Real-Time Imaging of Cellulose Reorientation during Cell Wall Expansion in Arabidopsis Roots    

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.

Anisotropic Cell Expansion Is Affected through the Bidirectional Mobility of Cellulose Synthase Complexes and Phosphorylation at Two Critical Residues on CESA3

“…In this study, atomic force microscopy was applied to image the primary cell wall from the upper hypocotyls of dark-grown seedlings. It was confirmed by this study that cellulose existed mainly in the form of fibrillar bundles in the cell wall of expanding hypocotyl cells (Anderson et al, 2010;Ding et al, 2014), and the diameter of the fibrillar bundles was in the range of 20 to 150 nm (Fig. 5, B-F), comparable to the size observed in the primary cell wall from maize (Ding and Himmel, 2006).…”

“…5; Anderson et al, 2010;Ding et al, 2014), which might be an important mechanism for the regulation of cellulose-cellulose (Thomas et al, 2013), cellulose-pectin (Wang et al, 2015), and cellulosexyloglucan interactions. It has been observed that ,8% of the cellulose surface is available for cross-bridges with noncellulosic polymers (Bootten et al, 2004;Dick-Pérez et al, 2011).…”

“…Real-time imaging of cellulose using a fluorescence dye S4B has also shown that cellulose exists in large fibrillar bundles in the primary cell wall of Arabidopsis (Anderson et al, 2010). Thus, it has been suggested that bundles of aggregated cellulose microfibrils, not single microfibrils, might be the key structural units in primary cell walls (Thomas et al, 2013).…”

“…Model for regulation of anisotropic cell wall expansion by bidirectional deposition of cellulose. A, Bidirectional synthesis and deposition of microfibrils results in cellulose aggregation to fibrillar bundles or macrofibrils (Anderson et al, 2010), which are predominantly ribbon like (Ding et al, 2014), and some microfibril segments remain in direct contact with one another (Thomas et al, 2013). B, In the case of mismatched lengths of oppositely oriented microfibrils, derived from discrepancy of bidirectional mobility of cellulose synthase complexes, the spaces between one microfibril or macrofibril and the next may increase, allowing more noncellulosic polymers (dashed lines) to cross-bridge adjacent microfibrils or macrofibrils and thus retard cell elongation.…”

Lack of α-Xylosidase Activity in Arabidopsis Alters Xyloglucan Composition and Results in Growth Defects

“…Apart from being slightly smaller, this mutant has defective root hairs, but it seems clear that Arabidopsis must have alternative ways of regulating the separation of cellulose microfibrils. It is interesting nonetheless that microfibrils seem to be more irregularly spaced in the xyloglucan-deficient mutant (Anderson et al, 2010).…”