Plant Physiol. Drug Metab Dispos
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Published on September 24, 2004; 10.1104/pp.104.043752

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Received March 29, 2004
Returned for revision July 15, 2004
Accepted August 4, 2004

The Mechanics of Surface Expansion Anisotropy in Medicago truncatula Root Hairs

Jacques Dumais *, Sharon R. Long , and Sidney L. Shaw

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138
Department of Biological Sciences, Stanford University, Stanford, California 94305

* Corresponding author; email: jdumais{at}oeb.harvard.edu.

Wall expansion in tip-growing cells shows variations according to position and direction. In Medicago truncatula root hairs, wall expansion exhibits a strong meridional gradient with a maximum near the pole of the cell. Root hair cells also show a striking expansion anisotropy, i.e. over most of the dome surface the rate of circumferential wall expansion exceeds the rate of meridional expansion. Concomitant measurements of expansion rates and wall stresses reveal that the extensibility of the cell wall must vary abruptly along the meridian of the cell to maintain the gradient of wall expansion. To determine the mechanical basis of expansion anisotropy, we compared measurements of wall expansion with expansion patterns predicted from wall structural models that were either fully isotropic, transversely isotropic, or fully anisotropic. Our results indicate that a model based on a transversely isotropic wall structure can provide a good fit of the data although a fully anisotropic model offers the best fit overall. We discuss how such mechanical properties could be controlled at the microstructural level.






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