Tech.SightPLANT GENOMICS

Plant Genotoxicity

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2214
DOI: 10.1126/science.282.5397.2214

Several organisms have been used to monitor and test the effect of radiation on the genome, including Drosophila melanogaster and transgenic mice. In the case of both Drosophila and transgenic mice, testing of the effect of ionizing radiation is performed by monitoring the reversal of a specific gene mutation: the reversion of a recessive eye color mutation in Drosophila and the restoration of the lacZ gene in germ cells in transgenic mice.

A plant model is also available for testing radiation's effect on the genome. As with animal models, this model is based on the premise that exposure to radiation results in an increased frequency of homologous recombination. In this case, transgenic Arabidopsis thaliana plants carry two overlapping, nonfunctional truncated versions of a chimeric β-glucuronidase marker gene in inverted orientation (1). Homologous recombination at this locus restores a functional β-glucuronidase gene that can be detected by histochemical staining.

A recent report in Nature Biotechnology describes the use of this plant model to monitor the genotoxic effect of the radiation fallout from the Chernobyl, Ukraine, accident (2), which occurred on 26 April 1986, and contaminated a large region of the former Soviet Union, as well as Scandinavia and Eastern Europe. Following the accident, numerous studies reported genotoxicity in humans, animals, and plants around Chernobyl. Around the reactor itself, a 600-km2 zone has been closed to agriculture since the accident, because the average soil contamination is too high. This disaster, although very unfortunate, offers the opportunity to study the effect of ionizing radiation on living organisms and to compare data collected in a laboratory setting with data obtained from the field.

The authors collected soil from seven locations around Chernobyl in which they seeded A. thaliana in small pots in the laboratory. In parallel, they seeded plants on location in plots of 1 m2. For each experiment at the different geographical sites, they planted 200 plants and repeated the experiments at each site six times. Soil samples were assayed for various isotopes and were found to contain 137Cs, 90Sr, 241Am, as well as other radionuclides. The density of soil pollution in the soil samples ranged from 20 to 6000 Ci/km2, with gamma dose rates of 97 and 8500 microroentgen per hour, respectively. The plants were harvested 5 weeks after germination and vacuum infiltrated for 15 min in staining buffer containing 5-bromo-4-chloro-3-indolyl glucuronide substrate. The plants were then incubated for 48 hours at 37°C before scores were assigned under the binocular scope.

Germination rates of the seeds decreased with increasing density of soil pollution, and there was a high degree of correlation between laboratory and open-field experiments. Moreover, an increase in soil contamination was linked to an increase in the average number of recombinations (up to five per plant) in both open-field and laboratory experiments. The number of recombinations peaked at a soil contamination of about 1000 Ci/km2 and decreased at higher levels. The authors suggest that under higher radiation doses, the precision of the recombination process may decrease. Another explanation is that higher doses lead to more double-strand breaks that may be repaired more often through end-to-end joining (often referred to as illegitimate recombination). Cytological studies in other plants indicated a correlation between homologous recombination frequencies in A. thaliana and in the frequency of chromosomal bridges and fragments. Of note, the authors found that the dose-response curves they observed tended to be left-shifted for the field experiments compared with those for the laboratory experiments, that is, although the trends were identical, there was more damage in the field. They explain the differences by indicating that the plants in pots were surrounded by much less contaminated soil than the plants in the field.

This plant model has several advantages over animal-based models for studying the effects of environmental radiation: it is easy and sensitive, and reasonably fast, taking only 5 weeks before harvesting of plants. The method is also simple to perform—histochemical staining of the plant appears to be the most complicated step. Finally, the use of plants has economical and ethical advantages over the use of animals.

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