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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. Right: Moss from space shuttle Columbia (STS-87) shows
spiral growth patterns that emerged in low-gravity. 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. Below: 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. 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 indicated a region of amyloplast clustering. [more
information] 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. Left: Professor Fred Sack, Ohio State University. [more] 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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