Plant PA signaling via diacylglycerol kinase

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.

Melatonin Mitigates Lead-Induced Oxidative Stress and Modifies Phospholipid Profile in Tobacco BY-2 Suspension Cells

Many studies have shown that melatonin (an indoleamine) is an important molecule in plant physiology. It is known that this indoleamine is crucial during plant stress responses, especially by counteracting secondary oxidative stress (efficient direct and indirect antioxidant) and switching on different defense plant strategies. In this report, we present exogenous melatonin's potential to protect lipid profile modification and membrane integrity in Nicotiana tabacum L. line Bright Yellow 2 (BY-2) cell culture exposed to lead. There are some reports of the positive effect of melatonin on animal cell membranes; ours is the first to report changes in the lipid profile in plant cells. The experiments were performed in the following variants: LS: cells cultured on unmodified LS medium-control; (ii) MEL: BY-2 cells cultured on LS medium with melatonin added from the beginning of culture; (iii) Pb: BY-2 cells cultured on LS medium with Pb2+ added on the 4th day of culture; (iv) MEL+Pb: BY-2 cells cultured on LS medium with melatonin added from the start of culture and stressed with Pb2+ added on the 4th day of culture. Lipidomic analysis of BY-2 cells revealed the presence of 40 different phospholipids. Exposing cells to lead led to the overproduction of ROS, altered fatty acid composition and increased PLD activity and subsequently elevated the level of phosphatidic acid at the cost of dropping the phosphatidylcholine. In the presence of lead, double-bond index elevation, mainly by higher quantities of linoleic (C18:2) and linolenic (C18:3) acids in the log phase of growth, was observed. In contrast, cells exposed to heavy metal but primed with melatonin showed more similarities with the control. Surprisingly, the overproduction of ROS caused of lipid peroxidation only in the stationary phase of growth, although considerable changes in lipid profiles were observed in the log phase of growth-just 4 h after lead administration. Our results indicate that the pretreatment of BY-2 with exogenous melatonin protected tobacco cells against membrane dysfunctions caused by oxidative stress (lipid oxidation), but also findings on a molecular level suggest the possible role of this indoleamine in the safeguarding of the membrane lipid composition that limited lead-provoked cell death. The presented research indicates a new mechanism of the defense strategy of plant cells generated by melatonin.

Genetic interaction between TTG2 and AtPLC1 reveals a role for phosphoinositide signaling in a co-regulated suite of Arabidopsis epidermal pathways

The TTG2 transcription factor of Arabidopsis regulates a set of epidermal traits, including the differentiation of leaf trichomes, flavonoid pigment production in cells of the inner testa (or seed coat) layer and mucilage production in specialized cells of the outer testa layer. Despite the fact that TTG2 has been known for over twenty years as an important regulator of multiple developmental pathways, little has been discovered about the downstream mechanisms by which TTG2 co-regulates these epidermal features. In this study, we present evidence of phosphoinositide lipid signaling as a mechanism for the regulation of TTG2-dependent epidermal pathways. Overexpression of the AtPLC1 gene rescues the trichome and seed coat phenotypes of the ttg2-1 mutant plant. Moreover, in the case of seed coat color rescue, AtPLC1 overexpression restored expression of the TTG2 flavonoid pathway target genes, TT12 and TT13/AHA10. Consistent with these observations, a dominant AtPLC1 T-DNA insertion allele (plc1-1D) promotes trichome development in both wild-type and ttg2-3 plants. Also, AtPLC1 promoter:GUS analysis shows expression in trichomes and this expression appears dependent on TTG2. Taken together, the discovery of a genetic interaction between TTG2 and AtPLC1 suggests a role for phosphoinositide signaling in the regulation of trichome development, flavonoid pigment biosynthesis and the differentiation of mucilage-producing cells of the seed coat. This finding provides new avenues for future research at the intersection of the TTG2-dependent developmental pathways and the numerous molecular and cellular phenomena influenced by phospholipid signaling.

Transcriptomic and Lipidomic Analysis Reveals Complex Regulation Mechanisms Underlying Rice Roots' Response to Salt Stress

Rice (Oryza sativa L.), a crucial food crop that sustains over half the world's population, is often hindered by salt stress during various growth stages, ultimately causing a decrease in yield. However, the specific mechanism of rice roots' response to salt stress remains largely unknown. In this study, transcriptomics and lipidomics were used to analyze the changes in the lipid metabolism and gene expression profiles of rice roots in response to salt stress. The results showed that salt stress significantly inhibited rice roots' growth and increased the roots' MDA content. Furthermore, 1286 differentially expressed genes including 526 upregulated and 760 downregulated, were identified as responding to salt stress in rice roots. The lipidomic analysis revealed that the composition and unsaturation of membrane lipids were significantly altered. In total, 249 lipid molecules were differentially accumulated in rice roots as a response to salt stress. And most of the major phospholipids, such as phosphatidic acid (PA), phosphatidylcholine (PC), and phosphatidylserine (PS), as well as major sphingolipids including ceramide (Cer), phytoceramide (CerP), monohexose ceramide (Hex1Cer), and sphingosine (SPH), were significantly increased, while the triglyceride (TG) molecules decreased. These results suggested that rice roots mitigate salt stress by altering the fluidity and integrity of cell membranes. This study enhances our comprehension of salt stress, offering valuable insights into changes in the lipids and adaptive lipid remodeling in rice's response to salt stress.

DGK5β-derived phosphatidic acid regulates ROS production in plant immunity by stabilizing NADPH oxidase

In plant immunity, phosphatidic acid (PA) regulates reactive oxygen species (ROS) by binding to respiratory burst oxidase homolog D (RBOHD), an NADPH oxidase responsible for ROS production. Here, we analyze the influence of PA binding on RBOHD activity and the mechanism of RBOHD-bound PA generation. PA binding enhances RBOHD protein stability by inhibiting vacuolar degradation, thereby increasing chitin-induced ROS production. Mutations in diacylglycerol kinase 5 (DGK5), which phosphorylates diacylglycerol to produce PA, impair chitin-induced PA and ROS production. The DGK5 transcript DGK5β (but not DGK5α) complements reduced PA and ROS production in dgk5-1 mutants, as well as resistance to Botrytis cinerea. Phosphorylation of S506 residue in the C-terminal calmodulin-binding domain of DGK5β contributes to the activation of DGK5β to produce PA. These findings suggest that DGK5β-derived PA regulates ROS production by inhibiting RBOHD protein degradation, elucidating the role of PA-ROS interplay in immune response regulation.

Arabidopsis thaliana phosphoinositide-specific phospholipase C 2 is required for Botrytis cinerea proliferation

Phospholipase C (PLC) plays a key role in lipid signaling during plant development and stress responses. PLC activation is one of the earliest responses during pathogen perception. Arabidopsis thaliana contains seven PLC encoding genes (AtPLC1 to AtPLC7) and two pseudogenes (AtPLC8 and AtPLC9), being AtPLC2 the most abundant isoform with constitutive expression in all plant organs. PLC has been linked to plant defense signaling, in particular to the production of reactive oxygen species (ROS). Previously, we demonstrated that AtPLC2 is involved in ROS production via the NADPH oxidase isoforms RBOHD activation during stomata plant immunity. Here we studied the role of AtPLC2 on plant resistance against the necrotrophic fungus Botrytis cinerea, a broad host-range and serious agricultural pathogen. We show that the AtPLC2-silenced (amiR PLC2) or null mutant (plc2-1) plants developed smaller B. cinerea lesions. Moreover, plc2-1 showed less ROS production and an intensified SA-dependent signaling upon infection, indicating that B. cinerea uses AtPLC2-triggered responses for a successful proliferation. Therefore, AtPLC2 is a susceptibility (S) gene that facilitates B. cinerea infection and proliferation.

Genetic and lipidomic analyses reveal the key role of lipid metabolism for cold tolerance in maize

Lipid remodeling is crucial for cold tolerance in plants. However, the precise alternations of lipidomics during cold responses remain elusive, especially in maize (Zea mays L.). In addition, the key genes responsible for cold tolerance in maize lipid metabolism have not been identified. Here, we integrate lipidomic, transcriptomic, and genetic analysis to determine the profile of lipid remodeling caused by cold stress. We find that the homeostasis of cellular lipid metabolism is essential for maintaining cold tolerance of maize. Also, we detect 210 lipid species belonging to 13 major classes, covering phospholipids, glycerides, glycolipids, and free fatty acids. Various lipid metabolites undergo specific and selective alterations in response to cold stress, especially mono-/di-unsaturated lysophosphatidic acid, lysophosphatidylcholine, phosphatidylcholine, and phosphatidylinositol, as well as polyunsaturated phosphatidic acid, monogalactosyldiacylglycerol, diacylglycerol, and triacylglycerol. In addition, we identify a subset of key enzymes, including ketoacyl-acyl-carrier protein synthase II (KAS II), acyl-carrier protein 2 (ACP2), male sterility33 (Ms33), and stearoyl-acyl-carrier protein desaturase 2 (SAD2) involved in glycerolipid biosynthetic pathways are positive regulators of maize cold tolerance. These results reveal a comprehensive lipidomic profile during the cold response of maize and provide genetic resources for enhancing cold tolerance in crops.

Dual phosphorylation of DGK5-mediated PA burst regulates ROS in plant immunity

Phosphatidic acid (PA) and reactive oxygen species (ROS) are crucial cellular messengers mediating diverse signaling processes in metazoans and plants. How PA homeostasis is tightly regulated and intertwined with ROS signaling upon immune elicitation remains elusive. We report here that Arabidopsis diacylglycerol kinase 5 (DGK5) regulates plant pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). The pattern recognition receptor (PRR)-associated kinase BIK1 phosphorylates DGK5 at Ser-506, leading to a rapid PA burst and activation of plant immunity, whereas PRR-activated intracellular MPK4 phosphorylates DGK5 at Thr-446, which subsequently suppresses DGK5 activity and PA production, resulting in attenuated plant immunity. PA binds and stabilizes the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), regulating ROS production in plant PTI and ETI, and their potentiation. Our data indicate that distinct phosphorylation of DGK5 by PRR-activated BIK1 and MPK4 balances the homeostasis of cellular PA burst that regulates ROS generation in coordinating two branches of plant immunity.

Drought and heat stress mediated activation of lipid signaling in plants: a critical review

Lipids are a principal component of plasma membrane, acting as a protective barrier between the cell and its surroundings. Abiotic stresses such as drought and temperature induce various lipid-dependent signaling responses, and the membrane lipids respond differently to environmental challenges. Recent studies have revealed that lipids serve as signal mediators forreducing stress responses in plant cells and activating defense systems. Signaling lipids, such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, and N-acylethanolamines, are generated in response to stress. Membrane lipids are essential for maintaining the lamellar stack of chloroplasts and stabilizing chloroplast membranes under stress. However, the effects of lipid signaling targets in plants are not fully understood. This review focuses on the synthesis of various signaling lipids and their roles in abiotic stress tolerance responses, providing an essential perspective for further investigation into the interactions between plant lipids and abiotic stress.

Lipid Metabolomic and Transcriptomic Analyses Reveal That Phosphatidylcholine Enhanced the Resistance of Peach Seedlings to Salt Stress through Phosphatidic Acid

Soil salinity is a major conlinet limiting sustainable agricultural development in peach tree industry. In this study, lipid metabolomic pathway analysis indicated that phosphatidic acid is essential for root resistance to salt stress in peach seedlings. Through functional annotation analysis of differentially expressed genes in transcriptomics, we found that MAPK signaling pathway is closely related to peach tree resistance to salt stress, wherein PpMPK6 expression is significantly upregulated. Under salt conditions, the OE-PpMPK6 Arabidopsis thaliana (L.) Heynh. line showed higher resistance to salt stress than WT and KO-AtMPK6 lines. Furthermore, we found that the Na⁺ content in OE-PpMPK6 roots was significantly lower than that in WT and KO-AtMPK6 roots, indicating that phosphatidic acid combined with PpMPK6 activated the SOS1 (salt-overly-sensitive 1) protein to enhance Na⁺ efflux, thus alleviating the damage caused by NaCl in roots; these findings provide insight into the salt stress-associated transcriptional regulation.

Dynamic responses of PA to environmental stimuli imaged by a genetically encoded mobilizable fluorescent sensor

Membrane fluidity, permeability, and surface charges are controlled by phospholipid metabolism and transport. Despite the importance of phosphatidic acid (PA) as a bioactive molecule, the mechanical properties of PA translocation and subcellular accumulation are unknown. Here, we used a mobilizable, highly responsive genetically encoded fluorescent indicator, green fluorescent protein (GFP)-N160_RbohD, to monitor PA dynamics in living cells. The majority of GFP-N160_RbohD accumulated at the plasma membrane and sensitively responded to changes in PA levels. Cellular, pharmacological, and genetic analyses illustrated that both salinity and abscisic acid rapidly enhanced GFP-N160_RbohD fluorescence at the plasma membrane, which mainly depended on hydrolysis of phospholipase D. By contrast, heat stress induced nuclear translocation of PA indicated by GFP-N160_RbohD through a process that required diacylglycerol kinase activity, as well as secretory and endocytic trafficking. Strikingly, we showed that gravity triggers asymmetric PA distribution at the root apex, a response that is suppressed by PLDζ2 knockout. The broad utility of the PA sensor will expand our mechanistic understanding of numerous lipid-associated physiological and cell biological processes and facilitate screening for protein candidates that affect the synthesis, transport, and metabolism of PA.

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids - including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids - also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.

Mitochondrial Complex I Disruption Causes Broad Reorchestration of Plant Lipidome Including Chloroplast Lipids

Mitochondrial complex I (CI) plays a crucial role in oxidising NADH generated by the metabolism (including photorespiration) and thereby participates in the mitochondrial electron transfer chain feeding oxidative phosphorylation that generates ATP. However, CI mutations are not lethal in plants and cause moderate phenotypes, and therefore CI mutants are instrumental to examine consequences of mitochondrial homeostasis disturbance on plant cell metabolisms and signalling. To date, the consequences of CI disruption on the lipidome have not been examined. Yet, in principle, mitochondrial dysfunction should impact on lipid synthesis through chloroplasts (via changes in photorespiration, redox homeostasis, and N metabolism) and the endoplasmic reticulum (ER) (via perturbed mitochondrion-ER crosstalk). Here, we took advantage of lipidomics technology (by LC-MS), phospholipid quantitation by ³¹P-NMR, and total lipid quantitation to assess the impact of CI disruption on leaf, pollen, and seed lipids using three well-characterised CI mutants: CMSII in N. sylvestris and both ndufs4 and ndufs8 in Arabidopsis. Our results show multiple changes in cellular lipids, including galactolipids (chloroplastic), sphingolipids, and ceramides (synthesised by ER), suggesting that mitochondrial homeostasis is essential for the regulation of whole cellular lipidome via specific signalling pathways. In particular, the observed modifications in phospholipid and sphingolipid/ceramide molecular species suggest that CI activity controls phosphatidic acid-mediated signalling.

Identification and expression analysis of the lipid phosphate phosphatases gene family reveal their involvement in abiotic stress response in kiwifruit

Lipid phosphate phosphatases (LPPs) are a key enzyme in the production and degradation of phosphatidic acid (PA), which plays an important role in plant growth, development, stress resistance and plant hormone response. Thus far, little is known about the LPP family genes in kiwifruit (Actinidia spp.). According to this study, 7 members in the AcLPP family were identified from the whole genome of kiwifruit, the subcellular localization predictions were mainly on the plasma membrane. Chromosomal localization analysis showed that the AcLPP genes were unevenly distributed on 5 chromosomes, it was determined to have undergone strong purifying selection pressure. There were 5 duplicate gene pairs and all underwent segmental duplication events. The LPP genes of kiwifruit were conserved when compared with other plants, especially in terms of evolutionary relationships, conserved motifs, protein sequences, and gene structures. Cis-regulatory elements mainly included hormone response elements and abiotic response elements. Functional annotation of GO revealed that AcLPP genes were closely related to phosphatase/hydrolase activity, phosphorus metabolism and dephosphorylation. AcLPP genes family were predicted to be targets of miRNA. Transcript level analysis revealed that the AcLPP family played diverse functions in different tissues and during growth, development, and postharvest storage stages. qPCR analysis showed that the members of AcLPP gene family might be regulated by ETH, ABA, GA3, and IAA hormone signals. The family members were regulated by the stress of salt stress, osmotic stress, cold stress, and heat stress. These results would provide a basis and reference for studying the agricultural characteristics of kiwifruit and improving its stress resistance.

Cytidinediphosphate diacylglycerol synthase—Mediated phosphatidic acid metabolism is crucial for early embryonic development of Arabidopsis

Embryonic development is a key developmental event in plant sexual reproduction; however, regulatory networks of plant early embryonic development, particularly the effects and functional mechanisms of phospholipid molecules are still unknown due to the limitation of sample collection and analysis. We innovatively applied the microspore-derived in vitro embryogenesis of Brassica napus and revealed the dynamics of phospholipid molecules, especially phosphatidic acid (PA, an important second messenger that plays an important role in plant growth, development, and stress responses), at different embryonic developmental stages by using a lipidomics approach. Further analysis of Arabidopsis mutants deficiency of CDS1 and CDS2 (cytidinediphosphate diacylglycerol synthase, key protein in PA metabolism) revealed the delayed embryonic development from the proembryo stage, indicating the crucial effect of CDS and PA metabolism in early embryonic development. Decreased auxin level and disturbed polar localization of auxin efflux carrier PIN1 implicate that CDS-mediated PA metabolism may regulate early embryogenesis through modulating auxin transport and distribution. These results demonstrate the dynamics and importance of phospholipid molecules during embryo development, and provide informative clues to elucidate the regulatory network of embryogenesis.

As an important stage of plant life cycle, embryogenesis initiates with fertilization and terminates with embryo maturation, and plays important roles for seed development. In Arabidopsis, embryogenesis can be divided into three general phases during which distinct developmental and physiological events occur: proembryo stage that starts with asymmetric division of zygote, the transition from globular to heart stage, and late embryogenesis which is marked by organ expansion and maturation. Genetics studies have identified the genes and network that play important regulatory roles in embryogenesis, however, effects and functional mechanisms of phospholipids molecules, the major building blocks of cell membranes and signal transduction cascades in mammals and plants, in embryogenesis remain unknown. To overcome the limitation of sample collection, we innovatively applied the microspore-derived in vitro embryogenesis of Brassica napus and revealed the dynamics of phospholipid molecules, especially phosphatidic acid (PA) at different embryonic developmental stages by using a lipidomics approach. Further genetics analysis revealed the crucial effect of CDS (cytidinediphosphate diacylglycerol synthase, key protein in PA metabolism)—mediated PA metabolism in early embryonic development through modulating auxin transport and distribution, demonstrating the dynamics and importance of phospholipid molecules during embryo development.

A genetically encoded biosensor reveals spatiotemporal variation in cellular phosphate content in Brachypodium distachyon mycorrhizal roots

Arbuscular mycorrhizal (AM) symbiosis is accompanied by alterations to root cell metabolism and physiology, and to the pathways of orthophosphate (Pi) entry into the root, which increase with Pi delivery to cortical cells via arbuscules. How AM symbiosis influences the Pi content and Pi response dynamics of cells in the root cortex and epidermis is unknown. Using fluorescence resonance energy transfer (FRET)-based Pi biosensors, we mapped the relative cytosolic and plastidic Pi content of Brachypodium distachyon mycorrhizal root cells, analyzed responses to extracellular Pi and traced extraradical hyphae-mediated Pi transfer to colonized cells. Colonized cortical cells had a higher cytosolic Pi content relative to noncolonized cortical and epidermal cells, while plastidic Pi content was highest in cells at the infection front. Pi application to the entire mycorrhizal root resulted in transient changes in cytosolic Pi that differed in direction and magnitude depending on cell type and arbuscule status; cells with mature arbuscules showed a substantial transient increase in cytosolic Pi while those with collapsed arbuscules showed a decrease. Directed Pi application to extraradical hyphae resulted in measurable changes in cytosolic Pi of colonized cells 18 h after application. Our experiments reveal that cells within a mycorrhizal root vary in Pi content and Pi response dynamics.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

Phosphatidic Acid in Plant Hormonal Signaling: From Target Proteins to Membrane Conformations

Cells sense a variety of extracellular signals balancing their metabolism and physiology according to changing growth conditions. Plasma membranes are the outermost informational barriers that render cells sensitive to regulatory inputs. Membranes are composed of different types of lipids that play not only structural but also informational roles. Hormones and other regulators are sensed by specific receptors leading to the activation of lipid metabolizing enzymes. These enzymes generate lipid second messengers. Among them, phosphatidic acid (PA) is a well-known intracellular messenger that regulates various cellular processes. This lipid affects the functional properties of cell membranes and binds to specific target proteins leading to either genomic (affecting transcriptome) or non-genomic responses. The subsequent biochemical, cellular and physiological reactions regulate plant growth, development and stress tolerance. In the present review, we focus on primary (genome-independent) signaling events triggered by rapid PA accumulation in plant cells and describe the functional role of PA in mediating response to hormones and hormone-like regulators. The contributions of individual lipid signaling enzymes to the formation of PA by specific stimuli are also discussed. We provide an overview of the current state of knowledge and future perspectives needed to decipher the mode of action of PA in the regulation of cell functions.

The Role of Triacylglycerol in the Protection of Cells against Lipotoxicity under Drought in Lolium multiflorum/Festucaarundinacea Introgression Forms

Triacylglycerol is a key lipid compound involved in maintaining homeostasis of both membrane lipids and free fatty acids (FFA) in plant cells under adverse environmental conditions. However, its role in the process of lipid remodeling has not been fully recognized, especially in monocots, including grass species. For our study, two closely related introgression forms of Lolium multiflorum (Italian ryegrass) and Festuca arundinacea (tall fescue), distinct in their level of drought tolerance, were selected as plant models to study rearrangements in plant lipidome under water deficit and further re-watering. The low drought tolerant (LDT) form revealed an elevated level of cellular membrane damage accompanied by an increased content of polyunsaturated FFA and triacylglycerol under water deficit, compared with the high drought tolerant (HDT) form. However, the LDT introgression form demonstrated also the ability to regenerate its membranes after stress cessation. The obtained results clearly indicated that accumulation of triacylglycerol under advanced drought in the LDT form could serve as a cellular protective mechanism against overaccumulation of toxic polyunsaturated FFA and other lipid intermediates. Furthermore, accumulation of triacylglycerol under drought conditions could serve also as storage of substrates required for further regeneration of membranes after stress cessation. The rearrangements in triacylglycerol metabolism were supported by the upregulation of several genes, involved in a biosynthesis of triacylglycerol. With respect to this process, diacylglycerol O-acyltransferase DGAT2 seems to play the most important role in the analyzed grasses.

DIACYLGLYCEROL KINASE 5 regulates polar tip growth of tobacco pollen tubes

Pollen tubes require a tightly regulated pectin secretion machinery to sustain the cell wall plasticity required for polar tip growth. Involved in this regulation at the apical plasma membrane are proteins and signaling molecules, including phosphoinositides and phosphatidic acid (PA). However, the contribution of diacylglycerol kinases (DGKs) is not clear. We transiently expressed tobacco DGKs in pollen tubes to identify a plasma membrane (PM)-localized isoform, and then to study its effect on pollen tube growth, pectin secretion and lipid signaling. In order to potentially downregulate DGK5 function, we overexpressed an inactive variant. Only one of eight DGKs displayed a confined localization at the apical PM. We could demonstrate its enzymatic activity and that a kinase-dead variant was inactive. Overexpression of either variant led to differential perturbations including misregulation of pectin secretion. One mode of regulation could be that DGK5-formed PA regulates phosphatidylinositol 4-phosphate 5-kinases, as overexpression of the inactive DGK5 variant not only led to a reduction of PA but also of phosphatidylinositol 4,5-bisphosphate levels and suppressed related growth phenotypes. We conclude that DGK5 is an additional player of polar tip growth that regulates pectin secretion probably in a common pathway with PI4P 5-kinases.

Phosphatidic acid modulates MPK3- and MPK6-mediated hypoxia signaling in Arabidopsis

Phosphatidic acid (PA) is an important lipid essential for several aspects of plant development and biotic and abiotic stress responses. We previously suggested that submergence induces PA accumulation in Arabidopsis thaliana; however, the molecular mechanism underlying PA-mediated regulation of submergence-induced hypoxia signaling remains unknown. Here, we showed that in Arabidopsis, loss of the phospholipase D (PLD) proteins PLDα1 and PLDδ leads to hypersensitivity to hypoxia, but increased tolerance to submergence. This enhanced tolerance is likely due to improvement of PA-mediated membrane integrity. PA bound to the mitogen-activated protein kinase 3 (MPK3) and MPK6 in vitro and contributed to hypoxia-induced phosphorylation of MPK3 and MPK6 in vivo. Moreover, mpk3 and mpk6 mutants were more sensitive to hypoxia and submergence stress compared with wild type, and fully suppressed the submergence-tolerant phenotypes of pldα1 and pldδ mutants. MPK3 and MPK6 interacted with and phosphorylated RELATED TO AP2.12, a master transcription factor in the hypoxia signaling pathway, and modulated its activity. In addition, MPK3 and MPK6 formed a regulatory feedback loop with PLDα1 and/or PLDδ to regulate PLD stability and submergence-induced PA production. Thus, our findings demonstrate that PA modulates plant tolerance to submergence via both membrane integrity and MPK3/6-mediated hypoxia signaling in Arabidopsis.

NAC-mediated membrane lipid remodeling negatively regulates fruit cold tolerance

Low temperatures are known to destroy cell membranes' structural integrity by affecting the remodeling of their phospholipids. Fruits stored at low temperature are prone to chilling injury, characterized by discoloration, absence of ripening, surface pitting, growth inhibition, flavor loss, decay, and wilting. Phosphatidic acid, a vital second-messenger lipid in plants, is known to accumulate in response to different kinds of stress stimuli. However, the regulatory mechanism of its production from the degradation of phospholipids remains poorly understood. We identified two cold-responsive NAC (NAM/ATAF1/CUC2) transcription factors from bananas, namely, MaNAC25 and MaNAC28, which negatively regulated cold tolerance in banana fruits by upregulating the expression of phospholipid degradation genes in banana fruits. Furthermore, MaNAC25 and MaNAC28 formed a positive feedback loop to induce phospholipid degradation and produce phosphatidic acid. In contrast, ethylene directly inhibited the degradation of phospholipids in banana and transgenic tomato fruits. In addition, ethylene reduced the activity of MaNAC25 and MaNAC28, thereby inhibiting phospholipid degradation. To conclude, NAC-mediated membrane lipid remodeling negatively regulates the cold tolerance of banana and transgenic tomato fruits.

Genomic profiling and expression analysis of the diacylglycerol kinase gene family in heterologous hexaploid wheat

The inositol phospholipid signaling system mediates plant growth, development, and responses to adverse conditions. Diacylglycerol kinase (DGK) is one of the key enzymes in the phosphoinositide-cycle (PI-cycle), which catalyzes the phosphorylation of diacylglycerol (DAG) to form phosphatidic acid (PA). To date, comprehensive genomic and functional analyses of DGKs have not been reported in wheat. In this study, 24 DGK gene family members from the wheat genome (TaDGKs) were identified and analyzed. Each putative protein was found to consist of a DGK catalytic domain and an accessory domain. The analyses of phylogenetic and gene structure analyses revealed that each TaDGK gene could be grouped into clusters I, II, or III. In each phylogenetic subgroup, the TaDGKs demonstrated high conservation of functional domains, for example, of gene structure and amino acid sequences. Four coding sequences were then cloned from Chinese Spring wheat. Expression analysis of these four genes revealed that each had a unique spatial and developmental expression pattern, indicating their functional diversification across wheat growth and development processes. Additionally, TaDGKs were also prominently up-regulated under salt and drought stresses, suggesting their possible roles in dealing with adverse environmental conditions. Further cis-regulatory elements analysis elucidated transcriptional regulation and potential biological functions. These results provide valuable information for understanding the putative functions of DGKs in wheat and support deeper functional analysis of this pivotal gene family. The 24 TaDGKs identified and analyzed in this study provide a strong foundation for further exploration of the biological function and regulatory mechanisms of TaDGKs in response to environmental stimuli.

Characterization of the Glehnia littoralis Non-specific Phospholipase C Gene GlNPC3 and Its Involvement in the Salt Stress Response

Glehnia littoralis is a medicinal halophyte that inhabits sandy beaches and has high ecological and commercial value. However, the molecular mechanism of salt adaptation in G. littoralis remains largely unknown. Here, we cloned and identified a non-specific phospholipase C gene (GlNPC3) from G. littoralis, which conferred lipid-mediated signaling during the salt stress response. The expression of GlNPC3 was induced continuously by salt treatment. Overexpression of GlNPC3 in Arabidopsis thaliana increased salt tolerance compared to wild-type (WT) plants. GlNPC3-overexpressing plants had longer roots and higher fresh and dry masses under the salt treatment. The GlNPC3 expression pattern revealed that the gene was expressed in most G. littoralis tissues, particularly in roots. The subcellular localization of GlNPC3 was mainly at the plasma membrane, and partially at the tonoplast. GlNPC3 hydrolyzed common membrane phospholipids, such as phosphotidylserine (PS), phosphoethanolamine (PE), and phosphocholine (PC). In vitro enzymatic assay showed salt-induced total non-specific phospholipase C (NPC) activation in A. thaliana GlNPC3-overexpressing plants. Plant lipid profiling showed a significant change in the membrane-lipid composition of A. thaliana GlNPC3-overexpressing plants compared to WT after the salt treatment. Furthermore, downregulation of GlNPC3 expression by virus-induced gene silencing in G. littoralis reduced the expression levels of some stress-related genes, such as SnRK2, P5SC5, TPC1, and SOS1. Together, these results indicated that GlNPC3 and GlNPC3-mediated membrane lipid change played a positive role in the response of G. littoralis to a saline environment.

Novel QTL and Meta-QTL Mapping for Major Quality Traits in Soybean

Isoflavone, protein, and oil are the most important quality traits in soybean. Since these phenotypes are typically quantitative traits, quantitative trait locus (QTL) mapping has been an efficient way to clarify their complex and unclear genetic background. However, the low-density genetic map and the absence of QTL integration limited the accurate and efficient QTL mapping in previous researches. This paper adopted a recombinant inbred lines (RIL) population derived from 'Zhongdou27'and 'Hefeng25' and a high-density linkage map based on whole-genome resequencing to map novel QTL and used meta-analysis methods to integrate the stable and consentaneous QTL. The candidate genes were obtained from gene functional annotation and expression analysis based on the public database. A total of 41 QTL with a high logarithm of odd (LOD) scores were identified through composite interval mapping (CIM), including 38 novel QTL and 2 Stable QTL. A total of 660 candidate genes were predicted according to the results of the gene annotation and public transcriptome data. A total of 212 meta-QTL containing 122 stable and consentaneous QTL were mapped based on 1,034 QTL collected from previous studies. For the first time, 70 meta-QTL associated with isoflavones were mapped in this study. Meanwhile, 69 and 73 meta-QTL, respectively, related to oil and protein were obtained as well. The results promote the understanding of the biosynthesis and regulation of isoflavones, protein, and oil at molecular levels, and facilitate the construction of molecular modular for great quality traits in soybean.

Small signaling molecules in plant response to cold stress

Cold stress is one of the harsh environmental stresses that adversely affect plant growth and crop yields in the Qinghai-Tibet Plateau. However, plants have evolved mechanisms to overcome the impact of cold stress. Progress has been made in understanding how plants perceive and transduce low-temperature signals to tolerate cold stress. Small signaling molecules are crucial for cellular signal transduction by initiating the downstream signaling cascade that helps plants to respond to cold stress. These small signaling molecules include calcium, reactive oxygen species, nitric oxide, hydrogen sulfide, cyclic guanosine monophosphate, phosphatidic acid, and sphingolipids. The small signaling molecules are involved in many aspects of cellular and physiological functions, such as inducing gene expression and activating hormone signaling, resulting in upregulation of the antioxidant enzyme activities, osmoprotectant accumulation, malondialdehyde reduction, and photosynthesis improvement. We summarize our current understanding of the roles of the small signaling molecules in cold stress in plants, and highlight their crosstalk in cold signaling transduction. These discoveries help us understand how the plateau plants adapt to the severe alpine environment as well as to develop new crops tolerating cold stress in the Qinghai-Tibet Plateau.

Comparative RNA-Seq Analysis Reveals Potentially Resistance-Related Genes in Response to Bacterial Canker of Tomato

Tomato is one of the most important crops for human consumption. Its production is affected by the actinomycete Clavibacter michiganensis subsp. michiganensis (Cmm), one of the most devastating bacterial pathogens of this crop. Several wild tomato species represent a source of natural resistance to Cmm. Here, we contrasted the transcriptomes of the resistant wild tomato species Solanum arcanum LA2157 and the susceptible species Solanum lycopersicum cv. Ailsa Craig, during the first 24 h of challenge with Cmm. We used three analyses approaches which demonstrated to be complementary: mapping to S. lycopersicum reference genome SL3.0; semi de novo transcriptome assembly; and de novo transcriptome assembly. In a global context, transcriptional changes seem to be similar between both species, although there are some specific genes only upregulated in S. arcanum during Cmm interaction, suggesting that the resistance regulatory mechanism probably diverged during the domestication process. Although S. lycopersicum showed enriched functional groups related to defense, S. arcanum displayed a higher number of induced genes related to bacterial, oomycete, and fungal defense at the first few hours of interaction. This study revealed genes that may contribute to the resistance phenotype in the wild tomato species, such as those that encode for a polyphenol oxidase E, diacyl glycerol kinase, TOM1-like protein 6, and an ankyrin repeat-containing protein, among others. This work will contribute to a better understanding of the defense mechanism against Cmm, and the development of new control methods.

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

Synaptotagmins at the endoplasmic reticulum-plasma membrane contact sites maintain diacylglycerol homeostasis during abiotic stress

Endoplasmic reticulum-plasma membrane contact sites (ER-PM CS) play fundamental roles in all eukaryotic cells. Arabidopsis thaliana mutants lacking the ER-PM protein tether synaptotagmin1 (SYT1) exhibit decreased PM integrity under multiple abiotic stresses, such as freezing, high salt, osmotic stress, and mechanical damage. Here, we show that, together with SYT1, the stress-induced SYT3 is an ER-PM tether that also functions in maintaining PM integrity. The ER-PM CS localization of SYT1 and SYT3 is dependent on PM phosphatidylinositol-4-phosphate and is regulated by abiotic stress. Lipidomic analysis revealed that cold stress increased the accumulation of diacylglycerol at the PM in a syt1/3 double mutant relative to wild-type while the levels of most glycerolipid species remain unchanged. In addition, the SYT1-green fluorescent protein fusion preferentially binds diacylglycerol in vivo with little affinity for polar glycerolipids. Our work uncovers a SYT-dependent mechanism of stress adaptation counteracting the detrimental accumulation of diacylglycerol at the PM produced during episodes of abiotic stress.

Genome-wide analysis and expression patterns of lipid phospholipid phospholipase gene family in Brassica napus L

BACKGROUND

Lipid phosphate phosphatases (LPP) are critical for regulating the production and degradation of phosphatidic acid (PA), an essential signaling molecule under stress conditions. Thus far, the LPP family genes have not been reported in rapeseed (Brassica napus L.).

RESULTS

In this study, a genome-wide analysis was carried out to identify LPP family genes in rapeseed that respond to different stress conditions. Eleven BnLPPs genes were identified in the rapeseed genome. Based on phylogenetic and synteny analysis, BnLPPs were classified into four groups (Group I-Group IV). Gene structure and conserved motif analysis showed that similar intron/exon and motifs patterns occur in the same group. By evaluating cis-elements in the promoters, we recognized six hormone- and seven stress-responsive elements. Further, six putative miRNAs were identified targeting three BnLPP genes. Gene ontology analysis disclosed that BnLPP genes were closely associated with phosphatase/hydrolase activity, membrane parts, phosphorus metabolic process, and dephosphorylation. The qRT-PCR based expression profiles of BnLPP genes varied in different tissues/organs. Likewise, several gene expression were significantly up-regulated under NaCl, PEG, cold, ABA, GA, IAA, and KT treatments.

CONCLUSIONS

This is the first report to describe the comprehensive genome-wide analysis of the rapeseed LPP gene family. We identified different phytohormones and abiotic stress-associated genes that could help in enlightening the plant tolerance against phytohormones and abiotic stresses. The findings unlocked new gaps for the functional verification of the BnLPP gene family during stresses, leading to rapeseed improvement.

Transcriptome Co-expression Network Analysis Identifies Key Genes Regulating Conchosporangia Maturation of Pyropia haitanensis

Conchosporangia maturation is crucial for the yield of Pyropia/Porphyra. However, the molecular mechanisms underlying this process are poorly understood. In this study, we selected two strains of Pyropia haitanensis that show significant differences in conchosporangia maturation as materials to produce RNA-Seq libraries. Then, we identified key molecular pathways and genes involved in conchosporangia maturation by conducting a weighted gene co-expression network analysis. Two specific modules were identified, and included functions such as phosphorus metabolism, lipid metabolism, and the phosphatidylinositol signaling system. The hub genes that responded positively during conchosporangia maturation encoded diacylglycerol kinase (DGK) and phosphatidylinositol-3-phosphate-5-kinase, which are involved in the synthesis of phosphatidic acid, a key component of lipid metabolism. A full-length DGK sequence of P. haitanensis, designated as PhDGK1, was obtained by rapid-amplification of cDNA ends. Conserved motif and phylogenetic tree analyses showed that PhDGK1 belongs to DGK Cluster II. The transcript level of PhDGK1 increased during conchosporangia maturation in both strains, but increased earlier, and to higher levels, in the early-maturing strain than in the late-maturing strain. This pattern of gene expression was consistent with the patterns of maturity and changes in pigment contents. These results indicate that lipid metabolism plays a key role in regulating conchosporangia maturation in Pyropia spp., and that PhDGK1 might be a useful molecular marker for breeding new early-maturing strains.

Fight Hard or Die Trying: Current Status of Lipid Signaling during Plant-Pathogen Interaction

Plant diseases pose a substantial threat to food availability, accessibility, and security as they account for economic losses of nearly $300 billion on a global scale. Although various strategies exist to reduce the impact of diseases, they can introduce harmful chemicals to the food chain and have an impact on the environment. Therefore, it is necessary to understand and exploit the plants' immune systems to control the spread of pathogens and enable sustainable agriculture. Recently, growing pieces of evidence suggest a functional myriad of lipids to be involved in providing structural integrity, intracellular and extracellular signal transduction mediators to substantial cross-kingdom cell signaling at the host-pathogen interface. Furthermore, some pathogens recognize or exchange plant lipid-derived signals to identify an appropriate host or development, whereas others activate defense-related gene expression. Typically, the membrane serves as a reservoir of lipids. The set of lipids involved in plant-pathogen interaction includes fatty acids, oxylipins, phospholipids, glycolipids, glycerolipids, sphingolipids, and sterols. Overall, lipid signals influence plant-pathogen interactions at various levels ranging from the communication of virulence factors to the activation and implementation of host plant immune defenses. The current review aims to summarize the progress made in recent years regarding the involvement of lipids in plant-pathogen interaction and their crucial role in signal transduction.

Phospholipases C and D and Their Role in Biotic and Abiotic Stresses

Plants, as sessile organisms, have adapted a fine sensing system to monitor environmental changes, therefore allowing the regulation of their responses. As the interaction between plants and environmental changes begins at the surface, these changes are detected by components in the plasma membrane, where a molecule receptor generates a lipid signaling cascade via enzymes, such as phospholipases (PLs). Phospholipids are the key structural components of plasma membranes and signaling cascades. They exist in a wide range of species and in different proportions, with conversion processes that involve hydrophilic enzymes, such as phospholipase-C (PLC), phospholipase-D (PLD), and phospholipase-A (PLA). Hence, it is suggested that PLC and PLD are highly conserved, compared to their homologous genes, and have formed clusters during their adaptive history. Additionally, they generate responses to different functions in accordance with their protein structure, which should be reflected in specific signal transduction responses to environmental stress conditions, including innate immune responses. This review summarizes the phospholipid systems associated with signaling pathways and the innate immune response.

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.

Membrane Lipids' Metabolism and Transcriptional Regulation in Maize Roots Under Cold Stress

Low temperature is one of the major abiotic stresses that restrict the growth and development of maize seedlings. Membrane lipid metabolism and remodeling are key strategies for plants to cope with temperature stresses. In this study, an integrated lipidomic and transcriptomic analysis was performed to explore the metabolic changes of membrane lipids in the roots of maize seedlings under cold stress (5°C). The results revealed that major extraplastidic phospholipids [phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and phosphatidylinositol (PI)] were dominant membrane lipids in maize root tissues, accounting for more than 70% of the total lipids. In the transcriptome data of maize roots under cold stress, a total of 189 lipid-related differentially expressed genes (DEGs) were annotated and classified into various lipid metabolism pathways, and most of the DEGs were enriched in the "Eukaryotic phospholipid synthesis" (12%), "Fatty acid elongation" (12%), and "Phospholipid signaling" (13%) pathways. Under low temperature stress, the molar percentage of the most abundant phospholipid PC decreased around 10%. The significantly up-regulated expression of genes encoding phospholipase [phospholipase D (PLD)] and phosphatase PAP/LPP genes implied that PC turnover was triggered by cold stress mainly via the PLD pathway. Consequently, as the central product of PC turnover, the level of PA increased drastically (63.2%) compared with the control. The gene-metabolite network and co-expression network were constructed with the prominent lipid-related DEGs to illustrate the modular regulation of metabolic changes of membrane lipids. This study will help to explicate membrane lipid remodeling and the molecular regulation mechanism in field crops encountering low temperature stress.

Link between Lipid Second Messengers and Osmotic Stress in Plants

Plants are subject to different types of stress, which consequently affect their growth and development. They have developed mechanisms for recognizing and processing an extracellular signal. Second messengers are transient molecules that modulate the physiological responses in plant cells under stress conditions. In this sense, it has been shown in various plant models that membrane lipids are substrates for the generation of second lipid messengers such as phosphoinositide, phosphatidic acid, sphingolipids, and lysophospholipids. In recent years, research on lipid second messengers has been moving toward using genetic and molecular approaches to reveal the molecular setting in which these molecules act in response to osmotic stress. In this sense, these studies have established that second messengers can transiently recruit target proteins to the membrane and, therefore, affect protein conformation, activity, and gene expression. This review summarizes recent advances in responses related to the link between lipid second messengers and osmotic stress in plant cells.

Galactolipid and Phospholipid Profile and Proteome Alterations in Soybean Leaves at the Onset of Salt Stress

Salinity is a major environmental factor that constrains soybean yield and grain quality. Given our past observations using the salt-sensitive soybean (Glycine max [L.] Merr.) accession C08 on its early responses to salinity and salt-induced transcriptomic modifications, the aim of this study was to assess the lipid profile changes in this cultivar before and after short-term salt stress, and to explore the adaptive mechanisms underpinning lipid homeostasis. To this end, lipid profiling and proteomic analyses were performed on the leaves of soybean seedlings subjected to salt treatment for 0, 0.5, 1, and 2 h. Our results revealed that short-term salt stress caused dynamic lipid alterations resulting in recycling for both galactolipids and phospholipids. A comprehensive understanding of membrane lipid adaption following salt treatment was achieved by combining time-dependent lipidomic and proteomic data. Proteins involved in phosphoinositide synthesis and turnover were upregulated at the onset of salt treatment. Salinity-induced lipid recycling was shown to enhance jasmonic acid and phosphatidylinositol biosyntheses. Our study demonstrated that salt stress resulted in a remodeling of membrane lipid composition and an alteration in membrane lipids associated with lipid signaling and metabolism in C08 leaves.

Root membrane lipids as potential biomarkers to discriminate silage-corn genotypes cultivated on podzolic soils in boreal climate

Root membrane lipids are important biomolecules determining plant's ability to adapt to different growing environmental or climatic conditions. Herein, we demonstrate the potential use of root membrane lipids as biomarkers to discriminate silage-corn genotypes based on herbicide and insect/pest resistance genetic traits when cultivated on podzolic soils under short growing and moderately warm summer season in boreal climate. Lipids in root membranes of field grown silage-corn genotypes were previously quantified at crop maturity by ultra-high-performance liquid chromatography-hydrophilic interaction chromatography-heated electrospray ionization mass spectrometry. The lipid identified and quantified in silage-corn roots were phospholipids, glycolipids and sphingolipids. Following hierarchical cluster analysis, three groups of membrane lipids were observed to be very effective in segregating the five silage-corn genotypes. The first group consisted of hexosylceramide (HexCer), phosphatidylcholine (PC) and phosphatidylinositol (PI). The second group consisted of lysophosphatidic acid (LPA16:0) and lysophosphatidylcholine (LPC16:0), while the third group consisted of 37 molecular species from observed lipids (phospholipids, glycolipids, sphingolipids). Partial least squares-discriminant analysis (PLS-DA) based on 37 membrane lipid species, as well as principal component analysis using the variables important in projection derived from the PLS-DA segregated the five silage-corn genotypes into three groups according to their pesticide/herbicide resistant traits. This study is second to none using root lipidomics in discriminating different silage-corn genotypes based on their herbicide and insect/pest resistance genetic traits for cultivation in boreal climates. The segregated genotypes possess three different genetic traits for herbicide and insect/pest resistance including VT Double Pro (VT2P), VT Triple Pro Roundup Ready (VT3P/RR) and Roundup Ready-2 corn (RR2). These findings demonstrate that root membrane lipids could serve as appropriate chemical biosignatures to identify silage-corn genotypes based on herbicide and insect/pest resistance genetic traits suitable for cultivation in boreal climates.

GABA signaling in plants: targeting the missing pieces of the puzzle

The adaptation of plants to unstable environments relies on their ability to sense their surroundings and to generate and transmit corresponding signals to different parts of the plant to evoke changes necessary for optimizing growth and defense. Plants, like animals, contain a huge repertoire of intra- and intercellular signals, including organic and inorganic molecules. The occurrence of neurotransmitter-like signaling molecules in plants has been an intriguing field of research. Among these, γ-aminobutyric acid (GABA) was discovered in plants over half a century ago, and studies of its roles as a primary metabolite have been well documented, particularly in the context of stress responses. In contrast, evidence of the potential mechanism by which GABA acts as a signaling molecule in plants has only recently been reported. In spite of this breakthrough, the roles of GABA as a signaling molecule in plants have yet to be established and several aspects of the complexity of the GABA signaling system remain obscure. This review summarizes the uncertainties in GABA signaling in plants and suggests research directions and technologies that would help in answering unsolved questions.

Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.

Phosphatidic acid: an emerging versatile class of cellular mediators

Lipids function not only as the major structural components of cell membranes, but also as molecular messengers that transduce signals to trigger downstream signaling events in the cell. Phosphatidic acid (PA), the simplest and a minor class of glycerophospholipids, is a key intermediate for the synthesis of membrane and storage lipids, and also plays important roles in mediating diverse cellular and physiological processes in eukaryotes ranging from microbes to mammals and higher plants. PA comprises different molecular species that can act differently, and is found in virtually all organisms, tissues, and organellar membranes, with variations in total content and molecular species composition. The cellular levels of PA are highly dynamic in response to stimuli and multiple enzymatic reactions can mediate its production and degradation. Moreover, its unique physicochemical properties compared with other glycerophospholipids allow PA to influence membrane structure and dynamics, and interact with various proteins. PA has emerged as a class of new lipid mediators modulating various signaling and cellular processes via its versatile effects, such as membrane tethering, conformational changes, and enzymatic activities of target proteins, and vesicular trafficking.

Transcriptional Regulation of Lipid Catabolism during Seedling Establishment

Lipid catabolism in germinating seeds provides energy and substrates for initial seedling growth, but how this process is regulated is not well understood. Here, we show that an AT-hook motif-containing nuclear localized (AHL) protein regulates lipid mobilization and fatty acid β-oxidation during seed germination and seedling establishment. AHL4 was identified to directly interact with the lipid mediator phosphatidic acid (PA). Knockout (KO) of AHL4 enhanced, but overexpression (OE) of AHL4 attenuated, triacylglycerol (TAG) degradation and seedling growth. Normal seedling growth of the OE lines was restored by sucrose supplementation to the growth medium. AHL4-OE seedlings displayed decreased expression of genes involved in TAG hydrolysis and fatty acid oxidation, whereas the opposite was observed in AHL4-KOs. These genes contained AHL4-binding cis elements, and AHL4 was shown to bind to the promoter regions of genes encoding the TAG lipases SDP1 and DALL5 and acyl-thioesterase KAT5. These AHL4-DNA interactions were suppressed by PA species that bound to AHL4. These results indicate that AHL4 suppresses lipid catabolism by repressing the expression of specific genes involved in TAG hydrolysis and fatty acid oxidation, and that PA relieves AHL4-mediated suppression and promotes TAG degradation. Thus, AHL4 and PA together regulate lipid degradation during seed germination and seedling establishment.

Knockout of Pi21 by CRISPR/Cas9 and iTRAQ-Based Proteomic Analysis of Mutants Revealed New Insights into M. oryzae Resistance in Elite Rice Line

Rice blast (Magnaporthe oryzae) is a devastating disease affecting rice production globally. The development of cultivars with host resistance has been proved to be the best strategy for disease management. Several rice-resistance genes (R) have been recognized which induce resistance to blast in rice but R gene-mediated mechanisms resulting in defense response still need to be elucidated. Here, mutant lines generated through CRISPR/Cas9 based targeted mutagenesis to investigate the role of Pi21 against blast resistance and 17 mutant plants were obtained in T0 generation with the mutation rate of 66% including 26% bi-allelic, 22% homozygous, 12% heterozygous, and 3% chimeric and 17 T-DNA-free lines in T1 generation. The homozygous mutant lines revealed enhanced resistance to blast without affecting the major agronomic traits. Furthermore, comparative proteome profiling was adopted to study the succeeding proteomic regulations, using iTRAQ-based proteomic analysis. We identified 372 DEPs, among them 149 up and 223 were down-regulated, respectively. GO analysis revealed that the proteins related to response to stimulus, photosynthesis, carbohydrate metabolic process, and small molecule metabolic process were up-regulated. The most of DEPs were involved in metabolic, ribosomal, secondary metabolites biosynthesis, and carbon metabolism pathways. 40S ribosomal protein S15 (P31674), 50S ribosomal protein L4, L5, L6 (Q10NM5, Q9ZST0, Q10L93), 30S ribosomal protein S5, S9 (Q6YU81, Q850W6, Q9XJ28), and succinate dehydrogenase (Q9S827) were hub-proteins. The expression level of genes related to defense mechanism, involved in signaling pathways of jasmonic acid (JA), salicylic acid (SA), and ethylene metabolisms were up-regulated in mutant line after the inoculation of the physiological races of M. oryzae as compared to WT. Our results revealed the fundamental value of genome editing and expand knowledge about fungal infection avoidance in rice.

Functional analysis of phospholipase Dδ family in tobacco pollen tubes

Phosphatidic acid (PA), an important signalling and metabolic phospholipid, is predominantly localized in the subapical plasma membrane (PM) of growing pollen tubes. PA can be produced from structural phospholipids by phospholipase D (PLD), but the isoforms responsible for production of PM PA were not identified yet and their functional roles remain unknown. Following genome-wide bioinformatic analysis of the PLD family in tobacco, we focused on the pollen-overrepresented PLDδ class. Combining live-cell imaging, gene overexpression, lipid-binding and structural bioinformatics, we characterized five NtPLDδ isoforms. Distinct PLDδ isoforms preferentially localize to the cytoplasm or subapical PM. Using fluorescence recovery after photobleaching, domain deletion and swapping analyses we show that membrane-bound PLDδs are tightly bound to PM, primarily via the central catalytic domain. Overexpression analyses suggested isoform PLDδ3 as the most important member of the PLDδ subfamily active in pollen tubes. Moreover, only PLDδ3 shows significant constitutive PLD activity in vivo and, in turn, PA promotes binding of PLDδ3 to the PM. This forms a positive feedback loop leading to PA accumulation and the formation of massive PM invaginations. Tightly controlled production of PA generated by PLDδ3 at the PM is important for maintaining the balance between various membrane trafficking processes that are crucial for plant cell tip growth.

Testing Pollen Tube Proteins for In Vivo Binding to Phosphatidic Acid by n-Butanol Treatment and Confocal Microscopy

The general role of cellular membranes is to provide a barrier and to generate separate reaction spaces. However, additional functions of membrane domains enriched in certain classes of lipids have been discovered, which represent an important area of ongoing research. Such membrane domains can be found in cells at different size scales (e.g., nanodomains, microdomains), represent membrane regions with special physical properties and play important roles in the direct or indirect propagation of signaling processes. Domain formation within the plasma membrane (PM) does not only involve the accumulation of specific lipids, but also the recruitment of specific transmembrane or PM-associated peripheral proteins. Phosphatidic acid (PA) is increasingly recognized as an important signaling lipid and component of PM domains. This lipid is involved in the regulation not only of biotic or abiotic stress responses, but also of pollen tube tip growth and of other forms of polar cell expansion. Although many PA-binding proteins have been characterized, a conserved PA interaction motif could not be identified in these proteins. Consequently, protein binding to PA cannot be predicted based on sequence analysis, but has to be biochemically tested using lipid strip or liposome assays. Although these assays are often informative, they are generally based on the use of artificial model membranes, which compared to natural membranes contain fewer lipid types often at non-physiological concentrations. In this chapter, we describe an alternative in vivo assay that can be employed to analyze protein binding to PA at the PM of normally elongating tobacco pollen tubes. This assay is based on the use of n-butanol (n-ButOH), which inhibits phospholipase D (PLD) and thereby blocks a major biosynthetic pathway that generates PA within the PM from substrates like phosphatidylcholine (PC) or phosphatidylethanolamine (PE). PLD inhibition reduces the PA content of the PM and consequently the level of PM association of PA-binding proteins, which can be analyzed using fluorescence microscopy. Methods enabling n-ButOH treatment of cultured tobacco pollen tubes expressing YFP-tagged PA-binding proteins as well as the quantitative determination of the PM association of these proteins are described.

Comprehensive and quantitative profiling of lipid molecular species by LC-ESI-MS/MS of four native species from semiarid Patagonian Monte

The maintenance of lipid and fatty acids unsaturated composition has been described as one of the mechanisms associated to drought tolerance, but research about the lipid profile in native plants of semiarid environment is still limited. The primary objective was to study whether lipid profiles correlates with drought resistance strategies (tolerant or avoidant) of two life forms (shrubs and grasses). The lipid classes and molecular species of green leaves of Larrea divaricata and Lycium chilense shrubs and Pappostipa speciosa and Poa ligularis grasses were determined using LC-ESI-MS/MS. The soil water content was very low during spring and leaf relative water content was between 47 and 74% in the four species. Lipid profiling was different between both life forms. The prevalent compounds were digalactosyldiacylglycerol (DGDG), monogalactosyldiacylglycerol (MGDG) and phosphatidic acid (PA). The lipid signature shows that L. divaricata adjust its lipid composition to tolerate drought, increasing the content of: a) total lipids and total phospholipids, b) structural phospholipids (36:4 and 36:2-PC, phosphatidylcholine; 36:4-PE, phosphatidylethanolamine), c) chloroplast and mitochondria lipids (32:1 and 32:0-PG, phosphatidylglycerol; 34:3, 36:6 and 36:3-DGDG), d) signaling lipids (34:3, 34:2 and 36:5-PA and PI, phosphatidylinositol), and e) polyunsaturated fatty acids (PUFAs, 18:3 and 18:2) and long chain polyunsaturated fatty acids (LC-PUFAs, in 40:2 and 42:2-PS, phosphatidylserine). This membrane lipid composition contributes to membrane stabilization as metabolic-functional strategy for drought tolerance in the Patagonian Monte. In addition, the 18:3 present in lipids of both grasses could be incorporated to lamb fed based on pastures and result healthy for human dietary.

Potential role of root membrane phosphatidic acid in superior agronomic performance of silage-corn cultivated in cool climate cropping systems

The literature is replete with information describing the composition of the root lipidome in several plant species grown under various environmental conditions. However, it is unknown to what extent the root membrane lipidome vary between silage-corn genotypes, and how such variation could influence agronomic performances during field cultivation in cool climate. To address this issue, the root membrane lipidome and agronomic performance were assessed for five silage-corn genotypes (Fusion-RR, Yukon-R, A4177G3-RIB, DKC23-17RIB, DKC26-28RIB) cultivated under cool climatic conditions. Leaf area, plant height and biomass production were used as agronomic performance indicators. Varieties DKC26-28RIB and Yukon-R expressed significantly higher leaf area, plant height and biomass production compared to the other genotypes. A strong positive Spearman rank-order correlation (P = 0.001) was observed between biomass production and root phosphatidic acid (PA). The high correlation observed between PA and agronomic performance indicates PA could potentially be used as biomarker to assist in the selection of silage-corn genotypes with superior agronomic performance ideally suited for field cultivations in cool climatic conditions.

Membrane Lipid Remodeling in Response to Salinity

Salinity is one of the most decisive environmental factors threatening the productivity of crop plants. Understanding the mechanisms of plant salt tolerance is critical to be able to maintain or improve crop yield under these adverse environmental conditions. Plant membranes act as biological barriers, protecting the contents of cells and organelles from biotic and abiotic stress, including salt stress. Alterations in membrane lipids in response to salinity have been observed in a number of plant species including both halophytes and glycophytes. Changes in membrane lipids can directly affect the properties of membrane proteins and activity of signaling molecules, adjusting the fluidity and permeability of membranes, and activating signal transduction pathways. In this review, we compile evidence on the salt stress responses of the major membrane lipids from different plant tissues, varieties, and species. The role of membrane lipids as signaling molecules in response to salinity is also discussed. Advances in mass spectrometry (MS)-based techniques have largely expanded our knowledge of salt-induced changes in lipids, however only a handful studies have investigated the underlying mechanisms of membrane lipidome regulation. This review provides a comprehensive overview of the recent works that have been carried out on lipid remodeling of plant membranes under salt treatment. Challenges and future perspectives in understanding the mechanisms of salt-induced changes to lipid metabolisms are proposed.

Transcriptome analysis and codominant markers development in caper, a drought tolerant orphan crop with medicinal value

Caper (Capparis spinosa L.) is a xerophytic shrub cultivated for its flower buds and fruits, used as food and for their medicinal properties. Breeding programs and even proper taxonomic classification of the genus Capparis has been hampered so far by the lack of reliable genetic information and molecular markers. Here, we present the first genomic resource for C. spinosa, generated by transcriptomic approach and de novo assembly. The sequencing effort produced nearly 80 million clean reads assembled into 124,723 unitranscripts. Careful annotation and comparison with public databases revealed homologs to genes with a key role in important metabolic pathways linked to abiotic stress tolerance and bio-compounds production, such purine, thiamine and phenylpropanoid biosynthesis, α-linolenic acid and lipid metabolism. Additionally, a panel of genes involved in stomatal development/distribution and encoding for Stress Associated Proteins (SAPs) was also identified. We also used the transcriptomic data to uncover novel molecular markers for caper. Out of 50 SSRs tested, 14 proved polymorphic and represent the first set of SSR markers for the genus Capparis. This transcriptome will be an important contribution to future studies and breeding programs for this orphan crop, aiding to the development of improved varieties to sustain agriculture in arid conditions.