy [22])
Summary
Organisms show diverse radiosensitivity in a broad range [1,2] and are expected to possess various genome maintenance strategies. Terrestrial plants are often hyperresistant to ionizing radiation, the cells contain relatively large contents of genomic DNA, which is the most important biological target of radiation. Few studies have assessed the radiosensitivity of single plant cells and it remains unclear whether plants are equipped with hyperresistance to radiation at the cellular level. Ionizing radiation induces various types of DNA damage [6]. Among the types of damage, double-strand breaks (DSBs) are difficult to repair accurately and the impacts are the most serious biologically. We previously performed a comparative analysis of initial DSB yields between tobacco protoplasts and Chinese hamster CHO-K1 cells, and reported that the yield per Gb DNA per Gy (Gb DNA−1 Gy−1 ) in tobacco was only one-third of that induced in the Chinese hamster [8]
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