Plants experiencing chronic internal exposure to ionizing radiation exhibit higher frequency of homologous recombination than acutely irradiated plants

Ionizing radiation (IR) is a known mutagen responsible for causing DNA stra nd breaks in all living organisms. Strand breaks thus created can be repair ed by different mechanisms, including homologous recombination (HR), one of the key mechanisms maintaining genome stability [A. Britt, DNA damage and repair in plants, Annu. Rev. Plant. Phys. Plant Mel. Biol., 45 (1996) 75-10 0; H. Puchta, B. Hohn, From centiMorgans to basepairs: homologous recombina tion in plants, Trends Plant Sci., 1 (1996) 340-348.]. Acute or chronic exp osure to IR may have different influences on the genome integrity. Although in a radioactively contaminated environment plants are mostly exposed to c hronic pollution, evaluation of both kinds of influences is important. Esti mation of the frequency of HR in the exposed plants may serve as an indicat ion of genome stability. We used previously generated Arabidopsis thaliana and Nicotiana tabacum pla nts, transgenic for non-active versions of the beta-glucoronidase gene (iud A) IF. Swoboda, S. Gal, B. Hohn, H. Puchta, Intrachromosomal homologous rec ombination in whole plants, EMBO J., 13 (1994) 484-489; H. Puchta, P. Swobo da, B. Hohn, Induction of homologous DNA recombination in whole plants, Pla nt, 7 (1995) 203-210.] serving as a recombination substrate, to study the i nfluence of acute and chronic exposure to IR on the level of HR as example of genome stability in plants. Exposure of seeds and seedlings to 0.1 to 10 .0 Gy Co-60 resulted in increased HR frequency, although the effect was mor e pronounced in seedlings. For the study of the influence of chronic exposu re to IR, plants were grown on two chemically different types of soils, eac h artificially contaminated with equal amounts of Cs-137. We observed a str ong and significant correlation between the frequency of HR in plants, the radioactivity of the soil samples and the doses of radiation absorbed by pl ants tin all cases r > 0.9, n = 6, P < 0.05). In addition, we noted that pl ants grown in soils with different chemical composition, but equal radioact ivity, exhibited different levels of HR, dependent upon the absorbed dose o f radiation. Remarkably, we observed a much higher frequency of HR in plant s exposed to chronic irradiation when compared to acutely irradiated plants . Although acute application of 0.1-0.5 Gy did not lead to an increase of f requency of HR, the chronic exposure of the plants to several orders of mag nitude lower dose of 200 mu Gy led to a 5-6-fold induction of the frequency of WR as compared to the control. (C) 2000 Elsevier Science B.V. All right s reserved.