Extending the Microtubule/Microfibril Paradigm1

Abstract

The cortical microtubule array provides spatial information to the cellulose-synthesizing machinery within the plasma membrane of elongating cells. Until now data indicated that information is transferred from organized cortical microtubules to the cellulosesynthesizing complex, which results in the deposition of ordered cellulosic walls. How cortical microtubules become aligned is unclear. The literature indicates that biophysical forces, transmitted by the organized cellulose component of the cell wall, provide a spatial cue to orient cortical microtubules. This hypothesis was tested on tobacco (Nicotiana tabacum L.) protoplasts and suspension-cultured cells treated with the cellulose synthesis inhibitor isoxaben. Isoxaben (0.25-2.5 M) inhibited the synthesis of cellulose microfibrils (detected by staining with 1 g mL ؊1 fluorescent dye and polarized birefringence), the cells failed to elongate, and the cortical microtubules failed to become organized. The affects of isoxaben were reversible, and after its removal microtubules reorganized and cells elongated. Isoxaben did not depolymerize microtubules in vivo or inhibit the polymerization of tubulin in vitro. These data are consistent with the hypothesis that cellulose microfibrils, and hence cell elongation, are involved in providing spatial cues for cortical microtubule organization. These results compel us to extend the microtubule/microfibril paradigm to include the bidirectional flow of information.

Toward a Systems Approach to Understanding Plant Cell Walls

“…However, studies of the conditional mor1 mutant of Arabidopsis, which is deficient in microtubule polymerization at the nonpermissive temperature, have shown that ordered cellulose deposition is possible in the absence of assembled cortical microtubules and an existing cellulose template (51,52). It has also been observed that treatment of protoplasts with the cellulose synthase inhibitor isoxaben prevents characteristic orientation of the microtubules demonstrating cell wallto-cytoskeleton feedback (53). We believe that these seemingly contradictory lines of evidence may reflect a variable relationship between the cytoskeleton and the cellulose synthase complexes, depending on the stage of cell wall synthesis and expansion.…”

Wall-associated kinase 1 (WAK1) is crosslinked in endomembranes, and transport to the cell surface requires correct cell-wall synthesis

“…Immunodetection in protoplasts was done as described in Fisher and Cyr (Fisher and Cyr, 1998). Rat Jim5 and anti-rat Cy3 (Santa Cruz Biologicals, CA) were used at 1:100.…”

Multiple Cellulose Synthase Catalytic Subunits Are Required for Cellulose Synthesis in Arabidopsis

“…In expanding plant cells, the cellulose-xyloglucan network is considered the main load-bearing network that controls the extent of cell expansion, whereas the orientation of cellulose microfibril deposition has a major role in controlling the direction of cell expansion (Fisher and Cyr, 1998). After a period of expansion, certain cell types lay down a thick secondary cell wall inside the primary wall.…”