Functional analyses of differentially expressed isoforms of the Arabidopsis inositol phosphorylceramide synthase

Diverse INOSITOL PHOSPHORYLCERAMIDE SYNTHASE mutant alleles of Physcomitrium patens offer new insight into complex sphingolipid metabolism

Sphingolipids are widespread, abundant, and essential lipids in plants and in other eukaryotes. Glycosyl inositol phosphorylceramides (GIPCs) are the most abundant class of plant sphingolipids, and are enriched in the plasma membrane of plant cells. They have been difficult to study due to lethal or pleiotropic mutant phenotypes. To overcome this, we developed a CRISPR/Cas9-based method for generating multiple and varied knockdown and knockout populations of mutants in a given gene of interest in the model moss Physcomitrium patens. This system is uniquely convenient due to the predominantly haploid state of the Physcomitrium life cycle, and totipotency of Physcomitrium protoplasts used for transformation. We used this approach to target the INOSITOL PHOSPHORYLCERAMIDE SYNTHASE (IPCS) gene family, which catalyzes the first, committed step in the synthesis of GIPCs. We isolated knockout single mutants and knockdown higher-order mutants showing a spectrum of deficiencies in GIPC content. Remarkably, we also identified two mutant alleles accumulating inositol phosphorylceramides, the direct products of IPCS activity, and provide our best explanation for this unexpected phenotype. Our approach is broadly applicable for studying essential genes and gene families, and for obtaining unusual lesions within a gene of interest.

Root-associated Streptomyces produce galbonolides to modulate plant immunity and promote rhizosphere colonization

The rhizosphere, which serves as the primary interface between plant roots and the soil, constitutes an ecological niche for a huge diversity of microbial communities. Currently, there is little knowledge on the nature and the function of the different metabolites released by rhizospheric microbes to facilitate colonization of this highly competitive environment. Here, we demonstrate how the production of galbonolides, a group of polyene macrolides that inhibit plant and fungal inositol phosphorylceramide synthase (IPCS), empowers the rhizospheric Streptomyces strain AgN23, to thrive in the rhizosphere by triggering the plant's defence mechanisms. Metabolomic analysis of AgN23-inoculated Arabidopsis roots revealed a strong induction in the production of an indole alkaloid, camalexin, which is a major phytoalexin in Arabidopsis. By using a plant mutant compromised in camalexin synthesis, we show that camalexin production is necessary for the successful colonization of the rhizosphere by AgN23. Conversely, hindering galbonolides biosynthesis in AgN23 knock-out mutant resulted in loss of inhibition of IPCS, a deficiency in plant defence activation, notably the production of camalexin, and a strongly reduced development of the mutant bacteria in the rhizosphere. Together, our results identified galbonolides as important metabolites mediating rhizosphere colonization by Streptomyces.

The biological functions of sphingolipids in plant pathogenic fungi

Sphingolipids are critically significant in a range of biological processes in animals, plants, and fungi. In mammalian cells, they serve as vital components of the plasma membrane (PM) in maintaining its structure, tension, and fluidity. They also play a key role in a wide variety of biological processes, such as intracellular signal transduction, cell polarization, differentiation, and migration. In plants, sphingolipids are important for cell development and for cell response to environmental stresses. In pathogenic fungi, sphingolipids are crucial for the initiation and the development of infection processes afflicting humans. However, our knowledge on the metabolism and function of the sphingolipid metabolic pathway of pathogenic fungi affecting plants is still very limited. In this review, we discuss recent developments on sphingolipid pathways of plant pathogenic fungi, highlighting their uniqueness and similarity with plants and animals. In addition, we discuss recent advances in the research and development of fungal-targeted inhibitors of the sphingolipid pathway, to gain insights on how we can better control the infection process occurring in plants to prevent or/and to treat fungal infections in crops.

Diversity in sphingolipid metabolism across land plants

Sphingolipids are essential metabolites found in all plant species. They are required for plasma membrane integrity, tolerance of and responses to biotic and abiotic stresses, and intracellular signalling. There is extensive diversity in the sphingolipid content of different plant species, and in the identities and roles of enzymes required for their processing. In this review, we survey results obtained from investigations of the classical genetic model Arabidopsis thaliana, from assorted dicots with less extensive genetic toolkits, from the model monocot Oryza sativa, and finally from the model bryophyte Physcomitrium patens. For each species or group, we first broadly summarize what is known about sphingolipid content. We then discuss the most insightful and puzzling features of modifications to the hydrophobic ceramides, and to the polar headgroups of complex sphingolipids. Altogether, these data can serve as a framework for our knowledge of sphingolipid metabolism across the plant kingdom. This chemical and metabolic heterogeneity underpins equally diverse functions. With greater availability of different tools for analytical measurements and genetic manipulation, our field is entering an exciting phase of expanding our knowledge of the biological functions of this persistently cryptic class of lipids.

Detailed sphingolipid profile responded to salt stress in cotton root and the GhIPCS1 is involved in the regulation of plant salt tolerance

Sphingolipids are major structural components of membrane and active signaling molecules and play an important role in plant developmental processes and stress responses. As land salinization has increased globally, salinity has compromised the growth and productivity of crops such as cotton. Understanding the mechanisms of plant adaptation to salt stress is essential for breeding salt-tolerant crops. In this study, we explored the comprehensive metabolic profile of sphingolipids in cotton root under salt stress using lipidomics. 118 sphingolipid molecular species were identified, of which PhytoSph, PhytoCer, PhytoCer-OHFA, IPC, and GIPC were relatively high in content, and PhytoSph, PhytoCer, PhytoCer-OHFA, Phyto-GluCer, and IPC showed significant changes after salt stress, especially inositol phosphatidyl ceramide (IPC), which was significantly upregulated after salt treatment. Subsequently, we identified the genes encoding IPC synthase (IPCS), and ectopic expression of GhIPCS1 enhanced salt sensitivity in Arabidopsis, which might result from the disruption on the balance between various sphingolipid classes and/or molecular species. Overall, this study reveals key lipids and genes response to salt stress in cotton and provides a theoretical basis for the use of genetic engineering to improve cotton stress resistance.

Comparative Metabolomics Analysis Reveals Sterols and Sphingolipids Play a Role in Cotton Fiber Cell Initiation

Cotton fiber is a seed trichome that protrudes from the outer epidermis of cotton ovule on the day of anthesis (0 day past anthesis, 0 DPA). The initial number and timing of fiber cells are closely related to fiber yield and quality. However, the mechanism underlying fiber initiation is still unclear. Here, we detected and compared the contents and compositions of sphingolipids and sterols in 0 DPA ovules of Xuzhou142 lintless-fuzzless mutants (Xufl) and Xinxiangxiaoji lintless-fuzzless mutants (Xinfl) and upland cotton wild-type Xuzhou142 (XuFL). Nine classes of sphingolipids and sixty-six sphingolipid molecular species were detected in wild-type and mutants. Compared with the wild type, the contents of Sphingosine-1-phosphate (S1P), Sphingosine (Sph), Glucosylceramide (GluCer), and Glycosyl-inositol-phospho-ceramides (GIPC) were decreased in the mutants, while the contents of Ceramide (Cer) were increased. Detail, the contents of two Cer molecular species, d18:1/22:0 and d18:1/24:0, and two Phyto-Cer molecular species, t18:0/22:0 and t18:0/h22:1 were significantly increased, while the contents of all GluCer and GIPC molecular species were decreased. Consistent with this result, the expression levels of seven genes involved in GluCer and GIPC synthesis were decreased in the mutants. Furthermore, exogenous application of a specific inhibitor of GluCer synthase, PDMP (1-phenyl-2-decanoylamino-3-morpholino-1-propanol), in ovule culture system, significantly inhibited the initiation of cotton fiber cells. In addition, five sterols and four sterol esters were detected in wild-type and mutant ovules. Compared with the wild type, the contents of total sterol were not significantly changed. While the contents of stigmasterol and campesterol were significantly increased, the contents of cholesterol were significantly decreased, and the contents of total sterol esters were significantly increased. In particular, the contents of campesterol esters and stigmasterol esters increased significantly in the two mutants. Consistently, the expression levels of some sterol synthase genes and sterol ester synthase genes were also changed in the two mutants. These results suggested that sphingolipids and sterols might have some roles in the initiation of fiber cells. Our results provided a novel insight into the regulatory mechanism of fiber cell initiation.

Sphingolipid metabolism, transport, and functions in plants: Recent progress and future perspectives

Sphingolipids, which comprise membrane systems together with other lipids, are ubiquitous in cellular organisms. They show a high degree of diversity across plant species and vary in their structures, properties, and functions. Benefiting from the development of lipidomic techniques, over 300 plant sphingolipids have been identified. Generally divided into free long-chain bases (LCBs), ceramides, glycosylceramides (GlcCers) and glycosyl inositol phosphoceramides (GIPCs), plant sphingolipids exhibit organized aggregation within lipid membranes to form raft domains with sterols. Accumulating evidence has revealed that sphingolipids obey certain trafficking and distribution rules and confer unique properties to membranes. Functional studies using sphingolipid biosynthetic mutants demonstrate that sphingolipids participate in plant developmental regulation, stimulus sensing, and stress responses. Here, we present an updated metabolism/degradation map and summarize the structures of plant sphingolipids, review recent progress in understanding the functions of sphingolipids in plant development and stress responses, and review sphingolipid distribution and trafficking in plant cells. We also highlight some important challenges and issues that we may face during the process of studying sphingolipids.

Sphingolipids in plants: a guidebook on their function in membrane architecture, cellular processes, and environmental or developmental responses

Sphingolipids are fundamental lipids involved in various cellular, developmental and stress-response processes. As such, they orchestrate not only vital molecular mechanisms of living cells but also act in diseases, thus qualifying as potential pharmaceutical targets. Sphingolipids are universal to eukaryotes and are also present in some prokaryotes. Some sphingolipid structures are conserved between animals, plants and fungi, whereas others are found only in plants and fungi. In plants, the structural diversity of sphingolipids, as well as their downstream effectors and molecular and cellular mechanisms of action, are of tremendous interest to both basic and applied researchers, as about half of all small molecules in clinical use originate from plants. Here, we review recent advances towards a better understanding of the biosynthesis of sphingolipids, the diversity in their structures as well as their functional roles in membrane architecture, cellular processes such as membrane trafficking and cell polarity, and cell responses to environmental or developmental signals.

Transcriptome profiling of raspberry (Rubus idaeus Var. Amira) in response to infection by tomato ringspot virus (ToRSV)

Raspberry (Rubus sp.) is a berries fruit with an ongoing agricultural and commercial interest due to its high contents of flavonoids and nutrients beneficial for human health. The growing demand for raspberries is facing great challenges associated mainly with the dispersal of diseases, which produces a decrease in productivity and fruit quality. A broad range of genomic resources is available for other Rosaceae species; however, genomic resources for species of the Rubus genus are still limited. Here, we characterize the transcriptome of the Rubus idaeus (Var. Amira) in order to 1) provide clues in the transcriptional changes of R. idaeus against tomato ringspot virus (ToRSV); and 2) generate genomic resources for this economically important species. We generate more than 200 million sequencing reads from two mRNA samples of raspberry, infected and not infected by ToRSV, using Illumina technology. After de novo assembly, we obtained 68,853 predicted protein-coding sequences of which 71.3% and 61.3% were annotated using Gene Ontology and Pfam databases, respectively. Moreover, we find 2,340 genes with differential expression between raspberries infected and not infected by ToRSV. Analysis of these genes shows functional enrichments of the oxidation-reduction process, cell wall biogenesis, terpene synthase activity, and lyase activity. These genes could be involved in the raspberry immune response through the interaction of different metabolic pathways; however, this statement needs further investigations. Up-regulation of genes encoding terpene synthases, multicopper oxidases, laccases, and beta-glucosidases might suggest that these enzymes appear to be the predominant transcriptome immune response of R. idaeus against ToRSV. Furthermore, we identify thousands of molecular markers (i.e., SSRs and SNPs), increasing considerably the genomic resources currently available for raspberries. This study is the first report on investigating the transcriptional changes of R. idaeus against ToRSV.

Sphingolipids: towards an integrated view of metabolism during the plant stress response

Plants exist in an environment of changing abiotic and biotic stresses. They have developed a complex set of strategies to respond to these stresses and over recent years it has become clear that sphingolipids are a key player in these responses. Sphingolipids are not universally present in all three domains of life. Many bacteria and archaea do not produce sphingolipids but they are ubiquitous in eukaryotes and have been intensively studied in yeast and mammals. During the last decade there has been a steadily increasing interest in plant sphingolipids. Plant sphingolipids exhibit structural differences when compared with their mammalian counterparts and it is now clear that they perform some unique functions. Sphingolipids are recognised as critical components of the plant plasma membrane and endomembrane system. Besides being important structural elements of plant membranes, their particular structure contributes to the fluidity and biophysical order. Sphingolipids are also involved in multiple cellular and regulatory processes including vesicle trafficking, plant development and defence. This review will focus on our current knowledge as to the function of sphingolipids during plant stress responses, not only as structural components of biological membranes, but also as signalling mediators.

The identification of small molecule inhibitors of the plant inositol phosphorylceramide synthase which demonstrate herbicidal activity

Resistance to 157 different herbicides and 88% of known sites of action has been observed, with many weeds resistant to two or more modes. Coupled with tighter environmental regulation, this demonstrates the need to identify new modes of action and novel herbicides. The plant sphingolipid biosynthetic enzyme, inositol phosphorylceramide synthase (IPCS), has been identified as a novel, putative herbicide target. The non-mammalian nature of this enzyme offers the potential of discovering plant specific inhibitory compounds with minimal impact on animals and humans, perhaps leading to the development of new non-toxic herbicides. The best characterised and most highly expressed isoform of the enzyme in the model-dicot Arabidopsis, AtIPCS2, was formatted into a yeast-based assay which was then utilized to screen a proprietary library of over 11,000 compounds provided by Bayer AG. Hits from this screen were validated in a secondary in vitro enzyme assay. These studies led to the identification of a potent inhibitor that showed selectivity for AtIPCS2 over the yeast orthologue, and activity against Arabidopsis seedlings. This work highlighted the use of a yeast-based screening assay to discover herbicidal compounds and the status of the plant IPCS as a novel herbicidal target.

Expression levels of inositol phosphorylceramide synthase modulate plant responses to biotic and abiotic stress in Arabidopsis thaliana

This research was undertaken to investigate the global role of the plant inositol phosphorylceramide synthase (IPCS), a non-mammalian enzyme previously shown to be associated with the pathogen response. RNA-Seq analyses demonstrated that over-expression of inositol phosphorylceramide synthase isoforms AtIPCS1, 2 or 3 in Arabidopsis thaliana resulted in the down-regulation of genes involved in plant response to pathogens. In addition, genes associated with the abiotic stress response to salinity, cold and drought were found to be similarly down-regulated. Detailed analyses of transgenic lines over-expressing AtIPCS1-3 at various levels revealed that the degree of down-regulation is specifically correlated with the level of IPCS expression. Singular enrichment analysis of these down-regulated genes showed that AtIPCS1-3 expression affects biological signaling pathways involved in plant response to biotic and abiotic stress. The up-regulation of genes involved in photosynthesis and lipid localization was also observed in the over-expressing lines.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Functional Analyses of a Putative, Membrane-Bound, Peroxisomal Protein Import Mechanism from the Apicomplexan Protozoan Toxoplasma gondii

Peroxisomes are central to eukaryotic metabolism, including the oxidation of fatty acids—which subsequently provide an important source of metabolic energy—and in the biosynthesis of cholesterol and plasmalogens. However, the presence and nature of peroxisomes in the parasitic apicomplexan protozoa remains controversial. A survey of the available genomes revealed that genes encoding peroxisome biogenesis factors, so-called peroxins (Pex), are only present in a subset of these parasites, the coccidia. The basic principle of peroxisomal protein import is evolutionarily conserved, proteins harbouring a peroxisomal-targeting signal 1 (PTS1) interact in the cytosol with the shuttling receptor Pex5 and are then imported into the peroxisome via the membrane-bound protein complex formed by Pex13 and Pex14. Surprisingly, whilst Pex5 is clearly identifiable, Pex13 and, perhaps, Pex14 are apparently absent from the coccidian genomes. To investigate the functionality of the PTS1 import mechanism in these parasites, expression of Pex5 from the model coccidian Toxoplasma gondii was shown to rescue the import defect of Pex5-deleted Saccharomyces cerevisiae. In support of these data, green fluorescent protein (GFP) bearing the enhanced (e)PTS1 known to efficiently localise to peroxisomes in yeast, localised to peroxisome-like bodies when expressed in Toxoplasma. Furthermore, the PTS1-binding domain of Pex5 and a PTS1 ligand from the putatively peroxisome-localised Toxoplasma sterol carrier protein (SCP2) were shown to interact in vitro. Taken together, these data demonstrate that the Pex5–PTS1 interaction is functional in the coccidia and indicate that a nonconventional peroxisomal import mechanism may operate in the absence of Pex13 and Pex14.

Genome-wide analysis suggests divergent evolution of lipid phosphotases/phosphotransferase genes in plants

Genes of the LPPT (lipid phosphatase/phosphotransferase) family play important roles in lipid phosphorous transfer and triacylglycerol accumulation in plants. To provide overviews of the plant LPPT family and their overall relationships, here we carried out genome-wide identifications and analyses of plant LPPT family members. A total of 643 putative LPPT genes were identified from 48 sequenced plant genomes, among which 205 genes from 14 plants were chosen for further analyses. Plant LPPT genes belonged to three distinctive groups, namely the LPT (lipid phosphotransfease), LPP (lipid phosphatase), and pLPP (plastidic lipid phosphotransfease) groups. Genes of the LPT group could be further partitioned into three groups, two of which were only identified in terrestrial plants. Genes in the LPP and pLPP groups experienced duplications in early stages of plant evolution. Among 17 Zea mays LPPT genes, divergence of temporal-spatial expression patterns was revealed based on microarray data analysis. Peptide sequences of plant LPPT genes harbored different conserved motifs. A test of Branch Model versus One-ratio Model did not support significant selective pressures acting on different groups of LPPT genes, although quite different nonsynonymous evolutionary rates and selective pressures were observed. The complete picture of the plant LPPT family provided here should facilitate further investigations of plant LPPT genes and offer a better understanding of lipid biosynthesis in plants.

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.

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.

Transcriptome and hormone profiling reveals Eucalyptus grandis defence responses against Chrysoporthe austroafricana

BACKGROUND

Eucalyptus species and interspecific hybrids exhibit valuable growth and wood properties that make them a highly desirable commodity. However, these trees are challenged by a wide array of biotic stresses during their lifetimes. The Eucalyptus grandis reference genome sequence provides a resource to study pest and pathogen defence mechanisms in long-lived woody plants. E. grandis trees are generally susceptible to Chrysoporthe austroafricana, a causal agent of stem cankers on eucalypts. The aim of this study was to characterize the defence response of E. grandis against C. austroafricana.

RESULTS

Hormone profiling of susceptible and moderately resistant clonal E. grandis genotypes indicated a reduction in salicylic acid and gibberellic acid levels at 3 days post inoculation. We hypothesized that these signaling pathways may facilitate resistance. To further investigate other defence mechanisms at this time point, transcriptome profiling was performed. This revealed that cell wall modifications and response to oxidative stress form part of the defence responses common to both genotypes, whilst changes in the hormone signaling pathways may contribute to resistance. Additionally the expression of selected candidate defence response genes was induced earlier in moderately resistant trees than in susceptible trees, supporting the hypothesis that a delayed defence response may occur in the susceptible interaction.

CONCLUSION

The ability of a host to fine-tune its defence responses is crucial and the responses identified in this study extends our understanding of plant defence, gained from model systems, to woody perennials.

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.

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.

Sphingolipid synthesis and scavenging in the intracellular apicomplexan parasite, Toxoplasma gondii

Sphingolipids are essential components of eukaryotic cell membranes, particularly the plasma membrane, and are involved in a diverse array of signal transduction pathways. Mammals produce sphingomyelin (SM) as the primary complex sphingolipid via the well characterised SM synthase. In contrast yeast, plants and some protozoa utilise an evolutionarily related inositol phosphorylceramide (IPC) synthase to synthesise IPC. This activity has no mammalian equivalent and IPC synthase has been proposed as a target for anti-fungals and anti-protozoals. However, detailed knowledge of the sphingolipid biosynthetic pathway of the apicomplexan protozoan parasites was lacking. In this study bioinformatic analyses indicated a single copy orthologue of the putative SM synthase from the apicomplexan Plasmodium falciparum (the causative agent of malaria) was a bona fide sphingolipid synthase in the related model parasite, Toxoplasma gondii (TgSLS). Subsequently, TgSLS was indicated, by complementation of a mutant cell line, to be a functional orthologue of the yeast IPC synthase (AUR1p), demonstrating resistance to the well characterised AUR1p inhibitor aureobasidin A. In vitro, recombinant TgSLS exhibited IPC synthase activity and, for the first time, the presence of IPC was demonstrated in T. gondii lipid extracts by mass spectrometry. Furthermore, host sphingolipid biosynthesis was indicated to influence, but be non-essential for, T. gondii proliferation, suggesting that whilst scavenging does take place de novo sphingolipid synthesis may be important for parasitism.

Sphingolipids and plant defense/disease: the "death" connection and beyond

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.

Exploring Leishmania major inositol phosphorylceramide synthase (LmjIPCS): insights into the ceramide binding domain

The synthesis of set of ceramide analogues exploring hydrophobicity in the acyl chains and the degree and nature of hydroxylation is described. These have been assayed against the parasitic protozoan enzyme LmjIPCS. These studies showed that whilst the C-3 hydroxyl group was not essential for turnover it provided enhanced affinity. Reflecting the membrane bound nature of the enzyme a long (C(13)) hydrocarbon ceramide tail was necessary for both high affinity and turnover. Whilst the N-acyl chain also contributed to affinity, analogues lacking the amide linkage functioned as competitive inhibitors in both enzyme and cell-based assays. A model that accounts for this observation is proposed.