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First published online September 24, 2004; 10.1104/pp.104.043752

Plant Physiology 136:3266-3275 (2004)
© 2004 American Society of Plant Biologists

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CELL BIOLOGY AND SIGNAL TRANSDUCTION

The Mechanics of Surface Expansion Anisotropy in Medicago truncatula Root Hairs1

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

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 (J.D.); and Department of Biological Sciences, Stanford University, Stanford, California 94305 (S.R.L., S.L.S.)

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.

1 This work was supported by the Center for Computational Genetics and Biological Modeling, Stanford University (studentship to J.D.), by Dr. L. Mahadevan at the University of Cambridge (a research fellowship to J.D.), and by the U.S. Department of Energy (grant no. DE–FG03–90ER20010 to S.R.L.).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.043752.

* Corresponding author; e-mail jdumais{at}oeb.harvard.edu; fax 617–496–5854.

Received March 29, 2004; returned for revision July 15, 2004; accepted August 4, 2004.

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