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

Open Access
Author:
Huang, Shih-chun
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 21, 2015
Committee Members:
  • Janna Kay Maranas, Dissertation Advisor
  • Enrique Daniel Gomez, Committee Member
  • Seong H Kim, Committee Member
  • Daniel J Cosgrove, Committee Member
Keywords:
  • plant cell wall
  • cellulose
  • hemicellulose
  • neutron scattering
  • structure
  • dynamics
Abstract:
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 coating around cellulose microfibril is observed in in maize cell wall, presumably the mixed-linkage glucan play a similar role as xyloglucan in maize. This might be because the similar backbone to xyloglucan and cellulose microfibrils. To fully understand plant cell wall, we study not only cellulose-xyloglucan interaction between cellulose and the other polysaccharides, but also the interaction between cellulose and water. It should be noted that water makes up about 80% mass of cell wall. To remain active, cell wall requires an aqueous environment to permeate proteins and ions. Quasi-elastic neutron scattering is used to study the water dynamics. We investigate the state of water when it is in close contact with cellulose. On the surface of cellulose, two hydration layers are identified. Water in the inner hydration layer is directly constrained by the surface of cellulose. Thus the water molecules are confined and show relaxation times 100 times slower than bulk water. Water in the outer hydration layer is less confined but still has dynamics 5 times slower than bulk water. Despite the difference in the translational motion, both inner and outer hydration water shares similar rotational and vibrational diffusivity. To sum up, we bring an integrated result that includes the molecular interaction not only between cellulose and the non-cellulose polysaccharides but also between cellulose and water. Our results indicate cellulose in primary cell wall is about 3 nm thick, and sometimes the surface is covered by matrix polysaccharides if xyloglucan or mixed-linked glucan is decent. Most matrix polysaccharides are not capable to directly deposit on the cellulose surface. Instead, they form extended coils, presumably function as interfibril spacers. It should be noted that in the current picture of cell wall, water is often regarded as background. However, their very slow dynamics when it is in close contact with cellulose illustrates their ability to form direct bonding with cellulose. This should be taken into consideration in many experiment interpretations and building the cell wall model.