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Mossy Space Spirals
Samples of fire moss that travel onboard the space shuttle do something odd:
they spiral. Scientists say it's a clue to the fundamental inner workings of plant
cells.
July 16, 2002: Biologist Fred Sack carefully lifted a petri dish from the tray.
Inside were precious samples of moss just back from a two-week voyage aboard space
shuttle Columbia. He glanced at the growing clumps, blinked, then looked closer.
"It was not what I expected to see," recalls Sack. His moss was growing
in a spiral.
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Above: Moss from space shuttle
Columbia (STS-87) shows spiral growth patterns that emerged in low-gravity.
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On Earth, moss spores that take root send out hundreds of tiny filaments known
as protonemata. These filaments normally grow in an unruly fashion; they make
a messy-looking tangle. But the moss onboard Columbia did something different.
As if choreographed, the protonemata swirled together in the same direction. They
formed a distinctive clockwise spiral like no moss on Earth. 
"It's the kind of thing," says Sack, "where the shuttle lands,
they deliver your tray, you carefully take the dishes out of the hardware and
you're ready to start doing some photography. Then all of a sudden you just look
at the cultures, and WOW! You know right away it was not random growth."
That was four years ago. Sack, who is a professor at Ohio State University
(OSU), has studied the spirals ever since, yet they remain a mystery.
What was moss doing onboard the space shuttle? Sack and colleagues from NASA
had sent samples of Ceratodon purpureus - better known as fire moss - into
space to study the way plants sense gravity. Moss is good for such studies, explains
Sack, because it contains single cells that are gravitropic--that is, cells that
sense gravity and then grow either toward it or away from it. (Moss cells respond
to light, too, but the space-spirals formed in darkness. "Phototropism"
was not a factor.) Furthermore, moss is small; it doesn't take up much room on
the shuttle.
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Left: On Earth, moss grows away from gravity and toward
light. The left frame shows moss in a petri dish; the right frame shows fire moss
growing wild in a field.
Typically, when a gravitropic plant is sent into space, it gets confused; it
grows in a disoriented way. These odd spirals, Sack says, mark the first time
in space that a plant normally oriented by gravity has grown in a non-random pattern.
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No one knows exactly how plants sense gravity. One possibility, explains Sack,
is amyloplasts: tiny starch-filled particles that float within the cell. Because
these particles are heavy, gravity pulls them down. In some cells, like root cells,
the amyloplasts sink to the bottom, forming a kind of sediment. In the threadlike
protonemata that Sack studies, this sedimentation is more complex. It tends to
form not one layer of sediment, but many.
In space, the pattern of sedimentation is different than it is on the ground--an
important clue, says Sack. Actually, sedimentation shouldn't occur in space at
all, he added. In a zero-g environment, the amyloplasts ought to float around
at random. Instead, they bunch together. Why? It's part of the mystery.
When the normal pull of gravity vanishes, continues Sack, the position of these
starch particles is determined solely by structures inside the cell itself. An
example would be the cytoskeleton, a network of thin fibers that permeate some
cells. The fibers, made of proteins, give cells shape and hold the nucleus and
other organelles in place.
Above: This fire moss protonemata cell experienced
low gravity onboard space shuttle Columbia (STS-87) in 1997. The large arrow indicates
a region of amyloplast clustering.
Perhaps the cytoskeleton also causes amyloplasts to cluster together. It's
an idea Sack will test later this year when fire moss returns to space onboard
the shuttle Columbia (STS-107). Moss cells inside a device called the Biological
Research in Canisters experiment ("BRIC" for short) will be fed chemicals
that break down cytoskeletons. Will amyloplasts still form bunches? Or float at
random? "We'll see," says Sack with anticipation.
Even after researchers figure out why amyloplasts congregate in low gravity,
they'll still have more questions to answer. For example, what is the exact mechanism
that causes protonemata to spiral?
So far, Sack cautions, researchers can only speculate. Perhaps calcium ions,
which flow in and out of the cell, control the direction in which the tip of the
cell grows. The unique clustering of starch particles in low gravity might affect
the movement of these ions, which in turn might instruct moss filaments to curve
in a clockwise direction.
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The upcoming STS-107 research mission will provide lots of new data. Where
the first experiment (STS-87) involved only three moss cultures, this one will
involve 46 petri dishes, some of which will contain multiple cultures. "By
the time Columbia lands," predicts Sack, "there will be many thousands
of moss filaments grown on the shuttle for us to study."
Sack and his colleagues hope to learn more about the way gravity affects the
structure of the cell. Very often, says Sack, in biology you look at a very specialized
system--like fire moss--and you find some phenomena that turns out to be important
for all cells.
Gravity has been a powerful force shaping life on our planet, notes Sack. It's
so pervasive, he says, that people don't even recognize its influence. But one
trip to space can open your eyes; just take a look at the mysterious spirals of
fire moss.
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Above: Professor Fred Sack, Ohio
State University. + More
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Credits & Contacts
Authors: Karen Miller, Dr. Tony Phillips
Responsible NASA official: Ron Koczor
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
The Science Directorate at NASA's Marshall Space Flight Center sponsors the
Science@NASA web sites. The mission of Science@NASA is to help the public understand
how exciting NASA research is and to help NASA scientists fulfill their outreach
responsibilities.
Valuable Resources
NASA's Office of Biological
and Physical Research - supports studies of fundamental biology for the benefit
of humans in space and on Earth.
Space
Research and You - learn about all the experiments slated for flight onboard
shuttle Columbia during the STS-107 research mission, including the next fire
moss experiment.
Fred
D. Sack - professor Sack's home page at Ohio State University, includes Terrible
Anatomy Puns.
Time-lapse
video of a Ceratodon protonema. It grows up in the dark after turning to the
horizontal.
Curious
Cytoskeletons - (Science@NASA) Deep within the cells we're made of, squishy
skeletons feel the effects of gravity ... and respond in unexpected ways.
STS-87
- Fire moss first flew to space onboard a research mission that lasted from Nov.
19 - Dec 5, 1997 onboard space shuttle Columbia.
Amyloplasts
That Sediment in Protonemata of the Moss Ceratodon purpureus are Nonrandomly
Distributed in Microgravity -- Volker D. Kern, Jeffrey D. Smith, Jochen M. Schwuchow,
and Fred D. Sack, Plant Physiology, April 2001, Vol.125, pp. 2085-2094.
More about fire moss: Ceratodon
purpureus, vital statistics; Fire
moss, pictures and an introduction; a picture
of a protonemata.
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