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Plant’s Taste for Heavy Metal is in the Genes

Illustration by Sam DayPICTURE a landfill, rife with toxic heavy-metal wastes like cadmium, arsenic and mercury. (This does not require an overly active imagination: there are thousands of such sites in the United States today.) Now picture that same landfill covered with genetically engineered plants that absorb that waste and store it in what Dr. Philip Rea, associate professor of biology, refers to as their "intracellular landfills." Then picture those plants being harvested, the metals recovered or safely disposed of–and the landfill being removed from the EPA’s list of dangerously toxic sites without bankrupting the Treasury.
   
Phytoremediation–the use of plants to remove or neutralize toxic wastes from soils–is not a new idea. But its practical application recently got a huge boost from Rea and his team at Penn’s Plant Science Institute when they managed to isolate one of the "Holy Grails of phytoremediation"–a gene known as AtPCS1 that enables a plant to synthesize compounds called phytochelatins, which enable it to tolerate and absorb cadmium and other heavy metals.
   
AtPCS1 is shorthand for Arabidopsis thaliana phytochelatin synthase 1, and it was the Arabidopsis thaliana weed that yielded, after a painstaking search, the gene for Rea and his team. (The team has been comprised of Dr. Olena Vatamaniuk, Dr. Stéphane Mari and Dr. Yu-Ping Lu, all postdoctoral fellows in the biology department, though Lu has since left the University). While Arabidopsis is too small to be of practical value for phytoremediation, isolating the gene responsible for synthesizing phytochelatins opens the door to genetically engineering plants that can easily do what is otherwise a prohibitively expensive and difficult job.
   
"Arabidopsis is a very good model genetic system," says the wry, British-born Rea. "It’s a small plant, which is good if you want to have a large number of individuals [and] do real genetics. It has a small genome–it doesn’t have a lot of surplus DNA, which makes it very manipulable molecularly. Each plant produces a lot of progeny–some 10,000 seeds per plant. And it has a relatively short generation time."
   
While Rea and his team were not specifically trying to find the AtPCS1 gene, he says they had a "general interest in heavy-metal detoxification" in plants and fungal systems.
   
"We went after genes that regulated another family of genes that we were interested in at the time," he adds. "They happened to be genes that were involved in the transport of things into the vacuole." Vacuoles are intracellular compartments–"landfills," if you will –that represent some 40-70 percent of a cell’s volume.
   
By inserting Arabidopsis genes into fast-growing brewer’s yeast –some of it laced with cadmium, some without–Rea and his colleagues were able to figure out which of the plant’s genes could affect heavy-metal tolerance. Last summer, they narrowed down the list of suspects from more than 100 genes to just one –a breakthrough in itself, but not enough. They still had to figure out "how this gene is doing what it’s doing," in Rea’s words.
   
On November 14, they concluded that the gene "encoded" or encouraged an enzyme known as phytochelatin synthase. Exactly how they reached that conclusion is perhaps better left to the article they wrote for the June issue of the Proceedings of the National Academy of Sciences. But the long and short of it, says Rea, is that "we knew we had isolated the gene that encouraged the enzyme that is responsible for the synthesis of phytochelatins." And, he adds, "the things that activate the enzyme"–heavy metals–"are things that are themselves bound by phytochelatins."
   
Some nine months earlier, they had discovered that a lab headed by Dr. Julian Schroeder at the University of California-San Diego had "bumped into this same gene" and found that it conferred heavy-metal resistance when exposed to yeast. The Penn team and the UC-San Diego team agreed to publish their results simultaneously.
   
Then they learned that an Australian lab headed by Dr. Christopher Cobbett at the University of Melbourne, using a more time-consuming classical-genetics approach, had also been searching for the enzyme.
   
"In this race to find this enzyme," says Mari, "we were almost the latest ones. But with these yeast models, we were able to do within one year what they were trying to do in 10 years. Not that they were bad, but they were using classic genetics."
   
When they did figure out what the enzyme was doing, Rea decided to telephone Cobbett, whom he had never met, and let him know what his team had found. "I said, ‘Hi, is that Chris Cobbett? This is Phil Rea, calling from the University of Pennsylvania.’ And he just said, ‘I was expecting your call.’"
   
The three teams agreed to submit their papers simultaneously to three different journals. All members of the Penn team agree that the confluence of discoveries by the three labs made for good science–in part, as Vatamaniuk puts it, "because you can see how different labs, using different approaches, get to the same conclusion."
   
The Penn and UC-San Diego labs filed a joint patent to protect their initial discovery, which will almost certainly have commercial implications. Rea notes that they’ve been contacted by a company interested in buying up commercially valuable plant genes. Their next objective is to engineer plants in which the gene is "regulated" to ensure that the metals are only accumulated in certain parts of the plant.
   
"To have this gene is only part of the solution," he emphasizes. "It’s only one re-agent of many re-agents required to have plants that really do clean up contaminated soils. There are other conditions that have to be satisfied" –including finding ways to separate the heavy metals from soil particles such as clay micelles. Rea notes that DuPont and other chemical corporations have been experimenting with synthetic chelators that can be added to soils to free up the heavy metals.
   
Now, says Rea, "the race is on, because once a breakthrough like this is made, then a lot of the experiments become very obvious."

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Copyright 1999 The Pennsylvania Gazette Last modified 8/23/99