Bridging the divide between cytokinesis and cell expansion Steven K Backues 1 , Catherine A Konopka 1 , Colleen M McMichael 1 and Sebastian Y Bednarek Two of the most fundamental processes in plant development are cytokinesis, by which new cells are formed, and cell expansion, by which existing cells grow and establish their functional morphology. In this review we summarize recent progress in understanding the pathways necessary for cytokinesis and cell expansion, including the role of the cytoskeleton, cell wall biogenesis, and membrane trafficking. Here, we focus on genes and lipids that are involved in both cytokinesis and cell expansion and bridge the divide between these two processes. In addition, we discuss our understanding of and controversies surrounding the role of endocytosis in both of these processes. Addresses Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706, USA Corresponding author: Bednarek, Sebastian Y (bednarek@biochem.wisc.edu) 1 These authors contributed equally to this work. Current Opinion in Plant Biology 2007, 10:607–615 This review comes from a themed issue on Cell Biology Edited by Ben Scheres and Volker Lipka Available online 23rd October 2007 1369-5266/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2007.08.009 Introduction The de novo creation of plasma membrane and cell wall at the end of mitosis in plant cells requires a dynamic cytoskeletal array called the phragmoplast that directs vast movement of material, including lipids, proteins, and cell wall components, to and from the division plane to assemble the cytokinetic organelle known as the cell plate. The stages of somatic cell plate development include (1) creation of the phragmoplast from mitotic spindle remnants; (2) trafficking of vesicles to the division plane and their fusion to generate a tubular–vesicular network; (3) continued fusion of membrane tubules and their transformation into membrane sheets upon the deposition of callose, followed by deposition and organ- ization of cellulose and other cell wall components; (4) recycling of excess membrane and other material from the cell plate; and (5) fusion with the parental cell wall. These events are accompanied by the reorganization of other endomembrane systems including the ER. Although morphologically very different, there are many parallels between cell plate development and the process of cell expansion, which involves the addition of membrane to an existing plasma membrane and reorganization of the cell wall. It has become clear that these two processes have many similarities at a mechanistic level, with many of the same pathways and often even the same proteins being involved in both (reviewed in reference [1]). Here, we review recent progress in understanding cell expan- sion and cell plate formation, and the many new links between these two processes (Table 1). The role of the plant cytoskeleton in cytokinesis and cell expansion The dynamic organization of the plant cell actin and microtubule (MT) cytoskeletons is essential for the for- mation of the cell plate and plasma membrane dynamics. Cytoskeletal structure is largely regulated by cyto- skeletal-interacting proteins known as actin-binding proteins (ABPs) or microtubule-associated proteins (MAPs). These binding proteins regulate the assembly and disassembly of these polymers, determining length, stability, and organization. Some of these interacting proteins have been shown to have crucial roles in cyto- kinesis and cell expansion. Microtubules MICROTUBULE ORGANIZATION1 (MOR1) is a member of the highly conserved MAP215/DIS1 family of MAPs [2], which promote tubulin polymerization in vitro and regulate MT length by promoting dynamic instability. Arabidopsis mor1 mutants are severely stunted with short, radially swollen organs, hallmarks of defective polar growth. Temperature-sensitive mor1 mutants display defects in cortical MT arrays after 1.5 h at the restrictive temperature of 29 8C[2]. After 24–48 h at 30 8C, spindles and phragmoplasts are misaligned, discontinuous, branched, crooked, and contain short and aberrantly orga- nized MTs, resulting in the formation of incomplete cell walls (cell wall stubs) and internal cell wall inclusions, indicative of defects in cell plate biogenesis [3  ,4  ]. The malformed mor1 spindles and phragmoplasts persist longer than those observed in wild-type cells, delaying pro- gression of cell division, and many of the cells enter M phase without ever forming a preprophase band (PPB), while other cells contained a mislocalized or underdeve- loped PPB-like structure [4  ]. The gemini pollen1 (gem1) mutant, which displays altered cell division symmetry and ectopic cell plate growth in haploid gametophyte devel- opment resulting in a lack of proper microspore cell polarity establishment and subsequent aberrant cell fate [5–7], is an www.sciencedirect.com Current Opinion in Plant Biology 2007, 10:607–615