Anisotropic Contraction in Forisomes: Simple Models Won’t Fit Winfried S. Peters, 1 * Michael Knoblauch, 2 Stephen A. Warmann, 3 William F. Pickard, 4 and Amy Q. Shen 3 1 Department of Biology, Indiana University/Purdue University Fort Wayne, Fort Wayne, Indiana 2 School of Biological Science, Washington State University, Pullman, Washington 3 Department of Mechanical and Aerospace Engineering, Washington University, St. Louis, Missouri 4 Department of Electrical and Systems Engineering, Washington University, St. Louis, Missouri Forisomes are ATP-independent, Ca 21 -driven contractile protein bodies act- ing as reversible valves in the phloem of plants of the legume family. Fori- some contraction is anisotropic, as shrinkage in length is associated with radial expansion and vice versa. To test the hypothesis that changes in length and width are causally related, we monitored Ca 21 - and pH-dependent defor- mations in the exceptionally large forisomes of Canavalia gladiata by high- speed photography, and computed time-courses of derived geometric parame- ters (including volume and surface area). Soybean forisomes, which in the resting state resemble those of Canavalia geometrically but have less than 2% of the volume, were also studied to identify size effects. Calcium induced sixfold volume increases in forisomes of both species; in soybean, responses were completed in 0.15 s, compared to about 0.5 s required for a rapid response in Canavalia followed by slow swelling for several minutes. This size-dependent behavior supports the idea that forisome contractility might rest on similar mechanisms as those of polyelectrolyte gels, a class of artificial ‘‘smart’’ materials. In both species, time-courses of forisome length and diameter were variable and lacked correlation, arguing against a simple causal relationship between changes in length and width. Moreover, changes in the geometry of soybean forisomes differed qualitatively between Ca 21 - and pH-responses, suggesting that divalent cations and protons target differ- ent sites on the forisome proteins. Cell Motil. Cytoskeleton 65: 368–378, 2008. ' 2008 Wiley-Liss, Inc. Key words: Ca 21 -dependent contractility; contractile protein; contraction kinetics; pH-dependent contractility; Canavalia gladiata [Jacq.] DC.; Glycine max L. *Correspondence to: Winfried S. Peters, Indiana/Purdue University Fort Wayne, 2101 East Coliseum Boulevard, Fort Wayne, Indiana 46805-1499, USA. E-mail: petersw@ipfw.edu Contract grant sponsor: Indiana/Purdue University Fort Wayne; Con- tract grant sponsor: Federal Ministry of Education and Research, Ger- many; Contract grant number: 0312014A; Contract grant sponsor: National Science Foundation’s Sensors and Sensor Networks pro- gram; Contract grant number: 0510941. Received 31 July 2007; Accepted 15 January 2008 Published online 10 March 2008 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/cm.20266 ' 2008 Wiley-Liss, Inc. Cell Motility and the Cytoskeleton 65: 368–378 (2008)