Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response

Priming of defence genes for amplified response to secondary stress can be induced by application of the plant hormone salicylic acid or its synthetic analogue acibenzolar S‐methyl. In this study, we show that treatment with acibenzolar S‐methyl or pathogen infection of distal leaves induce chromatin modifications on defence gene promoters that are normally found on active genes, although the genes remain inactive. This is associated with an amplified gene response on challenge exposure to stress. Mutant analyses reveal a tight correlation between histone modification patterns and gene priming. The data suggest a histone memory for information storage in the plant stress response.     

From correlation to causation: The new frontier of transgenerational epigenetic inheritance

While heredity is predominantly controlled by what deoxyribonucleic acid (DNA) sequences are passed from parents to their offspring, a small but growing number of traits have been shown to be regulated in part by the non-genetic inheritance of information. Transgenerational epigenetic inheritance is defined as heritable information passed from parents to their offspring without changing the DNA sequence. Work of the past seven decades has transitioned what was previously viewed as rare phenomenology, into well-established paradigms by which numerous traits can be modulated. For the most part, studies in model organisms have correlated transgenerational epigenetic inheritance phenotypes with changes in epigenetic modifications. The next steps for this field will entail transitioning from correlative studies to causal ones. Here, we delineate the major molecules that have been implicated in transgenerational epigenetic inheritance in both mammalian and non-mammalian models, speculate on additional molecules that could be involved, and highlight some of the tools which might help transition this field from correlation to causation.     

Priming for enhanced ARGONAUTE2 activation accompanies induced resistance to cucumber mosaic virus in Arabidopsis thaliana

Systemic acquired resistance (SAR) is a broad-spectrum disease resistance response that can be induced upon infection from pathogens or by chemical treatment, such as with benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH). SAR involves priming for more robust activation of defence genes upon pathogen attack. Whether priming for SAR would involve components of RNA silencing remained unknown. Here, we show that upon leaf infiltration of water, BTH-primed Arabidopsis thaliana plants accumulate higher amounts of mRNA of ARGONAUTE (AGO)2 and AGO3, key components of RNA silencing. The enhanced AGO2 expression is associated with prior-to-activation trimethylation of lysine 4 in histone H3 and acetylation of histone H3 in the AGO2 promoter and with induced resistance to the yellow strain of cucumber mosaic virus (CMV[Y]). The results suggest that priming A. thaliana for enhanced defence involves modification of histones in the AGO2 promoter that condition AGO2 for enhanced activation, associated with resistance to CMV(Y). Consistently, the fold-reduction in CMV(Y) coat protein accumulation by BTH pretreatment was lower in ago2 than in wild type, pointing to reduced capacity of ago2 to activate BTH-induced CMV(Y) resistance. A role of AGO2 in pathogen-induced SAR is suggested by the enhanced activation of AGO2 after infiltrating systemic leaves of plants expressing a localized hypersensitive response upon CMV(Y) infection. In addition, local inoculation of SAR-inducing Pseudomonas syringae pv. maculicola causes systemic priming for enhanced AGO2 expression. Together our results indicate that defence priming targets the AGO2 component of RNA silencing whose enhanced expression is likely to contribute to SAR.     

Abscisic acid modulates salicylic acid biosynthesis for systemic acquired resistance in tomato

Among the regulatory mechanisms of systemic acquired resistance (SAR) in tomato, antagonistic interaction between salicylic acid (SA) and abscisic acid (ABA) signaling pathways was investigated. Treatment with 1,2-benzisothiazol-3(2H)-one1,1-dioxide (BIT) induced SAR in tomato thorough SA biosynthesis. Pretreatment of ABA suppressed BIT-induced SAR including SA accumulation, suggesting that ABA suppressed SAR by inhibiting SA biosynthesis.     

3-Acetonyl-3-hydroxyoxindole: a new inducer of systemic acquired resistance in plants

Systemic acquired resistance (SAR) is an inducible defence mechanism which plays a central role in protecting plants from microbial pathogen attack. Guided by bioassays, a new chemical inducer of SAR was isolated from the extracts of Strobilanthes cusia and identified to be 3-acetonyl-3-hydroxyoxindole (AHO), a derivative of isatin. Tobacco plants treated with AHO exhibited enhanced resistance to tobacco mosaic virus (TMV) and to the fungal pathogen Erysiphe cichoracearum (powdery mildew), accompanied by increased levels of pathogenesis-related gene 1 ( PR-1 ) expression, salicylic acid (SA) accumulation and phenylalanine ammonia-lyase activity. To study the mode of action of AHO, its ability to induce PR-1 expression and TMV resistance in nahG transgenic plants expressing salicylate hydroxylase, which prevents the accumulation of SA, was analysed. AHO treatment did not induce TMV resistance or PR-1 expression in nahG transgenic plants, suggesting that AHO acts upstream of SA in the SAR signalling pathway. In addition, using two-dimensional gel electrophoresis combined with mass spectrometry, five AHO-induced plant proteins were identified which were homologous to the effector proteins with which SA interacts. Our data suggest that AHO may represent a novel class of inducer that stimulates SA-mediated defence responses.     

Induced systemic resistance (ISR) against pathogens – a promising field for ecological research

Putative fitness costs provide an explanation for why ISR is induced instead of constitutive, and they might constrain the use of ISR as preventative protection of cultivated plants. Though ISR is mainly elicited by and effective against pathogens, further biotic agents such as leaf-chewing herbivores, leaf miners, aphids and even non-pathogenic root-colonising bacteria can induce systemic pathogen resistance, while some ISR traits can have a defensive effect against herbivores. ‘Cross-resistance’ elicited by and effective against non-microbial plant enemies thus might add significantly to the function of ISR. On the other hand, ‘trade-offs” have been reported, i.e. increased susceptibility to herbivores in ISR-expressing plants. Finally, ISR is a rather unspecific response, being active against different microbes. It thus might have effects on mutualistic bacteria and fungi, too. The question of how expression of ISR affects the large variety of mutualistic and antagonistic plant-microbe and plant-insect interactions cannot yet be answered. This knowledge is, however, needed to obtain a risk assessment for the use of chemically induced or genetically engineered ISR in crop protection. This review aims to provide an overview and to highlight some of the many open questions which require intensive ecological research.     

Induction of systemic acquired resistance in cotton by BTH has a negligible effect on phytophagous insects

Whether or not chemical changes in plants in response to pests (insects and pathogens) are general or specific remains unclear. Some evidence indicates that an induced response (IR) to arthropods via the octadecanoid pathway represents a distinct mechanism from the salicylic acid-based pathway of systemic acquired resistance (SAR) to pathogens. To further test this hypothesis, young cotton seedlings were activated with benzo (1,2,3) thiadiazole-7-carbothioic acid (S) methyl ester (BTH), an elicitor of SAR. The enzymatic activities of a number of pathogenesis-related (PR) proteins in young and old leaves of control and BTH treated plants were measured. BTH applications elicited marked increases in the activity levels of chitinase, peroxidase, and -1,3-glucanase both locally and systemically. The highest levels of induction were detected systemically in young leaves. Except for some local effects on whitefly oviposition, the induction of SAR by BTH had no effect on either host preference of whiteflies Bemisia tabaci (Gennadius) or on feeding efficiency of cotton bollworms Helicoverpa armigera (Hübner). We conclude that SAR induction via the salicylic acid pathway in Acala cotton has negligible effect on the tested insect herbivores.     

Transgenerational inheritance of acquired epigenetic signatures at CpG islands in mice

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

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

Induced systemic resistance (ISR) against pathogens in the context of induced plant defences

Induced systemic resistance (ISR) of plants against pathogens is a widespread phenomenon that has been intensively investigated with respect to the underlying signalling pathways as well as to its potential use in plant protection. Elicited by a local infection, plants respond with a salicylic-dependent signalling cascade that leads to the systemic expression of a broad spectrum and long-lasting disease resistance that is efficient against fungi, bacteria and viruses. Changes in cell wall composition, de novo production of pathogenesis-related-proteins such as chitinases and glucanases, and synthesis of phytoalexins are associated with resistance, although further defensive compounds are likely to exist but remain to be identified. In this Botanical Briefing we focus on interactions between ISR and induced resistance against herbivores that is mediated by jasmonic acid as a central signalling molecule. While many studies report cross-resistance, others have found trade-offs, i.e. inhibition of one resistance pathway by the other. Here we propose a framework that explains many of the thus far contradictory results. We regard elicitation separately from signalling and from production, i.e. the synthesis of defensive compounds. Interactions on all three levels can act independently from each other.     

Aqueous fruit extracts of Azadirachta indica induce systemic acquired resistance in barley against Drechslera graminea

The study reported here primarily focuses on whether fruit extracts of Azadirachta indica Juss. (Neem) can induce systemic acquired resistance (SAR) in Hordeum vulgare against Drechslera graminea. A single leaf from each of the 1-month-old seedlings grown in 50 pots was treated with neem extract. Seven samples were collected at 12-h intervals for estimation of salicylic acid (SA) content and activities of phenylalanine ammonia lyase (PAL) and peroxidase (PO). Disease incidence was recorded on uninoculated leaves after 2 weeks and on newly emerged leaves after 3 weeks of inoculation of spores of the pathogen. Treatment of single leaf of barley seedlings with aqueous fruit extract of neem could protect the untreated and later emerging leaves of these seedlings from infection by leaf stripe pathogen. The concentration of SA and activities of PAL and PO were significantly higher in untreated leaves of seedlings given a single leaf treatment with neem fruit extract. The results show that neem fruit extract induced SAR in barley seedlings against D. graminea. The results of the study are significant for developing an environment-friendly biocide, which could induce SAR in crop plants leading to efficient management of pathogens     

Insect eggs induce a systemic acquired resistance in Arabidopsis

Although they constitute an inert stage of the insect's life, eggs trigger plant defences that lead to egg mortality or attraction of egg parasitoids. We recently found that salicylic acid (SA) accumulates in response to oviposition by the Large White butterfly Pieris brassicae, both in local and systemic leaves, and that plants activate a response that is similar to the recognition of pathogen‐associated molecular patterns (PAMPs), which are involved in PAMP‐triggered immunity (PTI). Here we discovered that natural oviposition by P. brassicae or treatment with egg extract inhibit growth of different Pseudomonas syringae strains in Arabidopsis through the activation of a systemic acquired resistance (SAR). This egg‐induced SAR involves the metabolic SAR signal pipecolic acid, depends on ALD1 and FMO1, and is accompanied by a stronger induction of defence genes upon secondary infection. Although P. brassicae larvae showed a reduced performance when feeding on Pseudomonas syringae‐infected plants, this effect was less pronounced when infected plants had been previously oviposited. Altogether, our results indicate that egg‐induced SAR might have evolved as a strategy to prevent the detrimental effect of bacterial pathogens on feeding larvae.     

Nitric oxide and reactive oxygen species are required for systemic acquired resistance in plants

Systemic acquired resistance (SAR) is a form of broad-spectrum disease resistance that is induced in response to primary infection and that protects uninfected portions of the plant against secondary infections by related or unrelated pathogens. SAR is associated with an increase in chemical signals that operate in a collective manner to confer protection against secondary infections. These include, the phytohormone salicylic acid (SA), glycerol-3-phosphate (G3P), azelaic acid (AzA) and more recently identified signals nitric oxide (NO) and reactive oxygen species (ROS). NO, ROS, AzA and G3P function in the same branch of the SAR pathway, and in parallel to the SA-regulated branch. NO and ROS function upstream of AzA/G3P and different reactive oxygen species functions in an additive manner to mediate chemical cleavage of the C9 double bond on C18 unsaturated fatty acids to generate AzA. The parallel and additive functioning of various chemical signals provides important new insights in the overlapping pathways leading to SAR.     

A critical role for Arabidopsis MILDEW RESISTANCE LOCUS O2 in systemic acquired resistance

Members of the MILDEW RESISTANCE LOCUS O (MLO) gene family confer susceptibility to powdery mildews in different plant species, and their existence therefore seems to be disadvantageous for the plant. We recognized that expression of the Arabidopsis MLO2 gene is induced after inoculation with the bacterial pathogen Pseudomonas syringae, promoted by salicylic acid (SA) signaling, and systemically enhanced in the foliage of plants exhibiting systemic acquired resistance (SAR). Importantly, distinct mlo2 mutant lines were unable to systemically increase resistance to bacterial infection after inoculation with P. syringae, indicating that the function of MLO2 is necessary for biologically induced SAR in Arabidopsis. Our data also suggest that the close homolog MLO6 has a supportive but less critical role in SAR. In contrast to SAR, basal resistance to bacterial infection was not affected in mlo2. Remarkably, SAR‐defective mlo2 mutants were still competent in systemically increasing the levels of the SAR‐activating metabolites pipecolic acid (Pip) and SA after inoculation, and to enhance SAR‐related gene expression in distal plant parts. Furthermore, although MLO2 was not required for SA‐ or Pip‐inducible defense gene expression, it was essential for the proper induction of disease resistance by both SAR signals. We conclude that MLO2 acts as a critical downstream component in the execution of SAR to bacterial infection, being required for the translation of elevated defense responses into disease resistance. Moreover, our data suggest a function for MLO2 in the activation of plant defense priming during challenge by P. syringae.     

Overexpression of Arabidopsis MAP kinase kinase 7 leads to activation of plant basal and systemic acquired resistance

There is a growing body of evidence indicating that mitogen-activated protein kinase (MAPK) cascades are involved in plant defense responses. Analysis of the completed Arabidopsis thaliana genome sequence has revealed the existence of 20 MAPKs, 10 MAPKKs and 60 MAPKKKs, implying a high level of complexity in MAPK signaling pathways, and making the assignment of gene functions difficult. The MAP kinase kinase 7 (MKK7) gene of Arabidopsis has previously been shown to negatively regulate polar auxin transport. Here we provide evidence that MKK7 positively regulates plant basal and systemic acquired resistance (SAR). The activation-tagged bud1 mutant, in which the expression of MKK7 is increased, accumulates elevated levels of salicylic acid (SA), exhibits constitutive pathogenesis-related (PR) gene expression, and displays enhanced resistance to both Pseudomonas syringae pv. maculicola (Psm) ES4326 and Hyaloperonospora parasitica Noco2. Both PR gene expression and disease resistance of the bud1 plants depend on SA, and partially depend on NPR1. We demonstrate that the constitutive defense response in bud1 plants is a result of the increased expression of MKK7, and requires the kinase activity of the MKK7 protein. We found that expression of the MKK7 gene in wild-type plants is induced by pathogen infection. Reducing mRNA levels of MKK7 by antisense RNA expression not only compromises basal resistance, but also blocks the induction of SAR. Intriguingly, ectopic expression of MKK7 in local tissues induces PR gene expression and resistance to Psm ES4326 in systemic tissues, indicating that activation of MKK7 is sufficient for generating the mobile signal of SAR.     

Potential epigenetic biomarkers of obesity-related insulin resistance in human whole-blood

Obesity can increase the risk of complex metabolic diseases, including insulin resistance. Moreover, obesity can be caused by environmental and genetic factors. However, the epigenetic mechanisms of obesity are not well defined. Therefore, the identification of novel epigenetic biomarkers of obesity allows for a more complete understanding of the disease and its underlying insulin resistance. The aim of our study was to identify DNA methylation changes in whole-blood that were strongly associated with obesity and insulin resistance. Whole-blood was obtained from lean (n = 10; BMI = 23.6 ± 0.7 kg/m²) and obese (n = 10; BMI = 34.4 ± 1.3 kg/m²) participants in combination with euglycemic hyperinsulinemic clamps to assess insulin sensitivity. We performed reduced representation bisulfite sequencing on genomic DNA isolated from the blood. We identified 49 differentially methylated cytosines (DMCs; q < 0.05) that were altered in obese compared with lean participants. We identified 2 sites (Chr.21:46,957,981 and Chr.21:46,957,915) in the 5’ untranslated region of solute carrier family 19 member 1 (SLC19A1) with decreased methylation in obese participants (lean 0.73 ± 0.11 vs. obese 0.09 ± 0.05; lean 0.68 ± 0.10 vs. obese 0.09 ± 0.05, respectively). These 2 DMCs identified by obesity were also significantly predicted by insulin sensitivity (r = 0.68, P = 0.003; r = 0.66; P = 0.004). In addition, we performed a differentially methylated region (DMR) analysis and demonstrated a decrease in methylation of Chr.21:46,957,915–46,958,001 in SLC19A1 of ?34.9% (70.4% lean vs. 35.5% obese). The decrease in whole-blood SLC19A1 methylation in our obese participants was similar to the change observed in skeletal muscle (Chr.21:46,957,981, lean 0.70 ± 0.09 vs. obese 0.31 ± 0.11 and Chr.21:46,957,915, lean 0.72 ± 0.11 vs. obese 0.31 ± 0.13). Pyrosequencing analysis further demonstrated a decrease in methylation at Chr.21:46,957,915 in both whole-blood (lean 0.71 ± 0.10 vs. obese 0.18 ± 0.06) and skeletal muscle (lean 0.71 ± 0.10 vs. obese 0.30 ± 0.11). Our findings demonstrate a new potential epigenetic biomarker, SLC19A1, for obesity and its underlying insulin resistance.     

Compatibility of systemic acquired resistance and microbial biocontrol for suppression of plant disease in a laboratory assay

Systemic acquired resistance (SAR) and microbial biocontrol each hold promise as alternatives to pesticides for control of plant diseases. SAR and Bacillus cereus UW85, a microbial biocontrol agent, separately suppress seedling damping-off diseases caused by oomycete pathogens. The purposes of this study were to investigate how expression of SAR affected the efficacy of biocontrol by UW85 and if UW85 treatment of plants induced SAR. We devised a laboratory assay in which seedling damping-off disease, induction of SAR, and growth of UW85 could be quantified. Seedlings of Nicotiana tabacum Xanthi nc were germinated on moist filter paper and transferred after 7 days to water agar plates (40 seedlings per plate). Zoospores of oomycete pathogens (Pythium torulosum, Pythium aphanidermatum, or Phytophthora parasitica) were applied at concentrations that caused 80% seedling mortality within 10 days. Seedling mortality was dependent on zoospore inoculum concentration. The level of disease suppression caused by treatment with UW85 depended on the UW85 dose applied. SAR was induced with 0.5-mM salicylic acid or 0.1-mM 2,6-dichloroisonicotinic acid. Expression of an SAR-related gene was confirmed by northern analysis with a probe prepared from a tobacco PR-1a cDNA. Induction of SAR suppressed disease caused by each of the oomycete pathogens, but did not alter the growth of UW85 on roots. Treatment of seedlings with UW85 did not induce the expression of PR-1a. The combination of induction of SAR and treatment with UW85 resulted in additive suppression of disease as measured by seedling survival.     

Plastid omega3-fatty acid desaturase-dependent accumulation of a systemic acquired resistance inducing activity in petiole exudates of Arabidopsis thaliana is independent of jasmonic acid

Systemic acquired resistance (SAR) is an inducible defense mechanism that is activated throughout the plant, subsequent to localized inoculation with a pathogen. The establishment of SAR requires translocation of an unknown signal from the pathogen-inoculated leaf to the distal organs, where salicylic acid-dependent defenses are activated. We demonstrate here that petiole exudates (PeXs) collected from Arabidopsis leaves inoculated with an avirulent (Avr) Pseudomonas syringae strain promote resistance when applied to Arabidopsis, tomato ( Lycopersicum esculentum ) and wheat ( Triticum aestivum ). Arabidopsis FATTY ACID DESATURASE7 ( FAD7 ), SUPPRESSOR OF FATTY ACID DESATURASE DEFICIENCY1 ( SFD1 ) and SFD2 genes are required for accumulation of the SAR-inducing activity. In contrast to Avr PeX from wild-type plants, Avr PeXs from fad7 , sfd1 and sfd2 mutants were unable to activate SAR when applied to wild-type plants. However, the SAR-inducing activity was reconstituted by mixing Avr PeXs collected from fad7 and sfd1 with Avr PeX from the SAR-deficient dir1 mutant. Since FAD7 , SFD1 and SFD2 are involved in plastid glycerolipid biosynthesis and SAR is also compromised in the Arabidopsis monogalactosyldiacylglycerol synthase1 mutant we suggest that a plastid glycerolipid-dependent factor is required in Avr PeX along with the DIR1- encoded lipid transfer protein for long-distance signaling in SAR. FAD7 -synthesized lipids provide fatty acids for synthesis of jasmonic acid (JA). However, co-infiltration of JA and methylJA with Avr PeX from fad7 and sfd1 did not reconstitute the SAR-inducing activity. In addition, JA did not co-purify with the SAR-inducing activity confirming that JA is not the mobile signal in SAR.