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

Phospholipase-dependent signalling during the AvrRpm1- and AvrRpt2-induced disease resistance responses in Arabidopsis thaliana

Bacterial pathogens deliver type III effector proteins into plant cells during infection. On susceptible host plants, type III effectors contribute to virulence, but on resistant hosts they betray the pathogen to the plant's immune system and are functionally termed avirulence (Avr) proteins. Recognition induces a complex suite of cellular and molecular events comprising the plant's inducible defence response. As recognition of type III effector proteins occurs inside host cells, defence responses can be elicited by in planta expression of bacterial type III effectors. We demonstrate that recognition of either of two type III effectors, AvrRpm1 or AvrRpt2 from Pseudomonas syringae, induced biphasic accumulation of phosphatidic acid (PA). The first wave of PA accumulation correlated with disappearance of monophosphatidylinosotol (PIP) and is thus tentatively attributed to activation of a PIP specific phospholipase C (PLC) in concert with diacylglycerol kinase (DAGK) activity. Subsequent activation of phospholipase D (PLD) produced large amounts of PA from structural phospholipids. This later wave of PA accumulation was several orders of magnitude higher than the PLC-dependent first wave. Inhibition of phospholipases blocked the response, and feeding PA directly to leaf tissue caused cell death and defence-gene activation. Inhibitor studies ordered these events relative to other known signalling events during the plant defence response. Influx of extracellular Ca(2+) occurred downstream of PIP-degradation, but upstream of PLD activation. Production of reactive oxygen species occurred downstream of the phospholipases. The data presented indicate that PA is a positive regulator of RPM1- or RPS2-mediated disease resistance signalling, and that the biphasic PA production may be a conserved feature of signalling induced by the coiled-coil nucleotide binding domain leucine-rich repeat class of resistance proteins.

Nitric oxide enhances salt tolerance in maize seedlings through increasing activities of proton-pump and Na+/H+ antiport in the tonoplast

Nitric oxide (NO), an endogenous signaling molecule in animals and plants, mediates responses to abiotic and biotic stresses. Our previous work demonstrated that 100 microM sodium nitroprusside (SNP, an NO donor) treatment of maize seedlings increased K(+) accumulation in roots, leaves and sheathes, while decreasing Na(+) accumulation (Zhang et al. in J Plant Physiol Mol Biol 30:455-459, 2004b). Here we investigate how NO regulates Na(+), K(+) ion homeostasis in maize. Pre-treatment with 100 muM SNP for 2 days improved later growth of maize plants under 100 mM NaCl stress, as indicated by increased dry matter accumulation, increased chlorophyll content, and decreased membrane leakage from leaf cells. An NO scavenger, methylene blue (MB-1), blocked the effect of SNP. These results indicated that SNP-derived NO enhanced maize tolerance to salt stress. Further analysis showed that NaCl induced a transient increase in the NO level in maize leaves. Both NO and NaCl treatment stimulated vacuolar H(+)-ATPase and H(+)-PPase activities, resulting in increased H(+)-translocation and Na(+)/H(+) exchange. NaCl-induced H(+)-ATPase and H(+)-PPase activities were diminished by MB-1. 1-Butanol, an inhibitor of phosphatidic acid (PA) production by phospholipase D (PLD), reduced NaCl- and NO-induced H(+)-ATPase activation. In contrast, applied PA stimulated H(+)-ATPase activity. These results suggest that NO acts as a signal molecule in the NaCl response by increasing the activities of vacuolar H(+)-ATPase and H(+)-PPase, which provide the driving force for Na(+)/H(+) exchange. PLD and PA play an important role in this process.

Copper excess triggers phospholipase D activity in wheat roots

Wheat seedlings (Triticum durum Desf.) were incubated in 100muM Cu(2+) for different periods of time ranging from 1min up to 16h. Following metal addition a rapid intake of copper ions into the roots was observed. Cu(2+) induced an accumulation of both phosphatidic acid and phosphatidylbutanol within 1min of incubation, the latter indicating a very rapid induction of phospholipase D (PLD) activity. The highest PLD stimulation was detected after 2h from copper addition and decreased almost to the initial value at increasing times. Cycloheximide treatment of roots lowered phosphatidylbutanol accumulation because of a reduced PLD activity. The expression profile of a T. durum putative PLD-encoding gene showed a peak after 1h of treatment as well, indicating that enhanced gene expression contributed to the increase in PLD activity. In the absence of copper ions, roots treated with the G protein activator mastoparan showed increases in phosphatidic acid and phosphatidylbutanol similar to those detected with the metal. PLD activity was also stimulated by cholera toxin. Two putatively G protein alpha subunit encoding sequences were isolated and no significant differences in transcription activity following Cu(2+) addition were observed. In copper-treated roots an early production of superoxide generated both by total and membrane-bound NADPH oxidase occurred. The G protein inhibitor suramin as well as the PLD antagonist 1-butanol abolished copper-induced superoxide production.

Metabolomic approaches reveal that phosphatidic and phosphatidyl glycerol phospholipids are major discriminatory non-polar metabolites in responses by Brachypodium distachyon to challenge by Magnaporthe grisea

Metabolomic approaches were used to elucidate some key metabolite changes occurring during interactions of Magnaporthe grisea--the cause of rice blast disease--with an alternate host, Brachypodium distachyon. Fourier-transform infrared (FT-IR) spectroscopy provided a high-throughput metabolic fingerprint of M. grisea interacting with the B. distachyon accessions ABR1 (susceptible) and ABR5 (resistant). Principal component-discriminant function analysis (PC-DFA) allowed the differentiation between developing disease symptoms and host resistance. Alignment of projected 'test-set' on to 'training-set' data indicated that our experimental approach produced highly reproducible data. Examination of PC-DFA loading plots indicated that fatty acids were one chemical group that discriminated between responses by ABR1 and ABR5 to M. grisea. To identify these, non-polar extracts of M. grisea-challenged B. distachyon were directly infused into an electrospray ionization mass spectrometer (ESI-MS). PC-DFA indicated that M. grisea-challenged ABR1 and ABR5 were differentially clustered away from healthy material. Subtraction spectra and PC-DFA loadings plots revealed discriminatory analytes (m/z) between each interaction and seven metabolites were subsequently identified as phospholipids (PLs) by ESI-MS-MS. Phosphatidyl glycerol (PG) PLs were suppressed during both resistant and susceptible responses. By contrast, different phosphatidic acid PLs either increased or were reduced during resistance or during disease development. This suggests considerable and differential PL processing of membrane lipids during each interaction which may be associated with the elaboration/suppression of defence mechanisms or developing disease symptoms.

The highly charged region of plant beta-type phosphatidylinositol 4-kinase is involved in membrane targeting and phospholipid binding

In Arabidopsis thaliana and Oryza sativa, two types of PI 4-kinase (PI4Ks) have been isolated and functionally characterized. The alpha-type PI4Ks (approximately 220 kDa) contain a PH domain, which is lacking in beta-type PI4Ks (approximately 120 kDa). Beta-type PI4Ks, exemplified by Arabidopsis AtPI4Kbeta and rice OsPI4K2, contain a highly charged repetitive segment designated PPC (Plant PI4K Charged) region, which is an unique domain only found in plant beta-type PI4Ks at present. The PPC region has a length of approximately 300 amino acids and harboring 11 (AtPI4Kbeta) and 14 (OsPI4K2) repeats, respectively, of a 20-aa motif. Studies employing a modified yeast-based "Sequence of Membrane-Targeting Detection" system demonstrate that the PPC(OsPI4K2) region, as well as the former 8 and latter 6 repetitive motifs within the PPC region, are able to target fusion proteins to the plasma membrane. Further detection on the transiently expressed GFP fusion proteins in onion epidermal cells showed that the PPC(OsPI4K2) region alone, as well as the region containing repetitive motifs 1-8, was able to direct GFP to the plasma membrane, while the regions containing less repetitive motifs, i.e. 6, 4, 2 or single motif(s) led to predominantly intracellular localization. Agrobacterium-mediated transient expression of PPC-GFP fusion protein further confirms the membrane-targeting capacities of PPC region. In addition, the predominant plasma membrane localization of AtPI4Kbeta was mediated by the PPC region. Recombinant PPC peptide, expressed in E. coli, strongly binds phosphatidic acid, PI and PI4P, but not phosphatidylcholine, PI5P, or PI(4,5)P2 in vitro, providing insights into potential mechanisms for regulating sub-cellular localization and lipid binding for the plant beta-type PI4Ks.

Involvement of phospholipases C and D in early response to SAR and ISR inducers in Brassica napus plants

Phospholipid signaling is an important component in eukaryotic signal transduction pathways. In plants, it plays a key role in growth and development as well as in responses to environmental stresses, including pathogen attack. We investigated the involvement of both phospholipase C (PLC, EC 3.1.4.11) and D (PLD, EC 3.1.4.4) in early responses to the treatment of Brassica napus plants with the chemical inducers of systemic acquired resistance (SAR): salicylic acid (SA), benzothiadiazole (BTH), and with the inducer mediating the induced systemic resistance (ISR) pathway, methyl jasmonate (MeJA). Rapid activation (within 0.5-6 h treatment) of the in vitro activity level was found for phosphatidyl inositol 4,5 bisphosphate (PIP2)-specific PLC (PI-PLC) and three enzymatically different forms of PLD: conventional PLDalpha, PIP2-dependent PLD beta/gamma, and oleate-stimulated PLDdelta. The strongest response was found in case of cytosolic PIP2-dependent PLD beta/gamma after BTH treatment. PLDdelta was identified in B. napus leaves and was very rapidly activated after MeJA treatment with the highest degree of activation compared to the other PLD isoforms. Interestingly, an increase in the amount of protein was observed only for PLDgamma and/or delta after ISR induction, but later than the activation occurred. These results show that phospholipases are involved in very early processes leading to systemic responses in plants and that they are most probably initially first activated on post translational level.

Role of salicylic acid and fatty acid desaturation pathways in ssi2-mediated signaling

Stearoyl-acyl carrier protein desaturase-mediated conversion of stearic acid to oleic acid (18:1) is the key step that regulates the levels of unsaturated fatty acids (FAs) in cells. Our previous work with the Arabidopsis (Arabidopsis thaliana) ssi2/fab2 mutant and its suppressors demonstrated that a balance between glycerol-3-phosphate (G3P) and 18:1 levels is critical for the regulation of salicylic acid (SA)- and jasmonic acid-mediated defense signaling in the plant. In this study, we have evaluated the role of various genes that have an impact on SA, resistance gene-mediated, or FA desaturation (FAD) pathways on ssi2-mediated signaling. We show that ssi2-triggered resistance is dependent on EDS1, PAD4, EDS5, SID2, and FAD7 FAD8 genes. However, ssi2-triggered defects in the jasmonic acid pathway, morphology, and cell death phenotypes are independent of the EDS1, EDS5, PAD4, NDR1, SID2, FAD3, FAD4, FAD5, DGD1, FAD7, and FAD7 FAD8 genes. Furthermore, the act1-mediated rescue of ssi2 phenotypes is also independent of the FAD2, FAD3, FAD4, FAD5, FAD7, and DGD1 genes. Since exogenous application of glycerol converts wild-type plants into ssi2 mimics, we also studied the effect of exogenous application of glycerol on mutants impaired in resistance-gene signaling, SA, or fad pathways. Glycerol increased SA levels and induced pathogenesis-related gene expression in all but sid2, nahG, fad7, and fad7 fad8 plants. Furthermore, glycerol-induced phenotypes in various mutant lines correlate with a concomitant reduction in 18:1 levels. Inability to convert glycerol into G3P due to a mutation in the nho1-encoded glycerol kinase renders plants tolerant to glycerol and unable to induce the SA-dependent pathway. A reduction in the NHO1-derived G3P pool also results in a partial age-dependent rescue of the ssi2 morphological and cell death phenotypes in the ssi2 nho1 plants. The glycerol-mediated induction of defense was not associated with any major changes in the lipid profile and/or levels of phosphatidic acid. Taken together, our results suggest that glycerol application and the ssi2 mutation in various mutant backgrounds produce similar effects and that restoration of ssi2 phenotypes is not associated with the further desaturation of 18:1 to linoleic or linolenic acids in plastidal or extraplastidal lipids.

Mutation of the TGD1 chloroplast envelope protein affects phosphatidate metabolism in Arabidopsis

Phosphatidate (PA) is a central metabolite of lipid metabolism and a signaling molecule in many eukaryotes, including plants. Mutations in a permease-like protein, TRIGALACTOSYLDIACYLGLYCEROL1 (TGD1), in Arabidopsis thaliana caused the accumulation of triacylglycerols, oligogalactolipids, and PA. Chloroplast lipids were altered in their fatty acid composition consistent with an impairment of lipid trafficking from the endoplasmic reticulum (ER) to the chloroplast and a disruption of thylakoid lipid biosynthesis from ER-derived precursors. The process mediated by TGD1 appears to be essential as mutation of the protein caused a high incidence of embryo abortion. Isolated tgd1 mutant chloroplasts showed a decreased ability to incorporate PA into galactolipids. The TGD1 protein was localized to the inner chloroplast envelope and appears to be a component of a lipid transporter. As even partial disruption of TGD1 function has drastic consequences on central lipid metabolism, the tgd1 mutant provides a tool to explore regulatory mechanisms governing lipid homeostasis and lipid trafficking in plants.

Phosphatidic acid: a multifunctional stress signaling lipid in plants

Phosphatidic acid (PA) has only recently been identified as an important signaling molecule in both plants and animals. Nonetheless, it already promises to rival the importance of the classic second messengers Ca(2+) and cAMP. In plants, its formation is triggered in response to various biotic and abiotic stress factors, including pathogen infection, drought, salinity, wounding and cold. In general, PA signal production is fast (minutes) and transient. Recently, our understanding of the role of PA formation in stress responses as a result of phospholipases C and D activity has greatly increased. Moreover, the first protein targets of PA have been identified. Based on this recent work, potential mechanisms by which PA provokes downstream effects are emerging.

AtGLB1 enhances the tolerance of Arabidopsis to hydrogen peroxide stress

Hydrogen peroxide (H2O2) as a widespread molecule plays an important role in plant stress responses. Here, we showed that an Arabidopsis line overexpressing hemoglobin 1 (AtGLB1) can enhance its tolerance to severe hypoxic stress. In our research, Arabidopsis lines with different hemoglobin levels were employed to study the relationship between H2O2 level and the tolerance to hypoxic stress. The relatively low endogenous H2O2 level of AtGLB1-overexpressing plants could be one of the main factors for the increased tolerance of plants to hypoxic stress. Further investigation indicated that the activity of the antioxidant system involved in scavenging H2O2 increased in all three lines examined during hypoxic treatment, while only the line overexpressing AtGLB1 could retain these relatively high levels up to 48 h of the treatment, suggesting that the antioxidant system might play a role in the low H2O2 level of Arabidopsis overexpressing AtGLB1.

An important role of phosphatidic acid in ABA signaling during germination in Arabidopsis thaliana

Phosphatidic acid (PA) functions as a lipid signaling molecule in plants. Physiological analysis showed that PA triggers early signal transduction events that lead to responses to abscisic acid (ABA) during seed germination. We measured PA production during seed germination and found increased PA levels during early germination. To investigate the role of PA during seed germination, we focused on the PA catabolic enzyme lipid phosphate phosphatase (LPP). LPP catalyzes the conversion of PA to diacylglycerol (DAG). There are 4 LPP genes in the Arabidopsis genome. Among them, AtLPP2 and AtLPP3 are expressed during seed germination. Two AtLPP2 T-DNA insertional mutants (lpp2-1 and lpp2-2) showed hypersensitivity to ABA and significant PA accumulation during germination. Furthermore, double-mutant analysis showed that ABA-insensitive 4 (ABI4) is epistatic to AtLPP2 but ABA-insensitive 3 (ABI3) is not. These results suggest that PA is involved in ABA signaling and that AtLPP2 functions as a negative regulator upstream of ABI4, which encodes an AP2-type transcription factor, in ABA signaling during germination.

Presence and potential signaling function of N-acylethanolamines and their phospholipid precursors in the yeast Saccharomyces cerevisiae

N-acylethanolamines (NAEs) and N-acylphosphatidylethanolamines (NAPEs) are trace constituents of vertebrate cells and tissues and much is known about their metabolism and possible function in animals. Here we report for the first time the identification and quantification of NAEs and NAPEs in several strains of the yeast Saccharomyces cerevisiae. Gas chromatography-mass spectrometry of appropriate derivatives revealed 16:0, 16:1, 18:0 and 18:1 N-acyl groups in both NAE and NAPE whose levels, in wild-type cells, were 50 to 90 and 85 to 750 pmol/micromol lipid P, respectively (depending on the phase of growth). NAPE levels were reduced by 45 to 60% in a strain lacking three type B phospholipases, suggesting their involvement in NAPE synthesis by their known transacylation activity. A yeast strain lacking the YPL103c gene, which codes for a protein with 50.3% homology to human NAPE-specific phospholipase D, exhibited a 60% reduction in NAE, compared to wild-type controls. The exposure of various yeast strains to peroxidative stress, by incubation in media containing 0.6 mM H(2)O(2), resulted in substantial increases in NAE. Because yeast cells lack polyunsaturated fatty acids, they offer a useful system for the study of NAE generation and its potential signaling and cytoprotective effects in the absence of polyunsaturated ("endocannabinoid") congeners.

Mutations in the Arabidopsis phosphoinositide phosphatase gene SAC9 lead to overaccumulation of PtdIns(4,5)P2 and constitutive expression of the stress-response pathway

Phosphoinositides (PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P(2) is one of the best characterized PIs; studies in which PtdIns(4,5)P(2) localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P(2) and its derivative Ins(1,4,5)P(3) accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P(2) signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC) domain phosphatases. In some cases, these phosphatases also act on Ins(1,4,5)P(3). We have characterized the Arabidopsis (Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice (Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High-performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns(4,5)P(2) and Ins(1,4,5)P(3) as compared to wild-type plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress-induced genes and overaccumulate reactive-oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.

Elicitor-induced Activation of Phospholipases Plays an Important Role for the Induction of Defense Responses in Suspension-cultured Rice Cells

Biphasic generation of reactive oxygen species (ROS) induced by N-acetylchitooligosaccharide elicitor in rice cells was associated with the activation of phopholipase C (PLC) and phospholipase D (PLD). The activation of both enzymes was observed for the first phase of ROS generation, but only the activation of PLD was evident for the second response. Activation of PLD was associated with its recruitment to the membrane. Enzymatic products of these phospholipases, diacylglycerol (DG) and phosphatidic acid (PA), could induce ROS generation by themselves. Moreover, the addition of these lipids compensated the inhibition of the second phase of ROS generation by cycloheximide, indicating the involvement of the synthesis of PLD or related proteins in the second phase of ROS generation. DG and PA also induced the expression of elicitor-responsive genes in the absence of the elicitor. They could not induce phytoalexin biosynthesis by themselves but greatly enhanced the elicitor-induced phytoalexin accumulation. Further, the inhibition of PLD by 1-butanol inhibited the elicitor-induced phytoalexin accumulation, indicating the involvement of PLD and its reaction product, PA, in the induction of phytoalexin biosynthesis. These results indicated the importance of phospholipid signaling, especially by PLD and its product PA, in plant defense responses.

Hydrogen peroxide as a signal controlling plant programmed cell death

Hydrogen peroxide (H₂O₂) has established itself as a key player in stress and programmed cell death responses, but little is known about the signaling pathways leading from H₂O₂ to programmed cell death in plants. Recently, identification of key regulatory mutants and near-full genome coverage microarray analysis of H₂O₂-induced cell death have begun to unravel the complexity of the H₂O₂ network. This review also describes a novel link between H₂O₂ and sphingolipids, two signals that can interplay and regulate plant cell death.

Different signaling and cell death roles of heterotrimeric G protein alpha and beta subunits in the Arabidopsis oxidative stress response to ozone

Arabidopsis thaliana plants with null mutations in the genes encoding the alpha and beta subunits of the single heterotrimeric G protein are less and more sensitive, respectively, to O3 damage than wild-type Columbia-0 plants. The first peak of the bimodal oxidative burst elicited by O3 in wild-type plants is almost entirely missing in both mutants. The late peak is normal in plants lacking the Gbeta protein but missing in plants lacking the Galpha protein. Endogenous reactive oxygen species (ROS) are first detectable in chloroplasts of leaf epidermal guard cells. ROS production in adjacent cells is triggered by extracellular ROS signals produced by guard cell membrane-associated NADPH oxidases encoded by the AtrbohD and AtrbohF genes. The late, tissue damage-associated component of the oxidative burst requires only the Galpha protein and arises from multiple cellular sources. The early component of the oxidative burst, arising primarily from chloroplasts, requires signaling through the heterotrimer (or the Gbetagamma complex) and is separable from Galpha-mediated activation of membrane-bound NADPH oxidases necessary for both intercellular signaling and cell death.

Lipids, lipases, and lipid-modifying enzymes in plant disease resistance

Lipids and lipid metabolites influence pathogenesis and resistance mechanisms associated with plant-microbe interactions. Some microorganisms sense their presence on a host by perceiving plant surface waxes, whereas others produce toxins that target plant lipid metabolism. In contrast, plants have evolved to recognize microbial lipopolysaccharides (LPSs), sphingolipids, and lipid-binding proteins as elicitors of defense response. Recent studies have demonstrated that the plasma membrane provides a surface on which some plant resistance (R) proteins perceive pathogen-derived effectors and thus confer race-specific resistance. Plant cell membranes also serve as reservoirs from which biologically active lipids and precursors of oxidized lipids are released. Some of these oxylipins, for example jasmonic acid (JA), are important signal molecules in plant defense. Arabidopsis thaliana is an excellent model plant to elucidate the biosynthesis and metabolism of lipids and lipid metabolites, and the characterization of signaling mechanisms involved in the modulation of plant defense responses by phytolipids. This review focuses on recent studies that highlight the involvement of lipids and lipid metabolites, and enzymes involved in lipid metabolism and modification in plant disease resistance.

Evidence for and characterization of Ca2+ binding to the catalytic region of Arabidopsis thaliana phospholipase Dbeta

Most types of plant phospholipase D (PLD) require Ca(2+) for activity, but how Ca(2+) affects PLD activity is not well understood. We reported previously that Ca(2+) binds to the regulatory C2 domain that occurs in the N terminus of the Ca(2+)-requiring PLDs. Using Arabidopsis thaliana PLDbeta and C2-deleted PLDbeta (PLDbetacat), we now show that Ca(2+) also interacts with the catalytic regions of PLD. PLDbetacat exhibited Ca(2+)-dependent activity, was much less active, and required a higher level of Ca(2+) than the full-length PLDbeta. Ca(2+) binding of the proteins was stimulated by phospholipids; phosphatidylserine was the most effective among those tested. Scatchard plot analysis of Ca(2+) binding data yielded an estimate of 3.6 high affinity (K(d) = 29 mum) binding sites on PLDbeta. The Ca(2+)-PLDbetacat interaction increased the affinity of the protein for the activator, phosphatidylinositol 4,5-bisphosphate, but not for the substrate, phosphatidylcholine. This is in contrast to the effect of Ca(2+) binding to the C2 domain, which stimulates phosphatidylcholine binding but inhibits phosphatidylinositol 4,5-bisphosphate binding of the domain. These results demonstrate the contrasting and complementary effects of the Ca(2+)- and lipid-binding properties of the C2 and catalytic domains of plant PLD and provide insight into the mechanism by which Ca(2+) regulates PLD activity.

Activation of phospholipase D induced by hydrogen peroxide in suspension-cultured rice cells

Hydrogen peroxide (H2O2) (10-100 microM) induced rapid and transient accumulation of phosphatidic acid (PA) in suspension-cultured rice cells. When phospholipase activity in the cellular extract fraction prepared from rice cells treated with H2O2 was assayed in the presence of 1-butanol (0.1%), rapid and transient phosphatidylbutanol (PtdBut) formation was observed. Thus, the H2O2-activated phospholipase was concluded to be phospholipase D (PLD). Furthermore, H2O2 directly induced in vitro PLD activation in the cytosolic fraction without H2O2 treatment. In vitro and in vivo activation of PLD were completely suppressed in the presence of lavendustin A (0.05 mM), a potent inhibitor of protein tyrosine kinase. Phytoalexin biosynthesis induced by N-acetylchitooligosaccharide elicitor was enhanced in the presence of H2O2 (10-100 microM), whereas it was suppressed in the presence of tiron, a potent scavenger of O2-, 1-butanol (0.1%) and lavendustin A (0.05 mM). These results indicate that H2O2-inducible PLD activation enhances signal transduction leading to phytoalexin biosynthesis in rice cells.

The xipotl mutant of Arabidopsis reveals a critical role for phospholipid metabolism in root system development and epidermal cell integrity

Phosphocholine (PCho) is an essential metabolite for plant development because it is the precursor for the biosynthesis of phosphatidylcholine, which is the major lipid component in plant cell membranes. The main step in PCho biosynthesis in Arabidopsis thaliana is the triple, sequential N-methylation of phosphoethanolamine, catalyzed by S-adenosyl-l-methionine:phosphoethanolamine N-methyltransferase (PEAMT). In screenings performed to isolate Arabidopsis mutants with altered root system architecture, a T-DNA mutagenized line showing remarkable alterations in root development was isolated. At the seedling stage, the mutant phenotype is characterized by a short primary root, a high number of lateral roots, and short epidermal cells with aberrant morphology. Genetic and biochemical characterization of this mutant showed that the T-DNA was inserted at the At3g18000 locus (XIPOTL1), which encodes PEAMT (XIPOTL1). Further analyses revealed that inhibition of PCho biosynthesis in xpl1 mutants not only alters several root developmental traits but also induces cell death in root epidermal cells. Epidermal cell death could be reversed by phosphatidic acid treatment. Taken together, our results suggest that molecules produced downstream of the PCho biosynthesis pathway play key roles in root development and act as signals for cell integrity.

Phosphatidic acid accumulation is an early response in the Cf-4/Avr4 interaction

The Cladosporium fulvum (Cf)-4 gene of tomato confers resistance to the fungus C. fulvum, expressing the corresponding avirulence (Avr)4 gene, which codes for an elicitor protein. Little is known about how such mechanisms work, but previous studies have shown that elicitor recognition activates Ca(2+) signalling and protein kinases, such as mitogen-activated protein kinase (MAPK) and calcium-dependent protein kinase (CDPK). Here, we provide evidence that a new signalling component, the lipid second messenger phosphatidic acid (PA), is produced within a few minutes of AVR4/Cf-4 interaction. Using transgenic tobacco cells expressing the tomato Cf-4-resistance gene as a model system, phospholipid signalling pathways were studied by pre-labelling the cells with (32)P(i) and assaying for the formation of lipid signals after challenge with the fungal elicitor AVR4. A dramatic rapid response was an increase in (32)P-PA, together with its metabolic product diacylglycerol pyrophosphate (DGPP). AVR4 increased the levels of PA and DGPP in a Cf-4(+)-, time- and dose-dependent manner, while the non-matching elicitor AVR9 did not trigger any response. In general, PA signalling can be triggered by two different pathways: via phospholipase D (PLD), which generates PA directly by hydrolysing structural phospholipids like phosphatidylcholine (PC), or via PLC, which generates diacylglycerol (DAG) that is subsequently phosphorylated to PA by DAG kinase (DGK). To determine the origin of the AVR4-induced PA formation, a PLD-specific transphosphatidylation assay and a differential (32)P-labelling protocol were used. The results clearly demonstrated that most PA was produced via the phosphorylation of DAG. Neomycin and U73122, inhibitors of PLC activity, inhibited AVR4-induced PA accumulation, suggesting that the increase in DGK activity was because of increased PLC activity producing DAG. Lastly, evidence is provided that PLC signalling and, in particular, PA production could play a role in triggering responses, such as the AVR4-induced oxidative burst. For example, PLC inhibitors inhibited the oxidative burst, and when PA was added to cells, an oxidative burst was induced.

Phospholipase D alpha 1-derived phosphatidic acid interacts with ABI1 phosphatase 2C and regulates abscisic acid signaling

Phospholipase D (PLD) and protein phosphatase 2C (PP2C) both play a role in mediating plant responses to abscisic acid (ABA). In this article, we show that PLD alpha 1 and its product, phosphatidic acid (PA), regulate a PP2C, ABI1, which is a negative regulator of ABA responses in Arabidopsis. Leaves from a T-DNA insertional mutant of PLD alpha 1 and PLD alpha 1-antisense plants lose more water than do wild-type plants. The stomatal closure of PLD alpha 1-null leaves is insensitive to ABA but is promoted by PA. ABA treatment promotes an increase in PA from phosphatidylcholine in wild type but not in PLD alpha 1-null cells. PLD alpha 1-derived PA binds to ABI1; the PA-ABI1 binding is demonstrated by coprecipitating PA with ABI1 from plant cells, measuring binding of PA from vesicles to ABI1, and assaying ABI1 bound to PA immobilized on a filter. Deletion and site-specific mutational analyses show that arginine 73 in ABI1 is essential for PA-ABI1 binding. PA binding decreases the phosphatase activity of ABI1. The lack of ABA-induced production of PA in PLD alpha 1-null cells results in a decrease in the association of ABI1 with the plasma membrane in response to ABA. These results indicate that PA produced by PLD alpha 1 inhibits the function of the negative regulator ABI1, thus promoting ABA signaling. The identification of ABI1 as a direct target of the lipid messenger PA provides a functional link between the two families of important signaling enzymes, PLD and PP2C.

Lipid signaling

Various lipids are involved in mediating plant growth, development and responses to biotic and abiotic cues, and their production is regulated by lipid-signaling enzymes. Lipid-hydrolyzing enzymes play a pivotal role both in the production of lipid messengers and in other processes, such as cytoskeletal rearrangement, membrane trafficking, and degradation. Studies on the downstream targets and modes of action of lipid signals in plants are still in their early stages but distinguishing features of plant lipid-based signaling are being recognized. Phospholipase D enzymes and phosphatidic acid may play a broader role in lipid signaling in plants than in other systems.

Oleic acid levels regulated by glycerolipid metabolism modulate defense gene expression in Arabidopsis

Stearoyl-acyl-carrier-protein-desaturase-mediated conversion of stearic acid (18:0) to oleic acid (18:1) is a key step, which regulates levels of unsaturated fatty acids in cells. We previously showed that stearoyl-acyl-carrier-protein-desaturase mutants ssi2/fab2 carrying a loss-of-function mutation in the plastidial glycerol-3-phosphate (G3P) acyltransferase (act1) have elevated 18:1 levels and are restored in their altered defense signaling. Because G3P is required for the acylation of 18:1 by G3P acyltransferase, it was predicted that reduction of G3P levels should increase 18:1 levels and thereby revert ssi2-triggered phenotypes. Here we show that a mutation in G3P dehydrogenase restores both salicylic acid- and jasmonic acid-mediated phenotypes of ssi2 plants. The G3P dehydrogenase gene was identified by map-based cloning of the ssi2 suppressor mutant rdc8 (gly1-3) and confirmed by epistatic analysis of ssi2 with gly1-1. Restoration of ssi2-triggered phenotypes by the gly1-3 mutation was age-dependent and correlated with the levels of 18:1. Regeneration of G3P pools by glycerol application in ssi2 and ssi2 gly1-3 plants caused a marked reduction in the 18:1 levels, which rendered these plants hypersensitive to glycerol. This hypersensitivity in ssi2 was rescued by the act1 mutation. Furthermore, overexpression of the ACT1 gene resulted in enhanced sensitivity to glycerol. Glycerol application also lowered the 18:1 content in SSI2 plants and converted these into ssi2-mimics. Our results show that 18:1 levels in plastids are regulated by means of acylation with G3P, and a balance between G3P and 18:1 is critical for the regulation of salicylic acid- and jasmonic acid-mediated signaling pathways.

The plasma membrane–bound phospholipase Dδ enhances freezing tolerance in Arabidopsis thaliana

Freezing injury is a major environmental limitation on the productivity and geographical distribution of plants. Here we show that freezing tolerance can be manipulated in Arabidopsis thaliana by genetic alteration of the gene encoding phospholipase Ddelta (PLDdelta), which is involved in membrane lipid hydrolysis and cell signaling. Genetic knockout of the plasma membrane-associated PLDdelta rendered A. thaliana plants more sensitive to freezing, whereas overexpression of PLDdelta increased freezing tolerance. Lipid profiling revealed that PLDdelta contributed approximately 20% of the phosphatidic acid produced in wild-type plants during freezing, and overexpression of PLDdelta increased the production of phosphatidic acid species. The PLDdelta alterations did not affect the expression of the cold-regulated genes COR47 or COR78 or alter cold-induced increases in proline or soluble sugars, suggesting that the PLD pathway is a unique determinant of the response to freezing and may present opportunities for improving plant freezing tolerance.

Arabidopsis phospholipase Dalpha1 interacts with the heterotrimeric G-protein alpha-subunit through a motif analogous to the DRY motif in G-protein-coupled receptors

Phospholipase D (PLD) and heterotrimeric G-protein both play important, diverse roles in cellular regulation and signal transduction. Here we have determined the physical interaction between plant PLD and the only canonical alpha-subunit (Galpha) of the G-protein in Arabidopsis thaliana and the molecular basis for the interaction. PLDalpha1 expressed in either Escherichia coli or Arabidopsis was co-precipitated with Galpha. PLDalpha1 contains a sequence motif analogous to the G alpha-interacting DRY motif normally conserved in G-protein-coupled receptors. Mutation of the central Lys residue PLD(K564A) of this motif abolished the PLDalpha1-Galpha binding, whereas mutation of the two flanking residues PLD(E563A) and PLD(F565A) decreased the binding. Addition of Galpha to PLDalpha1 inhibited PLDalpha1 activity, whereas the PLD(K564A) mutation that disrupted the Galpha-PLDalpha1 binding abolished the inhibition. GTP relieved the Galpha inhibition of PLDalpha1 activity and also inhibited the binding between PLDalpha1 and Galpha. Meanwhile, the PLDalpha1-Galpha interaction stimulated the intrinsic GTPase activity of Galpha. Therefore, these results have demonstrated the direct binding between Galpha and PLDalpha1, identified the DRY motif on PLDalpha1 as the site for the interaction, and indicated that the interaction modulates reciprocally the activities of PLDalpha1 and Galpha.