Polarity and Orientation of Cell Division in Plants

Cell polarity is the development of asymmetry within a cell which can be monitored by physical changes in cell shape or localized distribution of molecular components. Cell polarity provides information for axis formation, patterning, growth and asymmetric cell division.

Asymmetric distribution of the plant hormone auxin is instrumental in regulating many polar growth and division responses at the tissue level (summarized in Dhonukshe et al. 2005) but is also implicated in specific cases of asymmetric cell division discussed here. Auxin is actively distributed within the plant by the combined action of AUX1 auxin influx carrier and PIN auxin efflux facilitators whose asymmetric subcellular localization has been correlated with the direction of auxin flow. Binding of auxin to Transport Inhibitor Response 1 (TIR1), an auxin receptor and subunit of an SCF-type ubiquitin ligase, promotes the degradation of a family of transcriptional repressors called Aux/IAA proteins. Aux/IAA proteins bind to Auxin Response Factor (ARF) proteins and inhibit the transcription of specific auxin response genes. Increased nuclear concentrations of auxin promote auxin binding to TIR1, causing the Aux/IAA proteins to associate with TIR1 and leading to their degradation by a proteasome-mediated pathway. The ARF protein is now free to activate transcription from its target promoter (reviewed in Jenik and Barton 2005). Studying the polar localization of PIN protein is aiding significantly in investigations on plant cell polarity and pinpointing the proteins involved (Xu and Scheres 2005).

Certain cell polarization events depend on ADP-Ribosylation Factor (ARF)-mediated vesicle trafficking to polarly localize Rho-related GTPases from plants (ROP). ROPs act as master switches in the transmission of various extracellular and intracellular signals and have classically been linked to the regulation of the cytoskeleton. ROPs control actin assembly and micro-tubule bundling through ROP-Interactive CRIB-motif (RIC) proteins and Wiskott-Aldrich syndrome protein family verprolin homologous/suppressor of cAMP receptor-actin related protein (WAVE/SCAR-ARP2/3) pathways (Gu et al. 2004; Burridge and Wennerberg 2004; Xu and Scheres 2005). In plants ARFs interact with Guanine nucleotide Exchange Factors (ARF-GEF) like GN0M/EMB30 for polar vesicle transport. gnom mutants display aberrant cell shape and abnormal orientation of cell division planes including the first division in the zygote (Sect. 4.1, Mayer U et al. 1993; Shevell et al. 1994; Geldner et al. 2003).

Another way to generate and maintain polarity may be altered sterol composition of the cell membrane which in yeast was shown to interfere with mating and may also be important for animal cell polarity (Bagnat and Simons 2002; Schuck and Simons 2004). A similar case has been made for plants as reported for the Arabidopsis mutant sterol methyltransferase

1/orc (smt1/orc), which is disturbed in the biosynthesis of plasma membrane sterols resulting in apolar distribution of cellular markers and aberrant division planes (Willemsen et al. 2003).

Clearly, disturbing the polarity of cells affects the orientation of the cell division plane. But how cell polarity is linked to division orientation in plants remains unclear. In animals the PARtitioning defective (PAR) proteins act downstream of polarization cues to stabilize polarity and they form the connection with the cytoskeleton to control asymmetric mitotic spindle positioning, determine the division plane and localize cell fate determinants to one side of the cell (reviewed in Wodarz 2002; McCarthy and Goldstein 2006). PAR genes were originally identified in a screen for mutants affecting the first asymmetric cell division of the Caenorhabditis elegans zygote and encode a diverse set of proteins consisting of Ser/Trh-kinases, PDZ-domain proteins and a 14-3-3 protein (Kemphues et al. 1988; Betschinger and Knoblich 2004).

Cell division is distinct in several ways in animals and plants. First, in animals, microtubule nucleation takes place at microtubule-organizing centers (MTOCs) such as the centrosomes associated with the poles of the mitotic spindle that determine the direction of chromosome segregation during mitosis. Higher plant cells lack discrete MTOCs but assemble highly ordered arrays of microtubules from nuclear polar caps that anticipate the mitotic spindle to coordinate cell division. Second, physical cell division or cytokinesis in animal cells involves inward constriction by an actinomyosin contractile ring that pulls in the plasma membrane whereas plant cell cytokinesis occurs at from the center toward the cell periphery. This process involves the "phragmoplast", the cytokinetic ring of the plant cell, consisting of antiparallel bundles of microtubules and actin that forms from the remains of the spindle between the two sets of chromosomes. The phrag-moplast delivers vesicles to the plane of cell division forming the outward growing cell plate. Interestingly, the future site of division in plants is predicted late in G2 by a transient cortical preprophase band (PPB) of co-aligned microtubules and actin filaments encircling the nucleus whereas in animals the site of cytokinesis is selected after chromosome separation. Although on the surface animal and plant division appear very different, the involved mechanisms and protein conservation indicate a common basis to both types of division (Jurgens 2005; Lloyd and Chan 2006 and references therein).

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