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The one-eyed pinhead gene functions in mesoderm and endoderm formation in zebra®sh and interacts with no tail. Development 124, 327±342 (1997). 27. Zhang, J., Talbot, W. S. & Schier, A. F. Positional cloning identi®es zebra®sh one-eyed pinhead asa permissive EGF-related ligand required during gastrulation. Cell 92, 241±251 (1998). 28. Stra Èhle, U. et al. one-eyed pinhead is required for development of the ventral midline of the zebra®sh (Danio rerio) neural tube. Genes Funct. 1, 131±148 (1997). Supplementary information is available on Nature's World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial of®ce of Nature. Acknowledgements We thank members of the Schier and Yelon laboratories for discussions; R. Burdine, A. Carmany-Rampey, K. Joubin, R. Lehmann, G. Struhl, W. Talbot and D. Yelon for comments on the manuscript; S. Zimmerman, R. Feeney and T. Bruno for ®sh care. Y.C. is the Rebecca Ridley Kry Fellow of the Cancer Research Fund of the Damon Runyon±Walter Winchell Foundation. A.F.S. is a Scholar of the McKnight Endowment Fund for Neuroscience and the Irma T. Hirschl Trust, and is supported by grants from the NIH. Correspondence and requests for materials should be addressed to A.F.S. (e-mail: schier@saturn.med.nyu.edu). letters to nature 610 NATURE | VOL 411 | 31 MAY 2001 | www.nature.com ................................................................. MOR1 is essential for organizing cortical microtubules in plants Angela T. Whittington, Oliver Vugrek, Ke Jun Wei, Nortrud G. Hasenbein, Keiko Sugimoto*, Madeleine C. Rashbrooke & Geoffrey O. Wasteneys Plant Cell Biology Group, Research School of Biological Sciences, The Australian National University, GPO Box 475, Canberra, ACT 2601, Australia .............................................................................................................................................. Microtubules orchestrate cell division and morphogenesis, but how they disassemble and reappear at different subcellular loca- tions is unknown. Microtubule organizing centres are thought to have an important role, but in higher plants microtubules assem- ble in ordered con®gurations even though microtubule organiz- ing centres are inconspicuous or absent. Plant cells generate highly organized microtubule arrays that coordinate mitosis, cytokinesis and expansion. Inhibiting microtubule assembly pre- vents chromosome separation 1 , blocks cell division 2 and impairs growth polarity 3 . Microtubules are essential for the formation of cell walls, through an array of plasma-membrane-associated cortical microtubules whose control mechanisms are unknown. Using a genetic strategy to identify microtubule organizing factors in Arabidopsis thaliana, we isolated temperature-sensitive *Present address: John Innes Centre, Department of Cell and Developmental Biology, Norwich NR4 7UH, UK. mutant alleles of the MICROTUBULE ORGANIZATION 1 (MOR1) gene. Here we show that MOR1 is the plant version of an ancient family of microtubule-associated proteins 4 . Point mutations that substitute single amino-acid residues in an amino-terminal HEAT repeat impart reversible temperature- dependent cortical microtubule disruption, showing that MOR1 is essential for cortical microtubule organization. In most plant cells that display diffuse rather than tip growth, microtubules localize to the cortical cytoplasm perpendicular to the major axis of expansion. Microtubules and cellulose micro®brils often have similar orientation patterns in elongating cells 5 and it is generally accepted, but not proven, that cortical microtubules control the alignment of cellulose micro®brils 6 . Identifying the factors that organize microtubule arrays at the periphery of plant cells is a necessary step towards understanding the mechanisms that underlie wall deposition and, hence, plant morphogenesis. To identify factors regulating cortical microtubule organization in plant cells, we used immuno¯uorescence microscopy to screen chemically mutagenized seedlings of A. thaliana for aberrant microtubule patterns. One mutant locus, mor1, causes tempera- ture-sensitive cortical microtubule shortening and disorganization (Fig. 1) and consequent morphological defects. We used ecotype- speci®c markers to identify MOR1 as a gene of around 14 kilobases (kb) that encodes a protein with a predicted relative molecular mass of 217,000 (M r 217K) (Fig. 2a) that has signi®cant deduced amino- acid sequence similarity to human TOGp 7 , Xenopus MAP215 Figure 1 Mutations in the MOR1 gene cause temperature-dependent microtubule disruption. We used anti-tubulin immuno¯uorescence to label cortical microtubules in epidermal cells of the ®rst true leaf of 21-day-old seedlings after incubating seedlings at 29 8C for 2 h before ®xation. a, Wild type. b, mor1-1 homozygote. Scale bar, 25 mm. a HR1 23 4 5 67 8 910 1,979 amino acids b E195K L174F mor1-1 mor1-2 Figure 2 MOR1 protein structure. a, Schematic representation with shaded boxes indicating conserved domains between MOR1, TOGp, XMAP215, MSPS and DdCP224. The shorter ZYG-9 and yeast proteins share the ®rst ®ve and ®rst four regions of homology, respectively. Black stripes show putative HEAT repeats (HR) in relation to these conserved domains. b, Deduced amino-acid sequence comparison of MOR1's HEAT repeat-1 with equivalent repeats in homologues. Black-shaded residues indicate identity, grey-shaded residues indicate similarity (40% threshold), outlined box indicates HEAT repeat. Mutations altering MOR1 amino-acid residues are indicated by arrowheads. © 2001 Macmillan Magazines Ltd