doi:10.1093/aob/mch071, available online at www.aob.oupjournals.org
INVITED REVIEW
A Post-genomic Approach to Understanding Sphingolipid Metabolism in
Arabidopsis thaliana
TERESA M. DUNN
1
, DANIEL V. LYNCH
2
, LOUISE V. MICHAELSON
3
and
JOHNATHAN A. NAPIER
3 ,
*
1
Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA,
2
Williams College, Williamstown,
MA 01267, USA and
3
Rothamsted Research, Harpenden, Herts AL5 2JQ, UK
Received: 28 October 2003 Returned for revision: 11 December 2003 Accepted: 6 January 2004 Published electronically: 22 March 2004
d
Aims To highlight the importance of sphingolipids and their metabolites in plant biology.
d
Scope The completion of the arabidopsis genome provides a platform for the identi®cation and functional
characterization of genes involved in sphingolipid biosynthesis. Using the yeast Saccharomyces cerevisiae as an
experimental model, this review annotates arabidopsis open reading frames likely to be involved in sphingolipid
metabolism. A number of these open reading frames have already been subject to functional characterization,
though the majority still awaits investigation. Plant-speci®c aspects of sphingolipid biology (such as enhanced
long chain base heterogeneity) are considered in the context of the emerging roles for these lipids in plant form
and function.
d
Conclusions Arabidopsis provides an excellent genetic and post-genomic model for the characterization of the
roles of sphingolipids in higher plants. ã 2004 Annals of Botany Company
Key words: Arabidopsis thaliana, ceramide, desaturase, lipid metabolism, long chain base, post-genomics,
Saccharomyces cerevisiae, signalling, sphingolipid.
INTRODUCTION
Sphingolipids are ubiquitous and essential components of
eukaryotic cells that were classically viewed as structural
components of the membrane. However, it is now clear that
sphingolipids and their metabolites are also dynamic
regulators of many cellular processes. In particular, studies
have shown that sphingolipids control crucial events in
mammalian cells that determine the normal development
and fate of living organisms, including proliferation,
differentiation and apoptosis. Far less is known about
sphingolipid functions in plants, but recent studies indicate
that they have important signalling roles in plants as well.
For example, sphingosine-1-phosphate plays a role in Ca
2+
-
mediated guard cell closure, a sphingosine transfer protein
is involved in programmed cell death, and plant resistance
to fungal toxins is mediated by a plant orthologue of a yeast
gene involved in ceramide synthesis (for recent reviews, see
Ng and Hetherington, 2001; Worrall et al., 2003). An
important step toward delineating the function of sphingo-
lipids has been the isolation of sphingolipid metabolism
mutants in Saccharomyces cerevisiae and the identi®cation
of yeast genes encoding the enzymes responsible for
sphingolipid biosynthesis and metabolism. These studies
provide the groundwork for investigating the functions of
sphingolipids in plants by a `reverse genetic' approach
because genome analysis indicates that many of the
enzymes have been conserved throughout evolution.
Arabidopsis thaliana has emerged as one of the best
model organisms for studying the biology of higher plants,
as well as the ®rst plant genome to be fully sequenced. The
aim of this review is to use this post-genomic platform to
examine the biosynthesis and metabolism of plant sphingo-
lipids, in particular in the context of S. cerevisiae, which
serves as the primary genetic and molecular model for
sphingo-biology. Moreover, this serves as a complement to
previous reviews, which have compared sphingolipid
metabolism in yeast and mammals (Dickson, 1998).
STRUCTURE AND OCCURRENCE OF PLANT
SPHINGOLIPIDS
Complex sphingolipids are formed by the addition of
various sugar residues or phosphate-containing headgroups
to a ceramide (Cer) backbone composed of a long-chain
base (LCB) that is amide-linked to a fatty acid; thus, an LCB
becomes acylated to generate a ceramide, which in turn is
modi®ed to yield a sphingolipid (see Fig. 1 for structural
information as well as biosynthetic pathway). Sphingolipids
are a diverse group of molecules with several hundred
different molecular species known. This complexity arises
from both the large array of possible polar headgroups, and
from differences in the chain lengths, degrees of unsatura-
tion, and hydroxylation states of both the LCB and fatty acid
moieties of the ceramide. The predominant sphingolipid
classes in mammals are sphingomyelin and the neutral and
acidic glycolipids, while inositolphosphorylceramides
(IPCs) are prevalent in yeast (Lester and Dickson, 1993).
The predominant sphingolipids in plant tissues are generally
considered to be glucosylceramides (GlcCers) (Lynch et al.,
1993b), although complex glycophosphosphingolipids,
Annals of Botany 93/5, ã Annals of Botany Company 2004; all rights reserved
* For correspondence. E-mail johnathan.napier@bbsrc.ac.uk
Annals of Botany 93: 483±497, 2004
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including IPCs, are also found in plant tissues (Lester and
Dickson, 1993; Lynch and Dunn, 2004). The absolute levels
of sphingolipid classes in plant tissues have not been subject
to unambiguous determination.
F I G . 1. A representation of the metabolic pathway displaying the enzymatic interconversions and structures of plant sphingolipids. Known yeast gene
orthologues are listed in parentheses following abbreviated enzyme designations. Plant genes predicted to be involved in sphingolipid metabolism are
not shown but are described in the text. Enzymatic steps modifying long-chain bases and amide-linked acyl chains via hydroxylation and/or
desaturation are thought to utilize ceramide, glucosylceramide or inositolphosphorylceramide as substrate, but acyl-chain elongation precedes ceramide
formation, and the hydroxylation of free sphinganine has been demonstrated. Inositolphosphorylceramide is thought to be the precursor to complex
glycophosphosphingolipids but the glycosyltransferases presumably required have not been identi®ed. In addition to free sphinganine (d18:0) and
4-hydroxysphinganine (t18:0, phytosphingosine), other long-chain bases present in plants include cis and trans isomers of 8-sphingenine (d18:1D
8
),
4,8-sphingadienine (d18:2D
4,8
) and 4-hydroxy-8-sphingenine (t18:1D
8
), shown here as constituents of ceramide, glucosylceramide and
inositolphosphorylceramide, respectively. Sphingosine (d18:1D
4trans
, 4-sphingenine) is shown as the phosphorylated derivative. The acyl chain
amide-linked in the ceramide shown is a nonhydroxy saturated C
24
chain (lignocerate), whereas that of inositolphosphorylceramide and
glucosylceramide is the a-hydroxylated counterpart. Enzyme abbreviations: CERase, ceramidase; GCase, glucosylceramidase; GCS, glucosylceramide
synthase; IPCS, inositolphophorylceramide synthase; KSR, 3-ketosphinganine reductase; LCBK, long-chain base kinase; LCBPL; long-chain
base-phosphate lyase; LCBPP, long-chain base-phosphate phosphatase; PLC, phospholipase C; SAT, sphinganine acyltransferase; SH, sphinganine
hydroxylase; SPT, serine palmitoyltransferase.
484 Dunn et al. Ð Sphingolipid Metabolism in Plants
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