CYTOSKELETAL CONTROL OF POLAR GROWTH IN PLANT-CELLS

There are two quite different modes of polar cell expansion in plant c ells, namely, diffuse growth and tip growth. The direction of diffuse growth is determined by the orientation of cellulose microfibrils in t he cell wall, which in turn are aligned by microtubules in the cell co rtex. The orientation of the cortical microtubule array changes in res ponse to developmental and environmental signals, and recent evidence indicates that microtubule disassembly/reassembly and microtubule tran slocation participate in reorientation of the array. Tip growth, in co ntrast, is governed mainly by F-actin, which has several putative form s and functions in elongating cells. Longitudinal cables are involved in vesicle transport to the expanding apical dome and, in some tip gro wers, a subapical ring of F-actin may participate in wall-membrane adh esions. The structure and function of F-actin within the apical dome m ay be variable, ranging from a dense meshwork to sparse single filamen ts. The presence of multiple F-actin structures in elongating tips sug gests extensive regulation of this cytoskeletal array.