BioEd Online

Samples of moss aboard the doomed space shuttle Columbia have survived to reveal an unusual growth pattern that gives clues to the plant's evolutionary history.

Samples of common roof moss (Ceratodon purpureus) grown in darkened containers on the shuttle put out wispy fronds in clockwise spirals. "This spiralling has never been seen in any other plant in space," says Fred Sack, plant biologist at Ohio State University, Columbus, Ohio, who led the study.

The Columbia shuttle broke apart on 1 February 2003, killing seven astronauts. The moss experiments fell 64 kilometres to Earth, and were found scattered across a five-mile area around Bronson, Texas, by retrieval crews during February and March of last year.

"Most of the samples were crushed," says Sack. But 11 of the 87 recovered cultures were usable. "It was really amazing that they survived the impact," comments John Kiss, a plant biologist from Miami University, Oxford, Ohio.

One of the dishes was linked to a temperature recorder, which shows a spike in the experiment's temperature as Columbia exploded, followed by the daily fluctuations of temperature between night and day ( see graph). Before returning to Earth, the astronauts had added chemicals that stopped the moss growing, so all its growth is known to have occurred in space alone.

Primitive motion

After an experiment flown on the Columbia shuttle in 1997 found spiral growth in two moss samples, Sack prepared 100 Petri dishes of moss seeds sealed inside aluminium containers for the Columbia's final mission.

On Earth, plants can tell which way is up and direct their growth accordingly. In microgravity, they generally grow in random directions. "I was very surprised," says Kiss. "You wouldn't expect to find such a distinctive pattern of growth in microgravity."

Common roof moss grows towards light. But in the dark, gravity takes over and the moss grows upwards, as if it were escaping from beneath a layer of soil.

Sack believes that removing both light and gravity reveals a more primitive mode of growing. "Perhaps spirals are a vestigial growth pattern that became masked when moss evolved to respond to gravity," he says. "That's the only explanation that makes sense so far." The research team has published its research online in the journal Planta¹.

Gravity mystery

The moss grows by sending out thin filaments; new growth occurs only at the tip. This means that a single cell at the end of the filament must sense gravity and direct the next cell's growth.

"We still don't know how plants sense gravity," says Sack. Biologists believe that gravity moves structures that sit between the cells. These may open or close channels that carry signalling chemicals to stimulate growth in particular areas of the plant.

"When we see something exceptional like this, it gives us a clue about how biological programming might be layered," Sack says. "I think that different plants have evolved their response to gravity separately."

A spiral is a very efficient way of spreading over a wide area, he adds, so this could have been the moss's original way of growing, ensuring that space was filled without parts of the plant crossing over each other and blocking light from filaments beneath.