Sphingolipids consist of hydrophobic ceramide backbones that are typically linked to polar sugar residues to form amphipathic lipid components of membranes (Lynch and Dunn 2004; Chen et al. 2010). The ceramide backbone contains a long chain amino alcohol referred to as a sphingoid long-chain base (LCB) linked through an amide bond to a fatty acid. LCBs are unique to sphingolipids.
In plants, LCBs typically have chain lengths of 18 carbon atoms and can contain double bonds in the Δ4 or Δ8 positions. The Δ4 double bond is found only in the trans configuration, while the Δ8 double bond can be found in either the trans or cis configurations. Following its initial synthesis, a LCB has two hydroxyl groups at the C-1 and C-3 carbons (Lynch and Dunn 2004; Chen et al. 2010). These LCBs are referred to as dihydroxy LCBs. A third hydroxyl group can be enzymatically added at the C-4 carbon to form a trihydroxy LCB. In the short-hand nomenclature, a dihydroxy LCB with 18 carbons and 1 double bond is referred to as “d18:1”, and a trihydroxy LCB with 18 carbons and 1 double bond is referred to as “t18:1”. LCBs can be phosphorylated at the C-1 position to form LCB-phosphates (LCB-P). Free LCBs and their phosphorylated forms are typically in low abundance in plant cells (Markham and Jaworski 2007; Markham et al. 2006). Instead, the majority of LCBs are found linked to fatty acids in ceramides.
Plant ceramide fatty acids exhibit a diverse range of chain lengths, spanning from 16 to 26 carbon atoms. The majority of which contain an enzymatically added hydroxyl group at the C-2 or α-position (Lynch and Dunn 2004; Chen et al. 2010). Analogous to the diacylglycerol backbone of glycerolipids, ceramides serve as the hydrophobic component of complex sphingolipids.
The polar head group of ceramides is attached at its C-1 position and can be a phosphate residue or a variety of sugar residues (Chen et al. 2010). The latter are referred to as glycosphingolipids. The simplest glycosphingolipid in plants is the glucosylceramide (GlcCer) with a single glucose residue and comprises approximately one-third of the glycosphingolipids of Arabidopsis leaves (Markham and Jaworski 2007; Markham et al. 2006). The most abundant glycosphingolipid in plants contains an inositol phosphate bound to the ceramide with up to seven additional hexose and pentose residues (Cacas et al. 2013). These molecules are referred to as glycosyl inositolphosphoceramides or GIPCs and comprise approximately two-thirds of the glycosphingolipids of Arabidopsis leaves (Markham and Jaworski 2007; Markham et al. 2006). The quantitative significance of GIPCs in plants was overlooked for many years due to the difficulty in their extraction using standard lipid analytical protocols because of the high polarity of their glycosylated head groups.
Between the different carbon chain-lengths and hydroxylation and unsaturation states of LCBs and fatty acids and the array of polar head groups, hundreds of potentially different sphingolipid species can occur in plants, the individual significance of which are only beginning to be elucidated (Markham et al. 2013; Bure et al. 2011).