Elastic properties of the forisome
Stephen A. Warmann A , William F. Pickard B and Amy Q. Shen A CA Department of Mechanical and Aerospace Engineering, Washington University in St Louis, St Louis, MO, USA.
B Department of Electrical and Systems Engineering, Washington University in St Louis, St Louis, MO, USA.
C Corresponding author. Email: aqshen@me.wustl.edu
Functional Plant Biology 34(10) 935-945 https://doi.org/10.1071/FP07132
Submitted: 23 May 2007 Accepted: 17 July 2007 Published: 13 September 2007
Abstract
Forisomes are elongate Ca2+-responsive contractile protein bodies and act as flow blocking gates within the phloem of legumes. Because an understanding of their mechanical properties in vitro underpins understanding of their physiology in vivo, we undertook, using a microcantilever method, microscopic tensile tests (incremental stress-relaxation measurements) on forisomes from Canavalia gladiata (Jacq.) DC Akanata Mame and Vicia faba L. Witkiem Major. Viscoelastic properties of forisomes in their longitudinal direction were investigated before and after Ca2+-induced contraction, but in the radial direction only before contraction. Forisomes showed mechanical properties typical of a biological material with a unidirectional fibrous structure, i.e. the modulus of elasticity in the direction of their fibers is much greater than in the radial direction. Creep data were collected in all tensile tests and fit with a three parameter viscoelastic model. The pre-contraction longitudinal elastic moduli of the forisomes were not differentiable between the two species (V. faba, 660 ± 360 kPa; C. gladiata, 600 ± 360 kPa). Both species showed a direction-dependent mechanical response: the elastic modulus was dramatically smaller in the radial direction than in the longitudinal direction, suggesting a weak protein cross-linking amongst longitudinal protein fibers. Activation of forisomes decreased forisome stiffness longitudinally, as evidenced by the loss of toe-region in the stress strain curve, suggesting that the forisome may have dispersed or disordered its protein structure in a controlled fashion. Contractile forces generated by single forisomes undergoing activation were also measured for V. faba (510 ± 390 nN) and C. gladiata (570 ± 310 nN).
Additional keywords: microcantilever, P-protein, sieve element, spasmoneme, viscoelasticity.
Acknowledgements
We thank the United States National Science Foundation for support under grant #0635352, Dr Shih-Chi Liu’s sensor program. Special thanks to Michael Knoblauch of Washington State University and Winfried Peters of Purdue/Indiana University for valuable assistance with forisome, to Frank Brozovich of the Mayo Clinic College of Medicine for detailed advice on gluing with polyurethane resin and to Bill McConnaughey for performing cross-calibrations with the Cell Poker in the Elson Laboratory at Washington University in Saint Louis.
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