Study of structure and dynamics of plant cell wall polysaccharides via neutron scattering

Plant cell wall plays important biological functions of plant bodies and constitutes many renewable sources that can be used to make paper, textile, polymer derivatives and alternative energy resources. At the nanoscale, plant cell wall is an extraordinarily complex composite composed of three polysaccharides, namely, cellulose, hemicellulose and pectins. Despite numerous works have been done to understand the wall polysaccharides over the past three centuries, many aspects of cell wall are still not clear. To enhance the understanding of plant cell wall, it is necessary to investigate cell wall at molecular level. Cell walls of both wild-type and xyloglucan-deficient mutant Arabidopsis thaliana are investigated. HPAEC experiment is performed to determine the fraction of cellulose, hemicellulose, and pectins. Cell wall structure at nanometer scale is characterized by small-angle neutron scattering (SANS). In this work, two methodologies are used to obtain information that was not available before. One is applying selective deuterium labeling on the non-cellulose polysaccharides in cell wall. Since the scattering of neutron from hydrogen is very different than that of deuterium, doing selective deuterium labeling create contrast between cellulose and the non-cellulose polysaccharides. Another method applied here is using contrast matching to detect structure of cellulose and the non-cellulose polysaccharides in turn. Models are utilized to fit the SANS profiles and to obtain the structural information of the cell wall samples. As expected, cellulose is high-aspect cylinders with diameters of about 3 nm and lengths of more than 100 nm. Partial surface of cellulose is covered by non-cellulose polysaccharides in wild-type Arabidopsis, which feature is negligible in xyloglucan-deficient mutant. This difference illustrates a unique role of xyloglucan in cell wall. However, not all xyloglucan is in close contact with cellulose, about half of xyloglucan has structure similar to the other non-cellulose polysaccharides, being solvated in water and only form extended coils, potentially function as spacers to separate cellulose microfibrils. The experiment method, used to determined cell wall structure of Arabidopsis, is extended to several plant species to obtain a more general picture of cell wall. One prominent difference among plants is cell wall composition. For example, non-graminaceous land plants contain a large fraction of xyloglucan whereas graminaceous monocot cell walls contain little xyloglucan but much mixed-linkage glucans and arabinoxylans. HPAEC and SANS are used to determine the composition and the cell wall structure of cucumber, onion, wheat and maize. The composition and the cell wall structure of cucumber and onion are similar to that is found in wild-type Arabidopsis: cellulose microfibrils are about 3 nm thick and partially coated by xyloglucan. Most of the non-cellulose polysaccharides form extended coils. Maize and wheat have little xyloglucan and decent amount of mixed-linkage glucan. A decent…