| The autoradiogram obtained from the exposed radicle showed two types of
nuclei, i.e. heavily labelled nuclei indicating the cells in the S phase,
and lightly labelled nuclei representing the cells undergoing unscheduled
DNA synthesis, which is typical of unscheduled DNA synthesis.7)
The frequencies of labelled nuclei were significantly increased in radicles exposed to gamma-radiation (Table 1, Fig. 1). The fraction of labelled nuclei was up to four times more than in the control for the first one hour, and about three times more for the first two hours of the post-irradiation periods. In unirradiated control, the labelled cells, in the presence of hydroxyurea, was 7% for one hour and 10% for two hours, a difference of less than two-fold, of the postirradiation periods. As seen in Fig. 1, the fraction of labelled nuclei induced by gamma-radiation increased in the first hour of the post-irradiation period and remained virtually constant in subsequent another one hour. This could result from the accelerated onset of DNA synthesis in some cells of the radicle meristem at the nonreplicative phase of the cell cycle. This unscheduled DNA synthesis was also revealed by enhanced 3H-TdR uptake into their DNA in the state of inhibited normal scheduled DNA synthesis after gamma-ray irradiation (Fig. 2). Both the increased frequency of labelled nuclei and the enhanced uptake of 3H-TdR into DNA were seen mainly for the first one hour after irradiation, and the amount of newly synthesized DNA were not increased more in the later postirradiation periods. The increase and enhancement of DNA synthesis in the radicle meristem cells after gamma-radiation seems to be more clearly seen by autoradiographic method than by biochemical method involving the direct measurement of the radioactivity. One of inhibitors of normal semiconservative DNA replication, hydroxyurea, did not affect this unscheduled DNA synthesis induced by gamma-radiation. It seems almost certain, thus, that unscheduled DNA synthesis observed here may not be a reflection of stimulated normal DNA replication, but a consequence of the performance of repair replication in the excision repair process after gamma-radiation. It seems also clear that wheat seed embryo cells possess excision repair system for gamma-radiation-induced lesions in DNA, which can function after gamma-irradiation. The present results, along with evidence from onion,15) barley.34,37) carrot,12,13) pea10) or Euglena19) cells showing excision reapir activity, strongly suggest that excision repair process seems to be widespread in plant species. However, the phylogenic distribution of this capability among plant cells has more and yet more to be determined to generalize this to most plant cells, because there are other reports indicating an absence of excision repair activity in Nicotiana,32) Haplapoppus,32) Vicia,25,35,36) Ginkgo33) or Chlamydomonas nuclear and chloroplast DNAs.30) The role of excision reapir system in the rejoining of chromosome breaks or in the formation of chromosome aberrations is also the type of experiment which is to be done in the future.2,17,20,35) |
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