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First published online November 14, 2002; 10.1104/pp.012534

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Plant Physiol, December 2002, Vol. 130, pp. 2095-2100

Tip-Growing Cells of the Moss Ceratodon purpureus Are Gravitropic in High-Density Media1

Jochen Michael Schwuchow, Volker Dieter Kern,2 and Fred David Sack*

Department of Plant Biology, Ohio State University, Columbus, Ohio 43210-1293

Gravity sensing in plants and algae is hypothesized to rely upon either the mass of the entire cell or that of sedimenting organelles (statoliths). Protonemata of the moss Ceratodon purpureus show upward gravitropism and contain amyloplasts that sediment. If moss sensing were whole-cell based, then media denser than the cell should prevent gravitropism or reverse its direction. Cells that were inverted or reoriented to the horizontal displayed distinct negative gravitropism in solutions of iodixanol with densities of 1.052 to 1.320 as well as in bovine serum albumin solutions with densities of 1.037 to 1.184 g cm-3. Studies using tagged molecules of different sizes and calculations of diffusion times suggest that both types of media penetrate through the apical cell wall. Estimates of the density of the apical cell range from 1.004 to 1.085. Because protonemata grow upward when the cells have a density that is lower than the surrounding medium, gravitropic sensing probably utilizes an intracellular mass in moss protonemata. These data provide additional support for the idea that sedimenting amyloplasts function as statoliths in gravitropism.

1 This work was supported by the National Aeronautics and Space Administration Fundamental Biology Program (grant nos. NAG10-0179 and NAG10-0263 to F.D.S.) and by the Deutsche Forschungsgemeinschaft (to J.M.S.).

2 Present address: Lockheed Martin Space Operations, National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035-0168.

* Corresponding author; e-mail sack.1{at}osu.edu; fax 614-292-6345.

© 2002 American Society of Plant Biologists


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