DuPont Challenge Science Essay Competition

Adaptation of Florae to Harmful Radiation

by Booker George

Bergen County Academies
Hackensack, New Jersey
Sponsoring Teacher: Judith Pinto

Third place, senior division, 2011

On April 26, 1986, an event occurred in Pripyat, Ukraine that still resonates in the public consciousness. That afternoon, accidents during unscheduled testing at the Chernobyl Nuclear Power Plant caused high pressure to build within a reactor. The resulting explosion exposed extremely flammable components to oxygen and created a blaze that launched clouds of radioactive material into the atmosphere. Winds spread these radioactive chemicals across Europe, but the majority of the material settled near the power plant, prominently in Russia, Ukraine, and Belarus. The rightful fear of cancer and radiation poisoning caused the evacuation of over 300,000 people from the areas with the highest radiation levels. Decontamination efforts, and the natural decay of radioisotopes, have lowered these radiation levels over the past 35 years, but the environment remains rich in radioactive material. Despite these adverse conditions, however, florae in the Chernobyl area are doing surprisingly well. When tested, plants from the area showed high levels of variation from other plants, particularly in regard to the proteins they produce, and displayed beneficial adaptations to their radioactive environment. Understanding these adaptations is a valuable component in understanding the abilities of plants to survive in mutagenic environments, and the long-term effect of radiation on ecosystems.

As expected from the deleterious effects of radiation on mammals, radiation is harmful to plants. The initial surge of fallout in 1986 killed approximately 400 hectares (about 1.54 square miles or 4 square kilometers) of a surrounding pine forest when the radiation burned or severely dried the trees (Sokolov, 1993). However, trees tested after the disaster were shown to have significant levels of developmental instability (deviation from the typical structure of their species). In this case, stability was measured through the level of asymmetry of leaves on the tree branches at places where a normal member of the species would be perfectly symmetric. Other studies in the Chernobyl area showed a high frequency of chromosome aberrations in wheat and winter rye(Kovalchuk et al, 2004). However, even though the plant life was harmed, the damage fell short of predictions. The predicted area of pine forest death was 2000 -3000 hectares (7.7 11.6 square miles or 20-30 square kilometers), making the expected range over five times the actual affected area (Sokolov, 1993).

What could have caused this level of error in the predictions? There was something about the flora that was not taken into account. Subsequent testing of plants from the Chernobyl region and from lab plots has revealed plant defense mechanisms that mitigated the effects of the radiation. One such investigation showed that the progeny of Arabidopsis plants in the Chernobyl were more resistant to mutagens than the offspring of Arabidopsis plants grown in normal soil (Kovalchuk et al, 2004). A discovery in the same study regarding homologous recombination frequencies offers insight into a process Arabidopsis plants use to stabilize their genome and become more resistant to mutagens (Kovalchuk et al, 2004). This change causes a process called homologous recombination to occur less frequently. Homologus recombination is a DNA process that carries the possibility of mutation; plant cells that perform it less frequently would therefore have a lesser chance of genetic mutation. This would be beneficial to a plant living in an environment with a heightened risk of mutation, such as the post-Chernobyl Ukranian forests. The offspring of other plants were observed to be similarly adapted to their radioactive environments: for example, seeds from flax plants grown in radioactive soil have been shown to carry different proteins than plants from a normal environment (Klubicova et al, 2010). The most prominent new proteins between these are those associated with DNA signaling and transcription (Klubicova et al, 2010). This indicates that redundancies in the DNA signaling processes are being used to avert mutations and ensure that the flax plants proteins are correctly dictated by the DNA.

Even with an understanding of how these plants are able to grow in mutagenic environments, the question remains: how these changes in plant chemistry occur? From portrayals of radiation in popular culture, one might think that the plant DNA was mutated by the contamination, but actual radiation is unlikely to result in beneficial mutations. On top of that, the ability to reproduce this effect in plants grown outside of the Chernobyl region proves that these are not novel evolutionary mutations in the DNA of the plants. The scientists conducting the previously mentioned study of irradiated Arabidopsis plants (Kovalchuk et al, 2004) believe they have an explanation for the changes in these plants. They found that the DNA of the plants from lineages grown in radioactive soil had increased levels of DNA methylation. DNA methylation is a process in which methyl groups are attached to parts of DNA to suppress the expression of certain genes. This means that the change is epigenetic, i.e. the changes in plant phenotype are the result of a difference in the genes that are being expressed, not the genes that exist.

Further investigation into the ability of plants to survive in the presence of mutagens is important to understanding the long-term effects of radiation. Obviously, this understanding is important in projecting the recovery process of existing disaster sites, but also has the potential to influence future decisions. As nuclear power becomes more widespread, the issue of storing nuclear waste becomes more significant. Therefore, accurately modeling the effects of a failure in a nuclear waste containment facility will become increasingly important. By extrapolation, the effect of radiation on surrounding plant life, ranging from the amount of radioactive material absorbed by flora, to the modeling of the recovery of the environment itself, will become important as well. This warrants more research into confirming the biological methods of plants of combating radiation; as testing different species to find patterns or constants amongst different kinds of flora would further the usefulness of this field in predicting the effects of mutagens in environments, and give insight into the associated DNA changes.

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