Structures in the cell wall that enable hygroscopic movement of wheat awns

Rivka Elbaum*, Stanislav Gorb, Peter Fratzl

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

78 Scopus citations

Abstract

The dispersal unit of wild wheat bears two prominent filaments called awns. The awns bend as they dry and straighten in a damp environment. This hygroscopic movement is explained by the orientation of the cellulose fibrils that build the cell wall, as follows. The stiff fibrils are embedded in a soft hygroscopic matrix. When the cell wall dries, the matrix shrinks but the fibrils do not. Therefore, the cell wall contracts in a direction perpendicular to the fibril orientation. Using X-ray scattering we identified a region at the base of the awn that contains fibrils aligned in all directions. This is the active part, which contracts as it dries and pulls the awn to a bent position. Cryo-scanning electron microscopy revealed sequential laminas which are rotated to form a nano-scale plywood construction, implying planar local order within the global isotropy. Water molecules absorbed into the matrix probably cause large microscopic distortions by expanding neighboring layers in perpendicular directions. This is thought to cause opening of tiny gaps between fiber layers, to facilitate the exchange and the transport of water through the cell wall, and thereby to increase the sensitivity of the actuating unit to moderate changes in humidity.

Original languageAmerican English
Pages (from-to)101-107
Number of pages7
JournalJournal of Structural Biology
Volume164
Issue number1
DOIs
StatePublished - Oct 2008
Externally publishedYes

Bibliographical note

Funding Information:
We thank Annemarie Martins for preparing the samples, Ingrid Zenke for measuring the WAXS, Cristine Pilz for the SAM images, Dirk Zerning for taking care of the plants, Eyal Shimoni for the help in producing the cryo SEM micrographs, and Moshe Feldman for supplying the wheat seeds. We thank Steve Weiner for his interesting ideas. R.E. thanks the Humboldt Foundation and the Charles Clore Program for fellowships, and the Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly for support. This work was supported by the Koshland Center for Basic Research.

Keywords

  • Awn
  • Cellulose
  • Dispersal
  • Hygroscopic movement
  • Plant material
  • Triticum turgidum
  • Water diffusion

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