NON-RANDOM ORIENTATION OF DARK-GROWN MOSS PROTONEMATA IN MICROGRAVITY (STS-87). V.D. Kern¹, and F.D. Sack². ¹Lockheed Martin Space Operations, NASA Ames Research Center, Moffett Field, CA, and ²Department of Plant Biology, Ohio State University, Columbus, OH.

     In darkness protonemata of the moss Ceratodon purpureus express negative gravitropism (they grow up) with high fidelity. When grown under microgravity conditions during spaceflight (STS-87, Nov./Dec. 1997), an essentially random orientation was observed during the first 7 days of cultivation. However, in cultures grown in space for 14 days, the protonemata grew in arcs and overall formed clockwise spirals.  Cultures grown on a slow-rotating clinostat on earth for 14 days also grew in clockwise spirals.  These spirals formed regardless of the orientation with respect to the acceleration force or to the direction of clinostat rotation (clock- vs. counter-clockwise).  Clinostat-grown cultures appeared to show less dramatic spirals than spaceflight cultures.  The presence of spirals in 14 d but not 7 d cultures could be due to culture age, stage, or size and/or to the duration of exposure to microgravity or clinostat rotation.  The phenomenon of protonemal phototropism allowed us to distinguish between these possibilities. When irradiated for 7 days, cultures displayed negative and positive phototropism while gravitropism was absent.  When these cultures were then kept in the dark for an additional 7 days, clockwise arcs and spirals formed.  This suggests that cultures may need to reach a certain stage before microgravity can induce spirals.  There are only a few reports of gravitropic organs or cells orienting non-randomly in space.  The presence of coordinated clockwise spiral growth in microgravity suggests that there is an endogenous growth polarity in Ceratodon that normally is suppressed by gravitropism.  A working hypothesis is that the spirals represent a residual spacing mechanism for controlling colony growth and the distribution of branches under some conditions and in some mosses.  An upcoming flight experiment on STS-107 scheduled for summer 2002 should allow us to learn more about the parameters that regulate clockwise spiral growth. 

     (Supported by NASA: NAG10-017).