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DEVELOPMENT AND HORMONE ACTION

A RING Domain Gene Is Expressed in Different Cell Types of Leaf Trace, Stem, and Juvenile Bundles in the Stem Vascular System of Zinnia¹

Department of Cell and Developmental Biology, John Innes Centre, Colney, Norwich NR4 7UH, United Kingdom

The in vitro zinnia (Zinnia elegans) mesophyll cell system, in which leaf mesophyll cells are induced to transdifferentiate into tracheary elements with high synchrony, has become an established model for studying xylogenesis. The architecture of the stem vascular system of zinnia cv Envy contains three anatomically distinct vascular bundles at different stages of development. Juvenile vascular strands of the subapical region develop into mature vascular strands with leaf trace segments and stem segments. Characteristic patterns of gene expression in juvenile, leaf trace, and stem bundles are revealed by a molecular marker, a RING domain-encoding gene, ZeRH2.1, originally isolated from a zinnia cDNA library derived from differentiating in vitro cultures. Using RNA in situ hybridization, we show that ZeRH2.1 is expressed preferentially in two specific cell types in mature zinnia stems. In leaf trace bundles, ZeRH2.1 transcript is abundant in xylem parenchyma cells, while in stem bundles it is abundant in phloem companion cells. Both of these cell types show wall ingrowths characteristic of transfer cells. In addition, ZeRH2.1 transcript is abundant in some phloem cells of juvenile bundles and in leaf palisade parenchyma. The complex and developmentally regulated expression pattern of ZeRH2.1 reveals heterogeneity in the vascular anatomy of the zinnia stem. We discuss a potential function for this gene in intercellular transport processes.

² Present address: Department of Agricultural Biotechnology, Agricultural University of Athens, Athens, Greece.

³ Present address: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907–1392.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.057901.

^* Corresponding author; e-mail mmccann{at}bilbo.bio.purdue.edu; fax 765–496–1496.

Received December 8, 2004; returned for revision March 23, 2005; accepted April 20, 2005.

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