These observations are consistent with the following model. The cellulose-synthesizing rosettes embedded in the plasma membrane spin out long cellulose molecules. As the synthesis of cellulose molecules and their self-assembly into microfibrils proceeds, the distal end of each microfibril presumably forms indirect cross-links to the previous layer of wall material, orienting the new microfibril in parallel with the old ones as it becomes integrated into the texture of the wall. Since the microfibril is stiff, the rosette at its growing, proximal end has to move as it deposits the new material. Traveling in the plane of the membrane, the rosette moves in the direction defined by the way in which the far end of the microfibril is anchored in the existing wall. In this way, each layer of microfibrils would tend to be spun out from the membrane in the same orientation as the layer laid down previously, with the rosettes following the direction of the preexisting oriented microfibrils outside the cell. Oriented microtubules inside the cell, however, can force a change in the direction in which the rosettes move: they can create boundaries in the plasma membrane that act like the banks of a canal to constrain rosette movement (Figure 19–65). In this view, cellulose synthesis can occur independently of microtubules; but it is constrained spatially when cortical microtubules are present to define membrane microdomains within which the enzyme complex can move.
Figure 19–65 One model of how the orientation of newly deposited cellulose microfibrils might be determined by the orientation of cortical microtubules.
(a) the large cellulose synthase complexes, or rosettes, are integral membrane proteins that continuously synthesize cellulose microfibrils on the outer face of the plasma membrane. the distal ends of the stiff microfibrils become integrated into the texture of the wall, and their elongation at the proximal end pushes the synthase complex along in the plane of the membrane. Because the cortical array of microtubules is attached to the plasma membrane in a way that confines this complex to defined membrane channels, the orientation of these microtubules— when they are present—determines the axis along which the new microfibrils are laid down. (B, C) two electron micrographs show the tight association of the cortical microtubules with the plasma membrane. One shows the microtubules in cross section while the other shows a microtubule in longitudinal section. Both emphasize the constant gap of about 20 nm between membrane and microtubule; the connecting molecules responsible remain obscure. (B and C, courtesy of andrew Staehelin.) 0.1 µm cellulose microfbril being added to preexisting wall plasma membrane microtubule attached to plasma membrane cellulose synthase complex CYTOSOL
In this way, plant cells can change their direction of expansion by a sudden What WE DON’t KNOW change in the orientation of their cortical array of microtubules. Because plant cells cannot move (being constrained by their walls), the entire morphology of • What are the regulatory mechanisms a multicellular plant presumably depends on a coordinated, highly patterned that control the rearrangement of deployment of cortical microtubule orientations during plant development. It is cell–cell junctions in epithelia during early development? What roles do not known how these orientations are controlled, although it has been shown that the microtubules can reorient rapidly in response to extracellular stimuli, includ these rearrangements?
ing plant growth regulators such as ethylene and auxins (discussed in Chapter 15).
Microtubules are not, however, the only cytoskeletal elements that influence • how do extracellular matrix proteins wall deposition. Local foci of cortical actin filaments can also direct the deposition and carbohydrates influence the of new wall material at specific sites on the cell surface, contributing to the elabo-localization and actions of extracellular rate final shaping of many differentiated plant cells.
receptors?