A post-genomic approach to understanding sphingolipid metabolism in Arabidopsis thaliana

Abstract

Aims: To highlight the importance of sphingolipids and their metabolites in plant biology.

Scope: The completion of the arabidopsis genome provides a platform for the identification 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-specific 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.

Conclusions: Arabidopsis provides an excellent genetic and post-genomic model for the characterization of the roles of sphingolipids in higher plants.

Fig. 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 identified. 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 (d18:1Δ⁸), 4,8‐sphingadienine (d18:2Δ^4,8) and 4‐hydroxy‐8‐sphingenine (t18:1Δ⁸), shown here as constituents of ceramide, glucosylceramide and inositolphosphorylceramide, respectively. Sphingosine (d18:1Δ^4trans, 4‐sphingenine) is shown as the phosphorylated derivative. The acyl chain amide‐linked in the ceramide shown is a nonhydroxy saturated C₂₄ chain (lignocerate), whereas that of inositolphosphorylceramide and glucosylceramide is the α‐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.

Fig. 2. Microsomal fatty acid elongation and genes involved in this process. Genes from either yeast or arabidopsis are annotated in the four sequential biochemical steps required for the C₂ elongation of an acyl‐CoA substrate. The molecular identity of the third reaction, the dehydratase, is currently unknown.

Fig. 3. Long‐chain base heterogeneity in plants is mediated by three different enzyme activities. Modifications to sphingolipid LCBs are mediated by three enzymes, as indicated above. These enzymes are (1) LCB Δ⁴‐desaturase, (2) LCB Δ⁸‐desaturase, (3) LCB C4‐hydroxylase (this latter enzyme is encoded by the orthologous SUR2 gene in yeast). The LCB Δ⁸‐desaturase appears to be the key determinant of enhanced heterogeneity, since not only is this enzyme stereo‐unselective (yielding both cis and trans double bonds at the Δ⁸ position), it also recognizes both di‐ and trihydroxy LCBs (d18:0 and t18:0) as substrates for desaturation. Allowing for the cis or trans orientation of the Δ⁸ double bond, nine different C₁₈ LCBs are possible in plant sphingolipids. The d18:2^4,8 LCBs appear to be enriched in GlcCers, whereas t18:1⁸ LCBs are likely to be major components of IPCs.