The cytoskeleton underlying side walls and cross walls in plants: molecules and macromolecular assemblies

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Abstract
Plant cells organize their growth by reinforcing side walls during interphase (causing them to elongate) and by positioning and orienting the cross wall at cytokinesis. In the first part of this presentation we review progress made in identifying different cytoskeletal components that underlie side walls and that are involved in the deposition of the cross wall. During interphase, the cortical microtubule arrays co-distribute with an antigen recognized by a ‘universal’ monoclonal antibody to intermediate filaments. Using rhodaminyl-lysine-phalloidin no F-actin could be detected at the cortex but endoplasmic, axial cables were found. The cytokinetic apparatus – the phragmoplast – contains microtubules and we find that F-actin and the intermediate filament antigen also co-distribute with this array. We describe the three-dimensional arrangement of microtubules forming the interphase array in cells enlarging by both tip-growth and intercalary growth. In root hairs of higher plants and in apical cells of the filamentous stage of moss Physcomitrella patens, microtubules (MT) are detected at the apices and it is suggested from this that fragmentation of microtubules and absence of MTs from the tip are preparation artefacts. Using human serum from a scleroderma patient, possible microtubule nucleating sites are detected in meristematic cells; these segregate with the broad spindle poles and they surround the nucleus during early interphase –implying a peri-nuclear origin for the cortical MT array. The interphase microtubule array is described in terms of a dynamic helical model, which proposes: that the MT array is an integral complex; that microtubules form helices; that helices can change their pitch – the array converting to the various conformations.