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.

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