Plant cells are confined by a network of cellulosic walls that imposes rigid control over the selection of division plane orientations, crucial for morphogenesis and genetically regulated. While in animal cells and yeast, the actin cytoskeleton is instrumental in the execution of cytokinesis, in plant cells the microtubule cytoskeleton is taking the lead in spatially controlling and executing cytokinesis by the formation of two unique, plant-specific arrays, the preprophase band (PPB) and the phragmoplast. The formation of microtubule arrays in plant cells is contingent on acentrosomal microtubule nucleation. At the onset of mitosis, the PPB defines the plane of cell division where the partitioning cell wall is later constructed by the cytokinetic phragmoplast, imposing a spatio-temporal relationship between the two processes. Current research progress in the field of plant cell division focuses on identifying and tying the links between early and late events in spatial control of cytokinesis and how microtubule array formation is regulated in plant cells.

During the early stages of Arabidopsis seed development, the endosperm is syncytial and proliferates rapidly through multiple rounds of mitosis in the absence of cytokinesis and cell wall formation. This stage of endosperm development is important in determining seed viability and size. To identify genes involved in syncytial endosperm development, we analyzed the endosperm transcriptome, obtained using laser capture microdissection of developing seeds at four days after pollination. Our results support the idea that similar sets of genes are required for conventional somatic mitosis with cytokinesis and syncytial proliferation. Furthermore, we identify cytoskeleton associated genes that may act to facilitate syncytial development thereby providing an important resource for further characterization of the processes involved in syncytial endosperm development.Seed development begins with double fertilization where the haploid egg cell and the double haploid central cell are both fertilized by haploid sperm cells contributed from a single pollen grain. This generates the diploid embryo and the triploid endosperm, respectively. The embryo and the endosperm grow rapidly in a coordinated manner that is influenced by the surrounding maternal integument tissues that later form the seed coat. During these early stages, maternal resources are used for the rapid cell division and growth that occurs as seed tissues form and develop. In many plant species, including Arabidopsis, the triploid primary endosperm nucleus undergoes several rounds of free-nuclear division, growing rapidly as a syncytium. The organization of microtubule arrays during this early stage of endosperm development is markedly different from those found in vegetative tissues.1 During the syncytial phase, interphase microtubules emanate from the nucleus into the cytoplasm and this nucleus-based radial microtubule system aids in positioning of nuclei and the designation of cytoplasm into nucleo-cytoplasmic domains. At about six days after pollination, cell wall deposition is initiated by the formation of phragmoplasts at the margins of nucleocytoplasmic domains. Interestingly, interzonal arrays which resemble phragmoplasts form between nuclei after karyokinesis, but they never participate in cell plate formation.2 This unusual form of cellular development is conducive to a rapid cellular proliferation; however, the molecular mechanisms that underlie the uncoupling of cell wall formation from mitosis remain elusive.To gain further insights into the early stages of endosperm biology, we have used laser capture microdissection to obtain RNA from syncytial stage endosperm at four days after pollination. At this stage of development, the endosperm has undergone six to seven rounds of nuclear division (~200 nucleocytoplasmic domains) but has not cellularized and the embryo is at the globular phase. Endosperm RNA was amplified using two-round in vitro transcription, then labeled and used, along with similarly treated silique amplified RNA, to probe long oligonucl...

The unequal (asymmetric) distribution of cell structures and proteins within a cell is designated as cell polarity. Cell polarity is a crucial prerequisite for morphogenetic processes such as oriented cell division and directed cell expansion. Rho-related plants (ROPs) are required for cellular morphogenesis through the reorganization of the cytoskeleton and vesicle transport in various tissues. Here, I review recent advances in ROP-dependent tip growth, vesicle transport, and tip architecture. I report on the regulatory mechanisms of ROP upstream regulators found in different cell types. It appears that these regulators assemble in nanodomains with specific lipid compositions and recruit ROPs for activation in a stimulus-dependent manner. Current models link mechanosensing/mechanotransduction to ROP polarity signaling involved in feedback mechanisms via the cytoskeleton. Finally, I discuss ROP signaling components that are upregulated by tissue-specific transcription factors and exhibit specific localization patterns during cell division, clearly suggesting ROP signaling in division plane alignment. Advances box: Progress has been made in the characterization of upstream regulators of ROPase signaling in diverse tissues, revealing a common theme of RopGEF phosphoregulation by diverse kinases initiating various ROP signaling cascadesNovel insights link mechanosensing/signal transduction pathways, ROP signaling, and cell wall-reinforcing feedback mechanismsIn response to different stimulants, ROP GTPases are recruited to distinct, pre-existing nanodomains composed of specific lipids and RopGEFs. Thus, one ROP GTPase may respond differently to different stimuliMaintenance of tip architecture in tip-growing cells requires secretory and endocytic trafficking, but the precise site of endocytosis may differ between cell types/speciesCell polarity signaling molecules locate at the cortical division site, or are particularly excluded from it; analysis of respective mutant phenotypes implicate the respective genes in division plane selection in different tissue contexts and diverse plant taxa, suggesting evolutionary conservation

No abstract

No abstract