Effects of mechanical stretching on average orientation of cellulose and pectin in onion epidermis cell wall: A polarized FT-IR study
- 617 Downloads
The organization of polysaccharides in plant cell walls is important for the mechanics of plant cells. Spectral analysis of cell walls by polarized IR can reveal polysaccharide organization, but may be complicated by dipoles not aligned with the backbone. For instance, analysis of uniaxially-aligned cellulose Iβ film revealed that the dipole transition vector of the 1160 cm−1 band involving stretch vibrations of glycosidic C1–O–C4 linkages is approximately at 30° with respect to the backbone of the cellulose chain, because of coupling with C5–O–C1 bonds in the six-membered rings. In the case of homogalacturonan, the dipole transition vector of the ester carbonyl group vibration (νC=O, 1745 cm−1) is expected to be nearly normal to the homogalacturonan backbone. Using this information and the dichroism equation, the change in net orientation of cell wall polymers upon mechanical stretch was determined by polarized IR analysis. Never-dried abaxial outer epidermal cell walls of the second scale of onion bulb were mechanically stretched along longitudinal or transverse directions with respect to the long axis of the cells and then dried while under mechanical stretch. The average orientations of both 1160 and 1745 cm−1 vibration transition dipoles were rotated by ~5° and ~4°, respectively, along the stretch direction from their initial random distributions upon longitudinal strain by 14%; and by ~4° and ~3°, respectively, upon transverse strain by 12%. These results imply that both cellulose microfibrils and pectins in the cell wall are passively realigned along the stretch direction by external mechanical force. The analytical methodology developed here will be useful to study how cell wall polymers might reorganize during cell wall growth and development.
KeywordsOnion epidermis Dichroism Cellulose Pectin Plant cell walls Molecular orientation Mechanical stretch Infra-red spectroscopy
This work was supported by the Center for Lignocellulose Structure and Formation (CLSF), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0001090. We would like to thank Dr. James Kubicki for visualization of the 1160 cm−1 vibration mode from our previous DFT-D2 calculations.
Supplementary material 2 (MP4 11212 kb)
- 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
- Smith-Moritz AM, Hao Z, Fernández-Niño SG, Fangel JU, Verhertbruggen Y, Holman H-YN, Willats WG, Ronald PC, Scheller HV, Heazlewood JL (2015) Structural characterization of a mixed-linkage glucan deficient mutant reveals alteration in cellulose microfibril orientation in rice coleoptile mesophyll cell walls. Front Plant Sci 6:628CrossRefGoogle Scholar
- Spatz H, Kohler L, Niklas K (1999) Mechanical behaviour of plant tissues: composite materials or structures? J Exp Biol 202(23):3269–3272Google Scholar
- Wilkes GL (1971) The measurement of molecular orientation in polymeric solids. In: Fortschritte der Hochpolymeren-Forschung. Advances in polymer science, vol 8. Springer, Berlin, pp 91–136. doi: 10.1007/3-540-05483-9_10