INTRODUCTION Cytokinesis partitions the cytoplasm of a dividing cell and involves a precisely orchestrated interaction between the cytoskeleton and vesicle trafficking that leads to synthesis of new plasma membrane. In higher plants, a specialised cytoskeletal structure supports the de novo formation of a cell wall during cytokinesis. The components of this cellular assembly, the phragmoplast, include microtubules, actin microfilaments and Golgi-derived vesicles (Staehelin and Hepler, 1996; Heese et al., 1998; Sylvester, 2000). At the end of telophase, elements for membrane and cell wall synthesis are transported by the phragmoplast to the interzone where the cell plate forms a membrane-bound compartment enclosing the immature wall. Phragmoplast and cell plate expand laterally until the cell plate fuses with the parental plasma membrane and cell wall. At the onset of mitosis, the site of fusion is marked by a transient array of microtubule, the preprophase band (PPB). The prominent role played by the microtubule cytoskeleton in this process is well supported by genetic evidence. Mutations in the Arabidopsis PILZ genes encoding the microtubule folding cofactor complex (Steinborn et al., 2002) severely disrupt microtubule organisation and result in a complete block of mitosis and cytokinesis both in embryo and endosperm (Mayer et al., 1999). Other mutations, such as tonneau1 and fass/tonneau2, more specifically affect organisation of cortical microtubules: fass/ton2 embryos, for example, lack both the cortical array of parallel microtubules and the PPB (Traas et al., 1995; McClinton and Sung, 1997). As a consequence, fass/ton2 cells undergo unordered expansion in all dimensions and form oblique irregular cell walls very unlike the well-ordered cell files that are representative of Arabidopsis embryos (Torres-Ruiz and Jürgens, 1994; Traas et al., 1995). Several mutations that affect cytokinesis in the Arabidopsis embryo have been isolated, including knolle and keule (Mayer et al., 1991). KNOLLE encodes a member of the syntaxin family, a class of membrane-bound receptors required for docking and fusion of vesicles at the target membrane (Lukowitz et al., 1996). The KNOLLE protein only accumulates at mitosis and localises to the plane of division, indicating that its specific role is to mediate membrane fusion at the cell plate (Lauber et al., 1997). The keule (Assaad et al., 1996) and hinkel (Strompen et al., 2002) mutants display cytokinesis defects during embryo development similar to 5567 Development 129, 5567-5576 © 2002 The Company of Biologists Ltd doi:10.1242/dev.00152 Distinct forms of cytokinesis characterise specific phases of development in plants. In Arabidopsis, as in many other species, the endosperm that nurtures the embryo in the seed initially develops as a syncytium. This syncytial phase ends with simultaneous partitioning of the multinucleate cytoplasm into individual cells, a process referred to as cellularisation. Our in vivo observations show that, as in cytokinesis, cellularisation of the Arabidopsis endosperm is coupled to nuclear division. A genetic analysis reveals that most Arabidopsis mutations affecting cytokinesis in the embryo also impair endosperm cellularisation. These results imply that cellularisation and cytokinesis share multiple components of the same basic machinery. We further report the identification of mutations in a novel gene, SPÄTZLE, that specifically interfere with cellularisation of the endosperm, but not with cytokinesis in the embryo. The analysis of this mutant might identify a specific checkpoint for the onset of cellularisation. Movies available on-line Key words: Arabidopsis thaliana, Endosperm, Seed, Cytokinesis, SPÄTZLE SUMMARY Cellularisation in the endosperm of Arabidopsis thaliana is coupled to mitosis and shares multiple components with cytokinesis Mikael Blom Sørensen 1,2 , Ulrike Mayer 3 , Wolfgang Lukowitz 4 , Hélène Robert 1 , Pierre Chambrier 1 , Gerd Jürgens 3 , Chris Somerville 4 , Loic Lepiniec 5 and Frédéric Berger 1, * 1 Laboratoire de Reproduction et Développement des Plantes, UMR 5667, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, F- 69364 Lyon, Cedex 07, France 2 Plant Research Department, PRD-301, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark 3 ZMBP – Center of Plant Molecular Biology, Developmental Genetics, University of Tübingen, Auf der Morgenstelle 3, D-72076 Tübingen, Germany 4 Carnegie Institution of Washington, Department of Plant Biology, 260 Panama Street, Stanford, California 94305, USA 5 Laboratoire de Biologie des Semences, UMR INRA/INA-PG, Route de Saint-Cyr, 78026 Versailles Cedex, France *Author for correspondence (e-mail: frederic.berger@ens-lyon.fr) Accepted 6 September 2002