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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

The GMD1 and GMD2 Genes of Arabidopsis Encode Isoforms of GDP-D-Mannose 4,6-Dehydratase with Cell Type-Specific Expression Patterns¹

Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06269 (C.P.B., G.F.V., W.-D.R.); and Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602 (G.F., M.G.H.)

L-Fucose (L-Fuc) is a monosaccharide constituent of plant cell wall polysaccharides and glycoproteins. The committing step in the de novo synthesis of L-Fuc is catalyzed by GDP-D-mannose 4,6-dehydratase, which, in Arabidopsis, is encoded by the GMD1 and GMD2 (MUR1) genes. To determine the functional significance of this genetic redundancy, the expression patterns of both genes were investigated via promoter--glucuronidase fusions and immunolocalization of a Fuc-containing epitope. GMD2 is expressed in most cell types of the root, with the notable exception of the root tip where strong expression of GMD1 is observed. Within shoot organs, GMD1::GUS expression is confined to stipules and pollen grains leading to fucosylation of the walls of these cell types in the mur1 mutant. These results suggest that GMD2 represents the major housekeeping gene for the de novo synthesis of GDP-L-Fuc, whereas GMD1 expression is limited to a number of specialized cell types. We conclude that the synthesis of GDP-L-Fuc is controlled in a cell-autonomous manner by differential expression of two isoforms of the same enzyme.

¹ This work was supported by the Department of Energy's Energy Biosciences Program (grant nos. DE–FG02–95ER20203 and DE–FG02–96ER20220).

² Present address: Department of Biochemistry and Molecular Biophysics, Columbia University, Hammer Health Sciences Bldg, Room 1104, 701 West 168th Street, New York, NY 10032.

³ Present address: National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401.

^* Corresponding author; e-mail wdreiter{at}uconnvm.uconn.edu; fax 860–486–4331.

Received February 19, 2003; returned for revision March 20, 2003; accepted March 27, 2003.

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