杏吧原创

WE鈥橰E a curious lot. Always looking for new stuff to do. Always looking for
new places to go. Maybe that鈥檚 why we feel it鈥檚 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鈥檙e 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鈥檚 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鈥檚 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鈥攆rom the ground up,
so to speak. And from the ground down, for that matter, since root systems are
just as important. When they鈥檙e 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鈥� 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鈥� 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鈥檚 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. 鈥淚t鈥檚 not so far-fetched to think that we can make
plants that are adapted,鈥� says team member Nina Allen, a plant cell biologist at
North Carolina State University in Raleigh. 鈥淚f you don鈥檛 start dreaming about
these things they are not going to happen.鈥� NASA already has a list of plants it
thinks could one day cut the mustard in space.

Candidate plants for space

Staple: Wheat, Soya, Potato, Sweet potato, Peanut, Rice, Quinoa, Pea, Sugar beet

Supplemental: Lettuce, Tomato, Spinach, Radish, Strawberry, Chard, Chufa nuts, Kale,
Onion, Carrot, Broccoli, Cabbage, Melon

Given NASA鈥檚 current priorities, Mars seems the likeliest first destination
for bionic plants. Probably not on the first crewed missions鈥攁t 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鈥檚 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鈥檒l 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 鈥�125 掳C. Sunlight is
only about half as intense as on Earth. And though the atmosphere is 95 per cent
CO₂鈥攖he raw material for photosynthesis鈥攁tmospheric
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 掳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鈥檙e not available on Earth?
Chris Somerville, a plant molecular biologist at Stanford University in
California, thinks they can. 鈥淲hat鈥檚 called genetic engineering right now is
really just genetic tinkering,鈥� he says. We splice genes from one plant or
animal to another, transferring traits that already exists in nature. But
Somerville believes it won鈥檛 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 杏吧原创, 2 December, p 36).
鈥淚鈥檓 pretty confident that in the 10 to 40-year time frame we鈥檙e going to have a lot of control over
every aspect of plants,鈥� 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鈥檚 gravity. On Mars, or anywhere else with a weaker gravitational pull,
those wouldn鈥檛 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鈥� 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鈥檚 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鈥� precious energy resources, so the light sources would have to be as
close as possible to the plants鈥� 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鈥檚 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鈥檚 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鈥檚 energy-gathering system by some other means, they could
flourish without the need to turn any of the colony鈥檚 precious energy into
light.

The next step would be to gain fine control over the plants鈥� 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鈥檚 Kennedy Space Center in Florida,
says that another intriguing option might be shifting plants鈥� 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鈥檚
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 杏吧原创, 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鈥檚 not hard to imagine a centralised control unit
that sends instructions to the plants via chemicals, light or some other means.
鈥淵ou would be using a pre-existing communication system,鈥� says Davies, 鈥渂ut
changing the outcome by having genes that you wanted activated rather than genes
that the plant wanted.鈥�

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鈥檚 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.

鈥淭hese sensing systems will all be molecule size with their own telemetry,鈥�
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, 鈥渁stronauts don鈥檛 have to become plant biologists to
survive in space鈥�. 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鈥檚 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鈥檚 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鈥檚 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鈥檛 worry
about those lonely space travellers. They鈥檒l always have some plants to talk
to.