A loss of resistance to avirulent bacterial pathogens in tobacco is associated with the attenuation of a salicylic acid-potentiated oxidative burst

PhWRKY30 activates salicylic acid biosynthesis to positively regulate antiviral defense response in petunia

Petunia (Petunia hybrida) plants are highly threatened by a diversity of viruses, causing substantial damage to ornamental quality and seed yield. However, the regulatory mechanism of virus resistance in petunia is largely unknown. Here, we revealed that a member of petunia WRKY transcription factors, PhWRKY30, was dramatically up-regulated following Tobacco rattle virus (TRV) infection. Down-regulation of PhWRKY30 through TRV-based virus-induced gene silencing increased green fluorescent protein (GFP)-marked TRV RNA accumulation and exacerbated the symptomatic severity. In comparison with wild-type (WT) plants, PhWRKY30-RNAi transgenic petunia plants exhibited a compromised resistance to TRV infection, whereas an enhanced resistance was observed in PhWRKY30-overexpressing (OE) transgenic plants. PhWRKY30 affected salicylic acid (SA) production and expression of arogenate dehydratase 1 (PhADT1), phenylalanine ammonia-lyase 1 (PhPAL1), PhPAL2b, nonexpressor of pathogenesis-related proteins 1 (PhNPR1), and PhPR1 in SA biosynthesis and signaling pathway. SA treatment restored the reduced TRV resistance to WT levels in PhWRKY30-RNAi plants, and application of SA biosynthesis inhibitor 2-aminoindan-2-phosphonic acid inhibited promoted resistance in PhWRKY30-OE plants. The protein-DNA binding assays showed that PhWRKY30 specifically bound to the promoter of PhPAL2b. RNAi silencing and overexpression of PhPAL2b led to decreased and increased TRV resistance, respectively. The transcription of a number of reactive oxygen species- and RNA silencing-associated genes was changed in PhWRKY30 and PhPAL2b transgenic lines. PhWRKY30 and PhPAL2b were further characterized to be involved in the resistance to Tobacco mosaic virus (TMV) invasion. Our findings demonstrate that PhWRKY30 positively regulates antiviral defense against TRV and TMV infections by modulating SA content.

Nitric Oxide Implication in Potato Immunity to Phytophthora infestans via Modifications of Histone H3/H4 Methylation Patterns on Defense Genes

Nitric oxide (NO) is an essential redox-signaling molecule operating in many physiological and pathophysiological processes. However, evidence on putative NO engagement in plant immunity by affecting defense gene expressions, including histone modifications, is poorly recognized. Exploring the effect of biphasic NO generation regulated by S-nitrosoglutathione reductase (GNSOR) activity after avr Phytophthora infestans inoculation, we showed that the phase of NO decline at 6 h post-inoculation (hpi) was correlated with the rise of defense gene expressions enriched in the TrxG-mediated H3K4me3 active mark in their promoter regions. Here, we report that arginine methyltransferase PRMT5 catalyzing histone H4R3 symmetric dimethylation (H4R3sme2) is necessary to ensure potato resistance to avr P. infestans. Both the pathogen and S-nitrosoglutathione (GSNO) altered the methylation status of H4R3sme2 by transient reduction in the repressive mark in the promoter of defense genes, R3a and HSR203J (a resistance marker), thereby elevating their transcription. In turn, the PRMT5-selective inhibitor repressed R3a expression and attenuated the hypersensitive response to the pathogen. In conclusion, we postulate that lowering the NO level (at 6 hpi) might be decisive for facilitating the pathogen-induced upregulation of stress genes via histone lysine methylation and PRMT5 controlling potato immunity to late blight.

OsNPR3.3-dependent salicylic acid signaling is involved in recessive gene xa5-mediated immunity to rice bacterial blight

Salicylic acid (SA) is a key natural component that mediates local and systemic resistance to pathogens in many dicotyledonous species. However, its function is controversial in disease resistance in rice plants. Here, we show that the SA signaling is involved in both pathogen-associated-molecular-patterns triggered immunity (PTI) and effector triggered immunity (ETI) to Xanthomonas oryzae pv. Oryzae (Xoo) mediated by the recessive gene xa5, in which OsNPR3.3 plays an important role through interacting with TGAL11. Rice plants containing homozygous xa5 gene respond positively to exogenous SA, and their endogenous SA levels are also especially induced upon infection by the Xoo strain, PXO86. Depletion of endogenous SA can significantly attenuate plant resistance to PXO86, even to 86∆HrpXG (mutant PXO86 with a damaged type III secretion system). These results indicated that SA plays an important role in disease resistance in rice plants, which can be clouded by high levels of endogenous SA and the use of particular rice varieties.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

Overlapping Yet Response-Specific Transcriptome Alterations Characterize the Nature of Tobacco-Pseudomonas syringae Interactions

In this study transcriptomic alterations of bacterially induced pattern triggered immunity (PTI) were compared with other types of tobacco-Pseudomonas interactions. In addition, using pharmacological agents we blocked some signal transduction pathways (Ca(2+) influx, kinases, phospholipases, proteasomic protein degradation) to find out how they contribute to gene expression during PTI. PTI is the first defense response of plant cells to microbes, elicited by their widely conserved molecular patterns. Tobacco is an important model of Solanaceae to study resistance responses, including defense mechanisms against bacteria. In spite of these facts the transcription regulation of tobacco genes during different types of plant bacterial interactions is not well-described. In this paper we compared the tobacco transcriptomic alterations in microarray experiments induced by (i) PTI inducer Pseudomonas syringae pv. syringae type III secretion mutant (hrcC) at earlier (6 h post inoculation) and later (48 hpi) stages of defense, (ii) wild type P. syringae (6 hpi) that causes effector triggered immunity (ETI) and cell death (HR), and (iii) disease-causing P. syringae pv. tabaci (6 hpi). Among the different treatments the highest overlap was between the PTI and ETI at 6 hpi, however, there were groups of genes with specifically altered activity for either type of defenses. Instead of quantitative effects of the virulent P. tabaci on PTI-related genes it influenced transcription qualitatively and blocked the expression changes of a special set of genes including ones involved in signal transduction and transcription regulation. P. tabaci specifically activated or repressed other groups of genes seemingly not related to either PTI or ETI. Kinase and phospholipase A inhibitors had highest impacts on the PTI response and effects of these signal inhibitors on transcription greatly overlapped. Remarkable interactions of phospholipase C-related pathways with the proteasomal system were also observable. Genes specifically affected by virulent P. tabaci belonged to various previously identified signaling routes, suggesting that compatible pathogens may modulate diverse signaling pathways of PTI to overcome plant defense.

Oxygen transfer rate identifies priming compounds in parsley cells

BACKGROUND

In modern agriculture, the call for an alternative crop protection strategy increases because of the desired reduction of fungicide and pesticide use and the continuously evolving resistance of pathogens and pests to agrochemicals. The direct activation of the plant immune system does not provide a promising plant protection measure because of high fitness costs. However, upon treatment with certain natural or synthetic compounds, plant cells can promote to a fitness cost-saving, primed state of enhanced defense. In the primed state, plants respond to biotic and abiotic stress with faster and stronger activation of defense, and this is often associated with immunity and abiotic stress tolerance. Until now, the identification of chemical compounds with priming-inducing activity (so-called plant activators) relied on tedious and invasive approaches, or required the late detection of secreted furanocoumarin phytoalexins in parsley cell cultures. Thus, simple, fast, straightforward, and noninvasive techniques for identifying priming-inducing compounds for plant protection are very welcome.

RESULTS

This report demonstrates that a respiration activity-monitoring system (RAMOS) can identify compounds with defense priming-inducing activity in parsley cell suspension in culture. RAMOS relies on the quasi-continuous, noninvasive online determination of the oxygen transfer rate (OTR). Treatment of parsley culture cells with the known plant activator salicylic acid (SA), a natural plant defense signal, resulted in an OTR increase. Addition of the defense elicitor Pep13, a cell wall peptide of Phythophthora sojae, induced two distinctive OTR peaks that were higher in SA-primed cells than in unprimed cells upon Pep13 challenge. Both, the OTR increase after priming with SA and the Pep13 challenge were dose-dependent. Furthermore, there was a close correlation of a compound's activity to enhance the oxygen consumption in parsley cells and its capacity to prime Pep13-induced furanocoumarin secretion as evaluated by fluorescence spectroscopy.

CONCLUSIONS

RAMOS noninvasively determines the OTR as a measure of the metabolic activity of plant cells. Chemical enhancement of oxygen consumption by salicylic derivatives in parsley cell suspension cultures correlates with the induction of the primed state of enhanced defense that enhances the quantity of Pep13-induced furanocoumarin phytoalexins. Treatment with the priming-active compounds methyl jasmonate and pyraclostrobin also resulted in an enhanced respiration activity. Thus, RAMOS is a novel technology for identifying priming-inducing compounds for agriculture.

Lipids in salicylic acid-mediated defense in plants: focusing on the roles of phosphatidic acid and phosphatidylinositol 4-phosphate

Plants have evolved effective defense strategies to protect themselves from various pathogens. Salicylic acid (SA) is an essential signaling molecule that mediates pathogen-triggered signals perceived by different immune receptors to induce downstream defense responses. While many proteins play essential roles in regulating SA signaling, increasing evidence also supports important roles for signaling phospholipids in this process. In this review, we collate the experimental evidence in support of the regulatory roles of two phospholipids, phosphatidic acid (PA), and phosphatidylinositol 4-phosphate (PI4P), and their metabolizing enzymes in plant defense, and examine the possible mechanistic interaction between phospholipid signaling and SA-dependent immunity with a particular focus on the immunity-stimulated biphasic PA production that is reminiscent of and perhaps mechanistically connected to the biphasic reactive oxygen species (ROS) generation and SA accumulation during defense activation.

De-novo assembly of mango fruit peel transcriptome reveals mechanisms of mango response to hot water treatment

Background

The mango belongs to the genus Mangifera, consisting of numerous tropical fruiting trees in the flowering plant family, Anacardiaceae. Postharvest treatment by hot water brushing (HWB) for 15–20 s was introduced commercially to improve fruit quality and reduce postharvest disease. This treatment enabled successful storage for 3–4 weeks at 12°C, with improved color and reduced disease development, but it enhanced lenticel discoloration on the fruit peel. We investigated global gene expression induced in fruit peel by HWB treatment, and identified key genes involved in mechanisms potentially associated with fruit resistance to pathogens, peel color improvement, and development of lenticel discoloration; this might explain the fruit’s phenotypic responses.

Results

The mango transcriptome assembly was created and characterized by application of RNA-seq to fruit-peel samples. RNA-seq-based gene-expression profiling identified three main groups of genes associated with HWB treatment: 1) genes involved with biotic and abiotic stress responses and pathogen-defense mechanisms, which were highly expressed; 2) genes associated with chlorophyll degradation and photosynthesis, which showed transient and low expression; and 3) genes involved with sugar and flavonoid metabolism, which were highly expressed.

Conclusions

We describe a new transcriptome of mango fruit peel of cultivar Shelly. The existence of three main groups of genes that were differentially expressed following HWB treatment suggests a molecular basis for the biochemical and physiological consequences of the postharvest HWB treatment, including resistance to pathogens, improved color development, and occurrence of lenticel discoloration.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-957) contains supplementary material, which is available to authorized users.

The secondary metabolism glycosyltransferases UGT73B3 and UGT73B5 are components of redox status in resistance of Arabidopsis to Pseudomonas syringae pv. tomato

Secondary metabolism plant glycosyltransferases (UGTs) ensure conjugation of sugar moieties to secondary metabolites (SMs) and glycosylation contributes to the great diversity, reactivity and regulation of SMs. UGT73B3 and UGT73B5, two UGTs of Arabidopsis thaliana (Arabidopsis), are involved in the hypersensitive response (HR) to the avirulent bacteria Pseudomonas syringae pv. tomato (Pst-AvrRpm1), but their function in planta is unknown. Here, we report that ugt73b3, ugt73b5 and ugt73b3 ugt73b5 T-DNA insertion mutants exhibited an accumulation of reactive oxygen species (ROS), an enhanced cell death during the HR to Pst-AvrRpm1, whereas glutathione levels increased in the single mutants. In silico analyses indicate that UGT73B3 and UGT73B5 belong to the early salicylic acid (SA)-induced genes whose pathogen-induced expression is co-regulated with genes related to cellular redox homeostasis and general detoxification. Analyses of metabolic alterations in ugt mutants reveal modification of SA and scopoletin contents which correlate with redox perturbation, and indicate quantitative modifications in the pattern of tryptophan-derived SM accumulation after Pst-AvrRpm1 inoculation. Our data suggest that UGT73B3 and UGT73B5 participate in regulation of redox status and general detoxification of ROS-reactive SMs during the HR to Pst-AvrRpm1, and that decreased resistance to Pst-AvrRpm1 in ugt mutants is tightly linked to redox perturbation.

Salicylic acid is an indispensable component of the Ny-1 resistance-gene-mediated response against Potato virus Y infection in potato

The purpose of the study was to investigate the role of salicylic acid (SA) signalling in Ny-1-mediated hypersensitive resistance (HR) of potato (Solanum tuberosum L.) to Potato virus Y (PVY). The responses of the Ny-1 allele in the Rywal potato cultivar and transgenic NahG-Rywal potato plants that do not accumulate SA were characterized at the cytological, biochemical, transcriptome, and proteome levels. Analysis of noninoculated and inoculated leaves revealed that HR lesions started to develop from 3 d post inoculation and completely restricted the virus spread. At the cytological level, features of programmed cell death in combination with reactive oxygen species burst were observed. In response to PVY infection, SA was synthesized de novo. The lack of SA accumulation in the NahG plants led to the disease phenotype due to unrestricted viral spreading. Grafting experiments show that SA has a critical role in the inhibition of PVY spreading in parenchymal tissue, but not in vascular veins. The whole transcriptome analysis confirmed the central role of SA in orchestrating Ny-1-mediated responses and showed that the absence of SA leads to significant changes at the transcriptome level, including a delay in activation of expression of genes known to participate in defence responses. Moreover, perturbations in the expression of hormonal signalling genes were detected, shown as a switch from SA to jasmonic acid/ethylene signalling. Viral multiplication in the NahG plants was accompanied by downregulation of photosynthesis genes and activation of multiple energy-producing pathways.

Pathogenesis related-10 proteins are small, structurally similar but with diverse role in stress signaling

Pathogenesis related-10 proteins are small proteins with cytosolic localization, conserved three dimensional structures and single intron at 185 bp position. These proteins have a broad spectrum of roles significantly in biotic and abiotic stresses. The RNase activity, ligand binding activity, posttranslational modification (phosphorylation) and phytohormone signaling provide some information into the mechanism of the regulation of PR-10 proteins, however the presence of isoforms makes it difficult to decipher its exact mode of function. The involvement of phosphorylation/dephosphorylation events in its activation is interesting and provides unique and unbiased insights into the complexity of its regulation. Studies on upstream region of different PR-10 genes indicate the presence of cis-acting elements for WRKY, RAVI, bZ1P, ERF, SEBF and Pti4 transcription factors indicating their role in regulating PR-10 promoter. In this review, we discuss in detail the structure and mechanism of regulation of PR-10 proteins.

The form of nitrogen nutrition affects resistance against Pseudomonas syringae pv. phaseolicola in tobacco

Different forms of nitrogen (N) fertilizer affect disease development; however, this study investigated the effects of N forms on the hypersensitivity response (HR)-a pathogen-elicited cell death linked to resistance. HR-eliciting Pseudomonas syringae pv. phaseolicola was infiltrated into leaves of tobacco fed with either NO₃⁻ or NH₄⁺. The speed of cell death was faster in NO₃⁻-fed compared with NH₄⁺-fed plants, which correlated, respectively, with increased and decreased resistance. Nitric oxide (NO) can be generated by nitrate reductase (NR) to influence the formation of the HR. NO generation was reduced in NH₄⁺-fed plants where N assimilation bypassed the NR step. This was similar to that elicited by the disease-forming P. syringae pv. tabaci strain, further suggesting that resistance was compromised with NH₄⁺ feeding. PR1a is a biomarker for the defence signal salicylic acid (SA), and expression was reduced in NH₄⁺-fed compared with NO₃⁻ fed plants at 24h after inoculation. This pattern correlated with actual SA measurements. Conversely, total amino acid, cytosolic and apoplastic glucose/fructose and sucrose were elevated in - treated plants. Gas chromatography/mass spectroscopy was used to characterize metabolic events following different N treatments. Following NO₃⁻ nutrition, polyamine biosynthesis was predominant, whilst after NH₄⁺ nutrition, flux appeared to be shifted towards the production of 4-aminobutyric acid. The mechanisms whereby feeding enhances SA, NO, and polyamine-mediated HR-linked defence whilst these are compromised with NH₄⁺, which also increases the availability of nutrients to pathogens, are discussed.

The signaling role of a mitochondrial superoxide burst during stress

Plant mitochondria are proposed to act as signaling organelles in the orchestration of defense responses to biotic stress and acclimation responses to abiotic stress. However, the primary signal(s) being generated by mitochondria and then interpreted by the cell are largely unknown. Recently, we showed that mitochondria generate a sustained burst of superoxide (O 2(-)) during particular plant-pathogen interactions. This O 2(-) burst appears to be controlled by mitochondrial components that influence rates of O 2(-) generation and scavenging within the organelle. The O 2(-) burst appears to influence downstream processes such as the hypersensitive response, indicating that it could represent an important mitochondrial signal in support of plant stress responses. The findings generate many interesting questions regarding the upstream factors required to generate the O 2(-) burst, the mitochondrial events that occur in support of and in parallel with this burst and the downstream events that respond to this burst.

Haemoglobin modulates salicylate and jasmonate/ethylene-mediated resistance mechanisms against pathogens

Nitric oxide (NO) plays a role in defence against hemibiotrophic pathogens mediated by salicylate (SA) and also necrotrophic pathogens influenced by jasmonate/ethylene (JA/Et). This study examined how NO-oxidizing haemoglobins (Hb) encoded by GLB1, GLB2, and GLB3 in Arabidopsis could influence both defence pathways. The impact of Hb on responses to the hemibiotrophic Pseudomonas syringae pathovar tomato (Pst) AvrRpm1 and the necrotrophic Botrytis cinerea were investigated using glb1, glb2, and glb3 mutant lines and also CaMV 35S GLB1 and GLB2 overexpression lines. In glb1, but not glb2 and glb3, increased resistance was observed to both pathogens but was compromised in the 35S-GLB1. A quantum cascade laser-based sensor measured elevated NO production in glb1 infected with Pst AvrRpm1 and B. cinerea, which was reduced in 35S-GLB1 compared to Col-0. SA accumulation was increased in glb1 and reduced in 35S-GLB1 compared to controls following attack by Pst AvrRpm1. Similarly, JA and Et levels were increased in glb1 but decreased in 35S-GLB1 in response to attack by B. cinerea. Quantitative PCR assays indicated reduced GLB1 expression during challenge with either pathogen, thus this may elevate NO concentration and promote a wide-ranging defence against pathogens.

Evidence of a role for foliar salicylic acid in regulating the rate of post-ingestive protein breakdown in ruminants and contributing to landscape pollution

Ruminant farming is important to global food security, but excessive proteolysis in the rumen causes inefficient use of nitrogenous plant constituents and environmental pollution. While both plant and microbial proteases contribute to ruminal proteolysis, little is known about post-ingestion regulation of plant proteases except that activity in the first few hours after ingestion of fresh forage can result in significant degradation of foliar protein. As the signal salicylic acid (SA) influences cell death during both biotic and abiotic stresses, Arabidopsis wild-type and mutants were used to test the effect of SA on proteolysis induced by rumen conditions (39 °C and anaerobic in a neutral pH). In leaves of Col-0, SA accumulation was induced by exposure to a rumen microbial inoculum. Use of Arabidopsis mutants with altered endogenous SA concentrations revealed a clear correlation with the rate of stress-induced proteolysis; rapid proteolysis occurred in leaves of SA-accumulating mutants cpr5-1 and dnd1-1 whereas there was little or no proteolysis in sid2-1 which is unable to synthesize SA. Reduced proteolysis in npr1-1 (Non-expressor of Pathogenesis Related genes) demonstrated a dependence on SA signalling. Slowed proteolysis in sid2-1 and npr1-1 was associated with the absence of a 34.6 kDa cysteine protease. These data suggest that proteolysis in leaves ingested by ruminants is modulated by SA. It is therefore suggested that influencing SA effects in planta could enable the development of forage crops with lower environmental impact and increased production potential.

A novel function for a redox-related LEA protein (SAG21/AtLEA5) in root development and biotic stress responses

SAG21/AtLEA5 belongs to the late embryogenesis-associated (LEA) protein family. Although it has been implicated in growth and redox responses, its precise roles remain obscure. To address this problem, we characterized root and shoot development and response to biotic stress in SAG21/AtLEA5 over-expressor (OEX) and antisense (AS) lines. AS lines exhibited earlier flowering and senescence and reduced shoot biomass. Primary root length was reduced in AS lines, as was the number of laterals relative to the primary root. Root hair number was unchanged but root hair length was proportional to SAG21/AtLEA5 expression level, with longer root hairs in OEX lines and shorter root hairs in AS, relative to wild type. Growth of the fungal nectroph, Botrytis cinerea and of a virulent bacterial pathogen (Pseudomonas syringae pv. tomato) was affected by SAG21/AtLEA5 expression; however, growth of an avirulent P.syringae strain was unaffected. A SAG21/AtLEA5-YFP fusion was localized to mitochondria, raising the intriguing possibility that SAG21 interacts with proteins involved in mitochondrial ROS signalling, which in turn, impacts on root development and pathogen responses.

Transformed tobacco (Nicotiana tabacum) plants over-expressing a peroxisome proliferator-activated receptor gene from Xenopus laevis (xPPARα) show increased susceptibility to infection by virulent Pseudomonas syringae pathogens

Transgenic tobacco plants capable of over-expressing Xenopus PPARα (xPPARα), a transcription factor known to be required for peroxisome proliferation in animals, were recently generated. These plants (herewith referred to as PPAR-OE) were found to have increased peroxisome abundance, higher peroxisomal acyl-CoA oxidase and catalase activity and modified fatty acid metabolism. Further characterization of PPAR-OE plants revealed a higher susceptibility to virulent and a partial loss of resistance to avirulent Pseudomonas syringae pathogens, whereas the basal resistance response remained unaffected. Biochemical- and defense-related gene expression analyses showed that increased susceptibility to bacterial invasion coincided with the generalized reduction in H(2)O(2) and salicylic acid (SA) levels observed within the first 24 h of bacterial contact. Decreased H(2)O(2) levels were correlated with modified activity levels of catalase and other antioxidant enzymes. A correspondence between a rapid (within 1-24 hpi; ACCO and AOC) and sustained increase (up to 6 days pi; ACCO) in the expression levels of ethylene (ACCO) and jasmonic acid (AOC) biosynthetic genes and a higher susceptibility to virulent bacterial invasion was also observed in PPAR-OE plants. Conversely, no apparent differences in the short- and/or long-term expression levels of markers for the hypersensitive-response, oxidative burst and systemic-acquired resistance were observed between wild type and PPAR-OE plants. The results suggest that peroxisome proliferation could lead to increased susceptibility to bacterial pathogens in tobacco by altering the redox balance of the plant and the expression pattern of key defense signaling pathway genes.

Accumulation of chlorophyll catabolites photosensitizes the hypersensitive response elicited by Pseudomonas syringae in Arabidopsis

• The staygreen (SGR) gene encodes a chloroplast-targeted protein which promotes chlorophyll degradation via disruption of light-harvesting complexes (LHCs). • Over-expression of SGR in Arabidopsis (SGR-OX) in a Columbia-0 (Col-0) background caused spontaneous necrotic flecking. To relate this to the hypersensitive response (HR), Col-0, SGR-OX and RNAi SGR (SGRi) lines were challenged with Pseudomonas syringae pv tomato (Pst) encoding the avirulence gene avrRpm1. Increased and decreased SGR expression, respectively, accelerated and suppressed the kinetics of HR-cell death. In Col-0, SGR transcript increased at 6  h after inoculation (hai) when tissue electrolyte leakage indicated the initiation of cell death. • Excitation of the chlorophyll catabolite pheophorbide (Pheide) leads to the formation of toxic singlet oxygen ((1)O(2)). Pheide was first detected at 6  hai with Pst avrRpm1 and was linked to (1)O(2) generation and correlated with reduced Pheide a oxygenase (PaO) protein concentrations. The maximum quantum efficiency of photosystem II (F(v)/F(m)), quantum yield of electron transfer at photosystem II (φPSII), and photochemical quenching (qP) decreased at 6  hai in Col-0 but not in SGRi. Disruption of photosynthetic electron flow will cause light-dependent H(2)O(2) generation at 6  hai. • We conclude that disruption of LHCs, possibly influenced by SGR, and absence of PaO produce phototoxic chlorophyll catabolites and oxidative stress leading to the HR.

Salicylic acid is involved in the Nb-mediated defense responses to Potato virus X in Solanum tuberosum

To evaluate the role of salicylic acid (SA) in Nb-mediated hypersensitive resistance to Potato virus X (PVX) avirulent strain ROTH1 in Solanum tuberosum, we have constructed SA-deficient transgenic potato plant lines by overexpressing the bacterial enzyme salicylate hydroxylase (NahG), which degrades SA. Evaluation of these transgenic lines revealed hydrogen peroxide accumulation and spontaneous lesion formation in an age- and light-dependent manner. In concordance, NahG potato plants were more sensitive to treatment with methyl viologen, a reactive oxygen species-generating compound. In addition, when challenged with PVX ROTH1, NahG transgenic lines showed a decreased disease-resistance response to infection and were unable to induce systemic acquired resistance. However, the avirulent viral effector, the PVX 25-kDa protein, does induce expression of the pathogenesis-related gene PR-1a in NahG potato plants. Taken together, our data indicate that SA is involved in local and systemic defense responses mediated by the Nb gene in Solanum tuberosum. This is the first report to show that basal levels of SA correlate with hypersensitive resistance to PVX.

Extracellular ATP is a regulator of pathogen defence in plants

In healthy plants extracellular ATP (eATP) regulates the balance between cell viability and death. Here we show an unexpected critical regulatory role of eATP in disease resistance and defensive signalling. In tobacco, enzymatic depletion of eATP or competition with non-hydrolysable ATP analogues induced pathogenesis-related (PR) gene expression and enhanced resistance to tobacco mosaic virus and Pseudomonas syringae pv. tabaci. Artificially increasing eATP concentrations triggered a drop in levels of the important defensive signal chemical salicylic acid (SA) and compromised basal resistance to viral and bacterial infection. Inoculating tobacco leaf tissues with bacterial pathogens capable of activating PR gene expression triggered a rapid decline in eATP. Conversely, inoculations with mutant bacteria unable to induce defence gene expression failed to deplete eATP. Furthermore, a collapse in eATP concentration immediately preceded PR gene induction by SA. Our study reveals a previously unsuspected role for eATP as a negative regulator of defensive signal transduction and demonstrates its importance as a key signal integrating defence and cell viability in plants.

Biphasic ethylene production during the hypersensitive response in Arabidopsis: a window into defense priming mechanisms?

The hypersensitive response (HR) is a cell death phenomenon associated with localized resistance to pathogens. Biphasic patterns in the generation of H(2)O(2), salicylic acid and ethylene have been observed in tobacco during the early stages of the HR. These biphasic models reflect an initial elicitation by pathogen-associated molecular patterns followed by a second phase, induced by pathogen-encoded avirulence gene products. The first phase has been proposed to potentiate the second, to increase the efficacy of plant resistance to disease. This potentiation is comparable to the "priming" of plant defenses which is seen when plants display systemic resistance to disease. The events regulating the generation of the biphasic wave, or priming, remains obscure, however recently we demonstrated a key role for nitric oxide in this process in a HR occurring in tobacco. Here we use laser photoacoustic detection to demonstrate that biphasic ethylene production also occurs during a HR occurring in Arabidopsis. We suggest that ethylene emanation during the HR represents a ready means of visualising biphasic events during the HR and that exploiting the genomic resources offered by this model species will facilitate the development of a mechanistic understanding of potentiating/priming processes.

Biphasic ethylene production during the hypersensitive response in Arabidopsis: a window into defense priming mechanisms?

The hypersensitive response (HR) is a cell death phenomenon associated with localized resistance to pathogens. Biphasic patterns in the generation of H(2)O(2), salicylic acid and ethylene have been observed in tobacco during the early stages of the HR. These biphasic models reflect an initial elicitation by pathogen-associated molecular patterns followed by a second phase, induced by pathogen-encoded avirulence gene products. The first phase has been proposed to potentiate the second, to increase the efficacy of plant resistance to disease. This potentiation is comparable to the "priming" of plant defenses which is seen when plants display systemic resistance to disease. The events regulating the generation of the biphasic wave, or priming, remains obscure, however recently we demonstrated a key role for nitric oxide in this process in a HR occurring in tobacco. Here we use laser photoacoustic detection to demonstrate that biphasic ethylene production also occurs during a HR occurring in Arabidopsis. We suggest that ethylene emanation during the HR represents a ready means of visualising biphasic events during the HR and that exploiting the genomic resources offered by this model species will facilitate the development of a mechanistic understanding of potentiating/priming processes.

Aspirin, salicylates, and cancer

Evidence from a wide range of sources suggests that individuals taking aspirin and related non-steroidal anti-inflammatory drugs have reduced risk of large bowel cancer. Work in animals supports cancer reduction with aspirin, but no long-term randomised clinical trials exist in human beings, and randomisation would be ethically unacceptable because vascular protection would have to be denied to a proportion of the participants. However, opportunistic trials of aspirin, designed to test vascular protection, provide some evidence of a reduction in cancer, but only after at least 10 years. We summarise evidence for the potential benefit of aspirin and natural salicylates in cancer prevention. Possible mechanisms of action and directions for further work are discussed, and implications for clinical practice are considered.

The herbicide flumioxazin stimulates pathogenesis-related gene expression and enzyme activities in Vitis vinifera

In this work, the capacity of the soil-applied herbicide flumioxazin (fmx) to trigger defence mechanisms was assessed using 6-week-old in vitro grown Vitis vinifera L. plantlets. Time-course studies demonstrated that the herbicide induced the expression of basic beta-1,3-glucanase (Vvglu), basic chitinase (Vvchit1b) and PR10 (VvPR10.3) genes encoding three pathogenesis-related (PR) proteins involved in grapevine defence against pathogens. Thus, all transcripts accumulated in grapevine tissues to reach maximum values after 24-72 h of herbicide exposure, except for VvPR10.3 gene expression, which was induced in roots and stems but not in leaves. Induction of PR genes was observed to a greater extent in roots and leaves, and its intensity diminished in the stems although still remained noteworthy. The activities of beta-1,3-glucanase and chitinase enzymes significantly increased in the whole plant after herbicide exposure and were still stimulated 21 days after the beginning of treatments. Similarly, the most remarkable effect occurred in roots. However, all enzyme activities tested were stimulated in the upper aerial tissues as well, indicating that fmx or a derived product acts systemically, likely via root uptake.

A Dynamic Mathematical Model To Clarify Signaling Circuitry Underlying Programmed Cell Death Control in Arabidopsis Disease Resistance

Plant cells undergo programmed cell death in response to invading pathogens. This cell death limits the spread of the infection and triggers whole plant antimicrobial and immune responses. The signaling network connecting molecular recognition of pathogens to these responses is a prime target for manipulation in genetic engineering strategies designed to improve crop plant disease resistance. Moreover, as alterations to metabolism can be misinterpreted as pathogen infection, successful plant metabolic engineering will ultimately depend on controlling these signaling pathways to avoid inadvertent activation of cell death. Programmed cell death resulting from infection of Arabidopsis thaliana with Pseudomonas syringae bacterial pathogens was chosen as a model system. Signaling circuitry hypotheses in this model system were tested by construction of a differential-equations-based mathematical model. Model-based simulations of time evolution of signaling components matched experimental measurements of programmed cell death and associated signaling components obtained in a companion study. Simulation of systems-level consequences of mutations used in laboratory studies led to two major improvements in understanding of signaling circuitry: (1) Simulations supported experimental evidence that a negative feedback loop in salicylic acid biosynthesis postulated by others does not exist. (2) Simulations showed that a second negative regulatory circuit for which there was strong experimental support did not affect one of two pathways leading to programmed cell death. Simulations also generated testable predictions to guide future experiments. Additional testable hypotheses were generated by results of individually varying each model parameter over 2 orders of magnitude that predicted biologically important changes to system dynamics. These predictions will be tested in future laboratory studies designed to further elucidate the signaling network control structure.

My health: whose responsibility? Low-dose aspirin and older people

The benefit of aspirin as a prophylactic after a thrombotic event was first observed 30 years ago. Its use after coronary or cerebral thrombosis, and in patients judged to be at increased risk of a thrombotic event, is now virtually mandatory, unless there are signs of intolerance. The present policy in the UK for cardiovascular protection by low-dose aspirin is dependent upon the identification of people at high vascular risk. The policy has had only very limited success, partly owing to the fact that only a relatively small proportion of people with levels of vascular risk factors that would justify aspirin prophylaxis are identified. In fact, it has been demonstrated that the application of accepted guidelines for aspirin prophylaxis to risk factor data in representative UK population samples gives a cost-effective evidence-base for a reasonable extension of prophylaxis to all people aged over approximately 50 years. It is possible that reductions in both dementia and cancer incidence could also follow the wider use of low-dose aspirin but further research on these outcomes is urgently required. The evidence on possible benefits and harm from low-dose aspirin should therefore be publicized widely, and everything possible should be done to stimulate discussion involving the general public. In the end, however, the preservation of health is one's own responsibility and, therefore, people should generally be encouraged to evaluate the evidence on health-promotion measures, including low-dose aspirin, and take responsibility for their own health.

Systemic acquired resistance

Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of "plant memory". Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.

Quantitative in situ assay of salicylic acid in tobacco leaves using a genetically modified biosensor strain of Acinetobacter sp. ADP1

Salicylic acid (SA) plays important roles in plants, most notably in the induction of systemic acquired resistance (SAR) against pathogens. A non-destructive in situ assay for SA would provide new insights into the functions of SA in SAR and other SA-regulated phenomena. We assessed a genetically engineered strain of Acinetobacter sp. ADP1, which proportionally produces bioluminescence in response to salicylates including SA and methylsalicylate, as a reporter for salicylate accumulation in the apoplast of plant leaves. SA was measured quantitatively in situ in NN genotype tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaves inoculated with tobacco mosaic virus (TMV). The biosensor revealed accumulation of apoplastic SA before the visible appearance of hypersensitive response (HR) lesions. When the biosensor was infiltrated into TMV-inoculated leaves displaying HR lesions at 90 and 168 h post-inoculation, salicylate accumulation was detected predominantly in tissues surrounding the lesions and in veins adjacent to HR lesions. These images are consistent with previous data demonstrating that SA accumulation occurs prior to and following the onset of visible HR lesions. We also used the biosensor to observe apoplastic SA accumulation in tobacco leaves inoculated with virulent and HR-eliciting strains of the bacterial plant pathogen Pseudomonas syringae. The work demonstrates that the Acinetobacter sp. ADP1 biosensor is a useful new tool to non-destructively assay salicylates in situ and to map their spatial distribution in plant tissues.

An essential role for salicylic acid in AtMYB30-mediated control of the hypersensitive cell death program in Arabidopsis

Salicylic acid (SA) plays a central role in resistance and defense induction in response to pathogen attack, but its role in the activation of the hypersensitive response (HR), a form of programmed cell death associated with resistance of plants, remains to be elucidated. AtMYB30, a R2R3-MYB transcriptional factor which acts as a positive regulator of the HR, is a good model for studying the role of SA in programmed cell death. Here, we demonstrate that AtMYB30 expression in response to an HR-inducing bacterial pathogen is dependent on SA accumulation, but NPR1-independent. Alterations of AtMYB30 expression (overexpression, depletion by antisense strategy, T-DNA insertion mutant) modulate SA levels and SA-associated gene expression. Additionally, mutants or transgenic lines altered in SA accumulation (nahG, sid1, sid2), but not those affected in SA signalling (npr1), abolish the accelerated cell death phenotype conferred by over-expression of AtMYB30. These results suggest that AtMYB30 is involved in an amplification loop or signalling cascade that modulates SA synthesis, which in turn modulates cell death.

Ozone-induced expression of the Arabidopsis FAD7 gene requires salicylic acid, but not NPR1 and SID2

The Arabidopsis FAD7 gene encodes a plastid omega-3 fatty acid desaturase that catalyzes the desaturation of dienoic fatty acids to trienoic fatty acids in chloroplast membrane lipids. The expression of FAD7 was rapidly and locally induced by ozone exposure, which causes oxidative responses equivalent to pathogen-induced hypersensitive responses and subsequently activates various defense-related genes. This induction was reduced in salicylic acid (SA)-deficient NahG plants expressing SA hydroxylase, but was unaffected in etr1 and jar1 mutants, which are insensitive to ethylene and jasmonic acid (JA), respectively. The SA dependence of the FAD7 induction was confirmed by the exogenous application of SA. SA-induced expression of FAD7 in the npr1 mutant which is defective in an SA signaling pathway occurred to the same extent as in the wild type. Furthermore, in the sid2 mutant which lacks an enzyme required for SA biosynthesis, the expression of FAD7 was induced by ozone exposure. These results suggest that the ozone-induced expression of FAD7 gene requires SA, but not ethylene, JA, NPR1 and SID2.

Effects of feeding Spodoptera littoralis on lima bean leaves. III. Membrane depolarization and involvement of hydrogen peroxide

In response to herbivore (Spodoptera littoralis) attack, lima bean (Phaseolus lunatus) leaves produced hydrogen peroxide (H(2)O(2)) in concentrations that were higher when compared to mechanically damaged (MD) leaves. Cellular and subcellular localization analyses revealed that H(2)O(2) was mainly localized in MD and herbivore-wounded (HW) zones and spread throughout the veins and tissues. Preferentially, H(2)O(2) was found in cell walls of spongy and mesophyll cells facing intercellular spaces, even though confocal laser scanning microscopy analyses also revealed the presence of H(2)O(2) in mitochondria/peroxisomes. Increased gene and enzyme activations of superoxide dismutase after HW were in agreement with confocal laser scanning microscopy data. After MD, additional application of H(2)O(2) prompted a transient transmembrane potential (V(m)) depolarization, with a V(m) depolarization rate that was higher when compared to HW leaves. In transgenic soybean (Glycine max) suspension cells expressing the Ca(2+)-sensing aequorin system, increasing amounts of added H(2)O(2) correlated with a higher cytosolic calcium ([Ca(2+)](cyt)) concentration. In MD and HW leaves, H(2)O(2) also triggered the increase of [Ca(2+)](cyt), but MD-elicited [Ca(2+)](cyt) increase was more pronounced when compared to HW leaves after addition of exogenous H(2)O(2). The results clearly indicate that V(m) depolarization caused by HW makes the membrane potential more positive and reduces the ability of lima bean leaves to react to signaling molecules.

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-beta-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10-100 microm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, alpha-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-beta-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

NO way to live; the various roles of nitric oxide in plant-pathogen interactions

Nitric oxide has attracted considerable interest from plant pathologists due its established role in regulating mammalian anti-microbial defences, particularly via programmed cell death (PCD). Although NO plays a major role in plant PCD elicited in response to certain types of pathogenic challenge, the race-specific hypersensitive response (HR), it is now evident that NO also acts in the regulation of non-specific, papilla-based resistance to penetration by plant cells that survive attack and, possibly, in systemic acquired resistance. Equally, the potential roles of NO signalling/scavenging within the pathogen are being recognized. This review will consider key defensive roles played by NO in living cells during plant-pathogen interactions, as well as in those undergoing PCD.

Yeast extract and methyl jasmonate-induced silymarin production in cell cultures of Silybum marianum (L.) Gaertn

The biosynthesis of the flavonolignan silymarin, a constitutive compound of the fruits of Silybum marianum with strong antihepatotoxic and hepatoprotective activities, is severely reduced in cell cultures of this species. It is well known that elicitation is one of the strategies employed to increase accumulation of secondary metabolites. Our study here reports on the effect of several compounds on the production of silymarin in S. marianum cultures. Yeast extract (YE), chitin and chitosan were compared with respect to their effects on silymarin accumulation in S. marianum suspensions and only yeast extract stimulated production. Jasmonic acid (JA) potentiated the yeast extract effect. One of the jasmonic acid derivatives, methyl jasmonate (MeJA), strongly promoted the accumulation of silymarin. Methyl jasmonate acted in a number of steps of the metabolic pathway of flavonolignans and its stimulating effect was totally dependent of "de novo" protein synthesis. Chalcone synthase (CHS) activity was enhanced by methyl jasmonate; however there did not appear to be a temporal relationship between silymarin accumulation and increase in enzyme activity. Also, this increase was not blocked by the protein synthesis inhibitor cycloheximide (CH). This study indicates that elicitor treatment promotes secondary metabolite production in S. marianum cultures and that jasmonic acid and its functional analogue plays a critical role in elicitation.

Induction of some defense-related genes and oxidative burst is required for the establishment of systemic acquired resistance in Capsicum annuum

The inoculation of primary pepper leaves with an avirulent strain of Xanthomonas campestris pv. vesicatoria induced systemic acquired resistance (SAR) in the non-inoculated, secondary leaves. This SAR response was accompanied by the systemic expression of the defense-related genes, a systemic microoxidative burst generating H2O2, and the systemic induction of both ion-leakage and callose deposition in the non-inoculated, secondary leaves. Some defense-related genes including those encoding PR-1, chitinase, osmotin, peroxidase, PR10, thionin, and SAR8.2 were markedly induced in the systemic leaves. The conspicuous systemic accumulation of H2O2 and the strong increase in peroxidase activity in the pepper leaves was suggested to play a role in the cell death process in the systemic micro-hypersensitive responses (HR), leading to the induction of the SAR. Treatment of the primary leaves with diphenylene iodinium (DPI), an inhibitor of oxidative burst, substantially reduced the induction of some of the defense-related genes, and lowered the activation of the oxidative bursts in the systemic leaves distant from the site of the avirulent pathogen inoculation and subsequently SAR. Overall, these results suggest that the induction of some defense-related genes as well as a rapid increase in oxidative burst is essential for establishing SAR in pepper plants.

Laser photoacoustic detection allows in planta detection of nitric oxide in tobacco following challenge with avirulent and virulent Pseudomonas syringae Pathovars

We demonstrate the use of laser photoacoustic detection (LPAD) as a highly sensitive method to detect in planta nitric oxide ((*)NO) production from tobacco (Nicotiana tabacum). LPAD calibration against (*)NO gas demonstrated a linear relationship over 2 orders of magnitude with a detection threshold of <20 pmol h(-1) (1 part per billion volume [ppbv]). The specificity of the photoacoustic signal for (*)NO when adding gas or the (*)NO donor, sodium nitroprusside, on injection into plant leaves, was demonstrated by its abolition with O(3) ((*)NO + O(3) --> NO(2) + O(2)). The utility of the LPAD method was shown by examination of a nonhost hypersensitive response and a disease induced by Pseudomonas syringae (P. s.) pv phaseolicola and P. s. pv tabaci in tobacco. (*)NO was detected within 40 min of challenge with P. s. pv phaseolicola, some 5 h before the initiation of visible tissue collapse. The wildfire tobacco pathogen P. s. pv tabaci initiated (*)NO generation at 2 h postinfection. The use of (*)NO donors, the scavenger CPTIO ([4-carboxyphenyl]-4,5-dihydro-4,4,5,5-tetramethyl-3-oxide), and the mammalian nitric oxide synthase inhibitor l-NMMA (N(G)-monomethyl-l-arginine) indicated that (*)NO influenced the kinetics of cell death and resistance to both avirulent and virulent bacteria in tobacco. These observations suggest that (*)NO is integral to the elicitation of cell death associated with these two bacterial pathogens in tobacco.

The AoPR10 promoter and certain endogenous PR10 genes respond to oxidative signals in Arabidopsis

SUMMARY The PR10 class of genes has been associated with plant defence. Previous studies with an asparagus PR10 gene (AoPR1) promoter in heterologous plants suggested that the AoPR10-GUS transgene was responsive to oxidative signals/stresses. Arabidopsis thaliana AoPR10-GUS transgenics allowed expression to be compared with that of a close homologue from the large family of PR10 class genes within the Arabidopsis genome. AoPR10-GUS was induced developmentally at sites of phenylpropanoid accumulation and by wounding, pathogen challenge and treatment with H(2)O(2) but not with salicylic acid (SA), ethylene, methyljasmonate or NO donors. Both wound- and pathogen-associated AoPR10-GUS expression could be suppressed by superoxide dismutase and the NADPH oxidase inhibitor, diphenylene iodonium. Northern blotting using an Arabidopsis PR10 homologue as a probe revealed transcript up-regulation by oxidative species generated by glucose oxidase and xanthine oxidase. In Arabidopsis, the AoPR10-GUS transgene was potentiated by SA and expressed systemically following wounding or challenge with avirulent bacteria. AoPR10-GUS x npr1-1 crosses revealed that potentiation and systemic expression was NPR1-independent. Systemic AoPR10-GUS expression following elicitation of a hypersensitive response but not wounding was abolished in NahG crosses, suggesting an SA-mediated potentiating action during SAR (systemic acquired resistance). These data suggest that the AoPR10 promoter reports the expression of reactive oxygen species-responsive PR10 genes and may indicate systemic changes in oxidative status following either wounding and/or the elicitation of a hypersensitive response in Arabidopsis.

Characterization of a proteinase inhibitor from Brachypodium distachyon suggests the conservation of defence signalling pathways between dicotyledonous plants and grasses

SUMMARY Proteinase inhibitors (PIs) are established markers for wound- and especially jasmonate-mediated signalling in dicot species such as tomato and potato. Differential screening of a cDNA library constructed from RNA isolated from wounded leaves of the grass Brachypodium distachyon led to the identification of a proteinase inhibitor gene (Bdpin1). Bdpin1 exhibited the highest homology to the subtilisin/chymotrypsin-inhibiting subgroup of the pin1 class of plant PIs. Northern analyses indicated that Bdpin1 was induced within 6 h at the site of wounding and systemically, by 24 h, thereby providing evidence for long-distance signalling in grasses. Bdpin1 also proved to be more rapidly induced in susceptible than in resistant ecotypes of B. distachyon following challenge with the Rice blast pathogen, Magnaporthe grisea. Screening with chemical signals indicated that Bdpin1 could be induced with MeJA but not with the putative mimic of salicylic acid, benzothiadiazole. Genomic Southern hybridization was consistent with Bdpin1 existing at a single locus, which was isolated following screening of a genomic cosmid library. DNA upstream of the Bdpin1 coding sequence was characterized via fusion to a GUS reporter and was found to confer wound-responsive transcription in B. distachyon and other cereals following biolistic bombardment. Both wound- and TMV-activated Bdpin1-GUS activity was detected in transgenic tobacco. Given that B. distachyon represents an ancestral grass species, our data suggest that there is considerable conservation in defence-associated signalling between dicots and grasses.

Feedback control of the Arabidopsis hypersensitive response

The plant hypersensitive response (HR) to avirulent bacterial pathogens results from programmed cell death of plant cells in the infected region. Ion leakage and changes in signaling components associated with HR progression were measured. These studies compared Arabidopsis mutants affecting feedback loops with wild-type plants, with timepoints taken hourly. In response to Pseudomonas syringae pv. tomato DC3000 x avrB, npr1-2 mutant plants showed increased ion leakage relative to wild-type plants. Hydrogen peroxide accumulation was similar to that in wild type, but salicylic acid accumulation was reduced at some timepoints. With DC3000 x avrRpt2, similar trends were seen. In response to DC3000 x avrB, ndr1-1 mutant plants showed more ion leakage than wild-type or npr1-2 plants. Hydrogen peroxide accumulation was delayed by approximately 1 h and reached half the level seen with wild-type plants. Salicylic acid accumulation was similar to npr1-2 mutant plants. With DC3000 x avrRpt2, ndr1-1 mutant plants showed no ion leakage, no hydrogen peroxide accumulation, and minimal salicylic acid accumulation. Results with a ndr1-1 and npr1-2 double mutant were similar to ndr1-1. A model consistent with these data is presented, in which one positive and two negative regulatory circuits control HR progression. Understanding this circuitry will facilitate HR manipulation for enhanced disease resistance.

Genetic evidence that expression of NahG modifies defence pathways independent of salicylic acid biosynthesis in the Arabidopsis-Pseudomonas syringae pv. tomato interaction

The salicylic acid (SA)-induction deficient (sid) mutants of Arabidopsis, eds5 and sid2 accumulate normal amounts of camalexin after inoculation with Pseudomonas syringae pv. tomato (Pst), while transgenic NahG plants expressing an SA hydroxylase that degrades SA have reduced levels of camalexin and exhibit a higher susceptibility to different pathogens compared to the sid mutants. SID2 encodes an isochorismate synthase necessary for the synthesis of SA. NahG was shown to act epistatically to the sid mutant phenotype regarding accumulation of camalexin after inoculation with Pst in eds5NahG and sid2NahG plants. The effect of the pad4 mutation on the sid mutant phenotype was furthermore tested in eds5pad4 and sid2pad4 double mutants, and it was demonstrated that PAD4 acts epistatically to EDS5 and SID2 regarding the production of camalexin after inoculation with Pst. NahG plants and pad4 mutants were also found to produce less ethylene (ET) after infection with Pst in comparison to the wild type (WT) and sid mutants. Both PAD4 and NahG acted epistatically to SID regarding the Pst-dependent production of ET that was found to be necessary for the accumulation of camalexin. Early production of jasmonic acid (JA) 12 h after inoculation with Pst/avrRpt2 was absent in all plants expressing NahG compared to the other mutants tested here. These genetic studies unravel pleiotropic changes in defence signalling of NahG plants that are unlikely to result from their low SA content. This adds unexpected difficulties in the interpretation of earlier findings based solely on NahG plants.

Capillary electrophoresis-based profiling and quantitation of total salicylic acid and related phenolics for analysis of early signaling in Arabidopsis disease resistance

A capillary electrophoresis-based method for quantitation of total salicylic acid levels in Arabidopsis leaves was developed. Direct comparison to previous high-performance liquid chromatography (HPLC)-based measurements showed similar levels of salicylic acid. Simultaneous quantitation of trans-cinnamic acid, benzoic acid, sinapic acid, and an internal recovery standard was achieved. A rapid, streamlined protocol with requirements for plant tissue reduced relative to those of HPLC-based protocols is presented. Complicated, multiparameter experiments were thus possible despite the labor-intensive nature of inoculating plants with bacterial pathogens. As an example of this sort of experiment, detailed time course studies of total salicylic acid accumulation by wild-type Arabidopsis and two lines with mutations affecting salicylic acid accumulation in response to either of two avirulent bacterial strains were performed. Accumulation in the first 12h was biphasic. The first phase was partially SID2 and NDR1 dependent with both bacterial strains. The second phase was largely independent of both genes with bacteria carrying avrB, but dependent upon both genes with bacteria carrying avrRpt2. Virulent bacteria did not elicit salicylic acid accumulation at these time points. Application of this method to various Arabidopsis pathosystems and the wealth of available disease resistance signaling mutants will refine knowledge of disease resistance and associated signal transduction.

cDNAs generated from individual epidermal cells reveal that differential gene expression predicting subsequent resistance or susceptibility to rust fungal infection occurs prior to the fungus entering the cell lumen

As the cowpea rust fungus penetrates the wall of a cowpea epidermal cell, resistant and susceptible plants exhibit different ultrastructural and cytochemical changes within the epidermal protoplast. To examine plant gene expression at this stage of infection, cytoplasm was extracted from individual inoculated or uninoculated epidermal cells before the fungal penetration peg reached the cell lumen. Initial differential colony hybridization screening of an expressed sequence tag library constructed from globally amplified cDNAs generated from the inoculated resistant cells resulted in 80 clones (out of 835) with a differential hybridization pattern. Further slot-blot screening and screening of the amplified cDNAs generated from inoculated or uninoculated, resistant or susceptible cells revealed 28 separate genes, mostly with no matching sequences in the databases, that were up-regulated in response to the growth of the fungus through the wall of resistant or susceptible cells. Five genes, including those coding for beta- and alpha-tubulin, were found to be down-regulated specifically in inoculated, susceptible cells, and five were specifically up-regulated in inoculated, resistant cells, including a PR-10 homolog and a phenylalanine ammonia-lyase gene. Probing the amplified cDNAs from each cell type for the expression of cell death-related genes revealed that an LLS1 homolog (vuLLS1), cloned from cowpea, was up-regulated by infection in both resistant and susceptible cells and that a homolog of HSR203J was differentially up-regulated in resistant cells. These data show that changes in gene expression predicting the subsequent expression of susceptibility or hypersensitive resistance to fungal infection occur prior to the fungus entering the cell lumen.

Priming in plant-pathogen interactions

Plants can acquire enhanced resistance to pathogens after treatment with necrotizing attackers, nonpathogenic root-colonizing pseudomonads, salicylic acid, beta-aminobutyric acid and many other natural or synthetic compounds. The induced resistance is often associated with an enhanced capacity to mobilize infection-induced cellular defence responses - a process called 'priming'. Although the phenomenon has been known for years, most progress in our understanding of priming has been made only recently. These studies show that priming often depends on the induced disease resistance key regulator NPR1 (also known as NIM1 or SAI1) and that priming has a major effect on the regulation of cellular plant defence responses.

Esa1, an Arabidopsis mutant with enhanced susceptibility to a range of necrotrophic fungal pathogens, shows a distorted induction of defense responses by reactive oxygen generating compounds

An Arabidopsis thaliana mutant, esa1, that shows enhanced susceptibility to the necrotrophic pathogens Alternaria brassicicola, Botrytis cinerea and Plectosphaerella cucumerina, but has wild-type levels of resistance to the biotrophic pathogens Pseudomonas syringae pv. tomato and Peronospora parasitica. The enhanced susceptibility towards necrotrophic pathogens correlated with a delayed induction of phytoalexin accumulation and delayed induction of the plant defensin gene PDF1.2 upon inoculation with pathogens. Two reactive oxygen generating compounds, paraquat and acifluorfen, were found to cause induction of both phytoalexin accumulation and PDF1.2 expression in wild-type plants, but this induction was almost completely abolished in esa1. This finding suggests that esa1 may somehow be involved in transduction of signals generated by reactive oxygen species.