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JXB Advance Access originally published online on August 1, 2006
Journal of Experimental Botany 2006 57(12):3091-3098; doi:10.1093/jxb/erl072
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© The Author [2006]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

The geometry of the forisome–sieve element–sieve plate complex in the phloem of Vicia faba L. leaflets

Winfried S. Peters1,2,*, Aart J. E. van Bel2 and Michael Knoblauch2,3,*

1Indiana University/Purdue University, Department of Biology, 2101 East Coliseum Boulevard, Fort Wayne, IN 46805-1499, USA
2Institut für Allgemeine Botanik, Justus-Liebig-Universität, Senckenbergstr. 17–21, D-35390 Gießen, Germany
3School of Biological Sciences, Washington State University, Pullman, WA 99164-4236, USA

*To whom correspondence should be addressed. E-mail: petersw{at}ipfw.edu, knoblauch{at}wsu.edu.

Forisomes are contractile protein bodies that appear to control flux rates in the phloem of faboid legumes by reversibly plugging the sieve tubes. Plugging is triggered by Ca2+ which induces an anisotropic deformation of forisomes, consisting of a longitudinal contraction and a radial expansion. By conventional light microscopy and confocal laser-scanning microscopy, the three-dimensional geometry of the forisome–sieve element–sieve plate complex in intact sieve tubes of leaflets of Vicia faba L. was reconstructed. Forisomes were mostly located close to sieve plates, and occasionally were observed drifting unrestrainedly along the sieve element, suggesting that they might be utilized as internal markers of flow direction. The diameter of forisomes in the resting state correlated with the diameter of their sieve elements, supporting the idea that radial expansion of forisomes is the geometric basis of reversible sieve tube plugging. Comparison of the present results regarding forisome geometry in situ with previously published data on forisome reactivity in vitro makes it questionable, however, whether forisomes are capable of completely sealing sieve tubes in V. faba leaves.

Key words: Ca2+-dependent contractility, contractile protein, forisome, phloem transport, sieve element plugging, sieve tube geometry, Vicia faba L


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