Poster: Tropisms

Abs # 655: Clockwise spiral growth in moss protonemata in microgravity and after clino-rotation

Presenter:	Kern, Volker D, vkern@mail.arc.nasa.gov
Authors	Kern, Volker D (A) (B)  Sack, Fred D (B)
Affiliations:	(A): NASA Ames Research Center (B): Department of Plant Biology, Ohio State University

Protonemata of the moss Ceratodon purpureus are a valuable system for studying how gravity regulates the polarity of tip-growth. Dark-grown protonemata express negative gravitropism with high fidelity. When grown in darkness in microgravity (STS-87) for 7 days, most cells were radially oriented. However, in older (14-d) cultures the protonemata grew in arcs and overall formed clockwise spirals. Cultures grown on a slow-rotating horizontal clinostat for 14 days also expressed spirals. Spirals were always clockwise and formed regardless of the orientation with respect to the acceleration force or to the direction of rotation. 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 clino-rotation. This was studied further by unilaterally irradiating cultures for 7 d, after which time cells were fully aligned in the light path due to both positive and negative phototropism. When such cultures were transferred to darkness for an additional 7 d, clockwise arcs and spirals formed. Thus, spiral formation requires only a 7-day dose of microgravity or clino-rotation, as long as the cultures are of a sufficient age or stage (7 days or less). Moreover, spirals form in response to a constantly changing g-vector as well as to a virtual removal of gravity. This suggests that there is an endogenous growth polarity in Ceratodon that is normally suppressed by gravitropism. A working hypothesis is that the spirals represent a residual spacing mechanism for controlling colony growth and the distribution of side branches.