Sphingolipid long chain base phosphates can regulate apoptotic-like programmed cell death in plants

Low temperature storage alleviates aging of paddy by reducing lipid degradation and peroxidation

The quality deterioration of paddy during storage is closely associated to lipid metabolism. To explore the effect of lipid metabolism on the texture of paddy, freshly harvested Nanjingxiangzhan (Indica rice) stored for 60 days at 15 °C and 25 °C for 60 days was investigated. Paddy stored at 15 °C showed higher contents of ATP, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, triacylglycerols, diacylglycerols, monogalactosyldiacylglycerol and digalactosyldiacylglycerol but lower levels of lysophosphatidylcholine, lysophosphatidylethanolamine, phosphatidic acid, ceramides and free fatty acids. Storage at 15 °C inhibited lipase, phospholipase D (PLD), lipoxygenase (LOX) activities and the corresponding gene expressions. Moreover, 15 °C storage retarded the rise of hardness, cohesiveness and chewiness, while delayed the reduction of gumminess and springiness. These findings suggested that maintenance in glycerophospholipids, glycerolipids and saccharolipids abundance, reduction in lysophospholipids, phosphatidic acid, ceramides and free fatty acids accumulation could contribute to enhanced internal resistance to aging in freshly harvested paddy at low temperature storage.

Sphingolipid long chain bases as mediators of cell death in olive fruit abscission

Plant sphingolipids are lipophilic membrane components essential for different cellular functions but they also act as signaling molecules in various aspects of plant development. However, the interaction between plant sphingolipids and abscission remains largely uncharacterized. Here, the possible role of sphingolipids in regulating fruit abscission was examined in the abscission zone (AZ) of olive fruit. To this end, sphingolipid levels were manipulated through the application of exogenous sphingolipid long-chain bases (LCBs) or biosynthesis inhibitors, and their effects on fruit abscission as well as sphingolipid LCB/gene expression, hormones, reactive oxygen species (ROS) and cell death levels were examined in the AZ of olive fruit. Our data indicated that exogenous sphinganine (d18:0) induced fruit abscission, whereas the application of sphingosine (d18:1) or phytosphingosine (t18:0) or their phosphorylated derivatives did not have an effect on fruit abscission. Moreover, inhibition of LCB kinase or ceramide synthase, which increases sphingolipid LCB levels in the AZ, reduced fruit break strength. This induction of fruit abscission is associated with elevated ROS levels and cell death in the AZ enriched in salicylic acid (SA) and jasmonic acid (JA). Along the same line, programmed cell death (PCD) was particularly evident on the distal side of the AZ. These data suggest that endogenous d18:0 plays a key cellular role as signaling molecule functioning upstream of the SA/JA signaling pathway in mediating PCD spatially regulated in the AZ during fruit abscission. Overall, the findings reported here provide insight into the complex connection between PCD and plant sphingolipid LCBs, uncovering their interaction in the abscission process.

Sphingolipid homeostasis: How do cells know when enough is enough? Implications for plant pathogen responses

Sphingolipid homeostatic regulation is important for balancing plant life and death. Plant cells finely tune sphingolipid biosynthesis to ensure sufficient levels to support growth through their basal functions as major components of endomembranes and the plasma membrane. Conversely, accumulation of sphingolipid biosynthetic intermediates, long-chain bases (LCBs) and ceramides, is associated with programmed cell death. Limiting these apoptotic intermediates is important for cell viability, while overriding homeostatic regulation permits cells to generate elevated LCBs and ceramides to respond to pathogens to elicit the hypersensitive response in plant immunity. Key to sphingolipid homeostasis is serine palmitoyltransferase (SPT), an endoplasmic reticulum-associated, multi-subunit enzyme catalyzing the first step in the biosynthesis of LCBs, the defining feature of sphingolipids. Across eukaryotes, SPT interaction with its negative regulator Orosomucoid-like (ORM) is critical for sphingolipid biosynthetic homeostasis. The recent cryo-electron microscopy structure of the Arabidopsis SPT complex indicates that ceramides bind ORMs to competitively inhibit SPT activity. This system provides a sensor for intracellular ceramide concentrations for sphingolipid homeostatic regulation. Combining the newly elucidated Arabidopsis SPT structure and mutant characterization, we present a model for the role of the 2 functionally divergent Arabidopsis ceramide synthase classes to produce ceramides that form repressive (trihydroxy LCB-ceramides) or nonrepressive (dihydroxy LCB-ceramides) ORM interactions to influence SPT activity. We describe how sphingolipid biosynthesis is regulated by the interplay of ceramide synthases with ORM-SPT when "enough is enough" and override homeostatic suppression when "enough is not enough" to respond to environmental stimuli such as microbial pathogen attack.

Balancing roles between phosphatidylinositols and sphingolipids in regulating immunity and ER stress responses in pi4kβ1,2

Plant immune regulation is complex. In addition to proteins, lipid molecules play critical roles in modulating immune responses. The mutant pi4kβ1,2 is mutated in two phosphatidylinositol 4-kinases PI4Kβ1 and β2 involved in the biosynthesis of phosphatidylinositol 4-phosphate (PI4P). The mutant displays autoimmunity, short roots, aberrant root hairs, and a heightened sensitivity to ER stress. In a forward genetic screen designed to dissect pi4kβ1,2 autoimmunity, we found that Orosomucoid-like 1 (ORM1) is required for the phenotypes of pi4kβ1,2, including short root and ER stress sensitivity. The orm1 mutations lead to increased long-chain base and ceramide levels in the suppressors. We also found that the basic region/leucine Zipper motif (bZIP) 28 and 60 transcription factors, central regulators of ER stress response, are required for its autoimmunity and root defect. In comparison, the defense-related phytohormones salicylic acid (SA) and N-hydroxypipecolic acid (NHP) are required for its autoimmunity but plays a minor role in its root phenotypes. Further, we found that wild-type plants overexpressing ORM1 are autoimmune, displaying short roots and increased ceramide levels. The autoimmunity of the ORM1 overexpression lines is dependent on SA, NHP, and bZIP60. As ORM1 is a known negative regulator of sphingolipid biosynthesis, our study uncovers a balancing role between PIs and sphingolipids in regulating immunity and ER stress responses in pi4kβ1,2.

Sphingosine Promotes Fiber Early Elongation in Upland Cotton

Sphingolipids play an important role in cotton fiber development, but the regulatory mechanism is largely unclear. We found that serine palmitoyltransferase (SPT) enzyme inhibitors, myriocin and sphingosine (dihydrosphingosine (DHS) and phytosphingosine (PHS)), affected early fiber elongation in cotton, and we performed a sphingolipidomic and transcriptomic analysis of control and PHS-treated fibers. Myriocin inhibited fiber elongation, while DHS and PHS promoted it in a dose-effect manner. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we found that contents of 22 sphingolipids in the PHS-treated fibers for 10 days were changed, of which the contents of 4 sphingolipids increased and 18 sphingolipids decreased. The transcriptome analysis identified 432 differentially expressed genes (238 up-regulated and 194 down-regulated) in the PHS-treated fibers. Among them, the phenylpropanoid biosynthesis pathway is the most significant enrichment. The expression levels of transcription factors such as MYB, ERF, LBD, and bHLH in the fibers also changed, and most of MYB and ERF were up-regulated. Auxin-related genes IAA, GH3 and BIG GRAIN 1 were up-regulated, while ABPs were down-regulated, and the contents of 3 auxin metabolites were decreased. Our results provide important sphingolipid metabolites and regulatory pathways that influence fiber elongation.

Bioactive Functions of Lipids in the Milk Fat Globule Membrane: A Comprehensive Review

The milk fat globule membrane (MFGM) is a complex tri-layer membrane that wraps droplets of lipids in milk. In recent years, it has attracted widespread attention due to its excellent bioactive functions and nutritional value. MFGM contains a diverse array of bioactive lipids, including cholesterol, phospholipids, and sphingolipids, which play pivotal roles in mediating the bioactivity of the MFGM. We sequentially summarize the main lipid types in the MFGM in this comprehensive review and outline the characterization methods used to employ them. In this comprehensive review, we sequentially describe the types of major lipids found in the MFGM and outline the characterization methods employed to study them. Additionally, we compare the structural disparities among glycerophospholipids, sphingolipids, and gangliosides, while introducing the formation of lipid rafts facilitated by cholesterol. The focus of this review revolves around an extensive evaluation of the current research on lipid isolates from the MFGM, as well as products containing MFGM lipids, with respect to their impact on human health. Notably, we emphasize the clinical trials encompassing a large number of participants. The summarized bioactive functions of MFGM lipids encompass the regulation of human growth and development, influence on intestinal health, inhibition of cholesterol absorption, enhancement of exercise capacity, and anticancer effects. By offering a comprehensive overview, the aim of this review is to provide valuable insights into the diverse biologically active functions exhibited by lipids in the MFGM.

Alkaline ceramidase (ClAC) inhibition enhances heat stress response in Cyrtorhinus lividipennis (Reuter)

Ceramidases (CDases) are vital sphingolipid enzymes involved in organismal growth and development. They have been reported as key mediators of thermal stress response. However, whether and how CDase responds to heat stress in insects remain unclear. Herein, we identified two CDase genes, C. lividipennis alkaline ceramidase (ClAC) and neutral ceramidase (ClNC), by searching the transcriptome and genome databases of the mirid bug, Cyrtorhinus lividipennis, an important natural predator of planthoppers. Quantitative PCR (qPCR) analysis showed that both ClNC and ClAC were highly expressed in nymphs than in adults. ClAC was especially highly expressed in the head, thorax, and legs, while ClNC was widely expressed in the tested organs. Only the ClAC transcription was significantly affected by heat stress. Knocking down ClAC increased the C. lividipennis nymph survival rate under heat stress. The transcriptome and lipidomics data showed that the RNA interference-mediated suppression of ClAC significantly upregulated the transcription level of catalase (CAT) and the content of long-chain base ceramides, including C16-, C18-, C24-, and C31- ceramides. In C. lividipennis nymphs, ClAC played an important role in heat stress response, and the upregulation of nymph survival rate might be caused by variation in the ceramide levels and transcriptional changes in CDase downstream genes. This study improves our understanding of the physiological functions of insect CDase under heat stress and provides valuable insights into the nature enemy application.

Sphingolipid Long-Chain Base Signaling in Compatible and Non-Compatible Plant-Pathogen Interactions in Arabidopsis

The chemical diversity of sphingolipids in plants allows the assignment of specific roles to special molecular species. These roles include NaCl receptors for glycosylinositolphosphoceramides or second messengers for long-chain bases (LCBs), free or in their acylated forms. Such signaling function has been associated with plant immunity, with an apparent connection to mitogen-activated protein kinase 6 (MPK6) and reactive oxygen species (ROS). This work used in planta assays with mutants and fumonisin B1 (FB1) to generate varying levels of endogenous sphingolipids. This was complemented with in planta pathogenicity tests using virulent and avirulent Pseudomonas syringae strains. Our results indicate that the surge of specific free LCBs and ceramides induced by FB1 or an avirulent strain trigger a biphasic ROS production. The first transient phase is partially produced by NADPH oxidase, and the second is sustained and is related to programmed cell death. MPK6 acts downstream of LCB buildup and upstream of late ROS and is required to selectively inhibit the growth of the avirulent but not the virulent strain. Altogether, these results provide evidence that a LCB- MPK6- ROS signaling pathway contributes differentially to the two forms of immunity described in plants, upregulating the defense scheme of a non-compatible interaction.

Overexpression of a ceramide synthase gene,GhCS1, inhibits fiber cell initiation and elongation by promoting the synthesis of ceramides containing dihydroxy LCB and VLCFA

Cotton is an important natural fiber crop worldwide. Cotton fiber cell is regarded as an ideal material for studying the growth and development of plant cells. Sphingolipids are important components of biomembrane and bioactive molecules which participate in many processes such as plant growth, development regulation, stimulus sensing, and stress response. However, the functions of sphingolipids in the cotton fiber development are still unclear. In the present study, we identified a cotton ceramide synthase gene, GhCS1, which is predominantly expressed in fiber cell. The GhCS1 is located in the endoplasmic reticulum and has the conserved domains of ceramide synthase. Overexpression of GhCS1 gene inhibited both vegetative and reproductive growth in cotton. Importantly, the fiber cell initiation and elongation were severely inhibited when compared with control. Comparison of the sphingolipid profile in the 0-DPA (days past anthesis) ovule (with fiber cell) between control and transgenic cotton plants showed that the content of sphingosines (Sph) decreased significantly in transgenic ovules, whereas the content of phyto-sphingosines (Phyto-Sph) had no change. Meanwhile, the content of ceramide containing Sph and very-long-chain fatty acid (VLCFA) increased significantly in transgenic ovules, while ceramide containing Phyto-Sph and long-chain fatty acids (LCFA)/VLCFA significantly decreased. These results indicated that GhCS1 was a functional ceramide synthase, which preferentially used Sph and VLCFA as substrates and was different from the Arabidopsis ceramide synthase AtLOH1/AtLOH3, which preferentially used Phyto-Sph and VLCFA as substrates, and also different from AtLOH2, which preferentially used Sph and LCFA as substrates. It is suggested that GhCS1 might be a new ceramide synthase gene in the plant, play some roles in the development of fiber cells and cotton plants.

Sphingolipid Long-Chain Base Phosphate Degradation Can Be a Rate-Limiting Step in Long-Chain Base Homeostasis

Sphingolipid long-chain bases (LCBs) are building blocks for membrane-localized sphingolipids, and are involved in signal transduction pathways in plants. Elevated LCB levels are associated with the induction of programmed cell death and pathogen-derived toxin-induced cell death. Therefore, levels of free LCBs can determine survival of plant cells. To elucidate the contribution of metabolic pathways regulating high LCB levels, we applied the deuterium-labeled LCB D-erythro-sphinganine-d7 (D₇-d18:0), the first LCB in sphingolipid biosynthesis, to Arabidopsis leaves and quantified labeled LCBs, LCB phosphates (LCB-Ps), and 14 abundant ceramide (Cer) species over time. We show that LCB D₇-d18:0 is rapidly converted into the LCBs d18:0P, t18:0, and t18:0P. Deuterium-labeled ceramides were less abundant, but increased over time, with the highest levels detected for Cer(d18:0/16:0), Cer(d18:0/24:0), Cer(t18:0/16:0), and Cer(t18:0/22:0). A more than 50-fold increase of LCB-P levels after leaf incubation in LCB D₇-d18:0 indicated that degradation of LCBs via LCB-Ps is important, and we hypothesized that LCB-P degradation could be a rate-limiting step to reduce high levels of LCBs. To functionally test this hypothesis, we constructed a transgenic line with dihydrosphingosine-1-phosphate lyase 1 (DPL1) under control of an inducible promotor. Higher expression of DPL1 significantly reduced elevated LCB-P and LCB levels induced by Fumonisin B₁, and rendered plants more resistant against this fungal toxin. Taken together, we provide quantitative data on the contribution of major enzymatic pathways to reduce high LCB levels, which can trigger cell death. Specifically, we provide functional evidence that DPL1 can be a rate-limiting step in regulating high LCB levels.

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.

Brassinosteroids in Plants: Crosstalk with Small-Molecule Compounds

Brassinosteroids (BRs) are known as the sixth type of plant hormone participating in various physiological and biochemical activities and play an irreplaceable role in plants. Small-molecule compounds (SMCs) such as nitric oxide (NO), ethylene, hydrogen peroxide (H₂O₂), and hydrogen sulfide (H₂S) are involved in plant growth and development as signaling messengers. Recently, the involvement of SMCs in BR-mediated growth and stress responses is gradually being discovered in plants, including seed germination, adventitious rooting, stem elongation, fruit ripening, and stress responses. The crosstalk between BRs and SMCs promotes plant development and alleviates stress damage by modulating the antioxidant system, photosynthetic capacity, and carbohydrate metabolism, as well as osmotic adjustment. In the present review, we try to explain the function of BRs and SMCs and their crosstalk in the growth, development, and stress resistance of plants.

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.

Sphingolipid Δ4-desaturation is an important metabolic step for glycosylceramide formation in Physcomitrium patens

Glycosylceramides are abundant membrane components in vascular plants and are associated with cell differentiation, organogenesis, and protein secretion. Long-chain base (LCB) Δ4-desaturation is an important structural feature for metabolic channeling of sphingolipids into glycosylceramide formation in plants and fungi. In Arabidopsis thaliana, LCB Δ4-unsaturated glycosylceramides are restricted to pollen and floral tissue, indicating that LCB Δ4-desaturation has a less important overall physiological role in A. thaliana. In the bryophyte Physcomitrium patens, LCB Δ4-desaturation is a feature of the most abundant glycosylceramides of the gametophyte generation. Metabolic changes in the P. patens null mutants for the sphingolipid Δ4-desaturase (PpSD4D) and the glycosylceramide synthase (PpGCS), sd4d-1 and gcs-1, were determined by ultra-performance liquid chromatography coupled with nanoelectrospray ionization and triple quadrupole tandem mass spectrometry analysis. sd4d-1 plants lacked unsaturated LCBs and the most abundant glycosylceramides. gcs-1 plants lacked all glycosylceramides and accumulated hydroxyceramides. While sd4d-1 plants mostly resembled wild-type plants, gcs-1 mutants were impaired in growth and development. These results indicate that LCB Δ4-desaturation is a prerequisite for the formation of the most abundant glycosylceramides in P. patens. However, loss of unsaturated LCBs does not affect plant viability, while blockage of glycosylceramide synthesis in gcs-1 plants causes severe plant growth and development defects.

Sphingolipid long-chain base hydroxylation influences plant growth and callose deposition in Physcomitrium patens

Sphingolipids are enriched in microdomains in the plant plasma membrane (PM). Hydroxyl groups in the characteristic long-chain base (LCB) moiety might be essential for the interaction between sphingolipids and sterols during microdomain formation. Investigating LCB hydroxylase mutants in Physcomitrium patens might therefore reveal the role of certain plant sphingolipids in the formation of PM subdomains. Physcomitrium patens mutants for the LCB C-4 hydroxylase S4H were generated by homologous recombination. Plants were characterised by analysing their sphingolipid and steryl glycoside (SG) profiles and by investigating different gametophyte stages. s4h mutants lost the hydroxyl group at the C-4 position of their LCB moiety. Loss of this hydroxyl group caused global changes in the moss sphingolipidome and in SG composition. Changes in membrane lipid composition may trigger growth defects by interfering with the localisation of membrane-associated proteins that are crucial for growth processes such as signalling receptors or callose-modifying enzymes. Loss of LCB-C4 hydroxylation substantially changes the P. patens sphingolipidome and reveals a key role for S4H during development of nonvascular plants. Physcomitrium patens is a valuable model for studying the diversification of plant sphingolipids. The simple anatomy of P. patens facilitates visualisation of physiological processes in biological membranes.

Disruption of long-chain base hydroxylation alters growth and impacts sphingolipid synthesis in Physcomitrella patens

Sphingolipids have roles as membrane structural components and as bioactive molecules in plants. In Physcomitrella patens, 4-hydroxysphinganine (phytosphingosine, t18:0) is the predominant sphingolipid long-chain base (LCB). To assess the functional significance of t18:0, CRISPR-Cas9 mutagenesis was used to generate mutant lines lacking the sole SPHINGOID BASE HYDROXYLASE (SBH) gene encoding the hydroxylase responsible for converting sphinganine (d18:0) to t18:0. Total sphingolipid content in sbh protonemata was 2.4-fold higher than in wild-type. Modest changes in glycosyl inositolphosphorylceramide (GIPC) glycosylation patterns occurred. Sphingolipidomic analyses of mutants lacking t18:0 indicated modest alterations in acyl-chain pairing with d18:0 in GIPCs and ceramides, but dramatic alterations in acyl-chain pairing in glucosylceramides, in which 4,8-sphingadienine (d18:2) was the principal LCB. A striking accumulation of free and phosphorylated LCBs accompanied loss of the hydroxylase. The sbh lines exhibited altered morphology, including smaller chloronemal cell size, irregular cell shape, reduced gametophore size, and increased pigmentation. In the presence of the synthetic trihydroxy LCB t17:0, the endogenous sphingolipid content of sbh lines decreased to wild-type levels, and the mutants exhibited phenotypes more similar to wild-type plants. These results demonstrate the importance of sphingolipid content and composition to Physcomitrella growth. They also illuminate similarities in regulating sphingolipid content but differences in regulating sphingolipid species composition between the bryophyte P. patens and angiosperm A. thaliana.

Mass Spectrometry-Based Profiling of Plant Sphingolipids from Typical and Aberrant Metabolism

Mass spectrometry has increasingly been used as a tool to complement studies of sphingolipid metabolism and biological functions in plants and other eukaryotes. Mass spectrometry is now essential for comprehensive sphingolipid analytical profiling because of the huge diversity of sphingolipid classes and molecular species in eukaryotes, particularly in plants. This structural diversity arises from large differences in polar head group glycosylation as well as carbon-chain lengths of fatty acids and desaturation and hydroxylation patterns of fatty acids and long-chain bases that together comprise the ceramide hydrophobic backbone of glycosphingolipids. The standard methods for liquid chromatography-mass spectrometry (LC-MS)-based analyses of Arabidopsis thaliana leaf sphingolipids profile >200 molecular species of four sphingolipid classes and free long-chain bases and their phosphorylated forms. While these methods have proven valuable for A. thaliana based sphingolipid research, we have recently adapted them for use with ultraperformance liquid chromatography separations of molecular species and to profile aberrant sphingolipid forms in pollen, transgenic lines, and mutants. This chapter provides updates to standard methods for LC-MS profiling of A. thaliana sphingolipids to expand the utility of mass spectrometry for plant sphingolipid research.

Speaking the language of lipids: the cross-talk between plants and pathogens in defence and disease

Lipids and fatty acids play crucial roles in plant immunity, which have been highlighted over the past few decades. An increasing number of studies have shown that these molecules are pivotal in the interactions between plants and their diverse pathogens. The roles played by plant lipids fit in a wide spectrum ranging from the first physical barrier encountered by the pathogens, the cuticle, to the signalling pathways that trigger different immune responses and expression of defence-related genes, mediated by several lipid molecules. Moreover, lipids have been arising as candidate biomarkers of resistance or susceptibility to different pathogens. Studies on the apoplast and extracellular vesicles have been highlighting the possible role of lipids in the intercellular communication and the establishment of systemic acquired resistance during plant-pathogen interactions. From the pathogen perspective, lipid metabolism and specific lipid molecules play pivotal roles in the pathogen's life cycle completion, being crucial during recognition by the plant and evasion from the host immune system, therefore potentiating infection. Studies conducted in the last years have contributed to a better understanding of the language of lipids during the cross-talk between plants and pathogens. However, it is essential to continue exploring the knowledge brought up to light by transcriptomics and proteomics studies towards the elucidation of lipid signalling processes during defence and disease. In this review, we present an updated overview on lipids associated to plant-pathogen interactions, exploiting their roles from the two sides of this battle.

Plant Cell Cultures as a Tool to Study Programmed Cell Death

Programmed cell death (PCD) is a genetically controlled suicide process present in all living beings with the scope of eliminating cells unnecessary or detrimental for the proper development of the organism. In plants, PCD plays a pivotal role in many developmental processes such as sex determination, senescence, and aerenchyma formation and is involved in the defense responses against abiotic and biotic stresses. Thus, its study is a main goal for plant scientists. However, since PCD often occurs in a small group of inaccessible cells buried in a bulk of surrounding uninvolved cells, its study in whole plant or complex tissues is very difficult. Due to their uniformity, accessibility, and reproducibility of application of stress conditions, cultured cells appear a useful tool to investigate the different aspects of plant PCD. In this review, we summarize how plant cell cultures can be utilized to clarify the plant PCD process.

Fumonisin B1: A Tool for Exploring the Multiple Functions of Sphingolipids in Plants

Fumonisin toxins are produced by Fusarium fungal pathogens. Fumonisins are structural analogs of sphingosine and potent inhibitors of ceramide synthases (CerSs); they disrupt sphingolipid metabolism and cause disease in plants and animals. Over the past three decades, researchers have used fumonisin B1 (FB1), the most common fumonisin, as a probe to investigate sphingolipid metabolism in yeast and animals. Although the physiological effects of FB1 in plants have yet to be investigated in detail, forward and reverse genetic approaches have revealed many genes involved in these processes. In this review, we discuss the intricate network of signaling pathways affected by FB1, including changes in sphingolipid metabolism and the effects of these changes, with a focus on our current understanding of the multiple effects of FB1 on plant cell death and plant growth. We analyze the major findings that highlight the connections between sphingolipid metabolism and FB1-induced signaling, and we point out where additional research is needed to fill the gaps in our understanding of FB1-induced signaling pathways in plants.

The dynamics and role of sphingolipids in eukaryotic organisms upon thermal adaptation

All living beings have an optimal temperature for growth and survival. With the advancement of global warming, the search for understanding adaptive processes to climate changes has gained prominence. In this context, all living beings monitor the external temperature and develop adaptive responses to thermal variations. These responses ultimately change the functioning of the cell and affect the most diverse structures and processes. One of the first structures to detect thermal variations is the plasma membrane, whose constitution allows triggering of intracellular signals that assist in the response to temperature stress. Although studies on this topic have been conducted, the underlying mechanisms of recognizing thermal changes and modifying cellular functioning to adapt to this condition are not fully understood. Recently, many reports have indicated the participation of sphingolipids (SLs), major components of the plasma membrane, in the regulation of the thermal stress response. SLs can structurally reinforce the membrane or/and send signals intracellularly to control numerous cellular processes, such as apoptosis, cytoskeleton polarization, cell cycle arresting and fungal virulence. In this review, we discuss how SLs synthesis changes during both heat and cold stresses, focusing on fungi, plants, animals and human cells. The role of lysophospholipids is also discussed.

Ceramide-Induced Cell Death Depends on Calcium and Caspase-Like Activity in Rice

Ceramide sphingolipids are major components of membranes. C2 and C6 ceramides induce programmed cell death (PCD) in animals and plants, and we previously showed that C2 and C6 ceramides induce PCD in rice (Oryza sativa) protoplasts. However, the mechanistic link between sphingolipids and PCD in rice remains unclear. Here, we observed that calcium levels increased rapidly after ceramide treatment. Moreover, the calcium channel inhibitor LaCl₃ and the intracellular calcium chelator acetoxymethyl-1, 2-bis (2-aminophenoxy) ethic acid (BAPTA-AM) inhibited this calcium increase and prevented ceramide-induced PCD. Moreover, caspase-3-like protease activity increased significantly in C6 ceramide-treated protoplasts, and a caspase-specific inhibitor prevented C6 ceramide-induced cell death. We also detected the other typical PCD events including ATP loss. DIDS (4, 49-diisothiocyanatostilbene- 2, 29-disulfonic acid), an inhibitor of voltage-dependent anion channels (VDACs), decreased C6 ceramide-induced cell death. Together, this evidence suggests that mitochondria played an important role in C6 ceramide-induced PCD.

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.

Survival factor 1 contributes to the oxidative stress response and is required for full virulence of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a devastating necrotrophic fungal pathogen that infects over 400 species of plants worldwide. Reactive oxygen species (ROS) modulations are critical for the pathogenic development of S. sclerotiorum. The fungus applies enzymatic and non-enzymatic antioxidants to cope with the oxidative stress during the infection processes. Survival factor 1 was identified and characterized to promote survival under conditions of oxidative stress in Saccharomyes cerevisiae. In this research, a gene named SsSvf1 was predicted to encode a survival factor 1 homologue in S. sclerotiorum. SsSvf1 transcripts showed high expression levels in hyphae under oxidative stress. Silencing of SsSvf1 resulted in increased sensitivity to oxidative stress in culture and increased levels of intracellular ROS. Transcripts of SsSvf1 showed a dramatic increase during the initial stage of infection and the gene-silenced strains displayed reduced virulence on oilseed rape and Arabidopsis thaliana. Inhibition of plant ROS production partially restores virulence of SsSvf1 gene-silenced strains. SsSvf1 gene-silenced strains exhibited normal oxalate production, but were impaired in compound appressorium formation and cell wall integrity. The results suggest that SsSvf1 is involved in coping with ROS during fungal-host interactions and plays a crucial role in the pathogenicity of S. sclerotiorum.

Elevated Levels of Phosphorylated Sphingobases Do Not Antagonize Sphingobase- or Fumonisin B1-Induced Plant Cell Death

Long-chain bases (LCBs), also termed sphingobases, are building blocks of sphingolipids, which make up a significant proportion of the cellular membrane system. They are also bioactive molecules regulating intracellular processes. Elevated levels of LCBs like phytosphingosine and dihydrosphingosine can induce cell death in plants and correlate with programmed cell death (PCD) reactions after pathogen recognition. We investigated the previously hypothesized antagonism between phosphorylated and nonphosphorylated LCBs with respect to cell death in Arabidopsis thaliana. Using HPLC-MS/MS, we determined levels of phosphorylated and nonphosphorylated LCBs after cell death induction by LCB application or by Fumonisin B1 (FB1) treatment. We show that previously reported antagonistic effects of phosphorylated LCBs after simultaneous application with nonphosphorylated LCBs are linked to reduced uptake of nonphosphorylated LCBs into the tissue. Furthermore, phosphorylated LCBs did not antagonize PCD induced by avirulence protein recognition. In a functional approach, we used Arabidopsis lines with perturbed levels of phosphorylated LCBs. In these plants, the degree of FB1-induced cell death did not consistently correlate negatively with levels of phosphorylated LCBs, but positively with levels of major nonphosphorylated LCBs phytosphingosine and dihydrosphingosine. As treatment with phosphorylated LCBs did not antagonize cell death, and elevated in vivo levels of these LCB species did not reduce FB1-induced cell death, we conclude that the hypothesized general cell death-antagonizing effect of phosphorylated LCBs in plant cell death reactions should be rejected. Instead, our time-course analysis of LCB levels during cell death reactions showed a positive correlation between levels of nonphosphorylated LCBs and cell death.

Emerging Roles of Sphingolipid Signaling in Plant Response to Biotic and Abiotic Stresses

Plant sphingolipids are not only structural components of the plasma membrane and other endomembrane systems but also act as signaling molecules during biotic and abiotic stresses. However, the roles of sphingolipids in plant signal transduction in response to environmental cues are yet to be investigated in detail. In this review, we discuss the signaling roles of sphingolipid metabolites with a focus on plant sphingolipids. We also mention some microbial sphingolipids that initiate signals during their interaction with plants, because of the limited literatures on their plant analogs. The equilibrium of nonphosphorylated and phosphorylated sphingolipid species determine the destiny of plant cells, whereas molecular connections among the enzymes responsible for this equilibrium in a coordinated signaling network are poorly understood. A mechanistic link between the phytohormone-sphingolipid interplay has also not yet been fully understood and many key participants involved in this complex interaction operating under stress conditions await to be identified. Future research is needed to fill these gaps and to better understand the signal pathways of plant sphingolipids and their interplay with other signals in response to environmental stresses.

Sphingosine kinase AtSPHK1 functions in fumonisin B1-triggered cell death in Arabidopsis

The fungal toxin Fumonisin B1 (FB1) is a strong inducer to trigger plant hypersensitive responses (HR) along with increased long chain bases (LCB) and long chain base phosphates (LCBP) contents, though the regulatory mechanism of FB1 action and how the LCB/LCBP signalling cassette functions during the process is still not fully understood. Here, we report sphingosine kinase 1 (SPHK1) as a key factor in FB1-induced HR by modulating the salicylic acid (SA) pathway and reactive oxygen species (ROS) accumulation in Arabidopsis thaliana. Overexpression of SPHK1 increases the FB1-induced accumulations of ROS and SA. The double mutant that simultaneously overexpresses SPHK1 and suppresses the SPPASE or DPL1, two enzymes are mainly responsible for Phyto-sphingosine-1-phosphate (Phyto-S1P) removal, showed enhanced susceptibility to FB1 killing and FB1-induced SA activation than the plants overexpress SPHK1 alone. Exogenous sphingosine-1-phosphate (S1P) can modulate the transcription of the SA-responsive marker gene PR1 in a concentration-dependent biphasic manner. Suppression of SPHK1 decreases SA production whereas promotes jasmonic acid (JA) biosynthesis in response to FB1 applications. Our findings indicate a role of SPHK1 in modulating FB1-triggered cell death via SA and JA pathway interactions.

The retraction of the protoplast during PCD is an active, and interruptible, calcium-flux driven process

The protoplast retracts during apoptosis-like programmed cell death (AL-PCD) and, if this retraction is an active component of AL-PCD, it should be used as a defining feature for this type of programmed cell death. We used an array of pharmacological and genetic tools to test if the rates of protoplast retraction in cells undergoing AL-PCD can be modulated. Disturbing calcium flux signalling, ATP synthesis and mitochondrial permeability transition all inhibited protoplast retraction and often also the execution of the death programme. Protoplast retraction can precede loss of plasma membrane integrity and cell death can be interrupted after the protoplast retraction had already occurred. Blocking calcium influx inhibited the protoplast retraction, reduced DNA fragmentation and delayed death induced by AL-PCD associated stresses. At higher levels of stress, where cell death occurs without protoplast retraction, blocking calcium flux had no effect on the death process. The results therefore strongly suggest that retraction of the protoplast is an active biological process dependent on an early Ca²⁺-mediated trigger rather than cellular disintegration due to plasma membrane damage. Therefore this morphologically distinct cell type is a quantifiable feature, and consequently, reporter of AL-PCD.

Synthesis and degradation of long-chain base phosphates affect fumonisin B1-induced cell death in Arabidopsis thaliana

Fumonisin B₁ (FB₁), an inducer of cell death, disrupts sphingolipid metabolism; large accumulations of de novo synthesized free long-chain bases (LCBs) are observed. However, it remains unclear whether tolerance to FB₁ toxicity in plants is connected with preventing the accumulation of free LCBs through their phosphorylation. Here a workflow for the extraction, detection and quantification of LCB phosphates (LCBPs) in Arabidopsis thaliana was developed. We studied the effect of expression of genes for three enzymes involved in the synthesis and degradation of LCBPs, LCB kinase (LCBK1), LCBP phosphatase (SPP1) and lyase (DPL1) on FB₁-induced cell death. As expected, large accumulations of saturated free LCBs, dihydrosphingosine and phytosphingosine, were observed in the FB₁-treated leaves. On the other hand, a high level of sphingenine phosphate was found in the FB₁-treated leaves even though free sphingenine was found in low amounts in these leaves. In comparison of WT and spp1 plants, the LCBP/LCB ratio is likely to be correlated with the degree of FB₁-induced cell death determined by trypan blue staining. The FB₁-treated leaves in dpl1 plants showed severe cell death and the elevation of free LCBs and LCBPs. LCBK1-OX and -KD plants showed resistance and sensitivity to FB₁, respectively, whereas free LCB and LCBP levels in FB₁-treated LCBK1-OX and -KD plants were moderately different to those in FB₁-treated WT plants. Overall, the findings described here suggest that LCBP/LCB homeostasis is an important topic that participates in the tolerance of plant cells to FB₁.

Long-chain bases from Cucumaria frondosa inhibit adipogenesis and regulate lipid metabolism in 3T3-L1 adipocytes

This study aims to investigate anti-adipogenic effects of long-chain bases from Cucumaria frondosa (Cf-LCBs) in vitro. Results showed that Cf-LCBs inhibited adipocyte differentiation and the expressions of CCAAT/enhancer binding proteins (C/EBPs) and peroxisome proliferators-activated receptor γ (PPARγ). Cf-LCBs increased β-catenin mRNA and nuclear translocation and increased its target genes, cyclin D1 and c-myc. Cf-LCBs enhanced fizzled and lipoprotein-receptor-related protein5/6 (LRP5/6) expressions, whereas wingless-type MMTV integration site10b (WNT10b) and glycogen syntheses kinase 3β (GSK3β) remained unchanged. Cf-LCBs also reduced adipogenesis and recovered WNT/β-catenin signaling in the cells suffering from 21H7, a β-catenin inhibitor. In addition, Cf-LCBs decreased triglyceride content and the expressions of lipogenesis genes. Cf-LCBs increased FFA levels and the expressions of lipidolytic factors. Cf-LCBs promoted the phosphorylation of adenosine-monophosphate-activated protein kinase (AMPK) and acetyl-CoA carboxylase. These findings indicate that Cf-LCBs inhibit adipogenesis through activation of WNT/β-catenin signaling and regulate lipid metabolism via activation of AMPK pathway.

Long-chain bases from sea cucumber mitigate endoplasmic reticulum stress and inflammation in obesity mice

Endoplasmic reticulum (ER) stress and inflammation can induce hyperglycemia. Long-chain bases (LCBs) from sea cucumber exhibit antihyperglycemic activities. However, their effects on ER stress and inflammation are unknown. We investigated the effects of LCBs on ER stress and inflammatory response in high-fat, fructose diet-induced obesity mice. Reactive oxygen species and free fatty acids were measured. Inflammatory cytokines in serum and their mRNA expressions in epididymal adipose tissues were investigated. Hepatic ER stress-related key genes were detected. c-Jun NH₂-terminal kinase and nuclear factor κB inflammatory pathways were also evaluated in the liver. Results showed that LCBs reduced serum and hepatic reactive oxygen species and free fatty acids concentrations. LCBs decreased serum proinflammatory cytokines levels, namely interleukin (IL)-1β, tumor necrosis factor-α, IL-6, macrophage inflammatory protein 1, and c-reactive protein, and increased anti-inflammatory cytokine IL-10 concentration. The mRNA and protein expressions of these cytokines in epididymal adipose tissues were regulated by LCBs as similar to their circulatory contents. LCBs inhibited phosphorylated c-Jun NH₂-terminal kinase and inhibitor κ kinase β, and nuclear factor κB nuclear translocation. LCBs also inhibited mRNA expression of ER stress markers glucose regulated protein, activating transcription factor 6, double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase, and X-box binding protein 1, and phosphorylation of eukaryotic initiation factor-α and inositol requiring enzyme 1α. These results indicate that LCBs can alleviate ER stress and inflammatory response. Nutritional supplementation with LCBs may offer an adjunctive therapy for RE stress-associated inflammation.

Metabolomics Reveals Cryptic Interactive Effects of Species Interactions and Environmental Stress on Nitrogen and Sulfur Metabolism in Seagrass

Eutrophication of estuaries and coastal seas is accelerating, increasing light stress on subtidal marine plants and changing their interactions with other species. To date, we have limited understanding of how such variations in environmental and biological stress modify the impact of interactions among foundational species and eventually affect ecosystem health. Here, we used metabolomics to assess the impact of light reductions on interactions between the seagrass Zostera marina, an important habitat-forming marine plant, and the abundant and commercially important blue mussel Mytilus edulis. Plant performance varied with light availability but was unaffected by the presence of mussels. Metabolomic analysis, on the other hand, revealed an interaction between light availability and presence of M. edulis on seagrass metabolism. Under high light, mussels stimulated seagrass nitrogen and energy metabolism. Conversely, in low light mussels impeded nitrogen and energy metabolism, and enhanced responses against sulfide toxicity, causing inhibited oxidative energy metabolism and tissue degradation. Metabolomic analysis thereby revealed cryptic changes to seagrass condition that could not be detected by traditional approaches. Our findings suggest that coastal eutrophication and associated reductions in light may shift seagrass-bivalve interactions from mutualistic to antagonistic, which is important for conservation management of seagrass meadows.

Classification and substrate head-group specificity of membrane fatty acid desaturases

Membrane fatty acid desaturases are a diverse superfamily of enzymes that catalyze the introduction of double bonds into fatty acids. They are essential in a range of metabolic processes, such as the production of omega-3 fatty acids. However, our structure-function understanding of this superfamily is still developing and their range of activities and substrate specificities are broad, and often overlapping, which has made their systematic characterization challenging. A central issue with characterizing these proteins has been the lack of a structural model, which has been overcome with the recent publication of the crystal structures of two mammalian fatty acid desaturases. In this work, we have used sequence similarity networks to investigate the similarity among over 5000 related membrane fatty acid desaturase sequences, leading to a detailed classification of the superfamily, families and subfamilies with regard to their function and substrate head-group specificity. This work will facilitate rapid prediction of the function and specificity of new and existing sequences, as well as forming a basis for future efforts to manipulate the substrate specificity of these proteins for biotechnology applications.

Genes encoding Δ(8)-sphingolipid desaturase from various plants: identification, biochemical functions, and evolution

∆(8)-sphingolipid desaturase catalyzes the C8 desaturation of a long chain base, which is the characteristic structure of various complex sphingolipids. The genes of 20 ∆(8)-sphingolipid desaturases from 12 plants were identified and functionally detected by using Saccharomyces cerevisiae system to elucidate the relationship between the biochemical function and evolution of this enzyme. Results showed that the 20 genes all can encode a functional ∆(8)-sphingolipid desaturase, which catalyzes different ratios of two products, namely, 8(Z) and 8(E)-C18-phytosphingenine. The coded enzymes could be divided into two groups on the basis of biochemical functions: ∆(8)-sphingolipid desaturase with a preference for an E-isomer product and ∆(8)-sphingolipid desaturase with a preference for a Z-isomer product. The conversion rate of the latter was generally lower than that of the former. Phylogenetic analysis revealed that the 20 desaturases could also be clustered into two groups, and this grouping is consistent with that of the biochemical functions. Thus, the biochemical function of ∆(8)-sphingolipid desaturase is correlated with its evolution. The two groups of ∆(8)-sphingolipid desaturases could arise from distinct ancestors in higher plants. However, they might have initially evolved from ∆(8)-sphingolipid desaturases in lower organisms, such as yeasts, which can produce E-isomer products only. Furthermore, almost all of the transgenic yeasts harboring ∆(8)-sphingolipid desaturase genes exhibit an improvement in aluminum tolerance. Our study provided new insights into the biochemical function and evolution of ∆(8)-sphingolipid desaturases in plants.

Lipid signalling in plant responses to abiotic stress

Lipids are one of the major components of biological membranes including the plasma membrane, which is the interface between the cell and the environment. It has become clear that membrane lipids also serve as substrates for the generation of numerous signalling lipids such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, N-acylethanolamines, free fatty acids and others. The enzymatic production and metabolism of these signalling molecules are tightly regulated and can rapidly be activated upon abiotic stress signals. Abiotic stress like water deficit and temperature stress triggers lipid-dependent signalling cascades, which control the expression of gene clusters and activate plant adaptation processes. Signalling lipids are able to recruit protein targets transiently to the membrane and thus affect conformation and activity of intracellular proteins and metabolites. In plants, knowledge is still scarce of lipid signalling targets and their physiological consequences. This review focuses on the generation of signalling lipids and their involvement in response to abiotic stress. We describe lipid-binding proteins in the context of changing environmental conditions and compare different approaches to determine lipid-protein interactions, crucial for deciphering the signalling cascades.

How Very-Long-Chain Fatty Acids Could Signal Stressful Conditions in Plants?

Although encountered in minor amounts in plant cells, very-long-chain fatty acids exert crucial functions in developmental processes. When their levels are perturbed by means of genetic approaches, marked phenotypic consequences that range from severe growth retardation to embryo lethality was indeed reported. More recently, a growing body of findings has also accumulated that points to a potential role for these lipids as signals in governing both biotic and abiotic stress outcomes. In the present work, we discuss the latter theory and explore the ins and outs of very-long-chain fatty acid-based signaling in response to stress, with an attempt to reconcile two supposedly antagonistic parameters: the insoluble nature of fatty acids and their signaling function. To explain this apparent dilemma, we provide new interpretations of pre-existing data based on the fact that sphingolipids are the main reservoir of very-long-chain fatty acids in leaves. Thus, three non-exclusive, molecular scenarii that involve these lipids as membrane-embedded and free entities are proposed.

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.

Substrate specificity, kinetic properties and inhibition by fumonisin B1 of ceramide synthase isoforms from Arabidopsis

Ceramide makes up the acyl-backbone of sphingolipids and plays a central role in determining the function of these essential membrane lipids. In Arabidopsis, the varied chemical composition of ceramide is determined by the specificity of three different isoforms of ceramide synthase, denoted LAG one homologue 1, -2 and -3 (LOH1, LOH2 and LOH3), for a range of long-chain base (LCB) and acyl-CoA substrates. The contribution of each of these isoforms to the synthesis of ceramide was investigated by in vitro ceramide synthase assays. The plant LCB phytosphingosine was efficiently used by the LOH1 and LOH3 isoforms, with LOH1 having the lowest Km for the LCB substrate of the three isoforms. In contrast, sphinganine was used efficiently only by the LOH2 isoform. Acyl-CoA specificity was also distinguished between the three isoforms with LOH2 almost completely specific for palmitoyl-CoA whereas the LOH1 isoform showed greatest activity with lignoceroyl- and hexacosanoyl-CoAs. Interestingly, unsaturated acyl-CoAs were not used efficiently by any isoform whereas unsaturated LCB substrates were preferred by LOH2 and 3. Fumonisin B1 (FB1) is a general inhibitor of ceramide synthases but LOH1 was found to have a much lower Ki than the other isoforms pointing towards the origin of FB1 sensitivity in plants. Overall, the data suggest distinct roles and modes of regulation for each of the ceramide synthases in Arabidopsis sphingolipid metabolism.

A mass spectrometry-based method for the assay of ceramide synthase substrate specificity

The acyl composition of sphingolipids is determined by the specificity of the enzyme ceramide synthase (EC 2.3.1.24). Ceramide contains a long-chain base (LCB) linked to a variety of fatty acids to produce a lipid class with potentially hundreds of structural variants. An optimized procedure for the assay of ceramide synthase in yeast microsomes is reported that uses mass spectrometry to detect any possible LCB and fatty acid combination synthesized from unlabeled substrates provided in the reaction. The assay requires the delivery of substrates with bovine serum albumin for maximum activity within defined limits of substrate concentration and specific methods to stop the reaction and extract the lipid that avoid the non-enzymatic synthesis of ceramide. The activity of ceramide synthase in yeast microsomes is demonstrated with the four natural LCBs found in yeast along with six saturated and two unsaturated fatty acyl-coenzyme As from 16 to 26 carbons in length. The procedure allows for the determination of substrate specificity and kinetic parameters toward natural substrates for ceramide synthase from potentially any organism.

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.

The TRANSPLANTA collection of Arabidopsis lines: a resource for functional analysis of transcription factors based on their conditional overexpression

Transcription factors (TFs) are key regulators of gene expression in all organisms. In eukaryotes, TFs are often represented by functionally redundant members of large gene families. Overexpression might prove a means to unveil the biological functions of redundant TFs; however, constitutive overexpression of TFs frequently causes severe developmental defects, preventing their functional characterization. Conditional overexpression strategies help to overcome this problem. Here, we report on the TRANSPLANTA collection of Arabidopsis lines, each expressing one of 949 TFs under the control of a β-estradiol-inducible promoter. Thus far, 1636 independent homozygous lines, representing an average of 2.6 lines for every TF, have been produced for the inducible expression of 634 TFs. Along with a GUS-GFP reporter, randomly selected TRANSPLANTA lines were tested and confirmed for conditional transgene expression upon β-estradiol treatment. As a proof of concept for the exploitation of this resource, β-estradiol-induced proliferation of root hairs, dark-induced senescence, anthocyanin accumulation and dwarfism were observed in lines conditionally expressing full-length cDNAs encoding RHD6, WRKY22, MYB123/TT2 and MYB26, respectively, in agreement with previously reported phenotypes conferred by these TFs. Further screening performed with other TRANSPLANTA lines allowed the identification of TFs involved in different plant biological processes, illustrating that the collection is a powerful resource for the functional characterization of TFs. For instance, ANAC058 and a TINY/AP2 TF were identified as modulators of ABA-mediated germination potential, and RAP2.10/DEAR4 was identified as a regulator of cell death in the hypocotyl-root transition zone. Seeds of TRANSPLANTA lines have been deposited at the Nottingham Arabidopsis Stock Centre for further distribution.

Epigenetic changes accompany developmental programmed cell death in tapetum cells

The tapetum, the nursing tissue inside anthers, undergoes cellular degradation by programmed cell death (PCD) during late stages of microspore-early pollen development. Despite the key function of tapetum, little is known about the molecular mechanisms regulating this cell death process in which profound nuclear and chromatin changes occur. Epigenetic features (DNA methylation and histone modifications) have been revealed as hallmarks that establish the functional status of chromatin domains, but no evidence on the epigenetic regulation of PCD has been reported. DNA methylation is accomplished by DNA methyltransferases, among which DNA methyl transferase 1 (MET1) constitutes one of the CG maintenance methyltransferase in plants, also showing de novo methyltransferase activity. In this work, the changes in epigenetic marks during the PCD of tapetal cells have been investigated by a multidisciplinary approach to reveal the dynamics of DNA methylation and the pattern of expression of MET1 in relation to the main cellular changes of this PCD process which have also been characterized in two species, Brassica napus and Nicotiana tabacum. The results showed that tapetum PCD progresses with the increase in global DNA methylation and MET1 expression, epigenetic changes that accompanied the reorganization of the nuclear architecture and a high chromatin condensation, activity of caspase 3-like proteases and Cyt c release. The reported data indicate a relationship between the PCD process and the DNA methylation dynamics and MET1 expression in tapetal cells, suggesting a possible new role for the epigenetic marks in the nuclear events occurring during this cell death process and providing new insights into the epigenetic control of plant PCD.

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

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.

Nitric oxide-sphingolipid interplays in plant signalling: a new enigma from the Sphinx?

Nitric oxide (NO) emerged as one of the major signaling molecules operating during plant development and plant responses to its environment. Beyond the identification of the direct molecular targets of NO, a series of studies considered its interplay with other actors of signal transduction and the integration of NO into complex signaling networks. Beside the close relationships between NO and calcium or phosphatidic acid signaling pathways that are now well-established, recent reports paved the way for interplays between NO and sphingolipids (SLs). This mini-review summarizes our current knowledge of the influence NO and SLs might exert on each other in plant physiology. Based on comparisons with examples from the animal field, it further indicates that, although SL-NO interplays are common features in signaling networks of eukaryotic cells, the underlying mechanisms and molecular targets significantly differ.

Cloning and characterization of two rice long-chain base kinase genes and their function in disease resistance and cell death

Sphingolipid metabolites such as long-chain base 1-phosphates (LCBPs) have been shown to play an important role in plants; however, little is known about their function in plant disease resistance and programmed cell death (PCD). In the present study, we cloned and identified two rice long-chain base kinase (LCBK) genes (OsLCBK1 and OsLCBK2), which are involved in biosynthesis of LCBPs, and performed functional analysis in transgenic tobacco. Expression of OsLCBK1 and OsLCBK2 was induced in rice seedlings after treatments with defense signaling molecules and after infection by Magnaporthe grisea, the causal agent of blast disease. Transgenic tobacco plants overexpressing OsLCBK1 were generated and disease resistance assays indicate that the OsLCBK1-overexpressing plants showed enhanced disease resistance against Pseudmonas syringae pv. tabacci, the causal agent of wildfire disease, and tobacco mosaic virus. Expression levels of some defense-related genes were constitutively up-regulated and further induced after pathogen infection in the OsLCBK1-overexpressing plants. Treatment with fungal toxin fumonisin B1, an effective inducer of PCD in plants, resulted in reduced level of cell death in the OsLCBK1-overexpressing plants, as indicated by cell death staining, leakage of electrolyte and expression of hypersensitive response indicator genes. These data suggest that rice LCBKs, probably through regulation of endogenous LCBP level, play important roles in disease resistance response and PCD in plants.

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.