Signals in plant disease resistance

Analysis of ginseng rusty root symptoms transcriptome and its pathogenesis directed by reactive oxygen species theory

Ginseng rusty root symptoms (GRS) is a primary disease of ginseng, which seriously decreases the yield and quality of ginseng and causes enormous losses to ginseng production. GRS prevention and control is still challenging due to its unclear etiology. In this study, the phloem tissue of healthy Panax ginseng (AG), the nonred tissue of the phloem epidermis around the lesion (BG), and the red lesion site tissue of GRS (CG) were extracted for mRNA transcriptomic analysis; 35,958 differentially expressed genes (DEGs) were identified and were associated with multiple stress resistance pathways, reactive oxygen species (ROS), and iron ion binding. Further study showed that the contents of O2 •-, H2O2, and malondialdehyde (MDA) were significantly increased in BG and CG tissues. Under anaerobic conditions caused by excessive soil moisture, the overproduction of ROS destroys cell membranes, simultaneously converting Fe2+ to Fe3+ and depositing it in the cell wall, which results in GRS, as evidenced by the success of the GRS induction test.

The biochemical changes in two moderately resistant and highly susceptible tomato cultivars at the later stages of Meloidogyne javanica infection

The most effective method of managing root-knot nematodes is employing resistant and tolerant cultivars. Investigating biochemical changes can help determine the cause of resistance or susceptibility of plants to nematodes. In this study, resistance levels of some tomato cultivars, ‘ALYSTE F-1’, ‘ARYZA F-1’, ‘Early Urbana’, ‘Rutgers’, ‘Dutch Mobil’ and ‘Hungarian Mobil’, were evaluated based on nematode reproduction indices under glasshouse conditions. After selecting the most susceptible and resistant cultivar, comparisons of activity of defence enzymes (guaiacol peroxidase, catalase, ascorbate peroxidase, superoxide dismutase, polyphenol oxidase and phenylalanine ammonia-lyase), and phenolic and lignin contents in leaves and roots were investigated. Analysis of nematode reproductive traits revealed that ‘ALYSTE F-1’ had the lowest number of galls per root system, egg masses per root system, eggs per root system, and second-stage juveniles per 1.5 kg of soil and, consequently, the lowest number of nematode populations. Finally, ‘ALYSTE F-1’ and ‘Dutch Mobil’ (based on reproduction factor, gall index and resistance index) were selected as moderately resistant and highly susceptible cultivars, respectively, for biochemical analysis. Biochemical analysis of leaves and roots showed that most of the defence compounds in ‘ALYSTE F-1’ were higher than ‘Dutch Mobil’. These results also showed that ‘ALYSTE F-1’ reacted to nematode attack more rapidly than ‘Dutch Mobil’.

Regulation of ascorbate-glutathione cycle by exogenous nitric oxide and hydrogen peroxide in soybean roots under arsenate stress

The role of nitric oxide (NO) and hydrogen peroxide (H2O2) is well known for regulating plant abiotic stress responses. However, underlying mechanisms are still poorly understood. Therefore, the present study investigated the involvement of NO and H2O2 signalling in the regulation of arsenate toxicity (AsV) in soybean roots employing a pharmacological approach. Results show that AsV toxicity declined root length and biomass due to greater As accumulation in the cell wall and cellular organelles. Arsenate induced cell death due to enhanced levels of reactive oxygen species, lipid and protein oxidation and down-regulation in ascorbate-glutathione cycle and redox states of ascorbate and glutathione. These results correlate with lower endogenous level of NO. Interestingly, addition of L-NAME increased AsV toxicity. However, addition of SNP reverses effect of L-NAME, suggesting that endogenous NO has a role in mitigating AsV toxicity. Exogenous H2O2 also demonstrated capability of alleviating AsV stress, while NAC reversed the protective effect of H2O2. Furthermore, DPI application further increased AsV toxicity, suggesting that endogenous H2O2 is also implicated in mitigating AsV stress. SNP was not able to mitigate AsV toxicity in the presence of DPI, suggesting that H2O2 might have acted downstream of NO in accomplishing amelioration of AsV toxicity.

Production of salicylic acid; a potent pharmaceutically active agent and its future prospects

Salicylic acid is one of the potent pharmaceutical organic acids that have various applications in the medical field. It acts as a plant hormone and helps in plant's growth & defence against pathogens. Beyond its numerous functions in plants, SA has great pharmaceutical importance since it acts as an intermediate for the synthesis of various drugs and dyes e.g. aspirin. At the industrial scale, chemical methods are used for the synthesis of SA but presently, several other sources are available that have the capability to alternate the chemical process which will be a step forward toward green synthesis. Aim of this paper is to provide comprehensive knowledge of SA production and its biological application.

Biochemical study of the effect of stress conditions on the mandelonitrile-associated salicylic acid biosynthesis in peach

Salicylic acid (SA) plays a central role in plant responses to environmental stresses. In a recent study, we suggested a third pathway for SA biosynthesis from mandelonitrile (MD) in peach plants. This pathway is an alternative to the phenylalanine ammonia-lyase pathway and links SA biosynthesis and cyanogenesis. In the present work, using biochemical approaches, we studied the effect of salt stress and Plum pox virus (PPV) infection on this proposed SA biosynthetic pathway from MD. Peach plants were submitted to salt stress and Plum pox virus (PPV) infection. We studied the levels of SA and its intermediates/precursors (phenylalanine, MD, amygdalin and benzoic acid) in in vitro shoots. Moreover, in peach seedlings, we analysed the content of H₂ O₂ -related enzymes, SA and the stress-related hormones abscisic acid and jasmonic acid. We showed that the contribution of this SA biosynthetic pathway from MD to the total SA pool does not seem to be important under the stress conditions assayed. Nevertheless, MD treatment not only affected the SA content, but also had a pleiotropic effect on abscisic acid and jasmonic acid levels. Furthermore, MD modulates the antioxidative metabolism via SA-dependent or -independent redox-related signalling pathways. Even though the proposed SA biosynthetic pathway seems to be functional under stress conditions, MD, and hence cyanogenic glycosides, may be operating more broadly than by influencing SA pathways and signalling. Thus, the physiological function of the proposed SA biosynthetic pathway remains to be elucidated.

Transcriptome analysis of an incompatible Persea americana-Phytophthora cinnamomi interaction reveals the involvement of SA- and JA-pathways in a successful defense response

Phytophthora cinnamomi Rands (Pc) is a hemibiotrophic oomycete and the causal agent of Phytophthora root rot (PRR) of the commercially important fruit crop avocado (Persea americana Mill.). Plant defense against pathogens is modulated by phytohormone signaling pathways such as salicylic acid (SA), jasmonic acid (JA), ethylene (ET), auxin and abscisic acid. The role of specific signaling pathways induced and regulated during hemibiotroph-plant interactions has been widely debated. Some studies report SA mediated defense while others hypothesize that JA responses restrict the spread of pathogens. This study aimed to identify the role of SA- and JA- associated genes in the defense strategy of a resistant avocado rootstock, Dusa in response to Pc infection. Transcripts associated with SA-mediated defense pathways and lignin biosynthesis were upregulated at 6 hours post-inoculation (hpi). Results suggest that auxin, reactive oxygen species (ROS) and Ca2+ signaling was also important during this early time point, while JA signaling was absent. Both SA and JA defense responses were shown to play a role during defense at 18 hpi. Induction of genes associated with ROS detoxification and cell wall digestion (β-1-3-glucanase) was also observed. Most genes induced at 24 hpi were linked to JA responses. Other processes at play in avocado at 24 hpi include cell wall strengthening, the formation of phenolics and induction of arabinogalactan, a gene linked to Pc zoospore immobility. This study represents the first transcriptome wide analysis of a resistant avocado rootstock treated with SA and JA compared to Pc infection. The results provide evidence of a biphasic defense response against the hemibiotroph, which initially involves SA-mediated gene expression followed by the enrichment of JA-mediated defense from 18 to 24 hpi. Genes and molecular pathways linked to Pc resistance are highlighted and may serve as future targets for manipulation in the development of PRR resistant avocado rootstocks.

The Fight Against Panax notoginseng Root-Rot Disease Using Zingiberaceae Essential Oils as Potential Weapons

The root of Panax notoginseng (P. notoginseng) is one of the most highly valuable medicinal herbs in China owing to its pronounced hemostatic and restorative properties. Despite this important fact, growing P. notoginseng is seriously limited by root-rot diseases. In studies aimed at developing a solution to this problem, environment-friendly essential oils (EOs) of five medicinal plants of the family Zingiberaceae were tested for their inhibitory effects on the growth of three main soil pathogens associated with the root-rot diseases of P. notoginseng. The results showed that the EOs of Alpinia katsumadai Hayata and Zingiber officinale Roscoe promote significant reductions in the mycelium growth of the pathogen in vitro at a concentration of 50 mg mL-1, which is much higher than that needed (5 mg mL-1) to reduce growth by the positive control, flutriafol. Furthermore, the chemical components of the two EOs were determined by using GC-MS analysis. Eucalyptol was found to account for more than 30% of the oils of the two plants, with the second major components being geranyl acetate and α-terpineol. These substances display different degrees of fungistasis in vitro. To further determine the effects of the EO of Zingiber officinale (Z. officinale) in vivo, soilless cultivation of P. notoginseng with pathogen inoculation was conducted in a greenhouse. Addition of the petroleum ether extract (approximately equal to EO) of Z. officinale to the culture matrix causes a large decrease in both the occurrence and severity of the P. notoginseng root-rot disease. The decreasing trend of net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) were all alleviated. In addition, the activities of catalase (CAT), peroxidase (POD), and the malondialdehyde (MDA) content were also largely reduced after pathogen infection, with the root activity being higher than that of the control. Taken together, the findings reveal that the EOs from plants might serve as promising sources of eco-friendly natural pesticides with less chemical resistance.

Hydrogen Peroxide Pretreatment Mitigates Cadmium-Induced Oxidative Stress in Brassica napus L.: An Intrinsic Study on Antioxidant Defense and Glyoxalase Systems

Cadmium (Cd) is considered as one of the most toxic metals for plant growth and development. In the present study, we investigated the role of externally applied hydrogen peroxide (H2O2) in regulating the antioxidant defense and glyoxalase systems in conferring Cd-induced oxidative stress tolerance in rapeseed (Brassica napus L.). Seedlings were pretreated with 50 μM H2O2 for 24 h. These pretreated seedlings as well as non-pretreated seedlings were grown for another 48 h at two concentrations of CdCl2 (0.5 and 1.0 mM). Both the levels of Cd increased MDA and H2O2 levels and lipoxygenase activity while ascorbate (AsA) declined significantly. However, reduced glutathione (GSH) content showed an increase at 0.5 mM CdCl2, but glutathione disulfide (GSSG) increased at any level of Cd with a decrease in GSH/GSSG ratio. The activities of ascorbate peroxidase (APX) and glutathione S-transferase (GST) upregulated due to Cd treatment in dose-dependent manners, while glutathione reductase (GR) and glutathione peroxidase (GPX) increased only at 0.5 mM CdCl2 and decreased at higher dose. The activity of monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) decreased under Cd stress. On the other hand, H2O2 pretreated seedlings, when exposed to Cd, AsA and GSH contents and GSH/GSSG ratio increased noticeably. H2O2 pretreatment increased the activities of APX, MDHAR, DHAR, GR, GST, GPX, and CAT of Cd affected seedlings. Thus enhancement of both the non-enzymatic and enzymatic antioxidants helped to decrease the oxidative damage as indicated by decreased levels of H2O2 and MDA. The seedlings which were pretreated with H2O2 also showed enhanced glyoxalase system. The activities of Gly I, and Gly II and the content of GSH increased significantly due to H2O2 pretreatment in Cd affected seedlings, compared to the Cd-stressed plants without H2O2 pretreatment which were vital for methylglyoxal detoxification. So, the major roles of H2O2 were improvement of antioxidant defense system and glyoxalase system which protected plants from the damage effects of ROS and MG. The mechanism of H2O2 to induce antioxidant defense and glyoxalase system and improving physiology under stress condition is not known clearly which should be elucidated. The signaling roles of H2O2 and its interaction with other signaling molecules, phytohormones or other biomolecules and their roles in stress protection should be explored.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

De novo sequencing, assembly, and analysis of the root transcriptome of Persea americana (Mill.) in response to Phytophthora cinnamomi and flooding

Avocado is a diploid angiosperm containing 24 chromosomes with a genome estimated to be around 920 Mb. It is an important fruit crop worldwide but is susceptible to a root rot caused by the ubiquitous oomycete Phytophthora cinnamomi. Phytophthora root rot (PRR) causes damage to the feeder roots of trees, causing necrosis. This leads to branch-dieback and eventual tree death, resulting in severe losses in production. Control strategies are limited and at present an integrated approach involving the use of phosphite, tolerant rootstocks, and proper nursery management has shown the best results. Disease progression of PRR is accelerated under high soil moisture or flooding conditions. In addition, avocado is highly susceptible to flooding, with even short periods of flooding causing significant losses. Despite the commercial importance of avocado, limited genomic resources are available. Next generation sequencing has provided the means to generate sequence data at a relatively low cost, making this an attractive option for non-model organisms such as avocado. The aims of this study were to generate sequence data for the avocado root transcriptome and identify stress-related genes. Tissue was isolated from avocado infected with P. cinnamomi, avocado exposed to flooding and avocado exposed to a combination of these two stresses. Three separate sequencing runs were performed on the Roche 454 platform and produced approximately 124 Mb of data. This was assembled into 7685 contigs, with 106 448 sequences remaining as singletons. Genes involved in defence pathways such as the salicylic acid and jasmonic acid pathways as well as genes associated with the response to low oxygen caused by flooding, were identified. This is the most comprehensive study of transcripts derived from root tissue of avocado to date and will provide a useful resource for future studies.

RBOH1-dependent H2O2 production and subsequent activation of MPK1/2 play an important role in acclimation-induced cross-tolerance in tomato

H2O2 and mitogen-activated protein kinase (MAPK) cascades play important functions in plant stress responses, but their roles in acclimation response remain unclear. This study examined the functions of H2O2 and MPK1/2 in acclimation-induced cross-tolerance in tomato plants. Mild cold, paraquat, and drought as acclimation stimuli enhanced tolerance to more severe subsequent chilling, photooxidative, and drought stresses. Acclimation-induced cross-tolerance was associated with increased transcript levels of RBOH1 and stress- and defence-related genes, elevated apoplastic H2O2 accumulation, increased activity of NADPH oxidase and antioxidant enzymes, reduced glutathione redox state, and activation of MPK1/2 in tomato. Virus-induced gene silencing of RBOH1, MPK1, and MPK2 or MPK1/2 all compromised acclimation-induced cross-tolerance and associated stress responses. Taken together, these results strongly suggest that acclimation-induced cross-tolerance is largely attributed to RBOH1-dependent H2O2 production at the apoplast, which may subsequently activate MPK1/2 to induce stress responses.

Transcriptional responses to cantharidin, a protein phosphatase inhibitor, in Arabidopsis thaliana reveal the involvement of multiple signal transduction pathways

Cantharidin is a natural compound isolated from the blister beetle (Epicauta spp.). It is a potent inhibitor of protein serine/threonine phosphatases (PPPs), especially PP2A and PP4. Protein phosphatases and kinases maintain a sensitive balance between dephosphorylated and phosphorylated forms of appropriate proteins, thereby playing important roles in signal transduction pathways and regulation of gene expression, cellular proliferation, cell differentiation, apoptosis and other processes. The foliage of 12-day-old Arabidopsis thaliana seedlings was treated with 200 µM (IC(30) ) of the PPP inhibitor cantharidin, and the entire transcriptome profile was determined by microarray analysis at 2, 10 and 24 h after treatment. The transcription of approximately 10% (2577) of the 24 000 genes of Arabidopsis changed significantly (P≤ 0.05 and signal log ratios: ≥1 or ≤-1) after treatment. Inhibition of PPPs significantly reduced transcription of genes associated with auxin and light signaling and induced expression of genes involved in the hypersensitive response and in flagellin and abscisic acid signaling. The great variety of up- and downregulated genes in this microarray experiment implied that cantharidin interfered with the activities of PPPs that interact directly or indirectly with receptors or are located near the beginning of signal transduction pathways. In many cases, PPPs interact with protein complexes of various receptors such as ethylene or light sensors localized in different cell compartments. They function as negative regulators modifying receptor functions, thus altering signaling that influences transcriptional responses.

Salicylic Acid biosynthesis and metabolism

Salicylic acid (SA) has been shown to regulate various aspects of growth and development; it also serves as a critical signal for activating disease resistance in Arabidopsis thaliana and other plant species. This review surveys the mechanisms involved in the biosynthesis and metabolism of this critical plant hormone. While a complete biosynthetic route has yet to be established, stressed Arabidopsis appear to synthesize SA primarily via an isochorismate-utilizing pathway in the chloroplast. A distinct pathway utilizing phenylalanine as the substrate also may contribute to SA accumulation, although to a much lesser extent. Once synthesized, free SA levels can be regulated by a variety of chemical modifications. Many of these modifications inactivate SA; however, some confer novel properties that may aid in long distance SA transport or the activation of stress responses complementary to those induced by free SA. In addition, a number of factors that directly or indirectly regulate the expression of SA biosynthetic genes or that influence the rate of SA catabolism have been identified. An integrated model, encompassing current knowledge of SA metabolism in Arabidopsis, as well as the influence other plant hormones exert on SA metabolism, is presented.

Hydrogen peroxide in plants: a versatile molecule of the reactive oxygen species network

Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species (ROS), which are subsequently converted to hydrogen peroxide (H(2)O(2)). Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H(2)O(2,) play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H(2)O(2) has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H(2)O(2) function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.

Salicylic acid influences Hsp70/Hsc70 expression in Lycopersicon esculentum: dose- and time-dependent induction or potentiation

Overexpression of heat-shock (HS) proteins (HSP) is often sufficient to protect against lethal environmental stresses. Anti-inflammatory salicylates potentiate the induction of the 70 kDa HSP (Hsp70) in mammals in response to HS and enhance thermotolerance. In plants, salicylic acid (SA) is a natural signalling molecule, mediating resistance in response to avirulent pathogens. The influence of SA on the HS response in plants is, however, unknown. We investigated the effect of SA, alone or with HS, on Hsp70/Hsc70 expression in tomato cells using biometabolic labelling and Western blotting. A dose- and time-dependent influence on Hsp70/Hsc70 accumulation was observed: SA at 1.0 mM (3 h) potentiated heat-induced accumulation, while 1.0 mM (5 h) and 0.5 mM (8 h) induced expression, the latter preceded by increased membrane permeability. These results suggest that in plants, as in mammals, low SA concentrations do not induce Hsp70/Hsc70 expression but potentiate HS induction and confer membrane protection, while cytotoxic levels induce Hsp70/Hsc70.

Inhibition of plant viral systemic infection by non-toxic concentrations of cadmium

Heavy metal, such as cadmium, have a significant impact on plant physiology. However, their potential effect on plant-pathogen interaction, an important biological process, has not been examined. This study shows that exposure of tobacco plants to non-toxic concentrations of cadmium completely blocked viral disease caused by turnip vein clearing virus. Cadmium-mediated viral protection was due to inhibition of the systemic movement of the virus, i.e. its spread from the inoculated into uninoculated leaves. Exposure of plants to cadmium had no effect on viral replication, assembly and local movement within the inoculated leaf. Analysis of the viral presence in different tissues suggested that cadmium treatment inhibited virus exit from the vascular tissue into uninoculated leaves rather than its entry into the host plant vasculature. Higher, toxic levels of cadmium did not produce this inhibitory effect on viral movement, allowing the systemic spread of the virus and development of the viral disease. These observations suggest that cadmium-induced viral protection requires a relatively healthy, unpoisoned plant in which non-toxic levels of cadmium may trigger the production of cellular factors which interfere with the viral systemic movement.

The myrosinase-binding protein from Brassica napus seeds possesses lectin activity and has a highly similar vegetatively expressed wound-inducible counterpart

This communication demonstrates that proteins in the family of myrosinase-binding proteins (MBP) present in seeds of Brassica napus possess lectin activity, binding most efficiently to p-aminophenyl alpha-D-mannopyranoside-agarose, and to some extent to N-acetylglucosamine-agarose. A cDNA encoding a vegetatively expressed, wound-inducible counterpart to these seed MBP was isolated and characterised. Upon wounding, this MBP transcript accumulated in old and young leaves, and was systemically expressed in the young plant. Additionally, the wound-induced MBP transcript increased in abundance after treating the young plants with methyl jasmonate (MeJA), jasmonic acid (JA) or abscisic acid (ABA), and to some extent in response to the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. Expression induced by wounding, ABA or JA was antagonised by simultaneous feeding of the plants with salicylic acid. MBP polypeptides accumulated in MeJA-treated plants. The myrosinases redistributed from the soluble fraction into the insoluble fraction of a tissue extract after induction. The most abundant MBP (94 kDa) partitioned in the insoluble fraction, while two larger MBP (103 kDa and 108 kDa) were present only in the soluble fraction of extracts obtained from the control or MeJA-treated plant tissues.

Regulation of the wound-induced myrosinase-associated protein transcript in Brassica napus plants

Two slightly differing cDNA clones corresponding to the wound-inducible form of a previously characterized seed myrosinase-associated protein (MyAP) have been isolated from Brassica napus L. The transcripts corresponding to the induced MyAP (iMyAP) were found to be developmentally regulated in various vegetative organs. Both young and old leaves exhibited wound-inducible iMyAP expression. Furthermore, in the young plant, wounding resulted in a systemic increase in leaves located both acropetally and basipetally to the wounded leaf. Also, the iMyAP transcripts were induced by methyl jasmonate, jasmonic acid and abscisic acid. The different inductions could be antagonized by salicylic acid. A general responsiveness in methyl-jasmonate-treated leaves was demonstrated by in situ hybridization. No effect on the amount of iMyAP transcript was detected after feeding the plants with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. The similarity between MyAP and a lipase from Arabidopsis thaliana indicated a possible function in liberating acylated glucosinolates from their acyl group, thereby making them available for hydrolysis by the myrosinase enzymes. We also report on a reduction in the amount of myrosinase transcripts derived from the vegetatively expressed MB-gene family after treatment with exogenously applied salicylic acid or abscisic acid.

Characterization of a salicylic acid-insensitive mutant (sai1) of Arabidopsis thaliana, identified in a selective screen utilizing the SA-inducible expression of the tms2 gene

Salicylic acid (SA) plays an important signaling role in the resistance of many plants to pathogen invasion. Increases in endogenous SA levels have been associated with the hypersensitive response as well as systemic acquired resistance (SAR). SA also induces the expression of a subset of the pathogenesis-related (PR) genes. However, relatively little is known about the events occurring subsequent to SA accumulation during a resistance response. In order to identify mutations in components of the SA signal transduction pathway, we have developed a genetic screen in Arabidopsis thaliana that utilizes the Agrobacterium tumefaciens tms2 gene as a counter-selectable marker. SA-inducible expression of the tms2 gene from the tobacco PR-1a promoter confers sensitivity to alpha-naphthalene acetamide (alpha-NAM), resulting in inhibition of root growth in germinating transgenic Arabidopsis seedlings. Mutants in which root growth is insensitive to alpha-NAM have been selected from this PR-1a:tms2 transgenic line with the expectation that a subset will lack a regulatory component downstream of SA. The sail mutant so identified expressed neither the PR-1a:tms2 transgene nor the endogenous Arabidopsis PR-1, PR-2, and PR-5 genes in response to SA. These genes also were not induced in sai1 by 2,6-dichloroisonicotinic acid (INA) or benzothiadiazole (BTH), two chemical inducers of SAR. As expected of a mutation acting downstream of SA, sai1 plants accumulate SA and its glucoside in response to infection with an avirulent pathogen and are more susceptible to this avirulent pathogen than the wild-type parent. sai1 is allelic to npr1, a previously identified SA-noninducible mutation. The recessive nature of the noninducible sai1 mutation suggests that the wild-type SAI1 gene acts as a positive regulator in the SA signal transduction pathway.

Salicylic acid is a modulator of tobacco and mammalian catalases

Salicylic acid (SA) plays a key role in the establishment of resistance to microbial pathogens in many plants. The discovery that SA inhibits catalase from tobacco led us to suggest that H2O2 acts as second messenger to activate plant defenses. Detailed analyses of SA's interaction with tobacco and mammalian catalases indicate that SA acts as an electron donor for the peroxidative cycle of catalase. When H2O2 fluxes were relatively low (1 microM/min or less), SA inhibited catalase, consistent with its suggested signaling function via H2O2. However, significant inhibition was only observed at 100 microM SA or more, a level reached in infected, but not in uninfected, leaves. This inhibition was probably due to siphoning catalase into the slow peroxidative reaction. Surprisingly, SA was also able to protect catalase from inactivation by damaging levels of H2O2 (lower millimolar range), which is generally assumed to reflect accumulation of inactive ferro-oxy intermediates. SA did so by supporting or substituting for the protective function of catalase-bound NADPH. These results add new features to SA's interaction with heme enzymes and its in vivo redox properties. Thus, SA, in addition to its proposed signaling function, may also have an important antioxidant role in containing oxidative processes associated with plant defense responses.