February 23, 2006
Growing Food on a Frozen Desert Planet
One day, humans will step foot on Mars. And they'll be hungry. Growing food on a frozen desert planet with a suffocatingly thin atmosphere, however, will be a challenge. Scientists are now bioengineering plants that can grow on mars.
NASA -- Plants and animals are fragile life forms. Dry them out, freeze them, expose them to high doses of radiation - they don't do so well. But not all organisms are so picky. Many archaeans, for example, are distinguished by their ability to adapt to a variety of extreme environments. It's in their genes.
Archaeans are single-celled organisms. Under a microscope they look like bacteria. But genetically they're as different from bacteria as you are. May of them are also extremophiles. They thrive under conditions that, until the 1970s, biologists thought were completely inhospitable to life. How do they do it? With a kind of genetic band-aid. Their DNA produces chemicals (enzymes) that repair the cell damage caused by environmental stresses.
There are plenty of harsh environments for life here on Earth. But when it comes to environmental stress, Mars has a corner on the market. The average temperature on the martian surface is about -63 C (-81 F); the atmosphere is a mere wisp of a thing, some 100 times thinner than Earth's; the planet is dry as a bone; and the surface is bathed in damaging ultraviolet radiation.
Some day humans will travel to Mars. Not only will they have to protect themselves from Mars' harsh conditions, they'll need to protect the food they grow, as well. The obvious solution would be to build greenhouses that provide Earth-like growing conditions. But that would require a tremendous expenditure of precious energy resources. Another solution would to modify the plants so that they grow under martian conditions.
That's the challenge that Amy Grunden, an assistant professor of microbiology, and Wendy Boss, a botany professor, both at North Carolina State University, have set for themselves. They want to find out whether, by inserting genes from extremophile archaeans into plants, they can teach the plants to resist stress the way the archaeans do.
Plant cells respond to stressors like cold or dehydration by creating a burst of superoxide, a toxic form of oxygen. Making poison may seem like an odd way to handle stress, but, explains Boss, "It's a signaling mechanism. You get a small burst of reactive oxygen that tells the cell, Look, mount a defense, fight.'" But that can't last forever. "Plants can lose a few cells and it doesn't bother them," says Boss. But if the stress - and the toxic oxygen - continues, "eventually the whole plant will die."
Extremophile archaeans have found a way to deal with oxidative stress. They produce antioxidants. Through a series of chemical reactions, they turn superoxide into a more benign substance: water. These chemical reactions are initiated by enzymes, and the instructions for creating these enzymes are encoded in the organisms' DNA.
One organism capable of performing this feat is Pyrococcus furiosus, which makes its home in the boiling waters of deep-sea hydrothermal vents. Given its super-hot environment you might think that Pyrococcus furiosus is constantly producing antioxidants. But actually, when the organism is basking in the heat of the vent, there's no oxygen present. It's when cells get spewed out into cold sea water, where oxygen is present, that the antioxidant action takes place.
"It's been adapted to deal with oxygen at low temperature because that's when it sees it, when it gets in the cold sea water," says Grunden.
Several different enzymes are required to convert superoxide to water. What Grunden and Boss have been working on is injecting the genes that produce these enzymes into plants - actually, into a clump of tobacco cells in a Petri dish. So far, they have succeeded in transferring the gene that performs the first step in the detoxifying process; it produces the enzyme that converts superoxide to the less-toxic hydrogen peroxide. The tobacco cells not only survived the "invasion," they produced the desired enzyme. Boss and Grunden are now in the process of adding a second gene, which produces an enzyme that converts hydrogen peroxide to water.
The entire archaeal stress-reduction process involves a total of four genes. The researchers plan to work their way up to this four-gene cocktail, one gene at a time. Then they're going to try adding a gene from a bacterium, Colwellia psychrerythraea, which thrives at temperatures below freezing. Their goal is to produce a plant that can withstand the stress of freezing temperatures.
"What we're doing with having introduced the Pyrococcus gene is laying the foundation for being able to get over the initial shock of the extreme conditions. Now what we need to do is start adapting the plant to deal with the cold temperature conditions that you see on Mars," says Grunden. Eventually, they hope to add genes for surviving under low-pressure and low-water conditions as well. "The Mars environment represents a multiple-stress condition."
No-one has ever tried to do this before. And it may not work. "We may find that when we put the whole pathway in, the cell just drops dead like that, because it doesn't like all these foreign genes," Boss says. And then there's always the danger that these modified plants, if they got released into the wild, could have a negative impact on forest land or crops. Boss counters that their experiments are being kept strictly under lab-safe conditions. "We are not intending to put them out in the public," she says. "Nothing is escaping."
But Boss also sees potential benefit of the work that she and Grunden are doing. Eventually, she says, their experiments may result in crops that can "grow on poor soil with low water." Such crops might, for example. help people survive a drought.
"I really hope that this in four years will have a positive impact on agriculture, maybe even human health. Who knows? Maybe we can grow something from archaea in plants that will cure some disease."¦ There's just so much biology that's untapped out there. And these archaea make some interesting compounds. Maybe we need more of them."
On the Net: