Involvement of Free Calcium in Action of Cryptogein, a Proteinaceous Elicitor of Hypersensitive Reaction in Tobacco Cells

At the Nexus between Cytoskeleton and Vacuole: How Plant Cytoskeletons Govern the Dynamics of Large Vacuoles

Large vacuoles are a predominant cell organelle throughout the plant body. They maximally account for over 90% of cell volume and generate turgor pressure that acts as a driving force of cell growth, which is essential for plant development. The plant vacuole also acts as a reservoir for sequestering waste products and apoptotic enzymes, thereby enabling plants to rapidly respond to fluctuating environments. Vacuoles undergo dynamic transformation through repeated enlargement, fusion, fragmentation, invagination, and constriction, eventually resulting in the typical 3-dimensional complex structure in each cell type. Previous studies have indicated that such dynamic transformations of plant vacuoles are governed by the plant cytoskeletons, which consist of F-actin and microtubules. However, the molecular mechanism of cytoskeleton-mediated vacuolar modifications remains largely unclear. Here we first review the behavior of cytoskeletons and vacuoles during plant development and in response to environmental stresses, and then introduce candidates that potentially play pivotal roles in the vacuole-cytoskeleton nexus. Finally, we discuss factors hampering the advances in this research field and their possible solutions using the currently available cutting-edge technologies.

Calcium signaling in plant immunity: a spatiotemporally controlled symphony

Calcium ions (Ca²⁺) are prominent intracellular messengers in all eukaryotic cells. Recent studies have emphasized the crucial roles of Ca²⁺ in plant immunity. Here, we review the latest progress on the spatiotemporal control of Ca²⁺ function in plant immunity. We discuss discoveries of how Ca²⁺ influx is triggered upon the activation of immune receptors, how Ca²⁺-permeable channels are activated, how Ca²⁺ signals are decoded inside plant cells, and how these signals are switched off. Despite recent advances, many open questions remain and we highlight the existing toolkit and the new technologies to address the outstanding questions of Ca²⁺ signaling in plant immunity.

Factors Involved on Tiger-Stripe Foliar Symptom Expression of Esca of Grapevine

Esca of grapevine causes yield losses correlated with incidence and severity symptom expression. Factors associated with leaf symptom mechanisms are yet to be fully clarified. Therefore, in 2019 and 2020, macro and microelement analyses and leaf reflectance measurements were carried out on leaves at different growth stages in a vineyard located in Abruzzo, central Italy. Surveys were carried out on leaves of both never leaf-symptomatic vines and different categories of diseased vine shoots. Never leaf-symptomatic and diseased vines were also treated with a fertilizer mixture that proved to be able to limit the symptom expression. Results showed that untreated asymptomatic diseased vines had high calcium contents for most of the vegetative season. On the contrary, treated asymptomatic diseased vines showed higher contents of calcium, magnesium, and sodium, at berries pea-sized, before the onset of symptoms. These vines had better physiological efficiency showing higher water index (WI), normalized difference vegetation index (NDVI), and green normalized difference vegetation index (GNDVI) values, compared to untreated asymptomatic vines, at fruit set. Results confirmed the strong response of the plant to symptom expression development and the possibility of limiting this response with calcium and magnesium applications carried out before the symptom onset.

The Effect of Plant Genotype, Growth Stage, and Mycosphaerella graminicola Strains on the Efficiency and Durability of Wheat-Induced Resistance by Paenibacillus sp. Strain B2

Plant-growth-promoting rhizobacteria are known as potential biofertilizers and plant-resistance inducers. The current work aims to study the durability of the resistance induced as a response to the inoculation of wheat grains with Paenibacillus sp. strain B2 (PB2) and its influence by plant genotype, growth stage, and Mycosphaerella graminicola strain (the causal agent of Septoria tritici blotch or STB). The results of the plate-counting method showed that PB2 has high potential for wheat-root external colonization [>106 colony-forming unit (CFU)/g of root], and the quantitative real-time polymerase chain reaction (qPCR) analysis demonstrated its internal root-colonization capacity on all tested cultivars. However, the colonization seems to be dependent on wheat-growth stage. The durability of PB2-induced resistance (PB2-IR) was tested at the 3-leaf, tillering, and flag-leaf-growth stages. Additionally, the results showed that the PB2-IR is durable and able to protect the flag leaf, the most important leaf layer during grain fill. It conferred a high protection efficiency (55-94%) against four virulent strains of M. graminicola and over 11 wheat cultivars with different resistance levels to STB. Although, PB2-IR is dependent on M. graminicola strains, wheat genotypes and growth stages, its efficiency, under field conditions, at protecting the last wheat-leaf layers was not an influence. However, it showed 71-79% of protection and reached 81-94% in association with half of the recommended dose of Cherokee® fungicide. This may be explained using laboratory results by its direct impact on M. graminicola strains in these leaf layers and by the indirect reduction of the inoculum coming from leaves infected during the earlier growth stages. Gene expression results showed that PB2-IR is correlated to upregulation of genes involved in defense and cell rescue and a priming effect in the basal defense, jasmonic acid signaling, phenylpropanoids and phytoalexins, and reactive oxygen species gene markers. To conclude, PB2 induces a high and durable resistance against M. graminicola under controlled and field conditions. The PB2-IR is a pathogen strain and is plant-growth-stage and genotype dependent. These results highlight the importance of taking into consideration these factors so as to avoid losing the effectiveness of induced resistance under field conditions.

The Phytophthora RXLR Effector Avrblb2 Modulates Plant Immunity by Interfering With Ca2+ Signaling Pathway

In plants, subcellular fluctuations in Ca²⁺ ion concentration are among the earliest responses to biotic and abiotic stresses. Calmodulin, which is a ubiquitous Ca²⁺ ion sensor in eukaryotes, plays a major role in translating these Ca²⁺ signatures to cellular responses by interacting with numerous proteins located in plasma membranes, cytoplasm, organelles and nuclei. In this report, we show that one of the Phytophthora RXLR effector, Avrblb2, interacts with calmodulin at the plasma membrane of the plant cells. Using deletion and single amino acid mutagenesis, we found that calmodulin binds to the effector domain of Avrblb2. In addition, we show that most known homologs of Avrblb2 in three different Phytophthora species interact with different isoforms of calmodulin. Type of amino acids at position 69 in Avrblb2, which determines Rbi-blb2 resistance protein-mediated defense responses, is not involved in the Avrblb2-calmodulin interaction. Using in planta functional analyses, we show that calmodulin binding to Avrblb2 is required for its recognition by Rpi-blb2 to incite hypersensitive response. These findings suggest that Avrblb2 by interacting with calmodulin interfere with plant defense associated Ca²⁺ signaling in plants.

Laboratory Microprobe X-Ray Fluorescence in Plant Science: Emerging Applications and Case Studies

In vivo and micro chemical analytical methods have the potential to improve our understanding of plant metabolism and development. Benchtop microprobe X-ray fluorescence spectroscopy (μ-XRF) presents a huge potential for facing this challenge. Excitation beams of 30 μm and 1 mm in diameter were employed to address questions in seed technology, phytopathology, plant physiology, and bioremediation. Different elements were analyzed in several situations of agronomic interest: (i) Examples of μ-XRF yielding quantitative maps that reveal the spatial distribution of zinc in common beans (Phaseolus vulgaris) primed seeds. (ii) Chemical images daily recorded at a soybean leaf (Glycine max) infected by anthracnose showed that phosphorus, sulfur, and calcium trended to concentrate in the disease spot. (iii) In vivo measurements at the stem of P. vulgaris showed that under root exposure, manganese is absorbed and transported nearly 10-fold faster than iron. (iv) Quantitative maps showed that the lead distribution in a leaf of Eucalyptus hybrid was not homogenous, this element accumulated mainly in the leaf border and midrib, the lead hotspots reached up to 13,400 mg lead kg^-1 fresh tissue weight. These case studies highlight the ability of μ-XRF in performing qualitative and quantitative elemental analysis of fresh and living plant tissues. Thus, it can probe dynamic biological phenomena non-destructively and in real time.

Diacylglycerol kinases activate tobacco NADPH oxidase-dependent oxidative burst in response to cryptogein

Cryptogein is a 10 kDa protein secreted by the oomycete Phytophthora cryptogea that activates defence mechanisms in tobacco plants. Among early signalling events triggered by this microbial-associated molecular pattern is a transient apoplastic oxidative burst which is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity of the RESPIRATORY BURST OXIDASE HOMOLOG isoform D (RBOHD). Using radioactive [33 P]-orthophosphate labelling of tobacco Bright Yellow-2 suspension cells, we here provide in vivo evidence for a rapid accumulation of phosphatidic acid (PA) in response to cryptogein because of the coordinated onset of phosphoinositide-dependent phospholipase C and diacylglycerol kinase (DGK) activities. Both enzyme specific inhibitors and silencing of the phylogenetic cluster III of the tobacco DGK family were found to reduce PA production upon elicitation and to strongly decrease the RBOHD-mediated oxidative burst. Therefore, it appears that PA originating from DGK controls NADPH-oxidase activity. Amongst cluster III DGKs, the expression of DGK5-like was up-regulated in response to cryptogein. Besides DGK5-like is likely to be the main cluster III DGK isoform silenced in one of our mutant lines, making it a strong candidate for the observed response to cryptogein. The relevance of these results is discussed with regard to early signalling lipid-mediated events in plant immunity.

Nine things to know about elicitins

888 I. 888 II. 889 III. 889 IV. 889 V. 891 VI. 891 VII. 891 VIII. 892 IX. 892 X. 893 XI. 893 893 References 893 SUMMARY: Elicitins are structurally conserved extracellular proteins in Phytophthora and Pythium oomycete pathogen species. They were first described in the late 1980s as abundant proteins in Phytophthora culture filtrates that have the capacity to elicit hypersensitive (HR) cell death and disease resistance in tobacco. Later, they became well-established as having features of microbe-associated molecular patterns (MAMPs) and to elicit defences in a variety of plant species. Research on elicitins culminated in the recent cloning of the elicitin response (ELR) cell surface receptor-like protein, from the wild potato Solanum microdontum, which mediates response to a broad range of elicitins. In this review, we provide an overview on elicitins and the plant responses they elicit. We summarize the state of the art by describing what we consider to be the nine most important features of elicitin biology.

ABC transporter PEN3/PDR8/ABCG36 interacts with calmodulin that, like PEN3, is required for Arabidopsis nonhost resistance

Nonhost resistance (NHR) is the most prevalent form of plant immunity. In Arabidopsis, NHR requires membrane-localized ATP-binding cassette (ABC) transporter PENETRATION (PEN) 3. Upon perception of pathogen-associated molecular patterns, PEN3 becomes phosphorylated, suggestive of PEN3 regulation by post-translational modification. Here, we investigated the PEN3 protein interaction network. We probed the Arabidopsis protein microarray AtPMA-5000 with the N-terminal cytoplasmic domain of PEN3. Several of the proteins identified to interact with PEN3 in vitro represent cellular Ca(2+) sensors, including calmodulin (CaM) 3, CaM7 and several CaM-like proteins, pointing to the importance of Ca(2+) sensing to PEN3-mediated NHR. We demonstrated co-localization of PEN3 and CaM7, and we confirmed PEN3-CaM interaction in vitro and in vivo by PEN3 pull-down with CaM Sepharose, CaM overlay assay and bimolecular fluorescence complementation. We also show that just like in pen3, NHR to the nonadapted fungal pathogens Phakopsora pachyrhizi and Blumeria graminis f.sp. hordei is compromised in the Arabidopsis cam7 and pen3 cam7 mutants. Our study discloses CaM7 as a PEN3-interacting protein crucial to Arabidopsis NHR and emphasizes the importance of Ca(2+) sensing to plant immunity.

Redox regulation in plant immune function

SIGNIFICANCE

Production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) occurs rapidly in response to attempted pathogen invasion of potential host plants. Such reduction-oxidation (redox) changes are sensed and transmitted to engage immune function, including the hypersensitive response, a programmed execution of challenged plant cells.

RECENT ADVANCES

Pathogen elicitors trigger changes in calcium that are sensed by calmodulin, calmodulin-like proteins, and calcium-dependent protein kinases, which activate ROS and RNS production. The ROS and RNS production is compartmentalized within the cell and occurs through multiple routes. Mitogen-activated protein kinase (MAPK) cascades are engaged upstream and downstream of ROS and nitric oxide (NO) production. NO is increasingly recognized as a key signaling molecule, regulating downstream protein function through S-nitrosylation, the addition of an NO moiety to a reactive cysteine thiol.

CRITICAL ISSUES

How multiple sources of ROS and RNS are coordinated is unclear. The putative protein sensors that detect and translate fluxes in ROS and RNS into differential gene expression are obscure. Protein tyrosine nitration following reaction of peroxynitrite with tyrosine residues has been proposed as another signaling mechanism or as a marker leading to protein degradation, but the reversibility remains to be established.

FUTURE DIRECTIONS

Research is needed to identify the full spectrum of NO-modified proteins with special emphasis on redox-activated transcription factors and their cognate target genes. A systems approach will be required to uncover the complexities integral to redox regulation of MAPK cascades, transcription factors, and defense genes through the combined effects of calcium, phosphorylation, S-nitrosylation, and protein tyrosine nitration.

Evidence for the involvement of intracellular Ca(2+) ions in the elicitation mechanism of Bacillus licheniformis

Treatment of Bacillus licheniformis cultures with biotic oligosaccharide elicitors is known to increase the production of the antibiotic bacitracin A. The mechanism of the elicitation is currently under investigation and in this paper we provide evidence on the modulatory role of Ca(2+) ions during this process. Addition of elicitors, mannan oligosaccharides, oligoguluronate and oligomannuronate to the liquid cultures resulted in 9.0, 5.2 and 5.0% increase in cytosolic Ca(2+) levels in B. licheniformis, while the presence of verapamil (Ca(2+) channel blocker) resulted in 74% decrease in bacitracin A levels, as compared to the control culture. We propose that Ca(2+) ions may acts as a secondary messenger in the regulation of the bacitracin A synthesis in the elicited B. licheniformis cultures.

Genetic elicitation by inducible expression of β-cryptogein stimulates secretion of phenolics from Coleus blumei hairy roots

The accumulation of phenolic compounds in plants is often part of the defense response against stress and pathogen attack, which can be triggered and activated by elicitors. Oomycetal proteinaceous elicitor, β-cryptogein, induces hypersensitive response and systemic acquired resistance against some pathogens. In order to test the effect of endogenously synthesized cryptogein protein on phenolic compounds accumulation in tissue, and secretion into the culture medium, Coleus blumei hairy roots were generated. Agrobacterium rhizogenes was employed to insert synthetic crypt gene, encoding β-cryptogein, under the control of alcohol-inducible promoter. The expression of β-cryptogein, in C. blumei hairy roots, was controlled by application of 1% and 2% ethanol, during 21 days induction period. Ethanol-induced expression of β-cryptogein caused significant decrease of soluble phenolics and rosmarinic acid (RA) in hairy root lines and increase of phenolics, RA and caffeic acid in culture medium. These data suggest that β-cryptogein might be a potential regulatory factor for phenolics secretion from the roots.

Hrip1, a novel protein elicitor from necrotrophic fungus, Alternaria tenuissima, elicits cell death, expression of defence-related genes and systemic acquired resistance in tobacco

Here, we report the identification, purification, characterization and gene cloning of a novel hypersensitive response inducing protein secreted by necrotrophic fungus, Alternaria tenuissima, designated as hypersensitive response inducing protein 1 (Hrip1). The protein caused the formation of necrotic lesions that mimic a typical hypersensitive response and apoptosis-related events including DNA laddering. The protein-encoding gene was cloned by rapid amplification of cDNA ends (RACE) method. The sequence analysis revealed that the cDNA is 495 bp in length and the open reading frame (ORF) encodes for a polypeptide of 163 amino acids with theoretical pI of 5.50 and molecular weight of 17 562.5 Da. Hrip1 induced calcium influx, medium alkalinization, activation of salicylic acid-induced protein kinase and several defence-related genes after infiltration in tobacco leaves. Cellular damage, restricted to the infiltrated zone, occurred only several hours later, at a time when expression of defence-related genes was activated. After several days, systemic acquired resistance was also induced. The tobacco plant cells that perceived the Hrip1 generated a cascade of signals acting at local, short, and long distances, and caused the coordinated expression of specific defence responses in a way similar to hypersensitivity to tobacco mosaic virus. Thus, Hrip1 represents a powerful tool to investigate further the signals and their transduction pathways involved in induced disease resistance in necrotrophic fungi.

Calcium signatures and signaling in cytosol and organelles of tobacco cells induced by plant defense elicitors

Calcium signatures induced by two elicitors of plant defense reactions, namely cryptogein and oligogalacturonides, were monitored at the subcellular level, using apoaequorin-transformed Nicotiana tabacum var Xanthi cells, in which the apoaequorin calcium sensor was targeted either to cytosol, mitochondria or chloroplasts. Our study showed that both elicitors induced specific Ca(2+) signatures in each compartment, with the most striking difference relying on duration. Common properties also emerged from the analysis of Ca(2+) signatures: both elicitors induced a biphasic cytosolic [Ca(2+)] elevation together with a single mitochondrial [Ca(2+)] elevation concomitant with the first cytosolic [Ca(2+)] peak. In addition, both elicitors induced a chloroplastic [Ca(2+)] elevation peaking later in comparison to cytosolic [Ca(2+)] elevation. In cryptogein-treated cells, pharmacological studies indicated that IP(3) should play an important role in Ca(2+) signaling contrarily to cADPR or nitric oxide, which have limited or no effect on [Ca(2+)] variations. Our data also showed that, depending on [Ca(2+)] fluxes at the plasma membrane, cryptogein triggered a mitochondrial respiration increase and affected excess energy dissipation mechanisms in chloroplasts. Altogether the results indicate that cryptogein profoundly impacted cell functions at many levels, including organelles.

Transcriptome analysis of tobacco BY-2 cells elicited by cryptogein reveals new potential actors of calcium-dependent and calcium-independent plant defense pathways

Cryptogein is a proteinaceous elicitor secreted by the oomycete Phytophthora cryptogea, which induces a hypersensitive response in tobacco plants. We have previously reported that in tobacco BY-2 cells treated with cryptogein, most of the genes of the phenylpropanoid pathway were upregulated and cell wall-bound phenolics accumulated. Both events were Ca(2+) dependent. In this study, we designed a microarray covering a large proportion of the tobacco genome and monitored gene expression in cryptogein-elicited BY-2 cells to get a more complete view of the transcriptome changes and to assess their Ca(2+) dependence. The predominant functional gene categories affected by cryptogein included stress- and disease-related proteins, phenylpropanoid pathway, signaling components, transcription factors and cell wall reinforcement. Among the 3819 unigenes whose expression changed more than fourfold, 90% were Ca(2+) dependent, as determined by their sensitivity to lanthanum chloride. The most Ca(2+)-dependent transcripts upregulated by cryptogein were involved in defense responses or the oxylipin pathway. This genome-wide study strongly supports the importance of Ca(2+)-dependent transcriptional regulation of regulatory and defense-related genes contributing to cryptogein responses in tobacco.

Type-2 histone deacetylases as new regulators of elicitor-induced cell death in plants

• Plant resistance to pathogen attack is often associated with a localized programmed cell death called hypersensitive response (HR). How this cell death is controlled remains largely unknown. • Upon treatment with cryptogein, an elicitor of tobacco defence and cell death, we identified NtHD2a and NtHD2b, two redundant isoforms of type-2 nuclear histone deacetylases (HDACs). These HDACs are phosphorylated after a few minutes' treatment, and their rate of mRNAs are rapidly and strongly reduced, leading to a 40-fold decrease after 10 h of treatment. • By using HDAC inhibitors, RNAi- and overexpression-based approaches, we showed that HDACs, and especially NtHD2a/b, act as inhibitors of cryptogein-induced cell death. Moreover, in NtHD2a/b-silenced plants, infiltration with cryptogein led to HR-like symptoms in distal leaves. • Taken together, these results show for the first time that type-2 HDACs, which are specific to plants, act as negative regulators of elicitor-induced cell death in tobacco (Nicotiana tabacum), suggesting that the HR is controlled by post-translational modifications including (de)acetylation of nuclear proteins.

Mechanisms of xylanase-induced nitric oxide and phosphatidic acid production in tomato cells

The second messenger nitric oxide (NO), phosphatidic acid (PA) and reactive oxygen species (ROS) are involved in the plant defense response during plant-pathogen interactions. NO has been shown to participate in PA production in response to the pathogen-associated molecular pattern xylanase in tomato cell suspensions. Defense responses downstream of PA include ROS production. The goal of this work was to study the signaling mechanisms involved in PA production during the defense responses triggered by xylanase and mediated by NO in the suspension-cultured tomato cells. We analyzed the participation of protein kinases, guanylate cyclase and the NO-mediated posttranslational modification S-nitrosylation, by means of pharmacology and biochemistry. We showed that NO, PA and ROS levels are significantly diminished by treatment with the general protein kinase inhibitor staurosporine. This indicates that xylanase-induced protein phosphorylation events might be the important components leading to NO formation, and hence for the downstream regulation of PA and ROS levels. When assayed, a guanylate cyclase inhibitor or a cGMP analog did not alter the PA accumulation. These results suggest that a cGMP-mediated pathway is not involved in xylanase-induced PA formation. Finally, the inhibition of protein S-nitrosylation did not affect NO formation but compromised PA and ROS production. Data collectively indicate that upon xylanase perception, cells activate a protein kinase pathway required for NO formation and that, S-nitrosylation-dependent mechanisms are involved in downstream signaling leading to PA and ROS.

Cytosolic calcium rises and related events in ergosterol-treated Nicotiana cells

The typical fungal membrane component ergosterol was previously shown to trigger defence responses and protect plants against pathogens. Most of the elicitors mobilize the second messenger calcium, to trigger plant defences. We checked the involvement of calcium in response to ergosterol using Nicotiana plumbaginifolia and Nicotiana tabacum cv Xanthi cells expressing apoaequorin in the cytosol. First, it was verified if ergosterol was efficient in these cells inducing modifications of proton fluxes and increased expression of defence-related genes. Then, it was shown that ergosterol induced a rapid and transient biphasic increase of free [Ca²⁺](cyt) which intensity depends on ergosterol concentration in the range 0.002-10 μM. Among sterols, this calcium mobilization was specific for ergosterol and, ergosterol-induced pH and [Ca²⁺](cyt) changes were specifically desensitized after two subsequent applications of ergosterol. Specific modulators allowed elucidating some events in the signalling pathway triggered by ergosterol. The action of BAPTA, LaCl₃, nifedipine, verapamil, neomycin, U73122 and ruthenium red suggested that the first phase was linked to calcium influx from external medium which subsequently triggered the second phase linked to calcium release from internal stores. The calcium influx and the [Ca²⁺](cyt) increase depended on upstream protein phosphorylation. The extracellular alkalinization and ROS production depended on calcium influx but, the ergosterol-induced MAPK activation was calcium-independent. ROS were not involved in cytosolic calcium rise as described in other models, indicating that ROS do not systematically participate in the amplification of calcium signalling. Interestingly, ergosterol-induced ROS production is not linked to cell death and ergosterol does not induce any calcium elevation in the nucleus.

Coping with stresses: roles of calcium- and calcium/calmodulin-regulated gene expression

Abiotic and biotic stresses are major limiting factors of crop yields and cause billions of dollars of losses annually around the world. It is hoped that understanding at the molecular level how plants respond to adverse conditions and adapt to a changing environment will help in developing plants that can better cope with stresses. Acquisition of stress tolerance requires orchestration of a multitude of biochemical and physiological changes, and most of these depend on changes in gene expression. Research during the last two decades has established that different stresses cause signal-specific changes in cellular Ca(2+) level, which functions as a messenger in modulating diverse physiological processes that are important for stress adaptation. In recent years, many Ca(2+) and Ca(2+)/calmodulin (CaM) binding transcription factors (TFs) have been identified in plants. Functional analyses of some of these TFs indicate that they play key roles in stress signaling pathways. Here, we review recent progress in this area with emphasis on the roles of Ca(2+)- and Ca(2+)/CaM-regulated transcription in stress responses. We will discuss emerging paradigms in the field, highlight the areas that need further investigation, and present some promising novel high-throughput tools to address Ca(2+)-regulated transcriptional networks.

Hierarchy and roles of pathogen-associated molecular pattern-induced responses in Nicotiana benthamiana

Our current understanding of pathogen-associated molecular pattern (PAMP)-triggered immunity signaling pathways in plants is limited due to the redundancy of several components or the lethality of mutants in Arabidopsis (Arabidopsis thaliana). To overcome this, we used a virus-induced gene silencing-based approach in combination with pharmacological studies to decipher links between early PAMP-triggered immunity events and their roles in immunity following PAMP perception in Nicotiana benthamiana. Two different calcium influx inhibitors suppressed the reactive oxygen species (ROS) burst: activation of the mitogen-activated protein kinases (MAPKs) and PAMP-induced gene expression. The calcium burst was unaffected in plants specifically silenced for components involved in ROS generation or for MAPKs activated by PAMP treatment. Importantly, the ROS burst still occurred in plants silenced for the two major defense-associated MAPK genes NbSIPK (for salicylic acid-induced protein kinase) and NbWIPK (for wound-induced protein kinase) or for both genes simultaneously, demonstrating that these MAPKs are dispensable for ROS production. We further show that NbSIPK silencing is sufficient to prevent PAMP-induced gene expression but that both MAPKs are required for bacterial immunity against two virulent strains of Pseudomonas syringae and their respective nonpathogenic mutants. These results suggest that the PAMP-triggered calcium burst is upstream of separate signaling branches, one leading to MAPK activation and then gene expression and the other to ROS production. In addition, this study highlights the essential roles of NbSIPK and NbWIPK in antibacterial immunity. Unexpectedly, negative regulatory mechanisms controlling the intensity of the PAMP-triggered calcium and ROS bursts were also revealed by this work.

Elicitin-membrane interaction is driven by a positive charge on the protein surface: role of Lys13 residue in lipids loading and resistance induction

Elicitins are family of small proteins secreted by species of the pathogenic fungus Phytophthora inducing a defence reaction in plants. They contain a hydrophobic cavity capable of binding sterols and fatty acids, and on the basis of their pI they are classified as either α-elicitins or more necrotising β-elicitins. The residue Lys13 was previously identified as a key determinant of the necrotising activity of basic elicitins. In the present study we describe changes in the ability of cryptogein, a β-elicitin inducing a hypersensitive response in tobacco, to transfer sterols and fatty acids between micelles and liposomes upon Lys13Val mutation. We propose that the change in activity is influenced by the elimination of positive charge on the surface of cryptogein, which is significant for correct positioning of the protein during lipid loading, without adversely affecting the binding of sterol to the cavity of the protein. Compared to wild type cryptogein, mutation Lys13Val resulted in lowered expression of defence-related genes and compromised resistance to Phytophthora parasitica. Furthermore, resistance induced by Lys13Val mutant was similar to that induced by acidic elicitin capsicein containing at amino position 13 valine Determined results sustained a crucial role of positive lysine residues on the surface of basic elicitins and suggested their significant role in correct protein-membrane interaction and thus on their biological activity.

Stimulation of defense reactions in Medicago truncatula by antagonistic lipopeptides from Paenibacillus sp. strain B2

With the aim of obtaining new strategies to control plant diseases, we investigated the ability of antagonistic lipopolypeptides (paenimyxin) from Paenibacillus sp. strain B2 to elicit hydrogen peroxide (H₂O₂) production and several defense-related genes in the model legume Medicago truncatula. For this purpose, M. truncatula cell suspensions were used and a pathosystem between M. truncatula and Fusarium acuminatum was established. In M. truncatula cell cultures, the induction of H₂O₂ reached a maximum 20 min after elicitation with paenimyxin, whereas concentrations higher than 20 μM inhibited H₂O₂ induction and this was correlated with a lethal effect. In plant roots incubated with different concentrations of paenimyxin for 24 h before inoculation with F. acuminatum, paenimyxin at a low concentration (ca. 1 μM) had a protective effect and suppressed 95% of the necrotic symptoms, whereas a concentration higher than 10 μM had an inhibitory effect on plant growth. Gene responses were quantified in M. truncatula by semiquantitative reverse transcription-PCR (RT-PCR). Genes involved in the biosynthesis of phytoalexins (phenylalanine ammonia-lyase, chalcone synthase, chalcone reductase), antifungal activity (pathogenesis-related proteins, chitinase), or cell wall (invertase) were highly upregulated in roots or cells after paenimyxin treatment. The mechanisms potentially involved in plant protection are discussed.

Co-occurring increases of calcium and organellar reactive oxygen species determine differential activation of antioxidant and defense enzymes in Ulva compressa (Chlorophyta) exposed to copper excess

In order to analyse copper-induced calcium release and (reactive oxygen species) ROS accumulation and their role in antioxidant and defense enzymes activation, the marine alga Ulva compressa was exposed to 10 µM copper for 7 d. The level of calcium, extracellular hydrogen peroxide (eHP), intracellular hydrogen peroxide (iHP) and superoxide anions (SA) as well as the activities of ascorbate peroxidase (AP), glutathione reductase (GR), glutathione-S-transferase (GST), phenylalanine ammonia lyase (PAL) and lipoxygenase (LOX) were determined. Calcium release showed a triphasic pattern with peaks at 2, 3 and 12 h. The second peak was coincident with increases in eHP and iHP and the third peak with the second increase of iHP. A delayed wave of SA occurred after day 3 and was not accompanied by calcium release. The accumulation of iHP and SA was mainly inhibited by organellar electron transport chains inhibitors (OETCI), whereas calcium release was inhibited by ryanodine. AP activation ceased almost completely after the use of OETCI. On the other hand, GR and GST activities were partially inhibited, whereas defense enzymes were not inhibited. In contrast, PAL and LOX were inhibited by ryanodine, whereas AP was not inhibited. Thus, copper stress induces calcium release and organellar ROS accumulation that determine the differential activation of antioxidant and defense enzymes.

Stimulation of defense reactions in Medicago truncatula by antagonistic lipopeptides from Paenibacillus sp. strain B2

With the aim of obtaining new strategies to control plant diseases, we investigated the ability of antagonistic lipopolypeptides (paenimyxin) from Paenibacillus sp. strain B2 to elicit hydrogen peroxide (H₂O₂) production and several defense-related genes in the model legume Medicago truncatula. For this purpose, M. truncatula cell suspensions were used and a pathosystem between M. truncatula and Fusarium acuminatum was established. In M. truncatula cell cultures, the induction of H₂O₂ reached a maximum 20 min after elicitation with paenimyxin, whereas concentrations higher than 20 μM inhibited H₂O₂ induction and this was correlated with a lethal effect. In plant roots incubated with different concentrations of paenimyxin for 24 h before inoculation with F. acuminatum, paenimyxin at a low concentration (ca. 1 μM) had a protective effect and suppressed 95% of the necrotic symptoms, whereas a concentration higher than 10 μM had an inhibitory effect on plant growth. Gene responses were quantified in M. truncatula by semiquantitative reverse transcription-PCR (RT-PCR). Genes involved in the biosynthesis of phytoalexins (phenylalanine ammonia-lyase, chalcone synthase, chalcone reductase), antifungal activity (pathogenesis-related proteins, chitinase), or cell wall (invertase) were highly upregulated in roots or cells after paenimyxin treatment. The mechanisms potentially involved in plant protection are discussed.

Interaction of cryptogein with its binding sites in tobacco plasma membrane studied using the piezoelectric biosensor

Elicitins are low-molecular-weight proteins representing the elicitor family secreted by many species of the oomycete Phytophthora. Elicitins induce a hypersensitive reaction in tobacco, a process that is triggered by binding of elicitin to the high-affinity site on the plasma membrane. Specific interaction of cryptogein with the binding sites on tobacco plasma membranes was studied using the piezoelectric biosensor in real time in a flow-through mode. Cryptogeins (wild-type and mutant forms) were covalently immobilized on the sensing surface, and membrane vesicles containing receptors were in solution. Kinetic characterization of the interaction provided values of kinetic rate association (k(a))=5.74 . 10(6)M(1)s(-1) and kinetic rate dissociation (k(d))=6.8710(-4)s(-1) constants, respectively. The kinetic equilibrium dissociation constant was calculated as K(D)=12.0 nM. The piezoelectric biosensor appeared to be a convenient tool for studying interactions of receptors embedded in membrane vesicles.

Abscisic acid negatively regulates elicitor-induced synthesis of capsidiol in wild tobacco

In the Solanaceae, biotic and abiotic elicitors induce de novo synthesis of sesquiterpenoid stress metabolites known as phytoalexins. Because plant hormones play critical roles in the induction of defense-responsive genes, we have explored the effect of abscisic acid (ABA) on the synthesis of capsidiol, the major wild tobacco (Nicotiana plumbaginifolia) sesquiterpenoid phytoalexin, using wild-type plants versus nonallelic mutants Npaba2 and Npaba1 that are deficient in ABA synthesis. Npaba2 and Npaba1 mutants exhibited a 2-fold higher synthesis of capsidiol than wild-type plants when elicited with either cellulase or arachidonic acid or when infected by Botrytis cinerea. The same trend was observed for the expression of the capsidiol biosynthetic genes 5-epi-aristolochene synthase and 5-epi-aristolochene hydroxylase. Treatment of wild-type plants with fluridone, an inhibitor of the upstream ABA pathway, recapitulated the behavior of Npaba2 and Npaba1 mutants, while the application of exogenous ABA reversed the enhanced synthesis of capsidiol in Npaba2 and Npaba1 mutants. Concomitant with the production of capsidiol, we observed the induction of ABA 8'-hydroxylase in elicited plants. In wild-type plants, the induction of ABA 8'-hydroxylase coincided with a decrease in ABA content and with the accumulation of ABA catabolic products such as phaseic acid and dihydrophaseic acid, suggesting a negative regulation exerted by ABA on capsidiol synthesis. Collectively, our data indicate that ABA is not required per se for the induction of capsidiol synthesis but is essentially implicated in a stress-response checkpoint to fine-tune the amplification of capsidiol synthesis in challenged plants.

Early responses of tobacco suspension cells to rhizobacterial elicitors of induced systemic resistance

Colonization of roots by selected strains of fluorescent Pseudomonas spp. can trigger induced systemic resistance (ISR) against foliar pathogens in a plant species-specific manner. It has been suggested that early responses in cell suspension cultures in response to rhizobacterial elicitors, such as generation of active oxygen species (AOS) and extracellular medium alkalinization (MA), are linked to the development of ISR in whole plants. Perception of flagellin was demonstrated to elicit ISR in Arabidopsis, and bacterial lipopolysaccharides (LPS) have been shown to elicit several defense responses and to act as bacterial determinants of ISR in various plant species. In the present study, the LPS-containing cell walls, the pyoverdine siderophores, and the flagella of Pseudomonas putida WCS358, P. fluorescens WCS374, and P. fluorescens WCS417, which are all known to act as elicitors of ISR in selected plant species, were tested for their effects on the production of AOS, MA, elevation of cytoplasmic Ca(2+) ([Ca(2+)](cyt)), and defense-related gene expression in tobacco suspension cells. The LPS of all three strains, the siderophore of WCS374, and the flagella of WCS358 induced a single, transient, early burst of AOS, whereas the siderophores of WCS358 and WCS417 and the flagella of WCS374 and WCS417 did not. None of the compounds caused cell death. Once stimulated by the active compounds, the cells became refractory to further stimulation by any of the active elicitors, but not to the elicitor cryptogein from the oomycete Phytophthora cryptogea, indicating that signaling upon perception of the different rhizobacterial compounds rapidly converges into a common response pathway. Of all compounds tested, only the siderophores of WCS358 and WCS417 did not induce MA; the flagella of WCS374 and WCS417, although not active as elicitors of AOS, did induce MA. These results were corroborated by using preparations from relevant bacterial mutants. The active rhizobacterial elicitors led to a rapid increase in [Ca(2+)](cyt), peaking at 6 min, whereas the inactive siderophores of WCS358 and WCS417 elicited a single spike at 1 min. Elicitation of the cells by cell-wall LPS of WCS358 or the siderophore of WCS374 induced a weak, transient expression of several defense-related genes, including PAL and GST. The spectrum of early responses of the suspension cells was not matched by the expression of ISR in whole tobacco plants against Erwinia carotovora pv. carotovora. Of the live bacterial strains, only WCS358 elicited significant ISR, but application of the LPS or the siderophore of all three strains also elicited ISR. Notably, the absence of elicitation of AOS and MA in suspension-cultured cells but induction of ISR in whole plants by the siderophore of WCS358, which was lost upon treatment with the siderophore-minus mutant of WCS358, indicates that the early responses in suspension cells are not predictive of the ability to induce ISR in whole plants. Possible explanations for these discrepancies are discussed.

Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization

The water uptake capacity of plant roots (i.e. their hydraulic conductivity, Lp(r)) is determined in large part by aquaporins of the plasma membrane intrinsic protein (PIP) subfamily. In the present work, we investigated two stimuli, salicylic acid (SA) and salt, because of their ability to induce an accumulation of reactive oxygen species (ROS) and an inhibition of Lp(r) concomitantly in the roots of Arabidopsis plants. The inhibition of Lp(r) by SA was partially counteracted by preventing the accumulation of hydrogen peroxide (H(2)O(2)) with exogenous catalase. In addition, exogenous H(2)O(2) was able to reduce Lp(r) by up to 90% in <15 min. Based on the lack of effects of H(2)O(2) on the activity of individual aquaporins in Xenopus oocytes, and on a pharmacological dissection of the action of H(2)O(2) on Lp(r), we propose that ROS do not gate Arabidopsis root aquaporins through a direct oxidative mechanism, but rather act through cell signalling mechanisms. Expression in transgenic roots of PIP-GFP fusions and immunogold labelling indicated that external H(2)O(2) enhanced, in <15 min, the accumulation of PIPs in intracellular structures tentatively identified as vesicles and small vacuoles. Exposure of roots to SA or salt also induced an intracellular accumulation of the PIP-GFP fusion proteins, and these effects were fully counteracted by co-treatment with exogenous catalase. In conclusion, the present work identifies SA as a novel regulator of aquaporins, and delineates an ROS-dependent signalling pathway in the roots of Arabidopsis. Several abiotic and biotic stress-related stimuli potentially share this path, which involves an H(2)O(2)-induced internalization of PIPs, to downregulate root water transport.

Vacuolar and cytoskeletal dynamics during elicitor-induced programmed cell death in tobacco BY-2 cells

Responses of plant cells to environmental stresses often involve morphological changes, differentiation and redistribution of various organelles and cytoskeletal network. Tobacco BY-2 cells provide excellent model system for in vivo imaging of these intracellular events. Treatment of the cell cycle-synchronized BY-2 cells with a proteinaceous oomycete elicitor, cryptogein, induces highly synchronous programmed cell death (PCD) and provide a model system to characterize vacuolar and cytoskeletal dynamics during the PCD. Sequential observation revealed dynamic reorganization of the vacuole and actin microfilaments during the execution of the PCD. We further characterized the effects cryptogein on mitotic microtubule organization in cell cycle-synchronized cells. Cryptogein treatment at S phase inhibited formation of the preprophase band, a cortical microtubule band that predicts the cell division site. Cortical microtubules kept their random orientation till their disruption that gradually occurred during the execution of the PCD twelve hours after the cryptogein treatment. Possible molecular mechanisms and physiological roles of the dynamic behavior of the organelles and cytoskeletal network in the pathogenic signal-induced PCD are discussed.

The plant defense elicitor cryptogein stimulates clathrin-mediated endocytosis correlated with reactive oxygen species production in bright yellow-2 tobacco cells

The plant defense elicitor cryptogein triggers well-known biochemical events of early signal transduction at the plasma membrane of tobacco (Nicotiana tabacum) cells, but microscopic observations of cell responses related to these early events were lacking. We determined that internalization of the lipophilic dye FM4-64, which is a marker of endocytosis, is stimulated a few minutes after addition of cryptogein to tobacco Bright Yellow-2 (BY-2) cells. This stimulation is specific to the signal transduction pathway elicited by cryptogein because a lipid transfer protein, which binds to the same receptor as cryptogein but without triggering signaling, does not increase endocytosis. To define the nature of the stimulated endocytosis, we quantified clathrin-coated pits (CCPs) forming on the plasma membrane of BY-2 cells. A transitory stimulation of this morphological event by cryptogein occurs within the first 15 min. In the presence of cryptogein, increases in both FM4-64 internalization and clathrin-mediated endocytosis are specifically blocked upon treatment with 5 microm tyrphostin A23, a receptor-mediated endocytosis inhibitor. The kinetics of the transient increase in CCPs at the plasma membrane coincides with that of transitory reactive oxygen species (ROS) production occurring within the first 15 min after elicitation. Moreover, in BY-2 cells expressing NtrbohD antisense cDNA, which are unable to produce ROS when treated with cryptogein, the CCP stimulation is inhibited. These results indicate that the very early endocytic process induced by cryptogein in tobacco is due, at least partly, to clathrin-mediated endocytosis and is dependent on ROS production by the NADPH oxidase NtrbohD.

Ethylene is required for elicitin-induced oxidative burst but not for cell death induction in tobacco cell suspension cultures

The signal compound ethylene and its relationships with oxidative burst and cell death were analyzed in cultured tobacco cells treated with the proteinaceous elicitor quercinin. Quercinin belongs to the protein family of elicitins and was isolated from the soil-born oak pathogen Phytophthora quercina. It was shown to induce a dose-dependent oxidative burst in tobacco cell culture in concentrations from 0.05 to 0.5 nM, and subsequently, cell death. The characteristics of quercinin-induced cell death included both membrane damage and DNA fragmentation in tobacco cell culture. At higher quercinin concentrations (2 nM), H(2)O(2) formation and ethylene biosynthesis were inhibited. Ethylene at low concentrations proved to be necessary for induction and maintenance of H(2)O(2) production in tobacco cells treated with quercinin. It was demonstrated that external addition of inhibitors of ethylene biosynthesis such as alpha-amino-oxy-acetic acid (AOA) and CoCl(2) also decreased or even inhibited the quercinin-induced oxidative burst, but did not influence cell death induction. These results demonstrate evidence for a requirement of the plant hormone ethylene for the onset of the quercinin-induced oxidative burst.

Cryptogein-induced anion effluxes: electrophysiological properties and analysis of the mechanisms through which they contribute to the elicitor-triggered cell death

Anion effluxes are amongst the earliest reactions of plant cells to elicitors of defence responses. However, their properties and their role in disease resistance remain almost unknown. We previously demonstrated that cryptogein, an elicitor of tobacco defence responses, induces a nitrate (NO(3) (-)) efflux. This efflux is an early prerequisite to the cryptogein-triggered hypersensitive response (HR). Here, we analyzed the electrophysiological properties of the elicitor-mediated NO(3) (-) efflux and clarified the mechanisms through which it contributes to cell death. Application of the discontinuous single electrode voltage-clamp technique in tobacco cells elicited with cryptogein enabled us to record the activation of slow-type deactivating anion channel currents. Cryptogein-induced plasma membrane depolarization and Ca(2+) influx, an essential component of elicitor signalling for HR cell death, were prevented by inhibiting the NO(3) (-) efflux. Similarly, pharmacological blocking of the anion efflux suppressed vacuolar collapse, a hallmark of cell death. The role of NO(3) (-) efflux in mediating proteases activation was further assessed. It is shown that cryptogein induced the activation of three proteases with apparent molecular masses of 95, 190 and 240 kDa. Their activation occurred independently on the anion efflux and, together with cell death, was strongly reduced by cycloheximide and the protease inhibitor PMSF. In contrast, the NO(3) (-) efflux was shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during the HR. Collectively, our data indicate that anion efflux is an early prerequisite to morphological and biochemical events participating to cell death.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Nitric oxide (NO) as an intermediate in the cryptogein-induced hypersensitive response--a critical re-evaluation

A hypersensitive response (HR) was induced in tobacco leaves and cell suspensions by the fungal elicitor cryptogein, and NO production was followed by chemiluminescence and occasionally by diaminofluorescein (DAF)-fluorescence. Results from both methods were at least partly consistent, but kinetics was different. NO emission was not induced by cryptogein in leaves, whereas in cell suspensions some weak NO emission was observed, which was nitrate reductase (NR)-dependent, but not required for cell death. Nitric oxide synthase (NOS) inhibitors did not prevent cell death, but PR-1 expression was weakened. In conclusion, neither NR nor NOS appear obligatory for the cryptogein-induced HR. However, a role for NO was still suggested by the fact that the NO scavenger cPTIO prevented the HR. Unexpectedly, cPTI, the reaction product of cPTIO and NO, also impaired the HR but without scavenging NO. Thus, prevention of the HR by cPTIO is not necessarily indicative for a role of NO. Further, even a 100-fold NO overproduction (over wild type) by a nitrite reductase-deficient mutant did not interfere with the cryptogein-induced HR. Accordingly, the role of NO in the HR should be reconsidered.

Proteinaceous and oligosaccharidic elicitors induce different calcium signatures in the nucleus of tobacco cells

We previously reported elevated cytosolic calcium levels in tobacco cells in response to elicitors [D. Lecourieux, C. Mazars, N. Pauly, R. Ranjeva, A. Pugin, Analysis and effects of cytosolic free calcium elevations in response to elicitors in Nicotiana plumbaginifolia cells, Plant Cell 14 (2002) 2627-2641]. These data suggested that in response to elicitors, Ca2+, as a second messenger, was involved in both systemic acquired resistance (RSA) and/or hypersensitive response (HR) depending on calcium signature. Here, we used transformed tobacco cells with apoaequorin expressed in the nucleus to monitor changes in free nuclear calcium concentrations ([Ca2+](nuc)) in response to elicitors. Two types of elicitors are compared: proteins leading to necrosis including four elicitins and harpin, and non-necrotic elicitors including flagellin (flg22) and two oligosaccharidic elicitors, namely the oligogalacturonides (OGs) and the beta-1,3-glucan laminarin. Our data indicate that the proteinaceous elicitors induced a pronounced and sustainable [Ca2+](nuc) elevation, relative to the small effects of oligosaccharidic elicitors. This [Ca2+](nuc) elevation, which seems insufficient to induce cell death, is unlikely to result directly from the diffusion of calcium from the cytosol. The [Ca2+](nuc) rise depends on free cytosolic calcium, IP3, and active oxygen species (AOS) but is independent of nitric oxide.

Modulation of reactive oxygen species production during osmotic stress in Arabidopsis thaliana cultured cells: involvement of the plasma membrane Ca2+-ATPase and H+-ATPase

In Arabidopsis thaliana cells, hypoosmotic treatment initially stimulates Ca2+ influx and inhibits its efflux and, concurrently, promotes a large H2O2 accumulation in the external medium, representative of reactive oxygen species (ROS) production. After the first 10-15 min, Ca2+ influx rate is, however, lowered, and a large rise in Ca2+ efflux, concomitant with a rapid decline in H2O2 level, takes place. The drop of the H2O2 peak, as well as the efflux of Ca2+, are prevented by treatment with submicromolar concentrations of eosin yellow (EY), selectively inhibiting the Ca2+-ATPase of the plasma membrane (PM). Comparable changes of Ca2+ fluxes are also induced by hyperosmotic treatment. However, in this case, the H2O2 level does not rise, but declines below control levels when Ca2+ efflux is activated. Also K+ and H+ net fluxes across the PM and cytoplasmic pH (pH(cyt)) are very differently influenced by the two opposite stresses: strongly decreased by hypoosmotic stress and increased under hyperosmotic treatment. The H2O2 accumulation kinetics, followed as a function of the pH(cyt) changes imposed by modulation of the PM H+-ATPase activity or weak acid treatment, show a close correlation between pH(cyt) and H2O2 formed, a larger amount being produced for changes towards acidic pH values. Overall, these results confirm a relevant role for the PM Ca2+-ATPase in switching off the signal triggering ROS production, and propose a role for the PM H+-ATPase in modulating the development of the oxidative wave through the pH(cyt) changes following the changes of its activity induced by stress conditions.

Differences in cell death induction by Phytophthora Elicitins are determined by signal components downstream of MAP kinase kinase in different species of Nicotiana and cultivars of Brassica rapa and Raphanus sativus

Elicitins are small, secreted proteins produced by species of the plant-pathogenic oomycete Phytophthora. They induce hypersensitive cell death in most Nicotiana species and in some cultivars of Brassica rapa and Raphanus sativus. In this study, two true-breeding Fast Cycling B. rapa lines were established that showed severe necrosis (line 7-R) or no visible response (line 18-NR) after treatment with elicitin. Unexpectedly, microscopic examination revealed localized cell death in line 18-NR plants, and expression levels of various defense-marker genes were comparable in both lines. These results suggested that both "responsive" and "nonresponsive" plants responded to elicitin but differed in the extent of the cell death response. Expression of a constitutively active form of Arabidopsis (Arabidopsis thaliana) MAP kinase kinase 4 (AtMEK4(DD)) also induced rapid development of confluent cell death in line 7-R, whereas line 18-NR showed no visible cell death. Similarly, elicitin-responsive Nicotiana species and R. sativus cultivars showed significantly stronger cell death responses following expression of AtMEK4(DD) compared with nonresponsive species/cultivars. Line 7-R also showed higher sensitivity to toxin-containing culture filtrates produced by Alternaria brassicicola, and toxin sensitivity cosegregated with elicitin responsiveness, suggesting that the downstream responses induced by elicitin and Alternaria toxin share factors that control the extent of cell death. Interestingly, elicitin responsiveness was shown to correlate with greater susceptibility to A. brassicicola (a necrotroph) in B. rapa but less susceptibility to Phytophthora nicotianae (a hemibiotroph) in Nicotiana, suggesting a more extensive cell death response could cause opposite effects on the outcomes of biotrophic versus necrotrophic plant-pathogen interactions.

Effects of scavengers for active oxygen species on cell death by cryptogein

The hypersensitive reaction is a type of programmed cell death in plants. Cryptogein is a proteinaceous elicitor secreted from Phythophthora cryptogea. In one current model, active oxygen species (AOS) trigger programmed cell death in plants. In this study, we examined a variety of AOS scavengers to elucidate the function of AOS in the death program. Most of these AOS scavengers, including tiron, a scavenger for superoxide radical, catalase for hydrogen peroxide, and hydroquinone, sodium ascorbate and propyl gallate for free radicals, almost completely removed extracellular AOS. However, none of the reagents completely blocked the cell death process. Other reagents, such as histidine and dimethylfuran, scavengers for singlet oxygen, and diphenyleneiodonium chloride, an inhibitor of NADPH oxidase, showed significant toxicity in BY-2 cells. These results indicate that AOS produced in the extracellular space do not play a role in hypersensitive cell death.

Cell cycle dependence of elicitor-induced signal transduction in tobacco BY-2 cells

The molecular links between the cell cycle and defense responses in plants are largely unknown. Using synchronized tobacco BY-2 cells, we analyzed the cell cycle dependence of elicitor-induced defense responses. In synchronized cultured apoaequorin-expressing cells, the increase in cytosolic free Ca2+ induced by a proteinaceous elicitor, cryptogein, was greatly suppressed during the G2 and M phases in comparison with G1 or S phases. Treatment with cryptogein during the G1 or S phases also induced biphasic (rapid/transient and slow/prolonged) responses in activation of mitogen-activated protein kinases (MAPKs) and production of reactive oxygen species (ROS). In contrast, elicitor treatment during the G2 or M phases induced only a rapid and transient phase of MAPK activation and ROS production. Their slow and prolonged phases as well as expression of defense-related genes, cell cycle arrest and cell death were induced only after the cell cycle progressed to the G1 phase; removal of the elicitor before the start of the G1 phase inhibited these responses. These results suggest that although cryptogein recognition occurred at all phases of the cell cycle, the recognition during the S or G1 phases, but not at the G2 or M phases, induces the prolonged activation of MAPKs and the prolonged production of ROS, followed by cell cycle arrest, accumulation of defense-related gene transcripts and cell death. Elicitor signal transduction depends on the cell cycle and is regulated differently at each phase.

Crosstalk between elicitor-induced cell death and cell cycle regulation in tobacco BY-2 cells

The molecular links between cell cycle control and the regulation of programmed cell death are largely unknown in plants. Here we studied the relationship between the cell cycle and elicitor-induced cell death using synchronized tobacco BY-2 cells. Flow cytometry and fluorescence microscopy of nuclear DNA, and RNA gel-blot analyses of cell cycle-related genes revealed that the proteinaceous elicitor cryptogein induced cell cycle arrest at the G1 or G2 phase before the induction of cell death. Furthermore, the patterns of cell death induction and defence-related genes were different in different phases of the cell cycle. Constitutive treatment with cryptogein induced cell cycle arrest and cell death at the G1 or G2 phase. With transient treatment for 2 h, cell cycle arrest and cell death were only induced by treatment with the elicitor during the S or G1 phase. By contrast, the elicitor-induced production of reactive oxygen species was observed during all phases of the cell cycle. These results indicate that although recognition of the elicitor signal is cell cycle-independent, the induction of cell cycle arrest and cell death depends on the phase of the cell cycle.

Lipid-binding form is a key conformation to induce a programmed cell death initiated in tobacco BY-2 cells by a proteinaceous elicitor of cryptogein

Cryptogein, a proteinaceous elicitor secreted by Phytophthora cryptogea, induces a remarkable hypersensitive cell death in tobacco cells. Two cryptogein mutants were analysed to characterize the induction mechanism of cell death; one was a newly synthesized mutant N93A whose 93rd Asn residue was changed to Ala, the other was K13V whose Lys at position 13 was replaced with Val. The effect of these mutations was evaluated in terms of extracellular alkalization, production of active oxygen species (AOS) and progression to death. The mutation N93A resulted in a reduction in activity to 71.0, 74.6 and 24.5% for original rates of extracellular alkalization, AOS production and cell death progression, respectively. In the case of the K13V mutation, these rates changed to 114, 3.38 and 7.40%, respectively. The lipid-binding activities of the mutants were analysed using fluorogenic lipid of dehydroergosterol. The results for N93A and K13V were 38.3 and 3.40% compared with the wild type, respectively. These findings indicate that the lipid-binding form was the only conformation to induce the production of AOS and programmed cell death in plants.

Analysis of nitric oxide signaling functions in tobacco cells challenged by the elicitor cryptogein

Nitric oxide (NO) has recently emerged as an important cellular mediator in plant defense responses. However, elucidation of the biochemical mechanisms by which NO participates in this signaling pathway is still in its infancy. We previously demonstrated that cryptogein, an elicitor of tobacco defense responses, triggers a NO burst within minutes in epidermal sections from tobacco leaves (Nicotiana tabacum cv Xanthi). Here, we investigate the signaling events that mediate NO production, and analyze NO signaling activities in the cryptogein transduction pathway. Using flow cytometry and spectrofluorometry, we observed that cryptogein-induced NO production in tobacco cell suspensions is sensitive to nitric oxide synthase inhibitors and may be catalyzed by variant P, a recently identified pathogen-inducible plant nitric oxide synthase. NO synthesis is tightly regulated by a signaling cascade involving Ca2+ influx and phosphorylation events. Using tobacco cells constitutively expressing the Ca2+ reporter apoaequorin in the cytosol, we have shown that NO participates in the cryptogein-mediated elevation of cytosolic free Ca2+ through the mobilization of Ca2+ from intracellular stores. The NO donor diethylamine NONOate promoted an increase in cytosolic free Ca2+ concentration, which was sensitive to intracellular Ca2+ channel inhibitors. Moreover, NO appears to be involved in the pathway(s) leading to the accumulation of transcripts encoding the heat shock protein TLHS-1, the ethylene-forming enzyme cEFE-26, and cell death. In contrast, NO does not act upstream of the elicitor-induced activation of mitogen-activated protein kinase, the opening of anion channels, nor expression of GST, LOX-1, PAL, and PR-3 genes. Collectively, our data indicate that NO is intimately involved in the signal transduction processes leading to cryptogein-induced defense responses.

Ergosterol elicits oxidative burst in tobacco cells via phospholipase A2 and protein kinase C signal pathway

Ergosterol, a typical fungal sterol, induced in tobacco (Nicotiana tabacum L. cv. Xanthi) suspension cells the synthesis of reactive oxygen species and alkalization of the external medium that are dependent on the mobilization of calcium from internal stores. We used specific inhibitors to elucidate the signal pathway triggered by ergosterol compared with cryptogein, a proteinaceous elicitor of Phytophthora cryptogea. Herbimycin A and genistein, inhibitors of tyrosine protein kinases, had no effect on the oxidative burst and pH changes induced by both elicitors. Similarly, H-89, an inhibitor of protein kinase A, had no effect on the induction of these defense reactions. However, the response to both elicitors was completely blocked by NPC-15437, a specific inhibitor of animal protein kinase C (PKC). The responses induced by cryptogein but not those induced by ergosterol were inhibited by U73122 and neomycin, inhibitors of phospholipase C (PLC). On the other hand, the activity of phospholipase A2 (PLA2) measured using a fluorogenic substrate was stimulated by ergosterol and not by cholesterol and cryptogein. A specific inhibitor of PLA2, arachidonic acid trifluoromethyl ketone (AACOCF3), inhibited the pathway stimulated by ergosterol but not that induced by cryptogein. These results suggest that the cryptogein-induced signal pathway leading to the oxidative burst and DeltapH changes includes PLC and PKC, whereas this response induced by ergosterol includes PLA2 and PKC.

Cryptogein-induced initial events in tobacco BY-2 cells: pharmacological characterization of molecular relationship among cytosolic Ca(2+) transients, anion efflux and production of reactive oxygen species

Ion fluxes and the production of reactive oxygen species (ROS) are early events that follow elicitor treatment or microbial infection. However, molecular mechanisms for these responses as well as their relationship have been controversial and still largely unknown. We here simultaneously monitored the temporal sequence of initial events at the plasma membrane in suspension-cultured tobacco cells (cell line BY-2) in response to a purified proteinaceous elicitor, cryptogein, which induced hypersensitive cell death. The elicitor induced transient rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) showing two distinct peaks, followed by biphasic (rapid/transient and slow/prolonged) Cl(-) efflux and H(+) influx. Pharmacological analyses suggested that the two phases of the [Ca(2+)](cyt) response correspond to Ca(2+) influx through the plasma membrane and an inositol 1,4,5-trisphophate-mediated release of Ca(2+) from intracellular Ca(2+) stores, respectively, and the [Ca(2+)](cyt) transients and the Cl(-) efflux were mutually dependent events regulated by protein phosphorylation. The elicitor also induced production of ROS including (*)O(2)(-) and H(2)O(2), which initiated after the [Ca(2+)](cyt) rise and required Ca(2+) influx, Cl(-) efflux and protein phosphorylation. An inhibitor of NADPH oxidase, diphenylene iodonium, completely inhibited the elicitor-induced production of (*)O(2)(-) and H(2)O(2), but did not affect the [Ca(2+)](cyt) transients. These results suggest that cryptogein-induced plasma membrane Ca(2+) influx is independent of ROS, and NADPH oxidase dependent ROS production is regulated by these series of ion fluxes.

Rac regulation of NtrbohD, the oxidase responsible for the oxidative burst in elicited tobacco cell

Five cDNAs encoding Rac protein homologues to the Rho-related proteins from plants (Rop) were isolated in tobacco, and the function of one of them, Ntrac5, was studied. The Ntrac5 mRNA is repressed when tobacco leaves and cells are treated with the fungal elicitor cryptogein. Tobacco cells were transformed with sense constructs of Ntrac5 or Ntrac5V15, encoding the native GTP/GDP-bound form of this Rac protein homologue or the constitutively active mutant in its GTP-bound form, respectively. Immunological studies indicate that the corresponding protein is continuously located on the plasma membrane (PM). Both types of transformed cells show the same extra-cellular alkalinization as the control, but a high decrease in the active oxygen species (AOS) production after elicitation with cryptogein. Moreover, the regulation of NtrbohD, the oxidase involved in AOS production upon elicitation, is affected at both transcriptional and translational levels in cells overexpressing Ntrac5. Thus, Ntrac5 could be considered as a negative regulator of NtrbohD.

Laminarin elicits defense responses in grapevine and induces protection against Botrytis cinerea and Plasmopara viticola

Grapevine (Vitis vinifera L.) is susceptible to many pathogens, such as Botrytis cinerea, Plasmopara viticola, Uncinula necator, and Eutypa lata. Phytochemicals are used intensively in vineyards to limit pathogen infections, but the appearance of pesticide-resistant pathogen strains and a desire to protect the environment require that alternative strategies be found. In the present study, the beta-1,3-glucan laminarin derived from the brown algae Laminaria digitata was shown both to be an efficient elicitor of defense responses in grapevine cells and plants and to effectively reduce B. cinerea and P. viticola development on infected grapevine plants. Defense reactions elicited by laminarin in grapevine cells include calcium influx, alkalinization of the extracellular medium, an oxidative burst, activation of two mitogen-activated protein kinases, expression of 10 defense-related genes with different kinetics and intensities, increases in chitinase and beta-1,3-glucanase activities, and the production of two phytoalexins (resveratrol and epsilon-viniferin). Several of these effects were checked and confirmed in whole plants. Laminarin did not induce cell death. When applied to grapevine plants, laminarin reduced infection by B. cinerea and P. viticola by approximately 55 and 75%, respectively. Our data describing a large set of defense reactions in grapevine indicate that the activation of defense responses using elicitors could be a valuable strategy to protect plants against pathogens.