Alignment of Cortical Microtubules by Anisotropic Wall Stresses

Abstract

Anisotropic mechanical forces exist in the walls of all turgid plant cells except those of spherical unicells. These forces potentially offer the cell important directional information regarding its major and minor axes, and the more complicated force patterns expected in multicellular organs could offer location-specific information. A mechanism is proposed to measure directional forces in the wall as deformations (strain) of molecules associated with cellulose microfibrils and transmit the information across the plasma membrane to orient cortical microtubules. Because microtubules in turn orient the synthesis of the cellulose microfibrils that determine the direction of future cell expansion, a feedback loop is established relating future cell shape to present cell shape and cell position. The loop can provide positive feedback to magnify a small asymmetry in shape, to reinforce an existing growth axis in a cylindrical cell or, by modification of the proteins postulated to convey directional information between wall and cytoplasm, the loop can be broken and the same directional information used to establish a new orientation for cortical microtubules. In this way, modification of a single protein replaces a transverse microtubule array with a helical one.