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Like Flower, Like Son
7 August 2006 (All day)
It's not easy being a plant. Shrubs, flowers, and trees must regularly fend off attacks from animals and pathogens, to say nothing of their constant struggle to find enough light and nutrients. Plants react to these stresses by modifying their genome. Now, researchers show that the urge to make these modifications can be passed down to offspring for as many as four generations.
Stress makes plants rejigger their genomes in several ways. If plants are pummeled by ultraviolet radiation, for example, their genomes undergo homologous recombination, whereby pieces of DNA are swapped between similar genomic strands. Scientists think that these changes may help the plant cope, possibly by protecting it from DNA damage. If a plant's progeny grow up with the same stresses, it would make sense for them to inherit the ability to deal with these insults.
To see whether that's true, plant biologists Jean Molinier and Barbara Hohn of the Friedrich Miescher Institute in Basel, Switzerland, genetically modified the thale cress plant (Arabidopsis thaliana) to produce blue spots if homologous recombination occurs. The team then exposed the modified plants to ultraviolet light or a fragment of a bacterial protein dubbed flagellin that mimics a bacterial infection. As expected, stressed plants showed about 2- to 4-fold higher levels of recombination than unstressed ones.
Surprisingly, though, the offspring of these plants had similar amounts of recombination as their parents, despite a lack of stress in their environment. "It's like the progeny simply memorized the insults the parents had suffered," says Hohn. What's more, the increased levels persisted for at least four generations and were transmitted irrespective of whether the male or the female parent was stressed, the team reports online 6 August in Nature.
Because homologous recombination was seen in all progeny--and not in just a fraction of plants as would be expected if classic Mendelian genetics was operating-- the team says that this unusual transmission is not simply a case of a parent passing a modified DNA sequence to its offspring. "It has to be happening at a higher level", says Hohn. This could mean so-called epigenetic changes, such as a boost in the level of enzymes that promote recombination. Further tests should reveal if and exactly how homologous recombination protects plants and their progeny, says Hohn.
The findings add a twist to natural selection, says plant geneticist Avraham Levy of the Weizman Institute of Science in Rehovot, Israel. "Contrary to the view that selection only works on the DNA sequence, the study shows that the environment can induce genetic as well as epigenetic changes, some of which may end up being selected", he says.