Molecular Rigidity in Dry and Hydrated Onion Cell Walls

Abstract

Solid-state nuclear magnetic resonance relaxation experiments can provide information on the rigidity of individual molecules within a complex structure such as a cell wall, and thus show how each polymer can potentially contribute to the rigidity of the whole structure. We measured the proton magnetic relaxation parameters T2 (spin-spin) and T,p (spin-lattice) through the 13C-nuclear magnetic resonance spectra of dry and hydrated cell walls from onion (Allium cepa L.) bulbs. Dry cell walls behaved as rigid solids. The form of their T, decay curves varied on a continuum between Caussian, as in crystalline solids, and exponential, as in more mobile materials. The degree of molecular mobility that could be inferred from the T2 and Tlp decay patterns was consistent with a crystalline state for cellulose and a glassy state for dry pectins. The theory of composite materials may be applied t o explain the rigidity of dry onion cell walls in terms of their components. Hydration made little difference to the rigidity of cellulose and most of the xyloglucan shared this rigidity, but the pectic fraction became much more mobile. Therefore, the cellulose/ xyloglucan microfibrils behaved as solid rods, and the most significant physical distinction within the hydrated cell wall was between the microfibrils and the predominantly pectic matrix. A minor xyloglucan fraction was much more mobile than the microfibrils and probably corresponded to cross-links between them. Away from the microfibrils, pectins expanded upon hydration into a nonhomogeneous, but much softer, almost-liquid gel. These data are consistent with a model for the stress-bearing hydrated cell wall in which pectins provide limited stiffness across the thickness of the wall, whereas the cross-linked microfibril network provides much greater rigidity in other directions.Little is known about the details of how the structure of plant cell walls gives them their rigidity and strength, and thus their ability to support the plant against the stresses of weather, gravity, and transpiration (Raven, 1977; Preston, 1979). Solid-state NMR spectrometry has the potential to provide a window into the internal stress-bearing properties of the cell wall. Magnetic relaxation data derived from NMR experiments allow the relative mobilities of individual polymers, and of functional groups within them, to be inferred (Abragam, 1961).Much of the relevant NMR methodology, and the theory for linking polymer mobility to bulk mechanical properties, have been developed for synthetic macromolecules (Schaefer et al., 1977;Kenwright and Say, 1993;McBrierty and Packer, 1994). Similar experiments have been carried out on plant cell walls (Irwin et al., 1984(Irwin et al., , 1985Newman et al., 1994Newman et al., , 1996 Foster et al., 1996;Ha et al., 1996), but the full transfer of the technology from synthetic polymers to hydrated biological materials such as the cell wall is not likely to be simple. It will require solutions to substantial experimental problems arising from the presence of wa...

Macromolecular biophysics of the plant cell wall: Concepts and methodology

“…It also rules out the possibility of studying the effects of controlled mechanical stress on the sample, and normally restricts the sample size to about 0.1 g so that some thousands of repetitions of each spectral measurement must be averaged to obtain an adequate signal/noise ratio. This can lead to very long durations for 13 C NMR experiments. These can be reduced by …”

“…It does this through the tiny magnetic fields created at the nucleus by orbiting electrons. Cell-wall polysaccharides in solution give detailed and highly informative 13 C and 1 H NMR spectra. However, if intact cell walls are examined in a conventional, solution-state NMR spectrometer, the 13 C or proton spectrum is so broad that it cannot be seen.…”

“…Solid-state 13 C NMR became a practical possibility in the late 1970s with the introduction of high-speed magic-angle spinning (MAS) and high-power dipolar decoupling, which provide alternatives to thermal averaging of these broadening effects [51]. This is achieved at a cost.…”

“…cross-polarisation (CP) or transfer of magnetisation from 1 H to 13 C for measurement, mainly because 1 H magnetisation returns to equilibrium much faster than 13 C so that each repetition requires about 1 s rather than 1 min. The CP-MAS experiment, with highpower decoupling, has become the standard way of recording NMR spectra from organic solids [51].…”

Group III-AXTHGenes of Arabidopsis Encode Predominant Xyloglucan Endohydrolases That Are Dispensable for Normal Growth

“…Classical models of the primary cell wall in land plants describe the wall as a composite structure composed of load-bearing, semicrystalline cellulose fibrils surrounded by an amorphous matrix of cross-linking glycans (hemicelluloses and pectins), structural (glyco)proteins, and, in some cases, polyphenolics (Carpita and Gibeaut, 1993;Carpita and McCann, 2000;Cosgrove, 2005;Jarvis, 2009). Moreover, primary plant cell walls have a high water content (approximately 80% in primary walls; Jarvis, 2009), which is responsible for maintaining the structure in a dynamic, hydrogel-like state (Ha et al, 1997;Zwieniecki et al, 2001;Jarvis, 2009). Contemporary structural analysis, using increasingly sophisticated probes, is refining our understanding of the exquisite spatial localization of cell wall polymers (Knox, 2008;Yarbrough et al, 2009).…”