Lack of mitotic delays at the onset of proliferation in dormant root primordia challenged by ionizing radiation

X-rays at doses between 2.5 and 20 Gy were applied to Allium cepa L. bulbs containing either dormant root primordia (before water imbibition) or actively proliferating meristems. Irradiation of the primordia that were enriched in G0 cells neither delayed proliferation onset nor root sprouting. Under both protocols, irradiation decreased the final length of the roots to about 60% (at 20 Gy) of that reached by the unirradiated controls. Irradiation of the proliferating meristems increased the mitotic index at some fixation times. This could not be due to a rise in the cell entry into mitosis, as the rate of root growth decreased simultaneously. The increased mitotic index should be the consequence of a delay in the relative time taken by mitosis in the whole cycle time. Lengthened mitosis probably allows the post-replicative repair of most DNA lesions, as the frequency of interphases with micronuclei was higher in the cells which were irradiated when still dormant than in those irradiated when cycling. Thus, the mitotic delays should be the consequence of a checkpoint pathway activated by the presence of DNA damage. This feedback mechanism seems only to develop after cell proliferation is restored.