Reactive oxygen species as transducers of sphinganine-mediated cell death pathway

Exploring lipid signaling in plant physiology: From cellular membranes to environmental adaptation

Lipids have evolved as versatile signaling molecules that regulate a variety of physiological processes in plants. Convincing evidence highlights their critical role as mediators in a wide range of plant processes required for survival, growth, development, and responses to environmental conditions such as water availability, temperature changes, salt, pests, and diseases. Understanding lipid signaling as a critical process has helped us expand our understanding of plant biology by explaining how plants sense and respond to environmental cues. Lipid signaling pathways constitute a complex network of lipids, enzymes, and receptors that coordinate important cellular responses and stressing plant biology's changing and adaptable traits. Plant lipid signaling involves a wide range of lipid classes, including phospholipids, sphingolipids, oxylipins, and sterols, each of which contributes differently to cellular communication and control. These lipids function not only as structural components, but also as bioactive molecules that transfer signals. The mechanisms entail the production of lipid mediators and their detection by particular receptors, which frequently trigger downstream cascades that affect gene expression, cellular functions, and overall plant growth. This review looks into lipid signaling in plant physiology, giving an in-depth look and emphasizing its critical function as a master regulator of vital activities.

The function of sphingolipids in membrane trafficking and cell signaling in plants, in comparison with yeast and animal cells

Sphingolipids are essential membrane components involved in a wide range of cellular, developmental and signaling processes. Sphingolipids are so essential that knock-out mutation often leads to lethality. In recent years, conditional or weak allele mutants as well as the broadening of the pharmacological catalog allowed to decipher sphingolipid function more precisely in a less invasive way. This review intends to provide a discussion and point of view on the function of sphingolipids with a main focus on endomembrane trafficking, Golgi-mediated protein sorting, cell polarity, cell-to-cell communication and cell signaling at the plasma membrane. While our main angle is the plant field research, we will constantly refer to and compare with the advances made in the yeast and animal field. In this review, we will emphasize the role of sphingolipids not only as a membrane component, but also as a key player at a center of homeostatic regulatory networks involving direct or indirect interaction with other lipids, proteins and ion fluxes.

Heat stress, especially when coupled with high light, accelerates the decline of tropical seagrass (Enhalus acoroides) meadows

Heat stress threatens the survival of seagrass, but its damage mechanisms are unclear. In this study, the results reveal that heat stress exceeding 36 °C in the dark caused inactivation of the PSII reaction center, damaging both the PSII donor and acceptor sides in Enhalus acoroides. High light further increased the damage to the photosynthetic apparatus under heat stress. The stronger the heat stress under high light, the harder the recovery of photosynthetic activity. Therefore, during ebb tide at noon in nature, heat stress combined with strong light would induce a significant, even irreversible decrease in photosynthetic activity. Moreover, the heat stress hindered the transcription of psbA and RuBisCO, enhanced respiratory O₂, and induced severe peroxidation even if the SOD, APX, and GPX activities significantly improved. The results clearly suggest that heat stress, especially when coupled with high light, may be an important cause for the decline of E. acoroides meadows.

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.

Cell death induced by mycotoxin fumonisin B1 is accompanied by oxidative stress and transcriptional modulation in Arabidopsis cell culture

Fumonisin B₁ induces rapid programmed cell death in Arabidopsis cells, oxidative and nitrosative bursts, and differentially modulates cell death responsive genes. Glutathione is the main antioxidant involved in the stress response. Fumonisin B₁ (FB₁) is a fungal toxin produced by Fusarium spp. able to exert pleiotropic toxicity in plants. FB₁ is known to be a strong inducer of the programmed cell death (PCD); however, the exact mechanism underling the plant-toxin interactions and the molecular events that lead to PCD are still unclear. Therefore, in this work, we provided a comprehensive investigation of the response of the model organism Arabidopsis thaliana at the nuclear, transcriptional, and biochemical level after the treatment with FB₁ at two different concentrations, namely 1 and 5 µM during a time-course of 96 h. FB₁ induced oxidative and nitrosative bursts and a rapid cell death in Arabidopsis cell cultures, which resembled a HR-like PCD event. Different genes involved in the regulation of PCD, antioxidant metabolism, photosynthesis, pathogenesis, and sugar transport were upregulated, especially during the late treatment time and with higher FB₁ concentration. Among the antioxidant enzymes and compounds studied, only glutathione appeared to be highly induced in both treatments, suggesting that it might be an important stress molecule induced during FB₁ exposure. Collectively, these findings highlight the complexity of the signaling network of A. thaliana and provide information for the understanding of the physiological, molecular, and biochemical responses to counteract FB₁-induced toxicity.

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.

Decreased R:FR Ratio in Incident White Light Affects the Composition of Barley Leaf Lipidome and Freezing Tolerance in a Temperature-Dependent Manner

In cereals, C-repeat binding factor genes have been defined as key components of the light quality-dependent regulation of frost tolerance by integrating phytochrome-mediated light and temperature signals. This study elucidates the differences in the lipid composition of barley leaves illuminated with white light or white light supplemented with far-red light at 5 or 15 °C. According to LC-MS analysis, far-red light supplementation increased the amount of monogalactosyldiacylglycerol species 36:6, 36:5, and 36:4 after 1 day at 5 °C, and 10 days at 15 °C resulted in a perturbed content of 38:6 species. Changes were observed in the levels of phosphatidylethanolamine, and phosphatidylserine under white light supplemented with far-red light illumination at 15 °C, whereas robust changes were observed in the amount of several phosphatidylserine species at 5 °C. At 15 °C, the amount of some phosphatidylglycerol species increased as a result of white light supplemented with far-red light illumination after 1 day. The ceramide (42:2)-3 content increased regardless of the temperature. The double-bond index of phosphatidylglycerol, phosphatidylserine, phosphatidylcholine ceramide together with total double-bond index changed when the plant was grown at 15 °C as a function of white light supplemented with far-red light. white light supplemented with far-red light increased the monogalactosyldiacylglycerol/diacylglycerol ratio as well. The gene expression changes are well correlated with the alterations in the lipidome.

Organized Disassembly of Photosynthesis During Programmed Cell Death Mediated By Long Chain Bases

In plants, pathogen triggered programmed cell death (PCD) is frequently mediated by polar lipid molecules referred as long chain bases (LCBs) or ceramides. PCD interceded by LCBs is a well-organized process where several cell organelles play important roles. In fact, light-dependent reactions in the chloroplast have been proposed as major players during PCD, however, the functional aspects of the chloroplast during PCD are largely unknown. For this reason, we investigated events that lead to disassembly of the chloroplast during PCD mediated by LCBs. To do so, LCB elevation was induced with Pseudomonas syringae pv. tomato (a non-host pathogen) or Fumonisin B1 in Phaseolus vulgaris. Then, we performed biochemical tests to detect PCD triggering events (phytosphingosine rises, MPK activation and H2O2 generation) followed by chloroplast structural and functional tests. Observations of the chloroplast, via optical phenotyping methods combined with microscopy, indicated that the loss of photosynthetic linear electron transport coincides with the organized ultrastructure disassembly. In addition, structural changes occurred in parallel with accumulation of H2O2 inside the chloroplast. These features revealed the collapse of chloroplast integrity and function as a mechanism leading to the irreversible execution of the PCD promoted by LCBs.

Ceramide synthase inhibition by fumonisins: a perfect storm of perturbed sphingolipid metabolism, signaling, and disease

Fumonisins are mycotoxins that cause diseases of plants and, when consumed by animals, can damage liver, kidney, lung, brain, and other organs, alter immune function, and cause developmental defects and cancer. They structurally resemble sphingolipids (SLs), and studies nearly 30 years ago discovered that the most prevalent fumonisin [fumonisin B1 (FB1)] potently inhibits ceramide synthases (CerSs), enzymes that use fatty acyl-CoAs to N-acylate sphinganine (Sa), sphingosine (So), and other sphingoid bases. CerS inhibition by FB1 triggers a "perfect storm" of perturbations in structural and signaling SLs that include: reduced formation of dihydroceramides, ceramides, and complex SLs; elevated Sa and So and their 1-phosphates, novel 1-deoxy-sphingoid bases; and alteration of additional lipid metabolites from interrelated pathways. Moreover, because the initial enzyme of sphingoid base biosynthesis remains active (sometimes with increased activity), the impact is multiplied by the continued production of damaging metabolites. Evidence from many studies, including characterization of knockout mice for specific CerSs and analyses of human blood (which found that FB1 intake is associated with elevated Sa 1-phosphate), has consistently pointed to CerS as the proximate target of FB1 It is also apparent that the changes in multiple bioactive lipids and related biologic processes account for the ensuing spectrum of animal and plant disease. Thus, the diseases caused by fumonisins can be categorized as "sphingolipidoses" (in these cases, due to defective SL biosynthesis), and the lessons learned about the consequences of CerS inhibition should be borne in mind when contemplating other naturally occurring and synthetic compounds (and genetic manipulations) that interfere with SL metabolism.

Aging-related elevation of sphingoid bases shortens yeast chronological life span by compromising mitochondrial function

Sphingoid bases (SBs) as bioactive sphingolipids, have been implicated in aging in yeast. However, we know neither how SBs are regulated during yeast aging nor how they, in turn, regulate it. Herein, we demonstrate that the yeast alkaline ceramidases (YPC1 and YDC1) and SB kinases (LCB4 and LCB5) cooperate in regulating SBs during the aging process and that SBs shortens chronological life span (CLS) by compromising mitochondrial functions. With a lipidomics approach, we found that SBs were increased in a time-dependent manner during yeast aging. We also demonstrated that among the enzymes known for being responsible for the metabolism of SBs, YPC1 was upregulated whereas LCB4/5 were downregulated in the course of aging. This inverse regulation of YPC1 and LCB4/5 led to the aging-related upregulation of SBs in yeast and a reduction in CLS. With the proteomics-based approach (SILAC), we revealed that increased SBs altered the levels of proteins related to mitochondria. Further mechanistic studies demonstrated that increased SBs inhibited mitochondrial fusion and caused fragmentation, resulting in decreases in mtDNA copy numbers, ATP levels, mitochondrial membrane potentials, and oxygen consumption. Taken together, these results suggest that increased SBs mediate the aging process by impairing mitochondrial structural integrity and functions.

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.

Silencing of a Germin-Like Protein Gene (CchGLP) in Geminivirus-Resistant Pepper (Capsicum chinense Jacq.) BG-3821 Increases Susceptibility to Single and Mixed Infections by Geminiviruses PHYVV and PepGMV

Germin-like proteins (GLPs) are encoded by a family of genes found in all plants, and in terms of function, the GLPs are implicated in the response of plants to biotic and abiotic stresses. CchGLP is a gene encoding a GLP identified in a geminivirus-resistant Capsicum chinense Jacq accession named BG-3821, and it is important in geminivirus resistance when transferred to susceptible tobacco in transgenic experiments. To characterize the role of this GLP in geminivirus resistance in the original accession from which this gene was identified, this work aimed at demonstrating the possible role of CchGLP in resistance to geminiviruses in Capsicum chinense Jacq. BG-3821. Virus-induced gene silencing studies using a geminiviral vector based in PHYVV component A, displaying that silencing of CchGLP in accession BG-3821, increased susceptibility to geminivirus single and mixed infections. These results suggested that CchGLP is an important factor for geminivirus resistance in C. chinense BG-3821 accession.

Long-chain bases, phosphatidic acid, MAPKs, and reactive oxygen species as nodal signal transducers in stress responses in Arabidopsis

Due to their sessile condition, plants have developed sensitive, fast, and effective ways to contend with environmental changes. These mechanisms operate as informational wires conforming extensive and intricate networks that are connected in several points. The responses are designed as pathways orchestrated by molecules that are transducers of protein and non-protein nature. Their chemical nature imposes selective features such as specificity, formation rate, and generation site to the informational routes. Enzymes such as mitogen-activated protein kinases and non-protein, smaller molecules, such as long-chain bases, phosphatidic acid, and reactive oxygen species are recurrent transducers in the pleiotropic responses to biotic and abiotic stresses in plants. In this review, we considered these four components as nodal points of converging signaling pathways that start from very diverse stimuli and evoke very different responses. These pleiotropic effects may be explained by the potentiality that every one of these four mediators can be expressed from different sources, cellular location, temporality, or magnitude. Here, we review recent advances in our understanding of the interplay of these four specific signaling components in Arabidopsis cells, with an emphasis on drought, cold and pathogen stresses.

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

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

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

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

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

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

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.