Negative cross-talk between salicylate- and jasmonate-mediated pathways in the Wassilewskija ecotype of Arabidopsis thaliana

Effects of Plant Hormones, Metal Ions, Salinity, Sugar, and Chemicals Pollution on Glucosinolate Biosynthesis in Cruciferous Plant

Cruciferous vegetable crops are grown widely around the world, which supply a multitude of health-related micronutrients, phytochemicals, and antioxidant compounds. Glucosinolates (GSLs) are specialized metabolites found widely in cruciferous vegetables, which are not only related to flavor formation but also have anti-cancer, disease-resistance, and insect-resistance properties. The content and components of GSLs in the Cruciferae are not only related to genotypes and environmental factors but also are influenced by hormones, plant growth regulators, and mineral elements. This review discusses the effects of different exogenous substances on the GSL content and composition, and analyzes the molecular mechanism by which these substances regulate the biosynthesis of GSLs. Based on the current research status, future research directions are also proposed.

Disclosure of salicylic acid and jasmonic acid-responsive genes provides a molecular tool for deciphering stress responses in soybean

Hormones orchestrate the physiology of organisms. Measuring the activity of defense hormone-responsive genes can help understanding immune signaling and facilitate breeding for plant health. However, different from model species like Arabidopsis, genes that respond to defense hormones salicylic acid (SA) and jasmonic acid (JA) have not been disclosed in the soybean crop. We performed global transcriptome analyses to fill this knowledge gap. Upon exogenous application, endogenous levels of SA and JA increased in leaves. SA predominantly activated genes linked to systemic acquired resistance and defense signaling whereas JA mainly activated wound response-associated genes. In general, SA-responsive genes were activated earlier than those responding to JA. Consistent with the paradigm of biotrophic pathogens predominantly activating SA responses, free SA and here identified most robust SA marker genes GmNIMIN1, GmNIMIN1.2 and GmWRK40 were induced upon inoculation with Phakopsora pachyrhizi, whereas JA marker genes did not respond to infection with the biotrophic fungus. Spodoptera exigua larvae caused a strong accumulation of JA-Ile and JA-specific mRNA transcripts of GmBPI1, GmKTI1 and GmAAT whereas neither free SA nor SA-marker gene transcripts accumulated upon insect feeding. Our study provides molecular tools for monitoring the dynamic accumulation of SA and JA, e.g. in a given stress condition.

Integrating Transcriptome and Coexpression Network Analyses to Characterize Salicylic Acid- and Jasmonic Acid-Related Genes in Tolerant Poplars Infected with Rust

Melampsora larici-populina causes serious poplar foliar diseases called rust worldwide. Salicylic acid (SA) and jasmonic acid (JA) are important phytohormones that are related to plant defence responses. To investigate the transcriptome profiles of SA- and JA-related genes involved in poplar rust interaction, two tolerant poplars and one intolerant poplar were selected for this study. Weighted gene coexpression network analysis (WGCNA) was applied to characterize the changes in the transcriptome profiles and contents of SA and JA after infection with the virulent E4 race of M. larici-populina. In response to infection with the E4 race of M. larici-populina, tolerant symptoms were correlated with the expression of genes related to SA and JA biosynthesis, the levels of SA and JA, and the expression of defence-related genes downstream of SA and JA. Tolerant poplars could promptly regulate the occurrence of defence responses by activating or inhibiting SA or JA pathways in a timely manner, including regulating the expression of genes related to programmed cell death, such as Kunitz-type trypsin inhibitor (KTI), to limit the growth of E4 and protect themselves. WGCNA suggested that KTI might be regulated by a Cytochrome P450 family (CYP) gene. Some CYPs should play an important role in both JA- and SA-related pathways. In contrast, in intolerant poplar, the inhibition of SA-related defence signalling through increasing JA levels in the early stage led to continued inhibition of a large number of plant–pathogen interaction-related and signalling-related genes, including NBS-LRRs, EDS1, NDR1, WRKYs, and PRs. Therefore, timely activation or inhibition of the SA or JA pathways is the key difference between tolerant and intolerant poplars.

A Salivary Odorant-Binding Protein Mediates Nilaparvata lugens Feeding and Host Plant Phytohormone Suppression

Odorant-binding proteins (OBPs) typically act as transporters of odor molecules and play an important role in insect host location. Here, we identified an OBP in brown planthopper (BPH) Nilaparvata lugens salivary glands via transcriptome sequencing. Real-time quantitative PCR and Western blotting analysis results showed that NlugOBP11 was highly expressed in salivary glands and secreted into rice plant during feeding, suggesting that it assists in BPH feeding on rice. Functional analysis in N. lugens saliva revealed that silencing this gene by RNA interference decreased the BPH stylet performance in the phloem of rice plants, reduced sap sucking, and ultimately led to insect death. Moreover, overexpression of NlugOBP11 in rice protoplasts or Nicotiana benthamiana leaves inhibited the production of defense-related signaling molecule salicylic acid in rice plant. The results demonstrate that NlugOBP11 is not only essential for BPH feeding, but also acts as an effector that inhibits plant defense.

Local adaptation to continuous mowing makes the noxious weed Solanum elaeagnifolium a superweed candidate by improving fitness and defense traits

The role of disturbance in accelerating weed growth is well understood. While most studies have focused on soil mediated disturbance, mowing can also impact weed traits. Using silverleaf nightshade (Solanum elaeagnifolium), a noxious and invasive weed, through a series of field, laboratory, and greenhouse experiments, we asked whether continuous mowing influences growth and plant defense traits, expressed via different avenues, and whether they cascade into offspring. We found that mowed plants produced significantly less number of fruits, and less number of total seeds per plant, but had higher seed mass, and germinated more and faster. When three herbivores were allowed to feed, tobacco hornworm (Manduca sexta) caterpillars, gained more mass on seedlings from unmowed plants, while cow pea aphid (Aphis craccivora), a generalist, established better on mowed seedlings; however, leaf trichome density was higher on unmowed seedlings, suggesting possible negative cross talk in defense traits. Texas potato beetle (Leptinotarsa texana), a co-evolved specialist on S. elaeagnifolium, did not show any differential feeding effects. We also found that specific root length, an indicator of nutrient acquisition, was significantly higher in first generation seedlings from mowed plants. Taken together, we show that mowing is a selective pressure that enhances some fitness and defense traits and can contribute to producing superweeds.

Pervasive interactions between foliar microbes and soil nutrients mediate leaf production and herbivore damage in a tropical forest

Producing and retaining leaves underlie the performance and survivorship of seedlings in deeply shaded tropical forests. These habitats are characterized by conditions ideal for foliar bacteria, which can be potent plant pathogens. Leaf production, retention and susceptibility to enemies may ultimately depend upon interactions among soil nutrients and foliar microbes, yet this has never been tested. We experimentally evaluated the degree that foliar bacteria and soil resource supply mediate leaf dynamics for five common tree species (five different families) in a Panamanian forest. We reduced foliar bacteria with antibiotics for 29 months and measured leaf production, retention and damage for seedlings nested within a replicated 15-yr factorial nutrient enrichment experiment (nitrogen, N; phosphorus, P; potassium, K). Our results demonstrate that when we applied antibiotics, soil nutrients - particularly N - always regulated seedling leaf production (and to a lesser extent herbivore damage) for all five tree species. In addition, it was common for two macronutrients together to negate or completely reverse the impact of applying either one alone. Our findings of frequent plant-microbe-nutrient interactions are novel and suggest that these interactions may reinforce plant species-environment associations, thereby creating a fairly cryptic and fine-scale dimension of niche differentiation for coexisting tree species.

Interactive Effects of Cabbage Aphid and Caterpillar Herbivory on Transcription of Plant Genes Associated with Phytohormonal Signalling in Wild Cabbage

Plants are commonly attacked by a variety of insect herbivores and have developed specific defenses against different types of attackers. At the molecular level, herbivore-specific signalling pathways are activated by plants in response to attackers with different feeding strategies. Feeding by leaf-chewing herbivores predominantly activates jasmonic acid (JA)-regulated defenses, whereas feeding by phloem-sucking herbivores generally activates salicylic acid (SA)-regulated defenses. When challenged sequentially by both phloem-sucking and leaf-chewing herbivores, SA-JA antagonism may constrain the plant's ability to timely and adequately divert defense to the second herbivore that requires activation of a different defensive pathway. We investigated the effect of the temporal sequence of infestation by the aphid Brevicoryne brassicae and three caterpillar species, Plutella xylostella, Pieris brassicae, and Mamestra brassicae, on the interaction between JA and SA signal-transduction pathways in three wild cabbage populations. We found no support for SA-JA antagonism, irrespective of the temporal sequence of herbivore introduction or the identity of the caterpillar species based on the transcript levels of the JA- and SA-regulated marker genes LOX and PR-1, respectively, at the examined time points, 6, 24, and 48 h. In general, infestation with aphids alone had little effect on the transcript levels of the two marker genes, whereas the three caterpillar species upregulated not only LOX but also PR-1. Transcriptional changes were different for plants from the three different natural cabbage populations.

Constitutive camalexin production and environmental stress response variation in Arabidopsis populations from the Iberian Peninsula

Optimal defense theory predicts that induction of defensive secondary metabolites in plants will be inversely correlated with constitutive expression of those compounds. Here, we asked whether camalexin, an important defense against fungal and bacterial pathogens, support this prediction in structured natural populations of Arabidopsis thaliana from the Iberian Peninsula. In common garden experiments, we found that genotypes from the VIE population constitutively hyper-accumulated camalexin. Camalexin concentrations were not induced significantly when plants were exposed to a temperature of 10°C for 48h. However, they were induced when plants were exposed to 48h of infection by the virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000. Genotypes from the VIE population with the hyper-accumulation of camalexin were significantly more resistant to bacterial growth. Induction of camalexin was negatively correlated with constitutive camalexin concentrations following log transformation and two different corrections for autocorrelation, thus supporting the tradeoff predicted by optimal defense theory. Constitutive overexpression of camalexin was not explained by the only known natural genetic polymorphism at the Accelerated Cell Death 6, ACD6, locus. Collectively, the results support an important role of camalexin in defense against P. syringae as well as significant structured variation in defense levels within wild populations.

Interactions between the jasmonic and salicylic acid pathway modulate the plant metabolome and affect herbivores of different feeding types

The phytohormones jasmonic acid (JA) and salicylic acid (SA) mediate induced plant defences and the corresponding pathways interact in a complex manner as has been shown on the transcript and proteine level. Downstream, metabolic changes are important for plant-herbivore interactions. This study investigated metabolic changes in leaf tissue and phloem exudates of Plantago lanceolata after single and combined JA and SA applications as well as consequences on chewing-biting (Heliothis virescens) and piercing-sucking (Myzus persicae) herbivores. Targeted metabolite profiling and untargeted metabolic fingerprinting uncovered different categories of plant metabolites, which were influenced in a specific manner, indicating points of divergence, convergence, positive crosstalk and pronounced mutual antagonism between the signaling pathways. Phytohormone-specific decreases of primary metabolite pool sizes in the phloem exudates may indicate shifts in sink-source relations, resource allocation, nutrient uptake or photosynthesis. Survival of both herbivore species was significantly reduced by JA and SA treatments. However, the combined application of JA and SA attenuated the negative effects at least against H. virescens suggesting that mutual antagonism between the JA and SA pathway may be responsible. Pathway interactions provide a great regulatory potential for the plant that allows triggering of appropriate defences when attacked by different antagonist species.

Signaling cross-talk in plant disease resistance

Hormone signaling crosstalk plays a major role in plant defense against a wide range of both biotic and abiotic stresses. While many reviews on plant-microbe interactions have well described the general trends of signaling pathways in shaping host responses to pathogens, few discussions have considered a synthesis of positive versus negative interactions among such pathways, or variations in the signaling molecules themselves. This review deals with the interaction trends between salicylic, jasmonic, and abscisic acids in the signaling pathways, as well as exceptions to such trends. Here we focused on antagonistic versus cooperative interactions between salicylic and jasmonic acids, two major disease resistance signaling molecules, and some interactions with abscisic acid, a known abiotic stress hormone, and another player in plant defense mechanisms. We provide a set of examples materializing either antagonism or cooperation for each interaction between two pathways, thereby showing the trends and pinpointing the exceptions. Such analyses are practical for researchers working on the subject and essential for a better exploitation of the data already available in plant disease resistance signaling, both in Arabidopsis and crop species, toward the development of better disease management strategies for economically important crops.

Protease inhibitor (PI) mediated defense in leaves and flowers of pigeonpea (protease inhibitor mediated defense in pigeonpea)

More than 200 insect pests are found growing on pigeonpea. Insects lay eggs, attack and feed on leaves, flowers and developing pods. Plants have developed elaborate defenses against these insect pests. The present work evaluates protease inhibitor (PI) based defense of pigeonpea in leaves and flowers. PIs in the extracts of these tender tissues were detected by using gel X-ray film contact print method. Up to three PIs (PI-3, PI-4 and PI-5) were detected in these tissues as against nine (PI-1-PI-9) in mature seeds. PI-3 is the major component of these tissues. Mechanical wounding, insect chewing, fungal pathogenesis and application of salicylic acid induced PIs in pigeonpea in these tissues. Induction was found to be local as well as systemic but local response was stronger than systemic response. During both local and systemic induction, PI-3 appeared first. In spite of the presence and induction of PIs in these tender tissues and seeds farmers continue to suffer yield loses. This is due to the weak expression of PIs. However the ability of the plant to respond to external stimuli by producing defense proteins does not seem to be compromised. This study therefore indicates that PIs are components of both constitutive and inducible defense and provide a ground for designing stronger inducible defense (PIs or other insect toxin based) in pigeonpea.

Jasmonate-dependent and COI1-independent defense responses against Sclerotinia sclerotiorum in Arabidopsis thaliana: auxin is part of COI1-independent defense signaling

The jasmonate receptor COI1 is known to facilitate plant defense responses against necrotrophic pathogens, including the ascomycete Sclerotinia sclerotiorum. However, it is not known to what extent jasmonates contribute to defense nor have COI1-independent defense pathways been sufficiently characterized. Here we show that the susceptibility to S. sclerotiorum of the aos mutant, deficient in biosynthesis of jasmonic acid (JA) and its precursor 12-oxophytadienoic acid, was elevated to a level reminiscent of that of hypersusceptible coi1 mutants. In contrast, susceptibility of the JA-deficient opr3 mutant was comparable with that of the wild type. A set of 99 genes responded similarly to infection with S. sclerotiorum in wild-type and coi1 mutant leaves. Expression of this COI1-independent gene set correlated with known differences in gene expression between wild-type plants and a mutant in the transcriptional repressor auxin response factor 2 (arf2). Susceptibility to S. sclerotiorum was reduced in two arf2 mutants early during infection, implicating ARF2 as a negative regulator of defense responses against this pathogen. Hypersusceptibility of an axr1 mutant to S. sclerotiorum confirmed the contribution of auxin action to defense responses against this fungal pathogen.

Mining for treatment-specific and general changes in target compounds and metabolic fingerprints in response to herbivory and phytohormones in Plantago lanceolata

Induction studies focusing on target metabolites may not reveal metabolic changes occurring in plants after various challenges. By contrast, metabolic fingerprinting can be a powerful tool to find patterns that are either treatment-specific or general and was therefore used to depict plant responses after various challenges. Plants of Plantago lanceolata were challenged by mechanical damage, specialist herbivores (aphids or sawfly larvae), generalist herbivores (Lepidopteran caterpillars) or phytohormones (jasmonic or salicylic acid). After 3 d of treatment, local and systemic leaves were analyzed for characteristic target metabolites (iridoid glucosides and verbascoside) by gas chromatography coupled with mass spectrometry (GC-MS) and for metabolic fingerprints by liquid chromatography coupled with time of flight mass spectrometry (LC-TOF-MS). Whereas only marginal changes in target metabolite concentrations were found, metabolic fingerprints were substantially affected especially by generalist and phytohormone treatments. By contrast, mechanical damage and specialist herbivory caused fewer changes. Responses to generalists partly overlapped with the changes caused by jasmonic acid, but many additional peaks were up-regulated. Furthermore, many peaks were co-induced by jasmonic and salicylic acid. The surprisingly high co-induction of peaks by both phytohormones suggests that the signaling pathways regulate a set of common targets. Furthermore, only metabolic fingerprinting could reveal that herbivores induce additional species-specific pathways beyond these phytohormone responses.

Root secretion of defense-related proteins is development-dependent and correlated with flowering time

Proteins found in the root exudates are thought to play a role in the interactions between plants and soil organisms. To gain a better understanding of protein secretion by roots, we conducted a systematic proteomic analysis of the root exudates of Arabidopsis thaliana at different plant developmental stages. In total, we identified 111 proteins secreted by roots, the majority of which were exuded constitutively during all stages of development. However, defense-related proteins such as chitinases, glucanases, myrosinases, and others showed enhanced secretion during flowering. Defense-impaired mutants npr1-1 and NahG showed lower levels of secretion of defense proteins at flowering compared with the wild type. The flowering-defective mutants fca-1, stm-4, and co-1 showed almost undetectable levels of defense proteins in their root exudates at similar time points. In contrast, root secretions of defense-enhanced cpr5-2 mutants showed higher levels of defense proteins. The proteomics data were positively correlated with enzymatic activity assays for defense proteins and with in silico gene expression analysis of genes specifically expressed in roots of Arabidopsis. In conclusion, our results show a clear correlation between defense-related proteins secreted by roots and flowering time.

Reproducible RNA preparation from sugarcane and citrus for functional genomic applications

High-throughput functional genomic procedures depend on the quality of the RNA used. Copurifying molecules can negatively impact the functionality of some plant RNA preparations employed in these procedures. We present a simplified, rapid, and scalable SDS/phenol-based method that provides the high-quantity and -quality RNA required by the newly emerging biotechnology applications. The method is applied to isolating RNA from tissues of two biotechnologically important crop plants, sugarcane and citrus, which provide a challenge due to the presence of fiber, polysaccharides, or secondary metabolites. The RNA isolated by this method is suitable for several downstream applications including northern blot hybridization, microarray analysis, and quantitative RT-PCR. This method has been used in a diverse range of projects ranging from screening plant lines overexpressing mammalian genes to analyzing plant responses to viral infection and defense signaling molecules.

Continua of specificity and virulence in plant host-pathogen interactions: causes and consequences

Ecological, evolutionary and molecular models of interactions between plant hosts and microbial pathogens are largely based around a concept of tightly coupled interactions between species pairs. However, highly pathogenic and obligate associations between host and pathogen species represent only a fraction of the diversity encountered in natural and managed systems. Instead, many pathogens can infect a wide range of hosts, and most hosts are exposed to more than one pathogen species, often simultaneously. Furthermore, outcomes of pathogen infection vary widely because host plants vary in resistance and tolerance to infection, while pathogens are also variable in their ability to grow on or within hosts. Environmental heterogeneity further increases the potential for variation in plant host-pathogen interactions by influencing the degree and fitness consequences of infection. Here, we describe these continua of specificity and virulence inherent within plant host-pathogen interactions. Using this framework, we describe and contrast the genetic and environmental mechanisms that underlie this variation, outline consequences for epidemiology and community structure, explore likely ecological and evolutionary drivers, and highlight several key areas for future research.

Polymorphism in jasmonate signaling partially accounts for the variety of volatiles produced by Nicotiana attenuata plants in a native population

Herbivore- and jasmonate-induced volatile organic compounds (VOCs), which mediate indirect defense, must provide reliable information for predators that frequently learn to associate their release with feeding herbivores. Yet little is known about variation of these cues within populations of native plants, on a scale encountered by predators. We examined variation in herbivore-elicited VOC emissions and patterns of herbivore-induced jasmonate signaling from accessions of Nicotiana attenuata co-occurring in a native population. VOC emissions elicited by herbivore oral secretions (OS) and by methyl jasmonate (MJ) were characterized using gas chromatography-mass spectrometry (GC-MS), high-resolution two-dimensional gas chromatography-time-of-flight mass spectrometry (GCxGC-ToF-MS) and micro-hydrolysis and micro-hydrogenation reactions. Accessions varied in emissions of abundant (trans-alpha-bergamotene, alpha-duprezianene, trans-beta-ocimene, and cis-3-hexenol) and total detectable VOCs, as well as the accumulation of jasmonates, the jasmonate antagonist salicylic acid (SA), abscisic acid (ABA) and jasmonate signaling-related transcripts after OS elicitation. Yet MJ treatment exacerbated differences in VOC emission, suggesting that much variation in VOC emission is caused by processes downstream of jasmonate signaling. Co-occurring N. attenuata plants emit different VOCs following simulated herbivore elicitation as a result in part of differences in jasmonate production and responsiveness, which could reduce the effectiveness of induced indirect defense.

The plastidic bile acid transporter 5 is required for the biosynthesis of methionine-derived glucosinolates in Arabidopsis thaliana

Aliphatic glucosinolate biosynthesis is highly compartmentalized, requiring import of 2-keto acids or amino acids into chloroplasts for side chain elongation and export of the resulting compounds into the cytosol for conversion into glucosinolate. Aliphatic glucosinolate biosynthesis in Arabidopsis thaliana is regulated by three R2R3-MYB transcription factors, the major player being High Aliphatic Glucosinolate 1 (HAG1/MYB28). Here, we show that BAT5, which belongs to the putative bile acid transporter family, is the only member of this family that is transactivated by HAG1/MYB28, HAG2/MYB76, and HAG3/MYB29. Furthermore, two isopropylmalate isomerases genes, IPMI1 and IPMI2, and the isopropylmalate dehydrogenase gene, IPMDH1, were identified as targets of HAG1/MYB28 and the corresponding proteins localized to plastids, suggesting a role in plastidic chain elongation reactions. The BAT proteins also localized to plastids; however, only mutants defective in BAT5 function contained strongly reduced levels of aliphatic glucosinolates. The bat5 mutant chemotype was rescued by induced overexpression of BAT5. Feeding experiments using 2-keto acids and amino acids of different chain length suggest that BAT5 is a plastidic transporter of (chain-elongated) 2-keto acids. Mechanical stimuli and methyl jasmonate transiently induced BAT5 expression in inflorescences and leaves. Thus, BAT5 was identified as the first transporter component of the aliphatic glucosinolate biosynthetic pathway.

Networking by small-molecule hormones in plant immunity

Plants live in complex environments in which they intimately interact with a broad range of microbial pathogens with different lifestyles and infection strategies. The evolutionary arms race between plants and their attackers provided plants with a highly sophisticated defense system that, like the animal innate immune system, recognizes pathogen molecules and responds by activating specific defenses that are directed against the invader. Recent advances in plant immunity research have provided exciting new insights into the underlying defense signaling network. Diverse small-molecule hormones play pivotal roles in the regulation of this network. Their signaling pathways cross-communicate in an antagonistic or synergistic manner, providing the plant with a powerful capacity to finely regulate its immune response. Pathogens, on the other hand, can manipulate the plant's defense signaling network for their own benefit by affecting phytohormone homeostasis to antagonize the host immune response.

Jasmonate- and salicylate-mediated plant defense responses to insect herbivores, pathogens and parasitic plants

While extensive research has examined plant defense responses to attack by herbivores and pathogens, plant responses to parasitism by other plants are not well characterized. The expression of induced plant defenses is mediated by complex signaling networks in which the plant hormones jasmonic acid (JA) and salicylic acid (SA) play key roles. In general, JA-mediated signaling pathways are implicated in the regulation of antiherbivore defenses, while the SA pathway is associated with defense responses against pathogens. However, there are many exceptions to this basic framework, and recent work suggests that interactions between the JA and SA pathways may play important roles in fine-tuning defense responses. The few studies that have addressed plant responses to parasitism by other plants suggest that both salicylates and jasmonates can mediate effective defenses. Here, a review is given of recent work on these defense signaling molecules with relevance for understanding their role in plant responses to parasitism.

Methyl salicylate production and jasmonate signaling are not essential for systemic acquired resistance in Arabidopsis

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.

An important role of a BAHD acyl transferase-like protein in plant innate immunity

Salicylic acid (SA) is an important regulator of plant resistance to biotrophic and hemi-biotrophic pathogens. The enhanced pseudomonas susceptibility 1 (eps1) mutant in Arabidopsis thaliana is hypersusceptible to both virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae. Through positional cloning, the EPS1 gene was isolated and found to encode a novel member of the BAHD acyltransferase superfamily. Pathogen-induced accumulation of SA and expression of pathogenesis-related (PR) genes were compromised in the eps1 mutant. SA could induce PR1 gene expression and restore disease resistance in the eps1 mutant. These results suggest that EPS1 functions upstream of SA and may be involved directly in synthesis of a precursor or a regulatory molecule for SA biosynthesis. Mutations of EPS1 or other genes important for SA accumulation or signaling conferred enhanced resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola in the Nossen-0 background but had little effect in the Columbia-0 background. These results suggest that there is natural variation among Arabidopsis ecotypes with respect to the antagonistic cross-talk between defense signaling pathways against various types of microbial pathogens.

Comparing induction at an early and late step in signal transduction mediating indirect defence in Brassica oleracea

The induction of plant defences involves a sequence of steps along a signal transduction pathway, varying in time course. In this study, the effects of induction of an early and a later step in plant defence signal transduction on plant volatile emission and parasitoid attraction are compared. Ion channel-forming peptides represent a class of inducers that induce an early step in signal transduction. Alamethicin (ALA) is an ion channel-forming peptide mixture from the fungus Trichoderma viride that can induce volatile emission and increase endogenous levels of jasmonic acid (JA) and salicylic acid in plants. ALA was used to induce an early step in the defence response in Brussels sprouts plants, Brassica oleracea var. gemmifera, and to study the effect on volatile emission and on the behavioural response of parasitoids to volatile emission. The parasitoid Cotesia glomerata was attracted to ALA-treated plants in a dose-dependent manner. JA, produced through the octadecanoid pathway, activates a later step in induced plant defence signal transduction, and JA also induces volatiles that are attractive to parasitoids. Treatment with ALA and JA resulted in distinct volatile blends, and both blends differed from the volatile blends emitted by control plants. Even though JA treatment of Brussels sprouts plants resulted in higher levels of volatile emission, ALA-treated plants were as attractive to C. glomerata as JA-treated plants. This demonstrates that on a molar basis, ALA is a 20 times more potent inducer of indirect plant defence than JA, although this hormone has more commonly been used as a chemical inducer of plant defence.

A comparison of two Nicotiana attenuata accessions reveals large differences in signaling induced by oral secretions of the specialist herbivore Manduca sexta

Genetic variation within and among populations provides the raw material for evolution. Although many studies describe inter- and intraspecific variation of defensive metabolites, little is known about variation among plant populations within early signaling responses elicited by herbivory or by herbivore oral secretions (OS) introduced into wounds during feeding. In this study, we compare the OS-elicited early responses as well as the antiherbivore defensive metabolites in two accessions of the wild tobacco Nicotiana attenuata and show that, compared with an accession collected from Utah, an Arizona accession has lower herbivore-elicited activity of the salicylic acid-induced protein kinase, an important mitogen-activated protein kinase involved in herbivore resistance. These differences in salicylic acid-induced protein kinase activity were associated with substantially different levels of OS-elicited jasmonic acid, jasmonic acid-isoleucine conjugate, and ethylene bursts. Gene expression level polymorphism (ELP) determines phenotypic variation among populations, and we found the two accessions to have significantly different ELPs in the genes involved in early signaling responses to herbivory. In addition, we found differences between the Utah and the Arizona accessions in the concentrations of several secondary metabolites that contribute to N. attenuata's direct and indirect defenses. This study demonstrates significant natural variation in regulatory elements that mediate plant responses to herbivore attack, highlighting the role of ELP in producing a diversity of plant defense phenotypes.

Phosphoinositide and inositolpolyphosphate signalling in defense responses of Arabidopsis thaliana challenged by mechanical wounding

Various biochemical signals are implicated in Arabidopsis wound signalling, including jasmonic acid (JA), salicylic acid, auxin, and Ca2+. Here, we report on cross-talk of phytohormones with phosphoinositide signals not previously implicated in plant wound responses. Within 30 min of mechanical wounding of Arabidopsis rosette-leaves, the levels of the lipid-derived soluble inositolpolyphosphate, inositol 1,4,5-trisphosphate (InsP(3)), increased four to five-fold. Concomitantly, the precursor lipids, phosphatidylinositol 4,5-bisphosphate, phosphatidylinositol 4-phosphate and phosphatidylinositol transiently depleted, followed by re-synthesis after 30-60 min of stimulation. Increased InsP(3) levels with wounding coincided with JA increases over the first hours of stimulation. In dde2-2-mutant plants deficient in JA biosynthesis, no InsP(3) increase was observed upon wounding, indicating that JA was required for InsP(3) formation, and InsP(3) levels increased in wild-type plants challenged with sorbitol, increasing endogenous JA levels. In InsP 5-ptase plants with attenuated phosphoinositide signalling, the induction of wounding-inducible genes was diminished compared with wild-type plants, suggesting a role for phosphoinositide signalling in mediating plant wound responses. The gene-expression patterns suggest that phosphoinositides contribute to both JA-dependent and JA-independent aspects of wound signalling. Weight gain of Plutella xylostella caterpillars feeding on InsP 5-ptase plants was increased compared with that of caterpillars feeding on wild-type plants. The ecophysiological relevance of phosphoinositide signals in plant defense responses to herbivory is discussed in light of recent findings of inositolpolyphosphate involvement in phytohormone-receptor function.

Low levels of polymorphism in genes that control the activation of defense response in Arabidopsis thaliana

Plants use signaling pathways involving salicylic acid, jasmonic acid, and ethylene to defend against pathogen and herbivore attack. Many defense response genes involved in these signaling pathways have been characterized, but little is known about the selective pressures they experience. A representative set of 27 defense response genes were resequenced in a worldwide set of 96 Arabidopsis thaliana accessions, and patterns of single nucleotide polymorphisms (SNPs) were evaluated in relation to an empirical distribution of SNPs generated from either 876 fragments or 236 fragments with >400 bp coding sequence (this latter set was selected for comparisons with coding sequences) distributed across the genomes of the same set of accessions. Defense response genes have significantly fewer protein variants, display lower levels of nonsynonymous nucleotide diversity, and have fewer nonsynonymous segregating sites. The majority of defense response genes appear to be experiencing purifying selection, given the dearth of protein variation in this set of genes. Eight genes exhibit some evidence of partial selective sweeps or transient balancing selection. These results therefore provide a strong contrast to the high levels of balancing selection exhibited by genes at the upstream positions in these signaling pathways.

Constitutive and herbivore-inducible glucosinolate concentrations in oilseed rape (Brassica napus) leaves are not affected by Bt Cry1Ac insertion but change under elevated atmospheric CO2 and O3

Glucosinolates are plant secondary compounds involved in direct chemical defence by cruciferous plants against herbivores. The glucosinolate profile can be affected by abiotic and biotic environmental stimuli. We studied changes in glucosinolate patterns in leaves of non-transgenic oilseed rape (Brassica napus ssp. oleifera) under elevated atmospheric CO2 or ozone (O3) concentrations and compared them with those from transgenic for herbivore-resistance (Bacillus thuringiensis Cry1Ac endotoxin), to assess herbivory dynamics. Both elevated CO2 and O3 levels decreased indolic glucosinolate concentrations in transgenic and non-transgenic lines, whereas O3 specifically increased the concentration of an aromatic glucosinolate, 2-phenylethylglucosinolate. The herbivore-inducible indolic glucosinolate response was reduced in elevated O3 whereas elevated CO2 altered the induction dynamics of indolic and aliphatic glucosinolates. Herbivore-resistant Bt plants experienced minimal leaf damage after target herbivore Plutella xylostella feeding, but exhibited comparatively similar increase in glucosinolate concentrations after herbivory as non-transgenic plants, indicating that the endogenous glucosinolate defence was not severely compromised by transgenic modifications. The observed differences in constitutive and inducible glucosinolate concentrations of oilseed rape under elevated atmospheric CO2 and O3 might have implications for plant-herbivore interactions in Brassica crop-ecosystems in future climate scenarios.

Maize 9-lipoxygenase ZmLOX3 controls development, root-specific expression of defense genes, and resistance to root-knot nematodes

Root-knot nematodes (RKN) are severe pests of maize. Although lipoxygenase (LOX) pathways and their oxylipin products have been implicated in plant-nematode interactions, prior to this report there was no conclusive genetic evidence for the function of any plant LOX gene in such interactions. We showed that expression of a maize 9-LOX gene, ZmLOX3, increased steadily and peaked at 7 days after inoculation with Meloidogyne incognita RKN. Mu-insertional lox3-4 mutants displayed increased attractiveness to RKN and an increased number of juveniles and eggs. A set of jasmonic acid (JA)- and ethylene (ET)-responsive and biosynthetic genes as well as salicylic acid (SA)-dependent genes were overexpressed specifically in the roots of lox3-4 mutants. Consistent with this, levels of JA, SA, and ET were elevated in lox3-4 mutant roots, but not in leaves. Unlike wild types, in lox3-4 mutant roots, a phenylalanine ammonia lyase (PAL) gene was not RKN-inducible, suggesting a role for PAL-mediated metabolism in nematode resistance. In addition to these alterations in the defense status of roots, lox3-4 knockout mutants displayed precocious senescence and reduced root length and plant height compared with the wild type, suggesting that ZmLOX3 is required for normal plant development. Taken together, our data indicate that the ZmLOX3-mediated pathway may act as a root-specific suppressor of all three major defense signaling pathways to channel plant energy into growth processes, but is required for normal levels of resistance against nematodes.

Salicylic acid and jasmonic acid signaling defense pathways reduce natural bacterial diversity on Arabidopsis thaliana

Terrestrial plants serve as large and diverse habitats for a wide range of pathogenic and nonpathogenic microbes, yet these communities are not well described and little is known about the effects of plant defense on microbial communities in nature. We designed a field experiment to determine how variation in two plant defense signaling pathways affects the size, diversity, and composition of the natural endophytic and epiphytic bacterial communities of Arabidopsis thaliana. To do this, we provide an initial characterization of these bacterial communities in one population in southwestern Michigan, United States, and we compare these two communities among A. thaliana mutants deficient in salicylic acid (SA) and jasmonic acid (JA) signaling defense pathways, controls, and plants with artificially elevated levels of defense. We identified 30 distinct bacterial groups on A. thaliana that differ in colony morphology and 16S rRNA sequence. We show that induction of SA-mediated defenses reduced endophytic bacterial community diversity, whereas plants deficient in JA-mediated defenses experienced greater epiphytic bacterial diversity. Furthermore, there was a positive relationship between total community size and diversity, indicating that relatively susceptible plants should, in general, harbor higher bacterial diversity. This experiment provides novel information about the ecology of bacteria on A. thaliana and demonstrates that variation in two specific plant-signaling defense pathways can influence bacterial diversity on plants.

Plant interactions with microbes and insects: from molecular mechanisms to ecology

Plants are members of complex communities and interact both with antagonists and beneficial organisms. An important question in plant defense-signaling research is how plants integrate signals induced by pathogens, beneficial microbes and insects into the most appropriate adaptive response. Molecular and genomic tools are now being used to uncover the complexity of the induced defense signaling networks that have evolved during the arms races between plants and their attackers. Molecular biologists and ecologists are joining forces to place molecular mechanisms of plant defense into an ecological perspective. Here, we review our current understanding of the molecular mechanisms of induced plant defense and their potential ecological relevance in nature.

SAR increases fitness of Arabidopsis thaliana in the presence of natural bacterial pathogens

Given the substantial costs of plant defenses against pathogens, there should be corresponding benefits that prevent resistance from being lost in natural plant populations. Here, we present evidence that systemic acquired resistance (SAR) benefits plants attacked by pathogenic bacteria in nature. In a large field experiment, we found that Arabidopsis thaliana treated with salicylic acid exhibited reduced titers of bacteria in their leaves and elevated fitness relative to controls. Most common members of the culturable bacterial community suffered this decrease, consistent with the role of SAR as a broad spectrum defense. We found no evidence of negative interactions between SAR and jasmonate-dependent resistance. Plants treated with jasmonic acid received significantly lower insect damage to their siliques, but exhibited no differences in bacterial growth or fitness relative to controls. Collectively, these data suggest a likely role of pathogenic bacteria in the maintenance of SAR, but not jasmonate-dependent resistance, in nature.

Before gene expression: early events in plant-insect interaction

Successful defense depends on the ability of the plant to recognize an attacking 'enemy' as early as possible. Early defense responses require enemy-initiated signaling cascades. Their activation ensures an induced response that is quantitative, timely and coordinated with other activities of the host cells. Damage-induced ion imbalances and modulations of channel activities are the first events occurring in the plasma membrane and result in rapid perturbations of the plasma membrane potential (V(m)) involving variations of cytosolic Ca(2+) concentrations. Interacting downstream networks of kinases and phytohormones mediate the signal and result in concerted gene activation. Here we review and discuss early events occurring before herbivore attack-related gene expression that are responsible for cascades of events and signal transductions, eventually leading to indirect and direct plant responses.

Novel extracellular chitinases rapidly and specifically induced by general bacterial elicitors and suppressed by virulent bacteria as a marker of early basal resistance in tobacco

Early basal resistance (EBR, formerly known as early induced resistance) is triggered by general bacterial elicitors. EBR has been suggested to inhibit or retard expression of the type III secretion system of pathogenic bacteria and may also prevent nonpathogenic bacteria from colonizing the plant tissue. The quickness of EBR here plays a crucial role, compensating for a low bactericidal efficacy. This inhibitory activity should take place in the cell wall, as bacteria do not enter living plant cells. We found several soluble proteins in the intercellular fluid of tobacco leaf parenchyma that coincided with EBR under different environmental (light and temperature) conditions known to affect EBR. The two most prominent proteins proved to be novel chitinases (EC 3.2.1.14) that were transcriptionally induced before and during EBR development. Their expression in the apoplast was fast and not stress-regulated as opposed to many pathogenesis-related proteins. Nonpathogenic, saprophytic, and avirulent bacteria all induced EBR and the chitinases. Studies using these chitinases as EBR markers revealed that the virulent Pseudomonas syringae pv. tabaci, being sensitive to EBR, must suppress it while suppressing the chitinases. EBR, the chitinases, as well as their suppression are quantitatively related, implying a delicate balance determining the outcome of an infection.

Glucosinolate metabolism and its control

Glucosinolates and their associated degradation products have long been recognized for their distinctive benefits to human nutrition and plant defense. Because most of the structural genes of glucosinolate metabolism have been identified and functionally characterized in Arabidopsis thaliana, current research increasingly focuses on questions related to the regulation of glucosinolate synthesis, distribution and degradation as well as to the feasibility of engineering customized glucosinolate profiles. Here, we highlight recent progress in glucosinolate research, with particular emphasis on the biosynthetic pathway and its metabolic relationships to auxin homeostasis. We further discuss emerging insight into the signaling networks and regulatory proteins that control glucosinolate accumulation during plant development and in response to environmental challenge.

Plant-mediated interactions between pathogenic microorganisms and herbivorous arthropods

Plant-mediated interactions between pathogenic microorganisms and arthropod herbivores occur when arthropod infestation or pathogen infection changes the shared host plant in ways that affect a subsequent attacker of the opposite type. Interest in such "tripartite" interactions has increased as the ecological and plant physiological framework for understanding and contextualizing them has developed. The outcomes of plant-mediated interactions are variable, and only a few provisional patterns can be identified at present. However, these interactions can have important consequences not only for individual pathogens and herbivores, but also for the population dynamics of both types of organisms in managed and natural ecosystems. Research has focused on the role of two plant response pathways in mediating tripartite interactions, one involving jasmonic acid and the other salicylic acid. Further studies of plant-mediated interactions will facilitate an understanding of how plants coordinate and integrate their defenses against multiple biotic threats.

Duplicate maize 13-lipoxygenase genes are differentially regulated by circadian rhythm, cold stress, wounding, pathogen infection, and hormonal treatments

Most plant oxylipins, a large class of diverse oxygenated polyunsaturated fatty acids and their derivatives, are produced through the lipoxygenase (LOX) pathway. Recent progress in dicots has highlighted the biological roles of oxylipins in plant defence responses to pathogens and pests. By contrast, the physiological function of LOXs and their metabolites in monocots is poorly understood. Two maize LOXs, ZmLOX10 and ZmLOX11 that share >90% amino acid sequence identity but are localized on different chromosomes, were cloned and characterized. Phylogenetic analysis revealed that ZmLOX10 and ZmLOX11 cluster together with well-characterized plastidic type 2 linoleate 13-LOXs from diverse plant species. Regio-specificity analysis of recombinant ZmLOX10 protein overexpressed in Escherichia coli proved it to be a linoleate 13-LOX with a pH optimum at approximately pH 8.0. Both predicted proteins contain putative transit peptides for chloroplast import. ZmLOX10 was preferentially expressed in leaves and was induced in response to wounding, cold stress, defence-related hormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), and inoculation with an avirulent strain of Cochliobolus carbonum. These data suggested a role for this gene in maize adaptation to abiotic stresses and defence responses against pathogens and pests. ZmLOX11 was preferentially expressed in silks and was induced in leaves only by ABA, indicating its possible involvement in responses to osmotic stress. In leaves, mRNA accumulation of ZmLOX10 is strictly regulated by a circadian rhythm, with maximal expression coinciding temporally with the highest photosynthetic activity. This study reveals the evolutionary divergence of physiological roles for relatively recently duplicated genes. Possible physiological functions of these 13-LOXs are suggested.

Expression of constitutive and inducible chemical defenses in native and invasive populations of Alliaria petiolata

The Evolution of Increased Competitive Ability (EICA) hypothesis posits that invasive plants in introduced habitats with reduced herbivore pressure will evolve reduced levels of costly resistance traits. In light of this hypothesis, we examined the constitutive and inducible expression of five chemical defense traits in Alliaria petiolata from four invasive North American and seven native European populations. When grown under common conditions, significant variation among populations within continents was found for trypsin inhibitors and peroxidase activity, and glucosinolates and trypsin inhibitors were significantly jasmonate-inducible across populations. Across populations, constitutive levels of glucosinolates and trypsin inhibitors were negatively correlated with their degree of induction, with three North American populations tending to have lower constitutive levels and higher inducibility of glucosinolates than the seven European populations. Alliarinoside and isovitexin 6"-O-beta-glucopyranoside levels were both higher in North American plants than in European plants, but levels of these compounds were generally increased by jasmonate in European plants and decreased by the same treatment in North American plants. Aside from the tendency for invasive populations to have reduced constitutive glucosinolate levels coupled with increased inducibility, little support for the predictions of EICA was evident in the chemical defenses that we studied.

Testing the optimal defense theory and the growth-differentiation balance hypothesis in Arabidopsis thaliana

Two prominent theories proposed to explain patterns of chemical defense expression in plants are the optimal defense theory (ODT) and the growth-differentiation balance hypothesis (GDBH). The ODT predicts that plant parts with high fitness value will be highly defended, and the GDBH predicts that slow growing plant parts will have more resources available for defense and thus will have higher defense levels than faster growing tissues. We examined growth rate, fitness value, and defense protein levels in leaves of a wild and lab ecotype of Arabidopsis thaliana to address whether patterns of defense protein expression in this plant conform to predictions of either the ODT or the GDBH. We divided leaves of A. thaliana into six leaf classes based on three developmental stages: vegetative, bolting, and flowering; with two leaf ages at each stage: young and old. We assessed the fitness value of leaves by determining the impact of the removal of each leaf class on total seed production and germination rates. Although A. thaliana was highly tolerant to defoliation, young leaves were more valuable than old in general, and young leaves on bolting plants were the most valuable leaf class in particular. Young leaves on vegetative plants grew fastest in both ecotypes, while old leaves on bolting and flowering plants grew slowest. Finally, defense levels were assessed in each leaf class by quantifying the constitutive and inducible expression of four defense-related proteins. Expression of guaiacol peroxidase and chitinase activity conformed largely to GDBH predictions. Expression of trypsin inhibitor and polyphenoloxidase activity varied by leaf class and treatment, but conformed to neither GDBH nor ODT predictions.

Fitness costs of mutations affecting the systemic acquired resistance pathway in Arabidopsis thaliana

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.

Management of Xanthomonas Leaf Blight of Onion with a Plant Activator, Biological Control Agents, and Copper Bactericides

Xanthomonas leaf blight (Xanthomonas axonopodis pv. allii) is a yield-limiting disease of onion (Allium cepa) in the western United States. Frequent applications of copper-based bactericides amended with an ethylenebisdithiocarbamate fungicide (e.g., maneb or mancozeb, class B2 carcinogens) provide some disease suppression, but strategies to reduce conventional bactericide use are needed to minimize grower costs, environmental impact, and public exposure to class B2 pesticides. Applications of acibenzolar-S-methyl reduced in planta and epiphytic populations of X. axonopodis pv. allii as effectively as applications of copper hydroxide-mancozeb in growth chamber studies. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced severity of Xanthomonas leaf blight as or more effectively than 9 to 12 weekly applications of copper hydroxide or copper hydroxide-mancozeb. Acibenzolar-S-methyl applications did not increase bulb yield or grade compared with copper bactericide treatments. However, bulb yield was reduced 22 to 27% when 10 weekly applications of acibenzolar-S-methyl were made in the absence of disease. Application of a commercial formulation of both Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 reduced severity of Xanthomonas leaf blight in field experiments. Weekly copper hydroxide applications starting 1 to 2 weeks before bulb initiation were as effective as weekly applications started 3 to 4 weeks before bulb initiation, irrespective of the maneb rate used. Integration of acibenzolar-S-methyl and biological control agents with copper hydroxide in a carefully timed spray program may eliminate the use of the class B2 carcinogens maneb and mancozeb on onion without compromising efficacy for management of Xanthomonas leaf blight.

The glucosinolate-myrosinase system in an ecological and evolutionary context

Functional analysis of natural variation in the model species Arabidopsis thaliana has enabled the cloning of many glucosinolate biosynthesis and hydrolysis genes. Variation in these genes is central to understanding the ecological role of the glucosinolate-myrosinase defense system, and allows us to dissect the evolutionary and ecological forces that shape polymorphism at underlying loci. These same genes are also variable in other crucifer species, suggesting the presence of recurring selection, possibly mediated by insects. By utilizing the genomic tools available in A. thaliana to investigate these loci fully, it might be possible to generate detailed evolutionary or ecological models to apply to other species.

Analysis of the glucosinolate pattern ofArabidopsis thalianaseeds by capillary zone electrophoresis coupled to electrospray ionization-mass spectrometry

An easy and rapid method for the analysis of intact, non-desulfated glucosinolates by capillary zone electrophoresis (CZE) coupled to electrospray ionization-time of flight-mass spectrometry (ESI-TOF-MS) is described. Surprisingly, an electrolyte and a sheath liquid based on formic acid provided the best results. In this strongly acidic system, the glucosinolates were separated and detected as anions, resulting in an excellent selectivity. Thus, crude plant extracts could be analyzed without any interference of matrix constituents. The sensitivity together with mass accuracy and true isotopic pattern of the TOF-MS allowed identification of a broad series of glucosinolates in Arabidopsis thaliana seeds.

Differential activity of peroxidase isozymes in response to wounding, gypsy moth, and plant hormones in northern red oak (Quercus rubra L.)

We measured total peroxidase activity and the activities of peroxidase isoforms in leaves of red oak (Quercus rubra L.) seedlings exposed to wounding and plant hormones in the greenhouse. Activity of specific peroxidase isoforms was induced differentially by gypsy moth wounding, mechanical wounding, and the wound-associated plant hormone jasmonic acid. Activity of one isoform was enhanced modestly by treatment with salicylate. A study of peroxidase activity in naturally occurring galls elicited on red oak leaves by 12 hymenopteran and dipteran insect species found 16 POD isoforms, 11 of which were differentially induced or suppressed in galls compared with leaves. In both studies, total peroxidase activity as measured spectrophotometrically was not clearly related to activity of these isoforms. These results indicate that red oak seedlings and trees may respond specifically to wounding, particular insects, and plant signals through changes in the activities of individual isozymes.

Pseudomonas syringae manipulates systemic plant defenses against pathogens and herbivores

Many pathogens are virulent because they specifically interfere with host defense responses and therefore can proliferate. Here, we report that virulent strains of the bacterial phytopathogen Pseudomonas syringae induce systemic susceptibility to secondary P. syringae infection in the host plant Arabidopsis thaliana. This systemic induced susceptibility (SIS) is in direct contrast to the well studied avirulence/R gene-dependent resistance response known as the hypersensitive response that elicits systemic acquired resistance. We show that P. syringae-elicited SIS is caused by the production of coronatine (COR), a pathogen-derived functional and structural mimic of the phytohormone jasmonic acid (JA). These data suggest that SIS may be a consequence of the previously described mutually antagonistic interaction between the salicylic acid and JA signaling pathways. Virulent P. syringae also has the potential to induce net systemic susceptibility to herbivory by an insect (Trichoplusia ni, cabbage looper), but this susceptibility is not caused by COR. Rather, consistent with its role as a JA mimic, COR induces systemic resistance to T. ni. These data highlight the complexity of defense signaling interactions among plants, pathogens, and herbivores.

Signal crosstalk and induced resistance: straddling the line between cost and benefit

This review discusses recent progress in our understanding of signaling in induced plant resistance and susceptibility to pathogens and insect herbivores, with a focus on the connections and crosstalk among phytohormone signaling networks that regulate responses to these and other stresses. Multiple stresses, often simultaneous, reduce growth and yield in plants. However, prior challenge by a pathogen or insect herbivore also can induce resistance to subsequent challenge. This resistance, or failure of susceptibility, must be orchestrated within a larger physiological context that is strongly influenced by other biotic agents and by abiotic stresses such as inadequate light, temperature extremes, drought, nutrient limitation, and soil salinity. Continued research in this area is predicated on the notion that effective utilization of induced resistance in crop protection will require a functional understanding of the physiological consequences of the "induced" state of the plant, coupled with the knowledge of the specificity and compatibility of the signaling systems leading to this state. This information may guide related strategies to improve crop performance in suboptimal environments, and define the limits of induced resistance in certain agricultural contexts.

Evaluation of natural and synthetic stimulants of plant immunity by microarray technology

Induction of local defence, as well as systemic resistance, of plants is associated with transcriptional reprogramming. Here we report on defence gene induction by natural and synthetic stimulants of plant immunity. Gene expression changes in Arabidopsis thaliana were monitored in response to several plant immunity stimulants (plant activators) using Northern blotting and an application-based array representing c. 750 genes involved in several aspects of plant defence and/or plant stress. The commercial plant activators Bio-S, Neudo-Vital and PRORADIX have been shown to induce systemic resistance. Here, Neudo-Vital, PRORADIX and Bio-S treatment induced different patterns of salicylic acid (SA) and jasmonic acid (JA) accumulation. Gene induction by these plant activators proved to be very complex. Rather than simply mimicking one of the known defence pathways induced by SA or JA, the response to the plant activators showed aspects of both major defence systems. A general feature was the transient activation of JA biosynthesis genes, combined with a much more sustained SA-associated defence gene induction. Our results demonstrate that plant immunity stimulants activate systemic immunity at the transcriptional level, and they provide insight into the coordinated transcriptional regulation of several classes of plant defence genes.

Salicylic acid inhibits jasmonic acid-induced resistance of Arabidopsis thaliana to Spodoptera exigua

The role of salicylic acid (SA) in plant responses to pathogens has been well documented, but its direct and indirect effects on plant responses to insects are not so well understood. We examined the effects of SA, alone and in combination with jasmonic acid (JA), on the performance of the generalist herbivore, Spodoptera exigua, in wild-type and mutant Arabidopsis thaliana genotypes that varied genetically in their ability to mount SA- and JA-mediated defence responses. In one experiment, growth of S. exigua larvae was highest on the Wassilewskija wild-type, intermediate on the Columbia wild-type and the JA-deficient fad mutant, and lowest on the nim1-1 and jar1-mutants, which are defective in the SA and JA pathways, respectively. Activity of guaiacol peroxidase, polyphenoloxidase, n-acetylglucosaminidase, and trypsin inhibitor varied by genotype but did not correlate with insect performance. SA treatment increased growth of S. exigua larvae by approximately 35% over all genotypes, but had no discernable effect on activities of the four defence proteins. In a second experiment, growth of S. exigua was highest across treatments on the cep1 mutant, a constitutive expressor of high SA levels and systemic acquired resistance, and lowest on the fad mutant, which is JA-deficient. JA treatment generally increased activity of all four defence proteins, increased total glucosinolate levels and reduced insect growth by approximately 25% over all genotypes. SA generally inhibited expression of JA-induced resistance to S. exigua when both hormones were applied simultaneously. Across genotypes and treatments, larval mass was negatively correlated with the activity of trypsin inhibitor and polyphenoloxidase and with total glucosinolate levels, and insect damage was negatively correlated with the activity of polyphenoloxidase. SA had little effect on the induction of defence protein activity by JA. However, SA attenuated the induction of glucosinolates by JA and therefore may explain better the interactive effects of SA and JA on insect performance. This study illustrates that direct and indirect cross-effects of SA on resistance to S. exigua can occur in A. thaliana. Effects of SA may be mediated through effects on plant defence chemistry or other aspects of the suitability of foliage for insect feeding and growth.

Interactive effects of jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis

Leaf trichomes protect plants from attack by insect herbivores and are often induced following damage. Hormonal regulation of this plant induction response has not been previously studied. In a series of experiments, we addressed the effects of artificial damage, jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis. Artificial damage and jasmonic acid caused significant increases in trichome production of leaves. The jar1-1 mutant exhibited normal trichome induction following treatment with jasmonic acid, suggesting that adenylation of jasmonic acid is not necessary. Salicylic acid had a negative effect on trichome production and consistently reduced the effect of jasmonic acid, suggesting negative cross-talk between the jasmonate and salicylate-dependent defense pathways. Interestingly, the effect of salicylic acid persisted in the nim1-1 mutant, suggesting that the Npr1/Nim1 gene is not downstream of salicylic acid in the negative regulation of trichome production. Last, we found that gibberellin and jasmonic acid had a synergistic effect on the induction of trichomes, suggesting important interactions between these two compounds.