Unique Characteristics of Cell Division in Vascular Plants

In eukaryotes, both microtubules (MTs) and microfilaments (MFs) are cytoskeletal polymers involved in meristematic cell proliferation. While animal cells build their MT arrays from structured organelles, such as centrosomes, and while they depolymerise their MFs and become round during mitosis, vascular plant cells lack centrosomes, maintain a filamentous actin cage around the spindle and are surrounded by a cell wall, preventing cellular mobility. During the cell cycle, plants activate specific dispersed MT nucleating sites, revealing successive plant-specific cytoskeletal arrays. The MF meshwork surrounding the spindle eventually drives the centrifugal growth of MTs, which leads Golgi-derived vesicles to fuse and separate each daughter cell during cytokinesis. The main orientation of actin fibres is parallel to that of spindle MTs, while a perpendicular constriction ring ensures animal daughter cell separation. However, despite the differences in their cytoskeleton behaviour and dynamics, both cell types succeed in controlled chromosome segregation. Key Concepts Mitosis is a short cell cycle period during which the cytoskeleton ensures balanced segregation of duplicated sister chromatids into two daughter cells. Plant cells have to deal with their pecto-cellulosic cell wall, constraining the orientation of the division axis in order to control morphogenesis. In plants, primary stem cells are located in shoot and root apical meristems (SAM and RAM) and secondary growth is ensured by cambium activation. Besides the conservation of microtubules (MTs) and actin filaments (MFs), required for successful karyokinesis and cytokinesis, respectively, in all eukaryotes, plant mitotic cells present an original cytoskeleton organisation. All the somatic cells of vascular plants lack centrosomes, and microtubule organising centres are spread all over the nuclear envelope, at the cell cortex and along pre-existing microtubules. Preceding the mitotic step per se, maintenance of centromere/kinetochore cohesion and integrity is required for the building of a bipolar spindle and accurate chromosome segregation. During mitosis, the activity of MT nucleating complexes (TuRCs) and MT-associated proteins (MAPs) leads to a succession of MT arrays that reorganise in the cytoplasm: cortical MTs/perinuclear MTs/pre-prophase band MTs/spindle MTs/phragmoplast MTs. The spindle apparatus, consisting of kinetochore fibres and interpolar MTs, is barrel-shaped and lacks polar asters. Some of the MAPs also connect MFs. Gamma-tubulin containing complexes and augmin complexes participate in the nucleation of new highly dynamic MTs that ensure spindle robustness. Actin filaments intermingle with MTs and do not depolymerise during mitosis, except in the PPB, which becomes an actin-deplete zone (ADZ). The cell plate expands centrifugally to the cell edges thanks to actin filaments, preceding MTs on which Golgi-derived vesicles move towards the equator and fuse together. Keywords: plants; cell division; mitosis; cytoskeleton; microtubules; actin microfilaments; preprophase band; spindle; phragmoplast; centromere/kinetochore; centromere cohesion