A protein kinase target of a PDK1 signalling pathway is involved in root hair growth in Arabidopsis

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.

Two Arabidopsis AGC kinases are critical for the polarized growth of pollen tubes

Reproduction of flowering plants requires the growth of pollen tubes to deliver immotile sperm for fertilization. Pollen tube growth resembles that of polarized metazoan cells, in that some molecular mechanisms underlying cell polarization and growth are evolutionarily conserved, including the functions of Rho GTPases and the dynamics of the actin cytoskeleton. However, a role for AGC kinases, crucial signaling mediators in polarized metazoan cells, has yet to be shown in pollen tubes. Here we demonstrate that two Arabidopsis AGC kinases are critical for polarized growth of pollen tubes. AGC1.5 and AGC1.7 are pollen-specific genes expressed during late developmental stages. Pollen tubes of single mutants had no detectable phenotypes during in vitro or in vivo germination, whereas those of double mutants were wider and twisted, due to frequent changes of growth trajectory in vitro. Pollen tubes of the double mutant also had reduced growth and were probably compromised in response to guidance cues in vivo. In the agc1.5 background, downregulation of AGC1.7 using an antisense construct phenocopied the growth defect of double mutant pollen tubes, providing additional support for a redundant function of AGC1.5/1.7 in pollen tube growth. Using the actin marker mouse Talin, we show that pollen tubes of double mutants had relatively unaffected longitudinal actin cables but had ectopic filamentous actin, indicating disturbed control of polarity. Our results demonstrate that AGC1.5 and AGC1.7 are critical components of the internal machinery of the pollen tube leading to polarized growth of pollen tubes.

Redox regulation in photosynthetic organisms: signaling, acclimation, and practical implications

Reactive oxygen species (ROS) have multifaceted roles in the orchestration of plant gene expression and gene-product regulation. Cellular redox homeostasis is considered to be an "integrator" of information from metabolism and the environment controlling plant growth and acclimation responses, as well as cell suicide events. The different ROS forms influence gene expression in specific and sometimes antagonistic ways. Low molecular antioxidants (e.g., ascorbate, glutathione) serve not only to limit the lifetime of the ROS signals but also to participate in an extensive range of other redox signaling and regulatory functions. In contrast to the low molecular weight antioxidants, the "redox" states of components involved in photosynthesis such as plastoquinone show rapid and often transient shifts in response to changes in light and other environmental signals. Whereas both types of "redox regulation" are intimately linked through the thioredoxin, peroxiredoxin, and pyridine nucleotide pools, they also act independently of each other to achieve overall energy balance between energy-producing and energy-utilizing pathways. This review focuses on current knowledge of the pathways of redox regulation, with discussion of the somewhat juxtaposed hypotheses of "oxidative damage" versus "oxidative signaling," within the wider context of physiological function, from plant cell biology to potential applications.

The effect of phospholipase Dalpha3 on Arabidopsis response to hyperosmotic stress and glucose

Membranes are the primary sites of perception for extracellular stimuli and are rich sources for signaling messengers. Phospholipase D (PLD) hydrolyzes membrane lipids to produce the messenger phosphatidic acid (PA), and the activation of PLD occurs under different hyperosmotic stresses, including dehydration and salt stress. We have recently found that PLDalpha3 that plays a positive role in hyperosmotic stress. PLDalpha3 hydrolyzes multiple substrates with distinguishable preferences. The involvement of PLDalpha3 in hyperosmotic stress is through a different mechanism from that PLDalpha1, which mediates the effect of abscisic acid on stomatal movements. PLDalpha3 enhances root growth and accelerates flowering time under hyperosmotic stress. Alterations of PLDalpha3 affect the level of PA, transcripts of TOR and AGC2.1, ABA-responsive genes, and phosphorylated S6K protein under hyperosmotic stress. Our further observation shows that PLDalpha3 is also involved in glucose response. PLDalpha3-KO seeds and seedlings are less sensitive to glucose whereas PLDalpha3-overepressed seeds are more sensitive than wild type. These results point to a possibility that PLDalpha3-mediated lipid signaling may play a role in integrating nutrient sensing, protein kinase activation, and hormones responses to regulate growth and development under hyperosmotic stress.

The involvement of phospholipases C and D in the asymmetric division of subsidiary cell mother cells of Zea mays

In the present study, the involvement of phospholipase C and D (PLC and PLD) pathways in the asymmetric divisions that produce the stomatal complexes of Zea mays was investigated. In particular, the polar organization of microtubules (MTs) and actin filaments (AFs) and the process of asymmetric division were studied in subsidiary cell mother cells (SMCs) treated with PLC and PLD modulators. In SMCs treated with butanol-1 (but-1), which blocks phosphatidic acid (PA) production via PLDs, AF-patch formation laterally to the inducing guard cell mother cell (GMC) and the subsequent asymmetric division were inhibited. In these SMCs, cell division plane determination, as expressed by MT preprophase band (MT-PPB) formation, was not disturbed. Exogenously applied PA partially relieved the but-1 effects on SMCs. In contrast to SMCs, but-1 did not affect the symmetric GMC division. Inhibition of the PLC catalytic activity by neomycin or U73122 resulted in inhibition of asymmetric SMC division, while AF-patch and MT-PPB were organized as in control SMCs. These data show that the PLC and PLD signaling pathways are involved in the transduction and/or perception of the inductive stimulus that is emitted by the GMCs and induces the polar AF organization and asymmetric SMC division. In contrast, division plane determination in SMCs, as expressed by MT-PPB formation, does not depend on PLC and PLD signaling pathways.

Genetic and physical interaction suggest that BARREN STALK 1 is a target of BARREN INFLORESCENCE2 in maize inflorescence development

Organogenesis in plants is controlled by polar auxin transport. In maize (Zea mays), barren inflorescence2 (bif2) encodes a co-ortholog of the serine/threonine protein kinase PINOID (PID), which regulates auxin transport in Arabidopsis. In this paper, we report that the basic helix-loop-helix transcription factor BARREN STALK1 (BA1) is a putative target of BIF2, revealing a previously unknown function of BIF2 in the nucleus. Both bif2 and ba1 are required for axillary meristem initiation during inflorescence and vegetative development in maize. Using a yeast two-hybrid approach, we identified BA1 as an interacting partner with BIF2. We confirmed the interaction by in vitro pull-down assays, and demonstrated that BIF2 phosphorylates BA1 in vitro. Previously, RNA in situ hybridization showed that bif2 and ba1 are both expressed during axillary meristem initiation. Here, we heterologously expressed BIF2 and BA1, and found that they co-localize in the nucleus. Based on these findings, we propose that in addition to regulating auxin transport at the cell periphery, BIF2 also functions in the nucleus by interacting with BA1 to promote axillary meristem initiation. Double mutant analysis is consistent with these results, showing that bif2 and ba1 have overlapping as well as unique roles in inflorescence development.

Post-translational modification of host proteins in pathogen-triggered defence signalling in plants

Microbial plant pathogens impose a continuous threat to global food production. Similar to animals, an innate immune system allows plants to recognize pathogens and swiftly activate defence. To activate a rapid response, receptor-mediated pathogen perception and subsequent downstream signalling depends on post-translational modification (PTM) of components essential for defence signalling. We discuss different types of PTMs that play a role in mounting plant immunity, which include phosphorylation, glycosylation, ubiquitination, sumoylation, nitrosylation, myristoylation, palmitoylation and glycosylphosphatidylinositol (GPI)-anchoring. PTMs are rapid, reversible, controlled and highly specific, and provide a tool to regulate protein stability, activity and localization. Here, we give an overview of PTMs that modify components essential for defence signalling at the site of signal perception, during secondary messenger production and during signalling in the cytoplasm. In addition, we discuss effectors from pathogens that suppress plant defence responses by interfering with host PTMs.

Nitric oxide triggers phosphatidic acid accumulation via phospholipase D during auxin-induced adventitious root formation in cucumber

Auxin and nitric oxide (NO) play fundamental roles throughout plant life. NO is a second messenger in auxin signal transduction leading to root developmental processes. The mechanisms triggered by auxin and NO that direct adventitious root (AR) formation are beginning to be unraveled. The goal of this work was to study phospholipid (PL) signaling during the auxin- and NO-induced AR formation in cucumber (Cucumis sativus) explants. Explants were labeled with 32P-inorganic phosphate and treated with the auxins indole-3-acetic acid or 1-naphthylacetic acid, or the NO donor S-nitroso N-acetyl penicillamine, in the presence or absence of the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. PLs were separated by thin-layer chromatography and quantified. We report that the signaling PLs phosphatidic acid (PA), phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulated within 1 min after auxin or NO treatment. Both auxin and NO evoked similar and transient time course responses, since signaling PLs returned to control levels after 20 or 30 min of treatment. The results indicate that auxin relies on NO in inducing PA, phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulation. Furthermore, we demonstrate that auxin and NO trigger PA formation via phospholipase D (PLD) activity. Explants treated for 10 min with auxin or NO displayed a 200% increase in AR number compared with control explants. In addition, PLD activity was required for the auxin- and NO-induced AR formation. Finally, exogenously applied PA increased up to 300% the number of ARs. Altogether, our data support the idea that PLD-derived PA is an early signaling event during AR formation induced by auxin and NO in cucumber explants.

Comparative proteomics analysis reveals an intimate protein network provoked by hydrogen peroxide stress in rice seedling leaves

Hydrogen peroxide (H2O2) plays a dual role in plants as the toxic by-product of normal cell metabolism and as a regulatory molecule in stress perception and signal transduction. However, a clear inventory as to how this dual function is regulated in plants is far from complete. In particular, how plants maintain survival under oxidative stress via adjustments of the intercellular metabolic network and antioxidative system is largely unknown. To investigate the responses of rice seedlings to H2O2 stress, changes in protein expression were analyzed using a comparative proteomics approach. Treatments with different concentrations of H2O2 for 6 h on 12-day-old rice seedlings resulted in several stressful phenotypes such as rolling leaves, decreased photosynthetic and photorespiratory rates, and elevated H2O2 accumulation. Analysis of approximately 2000 protein spots on each two-dimensional electrophoresis gel revealed 144 differentially expressed proteins. Of them, 65 protein spots were up-regulated, and 79 were down-regulated under at least one of the H2O2 treatment concentrations. Furthermore 129 differentially expressed protein spots were identified by mass spectrometry to match 89 diverse protein species. These identified proteins are involved in different cellular responses and metabolic processes with obvious functional tendencies toward cell defense, redox homeostasis, signal transduction, protein synthesis and degradation, photosynthesis and photorespiration, and carbohydrate/energy metabolism, indicating a good correlation between oxidative stress-responsive proteins and leaf physiological changes. The abundance changes of these proteins, together with their putative functions and participation in physiological reactions, produce an oxidative stress-responsive network at the protein level in H2O2-treated rice seedling leaves. Such a protein network allows us to further understand the possible management strategy of cellular activities occurring in the H2O2-treated rice seedling leaves and provides new insights into oxidative stress responses in plants.

Phospholipase Dalpha3 is involved in the hyperosmotic response in Arabidopsis

Rapid activation of phospholipase D (PLD), which hydrolyzes membrane lipids to generate phosphatidic acid (PA), occurs under various hyperosmotic conditions, including salinity and water deficiency. The Arabidopsis thaliana PLD family has 12 members, and the function of PLD activation in hyperosmotic stress responses has remained elusive. Here, we show that knockout (KO) and overexpression (OE) of previously uncharacterized PLDalpha3 alter plant response to salinity and water deficit. PLDalpha3 uses multiple phospholipids as substrates with distinguishable preferences, and alterations of PLDalpha3 result in changes in PA level and membrane lipid composition. PLDalpha3-KO plants display increased sensitivities to salinity and water deficiency and also tend to induce abscisic acid-responsive genes more readily than wild-type plants, whereas PLDalpha3-OE plants have decreased sensitivities. In addition, PLDalpha3-KO plants flower later than wild-type plants in slightly dry conditions, whereas PLDalpha3-OE plants flower earlier. These data suggest that PLDalpha3 positively mediates plant responses to hyperosmotic stresses and that increased PLDalpha3 expression and associated lipid changes promote root growth, flowering, and stress avoidance.

Specificity of RCN1-mediated protein phosphatase 2A regulation in meristem organization and stress response in roots

Protein dephosphorylation by the serine/threonine protein phosphatase 2A (PP2A) modulates a broad array of cellular functions. PP2A normally acts as a heterotrimeric holoenzyme complex comprising a catalytic subunit bound by regulatory A and B subunits. Characterization of the regulatory A subunit isoforms (ROOTS CURL IN NAPHTHYLPHTHALAMIC ACID1 [RCN1], PP2AA2, and PP2AA3) of Arabidopsis thaliana PP2A has shown that RCN1 plays a primary role in controlling root and hypocotyl PP2A activity in seedlings. Here we show that hypocotyl and root growth exhibit different requirements for RCN1-mediated regulation of PP2A activity. Roots of rcn1 mutant seedlings exhibit characteristic abnormalities in cell division patterns at the root apical meristem, as well as reduced growth under ionic, osmotic, and oxidative stress conditions. We constructed chimeric A subunit genes and found that restoration of normal root tip development in rcn1 plants requires both regulatory and coding sequences of RCN1, whereas the hypocotyl elongation defect of rcn1 plants can be complemented by either RCN1 or PP2AA3 transgenes. Furthermore, the RCN1 and PP2AA3 proteins exhibit ubiquitous subcellular localization patterns in seedlings and both associate with membrane compartments. Together, these results show that RCN1-containing PP2A has unique functions that cannot be attributed to isoform-specific expression and localization patterns. Postembryonic RCN1 function is required to maintain normal auxin distribution and stem cell function at the root apex. Our data show that RCN1-regulated phosphatase activity plays a unique role in regulating postembryonic root development and stress response.

The Arabidopsis Phosphatidylinositol Phosphate 5-Kinase PIP5K3 is a key regulator of root hair tip growth

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] functions as a site-specific signal on membranes to promote cytoskeletal reorganization and membrane trafficking. Localization of PtdIns(4,5)P2 to apices of growing root hairs and pollen tubes suggests that it plays an important role in tip growth. However, its regulation and mode of action remain unclear. We found that Arabidopsis thaliana PIP5K3 (for Phosphatidylinositol Phosphate 5-Kinase 3) encodes a phosphatidylinositol 4-phosphate 5-kinase, a key enzyme producing PtdIns(4,5)P2, that is preferentially expressed in growing root hairs. T-DNA insertion mutations that substantially reduced the expression of PIP5K3 caused significantly shorter root hairs than in the wild type. By contrast, overexpression caused longer root hairs and multiple protruding sites on a single trichoblast. A yellow fluorescent protein (YFP) fusion of PIP5K3, driven by the PIP5K3 promoter, complemented the short-root-hair phenotype. PIP5K3-YFP localized to the plasma membrane and cytoplasmic space of elongating root hair apices, to growing root hair bulges, and, notably, to sites about to form root hair bulges. The signal was greatest in rapidly growing root hairs and quickly disappeared when elongation ceased. These results provide evidence that PIP5K3 is involved in localizing PtdIns(4,5)P2 to the elongating root hair apex and is a key regulator of the machinery that initiates and promotes root hair tip growth.

A genetic regulatory network in the development of trichomes and root hairs

Trichomes and root hairs differentiate from epidermal cells in the aerial tissues and roots, respectively. Because trichomes and root hairs are easily accessible, particularly in the model plant Arabidopsis, their development has become a well-studied model of cell differentiation and growth. Molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of trichomes and root hair/hairless cells is regulated by similar molecular mechanisms. Transcriptional complexes regulate differentiation into trichome cells and root hairless cells, and formation of the transcriptional complexes is inhibited in neighboring cells. Control of cell growth after fate determination has also been analyzed using Arabidopsis mutants. The progression of endoreduplication cycles, reorientation of microtubules, and organization of the actin cytoskeleton play important roles in trichome growth. Various cellular components such as ion channels, the actin cytoskeleton, microtubules and cell wall materials, and intracellular signal transduction act to establish and maintain root hair tip growth.

Plant evolution: AGC kinases tell the auxin tale

The signaling molecule auxin is a central regulator of plant development, which instructs tissue and organ patterning, and couples environmental stimuli to developmental responses. Here, we discuss the function of PINOID (PID) and the phototropins, members of the plant specific AGCVIII protein kinases, and their role in triggering and regulating development by controlling PIN-FORMED (PIN) auxin transporter-generated auxin gradients and maxima. We propose that the AGCVIII kinase gene family evolved from an ancestral phototropin gene, and that the co-evolution of PID-like and PIN gene families marks the transition of plants from water to land. We hypothesize that the PID-like kinases function in parallel to, or downstream of, the phototropins to orient plant development by establishing the direction of polar auxin transport.

Conditional oxidative stress responses in the Arabidopsis photorespiratory mutant cat2 demonstrate that redox state is a key modulator of daylength-dependent gene expression, and define photoperiod as a crucial factor in the regulation of H2O2-induced cell death

Photorespiration is a light-dependent source of H(2)O(2) in the peroxisomes, where concentrations of this signalling molecule are regulated by catalase. Growth of Arabidopsis knock-out mutants for CATALASE2 (cat2) in ambient air caused severely decreased rosette biomass, intracellular redox perturbation and activation of oxidative signalling pathways. These effects were absent when cat2 was grown at high CO(2) levels to inhibit photorespiration, but were re-established following a subsequent transfer to air. Growth of cat2 in air at different daylengths revealed that photoperiod is a critical determinant of the oxidative stress response. Decreased growth was observed in 8-h, 12-h and 16-h photoperiods, but lesion development was dependent on long days. Experiments at different light fluence rates showed that cell death in cat2 was linked to long days and not to total light exposure or the severity of oxidative stress. Perturbed intracellular redox state and oxidative signalling pathway induction were more prominent in short days than in long days, as evidenced by glutathione status and induction of defence genes and oxidative stress-responsive transcripts. Similar daylength-dependent effects were observed in the response of mature plants transferred from short days in high CO(2) conditions to ambient air conditions. Prior growth of plants with short days in air alleviated the cat2 cell-death phenotype in long days. Together, the data reveal the influence of photoperiodic events on redox signalling, and define distinct photoperiod-dependent strategies in the acclimation versus cell-death decision in stress conditions.

The AGC Kinase MtIRE: A Link to Phospholipid Signaling During Nodulation?

The development of nitrogen fixing root nodules is complex and involves an interplay of signaling processes. During maturation of plant host cells and their endocytosed rhizobia in symbiosomes, host cells and symbiosomes expand. This expansion is accompanied by a large quantity of membrane biogenesis. We recently characterized an AGC kinase gene, MtIRE, that could play a role in this expansion. MtIRE's expression coincides with host cell and symbiosome expansion in the proximal side of the invasion zone in developing Medicago truncatula nodules. MtIRE's closest homolog is the Arabidopsis AGC kinase family IRE gene, which regulates root hair elongation. AGC kinases are regulated by phospholipid signaling in animals and fungi as well as in the several instances where they have been studied in plants. Here we suggest that a phospholipid signaling pathway may also activate MtIRE activity and propose possible upstream activators of MtIRE protein's presumed AGC kinase activity.

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.

An IRE-like AGC kinase gene, MtIRE, has unique expression in the invasion zone of developing root nodules in Medicago truncatula

The AGC protein kinase family (cAMP-dependent protein kinases A, cGMP-dependent protein kinases G, and phospholipid-dependent protein kinases C) have important roles regulating growth and development in animals and fungi. They are activated via lipid second messengers by 3-phosphoinositide-dependent protein kinase coupling lipid signals to phosphorylation of the AGC kinases. These phosphorylate downstream signal transduction protein targets. AGC kinases are becoming better studied in plants, especially in Arabidopsis (Arabidopsis thaliana), where specific AGC kinases have been shown to have key roles in regulating growth signal pathways. We report here the isolation and characterization of the first AGC kinase gene identified in Medicago truncatula, MtIRE. It was cloned by homology with the Arabidopsis INCOMPLETE ROOT HAIR ELONGATION (IRE) gene. Semiquantitative reverse transcription-polymerase chain reaction analysis shows that, unlike its Arabidopsis counterpart, MtIRE is not expressed in uninoculated roots, but is expressed in root systems that have been inoculated with Sinorhizobium meliloti and are developing root nodules. MtIRE expression is also found in flowers. Expression analysis of a time course of nodule development and of nodulating root systems of many Medicago nodulation mutants shows MtIRE expression correlates with infected cell maturation during nodule development. During the course of these experiments, nine Medicago nodulation mutants, including sli and dnf1 to 7 mutants, were evaluated for the first time for their microscopic nodule phenotype using S. meliloti constitutively expressing lacZ. Spatial localization of a pMtIRE-gusA transgene in transformed roots of composite plants showed that MtIRE expression is confined to the proximal part of the invasion zone, zone II, found in indeterminate nodules. This suggests MtIRE is useful as an expression marker for this region of the invasion zone.

Manipulation of plant programmed cell death pathways during plant-pathogen interactions

The interaction of plants with bacterial pathogens involves the manipulation of programmed cell death (PCD) pathways. During a resistance interaction PCD is induced in a process termed the hypersensitive response (HR) which may function to limit pathogen spread. In a susceptible plant-pathogen interactions, the pathogen both inhibits and/or induces host PCD depending on the infection stage and lifestyle of the pathogen. Genes/pathways regulating PCD in plants have been difficult to identify due to a lack of homologous sequences in plants for mammalian genes that control apoptosis and possibly due to functional redundancy. Our labs study plant PCD pathways and bacterial speck disease in tomato which is caused by Pseudomonas syringae pv. tomato (Pst). We recently identified the tomato protein kinases Pdk1 and Adi3 as negative regulators of plant PCD. The plant Pdk1/Adi3 pathway appears to function similarly to the Pdk1/PKB (Akt) pathway in mammals which functions as a major apoptosis negative regulation pathway. Here we discuss regulation of Pdk1/Adi3 and targeting of this pathway during the tomato-Pst interaction for modulation of host PCD.

Kinome profiling of Arabidopsis using arrays of kinase consensus substrates

BACKGROUND

Kinome profiling aims at the parallel analysis of kinase activities in a cell. Novel developed arrays containing consensus substrates for kinases are used to assess those kinase activities. The arrays described in this paper were already used to determine kinase activities in mammalian systems, but since substrates from many organisms are present we decided to test these arrays for the determination of kinase activities in the model plant species Arabidopsis thaliana.

RESULTS

Kinome profiling using Arabidopsis cell extracts resulted in the labelling of many consensus peptides by kinases from the plant, indicating the usefulness of this kinome profiling tool for plants. Method development showed that fresh and frozen plant material could be used to make cell lysates containing active kinases. Dilution of the plant extract increased the signal to noise ratio and non-radioactive ATP enhances full development of spot intensities. Upon infection of Arabidopsis with an avirulent strain of the bacterial pathogen Pseudomonas syringae pv. tomato, we could detect differential kinase activities by measuring phosphorylation of consensus peptides.

CONCLUSION

We show that kinome profiling on arrays with consensus substrates can be used to monitor kinase activities in plants. In a case study we show that upon infection with avirulent P. syringae differential kinase activities can be found. The PepChip can for example be used to purify (unknown) kinases that play a role in P. syringae infection. This paper shows that kinome profiling using arrays of consensus peptides is a valuable new tool to study signal-transduction in plants. It complements the available methods for genomics and proteomics research.

Arabidopsis PLDzeta2 regulates vesicle trafficking and is required for auxin response

Phospholipase D (PLD) and its product, phosphatidic acid (PA), play key roles in cellular processes, including stress and hormonal responses, vesicle trafficking, and cytoskeletal rearrangements. We isolated and functionally characterized Arabidopsis thaliana PLDzeta2, which is expressed in various tissues and enhanced by auxin. A PLDzeta2-defective mutant, pldzeta2, and transgenic plants deficient in PLDzeta2 were less sensitive to auxin, had reduced root gravitropism, and suppressed auxin-dependent hypocotyl elongation at 29 degrees C, whereas transgenic seedlings overexpressing PLDzeta2 showed opposite phenotypes, suggesting that PLDzeta2 positively mediates auxin responses. Studies on the expression of auxin-responsive genes and observation of the beta-glucuronidase (GUS) expression in crosses between pldzeta2 and lines containing DR5-GUS indicated that PLDzeta2, or PA, stimulated auxin responses. Observations of the membrane-selective dye FM4-64 showed suppressed vesicle trafficking under PLDzeta2 deficiency or by treatment with 1-butanol, a PLD-specific inhibitor. By contrast, vesicle trafficking was enhanced by PA or PLDzeta2 overexpression. Analyses of crosses between pldzeta2 and lines containing PIN-FORMED2 (PIN2)-enhanced green fluorescent protein showed that PLDzeta2 deficiency had no effect on the localization of PIN2 but blocked the inhibition of brefeldin A on PIN2 cycling. These results suggest that PLDzeta2 and PA are required for the normal cycling of PIN2-containing vesicles as well as for function in auxin transport and distribution, and hence auxin responses.

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.

Polarized growth: maintaining focus on the tip

Tip growth, a spatially focused cell expansion, has been best characterized in two plant cell types: pollen tubes and root hairs. It has long been established that both cell types require three intracellular components for this process: a tip-high calcium gradient, a polarized actin cytoskeleton, and tip-directed vesicle trafficking. More recently, additional mechanistic parallels have been observed between the two cell types, including roles for ROP and Rab GTPase signaling, phosphoinositides, calcium-dependent protein kinases, and the exocyst. Uncovering pathways that control the three components is beginning to reveal a highly interconnected network, which we call the tip growth LENS (for localization enhancing network, self-sustaining), that coordinates the required cellular activities to allow regulated tip growth, and to maintain itself as the tip advances.

Vesicular trafficking, cytoskeleton and signalling in root hairs and pollen tubes

Root hairs and pollen tubes show strictly polar cell expansion called tip growth. Recent studies of tip growth in root hairs and pollen tubes have revealed that small GTPases of the Rab, Arf and Rho/Rac families, along with their regulatory proteins, are essential for spatio-temporal regulation of vesicular trafficking, cytoskeleton organization and signalling. ROP/RAC GTPases are involved in a multiplicity of functions including the regulation of cytoskeleton organization, calcium signalling and endocytosis in pollen tubes and root hairs. One of the most exciting recent discoveries is the preferential localization of vesicles of the trans-Golgi network (TGN), defined by specific RAB GTPases, in the apical "clear zone" and the definition of TGN as a bona fide organelle involved in both polarized secretion and endocytosis. The TGN is thought to serve the function of an early endosome in plants because it is involved in early endocytosis and rapid vesicular recycling of the plasma membrane in root epidermal cells.

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.

The Arabidopsis MAP kinase kinase MKK1 participates in defence responses to the bacterial elicitor flagellin

Plants sense pathogens through both pathogen-associated molecular patterns and recognition of race-specific virulence factors, which induce basal defence or an accelerated defence (often manifest in the form of local cell death), respectively. A mitogen-activated protein kinase (MAPK) module in Arabidopsis was previously proposed to signal from perception of the bacterial elicitor flagellin to the activation of basal defence-related genes. Here, we present evidence for a parallel MAPK-signalling pathway involved in the response to flg22, a peptide corresponding to the most conserved domain of flagellin. The endogenous Arabidopsis MAP kinase kinase MKK1 is activated in cells treated with flg22, phosphorylates the MAPK MPK4 in vitro, and activates it in vivo in protoplasts. In mkk1 mutant plants, the activation by flg22 of MPK4 and two other flg22-induced MAPKs (MPK3 and MPK6) is impaired. In the mkk1 mutant, a battery of both flg22-induced and flg22-repressed genes show altered expression, indicating that MKK1 negatively regulates the activity of flagellin-responsive genes. Intriguingly, in contrast to the mpk4 mutant, mkk1 shows no morphological anomalies and is compromised in resistance to both virulent and avirulent Pseudomonas syringae strains. Thus, the MKK1 signalling pathway modulates the expression of genes responding to elicitors and plays an important role in pathogen defence.

Quantitative profiling of Arabidopsis polar glycerolipids in response to phosphorus starvation. Roles of phospholipases D zeta1 and D zeta2 in phosphatidylcholine hydrolysis and digalactosyldiacylglycerol accumulation in phosphorus-starved plants

Phosphorus is an essential macronutrient that often limits plant growth and development. Under phosphorus-limited conditions, plants undergo substantial alterations in membrane lipid composition to cope with phosphorus deficiency. To characterize the changes in lipid species and to identify enzymes involved in plant response to phosphorus starvation, 140 molecular species of polar glycerolipids were quantitatively profiled in rosettes and roots of wild-type Arabidopsis (Arabidopsis thaliana) and phospholipase D knockout mutants pld zeta1, pld zeta2, and pld zeta1 pld zeta2. In response to phosphorus starvation, the concentration of phospholipids was decreased and that of galactolipids was increased. Phospholipid lost in phosphorus-starved Arabidopsis rosettes was replaced by an equal amount of galactolipid. The concentration of phospholipid lost in roots was much greater than in rosettes. Disruption of both PLD zeta1 and PLD zeta2 function resulted in a smaller decrease in phosphatidylcholine and a smaller increase in digalactosyldiacylglycerol in phosphorus-starved roots. The results suggest that hydrolysis of phosphatidylcholine by PLD zetas during phosphorus starvation contributes to the supply of inorganic phosphorus for cell metabolism and diacylglycerol moieties for galactolipid synthesis.

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.

Structural and functional insights into the regulation of Arabidopsis AGC VIIIa kinases

The AGCVIIIa kinases of Arabidopsis are members of the eukaryotic PKA, PKG, and PKC group of regulatory kinases. One AGCVIIIa kinase, PINOID (PID), plays a fundamental role in the asymmetrical localization of membrane proteins during polar auxin transport. The remaining 16 AGCVIIIa genes have not been associated with single mutant phenotypes, suggesting that the corresponding kinases function redundantly. Consistent with this idea, we find that the genes encoding the Arabidopsis AGCVIIIa kinases have spatially distinct, but overlapping, expression domains. Here we show that the majority of Arabidopsis AGCVIIIa kinases are substrates for the 3-phosphoinositide-dependent kinase 1 (PDK1) and that trans-phosphorylation by PDK1 correlates with activation of substrate AGCVIIIa kinases. Mutational analysis of two conserved regulatory domains was used to demonstrate that sequences located outside of the C-terminal PDK1 interaction (PIF) domain and the activation loop are required for functional interactions between PDK1 and its substrates. A subset of GFP-tagged AGCVIIIa kinases expressed in Saccharomyces cerevisiae and tobacco BY-2 cells were preferentially localized to the cytoplasm (AGC1-7), nucleus (WAG1 and KIPK), and the cell periphery (PID). We present evidence that PID insertion domain sequences are sufficient to direct the observed peripheral localization. We find that PID specifically but non-selectively binds to phosphoinositides and phosphatidic acid, suggesting that PID might directly interact with the plasma membrane through protein-lipid interactions. The initial characterization of the AGCVIIIa kinases presented here provides a framework for elucidating the physiological roles of these kinases in planta.

Reactive oxygen species signaling in plants

The evolution of aerobic metabolism such as respiration and photosynthesis resulted in the generation of reactive oxygen species (ROS). A common property of all ROS types is that they can cause oxidative damage to proteins, DNA, and lipids. This toxicity of ROS explains the evolution of complex arrays of nonenzymatic and enzymatic detoxification mechanisms in plants. However, increasing evidence indicates that plants also make use of ROS as signaling molecules for regulating development and various physiological responses. In this review, novel insights into the mechanisms of how plants sense and respond to ROS are discussed in the context of the biological effects and functions of ROS in plants.

Evolutionary conservation and regulation of particular alternative splicing events in plant SR proteins

Alternative splicing is an important mechanism for fine tuning of gene expression at the post-transcriptional level. SR proteins govern splice site selection and spliceosome assembly. The Arabidopsis genome encodes 19 SR proteins, several of which have no orthologues in metazoan. Three of the plant specific subfamilies are characterized by the presence of a relatively long alternatively spliced intron located in their first RNA recognition motif, which potentially results in an extremely truncated protein. In atRSZ33, a member of the RS2Z subfamily, this alternative splicing event was shown to be autoregulated. Here we show that atRSp31, a member of the RS subfamily, does not autoregulate alternative splicing of its similarily positioned intron. Interestingly, this alternative splicing event is regulated by atRSZ33. We demonstrate that the positions of these long introns and their capability for alternative splicing are conserved from green algae to flowering plants. Moreover, in particular alternative splicing events the splicing signals are embedded into highly conserved sequences. In different taxa, these conserved sequences occur in at least one gene within a subfamily. The evolutionary preservation of alternative splice forms together with highly conserved intron features argues for additional functions hidden in the genes of these plant-specific SR proteins.

Phospholipase DZ2 plays an important role in extraplastidic galactolipid biosynthesis and phosphate recycling in Arabidopsis roots

Low phosphate (Pi) availability is one of the major constraints for plant productivity in natural and agricultural ecosystems. Plants have evolved a myriad of developmental and biochemical mechanisms to increase internal Pi uptake and utilization efficiency. One important biochemical pathway leading to an increase in internal Pi availability is the hydrolysis of phospholipids. Hydrolyzed phospholipids are replaced by nonphosphorus lipids such as galactolipids and sulfolipids, which help to maintain the functionality and structure of membrane systems. Here we report that a member of the Arabidopsis phospholipase D gene family (PLDZ2) is gradually induced upon Pi starvation in both shoots and roots. From lipid content analysis we show that an Arabidopsis pldz2 mutant is defective in the hydrolysis of phospholipids and has a reduced capacity to accumulate galactolipids under limiting Pi conditions. Morphological analysis of the pldz2 root system shows a premature change in root architecture in response to Pi starvation. These results show that PLDZ2 is involved in the eukaryotic galactolipid biosynthesis pathway, specifically in hydrolyzing phosphatidylcholine and phosphatidylethanolamine to produce diacylglycerol for digalactosyldiacylglycerol synthesis and free Pi to sustain other Pi-requiring processes.

Phosphorylation and activation of PINOID by the phospholipid signaling kinase 3-phosphoinositide-dependent protein kinase 1 (PDK1) in Arabidopsis

Activity of the serine-threonine protein kinase PINOID (PID) has been implicated in the asymmetrical localization of the membrane-associated PINFORMED (PIN) family of auxin transport facilitators. However, the means by which PID regulates PIN protein distribution is unknown. We have used recombinant PID protein to dissect the regulation of PID activity in vitro. We demonstrate that intramolecular PID autophosphorylation is required for the ability of PID to phosphorylate an exogenous substrate. PID-like mammalian AGC kinases act in a phosphorylation cascade initiated by the phospholipid-associated kinase, 3-phosphoinositide-dependent protein kinase 1 (PDK1), which binds to the C-terminal hydrophobic PDK1-interacting fragment (PIF) domain found in PDK1 substrates. We find that Arabidopsis PDK1 interacts with PID, and that transphosphorylation by PDK1 increases PID autophosphorylation. We show that a PID activation loop serine is required for PDK1-dependent PID phosphorylation. This activation is rapid and requires the PIF domain. Cell extracts from flowers and seedling shoots dramatically increase PID phosphorylation in a tissue-specific manner. A PID protein variant in which the PIF domain was mutated failed to be activated by the seedling shoot extracts. PID immunoprecipitated from Arabidopsis cells in which PDK1 expression was inhibited by RNAi showed a dramatic reduction in transphosphorylation of myelin basic protein substrate. These results indicate that AtPDK1 is a potent enhancer of PID activity and provide evidence that phospholipid signaling may play a role in the signaling processes controlling polar auxin transport.

Heterodimeric capping protein from Arabidopsis is regulated by phosphatidic acid

The cytoskeleton is a key regulator of morphogenesis, sexual reproduction, and cellular responses to extracellular stimuli. Changes in the cellular architecture are often assumed to require actin-binding proteins as stimulus-response modulators, because many of these proteins are regulated directly by binding to intracellular second messengers or signaling phospholipids. Phosphatidic acid (PA) is gaining widespread acceptance as a major, abundant phospholipid in plants that is required for pollen tube tip growth and mediates responses to osmotic stress, wounding, and phytohormones; however, the number of identified effectors of PA is rather limited. Here we demonstrate that exogenous PA application leads to significant increases in filamentous actin levels in Arabidopsis suspension cells and poppy pollen grains. To investigate further these lipid-induced changes in polymer levels, we analyzed the properties of a key regulator of actin filament polymerization, the heterodimeric capping protein from Arabidopsis thaliana (AtCP). AtCP binds to PA with a K(d) value of 17 muM and stoichiometry of approximately 1:2. It also binds well to PtdIns(4,5)P(2), but not to several other phosphoinositide or acidic phospholipids. The interaction with PA inhibited the actin-binding activity of CP. In the presence of PA, CP is unable to block the barbed or rapidly growing and shrinking end of actin filaments. Precapped filament barbed ends can also be uncapped by addition of PA, allowing rapid filament assembly from an actin monomer pool that is buffered with profilin. The findings support a model in which the inhibition of CP activity in cells by elevated PA results in the stimulation of actin polymerization from a large pool of profilin-actin. Such regulation may be important for the response of plant cells to extracellular stimuli as well as for the normal process of pollen tube tip growth.

Keeping it together: co-ordinating plant growth

One of the most important demands of multicellularity is the co-ordination of cell proliferation and cell growth to allow the ultimate differentiation of functional organs and tissues. In plants, endogenously and exogenously generated developmental signals hone a basic patterning plan to the demands of a changing environment throughout the lifecycle. Recent advances have started to identify many signalling pathways and intermediates that are potentially implicated in controlling plant growth in response to developmentally important signals. These include pathways that are conserved in other eukaryotes, such as the Target Of Rapamycin (TOR) pathway and lipid signalling via S6-Kinases, as well as pathways that contain plant-specific elements, such as ERECTA-class receptor kinases and TCP-class transcription factors. Understanding how these elements are integrated to give co-ordinated growth remains one of the major challenges in plant biology.

Double knockouts of phospholipases Dzeta1 and Dzeta2 in Arabidopsis affect root elongation during phosphate-limited growth but do not affect root hair patterning

Root elongation and root hair formation are important in nutrient absorption. We found that two Arabidopsis (Arabidopsis thaliana) phospholipase Ds (PLDs), PLDzeta1 and PLDzeta2, were involved in root elongation during phosphate limitation. PLDzeta1 and PLDzeta2 are structurally different from the majority of plant PLDs by having phox and pleckstrin homology domains. Both PLDzetas were expressed more in roots than in other tissues. It was reported previously that inducible suppression or inducible overexpression of PLDzeta1 affected root hair patterning. However, gene knockouts of PLDzeta1, PLDzeta2, or the double knockout of PLDzeta1 and PLDzeta2 showed no effect on root hair formation. The expression of PLDzetas increased in response to phosphate limitation. The elongation of primary roots in PLDzeta1 and PLDzeta2 double knockout mutants was slower than that of wild type and single knockout mutants. The loss of PLDzeta2, but not PLDzeta1, led to a decreased accumulation of phosphatidic acid in roots under phosphate-limited conditions. These results indicate that PLDzeta1 and PLDzeta2 play a role in regulating root development in response to nutrient limitation.

Organization and function of the actin cytoskeleton in developing root cells

The actin cytoskeleton is a highly dynamic structure, which mediates various cellular functions in large part through accessory proteins that tilt the balance between monomeric G-actin and filamentous actin (F-actin) or by facilitating interactions between actin and the plasma membrane, microtubules, and other organelles. Roots have become an attractive model to study actin in plant development because of their simple anatomy and accessibility of some root cell types such as root hairs for microscopic analyses. Roots also exhibit a remarkable developmental plasticity and possess a delicate sensory system that is easily manipulated, so that one can design experiments addressing a range of important biological questions. Many facets of root development can be regulated by the diverse actin network found in the various root developmental regions. Various molecules impinge on this actin scaffold to define how a particular root cell type grows or responds to a specific environmental signal. Although advances in genomics are leading the way toward elucidating actin function in roots, more significant strides will be realized when such tools are combined with improved methodologies for accurately depicting how actin is organized in plant cells.

Adi3 is a Pdk1-interacting AGC kinase that negatively regulates plant cell death

Bacterial speck disease in tomato is caused by Pseudomonas syringae pv. tomato. Resistance to this disease is conferred by the host Pto kinase, which recognizes P. s. pv. tomato strains that express the effector AvrPto. We report here that an AvrPto-dependent Pto-interacting protein 3 (Adi3) is a member of the AGC family of protein kinases. In mammals, AGC kinases are regulated by 3-phosphoinositide-dependent protein kinase-1 (Pdk1). We characterized tomato Pdk1 and showed that Pdk1 and Pto phosphorylate Adi3. Gene silencing of Adi3 in tomato causes MAPKKKalpha-dependent formation of necrotic lesions. Use of a chemical inhibitor of Pdk1, OSU-03012, also implicates Pdk1 and Adi3 in plant cell death regulation. Adi3 thus appears to function analogously to the mammalian AGC kinase protein kinase B/Akt by negatively regulating cell death via Pdk1 phosphorylation. We speculate that the negative regulatory function of Adi3 might be subverted by interaction with Pto/AvrPto, leading to host cell death that is associated with pathogen attack.

Local interactions shape plant cells

Plant cell expansion is usually attributed to the considerable osmotic pressure that develops within and impinges upon the cell boundary. Whereas turgor containment within expandable walls explains global expansion, the scalar nature of turgor does not directly suggest a mechanism for achieving the localized, differential growth that is responsible for the diversity of plant-cell forms. The key to achieving local growth in plant cells appears to lie not in harnessing turgor but in using it to identify weak regions in the cell boundary and thus creating discrete intracellular domains for targeting the growth machinery. Membrane-interacting phospholipases, Rho-like proteins and their interactors, an actin-modulating ARP2/3 complex with its upstream regulators, and actin-microtubule interactions play important roles in the intracellular cooperation to shape plant cells.

Janus kinase-signal transducer and activator of transcription mediates phosphatidic acid-induced interleukin (IL)-1beta and IL-6 production

We have found previously that phosphatidic acid (PA) can induce inflammatory mediators such as cytokines, which implies that PA plays a role in inflammatory response. In the present study, we provide evidence of the PA-mediated activation of the Janus tyrosine kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway, which results in the production of interleukin (IL)-1beta and IL-6. PA elicited the rapid phosphorylations of JAK2 and STAT1/3, and the subsequent nuclear translocation. Macrophages that had been transiently transfected with a luciferase reporter construct containing eight consecutive gamma-interferon activating sequence (GAS) elements, a known STAT binding site, exhibited enhanced reporter gene activity in response to PA stimulation, which further supports the involvement of JAK-STAT activation in the PA-induced signaling pathway. Of the inflammatory cytokines, IL-1beta, IL-6, and tumor necrosis factor (TNF)-alpha were detected in media from macrophages stimulated with PA. Moreover, the JAK2 inhibitor alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG-490) abolished PA-induced IL-1beta and IL-6 release but not TNF-alpha production, which is consistent with the notion that IL-1beta and IL-6 but not TNF-alpha contain a STAT binding element in their promoter region. The knockdown of JAK2 in macrophages by small interfering RNA significantly attenuated PA-induced IL-1beta and IL-6 production. In addition, JAK2 inhibitor suppressed PA-induced Akt phosphorylation, and the Akt inhibitor 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) blocked GAS activation (GAS contains a promoter that responds to PA), suggesting that PA-mediated JAK2 activation leads to phosphatidylinositol 3-kinase/Akt phosphorylation and STAT activation, and the subsequent translocation of STAT to the nucleus. Together, our data demonstrate that PA-activated macrophages produce IL-1beta and IL-6 and that these processes require the activation of the JAK2-STAT1/3 or JAK2-Akt-STAT signaling pathways.

The cold-induced early activation of phospholipase C and D pathways determines the response of two distinct clusters of genes in Arabidopsis cell suspensions

In plants, a temperature downshift represents a major stress that will lead to the induction or repression of many genes. Therefore, the cold signal has to be perceived and transmitted to the nucleus. In response to a cold exposure, we have shown that the phospholipase D (PLD) and the phospholipase C (PLC)/diacylglycerol kinase pathways are simultaneously activated. The role of these pathways in the cold response has been investigated by analyzing the transcriptome of cold-treated Arabidopsis (Arabidopsis thaliana) suspension cells in the presence of U73122 or ethanol, inhibitors of the PLC/diacylglycerol kinase pathway and of the phosphatidic acid produced by PLD, respectively. This approach showed that the expression of many genes was modified by the cold response in the presence of such agents. The cold responses of most of the genes were repressed, thus correlating with the inhibitory effect of U73122 or ethanol. We were thus able to identify 58 genes that were regulated by temperature downshift via PLC activity and 87 genes regulated by temperature downshift via PLD-produced phosphatidic acid. Interestingly, each inhibitor appeared to affect different cold response genes. These results support the idea that both the PLC and PLD pathways are upstream of two different signaling pathways that lead to the activation of the cold response. The connection of these pathways with the CBF pathway, currently the most understood genetic system playing a role in cold acclimation, is discussed.