Cell Edges Accumulate Gamma Tubulin Complex Components and Nucleate Microtubules following Cytokinesis in Arabidopsis thaliana

Microtubules emanate from distinct organizing centers in fungal and animal cells. In plant cells, by contrast, microtubules initiate from dispersed sites in the cell cortex, where they then self-organize into parallel arrays. Previous ultrastructural evidence suggested that cell edges participate in microtubule nucleation but so far there has been no direct evidence for this. Here we use live imaging to show that components of the gamma tubulin nucleation complex (GCP2 and GCP3) localize at distinct sites along the outer periclinal edge of newly formed crosswalls, and that microtubules grow predominantly away from these edges. These data confirm a role for cell edges in microtubule nucleation, and suggest that an asymmetric distribution of microtubule nucleation factors contributes to cortical microtubule organization in plants, in a manner more similar to other kingdoms than previously thought

Actin microfilament and microtubule distribution patterns in the expanding root of Aribidopsis thaliana

Determination of the precise role(s) of actin microfilaments in the control of cell shape and elongation in the root tips of the model genetic system Arabidopsis thaliana (L.) Heynh is frustrated by inadequate microscopy imaging techniques. In this paper, we documented both microfilaments and microtubules in the root tips of Arabidopsis by double immunofluorescence labelling and computer-generated reconstruction of confocal image series. Our procedure, which complements the use of recently developed fluorescent reporter proteins, revealed hitherto undescribed aspects of the Arabidopsis microfilament cytoskeleton that may provide important clues about mechanisms behind cell elongation. We found that preservation of extensive arrays of transverse cortical microfilaments depends on unperturbed microtubule organization. Compared with ordinary epidermal cells, cells situated in the trichoblast or hair-forming cell files were comparatively devoid of endoplasmic microfilaments when in the distal elongation zone, well before hair formation begins. Computer-aided reconstructions also revealed that the nonexpanding end walls of cells in the distal elongation zone have radially oriented microtubules and randomly arranged microfilaments. In dividing cells, microfilaments became more prominent in the cell cortex, and subtle differences between microtubule and microfilament organization were seen within the phragmoplast. These observations will form the basis of understanding the roles of the cytoskeleton in controlling elongation in root tissues. In light of the many Arabidopsis mutants with altered root morphology, our methods offer a reliable approach to assess the function of cytoskeletal proteins and signalling systems in root morphogenesis

The cytoskeleton and co-ordination of directional expansion in a multicellular context

The cytoskeleton governs many critical processes in expanding plant cells, including the delivery of wall components and the establishment and maintenance of growth direction. This work describes how cytoskeletal arrays assemble, and how their spatial organization and dynamics regulate the anisotropic properties of plant cell walls. We describe the mechanisms that construct and organize transverse microtubule arrays, and explore how these arrays, and the direction of elongation, are influenced by hormones. We then consider how cortical microtubules regulate the mechanical properties of the load-bearing cellulose microfibrils, through interacting with cellulose synthase complexes, and by coordinating the secretion of wall proteins. Actin microfilaments form part of the machinery that controls polar auxin transport, and have critical functions in vesicle transport. In recent years, it has become increasingly clear that microtubules and actin microfilaments work in concert to coordinate cell expansion. This microfilament-microtubule coordination is mediated through the activity of Rop GTPase signalling switches. We highlight this process in the growth of pavement cells found in the epidermal layers of leaves