The essential nature of sphingolipids in plants as revealed by the functional identification and characterization of the Arabidopsis LCB1 subunit of serine palmitoyltransferase

ORM Expression Alters Sphingolipid Homeostasis and Differentially Affects Ceramide Synthase Activity

Sphingolipid synthesis is tightly regulated in eukaryotes. This regulation in plants ensures sufficient sphingolipids to support growth while limiting the accumulation of sphingolipid metabolites that induce programmed cell death. Serine palmitoyltransferase (SPT) catalyzes the first step in sphingolipid biosynthesis and is considered the primary sphingolipid homeostatic regulatory point. In this report, Arabidopsis (Arabidopsis thaliana) putative SPT regulatory proteins, orosomucoid-like proteins AtORM1 and AtORM2, were found to interact physically with Arabidopsis SPT and to suppress SPT activity when coexpressed with Arabidopsis SPT subunits long-chain base1 (LCB1) and LCB2 and the small subunit of SPT in a yeast (Saccharomyces cerevisiae) SPT-deficient mutant. Consistent with a role in SPT suppression, AtORM1 and AtORM2 overexpression lines displayed increased resistance to the programmed cell death-inducing mycotoxin fumonisin B₁, with an accompanying reduced accumulation of LCBs and C16 fatty acid-containing ceramides relative to wild-type plants. Conversely, RNA interference (RNAi) suppression lines of AtORM1 and AtORM2 displayed increased sensitivity to fumonisin B₁ and an accompanying strong increase in LCBs and C16 fatty acid-containing ceramides relative to wild-type plants. Overexpression lines also were found to have reduced activity of the class I ceramide synthase that uses C16 fatty acid acyl-coenzyme A and dihydroxy LCB substrates but increased activity of class II ceramide synthases that use very-long-chain fatty acyl-coenzyme A and trihydroxy LCB substrates. RNAi suppression lines, in contrast, displayed increased class I ceramide synthase activity but reduced class II ceramide synthase activity. These findings indicate that ORM mediation of SPT activity differentially regulates functionally distinct ceramide synthase activities as part of a broader sphingolipid homeostatic regulatory network.

Plasma membrane proteomics in the maize primary root growth zone: novel insights into root growth adaptation to water stress

Previous work on maize (Zea mays L.) primary root growth under water stress showed that cell elongation is maintained in the apical region of the growth zone but progressively inhibited further from the apex. These responses involve spatially differential and coordinated regulation of osmotic adjustment, modification of cell wall extensibility, and other cellular growth processes that are required for root growth under water-stressed conditions. As the interface between the cytoplasm and the apoplast (including the cell wall), the plasma membrane likely plays critical roles in these responses. Using a simplified method for enrichment of plasma membrane proteins, the developmental distribution of plasma membrane proteins was analysed in the growth zone of well-watered and water-stressed maize primary roots. The results identified 432 proteins with differential abundances in well-watered and water-stressed roots. The majority of changes involved region-specific patterns of response, and the identities of the water stress-responsive proteins suggest involvement in diverse biological processes including modification of sugar and nutrient transport, ion homeostasis, lipid metabolism, and cell wall composition. Integration of the distinct, region-specific plasma membrane protein abundance patterns with results from previous physiological, transcriptomic and cell wall proteomic studies reveals novel insights into root growth adaptation to water stress.

Plant sphingolipids: Their importance in cellular organization and adaption

Sphingolipids and their phosphorylated derivatives are ubiquitous bio-active components of cells. They are structural elements in the lipid bilayer and contribute to the dynamic nature of the membrane. They have been implicated in many cellular processes in yeast and animal cells, including aspects of signaling, apoptosis, and senescence. Although sphingolipids have a better defined role in animal systems, they have been shown to be central to many essential processes in plants including but not limited to, pollen development, signal transduction and in the response to biotic and abiotic stress. A fuller understanding of the roles of sphingolipids within plants has been facilitated by classical biochemical studies and the identification of mutants of model species. Recently the development of powerful mass spectrometry techniques hailed the advent of the emerging field of lipidomics enabling more accurate sphingolipid detection and quantitation. This review will consider plant sphingolipid biosynthesis and function in the context of these new developments. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Acyl carrier proteins from sunflower (Helianthus annuus L.) seeds and their influence on FatA and FatB acyl-ACP thioesterase activities

The kinetics of acyl-ACP thioesterases from sunflower importantly changed when endogenous ACPs were used. Sunflower FatB was much more specific towards saturated acyl-ACPs when assayed with them. Acyl carrier proteins (ACPs) are small (~9 kDa), soluble, acidic proteins involved in fatty acid synthesis in plants and bacteria. ACPs bind to fatty acids through a thioester bond, generating the acyl-ACP lipoproteins that are substrates for fatty acid synthase (FAS) complexes, and that are required for fatty acid chain elongation, acting as important intermediates in de novo fatty acid synthesis in plants. Plants, usually express several ACP isoforms with distinct functionalities. We report here the cloning of three ACPs from developing sunflower seeds: HaACP1, HaACP2, and HaACP3. These proteins were plastidial ACPs expressed strongly in seeds, and as such they are probably involved in the synthesis of sunflower oil. The recombinant sunflower ACPs were expressed in bacteria but they were lethal to the prokaryote host. Thus, they were finally produced using the GST gene fusion system, which allowed the apo-enzyme to be produced and later activated to the holo form. Radiolabelled acyl-ACPs from the newly cloned holo-ACP forms were also synthesized and used to characterize the activity of recombinant sunflower FatA and FatB thioesterases, important enzymes in plant fatty acids synthesis. The activity of these enzymes changed significantly when the endogenous ACPs were used. Thus, FatA importantly increased its activity levels, whereas FatB displayed a different specificity profile, with much high activity levels towards saturated acyl-CoA derivatives. All these data pointed to an important influence of the ACP moieties on the activity of enzymes involved in lipid synthesis.

Virulence Program of a Bacterial Plant Pathogen: The Dickeya Model

The pectinolytic Dickeya spp. are Gram-negative bacteria causing severe disease in a wide range of plant species. Although the Dickeya genus was initially restricted to tropical and subtropical areas, two Dickeya species (D. dianthicola and D. solani) emerged recently in potato cultures in Europe. Soft-rot, the visible symptoms, is caused by plant cell wall degrading enzymes, mainly pectate lyases (Pels) that cleave the pectin polymer. However, an efficient colonization of the host requires many additional elements including early factors (eg, flagella, lipopolysaccharide, and exopolysaccharide) that allow adhesion of the bacteria and intermediate factors involved in adaptation to new growth conditions encountered in the host (eg, oxidative stress, iron starvation, and toxic compounds). To facilitate this adaptation, Dickeya have developed complex regulatory networks ensuring appropriate expression of virulence genes. This review presents recent advances in our understanding of the signals and genetic circuits impacting the expression of virulence determinants. Special attention is paid to integrated control of virulence functions by variations in the superhelical density of chromosomal DNA, and the global and specific regulators, making the regulation of Dickeya virulence an especially attractive model for those interested in relationships between the chromosomal dynamics and gene regulatory networks.

Toxin distribution and sphingoid base imbalances in Fusarium verticillioides-infected and fumonisin B1-watered maize seedlings

Fusarium verticillioides is a major maize pathogen and there are susceptible and resistant cultivars to this fungal infection. Recent studies suggest that its main mycotoxin fumonisin B1 (FB1) may be involved in phytopathogenicity, but the underlying mechanisms are mostly still unknown. This work was aimed at assessing whether FB1 disseminates inside the plants, as well as identifying possible correlations between the maize resistant/susceptible phenotype and the unbalances of the FB1-structurally-related sphingoid base sphinganine (Sa) and phytosphingosine (Pso) due to toxin accumulation. Resistant (RH) and susceptible hybrid (SH) maize seedlings grown from seeds inoculated with a FB1-producer F. verticillioides and from uninoculated ones irrigated with FB1 (20 ppm), were harvested at 7, 14 and 21 days after planting (dap), and the FB1, Sa and Pso levels were quantified in roots and aerial parts. The toxin was detected in roots and aerial parts for inoculated and FB1-irrigated plants of both hybrids. However, FB1 levels were overall higher in SH seedlings regardless of the treatment (infection or watering). Sa levels increased substantially in RH lines, peaking at 54-fold in infected roots at 14 dap. In contrast, the main change observed in SH seedlings was an increase of Pso in infected roots at 7 dap. Here, it was found that FB1 disseminates inside seedlings in the absence of FB1-producer fungal infections, perhaps indicating this might condition the fungus-plant interaction before the first contact. Furthermore, the results strongly suggest the existence of at least two ceramide synthase isoforms in maize with different substrate specificities, whose differential expression after FB1 exposure could be closely related to the susceptibility/resistance to F. verticillioides.

Synthesis and degradation pathways, functions, and pathology of ceramides and epidermal acylceramides

Ceramide (Cer) is a structural backbone of sphingolipids and is composed of a long-chain base and a fatty acid. Existence of a variety of Cer species, which differ in chain-length, hydroxylation status, and/or double bond number of either of their hydrophobic chains, has been reported. Ceramide is produced by Cer synthases. Mammals have six Cer synthases (CERS1-6), each of which exhibits characteristic substrate specificity toward acyl-CoAs with different chain-lengths. Knockout mice for each Cer synthase show corresponding, isozyme-specific phenotypes, revealing the functional differences of Cers with different chain-lengths. Cer diversity is especially prominent in epidermis. Changes in Cer levels, composition, and chain-lengths are associated with atopic dermatitis. Acylceramide (acyl-Cer) specifically exists in epidermis and plays an essential role in skin permeability barrier formation. Accordingly, defects in acyl-Cer synthesis cause the cutaneous disorder ichthyosis with accompanying severe skin barrier defects. Although the molecular mechanism by which acyl-Cer is generated was long unclear, most genes involved in its synthesis have been identified recently. In Cer degradation pathways, the long-chain base moiety of Cer is converted to acyl-CoA, which is then incorporated mainly into glycerophospholipids. This pathway generates the lipid mediator sphingosine 1-phosphate. This review will focus on recent advances in our understanding of the synthesis and degradation pathways, physiological functions, and pathology of Cers/acyl-Cers.

Plant Sphingolipid Metabolism and Function

Sphingolipids, a once overlooked class of lipids in plants, are now recognized as abundant and essential components of plasma membrane and other endomembranes of plant cells. In addition to providing structural integrity to plant membranes, sphingolipids contribute to Golgi trafficking and protein organizational domains in the plasma membrane. Sphingolipid metabolites have also been linked to the regulation of cellular processes, including programmed cell death. Advances in mass spectrometry-based sphingolipid profiling and analyses of Arabidopsis mutants have enabled fundamental discoveries in sphingolipid structural diversity, metabolism, and function that are reviewed here. These discoveries are laying the groundwork for the tailoring of sphingolipid biosynthesis and catabolism for improved tolerance of plants to biotic and abiotic stresses.

Regulation of Sphingolipid Biosynthesis by the Morphogenesis Checkpoint Kinase Swe1

Sphingolipid (SL) biosynthesis is negatively regulated by the highly conserved endoplasmic reticulum-localized Orm family proteins. Defective SL synthesis in Saccharomyces cerevisiae leads to increased phosphorylation and inhibition of Orm proteins by the kinase Ypk1. Here we present evidence that the yeast morphogenesis checkpoint kinase, Swe1, regulates SL biosynthesis independent of the Ypk1 pathway. Deletion of the Swe1 kinase renders mutant cells sensitive to serine palmitoyltransferase inhibition due to impaired sphingoid long-chain base synthesis. Based on these data and previous results, we suggest that Swe1 kinase perceives alterations in SL homeostasis, activates SL synthesis, and may thus represent the missing regulatory link that controls the SL rheostat during the cell cycle.

The AvrE superfamily: ancestral type III effectors involved in suppression of pathogen-associated molecular pattern-triggered immunity

The AvrE superfamily of type III effectors (T3Es) is widespread among type III-dependent phytobacteria and plays a crucial role during bacterial pathogenesis. Members of the AvrE superfamily are vertically inherited core effectors, indicating an ancestral acquisition of these effectors in bacterial plant pathogens. AvrE-T3Es contribute significantly to virulence by suppressing pathogen-associated molecular pattern (PAMP)-triggered immunity. They inhibit salicylic acid-mediated plant defences, interfere with vesicular trafficking and promote bacterial growth in planta. AvrE-T3Es elicit cell death in both host and non-host plants independent of any known plant resistance protein, suggesting an original interaction with the plant immune system. Recent studies in yeast have indicated that they activate protein phosphatase 2A and inhibit serine palmitoyl transferase, the first enzyme of the sphingolipid biosynthesis pathway. In this review, we describe the current picture that has emerged from studies of the different members of this fascinating large family.

Overexpression of Arabidopsis Ceramide Synthases Differentially Affects Growth, Sphingolipid Metabolism, Programmed Cell Death, and Mycotoxin Resistance

Ceramide synthases catalyze an N-acyltransferase reaction using fatty acyl-coenzyme A (CoA) and long-chain base (LCB) substrates to form the sphingolipid ceramide backbone and are targets for inhibition by the mycotoxin fumonisin B1 (FB1). Arabidopsis (Arabidopsis thaliana) contains three genes encoding ceramide synthases with distinct substrate specificities: LONGEVITY ASSURANCE GENE ONE HOMOLOG1 (LOH1; At3g25540)- and LOH3 (At1g19260)-encoded ceramide synthases use very-long-chain fatty acyl-CoA and trihydroxy LCB substrates, and LOH2 (At3g19260)-encoded ceramide synthase uses palmitoyl-CoA and dihydroxy LCB substrates. In this study, complementary DNAs for each gene were overexpressed to determine the role of individual isoforms in physiology and sphingolipid metabolism. Differences were observed in growth resulting from LOH1 and LOH3 overexpression compared with LOH2 overexpression. LOH1- and LOH3-overexpressing plants had enhanced biomass relative to wild-type plants, due in part to increased cell division, suggesting that enhanced synthesis of very-long-chain fatty acid/trihydroxy LCB ceramides promotes cell division and growth. Conversely, LOH2 overexpression resulted in dwarfing. LOH2 overexpression also resulted in the accumulation of sphingolipids with C16 fatty acid/dihydroxy LCB ceramides, constitutive induction of programmed cell death, and accumulation of salicylic acid, closely mimicking phenotypes observed previously in LCB C-4 hydroxylase mutants defective in trihydroxy LCB synthesis. In addition, LOH2- and LOH3-overexpressing plants acquired increased resistance to FB1, whereas LOH1-overexpressing plants showed no increase in FB1 resistance, compared with wild-type plants, indicating that LOH1 ceramide synthase is most strongly inhibited by FB1. Overall, the findings described here demonstrate that overexpression of Arabidopsis ceramide synthases results in strongly divergent physiological and metabolic phenotypes, some of which have significance for improved plant performance.

Misexpression of the Niemann-Pick disease type C1 (NPC1)-like protein in Arabidopsis causes sphingolipid accumulation and reproductive defects

Misexpression of the AtNPC1 - 1 and AtNPC1 - 2 genes leads to altered sphingolipid metabolism, growth impairment, and male reproductive defects in a hemizygous Arabidopsis thaliana (L.) double-mutant population. Abolishing the expression of both gene copies has lethal effects. Niemann-Pick disease type C1 is a lysosomal storage disorder caused by mutations in the NPC1 gene. At the cellular level, the disorder is characterized by the accumulation of storage lipids and lipid trafficking defects. The Arabidopsis thaliana genome contains two genes (At1g42470 and At4g38350) with weak homology to mammalian NPC1. The corresponding proteins have 11 predicted membrane-spanning regions and contain a putative sterol-sensing domain. The At1g42470 protein is localized to the plasma membrane, while At4g38350 protein has a dual localization in the plasma and tonoplast membranes. A phenotypic analysis of T-DNA insertion mutants indicated that At1g42470 and At4g38350 (designated AtNPC1-1 and AtNPC1-2, respectively) have partially redundant functions and are essential for plant reproductive viability and development. Homozygous plants impaired in the expression of both genes were not recoverable. Plants of a hemizygous AtNPC1-1/atnpc1-1/atnpc1-2/atnpc1-2 population were severely dwarfed and exhibited male gametophytic defects. These gene disruptions did not have an effect on sterol concentrations; however, hemizygous AtNPC1-1/atnpc1-1/atnpc1-2/atnpc1-2 mutants had increased fatty acid amounts. Among these, fatty acid α-hydroxytetracosanoic acid (h24:0) occurs in plant sphingolipids. Follow-up analyses confirmed the accumulation of significantly increased levels of sphingolipids (assayed as hydrolyzed sphingoid base component) in the hemizygous double-mutant population. Certain effects of NPC1 misexpression may be common across divergent lineages of eukaryotes (sphingolipid accumulation), while other defects (sterol accumulation) may occur only in certain groups of eukaryotic organisms.

Sphingolipid metabolism is strikingly different between pollen and leaf in Arabidopsis as revealed by compositional and gene expression profiling

Although sphingolipids are essential for male gametophytic development in Arabidopsis thaliana, sphingolipid composition and biosynthetic gene expression have not been previously examined in pollen. In this report, electrospray ionization (ESI)-MS/MS was applied to characterization of sphingolipid compositional profiles in pollen isolated from wild type Arabidopsis Col-0 and a long-chain base (LCB) Δ4 desaturase mutant. Pollen fractions were highly enriched in glucosylceramides (GlcCer) relative to levels previously reported in leaves. Accompanying the loss of the Δ4 unsaturated LCB sphingadiene (d18:2) in the Δ4 desaturase mutant was a 50% reduction in GlcCer concentrations. In addition, pollen glycosylinositolphosphoceramides (GIPCs) were found to have a complex array of N-acetyl-glycosylated GIPCs, including species with up to three pentose units that were absent from leaf GIPCs. Underlying the distinct sphingolipid composition of pollen, genes for key biosynthetic enzymes for GlcCer and d18:2 synthesis and metabolism were more highly expressed in pollen than in leaves or seedlings, including genes for GlcCer synthase (GCS), sphingoid base C-4 hydroxylase 2 (SBH2), LCB Δ8 desaturases (SLD1 and SLD2), and LOH2 ceramide synthase (LOH2). Overall, these findings indicate strikingly divergent sphingolipid metabolism between pollen and leaves in Arabidopsis, the significance of which remains to be determined.

The Arabidopsis ceramidase AtACER functions in disease resistance and salt tolerance

Ceramidases hydrolyze ceramide into sphingosine and fatty acids. In mammals, ceramidases function as key regulators of sphingolipid homeostasis, but little is known about their roles in plants. Here we characterize the Arabidopsis ceramidase AtACER, a homolog of human alkaline ceramidases. The acer-1 T-DNA insertion mutant has pleiotropic phenotypes, including reduction of leaf size, dwarfing and an irregular wax layer, compared with wild-type plants. Quantitative sphingolipid profiling showed that acer-1 mutants and the artificial microRNA-mediated silenced line amiR-ACER-1 have high ceramide levels and decreased long chain bases. AtACER localizes predominantly to the endoplasmic reticulum, and partially to the Golgi complex. Furthermore, we found that acer-1 mutants and AtACER RNAi lines showed increased sensitivity to salt stress, and lines overexpressing AtACER showed increased tolerance to salt stress. Reduction of AtACER also increased plant susceptibility to Pseudomonas syringae. Our data highlight the key biological functions of ceramidases in biotic and abiotic stresses in plants.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Deciphering the link between salicylic acid signaling and sphingolipid metabolism

The field of plant sphingolipid biology has evolved in recent years. Sphingolipids are abundant in cell membranes, and genetic analyses revealed essential roles for these lipids in plant growth, development, and responses to abiotic and biotic stress. Salicylic acid (SA) is a key signaling molecule that is required for induction of defense-related genes and rapid and localized cell death at the site of pathogen infection (hypersensitive response) during incompatible host-pathogen interactions. Conceivably, while levels of SA rapidly increase upon pathogen infection for defense activation, they must be tightly regulated during plant growth and development in the absence of pathogens. Genetic and biochemical evidence suggest that the sphingolipid intermediates, long-chain sphingoid bases, and ceramides, play a role in regulating SA accumulation in plant cells. However, how signals generated from the perturbation of these key sphingolipid intermediates are transduced into the activation of the SA pathway has long remained to be an interesting open question. At least four types of molecules - MAP kinase 6, reactive oxygen species, free calcium, and nitric oxide - could constitute a mechanistic link between sphingolipid metabolism and SA accumulation and signaling.

Ethylene Modulates Sphingolipid Synthesis in Arabidopsis

Sphingolipids have essential structural and bioactive functions in membranes and in signaling. However, how plants regulate sphingolipid biosynthesis in the response to stress remains unclear. Here, we reveal that the plant hormone ethylene can modulate sphingolipid synthesis. The fungal toxin Fumonisin B1 (FB1) inhibits the activity of ceramide synthases, perturbing sphingolipid homeostasis, and thus inducing cell death. We used FB1 to test the role of ethylene signaling in sphingolipid synthesis in Arabidopsis thaliana. The etr1-1 and ein2 mutants, which have disrupted ethylene signaling, exhibited hypersensitivity to FB1; by contrast, the eto1-1 and ctr1-1 mutants, which have enhanced ethylene signaling, exhibited increased tolerance to FB1. Gene expression analysis showed that during FB1 treatment, transcripts of genes involved in de novo sphingolipid biosynthesis were down-regulated in ctr1-1 mutants but up-regulated in ein2 mutants. Strikingly, under normal conditions, ctr1-1 mutants contained less ceramides and hydroxyceramides, compared with wild type. After FB1 treatment, ctr1-1 and ein2 mutants showed a significant improvement in sphingolipid contents, except the ctr1-1 mutants showed little change in hydroxyceramide levels. Treatment of wild-type seedlings with the ethylene precursor 1-aminocyclopropane carboxylic acid down-regulated genes involved in the sphingolipid de novo biosynthesis pathway, thus reducing sphingolipid contents and partially rescuing FB1-induced cell death. Taking these results together, we propose that ethylene modulates sphingolipids by regulating the expression of genes related to the de novo biosynthesis of sphingolipids.

Arabidopsis ribosomal proteins control vacuole trafficking and developmental programs through the regulation of lipid metabolism

The vacuole is the most prominent compartment in plant cells and is important for ion and protein storage. In our effort to search for key regulators in the plant vacuole sorting pathway, ribosomal large subunit 4 (rpl4d) was identified as a translational mutant defective in both vacuole trafficking and normal development. Polysome profiling of the rpl4d mutant showed reduction in polysome-bound mRNA compared with wild-type, but no significant change in the general mRNA distribution pattern. Ribsomal profiling data indicated that genes in the lipid metabolism pathways were translationally down-regulated in the rpl4d mutant. Live imaging studies by Nile red staining suggested that both polar and nonpolar lipid accumulation was reduced in meristem tissues of rpl4d mutants. Pharmacological evidence showed that sterol and sphingolipid biosynthetic inhibitors can phenocopy the defects of the rpl4d mutant, including an altered vacuole trafficking pattern. Genetic evidence from lipid biosynthetic mutants indicates that alteration in the metabolism of either sterol or sphingolipid biosynthesis resulted in vacuole trafficking defects, similar to the rpl4d mutant. Tissue-specific complementation with key enzymes from lipid biosynthesis pathways can partially rescue both vacuole trafficking and auxin-related developmental defects in the rpl4d mutant. These results indicate that lipid metabolism modulates auxin-mediated tissue differentiation and endomembrane trafficking pathways downstream of ribosomal protein function.

Amino acids--a life between metabolism and signaling

Amino acids serve as constituents of proteins, precursors for anabolism, and, in some cases, as signaling molecules in mammalians and plants. This review is focused on new insights, or speculations, on signaling functions of serine, γ-aminobutyric acid (GABA) and phenylalanine-derived phenylpropanoids. Serine acts as signal in brain tissue and mammalian cancer cells. In plants, de novo serine biosynthesis is also highly active in fast growing tissues such as meristems, suggesting a similar role of serine as in mammalians. GABA functions as inhibitory neurotransmitter in the brain. In plants, GABA is also abundant and seems to be involved in sexual reproduction, cell elongation, patterning and cell identity. The aromatic amino acids phenylalanine, tyrosine, and tryptophan are precursors for the production of secondary plant products. Besides their pharmaceutical value, lignans, neolignans and hydroxycinnamic acid amides (HCAA) deriving from phenylpropanoid metabolism and, in the case of HCAA, also from arginine have been shown to fulfill signaling functions or are involved in the response to biotic and abiotic stress. Although some basics on phenylpropanoid-derived signaling have been described, little is known on recognition- or signal transduction mechanisms. In general, mutant- and transgenic approaches will be helpful to elucidate the mechanistic basis of metabolite signaling.

Overexpression of patatin-related phospholipase AIIIβ altered the content and composition of sphingolipids in Arabidopsis

In plants, fatty acids are primarily synthesized in plastids and then transported to the endoplasmic reticulum (ER) for synthesis of most of the complex membrane lipids, including glycerolipids and sphingolipids. The first step of sphingolipid synthesis, which uses a fatty acid and a serine as substrates, is critical for sphingolipid homeostasis; its disruption leads to an altered plant growth. Phospholipase As have been implicated in the trafficking of fatty acids from plastids to the ER. Previously, we found that overexpression of a patatin-related phospholipase, pPLAIIIβ, resulted in a smaller plant size and altered anisotropic cell expansion. Here, we determined the content and composition of sphingolipids in pPLAIIIβ-knockout and overexpression plants (pPLAIIIβ-KO and -OE). 3-keto-sphinganine, the product of the first step of sphingolipid synthesis, had a 26% decrease in leaves of pPLAIIIβ-KO while a 52% increase in pPLAIIIβ-OE compared to wild type (WT). The levels of free long-chain base species, dihydroxy-C18:0 and trihydroxy-18:0 (d18:0 and t18:0), were 38 and 97% higher, respectively, in pPLAIIIβ-OE than in WT. The level of complex sphingolipids ceramide d18:0-16:0 and t18:1-16:0 had a twofold increase in pPLAIIIβ-OE. The level of hydroxy ceramide d18:0-h16:0 was 72% higher in pPLAIIIβ-OE compared to WT. The levels of several species of glucosylceramide and glycosylinositolphosphoceramide tended to be higher in pPLAIIIβ-OE than in WT. The total content of the complex sphingolipids showed a slightly higher in pPLAIIIβ-OE than in WT. These results revealed an involvement of phospholipase-mediated lipid homeostasis in plant growth.

Molecular characterization of rice sphingosine-1-phosphate lyase gene OsSPL1 and functional analysis of its role in disease resistance response

Our results indicate that overexpression of OsSPL1 in transgenic tobacco plants attenuated disease resistance and facilitated programmed cell death. Long-chain base phosphates including sphingosine-1-phosphate have been shown to act as signaling mediators in regulating programmed cell death (PCD) and stress responses in mammals. In the present study, we characterized a rice gene OsSPL1, encoding a putative sphingosine-1-phosphate lyase that is involved in metabolism of sphingosine-1-phosphate. Expression of OsSPL1 was down-regulated in rice plants after treatments with salicylic acid, benzothiadiazole and 1-amino cyclopropane-1-carboxylic acid, but was induced by infection with a virulent strain of Magnaporthe oryzae, the causal agent of rice blast disease. Transgenic tobacco lines with overexpression of OsSPL1 were generated and analyzed for the possible role of OsSPL1 in disease resistance response and PCD. The OsSPL1-overexpressing tobacco plants displayed increased susceptibility to infection of Pseudomonas syringae pv. tabaci (Pst), the causal agent of wildfire disease, showing severity of disease symptom and bacterial titers in inoculated leaves, and attenuated pathogen-induced expression of PR genes after infection of Pst as compared to the wild-type and vector-transformed plants. Higher level of cell death, as revealed by dead cell staining, leakage of electrolyte and expression of hypersensitive response indicator genes, was observed in the OsSPL1-overexpressing plants after treatment with fumonisin B1, a fungal toxin that induces PCD in plants. Our results suggest that OsSPL1 has different functions in regulating disease resistance response and PCD in plants.

Serine in plants: biosynthesis, metabolism, and functions

Serine (Ser) has a fundamental role in metabolism and signaling in living organisms. In plants, the existence of different pathways of Ser biosynthesis has complicated our understanding of this amino acid homeostasis. The photorespiratory glycolate pathway has been considered to be of major importance, whereas the nonphotorespiratory phosphorylated pathway has been relatively neglected. Recent advances indicate that the phosphorylated pathway has an important function in plant metabolism and development. Plants deficient in this pathway display developmental defects in embryos, male gametophytes, and roots. We propose that the phosphorylated pathway is more important than was initially thought because it is the only Ser source for specific cell types involved in developmental events. Here, we discuss its importance as a link between metabolism and development in plants.

Association mapping of seed oil and protein content in Sesamum indicum L. using SSR markers

Sesame is an important oil crop for the high oil content and quality. The seed oil and protein contents are two important traits in sesame. To identify the molecular markers associated with the seed oil and protein contents in sesame, we systematically performed the association mapping among 369 worldwide germplasm accessions under 5 environments using 112 polymorphic SSR markers. The general linear model (GLM) was applied with the criteria of logP≥3.0 and high stability under all 5 environments. Among the 369 sesame accessions, the oil content ranged from 27.89%–58.73% and the protein content ranged from 16.72%–27.79%. A significant negative correlation of the oil content with the protein content was found in the population. A total of 19 markers for oil content were detected with a R² value range from 4% to 29%; 24 markers for protein content were detected with a R² value range from 3% to 29%, of which 19 markers were associated with both traits. Moreover, partial markers were confirmed using mixed linear model (MLM) method, which suggested that the oil and protein contents are controlled mostly by major genes. Allele effect analysis showed that the allele associated with high oil content was always associated with low protein content, and vice versa. Of the 19 markers associated with oil content, 17 presented near the locations of the plant lipid pathway genes and 2 were located just next to a fatty acid elongation gene and a gene encoding Stearoyl-ACP Desaturase, respectively. The findings provided a valuable foundation for oil synthesis gene identification and molecular marker assistant selection (MAS) breeding in sesame.

Identification of a sphingolipid α-glucuronosyltransferase that is essential for pollen function in Arabidopsis

Glycosyl inositol phosphorylceramide (GIPC) sphingolipids are a major class of lipids in fungi, protozoans, and plants. GIPCs are abundant in the plasma membrane in plants, comprising around a quarter of the total lipids in these membranes. Plant GIPCs contain unique glycan decorations that include a conserved glucuronic acid (GlcA) residue and various additional sugars; however, no proteins responsible for glycosylating GIPCs have been identified to date. Here, we show that the Arabidopsis thaliana protein INOSITOL PHOSPHORYLCERAMIDE GLUCURONOSYLTRANSFERASE1 (IPUT1) transfers GlcA from UDP-GlcA to GIPCs. To demonstrate IPUT1 activity, we introduced the IPUT1 gene together with genes for a UDP-glucose dehydrogenase from Arabidopsis and a human UDP-GlcA transporter into a yeast mutant deficient in the endogenous inositol phosphorylceramide (IPC) mannosyltransferase. In this engineered yeast strain, IPUT1 transferred GlcA to IPC. Overexpression or silencing of IPUT1 in Nicotiana benthamiana resulted in an increase or a decrease, respectively, in IPC glucuronosyltransferase activity in vitro. Plants in which IPUT1 was silenced accumulated IPC, the immediate precursor, as well as ceramides and glucosylceramides. Plants overexpressing IPUT1 showed an increased content of GIPCs. Mutations in IPUT1 are not transmitted through pollen, indicating that these sphingolipids are essential in plants.

Expression of the bacterial type III effector DspA/E in Saccharomyces cerevisiae down-regulates the sphingolipid biosynthetic pathway leading to growth arrest

Erwinia amylovora, the bacterium responsible for fire blight, relies on a type III secretion system and a single injected effector, DspA/E, to induce disease in host plants. DspA/E belongs to the widespread AvrE family of type III effectors that suppress plant defense responses and promote bacterial growth following infection. Ectopic expression of DspA/E in plant or in Saccharomyces cerevisiae is toxic, indicating that DspA/E likely targets a cellular process conserved between yeast and plant. To unravel the mode of action of DspA/E, we screened the Euroscarf S. cerevisiae library for mutants resistant to DspA/E-induced growth arrest. The most resistant mutants (Δsur4, Δfen1, Δipt1, Δskn1, Δcsg1, Δcsg2, Δorm1, and Δorm2) were impaired in the sphingolipid biosynthetic pathway. Exogenously supplied sphingolipid precursors such as the long chain bases (LCBs) phytosphingosine and dihydrosphingosine also suppressed the DspA/E-induced yeast growth defect. Expression of DspA/E in yeast down-regulated LCB biosynthesis and induced a rapid decrease in LCB levels, indicating that serine palmitoyltransferase (SPT), the first and rate-limiting enzyme of the sphingolipid biosynthetic pathway, was repressed. SPT down-regulation was mediated by dephosphorylation and activation of Orm proteins that negatively regulate SPT. A Δcdc55 mutation affecting Cdc55-PP2A protein phosphatase activity prevented Orm dephosphorylation and suppressed DspA/E-induced growth arrest.

Arabidopsis mutants in sphingolipid synthesis as tools to understand the structure and function of membrane microdomains in plasmodesmata

Plasmodesmata-intercellular channels that communicate adjacent cells-possess complex membranous structures. Recent evidences indicate that plasmodesmata contain membrane microdomains. In order to understand how these submembrane regions collaborate to plasmodesmata function, it is necessary to characterize their size, composition and dynamics. An approach that can shed light on these microdomain features is based on the use of Arabidopsis mutants in sphingolipid synthesis. Sphingolipids are canonical components of microdomains together with sterols and some glycerolipids. Moreover, sphingolipids are transducers in pathways that display programmed cell death as a defense mechanism against pathogens. The study of Arabidopsis mutants would allow determining which structural features of the sphingolipids are important for the formation and stability of microdomains, and if defense signaling networks using sphingoid bases as second messengers are associated to plasmodesmata operation. Such studies need to be complemented by analysis of the ultrastructure and the use of protein probes for plasmodesmata microdomains and may constitute a very valuable source of information to analyze these membrane structures.

Arabidopsis 56-amino acid serine palmitoyltransferase-interacting proteins stimulate sphingolipid synthesis, are essential, and affect mycotoxin sensitivity

Maintenance of sphingolipid homeostasis is critical for cell growth and programmed cell death (PCD). Serine palmitoyltransferase (SPT), composed of LCB1 and LCB2 subunits, catalyzes the primary regulatory point for sphingolipid synthesis. Small subunits of SPT (ssSPT) that strongly stimulate SPT activity have been identified in mammals, but the role of ssSPT in eukaryotic cells is unclear. Candidate Arabidopsis thaliana ssSPTs, ssSPTa and ssSPTb, were identified and characterized. Expression of these 56-amino acid polypeptides in a Saccharomyces cerevisiae SPT null mutant stimulated SPT activity from the Arabidopsis LCB1/LCB2 heterodimer by >100-fold through physical interaction with LCB1/LCB2. ssSPTa transcripts were more enriched in all organs and >400-fold more abundant in pollen than ssSPTb transcripts. Accordingly, homozygous ssSPTa T-DNA mutants were not recoverable, and 50% nonviable pollen was detected in heterozygous ssspta mutants. Pollen viability was recovered by expression of wild-type ssSPTa or ssSPTb under control of the ssSPTa promoter, indicating ssSPTa and ssSPTb functional redundancy. SPT activity and sensitivity to the PCD-inducing mycotoxin fumonisin B1 (FB1) were increased by ssSPTa overexpression. Conversely, SPT activity and FB1 sensitivity were reduced in ssSPTa RNA interference lines. These results demonstrate that ssSPTs are essential for male gametophytes, are important for FB1 sensitivity, and limit sphingolipid synthesis in planta.

Arabidopsis 56-amino acid serine palmitoyltransferase-interacting proteins stimulate sphingolipid synthesis, are essential, and affect mycotoxin sensitivity

Maintenance of sphingolipid homeostasis is critical for cell growth and programmed cell death (PCD). Serine palmitoyltransferase (SPT), composed of LCB1 and LCB2 subunits, catalyzes the primary regulatory point for sphingolipid synthesis. Small subunits of SPT (ssSPT) that strongly stimulate SPT activity have been identified in mammals, but the role of ssSPT in eukaryotic cells is unclear. Candidate Arabidopsis thaliana ssSPTs, ssSPTa and ssSPTb, were identified and characterized. Expression of these 56-amino acid polypeptides in a Saccharomyces cerevisiae SPT null mutant stimulated SPT activity from the Arabidopsis LCB1/LCB2 heterodimer by >100-fold through physical interaction with LCB1/LCB2. ssSPTa transcripts were more enriched in all organs and >400-fold more abundant in pollen than ssSPTb transcripts. Accordingly, homozygous ssSPTa T-DNA mutants were not recoverable, and 50% nonviable pollen was detected in heterozygous ssspta mutants. Pollen viability was recovered by expression of wild-type ssSPTa or ssSPTb under control of the ssSPTa promoter, indicating ssSPTa and ssSPTb functional redundancy. SPT activity and sensitivity to the PCD-inducing mycotoxin fumonisin B1 (FB1) were increased by ssSPTa overexpression. Conversely, SPT activity and FB1 sensitivity were reduced in ssSPTa RNA interference lines. These results demonstrate that ssSPTs are essential for male gametophytes, are important for FB1 sensitivity, and limit sphingolipid synthesis in planta.

Identification of a sphingolipid-specific phospholipase D activity associated with the generation of phytoceramide-1-phosphate in cabbage leaves

The structure and biosynthetic route for an unidentified lipid (lipid X) detected by TLC of cabbage (Brassica oleracea) lipids was determined. Lipid X is a phospholipid that is resistant to mild alkali and detectable by MALDI-TOF MS as an adduct with Phos-tag, a phosphate-capture zinc complex. Various α-hydroxy fatty acids (16:0, 22:0, 24:0 and 24:1) were detected by GC-MS of fatty acid methyl esters prepared from lipid X. The deacyl derivative of lipid X was determined to be 4-hydroxysphingenine (dehydrophytosphingosine)-1-phosphate by MALDI-TOF MS with Phos-tag. From these results, lipid X was determined to be phytoceramide-1-phosphate (PC1P) with an α-hydroxy fatty acid. When cabbage homogenates were incubated, PC1P was formed, with a concomitant decrease in the amount of glycosylinositol phosphoceramide (GIPC). The formation of PC1P from GIPC was confirmed by treatment of purified cabbage GIPC with a membrane fraction of cabbage homogenates. Using a partially purified enzyme fraction, we found that the enzyme hydrolyzes GIPC specifically, but not glycerophospholipids and sphingomyelin. Arabidopsis thaliana also had this enzyme activity. From these results, we conclude that a previously uncharacterized phospholipase D activity that specifically hydrolyzes GIPC produces PC1P in brassicaceous plants.

Plant sphingolipids: function follows form

Plant sphingolipids are structurally diverse molecules that are important as membrane components and bioactive molecules. An appreciation of the relationship between structural diversity and functional significance of plant sphingolipids is emerging through characterization of Arabidopsis mutants coupled with advanced analytical methods. It is increasingly apparent that modifications such as hydroxylation and desaturation of the sphingolipid nonpolar long-chain bases and fatty acids influence their metabolic routing to particular complex sphingolipid classes and their functions in signaling pathways and other cellular processes, such as membrane protein targeting. Here, we review recent reports investigating some of the more prevalent sphingolipid structural modifications and their functional importance in plants.

Arabidopsis 3-ketoacyl-coenzyme a synthase9 is involved in the synthesis of tetracosanoic acids as precursors of cuticular waxes, suberins, sphingolipids, and phospholipids

Very-long-chain fatty acids (VLCFAs) with chain lengths from 20 to 34 carbons are involved in diverse biological functions such as membrane constituents, a surface barrier, and seed storage compounds. The first step in VLCFA biosynthesis is the condensation of two carbons to an acyl-coenzyme A, which is catalyzed by 3-ketoacyl-coenzyme A synthase (KCS). In this study, amino acid sequence homology and the messenger RNA expression patterns of 21 Arabidopsis (Arabidopsis thaliana) KCSs were compared. The in planta role of the KCS9 gene, showing higher expression in stem epidermal peels than in stems, was further investigated. The KCS9 gene was ubiquitously expressed in various organs and tissues, including roots, leaves, and stems, including epidermis, silique walls, sepals, the upper portion of the styles, and seed coats, but not in developing embryos. The fluorescent signals of the KCS9::enhanced yellow fluorescent protein construct were merged with those of BrFAD2::monomeric red fluorescent protein, which is an endoplasmic reticulum marker in tobacco (Nicotiana benthamiana) epidermal cells. The kcs9 knockout mutants exhibited a significant reduction in C24 VLCFAs but an accumulation of C20 and C22 VLCFAs in the analysis of membrane and surface lipids. The mutant phenotypes were rescued by the expression of KCS9 under the control of the cauliflower mosaic virus 35S promoter. Taken together, these data demonstrate that KCS9 is involved in the elongation of C22 to C24 fatty acids, which are essential precursors for the biosynthesis of cuticular waxes, aliphatic suberins, and membrane lipids, including sphingolipids and phospholipids. Finally, possible roles of unidentified KCSs are discussed by combining genetic study results and gene expression data from multiple Arabidopsis KCSs.

Synthesis of very-long-chain fatty acids in the epidermis controls plant organ growth by restricting cell proliferation

The synthesis of very-long-chain fatty acids (VLCFAs) in the epidermis is essential for the proper control of cell growth in Arabidopsis. VLCFAs act via their ability to suppress cytokinin synthesis in the vasculature, thus preventing cell overproliferation in internal tissues.

Plant organ growth is controlled by inter-cell-layer communication, which thus determines the overall size of the organism. The epidermal layer interfaces with the environment and participates in both driving and restricting growth via inter-cell-layer communication. However, it remains unknown whether the epidermis can send signals to internal tissue to limit cell proliferation in determinate growth. Very-long-chain fatty acids (VLCFAs) are synthesized in the epidermis and used in the formation of cuticular wax. Here we found that VLCFA synthesis in the epidermis is essential for proper development of Arabidopsis thaliana. Wild-type plants treated with a VLCFA synthesis inhibitor and pasticcino mutants with defects in VLCFA synthesis exhibited overproliferation of cells in the vasculature or in the rib zone of shoot apices. The decrease of VLCFA content increased the expression of IPT3, a key determinant of cytokinin biosynthesis in the vasculature, and, indeed, elevated cytokinin levels. These phenotypes were suppressed in ipt3;5;7 triple mutants, and also by vasculature-specific expression of cytokinin oxidase, which degrades active forms of cytokinin. Our results imply that VLCFA synthesis in the epidermis is required to suppress cytokinin biosynthesis in the vasculature, thus fine-tuning cell division activity in internal tissue, and therefore that shoot growth is controlled by the interaction between the surface (epidermis) and the axis (vasculature) of the plant body.

The epidermis functions as an important interface with the environment, but in plants it is also essential for establishing and maintaining the primary plant body. Recent studies have shown that the epidermis participates in both driving and restricting plant growth via inter-cell-layer communication. However, it remains an open question as to whether the epidermis can send signals to internal plant tissues to control cell proliferation during development. Here we report that the synthesis of very-long-chain fatty acids (VLCFAs) in the epidermis is essential for the proper control of cell proliferation in the plant Arabidopsis thaliana. We find that defects in VLCFA synthesis cause cells in the vasculature or in the rib zone of shoot apices to overproliferate. When VLCFA levels decrease, we observe that the synthesis of the phytohormone cytokinin increases in the vasculature. We also find that when cytokinin is degraded by the expression of cytokinin oxidase in the vasculature, enhanced cell proliferation in internal tissues is suppressed, indicating that VLCFA synthesis in the epidermis is required to suppress cytokinin biosynthesis and thus cell overproliferation. Our results demonstrate that shoot growth is controlled by interactions between the surface (epidermis) and the axis (vasculature) of the plant body, and highlight a role for VLCFAs in this interaction.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Disruption of sphingolipid biosynthesis in Nicotiana benthamiana activates salicylic acid-dependent responses and compromises resistance to Alternaria alternata f. sp. lycopersici

Sphingolipids play an important role in signal transduction pathways that regulate physiological functions and stress responses in eukaryotes. In plants, recent evidence suggests that their metabolic precursors, the long-chain bases (LCBs) act as bioactive molecules in the immune response. Interestingly, the virulence of two unrelated necrotrophic fungi, Fusarium verticillioides and Alternaria alternata, which are pathogens of maize and tomato plants, respectively, depends on the production of sphinganine-analog mycotoxins (SAMs). These metabolites inhibit de novo synthesis of sphingolipids in their hosts causing accumulation of LCBs, which are key regulators of programmed cell death. Therefore, to gain more insight into the role of sphingolipids in plant immunity against SAM-producing necrotrophic fungi, we disrupted sphingolipid metabolism in Nicotiana benthamiana through virus-induced gene silencing (VIGS) of the serine palmitoyltransfersase (SPT). This enzyme catalyzes the first reaction in LCB synthesis. VIGS of SPT profoundly affected N. benthamiana development as well as LCB composition of sphingolipids. While total levels of phytosphingosine decreased, sphinganine and sphingosine levels increased in SPT-silenced plants, compared with control plants. Plant immunity was also affected as silenced plants accumulated salicylic acid (SA), constitutively expressed the SA-inducible NbPR-1 gene and showed increased susceptibility to the necrotroph A. alternata f. sp. lycopersici. In contrast, expression of NbPR-2 and NbPR-3 genes was delayed in silenced plants upon fungal infection. Our results strongly suggest that LCBs modulate the SA-dependent responses and provide a working model of the potential role of SAMs from necrotrophic fungi to disrupt the plant host response to foster colonization.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Overexpression of rice sphingosine-1-phoshpate lyase gene OsSPL1 in transgenic tobacco reduces salt and oxidative stress tolerance

Sphingolipids, including sphingosine-1-phosphate (S1P), have been shown to function as signaling mediators to regulate diverse aspects of plant growth, development, and stress response. In this study, we performed functional analysis of a rice (Oryza sativa) S1P lyase gene OsSPL1 in transgenic tobacco plants and explored its possible involvement in abiotic stress response. Overexpression of OsSPL1 in transgenic tobacco resulted in enhanced sensitivity to exogenous abscisic acid (ABA), and decreased tolerance to salt and oxidative stress, when compared with the wild type. Furthermore, the expression levels of some selected stress-related genes in OsSPL1-overexpressing plants were reduced after application of salt or oxidative stress, indicating that the altered responsiveness of stress-related genes may be responsible for the reduced tolerance in OsSPL1-overexpressing tobacco plants under salt and oxidative stress. Our results suggest that rice OsSPL1 plays an important role in abiotic stress responses.

Connections between sphingosine kinase and phospholipase D in the abscisic acid signaling pathway in Arabidopsis

Phosphatidic acid (PA) and phytosphingosine 1-phosphate (phyto-S1P) both are lipid messengers involved in plant response to abscisic acid (ABA). Our previous data indicate that PA binds to sphingosine kinase (SPHK) and increases its phyto-S1P-producing activity. To understand the cellular and physiological functions of the PA-SPHK interaction, we isolated Arabidopsis thaliana SPHK mutants sphk1-1 and sphk2-1 and characterized them, together with phospholipase Dα1 knock-out, pldα1, in plant response to ABA. Compared with wild-type (WT) plants, the SPHK mutants and pldα1 all displayed decreased sensitivity to ABA-promoted stomatal closure. Phyto-S1P promoted stomatal closure in sphk1-1 and sphk2-1, but not in pldα1, whereas PA promoted stomatal closure in sphk1-1, sphk2-1, and pldα1. The ABA activation of PLDα1 in leaves and protoplasts was attenuated in the SPHK mutants, and the ABA activation of SPHK was reduced in pldα1. In response to ABA, the accumulation of long-chain base phosphates was decreased in pldα1, whereas PA production was decreased in SPHK mutants, compared with WT. Collectively, these results indicate that SPHK and PLDα1 act together in ABA response and that SPHK and phyto-S1P act upstream of PLDα1 and PA in mediating the ABA response. PA is involved in the activation of SPHK, and activation of PLDα1 requires SPHK activity. The data suggest that SPHK/phyto-S1P and PLDα1A are co-dependent in amplification of response to ABA, mediating stomatal closure in Arabidopsis.

Sphingolipid Δ8 unsaturation is important for glucosylceramide biosynthesis and low-temperature performance in Arabidopsis

Plants contain a large diversity of sphingolipid structures, arising in part from C4 hydroxylation and Δ4 and Δ8 desaturation of the component long-chain bases (LCBs). Typically, 85-90% of sphingolipid LCBs in Arabidopsis leaves contain a cis or transΔ8 double bond produced by sphingoid LCB Δ8 desaturase (SLD). To understand the metabolic and physiological significance of Δ8 unsaturation, studies were performed using mutants of the Arabidopsis SLD genes AtSLD1 and AtSLD2. Our studies revealed that both genes are constitutively expressed, the corresponding polypeptides are ER-localized, and expression of these genes in Saccharomyces cerevisiae yields mixtures of cis/transΔ8 desaturation products, predominantly as trans isomers. Consistent in part with the higher expression of AtSLD1 in Arabidopsis plants, AtSLD1 T-DNA mutants showed large reductions in Δ8 unsaturated LCBs in all organs examined, whereas AtSLD2 mutants showed little change in LCB unsaturation. Double mutants of AtSLD1 and AtSLD2 showed no detectable LCB Δ8 unsaturation. Comprehensive analysis of sphingolipids in rosettes of these mutants revealed a 50% reduction in glucosylceramide levels and a corresponding increase in glycosylinositolphosphoceramides that were restored by complementation with a wild-type copy of AtSLD1. Double sld1 sld2 mutants lacked apparent growth phenotypes under optimal conditions, but displayed altered responses to certain stresses, including prolonged exposure to low temperatures. These results are consistent with a role for LCB Δ8 unsaturation in selective channeling of ceramides for the synthesis of complex sphingolipids and the physiological performance of Arabidopsis.

Reduced expression of FatA thioesterases in Arabidopsis affects the oil content and fatty acid composition of the seeds

Acyl-acyl carrier protein (ACP) thioesterases are enzymes that control the termination of intraplastidial fatty acid synthesis by hydrolyzing the acyl-ACP complexes. Among the different thioesterase gene families found in plants, the FatA-type fulfills a fundamental role in the export of the C18 fatty acid moieties that will be used to synthesize most plant glycerolipids. A reverse genomic approach has been used to study the FatA thioesterase in seed oil accumulation by screening different mutant collections of Arabidopsis thaliana for FatA knockouts. Two mutants were identified with T-DNA insertions in the promoter region of each of the two copies of FatA present in the Arabidopsis genome, from which a double FatA Arabidopsis mutant was made. The expression of both forms of FatA thioesterases was reduced in this double mutant (fata1 fata2), as was FatA activity. This decrease did not cause any evident morphological changes in the mutant plants, although the partial reduction of this activity affected the oil content and fatty acid composition of the Arabidopsis seeds. Thus, dry mutant seeds had less triacylglycerol content, while other neutral lipids like diacylglycerols were not affected. Furthermore, the metabolic flow of the different glycerolipid species into seed oil in the developing seeds was reduced at different stages of seed formation in the fata1 fata2 line. This diminished metabolic flow induced increases in the proportion of linolenic and erucic fatty acids in the seed oil, in a similar way as previously reported for the wri1 Arabidopsis mutant that accumulates oil poorly. The similarities between these two mutants and the origin of their phenotype are discussed in function of the results.

Sphingolipid and ceramide homeostasis: potential therapeutic targets

Sphingolipids are ubiquitous in eukaryotic cells where they have been attributed a plethora of functions from the formation of structural domains to polarized cellular trafficking and signal transduction. Recent research has identified and characterised many of the key enzymes involved in sphingolipid metabolism and this has led to a heightened interest in the possibility of targeting these processes for therapies against cancers, Alzheimer's disease, and numerous important human pathogens. In this paper we outline the major pathways in eukaryotic sphingolipid metabolism and discuss these in relation to disease and therapy for both chronic and infectious conditions.

Sphingosine in plants--more riddles from the Sphinx?

• Sphingolipids are emerging as important mediators of cellular and developmental processes in plants, and advances in lipidomics have yielded a wealth of information on the composition of plant sphingolipidomes. Studies using Arabidopsis thaliana showed that the dihydroxy long-chain base (LCB) is desaturated at carbon position 8 (d18:1(Δ8)). This raised important questions on the role(s) of sphingosine (d18:1(Δ4)) and sphingosine-1-phosphate (d18:1(Δ4)-P) in plants, as these LCBs appear to be absent in A. thaliana. • Here, we surveyed 21 species from various phylogenetic groups to ascertain the position of desaturation of the d18:1 LCB, in order to gain further insights into the prevalence of d18:1(Δ4) and d18:1(Δ8) in plants. • Our results showed that d18:1(Δ8) is common in gymnosperms, whereas d18:1(Δ4) is widespread within nonseed land plants and the Poales, suggesting that d18:1(Δ4) is evolutionarily more ancient than d18:1(Δ8) in Viridiplantae. Additionally, phylogenetic analysis indicated that the sphingolipid Δ4-desaturases from Viridiplantae form a monophyletic group, with Angiosperm sequences falling into two distinct clades, the Eudicots and the Poales. • We propose that efforts to elucidate the role(s) of d18:1(Δ4) and d18:1(Δ4)-P should focus on genetically tractable Viridiplantae species where the d18:1 LCB is desaturated at carbon position 4.

MPK6, sphinganine and the LCB2a gene from serine palmitoyltransferase are required in the signaling pathway that mediates cell death induced by long chain bases in Arabidopsis

Long chain bases (LCBs) are sphingolipid intermediates acting as second messengers in programmed cell death (PCD) in plants. Most of the molecular and cellular features of this signaling function remain unknown. We induced PCD conditions in Arabidopsis thaliana seedlings and analyzed LCB accumulation kinetics, cell ultrastructure and phenotypes in serine palmitoyltransferase (spt), mitogen-activated protein kinase (mpk), mitogen-activated protein phosphatase (mkp1) and lcb-hydroxylase (sbh) mutants. The lcb2a-1 mutant was unable to mount an effective PCD in response to fumonisin B1 (FB1), revealing that the LCB2a gene is essential for the induction of PCD. The accumulation kinetics of LCBs in wild-type (WT) and lcb2a-1 plants and reconstitution experiments with sphinganine indicated that this LCB was primarily responsible for PCD elicitation. The resistance of the null mpk6 mutant to manifest PCD on FB1 and sphinganine addition and the failure to show resistance on pathogen infection and MPK6 activation by FB1 and LCBs indicated that MPK6 mediates PCD downstream of LCBs. This work describes MPK6 as a novel transducer in the pathway leading to LCB-induced PCD in Arabidopsis, and reveals that sphinganine and the LCB2a gene are required in a PCD process that operates as one of the more effective strategies used as defense against pathogens in plants.

Genome-wide comparison of two poplar genotypes with different growth rates

The ecologically dominant and economically important genus Populus, with its available full genome sequence, has become an ideal woody species for genomic study. Rapid growth is one of the primary advantageous features of Populus, and extensive physiological research has been carried out on the growth of Populus throughout the growing period among different clones. However, the molecular information related to the mechanisms of rapid growth is rather limited. In this study, an Affymetrix poplar genome array was employed to analyze the transcriptomic changes from the pre-growth to the fast-growth phase in two poplar clones (P.deltoides × P.nigra, DN2, and P.nigra × (P.deltoides × P. nigra), NE19) with different growth rates. A total of 1,695 differently expressed genes were identified between two time points in NE19 and DN2 (two-way ANOVA, P < 0.01 and fold change ≥2). Except for genes changing in common for both clones, many transcripts were regulated specifically in one genotype. After functional analysis of the differentially expressed genes, distinct biological strategies seemed to be utilized by the two genotypes to accommodate their fast-growth phase. The faster-growing clone NE19, which has a higher photosynthetic rate and larger total leaf area, emphasized growth-related primary metabolism. However, the slower-growing DN2 tended to have more up-regulated genes involved in defense-related secondary metabolism and stress response. Emphasis of such divergent biological processes in two clones may explain their significant growth differences during the fast-growth phase.

Sphingolipid base modifying enzymes in sunflower (Helianthus annuus): cloning and characterization of a C4-hydroxylase gene and a new paralogous Δ8-desaturase gene

Sphingolipids are components of plant cell membranes that participate in the regulation of important physiological processes. Unlike their animal counterparts, plant sphingolipids are characterized by high levels of base C4-hydroxylation. Moreover, desaturation at the Δ8 position predominates over the Δ4 desaturation typically found in animal sphingolipids. These modifications are due to the action of C4-hydroxylases and Δ8-long chain base desaturases, and they are important for complex sphingolipids finally becoming functional. The long chain bases of sunflower sphingolipids have high levels of hydroxylated and unsaturated moieties. Here, a C4-long chain base hydroxylase was functionally characterized in sunflower plant, an enzyme that could complement the sur2Δ mutation when heterologously expressed in this yeast mutant deficient in hydroxylation. This hydroxylase was ubiquitously expressed in sunflower, with the highest levels found in the developing cotyledons. In addition, we identified a new Δ8-long base chain desaturase gene that displays strong homology to a previously reported desaturase gene. This desaturase was also expressed in yeast and was able to change the long chain base composition of the transformed host. We studied the expression of this desaturase and compared it with that of the other isoform described in sunflower. The desaturase form studied in this paper displayed higher expression levels in developing seeds.

A conserved cysteine motif is critical for rice ceramide kinase activity and function

BACKGROUND

Ceramide kinase (CERK) is a key regulator of cell survival in dicotyledonous plants and animals. Much less is known about the roles of CERK and ceramides in mediating cellular processes in monocot plants. Here, we report the characterization of a ceramide kinase, OsCERK, from rice (Oryza sativa spp. Japonica cv. Nipponbare) and investigate the effects of ceramides on rice cell viability.

PRINCIPAL FINDINGS

OsCERK can complement the Arabidopsis CERK mutant acd5. Recombinant OsCERK has ceramide kinase activity with Michaelis-Menten kinetics and optimal activity at 7.0 pH and 40°C. Mg2+ activates OsCERK in a concentration-dependent manner. Importantly, a CXXXCXXC motif, conserved in all ceramide kinases and important for the activity of the human enzyme, is critical for OsCERK enzyme activity and in planta function. In a rice protoplast system, inhibition of CERK leads to cell death and the ratio of added ceramide and ceramide-1-phosphate, CERK's substrate and product, respectively, influences cell survival. Ceramide-induced rice cell death has apoptotic features and is an active process that requires both de novo protein synthesis and phosphorylation, respectively. Finally, mitochondria membrane potential loss previously associated with ceramide-induced cell death in Arabidopsis was also found in rice, but it occurred with different timing.

CONCLUSIONS

OsCERK is a bona fide ceramide kinase with a functionally and evolutionarily conserved Cys-rich motif that plays an important role in modulating cell fate in plants. The vital function of the conserved motif in both human and rice CERKs suggests that the biochemical mechanism of CERKs is similar in animals and plants. Furthermore, ceramides induce cell death with similar features in monocot and dicot plants.

Sphingolipids involvement in plant endomembrane differentiation: the BY2 case

Sphingolipids play an essential role in the functioning of the secretory pathway in eukaryotic organisms. Their importance in the functional organization of plant cells has not been studied in any detail before. The sphingolipid synthesis inhibitor fumonisin B1 (FB1), a mycotoxin acting as a specific inhibitor of ceramide synthase, was tested for its effects on cell growth, cell polarity, cell shape, cell cycle and on the ultrastructure of BY2 cells. We used cell lines expressing different GFP-tagged markers for plant cell compartments, as well as a Golgi marker fused to the photoconvertible protein Kaede. Light and electron microscopy, combined with flow cytometry, were applied to analyse the morphodynamics and architecture of compartments of the secretory pathway. The results indicate that FB1 treatment had severe effects on cell growth and cell shape, and induced a delay in cell division processes. The cell changes were accompanied by the formation of the endoplasmic reticulum (ER)-derived tubular aggregates (FB1-induced compartments), together with an inhibition of cargo transport from the ER to the Golgi apparatus. A change in polar localization of the auxin transporter PIN1 was also observed, but endocytic processes were little affected. Electron microscopy studies confirmed that molecular FB1 targets were distinct from brefeldin A (BFA) targets. We propose that the reported effects of inhibition of ceramide biosynthesis reflect the importance of sphingolipids during cell growth and establishment of cell polarity in higher plant cells, notably through their contribution to the functional organization of the ER or its differentiation into distinct compartments.

Sphingolipids in the root play an important role in regulating the leaf ionome in Arabidopsis thaliana

Sphingolipid synthesis is initiated by condensation of Ser with palmitoyl-CoA producing 3-ketodihydrosphinganine (3-KDS), which is reduced by a 3-KDS reductase to dihydrosphinganine. Ser palmitoyltransferase is essential for plant viability. Arabidopsis thaliana contains two genes (At3g06060/TSC10A and At5g19200/TSC10B) encoding proteins with significant similarity to the yeast 3-KDS reductase, Tsc10p. Heterologous expression in yeast of either Arabidopsis gene restored 3-KDS reductase activity to the yeast tsc10Δ mutant, confirming both as bona fide 3-KDS reductase genes. Consistent with sphingolipids having essential functions in plants, double mutant progeny lacking both genes were not recovered from crosses of single tsc10A and tsc10B mutants. Although the 3-KDS reductase genes are functionally redundant and ubiquitously expressed in Arabidopsis, 3-KDS reductase activity was reduced to 10% of wild-type levels in the loss-of-function tsc10a mutant, leading to an altered sphingolipid profile. This perturbation of sphingolipid biosynthesis in the Arabidopsis tsc10a mutant leads an altered leaf ionome, including increases in Na, K, and Rb and decreases in Mg, Ca, Fe, and Mo. Reciprocal grafting revealed that these changes in the leaf ionome are driven by the root and are associated with increases in root suberin and alterations in Fe homeostasis.

How cotton fibers elongate: a tale of linear cell-growth mode

Cotton fibers (cotton lint) are single-celled trichomes that differentiate from the ovule epidermis. Unidirectional and fast-growing cells generally expand at the dome-shaped apical zone (tip-growth mode); however, previous studies suggest that elongating fiber cells expand via a diffuse-growth mode. Tip-localized Ca(2+) gradient and active secretary vesicle trafficking are two important phenomena of tip-growth. Recently, a high Ca(2+) gradient is found in the cytoplasm of fast-elongating cotton fiber cells near the growing tip. Several protein coding genes participating in vesicle coating and transport are highly expressed in elongating fiber cells. Taken together with the observation that ethylene acts as a positive regulator for cotton fiber and several Arabidopsis tissues that are known to elongate via tip growth prompted us to propose a linear-growth mode for similar cell types.

Lead genetic studies in Dictyostelium discoideum and translational studies in human cells demonstrate that sphingolipids are key regulators of sensitivity to cisplatin and other anticancer drugs

A Dictyostelium discoideum mutant with a disruption in the sphingosine-1-phosphate (S-1-P) lyase gene was obtained in an unbiased genetic analysis, using random insertional mutagenesis, for mutants with increased resistance to the widely used cancer chemotherapeutic drug cisplatin. This finding opened the way to extensive studies in both D. discoideum and human cells on the role and mechanism of action of the bioactive sphingolipids S-1-P and ceramide in regulating the response to chemotherapeutic drugs. These studies showed that the levels of activities of the sphingolipid metabolizing enzymes S-1-P lyase, sphingosine kinase and ceramide synthase, affect whether a cell dies or lives in the presence of specific drugs. The demonstration that multiple enzymes of this biochemical pathway were involved in regulating drug sensitivity provided new opportunities to test whether pharmacological intervention might increase sensitivity. Thus it is of considerable clinical significance that pharmacological inhibition of sphingosine kinase synergistically sensitizes cells to cisplatin, both in D. discoideum and human cells. Linkage to the p38 MAP kinase and protein kinase C (PKC) signaling pathways has been demonstrated. This work demonstrates the utility of D. discoideum as a lead genetic system to interrogate molecular mechanisms controlling the sensitivity of tumor cells to chemotherapeutic agents and for determining novel ways of increasing efficacy. The D. discoideum system could be easily adapted to a high throughput screen for novel chemotherapeutic agents.