Experiments on moss grown aboard two Space Shuttle missions showed the plants didn’t behave as scientists expected them to in the near-absence of gravity.

The common roof moss (Ceratodon purpureus) grew in striking, clockwise spirals, according to Fred Sack. He is the study’s lead investigator and professor of plant cellular and molecular biology at Ohio State University, Columbus, Ohio.

The researchers expected random, unorganized growth as seen with every other type of plant flown in space. “We don’t know why moss grew non-randomly in space, but we found distinct spiral patterns,” Sack said. The findings are reported in the online edition of the journal, Planta.

Common roof moss is a relatively primitive plant in which certain cells, called tip cells, are guided by gravity in their growth. This gravity response is only seen when moss is kept in the dark, as light overrides gravity’s effect.

Moss originates from chains of cells with growth only taking place in the tip-most cell of a chain. When grown in the dark, the tip cells grow away from gravity’s pull — this gets the cells out of the soil and into the light.

“The way these tip cells respond to gravity is exceptional,” Sack said. “In most plants, gravity guides the growth of roots or stems, which are made up of many cells. But in moss, it is just a single cell that both senses and responds to gravity,” he added.

Common roof moss was grown in Petri dishes in lockers aboard Shuttle missions in 1997 and 2003. The second mission was the Space Shuttle Columbia (STS-107), which broke apart during reentry on Feb. 1, 2003. Most of the hardware holding the plants was recovered, and 11 of the recovered moss cultures were usable.

Astronauts followed similar experimental procedures on both flights. They chemically “fixed” the moss cultures before each mission reentered Earth’s atmosphere. This process stopped all growth in the moss, capturing their state during flight.

Control studies conducted at NASA’s Kennedy Space Center (KSC) in Florida used hardware and procedures similar to those used aboard each flight. However, these moss cultures were either kept stationary or turned at a slow spin on a clinostat — a machine resembling a record turntable placed on its edge. It is used to negate the effects of gravity.

On Earth, gravity controls the direction of moss growth thoroughly; it grows straight away from the center of the Earth, just like shoots in a field of corn. In space, scientists expected the cells to grow erratically in all directions, since there was no gravity cue.

Instead, the moss grew non-randomly in two successive types of patterns: The first pattern resembled spokes in a wheel. The cells grew outward from where they were originally sown. Later, the tips of the filaments grew in arcs, so the entire culture showed clockwise spirals. The same patterns were found when the moss was grown on a clinostat on the ground.

“The results are unusual, as this is the first time researchers report seeing this kind of plant growth response in space,” Sack said.

“Unlike the ordered response of moss cells in space, other types of plants grow randomly,” he said. “So in moss, gravity must normally mask a default growth pattern. This pattern is only revealed when the gravity signal is lost or disrupted,” Sack added.

Sack conducted the study with Volker Kern, who is now at KSC. Kern was at Ohio State during the study; David Reed, with Bionetics Corp. based at KSC; former Ohio State colleagues Jeanette Nadeau, Jochen Schwuchow and Alexander Skripnikov; and with Jessica Lucas, a graduate student in Sack’s lab.