PLD: Phospholipase Ds in Plant Signaling

A stress-inducible protein regulates drought tolerance and flowering time in Brachypodium and Arabidopsis

To cope with environmental stresses and ensure maximal reproductive success, plants have developed strategies to adjust the timing of their transition to reproductive growth. This has a substantial impact on the stress resilience of crops and ultimately on agricultural productivity. Here, we report a previously uncharacterized, plant-specific gene family designated as Regulator of Flowering and Stress (RFS). Overexpression of the BdRFS gene in Brachypodium distachyon delayed flowering, increased biomass accumulation, and promoted drought tolerance, whereas clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated knockout mutants exhibited opposite phenotypes. A double T-DNA insertional mutant in the two Arabidopsis (Arabidopsis thaliana) homologs replicated the effects on flowering and water deprivation seen in the B. distachyon CRISPR knockout lines, highlighting the functional conservation of the family between monocots and dicots. Lipid analysis of B. distachyon and Arabidopsis revealed that digalactosyldiacylglycerol (DGDG) and phosphatidylcholine (PC) contents were significantly, and reciprocally, altered in overexpressor and knockout mutants. Importantly, alteration of C16:0-containing PC, a Flowering Locus T-interacting lipid, associated with flowering phenotype, with elevated levels corresponding to earlier flowering. Co-immunoprecipitation analysis suggested that BdRFS interacts with phospholipase Dα1 as well as several other abscisic acid-related proteins. Furthermore, reduction of C18:3 fatty acids in DGDG corresponded with reduced jasmonic acid metabolites in CRISPR mutants. Collectively, we suggest that stress-inducible RFS proteins represent a regulatory component of lipid metabolism that impacts several agronomic traits of biotechnological importance.

Phospholipase Ds in plants: Their role in pathogenic and symbiotic interactions

Phospholipase Ds (PLDs) are a heterogeneous group of enzymes that are widely distributed in organisms. These enzymes hydrolyze the structural phospholipids of the plasma membrane, releasing phosphatidic acid (PA), an important secondary messenger. Plant PLDs play essential roles in several biological processes, including growth and development, abiotic stress responses, and plant-microbe interactions. Although the roles of PLDs in plant-pathogen interactions have been extensively studied, their roles in symbiotic relationships are not well understood. The establishment of the best-studied symbiotic interactions, those between legumes and rhizobia and between most plants and mycorrhizae, requires the regulation of several physiological, cellular, and molecular processes. The roles of PLDs in hormonal signaling, lipid metabolism, and cytoskeletal dynamics during rhizobial symbiosis were recently explored. However, to date, the roles of PLDs in mycorrhizal symbiosis have not been reported. Here, we present a critical review of the participation of PLDs in the interactions of plants with pathogens, nitrogen-fixing bacteria, and arbuscular mycorrhizal fungi. We describe how PLDs regulate rhizobial and mycorrhizal symbiosis by modulating reactive oxygen species levels, hormonal signaling, cytoskeletal rearrangements, and G-protein activity.

Phospholipase Dα1 Acts as a Negative Regulator of High Mg2+-Induced Leaf Senescence in Arabidopsis

Magnesium (Mg2+) is a macronutrient involved in essential cellular processes. Its deficiency or excess is a stress factor for plants, seriously affecting their growth and development and therefore, its accurate regulation is essential. Recently, we discovered that phospholipase Dα1 (PLDα1) activity is vital in the stress response to high-magnesium conditions in Arabidopsis roots. This study shows that PLDα1 acts as a negative regulator of high-Mg2+-induced leaf senescence in Arabidopsis. The level of phosphatidic acid produced by PLDα1 and the amount of PLDα1 in the leaves increase in plants treated with high Mg2+. A knockout mutant of PLDα1 (pldα1-1), exhibits premature leaf senescence under high-Mg2+ conditions. In pldα1-1 plants, higher accumulation of abscisic and jasmonic acid (JA) and impaired magnesium, potassium and phosphate homeostasis were observed under high-Mg2+ conditions. High Mg2+ also led to an increase of starch and proline content in Arabidopsis plants. While the starch content was higher in pldα1-1 plants, proline content was significantly lower in pldα1-1 compared with wild type plants. Our results show that PLDα1 is essential for Arabidopsis plants to cope with the pleiotropic effects of high-Mg2+ stress and delay the leaf senescence.

Plant phospholipase D: novel structure, regulatory mechanism, and multifaceted functions with biotechnological application

Phospholipases D (PLDs) are important membrane lipid-modifying enzymes in eukaryotes. Phosphatidic acid, the product of PLD activity, is a vital signaling molecule. PLD-mediated lipid signaling has been the subject of extensive research leading to discovery of its crystal structure. PLDs are involved in the pathophysiology of several human diseases, therefore, viewed as promising targets for drug design. The availability of a eukaryotic PLD crystal structure will encourage PLD targeted drug designing. PLDs have been implicated in plants response to biotic and abiotic stresses. However, the molecular mechanism of response is not clear. Recently, several novel findings have shown that PLD mediated modulation of structural and developmental processes, such as: stomata movement, root growth and microtubule organization are crucial for plants adaptation to environmental stresses. Involvement of PLDs in regulating membrane remodeling, auxin mediated alteration of root system architecture and nutrient uptake to combat nitrogen and phosphorus deficiencies and magnesium toxicity is established. PLDs via vesicle trafficking modulate cytoskeleton and exocytosis to regulate self-incompatibility (SI) signaling in flowering plants, thereby contributes to plants hybrid vigor and diversity. In addition, the important role of PLDs has been recognized in biotechnologically important functions, including oil/TAG synthesis and maintenance of seed quality. In this review, we describe the crystal structure of a plant PLD and discuss the molecular mechanism of catalysis and activity regulation. Further, the role of PLDs in regulating plant development under biotic and abiotic stresses, nitrogen and phosphorus deficiency, magnesium ion toxicity, SI signaling and pollen tube growth and in important biotechnological applications has been discussed.

Structural insights into phospholipase D function

Phospholipase D (PLD) and its metabolic active product phosphatidic acid (PA) engage in a wide range of physiopathologic processes in the cell. PLDs have been considered as a potential and promising drug target. Recently, the crystal structures of PLDs in mammalian and plant have been solved at atomic resolution. These achievements allow us to understand the structural differences among different species of PLDs and the functions of their key domains. In this review, we summarize the sequence and structure of different species of PLD isoforms, and discuss the structural mechanisms for PLD interactions with their binding partners and the functions of each key domain in the regulation of PLDs activation and catalytic reaction.

Phospholipase Dα1 mediates the high-Mg2+ stress response partially through regulation of K+ homeostasis

Intracellular levels of Mg²⁺ are tightly regulated, as Mg²⁺ deficiency or excess affects normal plant growth and development. In Arabidopsis, we determined that phospholipase Dα1 (PLDα1) is involved in the stress response to high-magnesium conditions. The T-DNA insertion mutant pldα1 is hypersensitive to increased concentrations of magnesium, exhibiting reduced primary root length and fresh weight. PLDα1 activity increases rapidly after high-Mg²⁺ treatment, and this increase was found to be dose dependent. Two lines harbouring mutations in the HKD motif, which is essential for PLDα1 activity, displayed the same high-Mg²⁺ hypersensitivity of pldα1 plants. Moreover, we show that high concentrations of Mg²⁺ disrupt K⁺ homeostasis, and that transcription of K⁺ homeostasis-related genes CIPK9 and HAK5 is impaired in pldα1. Additionally, we found that the akt1, hak5 double mutant is hypersensitive to high-Mg²⁺ . We conclude that in Arabidopsis, the enzyme activity of PLDα1 is vital in the response to high-Mg²⁺ conditions, and that PLDα1 mediates this response partially through regulation of K⁺ homeostasis.

Dynamic changes in membrane lipid composition of leaves of winter wheat seedlings in response to PEG-induced water stress

BACKGROUND

Membrane lipid composition associates closely with membrane stability and fluidity under water stress. In this study, lipidomic analyses based on electrospray ionization mass spectrometry (ESI-MS/MS) were carried out to explore dynamic changes of membrane lipids in term of molecular species caused by PEG (Polyethylene glycol-6000)-induced water stress in wheat seedlings.

RESULTS

Among the main phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) are primary degradation targets, and PC was degraded in the largest degree. Membrane ion leakage dramatically increased later than the significant reduction of these phospholipids, indicating that the loss of membrane integrity lagged behind severe phospholipid degradation. Monogalactosyldiacylglycerol (MGDG) increased firstly and decreased later, while digalactosyldiacylglycerol (DGDG) ratcheted up with stress. DGDG/MGDG increased after stress for 3 days, and unsaturation of DGDG was promoted with stress. Variation trends of galactolipids differed among molecular species. The time when MGDG (34:3), DGDG (34:3) began to decline approached to the time when non-stomatal limitation impaired photosynthesis. While the two predominant molecular species MGDG (36:6) and DGDG (36:6) began to decline later. So we speculated that MGDG (34:3), DGDG (34:3) might be key components in photosynthesis apparatus and participate in photosynthesis directly. While the two predominant molecular species, MGDG (36:6) and DGDG (36:6) might locate in thylakoid lipid bilayer matrix and play roles in stabilizing the membrane. The research provides new insights into the dynamic response of lipid metabolism to PEG-induced water stress.

CONCLUSION

In wheat plants under water stress, the major molecular species of PC, PE and PG were degraded, MGDG and DGDG molecular species had differing degradation time courses.

Arabidopsis PLDs with C2-domain function distinctively in hypoxia

Hypoxia (oxygen deprivation) causes metabolic disturbances at physiological, biochemical and genetic levels and results in decreased plant growth and development. Phospholipase D (PLD)-mediated signaling was reported for abiotic and biotic stress signaling events in plants. To investigate the participatory role of PLDs also in hypoxia signaling, we used wild type of Arabidopsis thaliana and 10 pld isoform mutants containing C2-domain. Hypoxia-induced changes in three major signaling players, namely, cytosolic free calcium (Ca²⁺ _cyt ), reactive oxygen species (ROS) and phosphatidic acid (PA), were determined in mesophyll protoplasts. The Ca²⁺ _cyt and ROS levels were monitored by fluorescence microscopy and confocal imaging, while PA levels were quantified by an enzymatic method. Our findings reveal that the elevations of cytosolic calcium and PA are reduced in all the 10 mutants dysfunctional in PLD isoforms. The hypoxia-related changes in both calcium and ROS show different kinetic patterns depending on the type of PLD studied. Pharmacological experiments confirm that both external and internal sources contribute to calcium and ROS accumulation under hypoxia. PLDα1-3, PLDβ1 and PLDγ1-3 are likely involved in calcium signaling under hypoxia as well as in PA production, while all investigated PLDs, except for PLDγ3, take part in ROS elevation.

De novo transcriptome sequencing and analysis of genes related to salt stress response in Glehnia littoralis

Soil salinity is one of the major environmental stresses affecting plant growth, development, and reproduction. Salt stress also affects the accumulation of some secondary metabolites in plants. Glehnia littoralis is an endangered medicinal halophyte that grows in coastal habitats. Peeled and dried Glehnia littoralis roots, named Radix Glehniae, have been used traditionally as a Chinese herbal medicine. Although Glehnia littoralis has great ecological and commercial value, salt-related mechanisms in Glehnia littoralis remain largely unknown. In this study, we analysed the transcriptome of Glehnia littoralis in response to salt stress by RNA-sequencing to identify potential salt tolerance gene networks. After de novo assembly, we obtained 105,875 unigenes, of which 75,559 were annotated in public databases. We identified 10,335 differentially expressed genes (DEGs; false discovery rate <0.05 and |log₂ fold-change| ≥ 1) between NaCl treatment (GL2) and control (GL1), with 5,018 upregulated and 5,317 downregulated DEGs. To further this investigation, we performed Gene Ontology (GO) analysis and the Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis. DEGs involved in secondary metabolite biosynthetic pathways, plant signal transduction pathways, and transcription factors in response to salt stress were analysed. In addition, we tested the gene expression of 15 unigenes by quantitative real-time PCR (qRT-PCR) to confirm the RNA-sequencing results. Our findings represent a large-scale assessment of the Glehnia littoralis gene resource, and provide useful information for exploring its molecular mechanisms of salt tolerance. Moreover, genes enriched in metabolic pathways could be used to investigate potential biosynthetic pathways of active compounds by Glehnia littoralis.

The sesquiterpene botrydial from Botrytis cinerea induces phosphatidic acid production in tomato cell suspensions

The phytotoxin botrydial triggers PA production in tomato cell suspensions via PLD and PLC/DGK activation. PLC/DGK-derived PA is partially required for botrydial-induced ROS generation. Phosphatidic acid (PA) is a phospholipid second messenger involved in the induction of plant defense responses. It is generated via two distinct enzymatic pathways, either via phospholipase D (PLD) or by the sequential action of phospholipase C and diacylglycerol kinase (PLC/DGK). Botrydial is a phytotoxic sesquiterpene generated by the necrotrophic fungus Botrytis cinerea that induces diverse plant defense responses, such as the production of reactive oxygen species (ROS). Here, we analyzed PA and ROS production and their interplay upon botrydial treatments, employing tomato (Solanum lycopersicum) cell suspensions as a model system. Botrydial induces PA production within minutes via PLD and PLC/DGK. Either inhibition of PLC or DGK diminishes ROS generation triggered by botrydial. This indicates that PLC/DGK is upstream of ROS production. In tomato, PLC is encoded by a multigene family constituted by SlPLC1-SlPLC6 and the pseudogene SlPLC7. We have shown that SlPLC2-silenced plants have reduced susceptibility to B. cinerea. In this work, we studied the role of SlPLC2 on botrydial-induced PA production by silencing the expression of SlPLC2 via a specific artificial microRNA. Upon botrydial treatments, SlPLC2-silenced-cell suspensions produce PA levels similar to wild-type cells. It can be concluded that PA is a novel component of the plant responses triggered by botrydial.

Membrane Lipid Oscillation: An Emerging System of Molecular Dynamics in the Plant Membrane

Biological rhythm represents a major biological process of living organisms. However, rhythmic oscillation of membrane lipid content is poorly described in plants. The development of lipidomic technology has led to the illustration of precise molecular profiles of membrane lipids under various growth conditions. Compared with conventional lipid signaling, which produces unpredictable lipid changes in response to ever-changing environmental conditions, lipid oscillation generates a fairly predictable lipid profile, adding a new layer of biological function to the membrane system and possible cross-talk with the other chronobiological processes. This mini review covers recent studies elucidating membrane lipid oscillation in plants.

Identification and characterization of the phosphatidic acid-binding A. thaliana phosphoprotein PLDrp1 that is regulated by PLDα1 in a stress-dependent manner

Phospholipase D (PLD) and its cleavage product phosphatidic acid (PA) are crucial in plant stress-signalling. Although some targets of PLD and PA have been identified, the signalling pathway is still enigmatic. This study demonstrates that the phosphoprotein At5g39570, now called PLD-regulated protein1 (PLDrp1), from Arabidopsis thaliana is directly regulated by PLDα1. The protein PLDrp1 can be divided into two regions with distinct properties. The conserved N-terminal region specifically binds PA, while the repeat-rich C-terminal domain suggests interactions with RNAs. The expression of PLDrp1 depends on PLDα1 and the plant water status. Water stress triggers a pldα1-like phenotype in PLDrp1 mutants and induces the expression of PLDrp1 in pldα1 mutants. The regulation of PLDrp1 by PLDα1 and environmental stressors contributes to the understanding of the complex PLD regulatory network and presents a new member of the PA-signalling chain in plants.

Arabidopsis phospholipase D alpha 1-derived phosphatidic acid regulates microtubule organization and cell development under microtubule-interacting drugs treatment

Phospholipase D (PLD) and its product phosphatidic acid (PA) are emerging as essential regulators of cytoskeleton organization in plants. However, the underlying molecular mechanisms of PA-mediated microtubule reorganization in plants remain largely unknown. In this study, we used pharmacological and genetic approaches to analyze the function of Arabidopsis thaliana PLDα1 in the regulation of microtubule organization and cell development in response to microtubule-affecting drugs. Treatment with the microtubule-stabilizing drug paclitaxel resulted in less growth inhibition and decreased rightward slant of roots, longitudinal alignment of microtubules, and enhanced length of hypocotyl epidermal cells in the pldα1 mutant, the phenotype of which was rescued by exogenous application of PA. Moreover, the pldα1 mutant was sensitive to the microtubule-disrupting drugs oryzalin and propyzamide in terms of seedling survival ratio, left-skewing angle of roots and microtubule organization. In addition, both disruption and stabilization of microtubules induced by drugs activated PLDα1 activity. Our findings demonstrate that in A. thaliana, PLDα1/PA might regulate cell development by modulating microtubule organization in an activity-dependent manner.

Phloem Proteomics Reveals New Lipid-Binding Proteins with a Putative Role in Lipid-Mediated Signaling

Global climate changes inversely affect our ability to grow the food required for an increasing world population. To combat future crop loss due to abiotic stress, we need to understand the signals responsible for changes in plant development and the resulting adaptations, especially the signaling molecules traveling long-distance through the plant phloem. Using a proteomics approach, we had identified several putative lipid-binding proteins in the phloem exudates. Simultaneously, we identified several complex lipids as well as jasmonates. These findings prompted us to propose that phloem (phospho-) lipids could act as long-distance developmental signals in response to abiotic stress, and that they are released, sensed, and moved by phloem lipid-binding proteins (Benning et al., 2012). Indeed, the proteins we identified include lipases that could release a signaling lipid into the phloem, putative receptor components, and proteins that could mediate lipid-movement. To test this possible protein-based lipid-signaling pathway, three of the proteins, which could potentially act in a relay, are characterized here: (I) a putative GDSL-motif lipase (II) a PIG-P-like protein, with a possible receptor-like function; (III) and PLAFP (phloem lipid-associated family protein), a predicted lipid-binding protein of unknown function. Here we show that all three proteins bind lipids, in particular phosphatidic acid (PtdOH), which is known to participate in intracellular stress signaling. Genes encoding these proteins are expressed in the vasculature, a prerequisite for phloem transport. Cellular localization studies show that the proteins are not retained in the endoplasmic reticulum but surround the cell in a spotted pattern that has been previously observed with receptors and plasmodesmatal proteins. Abiotic signals that induce the production of PtdOH also regulate the expression of GDSL-lipase and PLAFP, albeit in opposite patterns. Our findings suggest that while all three proteins are indeed lipid-binding and act in the vasculature possibly in a function related to long-distance signaling, the three proteins do not act in the same but rather in distinct pathways. It also points toward PLAFP as a prime candidate to investigate long-distance lipid signaling in the plant drought response.

Regulation of developmental and environmental signaling by interaction between microtubules and membranes in plant cells

Cell division and expansion require the ordered arrangement of microtubules, which are subject to spatial and temporal modifications by developmental and environmental factors. Understanding how signals translate to changes in cortical microtubule organization is of fundamental importance. A defining feature of the cortical microtubule array is its association with the plasma membrane; modules of the plasma membrane are thought to play important roles in the mediation of microtubule organization. In this review, we highlight advances in research on the regulation of cortical microtubule organization by membrane-associated and membrane-tethered proteins and lipids in response to phytohormones and stress. The transmembrane kinase receptor Rho-like guanosine triphosphatase, phospholipase D, phosphatidic acid, and phosphoinositides are discussed with a focus on their roles in microtubule organization.

Proteomic profiling reveals insights into Triticeae stigma development and function

To our knowledge, this study represents the first high-throughput characterization of a stigma proteome in the Triticeae. A total of 2184 triticale mature stigma proteins were identified using three different gel-based approaches combined with mass spectrometry. The great majority of these proteins are described in a Triticeae stigma for the first time. These results revealed many proteins likely to play important roles in stigma development and pollen-stigma interactions, as well as protection against biotic and abiotic stresses. Quantitative comparison of the triticale stigma transcriptome and proteome showed poor correlation, highlighting the importance of having both types of analysis. This work makes a significant contribution towards the elucidation of the Triticeae stigma proteome and provides novel insights into its role in stigma development and function.

Thematic minireview series on phospholipase D and cancer

Phospholipase D (PLD) signaling plays a critical role in cell growth and proliferation, vesicular trafficking, secretion, and endocytosis. At the cellular level, PLD and its reaction product, phosphatidate, interact with a large number of protein partners that are directly related to the actin cytoskeleton and cell migration. Cancer invasion and metastasis rely heavily on cellular motility, and as such, they have put PLD at center stage in cancer research. This minireview series highlights some of the molecular mechanisms that provide evidence for the emerging tumorigenic potential of PLD, the role of the microenvironment, and putative connections with inflammation. PLD represents a potential target for the rational development of therapeutics against cancer and other diseases.