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

Phosphatidic acid produced by phospholipase Dα1 and Dδ is incorporated into the internal membranes but not involved in the gene expression of RD29A in the abscisic acid signaling network in Arabidopsis thaliana

Core protein components of the abscisic acid (ABA) signaling network, pyrabactin resistance (PYR), protein phosphatases 2C (PP2C), and SNF1-related protein kinase 2 (SnRK2) are involved in the regulation of stomatal closure and gene expression downstream responses in Arabidopsis thaliana. Phosphatidic acid (PA) produced by the phospholipases Dα1 and Dδ (PLDs) in the plasma membrane has been identified as a necessary molecule in ABA-inducible stomatal closure. On the other hand, the involvement of PA in ABA-inducible gene expression has been suggested but remains a question. In this study, the involvement of PA in the ABA-inducible gene expression was examined in the model plant Arabidopsis thaliana and the canonical RD29A ABA-inducible gene that possesses a single ABA-responsive element (ABRE) in the promoter. The promoter activity and accumulation of the RD29A mRNA during ABA exposure to the plants were analyzed under conditions in which the production of PA by PLDs is abrogated through chemical and genetic modification. Changes in the subcellular localization of PA during the signal transduction were analyzed with confocal microscopy. The results obtained in this study suggest that inhibition of PA production by the PLDs does not affect the promoter activity of RD29A. PA produced by the PLDs and exogenously added PA in the plasma membrane are effectively incorporated into internal membranes to transduce the signal. However, exogenously added PA induces stomatal closure but not RD29A expression. This is because PA produced by the PLDs most likely inhibits the activity of not all but only the selected PP2C family members, the negative regulators of the RD29A promoter. This finding underscores the necessity for experimental verifications to adapt previous knowledge into a signaling network model before its construction.

The intricate role of lipids in orchestrating plant defense responses

Plants are exposed to a variety of pests and pathogens that reduce crop productivity. Plants respond to such attacks by activating a sophisticated signaling cascade that initiates with the recognition of pests/pathogens and may culminate into a resistance response. Lipids, being the structural components of cellular membranes, function as mediators of these signaling cascades and thus are instrumental in the regulation of plant defense responses. Accumulating evidence indicates that various lipids such as oxylipins, phospholipids, glycolipids, glycerolipids, sterols, and sphingolipids, among others, are involved in mediating cell signaling during plant-pathogen interaction with each lipid exhibiting a specific biological relevance, follows a distinct biosynthetic mechanism, and contributes to specific signaling cascade(s). Omics studies have further confirmed the involvement of lipid biosynthetic enzymes including the family of phospholipases in the production of defense signaling molecules subsequent to pathogen attack. Lipids participate in stress signaling by (1) mediating the signal transduction, (2) acting as precursors for bioactive molecules, (3) regulating ROS formation, and (4) interacting with various phytohormones to orchestrate the defense response in plants. In this review, we present the biosynthetic pathways of different lipids, their specific functions, and their intricate roles upstream and downstream of phytohormones under pathogen attack to get a deeper insight into the molecular mechanism of lipids-mediated regulation of defense responses in plants.

Lipid Signaling Requires ROS Production to Elicit Actin Cytoskeleton Remodeling during Plant Innate Immunity

In terrestrial plants a basal innate immune system, pattern-triggered immunity (PTI), has evolved to limit infection by diverse microbes. The remodeling of actin cytoskeletal arrays is now recognized as a key hallmark event during the rapid host cellular responses to pathogen attack. Several actin binding proteins have been demonstrated to fine tune the dynamics of actin filaments during this process. However, the upstream signals that stimulate actin remodeling during PTI signaling remain poorly characterized. Two second messengers, reactive oxygen species (ROS) and phosphatidic acid (PA), are elevated following pathogen perception or microbe-associated molecular pattern (MAMP) treatment, and the timing of signaling fluxes roughly correlates with actin cytoskeletal rearrangements. Here, we combined genetic analysis, chemical complementation experiments, and quantitative live-cell imaging experiments to test the role of these second messengers in actin remodeling and to order the signaling events during plant immunity. We demonstrated that PHOSPHOLIPASE Dβ (PLDβ) isoforms are necessary to elicit actin accumulation in response to flg22-associated PTI. Further, bacterial growth experiments and MAMP-induced apoplastic ROS production measurements revealed that PLDβ-generated PA acts upstream of ROS signaling to trigger actin remodeling through inhibition of CAPPING PROTEIN (CP) activity. Collectively, our results provide compelling evidence that PLDβ/PA functions upstream of RBOHD-mediated ROS production to elicit actin rearrangements during the innate immune response in Arabidopsis.

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

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

Survival of Plants During Short-Term BOA-OH Exposure: ROS Related Gene Expression and Detoxification Reactions Are Accompanied With Fast Membrane Lipid Repair in Root Tips

For the characterization of BOA-OH insensitive plants, we studied the time-dependent effects of the benzoxazolinone-4/5/6/7-OH isomers on maize roots. Exposure of Zea mays seedlings to 0.5 mM BOA-OH elicits root zone-specific reactions by the formation of dark rings and spots in the zone of lateral roots, high catalase activity on root hairs, and no visible defense reaction at the root tip. We studied BOA-6-OH- short-term effects on membrane lipids and fatty acids in maize root tips in comparison to the benzoxazinone-free species Abutilon theophrasti Medik. Decreased contents of phosphatidylinositol in A. theophrasti and phosphatidylcholine in maize were found after 10-30 min. In the youngest tissue, α-linoleic acid (18:2), decreased considerably in both species and recovered within one hr. Disturbances in membrane phospholipid contents were balanced in both species within 30-60 min. Triacylglycerols (TAGs) were also affected, but levels of maize diacylglycerols (DAGs) were almost unchanged, suggesting a release of fatty acids for membrane lipid regeneration from TAGs while resulting DAGs are buildings blocks for phospholipid reconstitution, concomitant with BOA-6-OH glucosylation. Expression of superoxide dismutase (SOD2) and of ER-bound oleoyl desaturase (FAD2-2) genes were contemporaneously up regulated in contrast to the catalase CAT1, while CAT3 was arguably involved at a later stage of the detoxification process. Immuno-responses were not elicited in short-terms, since the expression of NPR1, POX12 were barely affected, PR4 after 6 h with BOA-4/7-OH and PR1 after 24 h with BOA-5/6-OH. The rapid membrane recovery, reactive oxygen species, and allelochemical detoxification may be characteristic for BOA-OH insensitive plants.

RESPUESTAS TEMPRANAS EN SIMPLASTO DE TALLO ASOCIADAS A LA RUTA DEL ÁCIDO SALICÍLICO EN LA INTERACCIÓN CLAVEL (Dianthus caryophyllus, caryophyllaceae)- FOD (Fusarium oxysporum f. sp. dianthi)

El marchitamiento vascular causado por Fusarium oxysporum f. sp. dianthi (Fod) es la enfermedad que más afecta el cultivo de clavel. Comprender la naturaleza de la interacción entre la planta y el patógeno permitirá el futuro desarrollo de nuevas alternativas de control de la enfermedad. Es por ello que se busca tener evidencia experimental que permita entender el papel de la ruta de señalización del ácido salicílico (SA) y enzimas asociadas con la resistencia de la planta como son fenilalanina amonio liasa (PAL), polifenoloxidasa (PFO), guayacol peroxidasa (GPX) y fosfolipasa D (PLD), a nivel del simplasto del tallo durante la interacción con él patógeno. Se estableció un ensayo in vivo utilizando dos variedades de clavel con diferentes niveles de resistencia a la enfermedad y se determinaron en simplasto de tallo los niveles de SA, MeSA (salicilato de metilo) y las enzimas objeto de estudio. Se presentó inducción de las enzimas estudiadas, evidenciando en el caso de la enzima GPX un aumento a nivel transcripcional. Así mismo, se presentó un incremento de MeSA en los 1 y 14 dpi, mientras que SA se acumuló en tiempos tardíos. La correlación de Pearson determinó que a este nivel existe una acumulación de la hormona MeSA al 1 dpi con los niveles de las enzimas GPX y PLD. Se propone que la respuesta en este órgano de clavel puede estar activada por la ruta de señalización que involucra SA, afectando el metabolismo secundario y la regulación de especies reactivas de oxígeno.

Lipases associated with plant defense against pathogens

When facing microbe invaders, plants activate genetic and metabolic defense mechanisms and undergo extracellular and intracellular changes to obtain a certain level of host resistance. Dynamic adjustment and adaptation occur in structures containing lipophilic compounds and cellular metabolites. Lipids encompassing fatty acids, fatty acid-based polymers, and fatty acid derivatives are part of the fundamental architecture of cells and tissues and are essential compounds in numerous biological processes. Lipid-associated plant defense responses are mostly facilitated by the activation of lipases (lipid hydrolyzing proteins), which cleave or transform lipid substrates in various subcellular compartments. In this review, several types of plant defense-associated lipases are described, including their molecular aspects, enzymatic actions, cellular functions, and possible functional relevance in plant defense. Defensive roles are discussed considering enzyme properties, lipid metabolism, downstream regulation, and phenotypic traits in loss-of-function mutants.

Phospholipases AtPLDζ1 and AtPLDζ2 function differently in hypoxia

Besides hydrolyzing different membrane phospholipids, plant phospholipases D and molecular species of their byproducts phosphatidic acids (PLDs/PAs) are involved in diverse cellular events such as membrane-cytoskeleton dynamics, hormone regulation and biotic and/or abiotic stress responses at cellular or subcellular levels. Among the 12 Arabidopsis PLD genes, PLDζ1 and PLDζ2 uniquely possess Ca²⁺ -independent phox (PX) and pleckstrin (PH) homology domains. Here, we report that mutants deficient in these PLDs, pldζ1 and pldζ2, show differential sensitivities to hypoxia stimulus. In the present study, we used protoplasts of wild type and mutants and compared the hypoxia-induced changes in the levels of three major signaling mediators such as cytoplasmic free calcium [Ca²⁺_cyt. ], hydrogen peroxide (H₂ O₂ ) and PA. The concentrations of cytosolic Ca²⁺ and H₂ O₂ were determined by fluorescence microscopy and the fluorescent dyes Fura 2-AM and CM-H₂ DCFDA, specific for calcium and H₂ O₂ , respectively, while PA production was analyzed by an enzymatic method. The study reveals that AtPLDζ1 is involved in reactive oxygen species (ROS) signaling, whereas AtPLDζ2 is involved in cytosolic Ca²⁺ signaling pathways during hypoxic stress. Hypoxia induces an elevation of PA level both in Wt and pldζ1, while the PA level is unchanged in pldζ2. Thus, it is likely that AtPLDζ2 is involved in PA production by a calcium signaling pathway, while AtPLDζ1 is more important in ROS signaling.

ROS homeostasis as a prerequisite for the accomplishment of plant cytokinesis

Reactive oxygen species (ROS) are emerging players in several biological processes. The present work investigates their potential involvement in plant cytokinesis by the application of reagents disturbing ROS homeostasis in root-tip cells of Triticum turgidum. In particular, the NADPH-oxidase inhibitor diphenylene iodonium, the ROS scavenger N-acetyl-cysteine, and menadione that leads to ROS overproduction were used. The effects on cytokinetic cells were examined using light, fluorescence, and transmission electron microscopy. ROS imbalance had a great impact on the cytokinetic process including the following: (a) formation of atypical "phragmoplasts" incapable of guiding vesicles to the equatorial plane, (b) inhibition of the dictyosomal and/or endosomal vesicle production that provides the developing cell plates with membranous and matrix polysaccharidic material, (c) disturbance of the fusion processes between vesicles arriving on the cell plate plane, (d) disruption of endocytic vesicle production that mediates the removal of the excess membrane material from the developing cell plate, and (e) the persistence of large callose depositions in treated cell plates. Consequently, either elevated or low ROS levels in cytokinetic root-tip cells resulted in a total inhibition of cell plate assembly or the formation of aberrant cell plates, depending on the stage of the affected cytokinetic cells. The latter failed to expand towards cell cortex and hence to give rise to complete daughter cell wall. These data revealed for the first time the necessity of ROS homeostasis for accomplishment of plant cytokinesis, since it seems to be a prerequisite for almost every aspect of this process.

The durably resistant rice cultivar Digu activates defence gene expression before the full maturation of Magnaporthe oryzae appressorium

Rice blast caused by the fungal pathogen Magnaporthe oryzae is one of the most destructive diseases worldwide. Although the rice-M. oryzae interaction has been studied extensively, the early molecular events that occur in rice before full maturation of the appressorium during M. oryzae invasion are unknown. Here, we report a comparative transcriptomics analysis of the durably resistant rice variety Digu and the susceptible rice variety Lijiangxintuanheigu (LTH) in response to infection by M. oryzae (5, 10 and 20 h post-inoculation, prior to full development of the appressorium). We found that the transcriptional responses differed significantly between these two rice varieties. Gene ontology and pathway analyses revealed that many biological processes, including extracellular recognition and biosynthesis of antioxidants, terpenes and hormones, were specifically activated in Digu shortly after infection. Forty-eight genes encoding receptor kinases (RKs) were significantly differentially regulated by M. oryzae infection in Digu. One of these genes, LOC_Os08g10300, encoding a leucine-rich repeat RK from the LRR VIII-2 subfamily, conferred enhanced resistance to M. oryzae when overexpressed in rice. Our study reveals that a multitude of molecular events occur in the durably resistant rice Digu before the full maturation of the appressorium after M. oryzae infection and that membrane-associated RKs play important roles in the early response.

Salicylic-acid elicited phospholipase D responses in Capsicum chinense cell cultures

The plant response to different stress types can occur through stimulus recognition and the subsequent signal transduction through second messengers that send information to the regulation of metabolism and the expression of defense genes. The phospholipidic signaling pathway forms part of the plant response to several phytoregulators, such as salicylic acid (SA), which has been widely used to stimulate secondary metabolite production in cell cultures. In this work, we studied the effects of SA treatment on [(32)-P]Pi phospholipid turnover and phospholipase D (PLD) activity using cultured Capsicum chinense cells. In cultured cells, the PIP2 turnover showed changes after SA treatment, while the most abundant phospholipids (PLs), such as phosphatidylcholine (PC), did not show changes during the temporal course. SA treatment significantly increased phosphatidic acid (PA) turnover over time compared to control cells. The PA accumulation in cells treated with 1-butanol showed a decrease in messengers; at the same time, there was a 1.5-fold increase in phosphatidylbutanol. These results suggest that the participation of the PLD pathway is a source of PA production, and the activation of this mechanism may be important in the cell responses to SA treatment.

Salicylic acid induces vanillin synthesis through the phospholipid signaling pathway in Capsicum chinense cell cultures

Signal transduction via phospholipids is mediated by phospholipases such as phospholipase C (PLC) and D (PLD), which catalyze hydrolysis of plasma membrane structural phospholipids. Phospholipid signaling is also involved in plant responses to phytohormones such as salicylic acid (SA). The relationships between phospholipid signaling, SA, and secondary metabolism are not fully understood. Using a Capsicum chinense cell suspension as a model, we evaluated whether phospholipid signaling modulates SA-induced vanillin production through the activation of phenylalanine ammonia lyase (PAL), a key enzyme in the biosynthetic pathway. Salicylic acid was found to elicit PAL activity and consequently vanillin production, which was diminished or reversed upon exposure to the phosphoinositide-phospholipase C (PI-PLC) signaling inhibitors neomycin and U73122. Exposure to the phosphatidic acid inhibitor 1-butanol altered PLD activity and prevented SA-induced vanillin production. Our results suggest that PLC and PLD-generated secondary messengers may be modulating SA-induced vanillin production through the activation of key biosynthetic pathway enzymes.

Involvement of phospholipases C and D in the defence responses of riboflavin-treated tobacco cells

Riboflavin is an activator of defence responses in plants that increases resistance against diseases caused by fungal, oomycete, bacterial and viral pathogens. However, the mechanisms driving defence activation by riboflavin are poorly understood. We investigated the signal transduction pathways of phospholipase C (PLC) and phospholipase D (PLD) in tobacco (Nicotiana tabacum) suspension cells using a pharmacological approach to confirm whether riboflavin-mediated activation of the defence response is dependent on both PLC and PLD. The expression patterns analysed by quantitative reverse transcription-polymerase chain reaction demonstrated that the tobacco PLC and PLD gene families were differentially expressed in riboflavin-treated tobacco cells. PLC and PLD expression accompanied defence responses including the expression of defence response genes (PAL, PR-1a and PR-1b), the production of hydrogen peroxide and the accumulation of the phytoalexin scopoletin in tobacco cells treated with riboflavin. These defence responses were significantly inhibited in the presence of the PLC inhibitor U73122 and the PLD inhibitor 1-butanol; however, inhibitor analogues had no effect. Moreover, treating tobacco cells with phosphatidic acid, a signalling molecule produced by phospholipase catalysis, induced the accumulation of the phytoalexin scopoletin and compensated for the suppressive effects of U73122 and 1-butanol on riboflavin-induced accumulation of the phytoalexin. These results offer pharmacological evidence that PLC and PLD play a role in riboflavin-induced defences of tobacco.

Mechanisms for calcium sensing receptor-regulated stomatal closure in response to the extracellular calcium signal

In Arabidopsis, extracellular calcium (Ca²⁺(o)) promotes intracellular calcium (Ca²⁺(i)) transients and stomatal closure, which has been found to be regulated by the calcium sensing receptor (CAS). However, the detailed pathways for transducting the Ca²⁺(o) signal by CAS are still unclear. We found that nitric oxide (NO) and the hydrogen peroxide (H₂O₂) accumulated in the guard cell chloroplast were the two elements that act downstream of the CAS signaling and trigger the stomatal closure by prolonging Ca²⁺(i) transients. Here we provide more commentary on CAS-regulated H₂O₂ generation from chloroplast and Ca²⁺(i) transients in response to Ca²⁺(o), as well as other potential mechanisms that may be involved in the CAS signaling pathway.

The salt stress-induced LPA response in Chlamydomonas is produced via PLA₂ hydrolysis of DGK-generated phosphatidic acid

The unicellular green alga Chlamydomonas has frequently been used as a eukaryotic model system to study intracellular phospholipid signaling pathways in response to environmental stresses. Earlier, we found that hypersalinity induced a rapid increase in the putative lipid second messenger, phosphatidic acid (PA), which was suggested to be generated via activation of a phospholipase D (PLD) pathway and the combined action of a phospholipase C/diacylglycerol kinase (PLC/DGK) pathway. Lysophosphatidic acid (LPA) was also increased and was suggested to reflect a phospholipase A₂ (PLA₂) activity based on pharmacological evidence. The question of PA's and LPA's origin is, however, more complicated, especially as both function as precursors in the biosynthesis of phospho- and galactolipids. To address this complexity, a combination of fatty acid-molecular species analysis and in vivo ³²P-radiolabeling was performed. Evidence is provided that LPA is formed from a distinct pool of PA characterized by a high α-linolenic acid (18:3n-3) content. This molecular species was highly enriched in the polyphosphoinositide fraction, which is the substrate for PLC to form diacylglycerol. Together with differential ³²P-radiolabeling studies and earlier PLD-transphosphatidylation and PLA₂-inhibitor assays, the data were consistent with the hypothesis that the salt-induced LPA response is primarily generated through PLA₂-mediated hydrolysis of DGK-generated PA and that PLD or de novo synthesis [via endoplasmic reticulum - or plastid-localized routes] is not a major contributor.

Innate immunity in rice

Advances in studies of rice innate immunity have led to the identification and characterization of host sensors encoding receptor kinases that perceive conserved microbial signatures. Receptor kinases that carry the non-orginine-aspartate domain, are highly expanded in rice (Oryza sativa) compared with Arabidopsis (Arabidopsis thaliana). Researchers have also identified a diverse array of microbial effectors from bacterial and fungal pathogens that triggers immune responses upon perception. These include effectors that indirectly target host Nucleotide binding site/Leucine rich repeat proteins and transcription activator-like effectors that directly bind promoters of host genes. Here we review the recognition and signaling events that govern rice innate immunity.

Pdk1 kinase regulates basal disease resistance through the OsOxi1-OsPti1a phosphorylation cascade in rice

The AGC kinase OsOxi1, which has been isolated as an interactor with OsPti1a, positively regulates basal disease resistance in rice. In eukaryotes, AGC kinase family proteins are regulated by 3-phosphoinositide-dependent protein kinase 1 (Pdk1). In Arabidopsis, AtPdk1 directly interacts with phosphatidic acid, which functions as a second messenger in both biotic and abiotic stress responses. However, the functions of Pdk1 are poorly understood in plants. We show here that OsPdk1 acts upstream of the OsOxi1-OsPti1a signal cascade in disease resistance in rice. OsPdk1 interacts with OsOxi1 and phosphorylates the Ser283 residue of OsOxi1 in vitro. To investigate whether OsPdk1 is involved in immunity that is triggered by microbial-associated molecular patterns, we analyzed the phosphorylation status of OsPdk1 in response to chitin elicitor. Like OsOxi1, OsPdk1 is rapidly phosphorylated in response to chitin elicitor, suggesting that OsPdk1 participates in signal transduction through pathogen recognition. The overexpression of OsPdk1 enhanced basal resistance against a blast fungus, Magnaporthe oryzae, and a bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). Taken together, these results suggest that OsPdk1 positively regulates basal disease resistance through the OsOxi1-OsPti1a phosphorylation cascade in rice.

Silymarin secretion and its elicitation by methyl jasmonate in cell cultures of Silybum marianum is mediated by phospholipase D-phosphatidic acid

The flavonolignan silymarin is released to the extracellular medium of Silybum marianum cultures and its production can be stimulated by the elicitor methyljasmonate (MeJA). The sequence of the signalling processes leading to this response is unknown at present. It is reported in this work that MeJA increased the activity of the enzyme phospholipase D (PLD). Treatment with mastoparan (Mst), a PLD activity stimulator, also enhanced PLD and caused a substantial increase in silymarin production. The application of the product of PLD activity, phosphatidic acid (PA) promoted silymarin accumulation. Altering PLD activity by introducing in cultures n-butanol (nBuOH), which inhibits PA production by PLD, prevented silymarin elicitation by MeJA or Mst and also impeded its release in non-elicited cultures. Treatment with iso-, sec- or tert- butanol had no effect on silymarin production. The exogenous addition of PA reversed the inhibitory action of nBuOH, both in control and MeJA-treated cultures. These results suggest that the enzyme PLD and its product PA mediate silymarin secretion to the medium of S. marianum cultures.

Biological elicitors of plant secondary metabolites: mode of action and use in the production of nutraceutics

Many secondary metabolites of interest for human health and nutrition are produced by plants when they are under attack of microbial pathogens or insects. Treatment with elicitors derived from phytopathogens can be an effective strategy to increase the yield of specific metabolites obtained from plant cell cultures. Understanding how plant cells perceive microbial elicitors and how this perception leads to the accumulation of secondary metabolites, may help us improve the production of nutraceutics in terms of quantity and of quality of the compounds. The knowledge gathered in the past decades on elicitor perception and transduction is now being combined to high-throughput methodologies, such as transcriptomics and metabolomics, to engineer plant cells that produce compounds of interest at industrial scale.

Differential responses of rice to inoculation with wild-type and non-pathogenic mutants of Magnaporthe oryzae

We analyzed the response of rice to Magnaporthe oryzae infection using two mutant strains deficient in Mgb1 and Mst12, which are essential for the development of appresoria and penetration pegs. Both mutants induced the much lower levels of accumulation of phytoalexins than wild-type, suggesting that the massive production of phytoalexins requires the fungal invasion of rice cells. Intense accumulation of H2O2 in a single whole cell also required fungal penetration. Microarray analysis of rice gene expression revealed mutant-specific gene expression, indicating that signal exchange between rice and M. oryzae commence before fungal penetration of the rice cell. In situ detection of mRNAs for peroxidase and beta-1,3-glucanase showed that expression of these genes also occurs after penetration as observed for phytoalexin production.

Suppression of a phospholipase D gene, OsPLDbeta1, activates defense responses and increases disease resistance in rice

Phospholipase D (PLD) plays an important role in plants, including responses to abiotic as well as biotic stresses. A survey of the rice (Oryza sativa) genome database indicated the presence of 17 PLD genes in the genome, among which OsPLDalpha1, OsPLDalpha5, and OsPLDbeta1 were highly expressed in most tissues studied. To examine the physiological function of PLD in rice, we made knockdown plants for each PLD isoform by introducing gene-specific RNA interference constructs. One of them, OsPLDbeta1-knockdown plants, showed the accumulation of reactive oxygen species in the absence of pathogen infection. Reverse transcription-polymerase chain reaction and DNA microarray analyses revealed that the knockdown of OsPLDbeta1 resulted in the up-/down-regulation of more than 1,400 genes, including the induction of defense-related genes such as pathogenesis-related protein genes and WRKY/ERF family transcription factor genes. Hypersensitive response-like cell death and phytoalexin production were also observed at a later phase of growth in the OsPLDbeta1-knockdown plants. These results indicated that the OsPLDbeta1-knockdown plants spontaneously activated the defense responses in the absence of pathogen infection. Furthermore, the OsPLDbeta1-knockdown plants exhibited increased resistance to the infection of major pathogens of rice, Pyricularia grisea and Xanthomonas oryzae pv oryzae. These results suggested that OsPLDbeta1 functions as a negative regulator of defense responses and disease resistance in rice.

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.

Expression analysis of a plum pathogenesis related 10 (PR10) protein during brown rot infection

Plant PR10 is one of the pathogenesis related proteins, induced upon exposure to different stress conditions including fungal infection. PR10 proteins have been implicated in fungal disease resistance in some species; however its transcriptional regulation is not well understood. In the present work we cloned a PR10 gene from European plums (Prunus domestica L.) and monitored the quantitative changes in its transcript levels as a result of fungal infection in two varieties. We also studied the possible involvement of the membrane degrading enzyme phospholipase D-alpha (PLDalpha). In the susceptible variety, 'Veeblue', infection with the brown rot fungus Monilinia fructicola induced PLDalpha and PR10 expression, while in the resistant variety, 'Violette', a constitutive expression of PLDalpha and PR10 transcripts levels were observed. Resistance to M. fructicola also coincides with a sharp decrease in the expression of ABI1, a protein phosphatase and elevated hydrogen peroxide content after infection. Further, inhibition of PLDalpha by hexanal treatment, up-regulated ABI1 and decreased PR10 expression, suggesting a possible relationship between the two. We further confirm these results in Arabidopsis abi1 mutant that shows a higher level of PR10 transcripts.

Nitric oxide-induced phosphatidic acid accumulation: a role for phospholipases C and D in stomatal closure

Stomatal closure is regulated by a complex network of signalling events involving numerous intermediates, among them nitric oxide (NO). Little is known about the signalling events occurring downstream of NO. Previous studies have shown that NO modulates cytosolic calcium concentration and the activation of plasma membrane ion channels. Here we provide evidence that supports the involvement of the lipid second messenger phosphatidic acid (PA) in NO signalling during stomatal closure. PA levels in Vicia faba epidermal peels increased upon NO treatment to maximum levels within 30 min, subsequently decreasing to control levels at 60 min. PA can be generated via phospholipase D (PLD) or via phospholipase C (PLC) in concerted action with diacylglycerol kinase (DGK). Our results showed that NO-induced PA is produced via the activation of both pathways. NO-induced stomatal closure was blocked either when PLC or PLD activity was inhibited. We have shown that PLC- and PLD-derived PA represents a downstream component of NO signalling cascade during stomatal closure.

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.

Early events in the signalling pathway for the activation of MAPKs in rice roots exposed to nickel

It is well known that small quantities of nickel (Ni) are essential for plant species, and higher concentrations of Ni retard plant growth. However, the molecular mechanisms responsible for the regulation of plant growth by Ni are not well understood. The aim of this study is to investigate the early signalling pathways activated by Ni on rice (Oryza sativa L.) root. We showed that Ni elicited a remarkable increase in myelin basic protein (MBP) kinase activities. By immunoblot and immunoprecipitation analyses, it is suggested that Ni-activated 40- and 42-kDa MBP kinases are mitogen-activated protein kinases (MAPKs). Pretreatment of rice roots with the antioxidant, glutathione (GSH), the phospholipase D (PLD) inhibitor, n-butanol, and the calmodulin and CDPK antagonist and W7 inhibited Ni-induced MAPK activation. These results suggest that various signalling components are involved in transduction of the Ni signal in rice roots.

Genome-wide analysis of the phospholipase D family in Oryza sativa and functional characterization of PLDβ1 in seed germination

Phospholipase D (PLD) plays a critical role in plant growth and development, as well as in hormone and stress responses. PLD encoding genes constitute a large gene family that are present in higher plants. There are 12 members of the PLD family in Arabidopsis thaliana and several of them have been functionally characterized; however, the members of the PLD family in Oryza sativa remain to be fully described. Through genome-wide analysis, 17 PLD members found in different chromosomes have been identified in rice. Protein domain structural analysis reveals a novel subfamily, besides the C2-PLDs and PXPH-PLDs, that is present in rice - the SP-PLD. SP-PLD harbors a signal peptide instead of the C2 or PXPH domains at the N-terminus. Expression pattern analysis indicates that most PLD-encoding genes are differentially expressed in various tissues, or are induced by hormones or stress conditions, suggesting the involvement of PLD in multiple developmental processes. Transgenic studies have shown that the suppressed expression of rice PLD beta 1 results in reduced sensitivity to exogenous ABA during seed germination. Further analysis of the expression of ABA signaling-related genes has revealed that PLD beta 1 stimulates ABA signaling by activating SAPK, thus repressing GAmyb expression and inhibiting seed germination.

Nitric Oxide Is Critical for Inducing Phosphatidic Acid Accumulation in Xylanase-elicited Tomato Cells

Nitric Oxide (NO) is a second messenger related to development and (a)biotic stress responses in plants. We have studied the role of NO in signaling during plant defense responses upon xylanase elicitation. Treatment of tomato cell cultures with the fungal elicitor xylanase resulted in a rapid and dose-dependent NO accumulation. We have demonstrated that NO is required for the production of the lipid second messenger phosphatidic acid (PA) via the activation of the phospholipase C (PLC) and diacylglycerol kinase (DGK) pathway. Defense-related responses downstream of PA were studied. PA and, correspondingly, xylanase were shown to induce reactive oxygen species production. Scavenging of NO or inhibition of either the PLC or the DGK enzyme diminished xylanase-induced reactive oxygen species production. Xylanase-induced PLDbeta1 and PR1 mRNA levels decreased when NO or PA production were compromised. Finally, we have shown that NO and PA are involved in the induction of cell death by xylanase. Treatment with NO scavenger cPTIO, PLC inhibitor U73122, or DGK inhibitor R59022 diminished xylanase-induced cell death. On the basis of biochemical and pharmacological experimental results, we have shown that PLC/DGK-derived PA represents a novel downstream component of NO signaling cascade during plant defense.

Phospholipase C/diacylglycerol kinase-mediated signalling is required for benzothiadiazole-induced oxidative burst and hypersensitive cell death in rice suspension-cultured cells

The involvement of phospholipase C/diacylglycerol kinase (PLC/DGK)-mediated signalling in oxidative burst and hypersensitive cell death was studied in rice suspension-cultured cells treated with benzothiadiazole (BTH) and infected by Xanthomonas oryza pv. oryza (Xoo), the causal agent of rice leaf blight disease. Treatment of rice suspension cells with BTH resulted in a significant oxidative burst, as indicated by accumulation of superoxide anion and H(2)O(2), and hypersensitive cell death, as determined by Evans blue staining. A peak in oxidative burst was detected 3-4 h after BTH treatment and hypersensitive cell death was observed 8 h after treatment. In addition, significant oxidative burst and hypersensitive cell death were detected in BTH-treated suspension cells, but not in untreated control cells, after Xoo infection. Scavengers and antioxidants of active oxygen species, e.g., superoxide dismutase, catalase, N-acetylcysteine, and flavone, reduced significantly the BTH-induced oxidative burst and hypersensitive cell death, indicating that oxidative burst is required for BTH-induced hypersensitive cell death. Expression of the PLC/DGK pathway genes, a diacylglycerol kinase gene, OsDAGK1, and a phosphoinositide-specific phospholipase C gene, OsPI-PLC1, and a defence-related EREBP transcriptional factor gene, OsBIERF3, was activated in rice cells after BTH treatment and in the BTH-treated cells after Xoo infection. Treatment of rice cells with phosphatidic acid, a phospholipid signalling molecule, resulted in the production of oxidative burst and hypersensitive cell death. However, neomycin, a PLC inhibitor, inhibited partially but not completely the production of oxidative burst, hypersensitive cell death, and expression of OsBIERF3 and OsDAGK1 induced by BTH in rice cells. These results suggest that PLC/DGK-mediated signalling plays an important role in BTH-induced oxidative burst, hypersensitive response, and activation of defence response in rice.

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.

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.

LePLDbeta1 activation and relocalization in suspension-cultured tomato cells treated with xylanase

Phospholipase D (PLD) has been implicated in various cellular processes including membrane degradation, vesicular trafficking and signal transduction. Previously, we described a PLD gene family in tomato (Lycopersicon esculentum) and showed that expression of one of these genes, LePLDbeta1, was induced by treatment with the fungal elicitor xylanase. To further investigate the function of this PLD, a gene-specific RNAi construct was used to knock down levels of LePLDbeta1 transcript in suspension-cultured tomato cells. Silenced cells exhibited a strong decrease in xylanase-induced PLD activity and responded to xylanase treatment with a disproportionate oxidative burst. Furthermore, LePLDbeta1-silenced cell-suspension cultures were found to have increased polyphenol oxidase activity, to secrete less of the beta-d-xylosidase LeXYL2 and to secrete and express more of the xyloglucan-specific endoglucanase inhibitor protein XEGIP. Using an LePLDbeta1-green fluorescent protein (GFP) fusion protein for confocal laser scanning microscopy-mediated localization studies, untreated cells displayed a cytosolic localization, whereas treatment with xylanase induced relocalization to punctuate structures within the cytosol. Possible functions for PLDbeta in plant-pathogen interactions are discussed.

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.

Elicitor signal transduction leading to production of plant secondary metabolites

Plant secondary metabolites are unique sources for pharmaceuticals, food additives, flavors, and other industrial materials. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Understanding signal transduction paths underlying elicitor-induced production of secondary metabolites is important for optimizing their commercial production. This paper summarizes progress made on several aspects of elicitor signal transduction leading to production of plant secondary metabolites, including: elicitor signal perception by various receptors of plants; avirulence determinants and corresponding plant R proteins; heterotrimeric and small GTP binding proteins; ion fluxes, especially Ca2+ influx, and Ca2+ signaling; medium alkalinization and cytoplasmic acidification; oxidative burst and reactive oxygen species; inositol trisphosphates and cyclic nucleotides (cAMP and cGMP); salicylic acid and nitric oxide; jasmonate, ethylene, and abscisic acid signaling; oxylipin signals such as allene oxide synthase-dependent jasmonate and hydroperoxide lyase-dependent C12 and C6 volatiles; as well as other lipid messengers such as lysophosphatidylcholine, phosphatidic acid, and diacylglycerol. All these signal components are employed directly or indirectly by elicitors for induction of plant secondary metabolite accumulation. Cross-talk between different signaling pathways is very common in plant defense response, thus the cross-talk amongst these signaling pathways, such as elicitor and jasmonate, jasmonate and ethylene, and each of these with reactive oxygen species, is discussed separately. This review also highlights the integration of multiple signaling pathways into or by transcription factors, as well as the linkage of the above signal components in elicitor signaling network through protein phosphorylation and dephosphorylation. Some perspectives on elicitor signal transduction and plant secondary metabolism at the transcriptome and metabolome levels are also presented.

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.