1. Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
2. Botanical Gardens, Graduate School of Science, The University of Tokyo, Hakusan 3-7-1, Bunkyo-ku, Tokyo 112-0001, Japan
3. Plant Science Center, RIKEN, Suehiro 1-7-22, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
4. Present address: Section of Plant Biology, Division of Biological Sciences, University of California, One Shields Avenue, Davis, California 95616, USA

Correspondence to: Hiroyasu Motose^1,4 Email: hmotose@ucdavis.edu
The sequence for ZeXYP1 is deposited in GenBank under accession number AB159560.

Abstract

Inductive cell–cell interactions are essential for controlling cell fate determination in both plants and animals¹; however, the chemical basis of inductive signals in plants remains little understood. A proteoglycan-like factor named xylogen mediates local and inductive cell–cell interactions required for xylem differentiation in Zinnia cells cultured in vitro^{2, 3}. Here we describe the purification of xylogen and cloning of its complementary DNA, and present evidence for its role in planta. The polypeptide backbone of xylogen is a hybrid-type molecule with properties of both arabinogalactan proteins and nonspecific lipid-transfer proteins. Xylogen predominantly accumulates in the meristem, procambium and xylem. In the xylem, xylogen has a polar localization in the cell walls of differentiating tracheary elements. Double knockouts of Arabidopsis lacking both genes that encode xylogen proteins show defects in vascular development: discontinuous veins, improperly interconnected vessel elements and simplified venation. Our results suggest that the polar secretion of xylogen draws neighbouring cells into the pathway of vascular differentiation to direct continuous vascular development, thereby identifying a molecule that mediates an inductive cell–cell interaction involved in plant tissue differentiation.

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