C2 domain is responsible for targeting rice phosphoinositide specific phospholipase C

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

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

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.

Genomic-Wide Analysis of the PLC Family and Detection of GmPI-PLC7 Responses to Drought and Salt Stresses in Soybean

Phospholipase C (PLC) performs significant functions in a variety of biological processes, including plant growth and development. The PLC family of enzymes principally catalyze the hydrolysis of phospholipids in organisms. This exhaustive exploration of soybean GmPLC members using genome databases resulted in the identification of 15 phosphatidylinositol-specific PLC (GmPI-PLC) and 9 phosphatidylcholine-hydrolyzing PLC (GmNPC) genes. Chromosomal location analysis indicated that GmPLC genes mapped to 10 of the 20 soybean chromosomes. Phylogenetic relationship analysis revealed that GmPLC genes distributed into two groups in soybean, the PI-PLC and NPC groups. The expression patterns and tissue expression analysis showed that GmPLCs were differentially expressed in response to abiotic stresses. GmPI-PLC7 was selected to further explore the role of PLC in soybean response to drought and salt stresses by a series of experiments. Compared with the transgenic empty vector (EV) control lines, over-expression of GmPI-PLC7 (OE) conferred higher drought and salt tolerance in soybean, while the GmPI-PLC7-RNAi (RNAi) lines exhibited the opposite phenotypes. Plant tissue staining and physiological parameters observed from drought- and salt-stressed plants showed that stress increased the contents of chlorophyll, oxygen free radical (O2 -), hydrogen peroxide (H2O2) and NADH oxidase (NOX) to amounts higher than those observed in non-stressed plants. This study provides new insights in the functional analysis of GmPLC genes in response to abiotic stresses.

Expression and evolution of the phospholipase C gene family in Brachypodium distachyon

BACKGROUND

Phospholipase C (PLC) is an enzyme that hydrolyzes phospholipids and plays an important role in plant growth and development. The Brachypodium distachyon is a model plant of Gramineae, but the research on PLC gene family of Brachypodium has not been reported.

OBJECTIVE

This study was performed to identify the PLC family gene in Brachypodium and to determine the expression profiles of PLCs under the abiotic stress.

METHODS

Complete genome sequences and transcriptomes of Brachypodium were downloaded from the PLAZA. The hidden Markov model-based profile of the conserved PLC domain was submitted as a query to identify all potential PLC domain sequences with HMMER software. Expression profiles of BdPLCs were obtained based on the qRT-PCR analysis.

RESULTS

There were 8 PLC genes in Brachypodium (BdPI-PLCs 1-4 and BdNPCs 1-4). All members of BdPI-PLC had three conserved domains of X, Y, and C2, and no EF-hand was found. All BdNPCs contained a phosphatase domain. BdPI-PLC genes were distributed on Chr1, Chr2 and Chr4, with different types and numbers of cis-regulatory elements in their respective gene promoters. Phylogenetic analysis showed that the genetic relationship between Brachypodium and rice was closer than Arabidopsis. The expression patterns of BdPI-PLC gene under abiotic stresses (drought, low temperature, high temperature and salt stress) were up-regulated, indicated their important roles in response to low temperature, high temperature, drought and salt stresses.

CONCLUSIONS

This study provides comprehensive information for the study of Brachypodium PLC gene family and lays a foundation for further research on the molecular mechanism of Brachypodium stress adaptation.

The quest for osmosensors in plants

Osmotic stress has severe effects on crop productivity. Since climate change is predicted to exacerbate this problem, the development of new crops that are tolerant to osmotic stresses, especially drought and salinity stress, is required. However, only limited success has been achieved to date, primarily because of the lack of a clear understanding of the mechanisms that facilitate osmosensing. Here, we discuss the potential mechanisms of osmosensing in plants. We highlight the roles of proteins such as receptor-like kinases, which sense stress-induced cell wall damage, mechanosensitive calcium channels, which initiate a calcium-induced stress response, and phospholipase C, a membrane-bound enzyme that is integral to osmotic stress perception. We also discuss the roles of aquaporins and membrane-bound histidine kinases, which could potentially detect changes in extracellular osmolarity in plants, as they do in prokaryotes and lower eukaryotes. These putative osmosensors have the potential to serve as master regulators of the osmotic stress response in plants and could prove to be useful targets for the selection of osmotic stress-tolerant crops.

Biochemical characterization of phospholipases C from Coffea arabica in response to aluminium stress

Signal transduction in plants determines their successful adaptation to diverse stress factors. Our group employed suspension cells to study the phosphoinositide pathway, which is triggered by aluminium stress. We investigated about members of the PI-specific phospholipase C (PLC) family and evaluated their transcription profiles in Coffea arabica (Ca) suspension cells after 14days of culture when treated or not with 100μM AlCl₃. The four CaPLC1-4 members showed changes in their transcript abundance upon AlCl₃ treatment. The expression profiles of CaPLC1/2 exhibited a rapid and transitory increase in abundance. In contrast, CaPLC3 and CaPLC4 showed that transcript levels were up-regulated in short times (at 30s), while only CaPLC4 kept high levels and CaPLC3 was reduced to basal after 3h of treatment. CaPLC proteins were heterologously expressed, and CaPLC2 and CaPLC4 were tested for in vitro activity in the presence or absence of AlCl₃ and compared to Arabidopsis PLC2 (AtPLC2). A crude extract was isolated from coffee cells. CaPLC2 showed a similar inhibition (30%) as in AtPLC2 and in the crude extract, while in CaPLC4, the activity was enhanced by AlCl₃. Additionally, we visualized the yellow fluorescent protein PH domain of human PLCδ1 (YFP-PH_PLCδ1) subcellular localization in cells that were treated or not with AlCl₃. In non-treated cells, we observed a polar fluorescence signal towards the fused membrane. However, when cells were treated with AlCl₃, these signals were disrupted. Finally, this is the first time that PLC activity has been shown to be stimulated in vitro by AlCl₃.

SGT1 is required in PcINF1/SRC2-1 induced pepper defense response by interacting with SRC2-1

PcINF1 was previously found to induce pepper defense response by interacting with SRC2-1, but the underlying mechanism remains uninvestigated. Herein, we describe the involvement of SGT1 in the PcINF1/SRC2-1-induced immunity. SGT1 was observed to be up-regulated by Phytophthora capsici inoculation and synergistically transient overexpression of PcINF1/SRC2-1 in pepper plants. SGT1-silencing compromised HR cell death, blocked H₂O₂ accumulation, and downregulated HR-associated and hormones-dependent marker genes’ expression triggered by PcINF1/SRC2-1 co-overexpression. The interaction between SRC2-1 and SGT1 was found by the yeast two hybrid system and was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation analyses. The SGT1/SRC2-1 interaction was enhanced by transient overexpression of PcINF1 and Phytophthora capsici inoculation, and SGT1-silencing attenuated PcINF1/SRC2-1 interaction. Additionally, by modulating subcellular localizations of SRC2-1, SGT1, and the interacting complex of SGT1/SRC2-1, it was revealed that exclusive nuclear targeting of the SGT1/SRC2-1 complex blocks immunity triggered by formation of SGT1/SRC2-1, and a translocation of the SGT1/SRC2-1 complex from the plasma membrane and cytoplasm to the nuclei upon the inoculation of P. capsici. Our data demonstrate that the SGT1/SRC2-1 interaction, and its nucleocytoplasmic partitioning, is involved in pepper’s immunity against P. capsici, thus providing a molecular link between Ca²⁺ signaling associated SRC2-1 and SGT1-mediated defense signaling.

Biochemical characterization of the tomato phosphatidylinositol-specific phospholipase C (PI-PLC) family and its role in plant immunity

Plants possess effective mechanisms to quickly respond to biotic and abiotic stresses. The rapid activation of phosphatidylinositol-specific phospholipase C (PLC) enzymes occurs early after the stimulation of plant immune-receptors. Genomes of different plant species encode multiple PLC homologs belonging to one class, PLCζ. Here we determined whether all tomato homologs encode active enzymes and whether they can generate signals that are distinct from one another. We searched the recently completed tomato (Solanum lycopersicum) genome sequence and identified a total of seven PLCs. Recombinant proteins were produced for all tomato PLCs, except for SlPLC7. The purified proteins showed typical PLC activity, as different PLC substrates were hydrolysed to produce diacylglycerol. We studied SlPLC2, SlPLC4 and SlPLC5 enzymes in more detail and observed distinct requirements for Ca(2+) ions and pH, for both their optimum activity and substrate preference. This indicates that each enzyme could be differentially and specifically regulated in vivo, leading to the generation of PLC homolog-specific signals in response to different stimuli. PLC overexpression and specific inhibition of PLC activity revealed that PLC is required for both specific effector- and more general "pattern"-triggered immunity. For the latter, we found that both the flagellin-triggered response and the internalization of the corresponding receptor, Flagellin Sensing 2 (FLS2) of Arabidopsis thaliana, are suppressed by inhibition of PLC activity. Altogether, our data support an important role for PLC enzymes in plant defence signalling downstream of immune receptors. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

SRC2-1 is required in PcINF1-induced pepper immunity by acting as an interacting partner of PcINF1

Elicitins are elicitors that can trigger hypersensitive cell death in most Nicotiana spp., but their underlying molecular mechanism is not well understood. The gene Phytophthora capsici INF1 (PcINF1) coding for an elicitin from P. capsici was characterized in this study. Transient overexpression of PcINF1 triggered cell death in pepper (Capsicum annuum L.) and was accompanied by upregulation of the hypersensitive response marker, Hypersensitive Induced Reaction gene 1 (HIR1), and the pathogenesis-related genes SAR82, DEF1, BPR1, and PO2. A putative PcINF1-interacting protein, SRC2-1, was isolated from a pepper cDNA library by yeast two-hybrid screening and was observed to target the plasma membrane. The interaction between PcINF1 and SRC2-1 was confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Simultaneous transient overexpression of SRC2-1 and PcINF1 in pepper plants triggered intensive cell death, whereas silencing of SRC2-1 by virus-induced gene silencing blocked the cell death induction of PcINF1 and increased the susceptibility of pepper plants to P. capsici infection. Additionally, membrane targeting of the PcINF1-SRC2-1 complex was required for cell death induction. The C2 domain of SRC2-1 was crucial for SRC2-1 plasma membrane targeting and the PcINF1-SRC2-1 interaction. These results suggest that SRC2-1 interacts with PcINF1 and is required in PcINF1-induced pepper immunity.

Plant phosphoinositide-dependent phospholipases C: variations around a canonical theme

Phosphoinositide-specific phospholipase C (PI-PLC) cleaves, in a Ca(2+)-dependent manner, phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) into diacylglycerol (DAG) and inositol triphosphate (IP3). PI-PLCs are multidomain proteins that are structurally related to the PI-PLCζs, the simplest animal PI-PLCs. Like these animal counterparts, they are only composed of EF-hand, X/Y and C2 domains. However, plant PI-PLCs do not have a conventional EF-hand domain since they are often truncated, while some PI-PLCs have no EF-hand domain at all. Despite this simple structure, plant PI-PLCs are involved in many essential plant processes, either associated with development or in response to environmental stresses. The action of PI-PLCs relies on the mediators they produce. In plants, IP3 does not seem to be the sole active soluble molecule. Inositol pentakisphosphate (IP5) and inositol hexakisphosphate (IP6) also transmit signals, thus highlighting the importance of coupling PI-PLC action with inositol-phosphate kinases and phosphatases. PI-PLCs also produce a lipid molecule, but plant PI-PLC pathways show a peculiarity in that the active lipid does not appear to be DAG but its phosphorylated form, phosphatidic acid (PA). Besides, PI-PLCs can also act by altering their substrate levels. Taken together, plant PI-PLCs show functional differences when compared to their animal counterparts. However, they act on similar general signalling pathways including calcium homeostasis and cell phosphoproteome. Several important questions remain unanswered. The cross-talk between the soluble and lipid mediators generated by plant PI-PLCs is not understood and how the coupling between PI-PLCs and inositol-kinases or DAG-kinases is carried out remains to be established.

Comprehensive genomic analysis and expression profiling of phospholipase C gene family during abiotic stresses and development in rice

BACKGROUND

Phospholipase C (PLC) is one of the major lipid hydrolysing enzymes, implicated in lipid mediated signaling. PLCs have been found to play a significant role in abiotic stress triggered signaling and developmental processes in various plant species. Genome wide identification and expression analysis have been carried out for this gene family in Arabidopsis, yet not much has been accomplished in crop plant rice.

METHODOLOGY/PRINCIPAL FINDINGS

An exhaustive in-silico exploration of rice genome using various online databases and tools resulted in the identification of nine PLC encoding genes. Based on sequence, motif and phylogenetic analysis rice PLC gene family could be divided into phosphatidylinositol-specific PLCs (PI-PLCs) and phosphatidylcholine- PLCs (PC-PLC or NPC) classes with four and five members, respectively. A comparative analysis revealed that PLCs are conserved in Arabidopsis (dicots) and rice (monocot) at gene structure and protein level but they might have evolved through a separate evolutionary path. Transcript profiling using gene chip microarray and quantitative RT-PCR showed that most of the PLC members expressed significantly and differentially under abiotic stresses (salt, cold and drought) and during various developmental stages with condition/stage specific and overlapping expression. This finding suggested an important role of different rice PLC members in abiotic stress triggered signaling and plant development, which was also supported by the presence of relevant cis-regulatory elements in their promoters. Sub-cellular localization of few selected PLC members in Nicotiana benthamiana and onion epidermal cells has provided a clue about their site of action and functional behaviour.

CONCLUSION/SIGNIFICANCE

The genome wide identification, structural and expression analysis and knowledge of sub-cellular localization of PLC gene family envisage the functional characterization of these genes in crop plants in near future.

Plant phosphoinositide-specific phospholipase C: an insight

Phosphoinositide-specific phospholipase C (PI-PLC) belongs to an important class of enzymes involved in signaling related to lipids. They hydrolyze a membrane-associated phospholipid, phosphatidylinositol-4,5-bisphosphate, to produce inositol-1,4,5-trisphosphate and diacylglycerol. The role of PI-PLC and the mechanism behind its functioning is well studied in animal system; however, mechanism of plant PI-PLC functioning remains largely obscure. Here, we attempted to summarize the understanding regarding plant PI-PLC mechanism of regulation, localization, and domain association. Using sedimentation based phospholipid binding assay and surface plasmon resonance spectroscopy, it was demonstrated that C2 domain of plant PI-PLC alone is capable of targeting membranes. Moreover, change in surface hydrophobicity upon calcium stimulus is the key element in targeting plant PI-PLC from soluble fractions to membranes. This property of altering surface hydrophobicity plays a pivot role in regulation of PI-PLC activity.

Regulation of lipid biosynthesis by phosphatidylinositol-specific phospholipase C through the transcriptional repression of upstream activating sequence inositol containing genes

The regulation of phospholipid biosynthesis in Saccharomyces cerevisiae through cis-acting upstream activating sequence inositol (UAS(ino)) and trans-acting elements, such as the INO2-INO4 complex and OPI1 by inositol supplementation in growth is thoroughly studied. In this study, we provide evidence for the regulation of lipid biosynthesis by phosphatidylinositol-specific phospholipase C (PLC) through UAS(ino) and the trans-acting elements. Gene expression analysis and radiolabelling experiments demonstrated that the overexpression of rice PLC in yeast cells altered phospholipid biosynthesis at the levels of transcriptional and enzyme activity. This is the first report implicating PLC in the direct regulation of lipid biosynthesis.