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First published online January 21, 2005; 10.1104/pp.104.055806 Plant Physiology 137:724-737 (2005) © 2005 American Society of Plant Biologists Arabidopsis Sphingosine Kinase and the Effects of Phytosphingosine-1-Phosphate on Stomatal Aperture1,[w]Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802–5301 (S.C., S.G., S.M.A.); Station de Génétique Végétale, Unité Mixte de Recherche 320 Institut National de la Recherche Agronomique, 8120 Centre National de la Recherche Scientifique, Université Paris XI, Institut National Agronomique de Paris-Grignon, 91190 Gif-sur-Yvette, France (S.C.); Department of Biochemistry, Virginia Commonwealth University Medical Center, Richmond, Virginia 23892–0614 (H.L.S., S.S.); Laboratoire d'Activation Cellulaire et Transduction des Signaux, Institut de Biochimie et de Biophysique Moléculaire et Cellulaire, Unité Mixte de Recherche 8619 Centre National de la Recherche Scientifique, Université Paris XI, 91405 Orsay cedex, France (H.L.S.); and Department of Biology, Williams College, Williamstown, Massachusetts 01267 (D.V.L.)
Sphingolipids are a major component of membrane lipids and their metabolite sphingosine-1-phosphate (S1P) is a potent lipid mediator in animal cells. Recently, we have shown that the enzyme responsible for S1P production, sphingosine kinase (SphK), is stimulated by the phytohormone abscisic acid in guard cells of Arabidopsis (Arabidopsis thaliana) and that S1P is effective in regulating guard cell turgor. We have now characterized SphK from Arabidopsis leaves. SphK activity was mainly associated with the membrane fraction and phosphorylated predominantly the
1 This work was supported by the Institut National de la Recherche Agronomique (grant to S.C.), by the National Science Foundation (grant nos. MCB–0312864 to D.V.L. and MCB–0209694 to S.M.A.), by the U.S. Department of Agriculture (grant nos. 01–35304–09898 to S.G. and 01–35304–09916 to S.M.A.), and by the National Cancer Institute (grant no. CA61774 to S.S.). 2 These authors contributed equally to the paper. 3 Present address: Laboratoire de Physiopathologie de la Nutrition, UMR 7059 CNRS, Université Paris VII, F–75251 Paris cedex 05, France. [w] The online version of this article contains Web-only data. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.055806. * Corresponding author; e-mail coursol{at}moulon.inra.fr; fax 33–1–69–33–23–40. Received October 29, 2004; returned for revision December 8, 2004; accepted December 8, 2004. Related articles in Plant Physiol.:
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4-unsaturated long-chain sphingoid bases sphingosine (Sph) and 4,8-sphingadienine, and to a lesser extent, the saturated long-chain sphingoid bases dihydrosphingosine and phytosphingosine (Phyto-Sph). 4-Hydroxy-8-sphingenine, which is a major sphingoid base in complex glycosphingolipids from Arabidopsis leaves, was a relatively poor substrate compared with the corresponding saturated Phyto-Sph. In contrast, mammalian SphK1 efficiently phosphorylated Sph, dihydrosphingosine, and 4,8-sphingadienine, but not the 4-hydroxylated long-chain bases Phyto-Sph and 4-hydroxy-8-sphingenine. Surface dilution kinetic analysis of Arabidopsis SphK with Sph presented in mixed Triton X-100 micelles indicated that SphK associates with the micellar surface and then with the substrate presented on the surface. In addition, measurements of SphK activity under different assay conditions combined with phylogenetic analysis suggest that multiple isoforms of SphK may be expressed in Arabidopsis. Importantly, we found that phytosphingosine-1-phosphate, similar to S1P, regulates stomatal apertures and that its action is impaired in guard cells of Arabidopsis plants harboring T-DNA null mutations in the sole prototypical G-protein 
-subunit gene, GPA1. 
