The oleate-stimulated phospholipase D, PLDdelta, and phosphatidic acid decrease H2O2-induced cell death in Arabidopsis

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.

Arabidopsis thaliana membrane lipid molecular species and their mass spectral analysis

Herein, current approaches to electrospray ionization mass spectrometry-based analyses of membrane lipid molecular species found in Arabidopsis thaliana are summarized. Additionally, the identities of over 500 reported membrane lipid molecular species are assembled.

Crosstalk between Phospholipase D and Sphingosine Kinase in Plant Stress Signaling

The activation of phospholipase D (PLD) produces phosphatidic acid (PA), whereas plant sphingosine kinase (SPHK) phosphorylates long-chain bases to generate long-chain base-1-phosphates such as phytosphingosine-1-phosphate (phyto-S1P). PA and phyto-S1P have been identified as lipid messengers. Recent studies have shown that PA interacts directly with SPHKs in Arabidopsis, and that the interaction promotes SPHK activity. However, SPHK and phyto-S1P act upstream of PLDα1 and PA in the stomatal response to abscisic acid (ABA). These findings indicate that SPHK/phyto-S1P and PLD/PA are co-dependent in the amplification of lipid messengers, and that crosstalk between the sphingolipid- and phospholipid-mediated signaling pathways may play important roles in plant stress signaling.

Disturbance of reactive oxygen species homeostasis induces atypical tubulin polymer formation and affects mitosis in root-tip cells of Triticum turgidum and Arabidopsis thaliana

In this study, the effects of disturbance of the reactive oxygen species (ROS) homeostasis on the organization of tubulin cytoskeleton in interphase and mitotic root-tip cells of Triticum turgidum and Arabidopsis thaliana were investigated. Reduced ROS levels were obtained by treatment with diphenylene iodonium (DPI) and N-acetyl-cysteine, whereas menadione was applied to achieve ROS overproduction. Both increased and low ROS levels induced: (a) Macrotubule formation in cells with low ROS levels and tubulin paracrystals under oxidative stress. The protein MAP65-1 was detected in treated cells, exhibiting a conformation comparable to that of the atypical tubulin polymers. (b) Disappearance of microtubules (MTs). (c) Inhibition of preprophase band formation. (d) Delay of the nuclear envelope breakdown at prometaphase. (e) Prevention of perinuclear tubulin polymer assembly in prophase cells. (f) Loss of bipolarity of prophase, metaphase and anaphase spindles. Interestingly, examination of the A. thaliana rhd2/At respiratory burst oxidase homolog C (rbohc) NADPH oxidase mutant, lacking RHD2/AtRBOHC, gave comparable results. Similarly to DPI, the decreased ROS levels in rhd2 root-tip cells, interfered with MT organization and induced macrotubule assembly. These data indicate, for first time in plants, that ROS are definitely implicated in: (a) mechanisms controlling the assembly/disassembly of interphase, preprophase and mitotic MT systems and (b) mitotic spindle function. The probable mechanisms, by which ROS affect these processes, are discussed.

Hydrogen peroxide modulates the dynamic microtubule cytoskeleton during the defence responses to Verticillium dahliae toxins in Arabidopsis

The molecular mechanisms of signal transduction of plants in response to infection by Verticillium dahliae (VD) are not well understood. We previously showed that NO may act as an upstream signalling molecule to trigger the depolymerization of cortical microtubules in Arabidopsis. In the present study, we used the wild-type, and atrbohD and atrbohF mutants of Arabidopsis to explore the mechanisms of action of H(2)O(2) signals and the dynamic microtubule cytoskeleton in defence responses. We demonstrated that H(2)O(2) may also act as an upstream signalling molecule to regulate cortical microtubule depolymerization. The depolymerization of the cortical microtubules played a functional role in the signalling pathway to mediate the expression of defence genes. The results indicate that H(2)O(2) modulates the dynamic microtubule cytoskeleton to trigger the expression of defence genes against V. dahliae toxins (VD-toxins) in Arabidopsis.

Inositol polyphosphate 5-phosphatase7 regulates the production of reactive oxygen species and salt tolerance in Arabidopsis

Plants possess remarkable ability to adapt to adverse environmental conditions. The adaptation process involves the removal of many molecules from organelles, especially membranes, and replacing them with new ones. The process is mediated by an intracellular vesicle-trafficking system regulated by phosphatidylinositol (PtdIns) kinases and phosphatases. Although PtdIns comprise a fraction of membrane lipids, they function as major regulators of stress signaling. We analyzed the role of PtdIns 5-phosphatases (5PTases) in plant salt tolerance. The Arabidopsis (Arabidopsis thaliana) genome contains 15 At5PTases. We analyzed salt sensitivity in nine At5ptase mutants and identified one (At5ptase7) that showed increased sensitivity, which was improved by overexpression. At5ptase7 mutants demonstrated reduced production of reactive oxygen species (ROS). Supplementation of mutants with exogenous PtdIns dephosphorylated at the D5' position restored ROS production, while PtdIns(4,5)P(2), PtdIns(3,5)P(2), or PtdIns(3,4,5)P(3) were ineffective. Compromised salt tolerance was also observed in mutant NADPH Oxidase, in agreement with the low ROS production and salt sensitivity of PtdIns 3-kinase mutants and with the inhibition of NADPH oxidase activity in wild-type plants. Localization of green fluorescent protein-labeled At5PTase7 occurred in the plasma membrane and nucleus, places that coincided with ROS production. Analysis of salt-responsive gene expression showed that mutants failed to induce the RD29A and RD22 genes, which contain several ROS-dependent elements in their promoters. Inhibition of ROS production by diphenylene iodonium suppressed gene induction. In summary, our results show a nonredundant function of At5PTase7 in salt stress response by regulating ROS production and gene expression.

Tobacco Transcription Factor NtWRKY12 Interacts with TGA2.2 in vitro and in vivo

The promoter of the salicylic acid-inducible PR-1a gene of Nicotiana tabacum contains binding sites for transcription factor NtWRKY12 (WK-box at position -564) and TGA factors (as-1-like element at position -592). Transactivation experiments in Arabidopsis protoplasts derived from wild type, npr1-1, tga256, and tga2356 mutant plants revealed that NtWRKY12 alone was able to induce a PR-1a::β-glucuronidase (GUS) reporter gene to high levels, independent of co-expressed tobacco NtNPR1, TGA2.1, TGA2.2, or endogenous Arabidopsis NPR1, TGA2/3/5/6. By in vitro pull-down assays with GST and Strep fusion proteins and by Fluorescence Resonance Energy Transfer assays with protein-CFP and protein-YFP fusions in transfected protoplasts, it was shown that NtWRKY12 and TGA2.2 could interact in vitro and in vivo. Interaction of NtWRKY12 with TGA1a or TGA2.1 was not detectable by these techniques. A possible mechanism for the role of NtWRKY12 and TGA2.2 in PR-1a gene expression is discussed.

The OXI1 kinase pathway mediates Piriformospora indica-induced growth promotion in Arabidopsis

Piriformospora indica is an endophytic fungus that colonizes roots of many plant species and promotes growth and resistance to certain plant pathogens. Despite its potential use in agriculture, little is known on the molecular basis of this beneficial plant-fungal interaction. In a genetic screen for plants, which do not show a P. indica- induced growth response, we isolated an Arabidopsis mutant in the OXI1 (Oxidative Signal Inducible1) gene. OXI1 has been characterized as a protein kinase which plays a role in pathogen response and is regulated by H₂O₂ and PDK1 (3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1). A genetic analysis showed that double mutants of the two closely related PDK1.1 and PDK1.2 genes are defective in the growth response to P. indica. While OXI1 and PDK1 gene expression is upregulated in P. indica-colonized roots, defense genes are downregulated, indicating that the fungus suppresses plant defense reactions. PDK1 is activated by phosphatidic acid (PA) and P. indica triggers PA synthesis in Arabidopsis plants. Under beneficial co-cultivation conditions, H₂O₂ formation is even reduced by the fungus. Importantly, phospholipase D (PLD)α1 or PLDδ mutants, which are impaired in PA synthesis do not show growth promotion in response to fungal infection. These data establish that the P. indica-stimulated growth response is mediated by a pathway consisting of the PLD-PDK1-OXI1 cascade.

Like many root-colonizing microbes, the primitive Basidiomycete fungus Piriformospora indica colonizes the roots of many plant species and promotes their growth. The lack of host specificity suggests that the plant response to this endopyhte is based on general signalling processes. In a genetic screen for Arabidopsis plants, which do not show a P. indica-induced growth response, we isolated a mutant in the OXI1 (Oxidative Signal Inducible1) gene. Previously, this protein kinase has been shown to play a role in pathogen response and is regulated by H₂O₂ and PDK1 (3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1). A genetic analysis showed that deletion of PDK1 also abolishes the growth response to P. indica. PDK1 is activated by phosphatidic acid (PA). P. indica triggers PA synthesis and mutants impaired in PA synthesis do not show growth promotion in response to fungal infection. Since defense processes are repressed by P. indica, we propose that a pathway consisting of the PLD-PDK1-OXI1 cascade mediates the P. indica-induced growth response.

Phospholipid and triacylglycerol profiles modified by PLD suppression in soybean seed

Phospholipase D (PLD) is capable of hydrolyzing membrane phospholipids, producing phosphatidic acid. To alter phospholipid profiles in soybean seed, we attenuated PLD enzyme activity by an RNA interference construct using the partial sequence from a soybean PLDα gene. Two transgenic soybean lines were established by particle inflow gun (PIG) bombardment by co-bombarding with pSPLDi and pHG1 vectors. The lines were evaluated for the presence and expression of transgenes thoroughly through the T(4) generation. PLD-suppressed soybean lines were characterized by decreased PLDα enzyme activity and decreased PLDα protein both during seed development and in mature seeds. There was no change in total phospholipid amount; however, the PLD-attenuated transgenic soybean seed had higher levels of di18:2 (dilinoleoyl)-phosphatidylcholine (PC) and -phosphatidylethanolamine (PE) in seeds than the non-transgenic lines. The increased polyunsaturation was at the expense of PC and PE species containing monounsaturated or saturated fatty acids. In addition to increased unsaturation in the phospholipids, there was a decrease in unsaturation of the triacylglycerol (TAG) fraction of the soybean seeds. Considering recent evidence for the notion that desaturation of fatty acids occurs in the PC fraction and that the PC→DAG (diacylglycerol)→TAG pathway is the major route of TAG biosynthesis in developing soybean seed, the current data suggest that PLDα suppression slows the conversion of PC to TAG. This would be consistent with PLD playing a positive role in that conversion. The data indicate that soybean PLD attenuation is a potentially useful approach to altering properties of edible and industrial soybean lecithin.

Molecular, cellular, and physiological responses to phosphatidic acid formation in plants

Phosphatidic acid (PA) is an essential phospholipid involved in membrane biosynthesis and signal transduction in all eukaryotes. This review focuses on its role as lipid second messenger during plant stress, metabolism, and development. The contribution of different individual isoforms of enzymes that generate and break down PA will be discussed and the downstream responses highlighted, with particular focus on proteins that bind PA. Through characterization of several of these PA targets, a molecular and genetic basis for PA's role in plant stress and development is emerging.

Patatin-related phospholipase A: nomenclature, subfamilies and functions in plants

The release of fatty acids from membrane glycerolipids has been implicated in a variety of cellular processes, but the enzymes involved and their regulation are poorly understood in plants. One large group of acyl-hydrolyzing enzymes is structurally related to patatins. Patatins are potato tuber proteins with acyl-hydrolyzing activity, and the patatin catalytic domain is widely spread in bacterial, yeast, plant and animal enzymes. Recent results have indicated that patatin-related enzymes are involved in different cellular functions, including plant responses to auxin, elicitors or pathogens, and abiotic stresses and lipid mobilization during seed germination. In this review, we highlight recent developments regarding these enzymes and propose the nomenclature pPLA for the patatin-related phospholipase A enzyme.

Exogenously induced expression of ethylene biosynthesis, ethylene perception, phospholipase D, and Rboh-oxidase genes in broccoli seedlings

In higher plants, copper ions, hydrogen peroxide, and cycloheximide have been recognized as very effective inducers of the transcriptional activity of genes encoding the enzymes of the ethylene biosynthesis pathway. In this report, the transcriptional patterns of genes encoding the 1-aminocyclopropane-1-carboxylate synthases (ACSs), 1-aminocyclopropane-1-carboxylate oxidases (ACOs), ETR1, ETR2, and ERS1 ethylene receptors, phospholipase D (PLD)-alpha1, -alpha2, -gamma1, and -delta, and respiratory burst oxidase homologue (Rboh)-NADPH oxidase-D and -F in response to these inducers in Brassica oleracea etiolated seedlings are shown. ACS1, ACO1, ETR2, PLD-gamma1, and RbohD represent genes whose expression was considerably affected by all of the inducers used. The investigations were performed on the seedlings with (i) ethylene insensitivity and (ii) a reduced level of the PLD-derived phosphatidic acid (PA). The general conclusion is that the expression of ACS1, -3, -4, -5, -7, and -11, ACO1, ETR1, ERS1, and ETR2, PLD-gamma 1, and RbohD and F genes is undoubtedly under the reciprocal cross-talk of the ethylene and PA(PLD) signalling routes; both signals affect it in concerted or opposite ways depending on the gene or the type of stimuli. The results of these studies on broccoli seedlings are in agreement with the hypothesis that PA may directly affect the ethylene signal transduction pathway via an inhibitory effect on CTR1 (constitutive triple response 1) activity.

Mutual regulation of plant phospholipase D and the actin cytoskeleton

Membrane lipids and cytoskeleton dynamics are intimately inter-connected in the eukaryotic cell; however, only recently have the molecular mechanisms operating at this interface in plant cells been addressed experimentally. Phospholipase D (PLD) and its product phosphatidic acid (PA) were discovered to be important regulators in the membrane-cytoskeleton interface in eukaryotes. Here we report the mechanistic details of plant PLD-actin interactions. Inhibition of PLD by n-butanol compromises pollen tube actin, and PA rescues the detrimental effect of n-butanol on F-actin, showing clearly the importance of the PLD-PA interaction for pollen tube F-actin dynamics. From various candidate tobacco PLDs isoforms, we identified NtPLDbeta1 as a regulatory partner of actin, by both activity and in vitro interaction assays. Similarly to published data, the activity of tobacco PIP(2)-dependent PLD (PLDbeta) is specifically enhanced by F-actin and inhibited by G-actin. We then identified the NtPLDbeta1 domain responsible for actin interactions. Using sequence- and structure-based analysis, together with site-directed mutagenesis, we identified Asn323 and Thr382 of NtPLDbeta1 as the crucial amino acids in the actin-interacting fold. The effect of antisense-mediated suppression of NtPLDbeta1 or NtPLDdelta on pollen tube F-actin dynamics shows that NtPLDbeta1 is the active partner in PLD-actin interplay. The positive feedback loop created by activation of PLDbeta by F-actin and of F-actin by PA provides an important mechanism to locally increase membrane-F-actin dynamics in the cortex of plant cells.

Phospholipase Dalpha from sunflower (Helianthus annuus): cloning and functional characterization

D type phospholipases (PLD) are enzymes that hydrolyze the head group of phospholipids to produce phosphatidic acid. This activity is ubiquitous in plant tissues, and has been isolated and characterized from different species and organs. Several families of these proteins have been described in plants on the basis of their gene sequences (PLD alpha, beta, gamma, delta, zeta and epsilon). They have been shown to be involved in many metabolic events, such as response to abiotic stress, signal transduction, and membrane lipid turnover and degradation. In the present study, PLD activity was measured in the soluble fractions isolated from different organs of this plant. A PLD of alpha type was cloned from leaf cDNA that was responsible for most of this activity. The gene encoding this 810 aa protein was heterologously expressed in E. coli. This protein was not lethal for the eukaryotic host, although it altered its phospholipid profile. PLDalpha was purified to almost homogeneity by His-tag affinity chromatography, displaying an optimum pH of 6.5 and strong dependence on the presence of Ca(2+) and SDS in the assay medium. The enzyme was active towards phosphatidylcholine, Phosphatidylethanolamine and phosphatidylglycerol. Furthermore, the HaPLDalpha gene was found to be expressed at high levels in leaf and stem tissues.

Comparing genomic expression patterns across plant species reveals highly diverged transcriptional dynamics in response to salt stress

Background

Rice and barley are both members of Poaceae (grass family) but have a marked difference in salt tolerance. The molecular mechanism underlying this difference was previously unexplored. This study employs a comparative genomics approach to identify analogous and contrasting gene expression patterns between rice and barley.

Results

A hierarchical clustering approach identified several interesting expression trajectories among rice and barley genotypes. There were no major conserved expression patterns between the two species in response to salt stress. A wheat salt-stress dataset was queried for comparison with rice and barley. Roughly one-third of the salt-stress responses of barley were conserved with wheat while overlap between wheat and rice was minimal. These results demonstrate that, at transcriptome level, rice is strikingly different compared to the more closely related barley and wheat. This apparent lack of analogous transcriptional programs in response to salt stress is further highlighted through close examination of genes associated with root growth and development.

Conclusion

The analysis provides support for the hypothesis that conservation of transcriptional signatures in response to environmental cues depends on the genetic similarity among the genotypes within a species, and on the phylogenetic distance between the species.

Phospholipase dalpha1 and phosphatidic acid regulate NADPH oxidase activity and production of reactive oxygen species in ABA-mediated stomatal closure in Arabidopsis

We determined the role of Phospholipase Dalpha1 (PLDalpha1) and its lipid product phosphatidic acid (PA) in abscisic acid (ABA)-induced production of reactive oxygen species (ROS) in Arabidopsis thaliana guard cells. The pldalpha1 mutant failed to produce ROS in guard cells in response to ABA. ABA stimulated NADPH oxidase activity in wild-type guard cells but not in pldalpha1 cells, whereas PA stimulated NADPH oxidase activity in both genotypes. PA bound to recombinant Arabidopsis NADPH oxidase RbohD (respiratory burst oxidase homolog D) and RbohF. The PA binding motifs were identified, and mutation of the Arg residues 149, 150, 156, and 157 in RbohD resulted in the loss of PA binding and the loss of PA activation of RbohD. The rbohD mutant expressing non-PA-binding RbohD was compromised in ABA-mediated ROS production and stomatal closure. Furthermore, ABA-induced production of nitric oxide (NO) was impaired in pldalpha1 guard cells. Disruption of PA binding to ABI1 protein phosphatase 2C did not affect ABA-induced production of ROS or NO, but the PA-ABI1 interaction was required for stomatal closure induced by ABA, H(2)O(2), or NO. Thus, PA is as a central lipid signaling molecule that links different components in the ABA signaling network in guard cells.

Suppression of the rice fatty-acid desaturase gene OsSSI2 enhances resistance to blast and leaf blight diseases in rice

Fatty acids and their derivatives play important signaling roles in plant defense responses. It has been shown that suppressing a gene for stearoyl acyl carrier protein fatty-acid desaturase (SACPD) enhances the resistance of Arabidopsis (SSI2) and soybean to multiple pathogens. In this study, we present functional analyses of a rice homolog of SSI2 (OsSSI2) in disease resistance of rice plants. A transposon insertion mutation (Osssi2-Tos17) and RNAi-mediated knockdown of OsSSI2 (OsSSI2-kd) reduced the oleic acid (18:1) level and increased that of stearic acid (18:0), indicating that OsSSI2 is responsible for fatty-acid desaturase activity. These plants displayed spontaneous lesion formation in leaf blades, retarded growth, slight increase in endogenous free salicylic acid (SA) levels, and SA/benzothiadiazole (BTH)-specific inducible genes, including WRKY45, a key regulator of SA/BTH-induced resistance, in rice. Moreover, the OsSSI2-kd plants showed markedly enhanced resistance to the blast fungus Magnaporthe grisea and leaf-blight bacteria Xanthomonas oryzae pv. oryzae. These results suggest that OsSSI2 is involved in the negative regulation of defense responses in rice, as are its Arabidopsis and soybean counterparts. Microarray analyses identified 406 genes that were differentially expressed (>or=2-fold) in OsSSI2-kd rice plants compared with wild-type rice and, of these, approximately 39% were BTH responsive. Taken together, our results suggest that induction of SA-responsive genes, including WRKY45, is likely responsible for enhanced disease resistance in OsSSI2-kd rice plants.

Function and regulation of phospholipid signalling in plants

As an important metabolic pathway, phosphatidylinositol metabolism generates both constitutive and signalling molecules that are crucial for plant growth and development. Recent studies using genetic and molecular approaches reveal the important roles of phospholipid molecules and signalling in multiple processes of higher plants, including root growth, pollen and vascular development, hormone effects and cell responses to environmental stimuli plants. The present review summarizes the current progress in our understanding of the functional mechanism of phospholipid signalling, with an emphasis on the regulation of Ins(1,4,5)P3-Ca2+ oscillation, the second messenger molecule phosphatidic acid and the cytoskeleton.

Phospholipase D epsilon and phosphatidic acid enhance Arabidopsis nitrogen signaling and growth

Activation of phospholipase D (PLD) produces phosphatidic acid (PA), a lipid messenger implicated in cell growth and proliferation, but direct evidence for PLD and PA promotion of growth at the organism level is lacking. Here we characterize a new PLD gene, PLD epsilon, and show that it plays a role in promoting Arabidopsis growth. PLD epsilon is mainly associated with the plasma membrane, and is the most permissive of all PLDs tested with respect to its activity requirements. Knockout (KO) of PLD epsilon decreases root growth and biomass accumulation, whereas over-expression (OE) of PLD epsilon enhances root growth and biomass accumulation. The level of PA was higher in OE plants, but lower in KO plants than in wild-type plants, and suppression of PLD-mediated PA formation by alcohol alleviated the growth-promoting effect of PLD epsilon. OE and KO of PLD epsilon had opposite effects on lateral root elongation in response to nitrogen. Increased expression of PLD epsilon also promoted root hair elongation and primary root growth under severe nitrogen deprivation. The results suggest that PLD epsilon and PA promote organism growth and play a role in nitrogen signaling. The lipid-signaling process may play a role in connecting membrane sensing of nutrient status to increased plant growth and biomass production.

Heme-independent soluble and membrane-associated peroxidase activity of a Zea mays annexin preparation

Annexins are cytosolic proteins capable of reversible, Ca(2+)-dependent membrane binding or insertion. Animal annexins form and regulate Ca(2+)-permeable ion channels and may therefore participate in signaling. Zea mays (maize) annexins (ZmANN33 and ZmANN35) have recently been shown to form a Ca(2+)-permeable conductance in planar lipid bilayers and also exhibit in vitro peroxidase activity. Peroxidases form a superfamily of intra- or extracellular heme-containing enzymes that use H(2)O(2) as the electron acceptor in a number of oxidative reactions. Maize annexin peroxidase activity appears independent of heme and persists after membrane association, the latter suggesting a role in reactive oxygen species signaling.

Implication of phospholipase D in response of Hordeum vulgare root to short-term potassium deprivation

To verify the possible involvement of lipids and several other compounds including hydrogen peroxide (H(2)O(2)) and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) in the response of Hordeum vulgare to early potassium deprivation, plants were grown in hydroponic conditions for 30d with a modified Hewitt nutrient solution containing 3mM K(+). They were then incubated for increasing periods of time ranging from 2 to 36h in the same medium deprived of K(+). In contrast to leaves, root K(+) concentration showed its greatest decrease after 6h of treatment. The main lipids of the control barley roots were phospholipids (PL), representing more than 50% of the total lipids. PL did not change with treatment, whereas free sterols (FS) decreased following K(+) deprivation, showing a reduction of approximately 17% after 36h. With respect to the individual PL, 30h K(+) deprivation led to a reduction in phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylinositol (PI) levels, whereas phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidic acid (PA) levels increased. The maximum PA accumulation and the highest phospholipase D (PLD) activation, estimated by an accumulation of phosphatidylbutanol (PtBut), were observed after 24h of K(+) starvation. At the root level, H(2)O(2) showed the maximum value after 6h of incubation in -K solution. In parallel, G3PDH activity reached its minimum. On the basis of a concomitant stimulation of PLD activity and, consequently, PA accumulation, enhancement of H(2)O(2) production, and inhibition of G3PDH activity, we suggest a possible involvement of these three compounds in an early response to K(+) deprivation.

Cerium elicitor-induced phosphatidic acid triggers apoptotic signaling development in Taxus cuspidata cell suspension cultures

Degradation of membrane phospholipids is associated with apoptotic responses, but the signaling development of this degradation is not well understood. Cerium (Ce(4+)), an important rare earth element, induces cellular apoptosis and taxol biosynthesis in Taxus cuspidata suspension cultures. Here, using mass spectrometry and biochemical technique, we demonstrated that the phospholipase D (PLD) was rapidly activated by Ce(4+) and hydrolyzed structural phospholipids to generate lipid signal molecule, phosphatidic acid (PA). 1-Butanol, an antagonist of PLD-dependent PA production, blocked the biphasic burst of superoxide anions (O2(*-)) and thus mitigated cellular apoptosis. The time-course analysis of PA accumulation and ERK-like mitogen-activated protein kinase (MAPK) regulation indicated PA generation preceded MAPK activation, suggesting that the rapid accumulation of PA might be required for the initial MAPK activity. After 2h of Ce(4+) elicitation, however, PA-induced O2(*-) burst, forming a negative regulation to MAPK activity, which in turn led to apoptotic signaling development.

Arabidopsis phospholipase Dδ as an initiator of cytoskeleton-mediated signalling to fundamental cellular processes

Phospholipase D (PLD), in combination with the cytoskeleton, plays a key role in plant signal transduction. One isotype of the multigene Arabidopsis PLD family, AtPLDδ, has been implicated in binding microtubules, although the molecular details of the mechanism and identities of potential interaction partners are unclear. We constructed a GFP-AtPLDδ reporter gene, stably transformed it into an Arabidopsis suspension cell line, and used epitope-tagged affinity pull-down assays to isolate a complex of co-purifying proteins. Mass spectrometry analysis of the complex revealed a set of proteins including β-tubulin, actin 7, HSP70, clathrin heavy chain, ATP synthase subunits, and a band 7-4/flotillin homologue. Sequence alignments with defined tubulin- and actin-binding regions from human HsPLD2 revealed highly homologous regions in all 12 AtPLD isotypes, suggesting direct interactions of AtPLDδ with tubulin and actin, while interactions with the remaining partners are likely to be mediated by the cytoskeleton. We propose that AtPLDδ acts through a complex of cytoskeletal and partner proteins to modulate fundamental cellular processes such as cytoskeletal rearrangements, vesicular trafficking, assembly of Golgi apparatus, mitosis and cytokinesis.

Changes in C12:0, C18:1, C18:2 and C20:2 fatty acid content in wheat treated with resistance inducers and infected by powdery mildew

This work presents a global investigation of total fatty acid (FA) content in wheat in relation to treatment with four inducers of resistance and to powdery mildew infection. Linolenic acid (C18:3), linoleic acid (C18:2) and palmitic acid (16:0) were the most abundant FAs in wheat leaves. We investigated the effect of the following inducers of resistance: Iodus40, heptanoyl salicylic acid (HSA), Milsana and trehalose on FA accumulation. Previous studies established that lipid metabolism is altered by these compounds, and we therefore aimed to characterise their impact at the FA level. During a time course experiment, content (quantitative analysis) and percentage (qualitative analysis) of FAs were compared in treated plants and in controls, as well as in plants inoculated with Blumeria graminis f. sp. tritici (i) and non-inoculated (ni) plants. No change in C18:3 content was observed. C18:1 in Iodus 40-treated (ni) plants showed a quantitative 1.2-fold increase. Lauric acid (C12:0) content quantitatively increased after Iodus 40 (2.8-fold), Milsana (4.8-fold) and trehalose (4.0-fold) treatment in (i) plants. However, eicosadienoic acid (C20:2) quantitatively decreased in (ni) plants after Iodus 40 (1.5-fold) and Milsana (2.3-fold) treatment. The amount of C18:2 increased (1.6-fold) after HSA treatment in (i) plants. All these variations in FA content were correlated with variations in the corresponding relative percentages. Our work provides the first evidence for alterations in C12:0, C18:1, C18:2 and C20:2 FA content caused by four resistance inducers. We also compared the amount and percentage of each FA in untreated (i) and (ni) plants. In (i) plants, eicosadienoic acid (C20:2) increased and C18:2 decreased slightly. The potential involvement of these FAs during induced resistance and infection is discussed.

Plant phospholipid signaling: "in a nutshell"

Since the discovery of the phosphoinositide/phospholipase C (PI/PLC) system in animal systems, we know that phospholipids are much more then just structural components of biological membranes. In the beginning, this idea was fairly straightforward. Receptor stimulation activates PLC, which hydrolyses phosphatidylinositol4,5-bisphosphate [PtdIns(4,5)P2] into two second messengers: inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DG). While InsP3 difuses into the cytosol and triggers the release of calcium from an internal store via ligand-gated calcium channels, DG remains in the membrane where it recruits and activates members of the PKC family. The increase in calcium, together with the change in phosphorylation status, (in)activates a variety of protein targets, leading to a massive reprogramming, allowing the cell to appropriately respond to the extracellular stimulus. Later, it became obvious that not just PLC, but a variety of other phospholipid-metabolizing enzymes were activated, including phospholipase A, phospholipase D, and PI 3-kinase. More recently, it has become apparent that PtdIns4P and PtdIns(4,5)P2 are not just signal precursors but can also function as signaling molecules themselves. While plants contain most of the components described above, and evidence for their role in cell signaling is progressively increasing, major differences between plants and the mammalian paradigms exist. Below, these are described "in a nutshell."

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Heterologous expression, and biochemical and cellular characterization of CaPLA1 encoding a hot pepper phospholipase A1 homolog

Phospholipid signaling has been recently implicated in diverse cellular processes in higher plants. We identified a cDNA encoding the phospholipase A1 homolog (CaPLA1) from 5-day-old early roots of hot pepper. The deduced amino acid sequence showed that the lipase-specific catalytic triad is well conserved in CaPLA1. In vitro lipase assays and site-directed mutagenesis revealed that CaPLA1 possesses PLA1 activity, which catalyzes the hydrolysis of phospholipids at the sn-1 position. CaPLA1 was selectively expressed in young roots, at days 4-5 after germination, and rapidly declined thereafter, suggesting that the expression of CaPLA1 is subject to control by a development-specific mechanism in roots. Because transgenic work was extremely difficult in hot peppers, in this study we overexpressed CaPLA1 in Arabidopsis so as to provide cellular information on the function of this gene. CaPLA1 overexpressors had significantly longer roots, leaves and petioles, and grew more rapidly than the wild-type plants, leading to an early bolting phenotype with prolonged inflorescence. Microscopic analysis showed that the vegetative tissues of 35S:CaPLA1 plants contained an increased number of small-sized cells, which resulted in highly populated cell layers. In addition, mRNAs for cell cycle-controlled proteins and fatty acid catabolizing enzymes were coordinately upregulated in CaPLA1-overexpressing plants. These results suggest that CaPLA1 is functionally relevant in heterologous Arabidopsis cells, and hence might participate in a subset of positive control mechanisms of cell and tissue growth in transgenic lines. We discuss possible biochemical and cellular functions of CaPLA1 in relation to the phospholipid signaling pathway in hot pepper and transgenic Arabidopsis plants.

Dual functions of phospholipase Dalpha1 in plant response to drought

Phospholipase Dalpha1 (PLDalpha1) has been shown to mediate the abscisic acid regulation of stomatal movements. Arabidopsis plants deficient in PLDalpha1 increased, whereas PLDalpha1-overexpressing tobacco decreased, transpirational water loss. In the early stage of drought, the decrease in water loss was associated with a rapid stomatal closure caused by a high level of PLD in PLDalpha1-overexpressing plants. However, in the late stage of drought, the overexpressing plants displayed more susceptibility to drought than control plants. PLDalpha1 activity in the overexpressing plants was much higher than that of control plants in which drought also induced an increase in PLDalpha1 activity. The high level of PLDalpha1 activity was correlated to membrane degradation in late stages of drought, as demonstrated by ionic leakage and lipid peroxidation. These findings indicate that a high level of PLDalpha1 expression has different effects on plant response to water deficits. It promotes stomatal closure at earlier stages, but disrupts membranes in prolonged drought stress. These findings are discussed in relation to the understanding of PLD functions and potential applications.

Excessive copper induces the production of reactive oxygen species, which is mediated by phospholipase D, nicotinamide adenine dinucleotide phosphate oxidase and antioxidant systems

Tobacco BY-2 suspension cells were used to study the chemical damage and its associated mechanisms caused by Cu2+. Treatment with 100 micromol/L Cu2+ generated a large amount of H2O2 and thiobarbituric acid-reactive substances (TBARS) in cells. Using phospholipase D (PLD) specific inhibitor (1-butanol) or phosphatidic acid (PA), we demonstrated that PLD plays an important role in the generation of H2O2 and TBARS. Semi-quantitative reverse-transcriptase polymerase chain reaction and enzyme activity assays with wild type and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-overexpressing BY-2 cells revealed that PLD and PA are the key factors leading to NADPH oxidase activation, which is responsible for H2O2 and TBARS production induced by Cu2+. Moreover, the content of ascorbic acid (AsA), an effective antioxidant, was sharply reduced in BY-2 cells exposed to excessive Cu2+. Furthermore, a significant downregulation of the enzymes of AsA biosynthesis and the antioxidant system was found. This evidence suggests that excessive Cu2+-elevated reactive oxygen species (ROS) production is caused by upregulated PLD that elevates the activity of NADPH oxidase and its collapsed antioxidant systems that scavenges ROS.

Guard-cell signalling for hydrogen peroxide and abscisic acid

Guard cells can integrate and process multiple complex signals from the environment and respond by opening and closing stomata in order to adapt to the environmental signal. Over the past several years, considerable research progress has been made in our understanding of the role of reactive oxygen species (ROS) as essential signal molecules that mediate abscisic acid (ABA)-induced stomatal closure. In this review, we discuss hydrogen peroxide (H2O2) generation and signalling, H2O2-induced gene expression, crosstalk and the specificity between ABA and H2O2 signalling, and the cellular mechanism for ROS sensing in guard cells. This review focuses especially on the points of connection between ABA and H2O2 signalling in guard cells. The fundamental progress in understanding the role of ABA and ROS in guard cells will continue to provide a rational basis for biotechnological improvements in the development of drought-tolerant crop plants with improved water-use efficiency.

Programmed cell death in plants: new insights into redox regulation and the role of hydrogen peroxide

Programmed cell death (PCD), the highly regulated dismantling of cells, is essential for plant growth and survival. PCD plays key roles in embryo development, formation and maturation of many cell types and tissues, and plant reaction/adaptation to environmental conditions. Reactive oxygen species (ROS) are not only toxic by products of aerobic metabolism with strictly controlled cellular levels, but they also function as signaling agents regulating many biological processes and producing pleiotropic effects. Over the last decade, ROS have become recognized as important modulators of plant PCD. Molecular genetic approaches using plant mutants and transcriptome studies related to ROS-mediated PCD have revealed a wide array of plant-specific cell death regulators and have contributed to unraveling the elaborate redox signaling network. This review summarizes the biological processes, in which plant PCD participates and discusses the signaling functions of ROS with emphasis on hydrogen peroxide.

Lipidomic analysis reveals differential defense responses of Taxus cuspidata cells to two elicitors, methyl jasmonate and cerium (Ce4+)

Methyl jasmonate (MeJA) and cerium (Ce(4+)) elicitation share common features of increasing taxol accumulation of Taxus cuspidata cells. Interestingly, Ce(4+) induces programmed cell death (PCD), but this phenomenon is not observed with MeJA elicitation. Here, using a lipidomic approach to measure more than 100 membrane glycerophospholipids of T. cuspidata cells quantitatively, we discovered that lysophosphatidylcholine (LysoPC), phosphatidic acid (PA) and phosphatidylcholine were three potential lipid markers that were responsible for the differences between Ce(4+)-induced cells and MeJA-induced cells. Compared with MeJA elicitation, marked increase of phospholipase D (PLD) activity was observed following Ce(4+) elicitation, suggesting that the PLD activation and high concentrations of PA production might mediate the PCD. Rapid increase of phospholipase A(2) (PLA(2)) activity caused the release of fatty acids and LysoPC following Ce(4+) elicitation, which enhanced endogenous jasmonic acid (JA) accumulation. In contrast, PLA(2) activity was poorly induced following MeJA elicitation. PLA(2) inhibitor suppressed not only JA accumulation but also taxol production, suggesting that the PLA(2) activation mediated Ce(4+)-induced taxol production partially through a JA-dependent signaling pathway. These results demonstrate that differential alternation of glycerolphospholipids caused by phospholipases constitutes an important step in cell death response to Ce(4+) and increasing taxol production.

Differential degradation of extraplastidic and plastidic lipids during freezing and post-freezing recovery in Arabidopsis thaliana

Changes in membrane lipid composition play important roles in plant adaptation to and survival after freezing. Plant response to cold and freezing involves three distinct phases: cold acclimation, freezing, and post-freezing recovery. Considerable progress has been made toward understanding lipid changes during cold acclimation and freezing, but little is known about lipid alteration during post-freezing recovery. We previously showed that phospholipase D (PLD) is involved in lipid hydrolysis and Arabidopsis thaliana freezing tolerance. This study was undertaken to determine how lipid species change during post-freezing recovery and to determine the effect of two PLDs, PLDalpha1 and PLDdelta, on lipid changes during post-freezing recovery. During post-freezing recovery, hydrolysis of plastidic lipids, monogalactosyldiacylglycerol and plastidic phosphatidylglycerol, is the most prominent change. In contrast, during freezing, hydrolysis of extraplastidic phospholipids, phosphatidylcholine and phosphatidylethanolamine, occurs. Suppression of PLDalpha1 decreased phospholipid hydrolysis and phosphatidic acid production in both the freezing and post-freezing phases, whereas ablation of PLDdelta increased lipid hydrolysis and phosphatidic acid production during post-freezing recovery. Thus, distinctly different changes in lipid hydrolysis occur in freezing and post-freezing recovery. The presence of PLDalpha1 correlates with phospholipid hydrolysis in both freezing and post-freezing phases, whereas the presence of PLDdelta correlates with reduced lipid hydrolysis during post-freezing recovery. These data suggest a negative role for PLDalpha1 and a positive role for PLDdelta in freezing tolerance.

EDR2 negatively regulates salicylic acid-based defenses and cell death during powdery mildew infections of Arabidopsis thaliana

BACKGROUND

The hypersensitive necrosis response (HR) of resistant plants to avirulent pathogens is a form of programmed cell death in which the plant sacrifices a few cells under attack, restricting pathogen growth into adjacent healthy tissues. In spite of the importance of this defense response, relatively little is known about the plant components that execute the cell death program or about its regulation in response to pathogen attack.

RESULTS

We isolated the edr2-6 mutant, an allele of the previously described edr2 mutants. We found that edr2-6 exhibited an exaggerated chlorosis and necrosis response to attack by three pathogens, two powdery mildew and one downy mildew species, but not in response to abiotic stresses or attack by the bacterial leaf speck pathogen. The chlorosis and necrosis did not spread beyond inoculated sites suggesting that EDR2 limits the initiation of cell death rather than its spread. The pathogen-induced chlorosis and necrosis of edr2-6 was correlated with a stimulation of the salicylic acid defense pathway and was suppressed in mutants deficient in salicylic acid signaling. EDR2 encodes a novel protein with a pleckstrin homology and a StAR transfer (START) domain as well as a plant-specific domain of unknown function, DUF1336. The pleckstrin homology domain binds to phosphatidylinositol-4-phosphate in vitro and an EDR2:HA:GFP protein localizes to endoplasmic reticulum, plasma membrane and endosomes.

CONCLUSION

EDR2 acts as a negative regulator of cell death, specifically the cell death elicited by pathogen attack and mediated by the salicylic acid defense pathway. Phosphatidylinositol-4-phosphate may have a role in limiting cell death via its effect on EDR2. This role in cell death may be indirect, by helping to target EDR2 to the appropriate membrane, or it may play a more direct role.

Enhancing seed quality and viability by suppressing phospholipase D in Arabidopsis

Seed aging decreases the quality of seed and grain and results in agricultural and economic losses. Alterations that impair cellular structures and metabolism are implicated in seed deterioration, but the molecular and biochemical bases for seed aging are not well understood. Ablation of the gene for a membrane lipid-hydrolyzing phospholipase D (PLDalpha1) in Arabidopsis enhanced seed germination and oil stability after storage or exposure of seeds to adverse conditions. The PLDalpha1-deficient seeds exhibited a smaller loss of unsaturated fatty acids and lower accumulation of lipid peroxides than did wild-type seeds. However, PLDalpha1-knockdown seeds were more tolerant of aging than were PLDalpha1-knockout seeds. The results demonstrate the PLDalpha1 plays an important role in seed deterioration and aging in Arabidopsis. A high level of PLDalpha1 is detrimental to seed quality, and attenuation of PLDalpha1 expression has the potential to improve oil stability, seed quality and seed longevity.

Comparative lipidomics analysis of cellular development and apoptosis in two Taxus cell lines

A comparative lipidomics approach was employed to investigate the changes in membrane phospholipids during the procession of cellular development and apoptosis of two plant cell lines, Taxus cuspidata and Taxus chinensis var. mairei. Analysis of lipids by LC/ESI/MS(n) showed more than 90 phospholipid molecular species and indicated significant differences in the abundance throughout a 3-week period. Phosphatidic acid (PA), phosphatidylcholine (PC) and lysophosphatidylcholine (LysoPC) were three important lipid groups that were responsible for the discrimination between the apoptotic T. chinensis var. mairei and living T. cuspidata cells. Continuous increase of phospholipase D (PLD) activity led to PA production in apoptotic T. chinensis var. mairei cells suggesting that the PLD activation and PA formation mediated the apoptosis. Comparison of the profiles of phosphatidylbutanol (PtdBut) with those of PC or phosphatidylethanolamine (PE) indicated that PC rather than PE was the major substrate of PLD in vivo. These results suggest that the alternation of membrane phospholipids may regulate apoptosis, triggering an increase in taxol production of T. chinensis var. mairei cells.

Spatial variation in H2O2 response of Arabidopsis thaliana root epidermal Ca2+ flux and plasma membrane Ca2+ channels

Hydrogen peroxide is an important regulatory agent in plants. This study demonstrates that exogenous H2O2 application to Arabidopsis thaliana root epidermis results in dose-dependent transient increases in net Ca2+ influx. The magnitude and duration of the transients were greater in the elongation zone than in the mature epidermis. In both regions, treatment with the cation channel blocker Gd3+ prevented H2O2-induced net Ca2+ influx, consistent with application of exogenous H2O2 resulting in the activation of plasma membrane Gd3+-sensitive Ca2+-influx pathways. Application of 10 mm H2O2 to the external plasma membrane face of elongation zone epidermal protoplasts resulted in the appearance of a hyperpolarization-activated Ca2+-permeable conductance. This conductance differed from that previously characterized as being responsive to extracellular hydroxyl radicals. In contrast, in mature epidermal protoplasts a plasma membrane hyperpolarization-activated Ca2+-permeable channel was activated only when H2O2 was present at the intracellular membrane face. Channel open probability increased with intracellular [H2O2] and at hyperpolarized voltages. Unitary conductance decreased thus: Ba2+ > Ca2+ (14.5 pS) > Mg2+ > Zn2+ (20 mM external cation, 1 mM H2O2). Lanthanides and Zn2+ (but not TEA+) suppressed the open probability without affecting current amplitude. The results suggest spatial heterogeneity and differential sensitivity of Ca2+ channel activation by reactive oxygen species in the root that could underpin signalling.

Early PLDalpha-mediated events in response to progressive drought stress in Arabidopsis: a transcriptome analysis

Phospholipase D (PLD) has been implicated in a variety of stresses including osmotic stress and wounding. PLDalpha1-derived phosphatidic acid interacts with ABI1 phosphatase 2C and promotes abscisic acid signalling. It has also been shown to regulate proline biosynthesis negatively. Plants with abrogated PLDalpha show insensitivity to abscisic acid (ABA) and impaired stomatal conductance. The goal in the present study was to identify early PLDalpha-mediated events in response to progressive drought stress in Arabidopsis. Water was withheld from 7-week-old Arabidopsis thaliana (Col-0) and antisense-PLDalpha1 (anti-PLDalpha) in a controlled environment chamber. Diurnal leaf water potential (LWP) and photosynthesis measurements were recorded five and three times a day, respectively. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and microarray analyses were conducted using RNA from shoots collected at the fourth LWP time point on the ninth day after stress imposition. Anti-PLDalpha experienced severe water stress (-1.28 MPa) at the same time period that Col-0 experienced less water stress (-0.31 MPa). Diurnal LWP measurements showed that anti-PLDalpha had a lower LWP than Col-0 in both control and drought-stress conditions. Photosynthesis was also more affected in anti-PLDalpha than in Col-0. Anti-PLDalpha plants recovered fully following rehydration after 10 d of stress. qRT-PCR revealed up to 18-fold lower values for PLDalpha transcripts in stressed anti-PLDalpha plants when compared with stressed Col-0. Microarray expression profiles revealed distinct gene expression patterns in Col-0 and anti-PLDalpha. No differences in gene expression were detected between the two genotypes in the absence of drought stress. ROP8, PLDdelta, and lipid transfer proteins were among the differentially expressed genes between the two genotypes.

The Arabidopsis Protein Kinase PTI1-2 Is Activated by Convergent Phosphatidic Acid and Oxidative Stress Signaling Pathways Downstream of PDK1 and OXI1

Arabidopsis PDK1 activity is regulated by binding to the lipid phosphatidic acid (PA) resulting in activation of the oxidative stress-response protein kinase OXI1/AGC2-1. Thus there is an inferred link between lipid signaling and oxidative stress signaling modules. Among a panel of hormones and stresses tested, we found that, in addition to PA, the fungal elicitor xylanase activated PDK1, suggesting that PDK1 has a role in plant pathogen defense mechanisms. The downstream OXI1 was activated by additional stress factors, including PA, H(2)O(2), and partially by xylanase. We have isolated an interacting partner of OXI1, a Ser/Thr kinase (PTI1-2), which is downstream of OXI1. Its sequence closely resembles the tomato Pti kinase, which has been implicated in the hypersensitive response, a localized programmed cell death that occurs at the site of pathogen infection. PTI1-2 is activated by the same stresses/elicitors as OXI1 and additionally flagellin. We have used RNA interference to knock out the expression of PDK1 and OXI1 and to study the effects on PTI1-2 activity. We show that specific lipid signaling pathways converge on PTI1-2 via the PDK1-OXI1 axis, whereas H(2)O(2) and flagellin signals to OXI1-PTI1-2 via a PDK1-independent pathway. PTI1-2 represents a new downstream component that integrates diverse lipid and reactive oxygen stress signals and functions closely with OXI1.

Reactive oxygen species as signals that modulate plant stress responses and programmed cell death

Reactive oxygen species (ROS) are known as toxic metabolic products in plants and other aerobic organisms. An elaborate and highly redundant plant ROS network, composed of antioxidant enzymes, antioxidants and ROS-producing enzymes, is responsible for maintaining ROS levels under tight control. This allows ROS to serve as signaling molecules that coordinate an astonishing range of diverse plant processes. The specificity of the biological response to ROS depends on the chemical identity of ROS, intensity of the signal, sites of production, plant developmental stage, previous stresses encountered and interactions with other signaling molecules such as nitric oxide, lipid messengers and plant hormones. Although many components of the ROS signaling network have recently been identified, the challenge remains to understand how ROS-derived signals are integrated to eventually regulate such biological processes as plant growth, development, stress adaptation and programmed cell death.

The role of phospholipase D in plant stress responses

Phospholipase D (PLD) has been implicated in multiple plant stress responses. Its gene transcription and activity increase upon exposure to various stresses, and manipulation of PLD protein levels leads to altered stress tolerance. The plant PLD family is relatively large and heterogeneous, and different PLD isoforms are involved in separate stress responses. PLD and its product, phosphatidic acid, exert their effects by functioning in signal transduction cascades and by influencing the biophysical state of lipid membranes.