Structure and nanomechanical properties of a wonderfully complex material, the primary cell wall of plants: recent progress based on AFM and FESEM

Abstract

Growing plant cells synthesize a strong yet extensible cell wall (=extracellular matrix) composed of long, thin cellulose microfibrils that are laterally bonded to one another in organized layers ~ 40 nm thick and that are embedded in a hydrated matrix consisting of complex polysaccharides (pectins, hemicelluloses). I will briefly review the biosynthetic origins of these wall components and what is known about their assembly to form a hierarchically‐structured hydrated material with diverse physical and chemical properties. Recent advances in atomic force microscopy (AFM), field effect scanning electron microscopy (FESEM), and solid‐state NMR have led to a rethinking of how these wall components interact with one another and how cells regulate the irreversible expansion (growth) of the cell wall 1‐4 . With AFM we have characterized the detailed organization of recently‐deposited cellulose microfibrils in never dried cell walls of onion epidermis. The interaction of microfibrils with matrix can be visualized in two‐color maps based height (which emphasizes microfibrils and modulus or adhesion, which highlight soft matrix. Additionally, when the cell walls are stretched to reveal microfibril re‐arrangements after plastic or elastic deformation or after enzyme‐mediated cell wall creep, we find different patterns of microfibril movements. FESEM images of the same material (except dried) detect only a surface layer of pectins that obscure the underlying microfibrils. These can be unveiled by enzyme digestions to selectively remove pectins. Some of the remaining unsolved problems in cell wall structure and polysaccharide interactions will be highlighted.