Cytoskeleton and integration of cellular function in cells of higher plants

Constraints and opportunities inherent in a cellular existence in which the living protoplast is surrounded by a more-or-less rigid wall have given rise to a number of uniquely botanical modes of cytoskeletal form and function. These are summarized here, with special reference to the concept of a structurally and functionally integrated cytoplasmic matrix . The cell wall possesses at least some ofthe shape-maintaining properties that, in animal cells, are based on the cytoskeleton . A simple demonstration of this is the inability of a plant cell protoplast to withstand the forces of surface tension that it meets when it is removed from within its cell wall : it generally rounds up to a sphere, irrespective of its original shape. The cell wall influences cytoskeletal functions in other, less obvious ways. For instance, because shared cell walls allow only slight adjustments to the relative positions ofneighboring cells, plants must regulate very precisely the site and plane in which they divide and insert new walls . The mechanism is cytoskeletal, involving the preprophase band (PPB),' a specialized array of microtubules (MTs) . The shape of the wall itself is largely determined by its directional yielding to hydrostatic turgor pressures, and in turn this geometry is related to the initial orientation of its cellulose microfibrils . MT arrays underlying the plasma membrane appear to participate in regulating wall microfibril orientation, providing an indirect cytoskeletal control over the shape that the cell will assume when it expands. Control of both wall insertion and wall shaping thus is at least partly mediated by MTs. Cells so large that diffusion is an insufficient means of transporting solutes along intracellular and intercellular pathways commonly develop, and in such cases intracellular motility based on actomyosin becomes prominent. Although presumably a modification of the system which, in animals, changes cell shape and drives cell migration, its function in large plant cells is primarily to distribute metabolites and organelles . Nevertheless, within the confines ofthe wall, the shape ofthe protoplast