Abstract
Cellulose forms the major load-bearing network of the plant cell wall, which simultaneously protects the cell and directs its growth. Although the process of cellulose synthesis has been observed, little is known about the behavior of cellulose in the wall after synthesis. Using Pontamine Fast Scarlet 4B, a dye that fluoresces preferentially in the presence of cellulose and has excitation and emission wavelengths suitable for confocal microscopy, we imaged the architecture and dynamics of cellulose in the cell walls of expanding root cells. We found that cellulose exists in Arabidopsis (Arabidopsis thaliana) cell walls in large fibrillar bundles that vary in orientation. During anisotropic wall expansion in wild-type plants, we observed that these cellulose bundles rotate in a transverse to longitudinal direction. We also found that cellulose organization is significantly altered in mutants lacking either a cellulose synthase subunit or two xyloglucan xylosyltransferase isoforms. Our results support a model in which cellulose is deposited transversely to accommodate longitudinal cell expansion and reoriented during expansion to generate a cell wall that is fortified against strain from any direction.
Highlights
Cellulose forms the major load-bearing network of the plant cell wall, which simultaneously protects the cell and directs its growth
Our results provide evidence that in some layers of the cell wall, cellulose microfibrils rotate in a transverse to longitudinal direction after their synthesis in anisotropically expanding root cells, as has been suggested for Arabidopsis hypocotyl cell walls (Refregier et al, 2004)
The average strain rate of 30.8% for fiber rotation we calculated is in agreement with previous calculations of strain rate for whole Arabidopsis root cell walls (Beemster and Baskin, 1998), which suggests that passive reorientation of the fibers within the wall matrix in response to turgor pressure-driven cell expansion could be sufficient to explain the rotation we observed
Summary
Previously described as cell wall stains for fungi, Solophenyl Flavine 7GFE (7GFE) and Pontamine Fast Scarlet 4B (S4B; Hoch et al, 2005), and Calcofluor, which has been widely used to stain cell wall components (Sauter et al, 1993; Fig. 1A; Supplemental Fig. S1). Leftward- and rightward-tilted diagonal fibers coexisted in single cells (Fig. 2C), indicating that the handedness of cellulose fiber tilting was not restricted to one direction These fibrillar staining patterns were reminiscent of wide-field images of Calcofluor staining in cotton (Gossypium hirsutum) fibers (Sauter et al, 1993) and of XET activity in root cells that likely reflects celluloseassociated xyloglucans (Vissenberg et al, 2005). – 1.00 6 0.281 19.3 6 2.84 1.47 6 0.428 0.965 6 0.230 1.47 6 0.0618 15.2 6 2.51 9.01 6 0.473 1.86 6 0.0875 3.19 6 1.50 0.901 6 0.113 1.04 6 0.0965 0.882 6 0.152 0.888 6 0.0929 0.915 6 0.113 27.9 6 1.59 1.34 6 0.297 4.09 6 0.259 117.6 6 3.74 0.873 6 0.0966 1.82 6 0.211 10.5 6 0.506 noticed bright, amorphous S4B staining at root hair primordia in trichoblasts (Fig. 2E), which possibly reflects changes in cell wall structure at these locations This bright staining could reflect an increase in the amount or accessibility of cellulose at these locations, but could reflect an accumulation of some other wall component that fluoresces more weakly with S4B such as xyloglucan or callose. For the time-lapse experiments described above, an S4B concentration that did not inhibit root growth (0.01%) was used
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