Non-Linearity of dose-effect relationship on the example of cytogenetic effects in plant cells at low level exposure to ionising radiation

Over several decades, modelling the effects of ionizing radiation on biological system has relied on the target principle [Timofeeff-Ressovsky et al., 1935], which assumes that cell damage or modification to genes appear as a direct consequence of the exposure of biological macromolecules to charged particles. Furthermore, it is assumed that there is no threshold for the induction of biological damage and that the effects observed are proportional to the energy absorbed. Following this principle, the average number of hits per target should increase linearly with dose, and the yield of mutations per unit of dose is assumed to be the same at both low and high doses (linearity of response). This principle has served as the scientific background for the linear no-threshold (LNT) concept that forms the basis for the radiological protection for the public and the environment [ICRP, 1990]. It follows from the LNT that there is an additional risk for human health from exposure to any radiation level, even below natural background. Since the mid 50's, however, the scientific basis for the LNT concept has been challenged as experimental data have shown that, at low doses, there was a non linear relationship in the dose response. Luchnik and Timofeeff-Ressovsky were the first who showed a non-linear response to a low dose exposure [Luchnik, 1957; Timofeeff-Ressovsky and Luchnik, 1960]. Since then, many data have been accumulated which contradict the LNT model at low doses and dose rates. However, the hit-effect paradigm has become such a strong and indissoluble fact that it has persisted even under the growing pressure of scientific evidence for phenomena at low dose exposure that can not be successfully accounted for by the LNT concept. In recent years, additional information on non-targeted effects of radiation has been accumulated following the first reports of an adaptive response in human lymphocytes [Olivieri et al., 1984] as well as bystander mutagenic effect of alpha-particles [Nagasawa and Little, 1992; Mothersill et al., 1995]. Other phenomena including genomic instability, low-dose hypersensitivity, and increased radiation resistance effects are also under study [Marples et al., 1997; Kadhim et al., 2004; Bonner, 2004]. The nonlinearity of the dose-effect relationship with low level exposures has been demonstrated in a number of studies where chromosome aberrations were considered as the endpoint of interest. For example, the number of radiation-induced dicentrics in human peripheral blood lymphocytes found in [Pohl-Ruling et al., 1983; Lloyd et al., 1988, 1992] did not exceed the control level at doses below 40 mGy, with some experimental points lying significantly below control values. Essential deviations of chromosome aberrations appearance from linearity in mammals were also shown at higher doses of 100-300 mGy [Luchnik and Sevankaev, 1976; Takahashi et al., 1982]. In other species, deviations of cytogenetic effect induced by low doses from linearity have also been reported. For example, the dose response for cytogenetic effects in Chinese hamster fibroblasts and Vicia faba germs at doses from 0 to 2.5 Gy was shown to be non linear with a plateau at low doses by [Zaichkina et al., 1992]. Dose-effect curves on chromosome aberrations in root meristem cells of Pisum sativum plantlets in the dose range of 0-10 Gy also showed non-linear responses with a plateau for doses up to 1 Gy [Zaka et al., 2002]. However the available information on dose-effect relationships at low doses for non-human species is scarce despite its importance. In their natural environment, some non-human species may be at a higher risk of impact than humans because of differences in ecological niches occupied. [Geras'kin et.al., 2003]. Currently, radiation protection of the environment and maintenance of ecosystem sustainability is of a special concern. and the development of a harmonized approach to human and biota protection has been recognized as a challenge for modern radiobiology and radioecology [Copplestone et al., 2000; Pentreath, 2002]. In this context, much more information on non-human species response to low level exposures is needed. This paper summarizes findings of several studies on the cytogenetic effects induced by low level exposure to external γ-radiation in plant meristem cells