Impact of acidic pH on plant cell wall polysaccharide structure and dynamics: insights into the mechanism of acid growth in plants from solid-state NMR
- 67 Downloads
Acidification of plant primary cell walls causes cell wall expansion and plant growth. To understand how acidic pH affects the molecular structure and dynamics of wall polysaccharides, we have now characterized and compared Arabidopsis thaliana primary cell walls in neutral (pH 6.8) and acidic (pH 4.0) conditions using solid-state NMR spectroscopy. Quantitative 13C solid-state NMR spectra indicate that the pH 4.0 cell wall has neutral galacturonic acid residues in homogalacturonan (HG) and rhamnogalacturonan (RG). 13C INEPT spectra, which selectively detect highly dynamic polymers, indicate that some of the HG and RG chains in the interfibrillar region have become more dynamic in the acidic wall compared to the neutral cell wall, whereas other chains have become more rigid. Consistent with this increased dynamic heterogeneity, C–H dipolar couplings and 2D 13C–13C correlation spectra indicate that some of the HG backbones are partially aggregated in the acidic cell wall. Moreover, 2D correlation spectra measured with long mixing times indicate that the acidic cell wall has weaker cellulose–pectin interactions, and water-polysaccharide 1H spin diffusion data show that cellulose microfibrils are better hydrated at low pH. Taken together, these results indicate a cascade of chemical and conformational changes of wall polysaccharides due to cell wall acidification. These changes start with neutralization of the pectic polysaccharides, which disrupts calcium crosslinking of HG, causes partial aggregation of the interfibrillar HG, weakens cellulose–pectin interactions, and increases the hydration of both cellulose microfibrils and matrix polysaccharides. These molecular-level structural and dynamical changes are expected to facilitate polysaccharide slippage, which underlies cell wall loosening and expansion, and may occur both independent of and as a consequence of protein-mediated wall loosening.
KeywordsPlant primary cell wall Solid-state NMR Acidic pH Acid growth Cellulose–pectin interaction
- Ara, A
- GalA, GA
Insensitive nuclei enhanced by polarization transfer
Interior crystalline cellulose
Proton-driven 13C–13C spin diffusion
- Rha R
Solid-state nuclear magnetic resonance
Surface amorphous cellulose
- Xyl, x
This research was supported by the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001090.
- Durachko DM, Cosgrove DJ (2009) Measuring plant cell wall extension (creep) induced by acidic pH and by alpha-expansin. J Vis Exp 25:1263Google Scholar
- Kohn R, Kovac P (1978) Dissociation-constants of D-galacturonic and D-glucuronic acid and their O-methyl derivatives. Chem Pap 32:478–485Google Scholar