Understanding the arrangement of cellulose and matrix polysaccharides in primary cell walls is an important prerequisite for a molecular-level understanding of cell wall properties and function. One view from the polymer physics community considers cell walls to be a two-phase composite of crystalline cellulose nanofibrils embedded in a hydrated, amorphous matrix, with the cellulose orientation determining the mechanical anisotropy and the matrix being the prime determinant of various viscoelastic properties. Another views considers the matrix to be more organized as a molecular network that irreversibly binds to the surfaces of cellulose nanofibrils � a markedly different concept for wall architecture. In this presentation we will consider other ways of thinking about wall architecture. We have been studying the nano-scale structure, mechanical anisotropy and enzyme-induced creep behavior of primary cell walls to understand how cell wall properties emerge from wall structure and the action of selective endoglucanases and expansins. With AFM analysis of submerged, never-dried cell walls from growing tissues, we have imaged individual cellulose microfibrils that merge into and out of cellulose bundles that appear to be coated with matrix polysaccharides. We hypothesize that this reticulated network of cellulose bundles is a key determinant of wall mechanics and will discuss some of the potential modes of surface extension of this material. We have also measured stress/strain and creep behavior of cell walls in unilateral and bilateral extensions under the action of increasing force and the loosening action of enzymes and expansins. The results reveal that wall anisotropy is not simply dependent on cellulose orientation and hint at selective means for controlling cell wall mechanics.

100 years of cellulose fiber diffraction and the emergence of complementary techniques: Bundling and anisotropic linking of cellulose microfibrils in primary cell walls – significance for mechanics and expansive growth

245th National Meeting (2013)

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