Plants inherit secret stashes of genetic information from their long-dead ancestors and can use them to correct errors in their own genes -- a startling capacity for DNA editing and self-repair wholly unanticipated by modern genetics, researchers said yesterday.
The newly discovered phenomenon, which resembles the caching of early versions of a computer document for viewing later, allows plants to archive copies of genes from generations ago, long assumed to be lost forever.
Then, in a move akin to choosing their parents, plants can apparently retrieve selected bits of code from that archive and use them to overwrite the genes they have inherited directly. The process could offer survival advantages to plants suddenly burdened with new mutations or facing environmental threats for which the older genes were better adapted.
Scientists predicted that by harnessing the still-mysterious mechanism they would be able to control plant diseases and create novel varieties of crops. If the mechanism can be invoked in animals -- as some tantalized scientists venture may be possible -- it could also offer a revolutionary way to correct the genetic flaws that lead to cancer and other diseases.
"We think this demonstrates that there's this parallel path of inheritance that we've overlooked for 100 years, and that's pretty cool," said Robert E. Pruitt, a professor of botany and plant pathology at Purdue University in West Lafayette, Ind., who oversaw the studies with co-worker Susan Lolle.
The finding represents a "spectacular discovery," wrote German molecular biologists Detlef Weigel and Gerd Jurgens in a commentary accompanying the research in tomorrow's issue of the journal Nature, released yesterday. The existence of an unorthodox inheritance system does not overturn the basic rules of genetics worked out by Austrian monk Gregor Mendel in the 1800s, they noted. But like a newly discovered room in a mansion of treasures, it opens up a mind-boggling world of possibilities and proves that genetics is still a young science.
Hothead gene
"It adds a level of biological complexity and flexibility we hadn't appreciated," said Lolle, who is on a leave from Purdue to serve at the National Science Foundation, which funded the work.
The Purdue team began to suspect that something strange was afoot while studying a mutation in the mustard family weed Arabidopsis thaliana, a popular workhorse of plant genetics.
The mutation was in a gene known as hothead -- one of many related genes, including fiddlehead, airhead, pothead and deadhead, that when mutated cause abnormalities in stems and flowers.
Arabidopsis plants typically self-fertilize. So when both copies of a gene are mutated in a plant, its offspring is bound to be similarly flawed -- in hothead's case, exhibiting the parent's mutant flowers.
Yet in the Pruitt-Lolle lab, a small but steady percentage of hothead offspring had normal flowers, like their grandparents'. Somehow the mutation -- a single misspelled "letter" of genetic code in a gene made of 1,782 molecular letters -- was being repaired.
"At first, we assumed there had to be a simple explanation," Pruitt said. But a series of tests over more than a year eliminated every easy explanation, such as known DNA repair mechanisms or windblown pollen from normal plants.
Instead, molecular studies indicated that the plants harbored molecular "memories" of versions of their genetic code going back at least four generations -- versions that the plant can somehow use as templates to correct the spelling of mutated stretches of DNA.
The team has not found the templates, but evidence suggests they are pieces of RNA, a molecular cousin of DNA that can be inherited separately from the chromosomes that carry the primary genetic code in cells.
Pruitt said others have occasionally noted the appearance of "revertant" plants but ignored them, assuming they were the result of sloppy technique or other errors. By contrast, Pruitt and Lolle took the observation seriously, said Elliot Meyerowitz, a pioneering arabidopsis researcher at California Institute of Technology.
Written off as random
"There are different sorts of scientists. Some like to ignore the exceptions, and others like to concentrate on them," Meyerowitz said, adding that he suspects the novel gene-fixing mechanism is present in a wide variety of organisms, including animals. He suspects the trick has been overlooked because it operates only some of the time and because scientists have been predisposed to write off the evidence as random events.
The discovery, he said, seems on par with a few others that have significantly modified scientists' understanding of genetics since Mendel. Studies in corn led to the discovery of an important gene-shuffling mechanism that has since been found in other plants and animals, including people. Studies in insects found a new mechanism for gene regulation that has since been found throughout the biological world. And a mechanism for turning off genes, first identified in soil-dwelling roundworms and since found in humans, too, is now one of the hottest topics in medical genetics because of its potential to shut down disease-causing genes.
"I won't be surprised," Meyerowitz said, if the new DNA editing mechanism is present in people, too.
Gerald Fink, a professor of genetics at the Whitehead Institute for Biomedical Research in Cambridge, Mass., said it would be important to identify exactly how the mechanism operates and whether it works in all kinds of genes. But he said he was convinced that "something weird is definitely going on." The work serves as a good reminder, he added, that the central genetic code by itself is only part of the mystery of how inheritance works.
"This gives the lie to the idea that you know everything once you sequence the genome. You don't."
Lolle said the trick is probably a lifesaver for plants, which cannot run away from radiation, environmental extremes and other insults to their DNA. It is probably especially important for self-pollinating plants such as arabidopsis, she said, which are constantly at risk of becoming seriously mutated as a result of inbreeding.
She described the mechanism as one that allows a plant to reach back in time for a version of a gene "that's already been road-tested."
Lolle said she foresees medical benefits as scientists learn to control the molecular counterpart she suspects is in humans.
"I'm very optimistic," she said. "Once the scientific community takes hold of this, it's going to work forward at a very rapid pace."
