Multiple forms of secretory phospholipase A2 in plants

The enzyme encoded by Myrmecia incisa, a green microalga, phospholipase A2 gene preferentially hydrolyzes arachidonic acid at the sn-2 position of phosphatidylcholine

The enzyme phospholipase A2 (PLA2) plays a crucial role in acyl remodeling of phospholipids via the Lands' cycle, and consequently alters fatty acid compositions in triacylglycerol (TAG). In this study, a full-length cDNA sequence coding Myrmecia incisa phospholipase A2 (MiPLA2) was cloned using the technique of rapid amplification of cDNA ends. Comparison of the 1082-bp cDNA with its corresponding cloned DNA sequence revealed that MiPLA2 contained 3 introns. Mature MiPLA2 (mMiPLA2) had a conserved Ca2+-binding loop and a catalytic site motif that has been recognized in plant secretory PLA2 (sPLA2) proteins. Correspondingly, phylogenetic analysis illustrated that MiPLA2 was clustered within GroupXIA of plant sPLA2 proteins. To ascertain the function of MiPLA2, the cDNA coding for mMiPLA2 was subcloned into the vector pET-32a to facilitate the production of recombinant mMiPLA2 in Escherichia coli. Recombinant mMiPLA2 was purified and used for the in vitro enzyme reaction. Thin-layer chromatography profiles of the catalytic products generated by recombinant mMiPLA2 indicated a specificity for cleaving sn-2 acyl chains from phospholipids, thereby functionally characterizing MiPLA2. Although recombinant mMiPLA2 displayed a strong preference for phosphatidylethanolamine, it preferentially hydrolyzes arachidonic acid (ArA) at the sn-2 position of phosphatidylcholine. Results from the fused expression of p1300-sp-EGFP-mMiPLA2 illustrated that MiPLA2 was localized in the intercellular space of onion epidermis. Furthermore, the positive correlation between MiPLA2 transcription and free ArA levels were established. Consequently, the role of mMiPLA2 in the biosynthesis of ArA-rich TAG was elucidated. This study helps to understand how M. incisa preferentially uses ArA to synthesize TAG.

Genome-wide screening and characterization of phospholipase A (PLA)-like genes in sorghum (Sorghum bicolor L.)

MAIN CONCLUSION

The characterisation of PLA genes in the sorghum genome using in-silico methods revealed their essential roles in cellular processes, providing a foundation for further detailed studies. Sorghum bicolor (L.) Moench is the fifth most cultivated crop worldwide, and it is used in many ways, but it has always gained less popularity due to the yield, pest, and environmental constraints. Improving genetic background and developing better varieties is crucial for better sorghum production in semi-arid tropical regions. This study focuses on the phospholipase A (PLA) family within sorghum, comprehensively characterising PLA genes and their expression across different tissues. The investigation identified 32 PLA genes in the sorghum genome, offering insights into their chromosomal localization, molecular weight, isoelectric point, and subcellular distribution through bioinformatics tools. PLA-like family genes are classified into three groups, namely patatin-related phospholipase A (pPLA), phospholipase A1 (PLA1), and phospholipase A2 (PLA2). In-silico chromosome localization studies revealed that these genes are unevenly distributed in the sorghum genome. Cis-motif analysis revealed the presence of several developmental, tissue and hormone-specific elements in the promoter regions of the PLA genes. Expression studies in different tissues such as leaf, root, seedling, mature seed, immature seed, anther, and pollen showed differential expression patterns. Taken together, genome-wide analysis studies of PLA genes provide a better understanding and critical role of this gene family considering the metabolic processes involved in plant growth, defence and stress response.

More than meets the eye: knowns and unknowns of the trafficking of small secreted proteins in Arabidopsis

Small proteins represent a significant portion of the cargo transported through plant secretory pathways, playing crucial roles in developmental processes, fertilization, and responses to environmental stresses. Despite the importance of small secreted proteins, substantial knowledge gaps persist regarding the regulatory mechanisms governing their trafficking along the secretory pathway, and their ultimate localization or destination. To address these gaps, we conducted a comprehensive literature review, focusing particularly on trafficking and localization of Arabidopsis small secreted proteins with potential biochemical and/or signaling roles in the extracellular space, typically those within the size range of 101-200 amino acids. Our investigation reveals that while at least six members of the 21 mentioned families have a confirmed extracellular localization, eight exhibit intracellular localization, including cytoplasmic, nuclear, and chloroplastic locations, despite the presence of N-terminal signal peptides. Further investigation into the trafficking and secretion mechanisms of small protein cargo could not only deepen our understanding of plant cell biology and physiology but also provide a foundation for genetic manipulation strategies leading to more efficient plant cultivation.

Exploration of the pearl millet phospholipase gene family to identify potential candidates for grain quality traits

BACKGROUND

Phospholipases constitute a diverse category of enzymes responsible for the breakdown of phospholipids. Their involvement in signal transduction with a pivotal role in plant development and stress responses is well documented.

RESULTS

In the present investigation, a thorough genome-wide analysis revealed that the pearl millet genome contains at least 44 phospholipase genes distributed across its 7 chromosomes, with chromosome one harbouring the highest number of these genes. The synteny analysis suggested a close genetic relationship of pearl millet phospholipases with that of foxtail millet and sorghum. All identified genes were examined to unravel their gene structures, protein attributes, cis-regulatory elements, and expression patterns in two pearl millet genotypes contrasting for rancidity. All the phospholipases have a high alpha-helix content and distorted regions within the predicted secondary structures. Moreover, many of these enzymes possess binding sites for both metal and non-metal ligands. Additionally, the putative promoter regions associated with these genes exhibit multiple copies of cis-elements specifically responsive to biotic and abiotic stress factors and signaling molecules. The transcriptional profiling of 44 phospholipase genes in two genotypes contrasting for rancidity across six key tissues during pearl millet growth revealed a predominant expression in grains, followed by seed coat and endosperm. Specifically, the genes PgPLD-alpha1-1, PgPLD-alpha1-5, PgPLD-delta1-7a, PgPLA1-II-1a, and PgPLD-delta1-2a exhibited notable expression in grains of both the genotypes while showing negligible expression in the other five tissues. The sequence alignment of putative promoters revealed several variations including SNPs and InDels. These variations resulted in modifications to the corresponding cis-acting elements, forming distinct transcription factor binding sites suggesting the transcriptional-level regulation for these five genes in pearl millet.

CONCLUSIONS

The current study utilized a genome-wide computational analysis to characterize the phospholipase gene family in pearl millet. A comprehensive expression profile of 44 phospholipases led to the identification of five grain-specific candidates. This underscores a potential role for at least these five genes in grain quality traits including the regulation of rancidity in pearl millet. Therefore, this study marks the first exploration highlighting the possible impact of phospholipases towards enhancing agronomic traits in pearl millet.

Lyso-phosphatidylethanolamine triggers immunity against necrotrophs by promoting JA-signaling and ROS-homeostasis

Modulation of the plant defense response by bioactive molecules is of increasing interest. However, despite plant cell lipids being one of the major cellular components, their role in plant immunity remains elusive. We found that the exogenous application of the cell-membrane localized phospholipid lyso-phosphatidylethanolamine (LPE) reprograms the plant transcript profile in favor of defense-associated genes thereby priming the plant immune system. Exogenous LPE application to different Arabidopsis accessions increases resistance against the necrotrophic pathogens, Botrytis cinerea and Cochliobolus heterostrophus. We found that the immunity-promoting effect of LPE is repealed in the jasmonic acid (JA) receptor mutant coi1, but multiplied in the JA-hypersensitive mutant feronia (fer-4). The JA-signaling repressor JAZ1 is degraded following LPE administration, suggesting that JA-signaling is promoted by LPE. Following LPE-treatment, reactive oxygen species (ROS) accumulation is affected in coi1 and fer-4. Moreover, FER signaling inhibitors of the RALF family are strongly expressed after LPE application, and RALF23 is internalized in stress granules, suggesting the LPE-mediated repression of FER-signaling by promoting RALF function. The in-situ increase of LPE-abundance in the LPE-catabolic mutants lpeat1 and lpeat2 elevates plant resistance to B. cinerea, in contrast to the endogenous LPE-deficient mutant pla2-alpha. We show that LPE increases plant resistance against necrotrophs by promoting JA-signaling and ROS-homeostasis, thereby paving the way for the LPE-targeted genomic engineering of crops to raise their ability to resist biotic threats.

The plant protection preparation GZM improves crop immunity, yield, and quality

Lauryl alcohol, a natural compound found in plants and other organisms, is widely used to make surfactants, food, and pharmaceuticals. GZM, a plant protection preparation with lauryl alcohol as its major component is thought to establish a physical barrier on the plant surface, but its physiological functions are unknown. Here, we show that GZM improves the performance of peanut (Arachis hypogaea) plants in both the laboratory and the field. We demonstrate that the treatment with GZM or lauryl alcohol raises the contents of several specific lysophospholipids and induces the biosynthesis of phenylpropanoids, flavonoids, and wax in various plant species. In the field, GZM improves crop immunity, yield, and quality. In addition, GZM and lauryl alcohol can inhibit the growth of some pathogenic fungi. Our findings provide insights into the physiological and biological effects of GZM treatment on plants and show that GZM and lauryl alcohol are promising preparations in agricultural production.

Comparative phylogenomic and structural analysis of canonical secretory PLA2 and novel PLA2-like family in plants

Plant secretory phospholipase A₂ (sPLA₂) is a family of lipolytic enzymes involved in the sn-2 hydrolysis of phospholipid carboxyester bonds, characterized by the presence of a conserved PA2c domain. PLA₂ produces free fatty acids and lysophospholipids, which regulate several physiological functions, including lipid metabolism, plant growth and development, signal transduction, and response to various environmental stresses. In the present work, we have performed a comparative analysis of PA2c domain-containing genes across plants, focusing on gene distribution, phylogenetic analysis, tissue-specific expression, and homology modeling. Our data revealed the widespread occurrence of multiple sPLA₂ in most land plants and documented single sPLA₂ in multiple algal groups, indicating an ancestral origin of sPLA₂. We described a novel PA2c-containing gene family present in all plant lineages and lacking secretory peptide, which we termed PLA₂-like. Phylogenetic analysis revealed two independent clades in canonical sPLA₂ genes referred to as α and β clades, whereas PLA₂-like genes clustered independently as a third clade. Further, we have explored clade-specific gene expressions showing that while all three clades were expressed in vegetative and reproductive tissues, only sPLA₂-β and PLA₂-like members were expressed in the pollen and pollen tube. To get insight into the conservation of the gene regulatory network of sPLA₂ and PLA₂-like genes, we have analyzed the occurrence of various cis-acting promoter elements across the plant kingdom. The comparative 3D structure analysis revealed conserved and unique features within the PA2c domain for the three clades. Overall, this study will help to understand the evolutionary significance of the PA2c family and lay the foundation for future sPLA₂ and PLA₂-like characterization in plants.

A secretory phospholipase A2 of a fungal pathogen contributes to lipid droplet homeostasis, assimilation of insect-derived lipids, and repression of host immune responses

Secretory phospholipase A2s (sPLA2s) are found in a wide range of organisms from bacteria to higher plants and animals and are involved in varied and cellular processes. However, roles of these enzymes in microbial pathogens remain unclear. Here, an sPLA2 (BbPLA2) was characterized in the filamentous insect pathogenic fungus, Beauveria bassiana. BbPLA2 was exclusively expressed in insect hemolymph-derived cells (hyphal bodies), and its expression was induced by insect-derived nutrients and lipids, and nutrient starvation. High levels of secretion of BbPLA2 were observed as well as its distribution in hyphal body lipid drops (LDs). Overexpression of BbPLA2 increased the ability of B. bassiana to utilize insect-derived nutrients and lipids, and promoted LD accumulation, indicating functions for BbPLA2 in mediating LD homeostasis and assimilation of insect-derived lipids. Strains overexpressing BbPLA2 showed moderately increased virulence, including more efficient penetration of the insect cuticle and evasion of host immune responses as compared to the wild type strain. In addition, B. bassiana-activated host immune genes were downregulated in the BbPLA2 overexpression strain, but upregulated by infections with a ΔBbPLA2 strain. These data demonstrate that BbPLA2 contributes to LD homeostasis, assimilation of insect-derived lipids, and repression of host immune responses.

Role of human group IIA secreted phospholipase A2 in malaria pathophysiology: Insights from a transgenic mouse model

We previously showed that injection of recombinant human group IIA secreted phospholipase A₂ (hGIIA sPLA₂) to Plasmodium chabaudi-infected mice lowers parasitaemia by 20%. Here, we show that transgenic (TG) mice overexpressing hGIIA sPLA₂ have a peak of parasitaemia about 30% lower than WT littermates. During infection, levels of circulating sPLA₂, enzymatic activity and plasma lipid peroxidation were maximal at day-14, the peak of parasitaemia. Levels of hGIIA mRNA increased in liver but not in spleen and blood cells, suggesting that liver may contribute as a source of circulating hGIIA sPLA₂. Before infection, baseline levels of leukocytes and pro-inflammatory cytokines were higher in TG mice than WT littermates. Upon infection, the number of neutrophils, lymphocytes and monocytes increased and were maximal at the peak of parasitaemia in both WT and TG mice, but were higher in TG mice. Similarly, levels of the Th1 cytokines IFN-γ and IL-2 increased in WT and TG mice, but were 7.7- and 1.7-fold higher in TG mice. The characteristic shift towards Th2 cytokines was observed during infection in both WT and TG mice, with increased levels of IL-10 and IL-4 at day-14. The current data are in accordance with our previous in vitro findings showing that hGIIA kills parasites by releasing toxic lipids from oxidized lipoproteins. They further show that hGIIA sPLA₂ is induced during mouse experimental malaria and has a protective in vivo role, lowering parasitaemia by likely releasing toxic lipids from oxidized lipoproteins but also indirectly by promoting a more sustained innate immune response.

Lyso-phosphatidylethanolamine primes the plant immune system and promotes basal resistance against hemibiotrophic pathogens

Background

Lyso-phosphatidylethanolamine (LPE) is a natural phospholipid that functions in the early stages of plant senescence. Plant innate immunity and early leaf senescence share molecular components. To reveal conserved mechanisms that link-up both processes, we tried to unravel to what extent LPE coordinates defense response and by what mode of action.

Result

We found that LPE-treatment induces signaling and biosynthesis gene expression of the defensive hormone salicylic acid (SA). However, jasmonic acid and ethylene triggered gene induction levels are indistinguishable from the control. In accordance with gene induction for SA, oxidative stress, and reactive oxygen species (ROS) production, we detected raised in-situ hydrogen peroxide levels following LPE-application. Yet, ROS-burst assays of LPE-pretreated plants revealed a reduced release of ROS after PAMP-administration suggesting that LPE interferes with an oxidative burst. Our data refer to a priming effect of LPE on SA/ROS-associated genomic loci that encode pivotal factors in early senescence and considerably improve plant basal immunity. Thus, we challenged Arabidopsis thaliana with the hemibiotrophic pathogen Pseudomonas syringae. Consistently, we found an increased resistance in the LPE-pretreated Arabidopsis plants compared to the mock-pretreated control.

Conclusions

Our results underscore a beneficial effect of LPE on plant innate immunity against hemibiotrophs. Given the resistance-promoting effect of exogenously applied LPE, this bio-agent bears the potential of being applied as a valuable tool for the genetic activation of defense-associated traits.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-020-00661-8.

Patatin-Related Phospholipase pPLAIIIγ Involved in Osmotic and Salt Tolerance in Arabidopsis

Patatin-related phospholipases (pPLAs) are acyl-hydrolyzing enzymes implicated in various processes, including lipid metabolism, signal transduction, plant growth and stress responses, but the function for many specific pPLAs in plants remains unknown. Here we determine the effect of patatin-related phospholipase A pPLAIIIγ on Arabidopsis response to abiotic stress. Knockout of pPLAIIIγ rendered plants more sensitive whereas overexpression of pPLAIIIγ enhanced plant tolerance to NaCl and drought in seed germination and seedling growth. The pPLAIIIγ-knockout and overexpressing seedlings displayed a lower and higher level of lysolipids and free fatty acids than that of wild-type plants in response to NaCl stress, respectively. These results indicate that pPLAIIIγ acts a positive regulator of salt and osmatic stress tolerance in Arabidopsis.

Analysis of Phospholipids, Lysophospholipids, and Their Linked Fatty Acyl Chains in Yellow Lupin Seeds (Lupinus luteus L.) by Liquid Chromatography and Tandem Mass Spectrometry

Hydrophilic interaction liquid chromatography (HILIC) and electrospray ionization (ESI) coupled to either Fourier-transform (FT) orbital-trap or linear ion-trap tandem mass spectrometry (LIT-MS/MS) was used to characterize the phospholipidome of yellow lupin (Lupinus luteus) seeds. Phosphatidylcholines (PC) were the most abundant species (41 ± 6%), which were followed by lyso-forms LPC (30 ± 11%), phosphatidylethanolamines (PE, 13 ± 4%), phosphatidylglycerols (PG, 5.1 ± 1.7%), phosphatidic acids (PA, 4.9 ± 1.8%), phosphatidylinositols (PI, 4.7 ± 1.1%), and LPE (1.2 ± 0.5%). The occurrence of both isomeric forms of several LPC and LPE was inferred by a well-defined fragmentation pattern observed in negative ion mode. An unprecedented characterization of more than 200 polar lipids including 52 PC, 42 PE, 42 PA, 35 PG, 16 LPC, 13 LPE, and 10 PI, is reported. The most abundant fatty acids (FA) as esterified acyl chains in PL were 18:1 (oleic), 18:2 (linoleic), 16:0 (palmitic), and 18:3 (linolenic) with relatively high contents of long fatty acyl chains such as 22:0 (behenic), 24:0 (lignoceric), 20:1 (gondoic), and 22:1 (erucic). Their occurrence was confirmed by reversed-phase (RP) LC-ESI-FTMS analysis of a chemically hydrolyzed sample extract in acid conditions at 100 °C for 45 min.

Secretory Phospholipases A2 in Plants

Secreted phospholipases (sPLA2s) in plants are a growing group of enzymes that catalyze the hydrolysis of sn-2 glycerophospholipids to lysophospholipids and free fatty acids. Until today, around only 20 sPLA2s were reported from plants. This review discusses the newly acquired information on plant sPLA2s including molecular, biochemical, catalytic, and functional aspects. The comparative analysis also includes phylogenetic, evolutionary, and tridimensional structure. The observations with emphasis in Glycine max sPLA2 are compared with the available data reported for all plants sPLA2s and with those described for animals (mainly from pancreatic juice and venoms sources).

Functional diversity of glycerolipid acylhydrolases in plant metabolism and physiology

Most current knowledge about plant lipid metabolism has focused on the biosynthesis of lipids and their transport between different organelles. However, lipid composition changes during development and in response to environmental cues often go beyond adjustments of lipid biosynthesis. When lipids have to be removed to adjust the extent of membranes during down regulation of photosynthesis, or lipid composition has to be adjusted to alter the biophysical properties of membranes, or lipid derived chemical signals have to be produced, lipid-degrading enzymes come into play. This review focuses on glycerolipid acylhydrolases that remove acyl groups from glycerolipids and will highlight their roles in lipid remodeling and lipid-derived signal generation. One emerging theme is that these enzymes are involved in the dynamic movement of acyl groups through different lipid pools, for example from polar membrane lipids to neutral lipids sequestered in lipid droplets during de novo triacylglycerol synthesis. Another example of acyl group sequestration in the form of triacylglycerols in lipid droplets is membrane lipid remodeling in response to abiotic stresses. Fatty acids released for membrane lipids can also give rise to potent signaling molecules and acylhydrolases are therefore often the first step in initiating the formation of these lipid signals.

Catch You on the Flip Side: A Critical Review of Flippase Mutant Phenotypes

Lipid flippases are integral membrane proteins that use ATP hydrolysis to power the generation of phospholipid asymmetry between the two leaflets of biological membranes, a process essential for cell survival. Although the first report of a plant lipid flippase was published in 2000, progress in the field has been slow, partially due to the high level of redundancy in this gene family. However, recently an increasing number of reports have examined the physiological function of lipid flippases, mainly in Arabidopsis thaliana. In this review we aim to summarize recent findings on the physiological relevance of lipid flippases in plant adaptation to a changing environment and caution against misinterpretation of pleiotropic effects in genetic studies of flippases.

Recent Advances in the Cellular and Developmental Biology of Phospholipases in Plants

Phospholipases (PLs) are lipid-hydrolyzing enzymes known to have diverse signaling roles during plant abiotic and biotic stress responses. They catalyze lipid remodeling, which is required to generate rapid responses of plants to environmental cues. Moreover, they produce second messenger molecules, such as phosphatidic acid (PA) and thus trigger or modulate signaling cascades that lead to changes in gene expression. The roles of phospholipases in plant abiotic and biotic stress responses have been intensively studied. Nevertheless, emerging evidence suggests that they also make significant contributions to plants' cellular and developmental processes. In this mini review, we summarized recent advances in the study of the cellular and developmental roles of phospholipases in plants.

A Novel Phospholipase A2 Isolated from Palythoa caribaeorum Possesses Neurotoxic Activity

Zoanthids of the genus Palythoa are distributed worldwide in shallow waters around coral reefs. Like all cnidarians, they possess nematocysts that contain a large diversity of toxins that paralyze their prey. This work was aimed at isolating and functionally characterizing a cnidarian neurotoxic phospholipase named A2-PLTX-Pcb1a for the first time. This phospholipase was isolated from the venomous extract of the zoanthid Palythoa caribaeorum. This enzyme, which is Ca²⁺-dependent, is a 149 amino acid residue protein. The analysis of the A2-PLTX-Pcb1a sequence showed neurotoxic domain similitude with other neurotoxic sPLA₂´s, but a different catalytic histidine domain. This is remarkable, since A2-PLTX-Pcb1a displays properties like those of other known PLA₂ enzymes.

Molecular details of secretory phospholipase A2 from flax (Linum usitatissimum L.) provide insight into its structure and function

Secretory phospholipase A₂ (sPLA₂) are low molecular weight proteins (12-18 kDa) involved in a suite of plant cellular processes imparting growth and development. With myriad roles in physiological and biochemical processes in plants, detailed analysis of sPLA₂ in flax/linseed is meagre. The present work, first in flax, embodies cloning, expression, purification and molecular characterisation of two distinct sPLA₂s (I and II) from flax. PLA₂ activity of the cloned sPLA₂s were biochemically assayed authenticating them as bona fide phospholipase A₂. Physiochemical properties of both the sPLA₂s revealed they are thermostable proteins requiring di-valent cations for optimum activity.While, structural analysis of both the proteins revealed deviations in the amino acid sequence at C- & N-terminal regions; hydropathic study revealed LusPLA₂I as a hydrophobic protein and LusPLA₂II as a hydrophilic protein. Structural analysis of flax sPLA₂s revealed that secondary structure of both the proteins are dominated by α-helix followed by random coils. Modular superimposition of LusPLA₂ isoforms with rice sPLA₂ confirmed monomeric structural preservation among plant phospholipase A₂ and provided insight into structure of folded flax sPLA₂s.

Molecular details of secretory phospholipase A2 from flax (Linum usitatissimum L.) provide insight into its structure and function

Secretory phospholipase A2 (sPLA2) are low molecular weight proteins (12-18 kDa) involved in a suite of plant cellular processes imparting growth and development. With myriad roles in physiological and biochemical processes in plants, detailed analysis of sPLA2 in flax/linseed is meagre. The present work, first in flax, embodies cloning, expression, purification and molecular characterisation of two distinct sPLA2s (I and II) from flax. PLA2 activity of the cloned sPLA2s were biochemically assayed authenticating them as bona fide phospholipase A2. Physiochemical properties of both the sPLA2s revealed they are thermostable proteins requiring di-valent cations for optimum activity.While, structural analysis of both the proteins revealed deviations in the amino acid sequence at C- & N-terminal regions; hydropathic study revealed LusPLA2I as a hydrophobic protein and LusPLA2II as a hydrophilic protein. Structural analysis of flax sPLA2s revealed that secondary structure of both the proteins are dominated by α-helix followed by random coils. Modular superimposition of LusPLA2 isoforms with rice sPLA2 confirmed monomeric structural preservation among plant phospholipase A2 and provided insight into structure of folded flax sPLA2s.

Origin and evolution of group XI secretory phospholipase A2 from flax (Linum usitatissimum) based on phylogenetic analysis of conserved domains

Phospholipase A₂ (PLA₂) belongs to class of lipolytic enzymes (EC 3.1.1.4). Lysophosphatidic acid (LPA) and free fatty acids (FFAs) are the products of PLA₂ catalyzed hydrolysis of phosphoglycerides at sn-2 position. LPA and FFA that act as second mediators involved in the development and maturation of plants and animals. Mining of flax genome identified two phospholipase A₂ encoding genes, viz., LusPLA ₂ I and LusPLA ₂ II (Linum usitatissimum secretory phospholipase A₂). Molecular simulation of LusPLA₂s with already characterized plant sPLA₂s revealed the presence of conserved motifs and signature domains necessary to classify them as secretory phospholipase A₂. Phylogenetic analysis of flax sPLA₂ with representative sPLA₂s from other organisms revealed that they evolved rapidly via gene duplication/deletion events and shares a common ancestor. Our study is the first report of detailed phylogenetic analysis for secretory phospholipase A₂ in flax. Comparative genomic analysis of two LusPLA₂s with earlier reported plant sPLA₂s, based on their gene architectures, sequence similarities, and domain structures are presented elucidating the uniqueness of flax sPLA₂.

Identification of a secretory phospholipase A2 from Papaver somniferum L. that transforms membrane phospholipids

The full-length sequence of a new secretory phospholipase A2 was identified in opium poppy seedlings (Papaver somniferum L.). The cDNA of poppy phospholipase A2, denoted as pspla2, encodes a protein of 159 amino acids with a 31 amino acid long signal peptide at the N-terminus. PsPLA2 contains a PLA2 signature domain (PA2c), including the Ca(2+)-binding loop (YGKYCGxxxxGC) and the catalytic site motif (DACCxxHDxC) with the conserved catalytic histidine and the calcium-coordinating aspartate residues. The aspartate of the His/Asp dyad playing an important role in animal sPLA2 catalysis is substituted by a serine residue. Furthermore, the PsPLA2 sequence contains 12 conserved cysteine residues to form 6 structural disulfide bonds. The calculated molecular weight of the mature PsPLA2 is 14.0 kDa. Based on the primary structure PsPLA2 belongs to the XIB group of PLA2s. Untagged recombinant PsPLA2 obtained by expression in Escherichia coli, renaturation from inclusion bodies and purification by cation-exchange chromatography was characterized in vitro. The pH optimum for activity of PsPLA2 was found to be pH 7, when using mixed micelles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and Triton X-100. PsPLA2 specifically cleaves fatty acids from the sn-2 position of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and shows a pronounced preference for PC over phosphatidyl ethanolamine, -glycerol and -inositol. The active recombinant enzyme was tested in vitro against natural phospholipids isolated from poppy plants and preferably released the unsaturated fatty acids, linoleic acid and linolenic acid, from the naturally occurring mixture of substrate lipids.

Bee Venom Phospholipase A2: Yesterday's Enemy Becomes Today's Friend

Bee venom therapy has been used to treat immune-related diseases such as arthritis for a long time. Recently, it has revealed that group III secretory phospholipase A₂ from bee venom (bee venom group III sPLA₂) has in vitro and in vivo immunomodulatory effects. A growing number of reports have demonstrated the therapeutic effects of bee venom group III sPLA₂. Notably, new experimental data have shown protective immune responses of bee venom group III sPLA₂ against a wide range of diseases including asthma, Parkinson’s disease, and drug-induced organ inflammation. It is critical to evaluate the beneficial and adverse effects of bee venom group III sPLA₂ because this enzyme is known to be the major allergen of bee venom that can cause anaphylactic shock. For many decades, efforts have been made to avoid its adverse effects. At high concentrations, exposure to bee venom group III sPLA₂ can result in damage to cellular membranes and necrotic cell death. In this review, we summarized the current knowledge about the therapeutic effects of bee venom group III sPLA₂ on several immunological diseases and described the detailed mechanisms of bee venom group III sPLA₂ in regulating various immune responses and physiopathological changes.

Cloning, expression analysis, and functional characterization of two secretory phospholipases A2 in durum wheat (Triticum durum Desf.)

We previously isolated four cDNAs in durum wheat, TdsPLA2I, TdsPLA2II, TdsPLA2III and TdsPLA2IV, that encode proteins with homology to plant secretory phospholipases A2 (sPLA2s) (Verlotta et al., Int. J. Mol. Sci., 14, 2013, 5146-5169). In this study, we have further characterized TdsPLA2II and TdsPLA2III sequences that, on the basis of our previous findings, might encode sPLA2 isoforms with different features. Functional analysis revealed that, similarly to other known sPLA2s, TdsPLA2II and TdsPLA2III have an optimum at pH 9.0, require Ca(2+), are heat stable, and are inhibited by the disulfide-bond-reducing agent dithiothreitol. However, differences emerged between these TdsPLA2 isoforms. Transcript analysis revealed that the TdsPLA2III gene is highly up-regulated under different environmental stresses; conversely, the TdsPLA2II gene is expressed at constant levels under almost all of the stress conditions examined. Moreover, TdsPLA2II is saturated at micromolar substrate and Ca(2+) concentrations, whereas TdsPLA2III requires millimolar concentrations to reach maximal activity. This suggests that TdsPLA2II normally functions under optimal conditions in vivo, whereas TdsPLA2III is only partially activated, depending on the specific phospholipid and Ca(2+) levels. Altogether these data lead to the hypothesis that in vivo TdsPLA2II and TdsPLA2III are differently regulated at both molecular and biochemical level and that TdsPLA2III plays a major role in durum wheat response to adverse environmental conditions.

A phospholipid uptake system in the model plant Arabidopsis thaliana

Plants use solar energy to produce lipids directly from inorganic elements and are not thought to require molecular systems for lipid uptake from the environment. Here we show that Arabidopsis thaliana Aminophospholipid ATPase10 (ALA10) is a P4-type ATPase flippase that internalizes exogenous phospholipids across the plasma membrane, after which they are rapidly metabolized. ALA10 expression and phospholipid uptake are high in the epidermal cells of the root tip and in guard cells, the latter of which regulate the size of stomatal apertures to modulate gas exchange. ALA10-knockout mutants exhibit reduced phospholipid uptake at the root tips and guard cells and are affected in growth and transpiration. The presence of a phospholipid uptake system in plants is surprising. Our results suggest that one possible physiological role of this system is to internalize lysophosphatidylcholine, a signalling lipid involved in root development and stomatal control.

Auxins action on Glycine max secretory phospholipase A2 is mediated by the interfacial properties imposed by the phytohormones

Secretory phospholipase A2 (sPLA2) are soluble enzymes that catalyze the conversion of phospholipids to lysophospholipids and free fatty acids at membrane interfaces. The effect of IAA and IPA auxins over the activity of recombinant sPLA2 isoforms from Glycine max was studied using membrane model systems including mixed micelles and Langmuir lipid monolayers. Both phytohormones stimulate the activity of both plant sPLA2 using DLPC/Triton mixed micelles as substrate. To elucidate the mechanism of action of the phytohormones, we showed that both auxins are able to self-penetrate lipid monolayers and cause an increment in surface pressure and an expansion of lipid/phytohormone mixed interfaces. The stimulating effect of auxins over phospholipase A2 activity was still present when using Langmuir mixed monolayers as organized substrate regardless of sPLA2 source (plant or animal). All the data suggest that the stimulating effect of auxins over sPLA2 is due to a more favorable interfacial environment rather to a direct effect over the enzyme.

A small phospholipase A2-α from castor catalyzes the removal of hydroxy fatty acids from phosphatidylcholine in transgenic Arabidopsis seeds

Ricinoleic acid, an industrially useful hydroxy fatty acid (HFA), only accumulates to high levels in the triacylglycerol fraction of castor (Ricinus communis) endosperm, even though it is synthesized on the membrane lipid phosphatidylcholine (PC) from an oleoyl ester. The acyl chains of PC undergo intense remodeling through the process of acyl editing. The identities of the proteins involved in this process, however, are unknown. A phospholipase A2 (PLA2) is thought to be involved in the acyl-editing process. We show here a role for RcsPLA2α in the acyl editing of HFA esterified to PC. RcsPLA2α was identified by its high relative expression in the castor endosperm transcriptome. Coexpression in Arabidopsis (Arabidopsis thaliana) seeds of RcsPLA2α with the castor fatty acid hydroxylase RcFAH12 led to a dramatic decrease in seed HFA content when compared with RcFAH12 expression alone in both PC and the neutral lipid fraction. The low-HFA trait was heritable and gene dosage dependent, with hemizygous lines showing intermediate HFA levels. The low seed HFA levels suggested that RcsPLA2α functions in vivo as a PLA2 with HFA specificity. Activity assays with yeast (Saccharomyces cerevisiae) microsomes showed a high specificity of RcsPLA2α for ricinoleic acid, superior to that of the endogenous Arabidopsis PLA2α. These results point to RcsPLA2α as a phospholipase involved in acyl editing, adapted to specifically removing HFA from membrane lipids in seeds.

Kinetic characterization, optimum conditions for catalysis and substrate preference of secretory phospholipase A2 from Glycine max in model membrane systems

Two secretory phospholipase A2 (sPLA2s) from Glycine max, GmsPLA2-IXA-1 and GmsPLA2-XIB-2, have been purified as recombinant proteins and the activity was evaluated in order to obtain the optimum conditions for catalysis using mixed micelles and lipid monolayers as substrate. Both sPLA2s showed a maximum enzyme activity at pH 7 and a requirement of Ca(2+) in the micromolar range. These parameters were similar to those found for animal sPLA2s but a surprising optimum temperature for catalysis at 60 °C was observed. The effect of negative interfacial charges on the hydrolysis of organized substrates was evaluated through initial rate measurements using short chain phospholipids with different head groups. The enzymes showed subtle differences in the specificity for phospholipids with different head groups (DLPC, DLPG, DLPE, DLPA) in presence or absence of NaCl. Both recombinant enzymes showed lower activity toward anionic phospholipids and a preference for the zwitterionic ones. The values of the apparent kinetic parameters (Vmax and KM) demonstrated that these enzymes have more affinity for phosphatidylcholine compared with phosphatidylglycerol, in contrast with the results observed for pancreatic sPLA2. A hopping mode of catalysis was proposed for the action of these sPLA2 on mixed phospholipid/triton micelles. On the other hand, Langmuir-monolayers assays indicated an optimum lateral surface pressure for activity in between 13 and 16 mN/m for both recombinant enzymes.

Lysophosphatidylcholine enhances susceptibility in signaling pathway against pathogen infection through biphasic production of reactive oxygen species and ethylene in tobacco plants

It was previously reported that the amounts of lysophosphatidylcholines (lysoPCs), which are naturally occurring bioactive lipid molecules, significantly increase following pathogen inoculation, as determined using ultraperformance liquid chromatography-quadrupole-time of flight/mass spectrometry analyses. Here, real-time quantitative RT-PCR was performed for the phospholipase A2 (PLA2) genes, Nt1PLA2 and Nt2PLA2, which are responsible for LysoPCs generation. The transcription level of Nt2PLA2 in pathogen-infected tobacco plants transiently peaked at 1h and 36 h, whereas induction of Nt1PLA2 transcription peaked at 36 h. A prominent biphasic ROS accumulation in lysoPC (C18:1(9Z))-treated tobacco leaves was also observed. Transcription of NtRbohD, a gene member of NADPH oxidase, showed biphasic kinetics upon lysoPC 18:1 treatment, as evidenced by an early transient peak in phase I at 1h and a massive peak in phase II at 12h. Each increase in NtACS2 and NtACS4 transcription, gene members of the ACC synthase family, was followed by biphasic peaks of ethylene production after lysoPC 18:1 treatment. This suggested that lysoPC (C18:1)-induced ethylene production was regulated at the transcriptional level of time-dependent gene members. LysoPC 18:1 treatment also rapidly induced cell damage. LysoPC 18:1-induced cell death was almost completely abrogated in ROS generation-impaired transgenic plants (rbohD-as and rbohF-as), ethylene production-impaired transgenic plants (CAS-AS and CAO-AS), and ethylene signaling-impaired transgenic plants (Ein3-AS), respectively. Taken together, pathogen-induced lysoPCs enhance pathogen susceptibility accompanied by ROS and ethylene biosynthesis, resulting in chlorophyll degradation and cell death. Expression of PR genes (PR1-a, PR-3, and PR-4b) and LOX3 was strongly induced in lysoPC 18:1-treated leaves, indicating the involvement of lysoPC 18:1 in the defense response. However, lysoPC 18:1 treatment eventually resulted in cell death, as evidenced by metacaspase gene expression. Therefore, a hypothesis is proposed that the antipathogenic potential of lysoPC 18:1 is dependent on how quickly it is removed from cells for avoidance of lysoPC toxicity.

The Magic Velvet Bean of Mucuna pruriens

Mucuna pruriens (Fabaceae) is an established herbal drug used for the management of male infertility, nervous disorders, and also as an aphrodisiac. It has been shown that its seeds are potentially of substantial medicinal importance. The ancient Indian medical system, Ayurveda, traditionally used M. pruriens, even to treat such things as Parkinson's disease. M. pruriens has been shown to have anti-parkinson and neuroprotective effects, which may be related to its anti-oxidant activity. In addition, anti-oxidant activity of M. pruriens has been also demonstrated in vitro by its ability to scavenge DPPH radicals and reactive oxygen species. In this review the medicinal properties of M. pruriens are summarized, taking in consideration the studies that have used the seeds extracts and the leaves extracts.

The pepper patatin-like phospholipase CaPLP1 functions in plant cell death and defense signaling

Phospholipases hydrolyze phospholipids into fatty acids and other lipophilic substances. Phospholipid signaling is crucial for diverse cellular processes in plants. However, the precise role of phospholipases in plant cell death and defense signaling is not fully understood. Here, we identified a pepper (Capsicum annuum) patatin-like phospholipase (CaPLP1) gene that is transcriptionally induced in pepper leaves by avirulent Xanthomonas campestris pv. vesicatoria (Xcv) infection. CaPLP1 containing an N-terminal signal peptide localized to the cytoplasm and plasma membrane, leading to the secretion into the apoplastic regions. Silencing of CaPLP1 in pepper conferred enhanced susceptibility to Xcv infection. Defense responses to Xcv, including the generation of reactive oxygen species (ROS), hypersensitive cell death and the expression of the salicylic acid (SA)-dependent marker gene CaPR1, were compromised in the CaPLP1-silenced pepper plants. Transient expression of CaPLP1 in pepper leaves induced the accumulation of fluorescent phenolics, expression of the defense marker genes CaPR1 and CaSAR82A, and generation of ROS, ultimately leading to the hypersensitive cell death response. Overexpression (OX) of CaPLP1 in Arabidopsis also conferred enhanced resistance to Pseudomonas syringae pv. tomato (Pst) and Hyaloperonospora arabidopsidis infection. CaPLP1-OX leaves showed reduced Pst growth, enhanced ROS burst and electrolyte leakage, induction of the defense response genes AtPR1, AtRbohD and AtGST, as well as constitutive activation of both the SA-dependent gene AtPR1 and the JA-dependent gene AtPDF1.2. Together, these results suggest that CaPLP1 is involved in plant defense and cell death signaling in response to microbial pathogens.

Secretory Phospholipases A2 in Durum Wheat (Triticum durum Desf.): Gene Expression, Enzymatic Activity, and Relation to Drought Stress Adaptation

Phospholipases A₂ (PLA₂s) are known to mediate signaling cascades during plant growth and development, as well as biotic and abiotic stress responses. In this context, the present study provides extensive characterization of specific PLA₂s in durum wheat, and assesses their involvement in durum wheat response to drought stress. In durum wheat leaves, four full-length expressed sequences encoding putative PLA₂s were isolated and characterized as belonging to the class of secretory PLA₂s (sPLA₂s): TdsPLA₂I, TdsPLA₂II, TdsPLA₂III and TdsPLA₂IV. PLA₂ activity was also detected, the characteristics of which resemble those of previously characterized plant sPLA₂s: strong preference for phospholipids; requirement for millimolar Ca²⁺ concentrations; optimal activity at basic pH; heat stability; and inhibition by the reducing agent dithiothreitol. With drought stress imposed at both the vegetative and reproductive stages, accumulation of TdsPLA₂I and TdsPLA₂III transcripts, and to a lesser extent of TdsPLA₂IV transcript, paralleled increased PLA₂ activity; both transcript levels and enzymatic activity decreased as a consequence of stress recovery. Consistently, free fatty acid analysis of drought-stressed leaves revealed increased linoleate, linolenate and palmitate contents, which were reversed by plant re-watering. Overall, these findings strongly suggest that there are inducible sPLA₂ isoforms in durum wheat that have roles in orchestrating the plant response to drought stress.

The existence of phospholipase A(2) activity in plant mitochondria and its activation by hyperosmotic stress in durum wheat (Triticum durum Desf.)

The activity of mitochondrial phospholipase A(2) (PLA(2)) was shown for the first time in plants. It was observed in etiolated seedlings from durum wheat, barley, tomato, spelt and green seedlings of maize, but not in potato and topinambur tubers and lentil etiolated seedlings. This result was achieved by a novel spectrophotometric assay based on the coupled PLA(2)/lipoxygenase reactions using 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine as substrate; the mitochondrial localisation was assessed by checking recovery of marker enzymes. Durum wheat mitochondrial PLA(2) (DWM-PLA(2)) showed maximal activity at pH 9.0 and 1mM Ca(2+), hyperbolic kinetics (K(m)=90±6μM, V(max)=29±1nmolmin(-1)mg(-1) of protein) and inhibition by methyl arachidonyl fluorophosphonate, 5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)pentanoic acid and palmityl trifluoromethyl ketone. Reactive oxygen species had no effect on DWM-PLA(2), that instead was activated by about 50% and 95%, respectively, under salt (0.21M NaCl) and osmotic (0.42M mannitol) stress imposed during germination. Contrarily, a secondary Ca(2+)-independent activity, having optimum at pH 7.0, was stress-insensitive. We propose that the activation of DWM-PLA(2) is responsible for the strong increase of free fatty acids recently measured in mitochondria under the same stress conditions [Laus, et al., J. Exp. Bot. 62 (2011) 141-154] that, in turn, activate potassium channel and uncoupling protein, able to counteract hyperosmotic stress.

In silico and in vitro characterization of phospholipase A₂ isoforms from soybean (Glycine max)

At the present, no secreted phospholipase A₂ (sPLA₂) from soybean (Glycine max) was investigated in detail. In this work we identified five sequences of putative secreted sPLA₂ from soybean after a BLAST search in G. max database. Sequence analysis showed a conserved PA2c domain bearing the Ca²⁺ binding loop and the active site motif. All the five mature proteins contain 12 cysteine residues, which are commonly conserved in plant sPLA₂s. We propose a phylogenetic tree based on sequence alignment of reported plant sPLA₂s including the novel enzymes from G. max. According to PLA₂ superfamily, two of G. max sPLA₂s are grouped as XIA and the rest of sequences as XIB, on the basis of differences found in their molecular weights and deviating sequences especially in the N- and C-terminal regions of the isoenzymes. Furthermore, we report the cloning, expression and purification of one of the putative isoenzyme denoted as GmsPLA₂-XIA-1. We demonstrate that this mature sPLA₂ of 114 residues had PLA₂ activity on Triton:phospholipid mixed micelles and determine the kinetic parameters for this system. We generate a model based on the known crystal structure of sPLA₂ from rice (isoform II), giving first insights into the three-dimensional structure of folded GmsPLA₂-XIA-1. Besides describing the spatial arrangement of highly conserved pair HIS-49/ASP-50 and the Ca⁺² loop domains, we propose the putative amino acids involved in the interfacial recognition surface. Additionally, molecular dynamics simulations indicate that calcium ion, besides its key function in the catalytic cycle, plays an important role in the overall stability of GmsPLA₂-XIA-1 structure.

Autoproteolytic Activation of a Symbiosis-regulated Truffle Phospholipase A2

Fungal phospholipases are members of the fungal/bacterial group XIV secreted phospholipases A(2) (sPLA(2)s). TbSP1, the sPLA(2) primarily addressed in this study, is up-regulated by nutrient deprivation and is preferentially expressed in the symbiotic stage of the ectomycorrhizal fungus Tuber borchii. A peculiar feature of this phospholipase and of its ortholog from the black truffle Tuber melanosporum is the presence of a 54-amino acid sequence of unknown functional significance, interposed between the signal peptide and the start of the conserved catalytic core of the enzyme. X-ray diffraction analysis of a recombinant TbSP1 form corresponding to the secreted protein previously identified in T. borchii mycelia revealed a structure comprising the five α-helices that form the phospholipase catalytic module but lacking the N-terminal 54 amino acids. This finding led to a series of functional studies that showed that TbSP1, as well as its T. melanosporum ortholog, is a self-processing pro-phospholipase A(2), whose phospholipase activity increases up to 80-fold following autoproteolytic removal of the N-terminal peptide. Proteolytic cleavage occurs within a serine-rich, intrinsically flexible region of TbSP1, does not involve the phospholipase active site, and proceeds via an intermolecular mechanism. Autoproteolytic activation, which also takes place at the surface of nutrient-starved, sPLA(2) overexpressing hyphae, may strengthen and further control the effects of phospholipase up-regulation in response to nutrient deprivation, also in the context of symbiosis establishment and mycorrhiza formation.

Characterization of secretory phospholipase A₂ with phospholipase A₁ activity in tobacco, Nicotiana tabacum (L.)

A cDNA encoding protein with homology to plant secretory phospholipase A₂ (sPLA₂), denoted as Nt1 PLA₂, was isolated from tobacco (Nicotiana tabacum). The cDNA encodes a mature protein of 118 amino acid residues with a putative signal peptide of 29 residues. The mature form of Nt1 PLA₂ has 12 cysteines, Ca²⁺ binding loop and catalytic site domain that are commonly conserved in plant sPLA₂s. The recombinant Nt1 PLA₂ was expressed as a fusion protein with thioredoxin in E. coli BL21 cells and was purified by an ion exchange chromatography after digestion of the fusion proteins by Factor Xa protease to obtain the mature form. Interestingly, Nt1 PLA₂ could hydrolyze the ester bond at the sn-1 position of glycerophospholipids as well as at the sn-2 position, when the activities were determined using mixed-micellar phospholipids with sodium cholate. Both activities for the sn-1 and -2 positions of glycerophospholipids required Ca²⁺ essentially, and maximal activities were found in an alkaline region when phosphatidylcholine, phosphatidylglycerol or phosphatidylethanolamine was used as a substrate. The level of Nt1 PLA₂ mRNA was detected at a higher level in tobacco flowers than stem, leaves and roots, and was induced by salicylic acid.

Rice phospholipase A superfamily: organization, phylogenetic and expression analysis during abiotic stresses and development

Background

Phospholipase A (PLA) is an important group of enzymes responsible for phospholipid hydrolysis in lipid signaling. PLAs have been implicated in abiotic stress signaling and developmental events in various plants species. Genome-wide analysis of PLA superfamily has been carried out in dicot plant Arabidopsis. A comprehensive genome-wide analysis of PLAs has not been presented yet in crop plant rice.

Methodology/Principal Findings

A comprehensive bioinformatics analysis identified a total of 31 PLA encoding genes in the rice genome, which are divided into three classes; phospholipase A₁ (PLA₁), patatin like phospholipases (pPLA) and low molecular weight secretory phospholipase A₂ (sPLA₂) based on their sequences and phylogeny. A subset of 10 rice PLAs exhibited chromosomal duplication, emphasizing the role of duplication in the expansion of this gene family in rice. Microarray expression profiling revealed a number of PLA members expressing differentially and significantly under abiotic stresses and reproductive development. Comparative expression analysis with Arabidopsis PLAs revealed a high degree of functional conservation between the orthologs in two plant species, which also indicated the vital role of PLAs in stress signaling and plant development across different plant species. Moreover, sub-cellular localization of a few candidates suggests their differential localization and functional role in the lipid signaling.

Conclusion/Significance

The comprehensive analysis and expression profiling would provide a critical platform for the functional characterization of the candidate PLA genes in crop plants.

Phospholipases: an overview

Phospholipids are present in all living organisms. They are a major component of all biological membranes, along with glycolipids and cholesterol. Enzymes aimed at cleaving the various bonds in phospholipids, namely phospholipases, are consequently widespread in nature, playing very diverse roles from aggression in snake venom to signal transduction, lipid mediators production, and digestion in humans. Although all phospholipases target phospholipids as substrates, they vary in the site of action on the phospholipids molecules, physiological function, mode of action, and their regulation. Significant studies on phospholipases characterization, physiological role, and industrial potential have been conducted worldwide. Some of them have been directed for biotechnological advances, such as gene discovery and functional enhancement by protein engineering. Others reported phospholipases as virulence factors and major causes of pathophysiological effects. In this introductory chapter, we provide brief details of different phospholipases.

Plant phospholipases: an overview

Plant phospholipases can be grouped into four major types, phospholipase D, phospholipase C, phospholipase A1 (PLA(1)), and phospholipase A2 (PLA(2)), that hydrolyze glycerophospholipids at different ester bonds. Within each type, there are different families or subfamilies of enzymes that can differ in substrate specificity, cofactor requirement, and/or reaction conditions. These differences provide insights into determining the cellular function of specific phospholipases in plants, and they can be explored for different industrial applications.

Phospholipases in action during plant defense signaling

Eukaryotic organisms rely on intricate signaling networks to connect recognition of microbes with the activation of efficient defense reactions. Accumulating evidence indicates that phospholipids are more than mere structural components of biological membranes. Indeed, phospholipid-based signal transduction is widely used in plant cells to relay perception of extracellular signals. Upon perception of the invading microbe, several phospholipid hydrolyzing enzymes are activated that contribute to the establishment of an appropriate defense response. Activation of phospholipases is at the origin of the production of important defense signaling molecules, such as oxylipins and jasmonates, as well as the potent second messenger phosphatidic acid (PA), which has been shown to modulate the activity of a variety of proteins involved in defense signaling. Here, we provide an overview of recent reports describing the different plant phospholipase pathways that are activated during the establishment of plant defense reactions in response to pathogen attack.

Endoplasmic reticulum- and Golgi-localized phospholipase A2 plays critical roles in Arabidopsis pollen development and germination

The phospholipase A(2) (PLA(2)) superfamily of lipolytic enzymes is involved in a number of essential biological processes, such as inflammation, development, host defense, and signal transduction. Despite the proven involvement of plant PLA(2)s in many biological functions, including senescence, wounding, elicitor and stress responses, and pathogen defense, relatively little is known about plant PLA(2)s, and their genes essentially remain uncharacterized. We characterized three of four Arabidopsis thaliana PLA(2) paralogs (PLA(2)-β, -γ, and -δ) and found that they (1) are expressed during pollen development, (2) localize to the endoplasmic reticulum and/or Golgi, and (3) play critical roles in pollen development and germination and tube growth. The suppression of PLA(2) using the RNA interference approach resulted in pollen lethality. The inhibition of pollen germination by pharmacological PLA(2) inhibitors was rescued by a lipid signal molecule, lysophosphatidyl ethanolamine. Based on these results, we propose that plant reproduction, in particular, male gametophyte development, requires the activities of the lipid-modifying PLA(2)s that are conserved in other organisms.

Phospholipase A(2) is required for PIN-FORMED protein trafficking to the plasma membrane in the Arabidopsis root

Phospholipase A(2) (PLA(2)), which hydrolyzes a fatty acyl chain of membrane phospholipids, has been implicated in several biological processes in plants. However, its role in intracellular trafficking in plants has yet to be studied. Here, using pharmacological and genetic approaches, the root hair bioassay system, and PIN-FORMED (PIN) auxin efflux transporters as molecular markers, we demonstrate that plant PLA(2)s are required for PIN protein trafficking to the plasma membrane (PM) in the Arabidopsis thaliana root. PLA(2)alpha, a PLA(2) isoform, colocalized with the Golgi marker. Impairments of PLA(2) function by PLA(2)alpha mutation, PLA(2)-RNA interference (RNAi), or PLA(2) inhibitor treatments significantly disrupted the PM localization of PINs, causing internal PIN compartments to form. Conversely, supplementation with lysophosphatidylethanolamine (the PLA(2) hydrolytic product) restored the PM localization of PINs in the pla(2)alpha mutant and the ONO-RS-082-treated seedling. Suppression of PLA(2) activity by the inhibitor promoted accumulation of trans-Golgi network vesicles. Root hair-specific PIN overexpression (PINox) lines grew very short root hairs, most likely due to reduced auxin levels in root hair cells, but PLA(2) inhibitor treatments, PLA(2)alpha mutation, or PLA(2)-RNAi restored the root hair growth of PINox lines by disrupting the PM localization of PINs, thus reducing auxin efflux. These results suggest that PLA(2), likely acting in Golgi-related compartments, modulates the trafficking of PIN proteins.

Light controls phospholipase A2alpha and beta gene expression in Citrus sinensis

The low-molecular weight secretory phospholipase A2alpha (CssPLA2alpha) and beta (CsPLA2beta) cloned in this study exhibited diurnal rhythmicity in leaf tissue of Citrus sinensis. Only CssPLA2alpha displayed distinct diurnal patterns in fruit tissues. CssPLA2alpha and CsPLA2beta diurnal expression exhibited periods of approximately 24 h; CssPLA2alpha amplitude averaged 990-fold in the leaf blades from field-grown trees, whereas CsPLA2beta amplitude averaged 6.4-fold. Diurnal oscillation of CssPLA2alpha and CsPLA2beta gene expression in the growth chamber experiments was markedly dampened 24 h after transfer to continuous light or dark conditions. CssPLA2alpha and CsPLA2beta expressions were redundantly mediated by blue, green, red and red/far-red light, but blue light was a major factor affecting CssPLA2alpha and CsPLA2beta expression. Total and low molecular weight CsPLA2 enzyme activity closely followed diurnal changes in CssPLA2alpha transcript expression in leaf blades of seedlings treated with low intensity blue light (24 micromol m(-2) s(-1)). Compared with CssPLA2alpha basal expression, CsPLA2beta expression was at least 10-fold higher. Diurnal fluctuation and light regulation of PLA2 gene expression and enzyme activity in citrus leaf and fruit tissues suggests that accompanying diurnal changes in lipophilic second messengers participate in the regulation of physiological processes associated with phospholipase A2 action.

Lipid deacylating enzymes in plants: old activities, new genes

Because lipids are major components of cellular membranes, their degradation under stress conditions compromises compartmentalization. However, in addition to having structural roles, membrane lipids are also implicated in signalling processes involving the activity of lipolytic enzymes. Phospholipases D and C, acting on the polar heads of phospholipids, have been relatively well characterized in plants. In contrast, knowledge of lipid deacylating enzymes remains limited. Lipid acyl hydrolases (LAH) are able to hydrolyse both fatty acid moieties of polar lipids. They differ from phospholipases A(1) or A(2) (PLA) acting on sn-1 or sn-2 positions of phospholipids, respectively, as well as from lipases which de-esterify triacylglycerols. The free polyunsaturated fatty acids generated by deacylating enzymes can be used in the biosynthesis of oxylipins and the lysophospholipids, provided by PLAs, are also bioactive molecules. In the four decades that have passed since the first description of LAH activities in plants some enzymes have been purified. In recent years, the widespread use of molecular approaches together with the attention paid to lipid signalling has contributed to a renewed interest in LAH and has led to the identification of different gene families and the characterization of new enzymes. Additionally, several proteins with putative lipase/esterase signatures have been identified. In the present paper we review currently available data on LAHs, PLAs, triacylglycerol lipases and other putative deacylating enzymes. The roles of lipid deacylating enzymes in plant growth, development and stress responses are discussed in the context of their involvement in membrane deterioration, lipid turnover and cellular signalling.

Crystal structure of a class XIB phospholipase A2 (PLA2): rice (oryza sativa) isoform-2 pla2 and an octanoate complex

Phospholipase A(2) catalyzes the specific hydrolysis of the sn-2 acyl bond of various glycerophospholipids, producing fatty acids and lysophospholipids. Phospholipase A(2)s (PLA(2)s) constitute a large superfamily of enzymes whose products are important for a multitude of signal transduction processes, lipid mediator release, lipid metabolism, development, plant stress responses, and host defense. The crystal structure of rice (Oryza sativa) isoform 2 phospholipase A(2) has been determined to 2.0 A resolution using sulfur SAD phasing, and shows that the class XIb phospholipases have a unique structure compared with other secreted PLA(2)s. The N-terminal half of the chain contains mainly loop structure, including the conserved Ca(2+)-binding loop, but starts with a short 3(10)-helix and also includes two short anti-parallel beta-strands. The C-terminal half is folded into three anti-parallel alpha-helices, of which the two first are also present in other secreted PLA(2)s and contain the conserved catalytic histidine and calcium liganding aspartate residues. The structure is stabilized by six disulfide bonds. The water structure around the calcium ion binding site suggests the involvement of a second water molecule in the mechanism for hydrolysis, the water-assisted calcium-coordinate oxyanion mechanism. The octanoate molecule in the complex structure is bound in a hydrophobic pocket, which extends to the likely membrane interface and is proposed to model the binding of the product fatty acid. Due to the differences in structure, the suggested surface for binding to the membrane has a different morphology in the rice PLA(2) compared with other phospholipases.