COLUMBUS, Ohio – An experiment scheduled for today’s Space
Shuttle Columbia mission may provide clues about just how
plant growth is guided by gravity.
The study, an extension of work from a previous shuttle mission
in 1997, should test whether the absence of gravity changes how
simple plants grow. The experiment will use common roof moss
(Ceratodon
purpureus), a plant that uses gravity to determine the
direction that single cells grow.
These tip cells will respond to both light and gravity, explained
Fred
Sack, professor of plant
biology at Ohio State University. Light is the stronger of
the two factors, but in the dark, the cells grow in the direction
opposite the attraction of gravity.
These are exceptional cells. It is rare for gravity to control
the direction that single cells grow instead of an entire plant,
Sack says. We wanted to know if they were placed in a near gravity-free
environment, would the plants grow in a random fashion. The space
shuttle offers us a laboratory to test that hypothesis.
Sacks team got a partial answer from their 1997 shuttle experiments.
Surprisingly, the cells growth was not random once gravity was
removed. Moss cultures grown in orbit for two weeks in the dark
during that flight produced elaborate clockwise spirals.
We suspect that those spirals resulted from a residual spacing
mechanism intended to control colony growth and the distribution
of branches, Sack says, a mechanism that is normally suppressed
by the stronger influence of gravity on earth.
The current shuttle experiments are aimed at answering some
of these questions. Forty-seven moss-containing Petri dishes
will be grown in a self-contained, mid-deck locker. In one treatment,
the moss will grow first in red light and then in the darkness
to see how quickly spirals develop.
At the center of the project is the question of how the cells
sense gravity and how that event controls individual cell growth.
Researchers have shown that it is likely that heavy organelles
in the tip cells fall and somehow signal growth in the opposite
direction.
On earth, these organelles never completely settle to the
bottom of the tip cells. They remain trapped in certain zones
within the cells, apparently supported and contained by an intricate
scaffold-like structure of microtubules and actin microfilaments
within the cell.
The earlier Shuttle experiment showed a surprising result
that the organelles aggregated in clusters instead of being randomly
located within the cell. One goal of this missions experiment
is to determine if that microscopic scaffolding controls the
clustering.
The researchers will test this by injecting drugs that selectively
break down the different types of protein fibers in the cells
scaffolding. The researchers hope to determine whether the organelles
still cluster together once the microscopic scaffolding is removed.
In space where there is minimal gravity, youd expect a more
random distribution of organelles within the cells," Sack
said. The presence of clusters in them while in space suggests
that internal forces exist within the cell that gravity ordinarily
overwhelms".
Sack says that "This fiber-organelle relationship may
be a specialization that the moss cells developed over time for
gravity sensing. But it may also relate to understanding how
most cells cope with internal mechanical forces.
After the experiments are completed, the astronauts will chemically
preserve the moss containers before returning to Earth. Upon
landing, the Ohio State research team will take thousands of
microscope pictures for later study.
This research was supported by Fundamental
Biology Program of the National
Aeronautics and Space Administration. Along with Sack, Volker
Kern, a postdoctoral fellow in plant biology at Ohio State, is
a co-principle investigator on the project.
Contact: Fred Sack, (614) 292-0896; sack.1@osu.edu
Written by Earle Holland (614) 292-8384; Holland.8@osu.edu
