Roles of Microtubules in Plant Tip Growing Cells

Dear Colleagues,

In plant cells, unlike in animals, cytoplasmic streaming and organelle movement are mostly attributed to the actin cytoskeleton. The specific role of actin filaments is thought to have been inherited by land plants from an ancestral alga, since in the giant internodal cells of Characean algae, myosin motors enable more efficient organelle and metabolite movement in large plant cells. Thus, while actin-dependent cytoplasmic streaming uniformly redistributes organelles in plant cells, the restriction of organelles to specific cell areas and fine membrane movements suggest the involvement of microtubules (MTs). Moreover, the maintenance of Golgi morphology and the biogenesis of the trans-Golgi network have been associated with MT function. It was also reported that MTs contribute to ER tubule elongation and anchoring in Arabidopsis. MTs could therefore have a role in controlling membrane trafficking and secretion in somatic plant cells.

The cytoskeleton also represents a determinant for establishing and maintaining cell polarity; polarized growth represents a model of cell extension common to several eukaryotes. In basal and higher land plants, different types of cells are characterized by polarized growth, such as moss protonemata, the rhizoids of fern gametophytes, root hairs and pollen tubes. Data derived from actin filament depolymerization and the phenotypic analysis of mutants missing different actin-binding proteins has evidenced that actin filament organization and dynamics are essential for polar secretion and growth in moss protonemata. Interestingly, MTs were also implicated in tip growth since depolymerization assays showed that they participate in the correct positioning of actin-nucleating class II formins at the tip and in actin organization.

MT nucleation has been also related to MT organization during cell differentiation, and the molecular mechanisms involved in this process have been studied in plant somatic cells and moss protonemata. Interestingly, beside g-tubulin-dependent MT nucleation, an additional mechanism of cytoplasmic MT establishment has been hypothesized.

In pollen tubes, recent evidence suggests that MTs could participate in plasma membrane internalization in the tip region and in endosome trafficking. In addition, MT dynamics have been hypothesized to regulate the movements of prevacuolar compartments toward vacuoles, in tobacco pollen tubes.

Studying the origin, organization and role of MTs represents an exciting field for unravelling the concerted action of cytoskeletal components in plant tip growing cells.

Prof. Dr. Alessandra Moscatelli
Guest Editor

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• microtubules
• plant tip growing cells
• actin cytoskeleton
• MT dynamics