Voyage of the space veggies

December 06, 2000

WE'RE a curious lot. Always looking for new stuff to do. Always looking for new places to go. Maybe that's why we feel it's our destiny to travel to other planets. And not just to drop in, dig around, grab some rocks and catch a ride home on the next feasible trajectory, but to settle in, maybe even build a colony.

Doing that will almost certainly require growing plants in space. Plants are the only option we have for food, beyond what we take with us. They're also natural water purifiers, oxygen generators and carbon dioxide scrubbers. In short, little life-support machines. But what kind of plants should we take into space? Some cereals, a few salad leaves and something pretty to spruce up the capsule? Probably not. It's true that plants have been doing a bang-up job of keeping this planet habitable for aeons, but conditions in space suggest that what grows here is not a good guide to what's needed out there.

With this in mind, an eclectic band of scientists recently converged on a quiet 210-year-old inn just outside Research Triangle Park in North Carolina. Their objective was to begin the process of redesigning plants to fulfil the needs of future space settlers. The group included specialists in nanotechnology, genomics, cell biology, engineering and botany. On the agenda: how to take living plants and turn them into programmable life-support machines for space. Bionic plants, if you will.

Their vision is a complete re-engineering of plants-from the ground up, so to speak. And from the ground down, for that matter, since root systems are just as important. When they're done, they hope to have plants that can survive, or even thrive, in the dramatically different conditions found off Earth. That will mean writing new, heritable traits into the plants' genetic code. In the process they would also like to add a few tricks to allow humans to control plant metabolism remotely. And for good measure, they envision implanting minuscule electronic sensors to collect data on the plants' health, so humans can intervene before something goes drastically wrong.

It is a tall order, but the group has some time to work it all out. The research is funded by NASA's Institute for Advanced Concepts (NIAC), which supports work not expected to come to fruition for 10 to 40 years. And the group reckons it can be done. "It's not so far-fetched to think that we can make plants that are adapted," says team member Nina Allen, a plant cell biologist at North Carolina State University in Raleigh. "If you don't start dreaming about these things they are not going to happen." NASA already has a list of plants it thinks could one day cut the mustard in space (see Table).

Given NASA's current priorities, Mars seems the likeliest first destination for bionic plants. Probably not on the first crewed missions-at three years or so, these would be short enough to take along everything the travellers needed. But for longer trips plants would be useful, maybe even indispensable. Team leader Chris Brown, a senior research scientist at environmental technology firm Dynamac of Durham, North Carolina, who is also a botanist at North Carolina State University, reckons the minimum requirement would be a mission longer than five years. It's really a question of whether bionic plants can compete economically with mechanical life-support systems, he says.

Assuming that the sums do work out, the next question is how to grow bionic plants on Mars. Nobody imagines that it'll just be a matter of running a hoe through the regolith and waiting for the harvest moons. Conditions on Mars are just too harsh. Temperatures regularly drop to -125 ¡C. Sunlight is only about half as intense as on Earth. And though the atmosphere is 95 per cent CO2-the raw material for photosynthesis-atmospheric pressure is less than 1 per cent that on Earth.

That means bionic plants would have to be housed in enclosed spaces such as converted caves or greenhouses. One idea is an inflatable greenhouse that could be maintained at a slightly elevated pressure to keep the structure simple and light, with enough heating to keep temperatures around 5 ¡C and enough lighting for the plants to photosynthesise. This might be built by an advance party of robots laying the groundwork for colonists.

Even inside this protective cocoon, bionic plants would have to be adapted for low pressure, dim light and relatively cool temperatures. There are plants on Earth that can handle the cold, but none has ever had to evolve for low pressure. Could traits be programmed in even if they're not available on Earth? Chris Somerville, a plant molecular biologist at Stanford University in California, thinks they can. "What's called genetic engineering right now is really just genetic tinkering," he says. We splice genes from one plant or animal to another, transferring traits that already exists in nature. But Somerville believes it won't be long before molecular biologists can sit down and design genes from scratch.

Like human genomics, plant genomics is ploughing onward. The genome of Arabidopsis, the laboratory workhorse of plant genetics, will be completely mapped soon, and gene functions should follow in short order (New Scientist, 2 December, p 36). "I'm pretty confident that in the 10 to 40-year time frame we're going to have a lot of control over every aspect of plants," says Somerville. When that happens, plants could be designed to grow in all sorts of planetary environments.

They could also be given useful new traits. One possibility, Somerville says, is to get rid of the rigid cell walls which evolved to allow plants to stand up in Earth's gravity. On Mars, or anywhere else with a weaker gravitational pull, those wouldn't be much use. Eliminating the cell wall would also make plants easier to digest. Another idea is to turn the plants into mini sewage systems. They already take in dirty water and clean it through the process of transpiration. This water could be harvested in greenhouses simply by using cooled coils to capture it, just like a dehumidifier. There are limits to the dirtiness of the water plants can process, but increase their tolerance to urea, for example, and they could thrive on the colonists' urine.

The researchers are also looking to give plants new attributes to make them grow more efficiently. Distant colonies might be so far from the Sun that plants could never hope to gather enough sunlight to meet their energy needs. Even on Mars, it's likely that the Sun would be too weak. One solution might be to grow the plants under artificial lights. But this would put a huge strain on the colonies' precious energy resources, so the light sources would have to be as close as possible to the plants' light-harvesting system to minimise losses.

Taking this idea to its logical conclusion could mean giving plants their own internal light source. The scenario goes something like this. A plant's genome would be manipulated to grow molecular lamps, possibly built of bioluminescent proteins from deep-sea fish, near its photosynthetic apparatus. Similar genes from jellyfish are already routinely spliced into plants and other organisms as a tag.

These light sources could garner their energy in entirely new ways. For example, molecular devices could be designed to absorb parts of the electromagnetic spectrum not normally used by plants, such as ultraviolet or infrared, and convert them to useful wavelengths. Or there might be some way to fuel these devices by providing energy through something other than light, such as chemical or electrical means. It depends on what's available. Bionic plants could even be engineered to grow in the dark. After all, photosynthesis is just a fancy way of moving electrons around. If those electrons could be injected into the plant's energy-gathering system by some other means, they could flourish without the need to turn any of the colony's precious energy into light.

The next step would be to gain fine control over the plants' metabolism. This could allow people to turn photosynthesis on or off to generate oxygen, or tell the plants to produce a new plastic or drug. Team member Ray Wheeler, who works on the Advanced Life Support Program at NASA's Kennedy Space Center in Florida, says that another intriguing option might be shifting plants' output from standard carbohydrates to an oil, for instance. This would increase their CO2 demands without raising oxygen output, allowing colonists to fine-tune the atmosphere.

But how on Earth (or elsewhere) could we take control of a plant's metabolism? Team member Eric Davies, a plant physiologist at North Carolina State University, says we should remember that plants already have extensive communication skills. When bugs start to munch on their leaves, for instance, they can use chemical, electrical and other means to warn the rest of their anatomy to begin producing a repellent. Some plants can send warnings to other individuals (New Scientist, 22 February 1997, p 16). They also detect and respond to changes in light levels, such as those associated with season shifts.

Plants react to these various signals by turning genes on and off, which is exactly what the NIAC team wants to do. So, once researchers figure out exactly how these mechanisms work, it's not hard to imagine a centralised control unit that sends instructions to the plants via chemicals, light or some other means. "You would be using a pre-existing communication system," says Davies, "but changing the outcome by having genes that you wanted activated rather than genes that the plant wanted."

Sending these instructions would have to be done remotely if bionic plants were part of an advance party for a human colony. The unit would have to obey radio signals from Earth, but NASA has already proved it can do something similar: on the 1997 Pathfinder mission to Mars, the Sojourner Rover was radio controlled.

If a human colony were relying on plants for food, air and water, it's safe to say that there would be a deep and abiding interest in their health. This leads to the final component of the bionic plant vision: feedback from plants to people. Plants could be fitted with sensors for vital indicators of health, such as pH, or for early-warning signals such as superoxides, which many plants produce in response to pathogens, wounds and other insults. The sensors would flag the onset of a problem before any visible signs appeared.

"These sensing systems will all be molecule size with their own telemetry," says Troy Nagle, a sensor expert at North Carolina State University who is also a group member. The sensors might send information to a computer system that would analyse the data and sound the alarm when problems arose. The computer system could also be programmed with information on how to respond to problems so, as Nagle puts it, "astronauts don't have to become plant biologists to survive in space". For monitoring, the group discussed the possibility of using nanomachines embedded in just a few plants to give an approximation of the health of a whole crop. Eventually they envisage engineering some sort of heritable sensing apparatus. Looking further ahead, some of the discussions even delved into the concept of designing entirely new cellular organelles to perform most of the control tasks. But that's a long way off. However, some spin-offs from the project could be put to use long before people start colonising space. Nagle and Nina Allen have already developed small plant sensors that measure pH and detect chloride and potassium ions. These can be inserted into a plant's tissues, or placed on its roots, to measure its condition and whether enough nutrients are available to it. Right now the sensors are a few millimetres wide and almost a centimetre long, but the ultimate goal is nanoscale devices that could be implanted in cells. Farmers could fit just a few plants in their crops with sensors and use the data to monitor the crop's progress.

In space, too, plants could be put to use straight away. Greenery seems to offer a psychological benefit when humans are cooped up, as evidenced by the popularity of the greenhouse at the South Pole research station. So don't worry about those lonely space travellers. They'll always have some plants to talk to.
-end-
Mark Schrope is a science writer based in Virginia

This Feature appears in New Scientist issue 9th December 2000

SOURCE - NEW SCIENTIST, http://www.newscientist.com

New Scientist

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