Overexpression of a novel peanut NBS-LRR gene AhRRS5 enhances disease resistance to Ralstonia solanacearum in tobacco

Bacterial wilt caused by Ralstonia solanacearum is a ruinous soilborne disease affecting more than 450 plant species. Efficient control methods for this disease remain unavailable to date. This study characterized a novel nucleotide-binding site-leucine-rich repeat resistance gene AhRRS5 from peanut, which was up-regulated in both resistant and susceptible peanut cultivars in response to R. solanacearum. The product of AhRRS5 was localized in the nucleus. Furthermore, treatment with phytohormones such as salicylic acid (SA), abscisic acid (ABA), methyl jasmonate (MeJA) and ethephon (ET) increased the transcript level of AhRRS5 with diverse responses between resistant and susceptible peanuts. Abiotic stresses such as drought and cold conditions also changed AhRRS5 expression. Moreover, transient overexpression induced hypersensitive response in Nicotiana benthamiana. Overexpression of AhRRS5 significantly enhanced the resistance of heterogeneous tobacco to R. solanacearum, with diverse resistance levels in different transgenic lines. Several defence-responsive marker genes in hypersensitive response, including SA, JA and ET signals, were considerably up-regulated in the transgenic lines as compared with the wild type inoculated with R. solanacearum. Nonexpressor of pathogenesis-related gene 1 (NPR1) and non-race-specific disease resistance 1 were also up-regulated in response to the pathogen. These results indicate that AhRRS5 participates in the defence response to R. solanacearum through the crosstalk of multiple signalling pathways and the involvement of NPR1 and R gene signals for its resistance. This study may guide the resistance enhancement of peanut and other economic crops to bacterial wilt disease.

Overexpression of Arabidopsis NIMIN1 results in salicylate intolerance

The transcriptional regulator NPR1 mediates salicylic acid (SA)-induced plant immunity. NPR1 is also required for tolerance to high concentrations of SA. NPR1-interacting protein, NIMIN1, represses immune response by interacting with and negating NPR1. We tested the salicylic acid tolerance of transgenic plants overexpressing NIMIN1 and found that these plants displayed SA intolerance, similar to the npr1 mutant, due to sequestration of NPR1 by NIMIN1. Plants overexpressing mutated NIMIN1 that cannot interact with NPR1 showed no SA tolerance defect. Gene expression analysis showed that NPR1 is required for SA-stress induced as well as pathogen-induced NIMIN1 expression. These results indicate that over-accumulation of a negative regulator renders plants hypersensitive to SA by limiting NPR1 function. Furthermore, NPR1 activates negative regulators such as NIMIN1 for feedback inhibition of SA signaling to maintain immune homeostasis.

Abscisic acid promotes proteasome-mediated degradation of the transcription coactivator NPR1 in Arabidopsis thaliana

Proteasome-mediated turnover of the transcription coactivator NPR1 is pivotal for efficient activation of the broad-spectrum plant immune responses known as localized acquired resistance (LAR) and systemic acquired resistance (SAR) in adjacent and systemic tissues, respectively, and requires the CUL3-based E3 ligase and its adaptor proteins, NPR3 and NPR4, which are receptors for the signaling molecule salicylic acid (SA). It has been shown that SA prevents NPR1 turnover under non-inducing and LAR/SAR-inducing conditions, but how cellular NPR1 homeostasis is maintained remains unclear. Here, we show that the phytohormone abscisic acid (ABA) and SA antagonistically influence cellular NPR1 protein levels. ABA promotes NPR1 degradation via the CUL3(NPR) (3/) (NPR) (4) complex-mediated proteasome pathway, whereas SA may protect NPR1 from ABA-promoted degradation through phosphorylation. Furthermore, we demonstrate that the timing and strength of SA and ABA signaling are critical in modulating NPR1 accumulation and target gene expression. Perturbing ABA or SA signaling in adjacent tissues alters the temporal dynamic pattern of NPR1 accumulation and target gene transcription. Finally, we show that sequential SA and ABA treatment leads to dynamic changes in NPR1 protein levels and target gene expression. Our results revealed a tight correlation between sequential SA and ABA signaling and dynamic changes in NPR1 protein levels and NPR1-dependent transcription in plant immune responses.

Transfer RNA Derived Small RNAs Targeting Defense Responsive Genes Are Induced during Phytophthora capsici Infection in Black Pepper (Piper nigrum L.)

Small RNAs derived from transfer RNAs were recently assigned as potential gene regulatory candidates for various stress responses in eukaryotes. In this study, we report on the cloning and identification of tRNA derived small RNAs from black pepper plants in response to the infection of the quick wilt pathogen, Phytophthora capsici. 5'tRFs cloned from black pepper were validated as highly expressed during P. capsici infection. A high-throughput systematic analysis of the small RNAome (sRNAome) revealed the predominance of 5'tRFs in the infected leaf and root. The abundance of 5'tRFs in the sRNAome and the defense responsive genes as their potential targets indicated their regulatory role during stress response in black pepper. The 5'Ala(CGC) tRF mediated cleavage was experimentally mapped at the tRF binding sites on the mRNA targets of Non-expresser of pathogenesis related protein (NPR1), which was down-regulated during pathogen infection. Comparative sRNAome further demonstrated sequence conservation of 5'Ala tRFs across the angiosperm plant groups, and many important genes in the defense response were identified in silico as their potential targets. Our findings uncovered the diversity, differential expression and stress responsive functional role of tRNA-derived small RNAs during Phytophthora infection in black pepper.

β-cyclocitral upregulates salicylic acid signalling to enhance excess light acclimation in Arabidopsis

β-cyclocitral (β-CC), a volatile oxidized derivative of β-carotene, can upregulate the expression of defence genes to enhance excess light (EL) acclimation. However, the signalling cascades underlying this process remain unclear. In this study, salicylic acid (SA) is involved in alleviating damage to promote β-CC-enhanced EL acclimation. In early stages of EL illumination, β-CC pretreatment induced SA accumulation and impeded reactive oxygen species (ROS) production in the chloroplast. A comparative analysis of two SA synthesis pathways in Arabidopsis revealed that SA concentration mainly increased via the isochorismate synthase 1 (ICS1)-mediated isochorismate pathway, which depended on essential regulative function of enhanced disease susceptibility 1 (EDS1). Further results showed that, in the process of β-CC-enhanced EL acclimation, nuclear localization of nonexpressor of pathogenesis-related genes 1 (NPR1) was regulated by SA accumulation and NPR1 induced subsequent transcriptional reprogramming of gluthathione-S-transferase 5 (GST5) and GST13 implicated in detoxification. In summary, β-CC-induced SA synthesis contributes to EL acclimation response by decreasing ROS production in the chloroplast, promoting nuclear localization of NPR1, and upregulating GST transcriptional expression. This process is a possible molecular regulative mechanism of β-CC-enhanced EL acclimation.

NPR1 is Instrumental in Priming for the Enhanced flg22-induced MPK3 and MPK6 Activation

Pathogen-associated molecular patterns (PAMPs) activate mitogen-activated protein kinases (MAPKs), essential components of plant defense signaling. Salicylic acid (SA) is also central to plant resistance responses, but its specific role in regulation of MAPK activation is not completely defined. We have investigated the role of SA in PAMP-triggered MAPKs pathways in Arabidopsis SA-related mutants, specifically in the flg22-triggered activation of MPK3 and MPK6. cim6, sid2, and npr1 mutants exhibited wild-type-like flg22-triggered MAPKs activation, suggesting that impairment of SA signaling has no effect on the flg22-triggered MAPKs activation. Pretreatment with low concentrations of SA enhanced flg22-induced MPK3 and MPK6 activation in all seedlings except npr1, indicating that NPR1 is involved in SA-mediated priming that enhanced flg22-induced MAPKs activation.

Phylogenetic and expression analysis of the NPR1-like gene family from Persea americana (Mill.)

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) forms an integral part of the salicylic acid (SA) pathway in plants and is involved in cross-talk between the SA and jasmonic acid/ethylene (JA/ET) pathways. Therefore, NPR1 is essential to the effective response of plants to pathogens. Avocado (Persea americana) is a commercially important crop worldwide. Significant losses in production result from Phytophthora root rot, caused by the hemibiotroph, Phytophthora cinnamomi. This oomycete infects the feeder roots of avocado trees leading to an overall decline in health and eventual death. The interaction between avocado and P. cinnamomi is poorly understood and as such limited control strategies exist. Thus uncovering the role of NPR1 in avocado could provide novel insights into the avocado - P. cinnamomi interaction. A total of five NPR1-like sequences were identified. These sequences were annotated using FGENESH and a maximum-likelihood tree was constructed using 34 NPR1-like protein sequences from other plant species. The conserved protein domains and functional motifs of these sequences were predicted. Reverse transcription quantitative PCR was used to analyze the expression of the five NPR1-like sequences in the roots of avocado after treatment with salicylic and jasmonic acid, P. cinnamomi infection, across different tissues and in P. cinnamomi infected tolerant and susceptible rootstocks. Of the five NPR1-like sequences three have strong support for a defensive role while two are most likely involved in development. Significant differences in the expression profiles of these five NPR1-like genes were observed, assisting in functional classification. Understanding the interaction of avocado and P. cinnamomi is essential to developing new control strategies. This work enables further classification of these genes by means of functional annotation and is a crucial step in understanding the role of NPR1 during P. cinnamomi infection.

Integrating data on the Arabidopsis NPR1/NPR3/NPR4 salicylic acid receptors; a differentiating argument

Salicylic acid (SA) is a mandatory plant metabolite in the deployment of systemic acquired resistance (SAR), a broad-spectrum systemic immune response induced by local inoculation with avirulent pathogens. The NPR1 transcription co-activator is the central node positively regulating SAR. SA was the last of the major hormones to be without a known receptor. Recently, NPR1 was shown to be the direct link between SA and gene activation. This discovery seems to be controversial. NPR1 being an SA-receptor is reminiscent of the mammalian steroid receptors, which are transcription factors whose binding to DNA is dependent on the interaction with a ligand. Unlike steroid receptors, NPR1 does not bind directly to DNA, but is recruited to promoters by the TGA family of transcription factors to form an enhanceosome. In Arabidopsis, NPR1 is part of a multigene family in which two other members, NPR3 and NPR4, have also been shown to interact with SA. NPR3/NPR4 are negative regulators of immunity and act as substrate adaptors for the recruitment of NPR1 to an E3-ubiquitin ligase, leading to its subsequent degradation by the proteasome. In this perspective, we will stress-test in a friendly way the current NPR1/NPR3/NPR4 model.

Phospholipase D affects translocation of NPR1 to the nucleus in Arabidopsis thaliana

Phytohormone salicylic acid (SA) is a crucial component of plant-induced defense against biotrophic pathogens. Although the key players of the SA pathway are known, there are still gaps in the understanding of the molecular mechanism and the regulation of particular steps. In our previous research, we showed in Arabidopsis suspension cells that n-butanol, which specifically modulates phospholipase D activity, significantly suppresses the transcription of the pathogenesis related (PR-1) gene, which is generally accepted as the SA pathway marker. In the presented study, we have investigated the site of n-butanol action in the SA pathway. We were able to show in Arabidopsis plants treated with SA that n-butanol inhibits the transcription of defense genes (PR-1, WRKY38). Fluorescence microscopy of Arabidopsis thaliana mutants expressing 35S::NPR1-GFP (nonexpressor pathogenesis related 1) revealed significantly decreased nuclear localization of NPR1 in the presence of n-butanol. On the other hand, n-butanol did not decrease the nuclear localization of NPR1 in 35S::npr1C82A-GFP and 35S::npr1C216A-GFP mutants constitutively expressing NPR1 monomers. Mass spectrometric analysis of plant extracts showed that n-butanol significantly changes the metabolic fingerprinting while t-butanol had no effect. We found groups of the plant metabolites, influenced differently by SA and n-butanol treatment. Thus, we proposed several metabolites as markers for n-butanol action.

Salicylic acid and reactive oxygen species interplay in the transcriptional control of defense genes expression

It is well established that salicylic acid (SA) plays a critical role in the transcriptional reprograming that occurs during the plant defense response against biotic and abiotic stress. In the course of the defense response, the transcription of different sets of defense genes is controlled in a spatio-temporal manner via SA-mediated mechanisms. Interestingly, different lines of evidence indicate that SA interplays with reactive oxygen species (ROS) and glutathione (GSH) in stressed plants. In this review we focus on the evidence that links SA, ROS, and GSH signals to the transcriptional control of defense genes. We discuss how redox modifications of regulators and co-regulators involved in SA-mediated transcriptional responses control the temporal patterns of gene expression in response to stress. Finally, we examine how these redox sensors are coordinated with the dynamics of cellular redox changes occurring in the defense response to biotic and abiotic stress.

Cullin-RING ubiquitin ligases in salicylic acid-mediated plant immune signaling

Plant immune responses against biotrophic pathogens are regulated by the signaling hormone salicylic acid (SA). SA establishes immunity by regulating a variety of cellular processes, including programmed cell death (PCD) to isolate and kill invading pathogens, and development of systemic acquired resistance (SAR) which provides long-lasting, broad-spectrum resistance throughout the plant. Central to these processes is post-translational modification of SA-regulated signaling proteins by ubiquitination, i.e., the covalent addition of small ubiquitin proteins. Emerging evidence indicates SA-induced protein ubiquitination is largely orchestrated by Cullin-RING ligases (CRLs), which recruit specific substrates for ubiquitination using interchangeable adaptors. Ligation of ubiquitin chains interlinked at lysine 48 leads to substrate degradation by the 26S proteasome. Here we discuss how CRL-mediated degradation of both nucleotide-binding/leucine-rich repeat domain containing immune receptors and SA-induced transcription regulators are critical for functional PCD and SAR responses, respectively. By placing these recent findings in context of knowledge gained in other eukaryotic model species, we highlight potential alternative roles for processive ubiquitination in regulating the activity of SA-mediated immune responses.

Ectopically expressed sweet pepper ferredoxin PFLP enhances disease resistance to Pectobacterium carotovorum subsp. carotovorum affected by harpin and protease-mediated hypersensitive response in Arabidopsis

Plant ferredoxin-like protein (PFLP) is a photosynthesis-type ferredoxin (Fd) found in sweet pepper. It contains an iron-sulphur cluster that receives and delivers electrons between enzymes involved in many fundamental metabolic processes. It has been demonstrated that transgenic plants overexpressing PFLP show a high resistance to many bacterial pathogens, although the mechanism remains unclear. In this investigation, the PFLP gene was transferred into Arabidopsis and its defective derivatives, such as npr1 (nonexpresser of pathogenesis-related gene 1) and eds1 (enhanced disease susceptibility 1) mutants and NAHG-transgenic plants. These transgenic plants were then infected with the soft-rot bacterial pathogen Pectobacterium carotovorum subsp. carotovorum (Erwinia carotovora ssp. carotovora, ECC) to investigate the mechanism behind PFLP-mediated resistance. The results revealed that, instead of showing soft-rot symptoms, ECC activated hypersensitive response (HR)-associated events, such as the accumulation of hydrogen peroxide (H2 O2 ), electrical conductivity leakage and expression of the HR marker genes (ATHSR2 and ATHSR3) in PFLP-transgenic Arabidopsis. This PFLP-mediated resistance could be abolished by inhibitors, such as diphenylene iodonium (DPI), 1-l-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E64) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), but not by myriocin and fumonisin. The PFLP-transgenic plants were resistant to ECC, but not to its harpin mutant strain ECCAC5082. In the npr1 mutant and NAHG-transgenic Arabidopsis, but not in the eds1 mutant, overexpression of the PFLP gene increased resistance to ECC. Based on these results, we suggest that transgenic Arabidopsis contains high levels of ectopic PFLP; this may lead to the recognition of the harpin and to the activation of the HR and other resistance mechanisms, and is dependent on the protease-mediated pathway.

Mitogen-activated protein kinase 6 regulates NPR1 gene expression and activation during leaf senescence induced by salicylic acid

Plant senescence is a highly regulated process that can be induced by a range of factors. The nonexpressor of pathogenesis-related genes 1 (npr1) mutant is defective in the salicylic acid (SA) signalling pathway, displaying delayed yellowing during developmental senescence. However, the regulating mechanism of NPR1 on exogenous SA-induced senescence in detached Arabidopsis leaves has not yet been clarified. It was shown here that mitogen-activated protein kinase 6 (MPK6) is involved in promoting exogenous SA-induced detached leaf senescence. During the process of SA-induced senescence, the expression of NPR1 and senescence-related transcription factor WRKY6 was suppressed in mpk6 mutant plants. Further analyses showed that the NPR1 mRNA level is reduced in wrky6 mutants and enhanced in WRKY6 overexpressing lines. Meanwhile, chromatin immunoprecipitation experiments revealed that WRKY6 binds directly to the NPR1 promoter containing W-box motifs. Moreover, inhibition of MPK6 function diminished SA-induced monomerization and nuclear localization of NPR1. In addition, the expression of Trx h5, which catalyses the SA-induced NPR1 activation, was suppressed in the mpk6 mutant, suggesting that MPK6 promotes NPR1 activation, possibly by regulating the expression of Trx h5. Collectively, MPK6-mediated WRKY6 and Trx h5 transcriptional activation co-regulated the expression of the NPR1 gene and the monomerization of NPR1 protein, allowing it to enter the nucleus, thereby promoting SA-induced leaf senescence. These results provide new insight into the mechanism of exogenous SA-induced detached leaf senescence.

Salicylic acid signaling in disease resistance

Salicylic acid (SA) is a key plant hormone that mediates host responses against microbial pathogens. Identification and characterization of SA-interacting/binding proteins is a topic which has always excited scientists studying microbial defense response in plants. It is likely that discovery of a true receptor for SA may greatly advance understanding of this important signaling pathway. SABP2 with its high affinity for SA was previously considered to be a SA receptor. Despite a great deal work we may still not have true a receptor for SA. It is also entirely possible that there may be more than one receptor for SA. This scenario is more likely given the diverse role of SA in various physiological processes in plants including, modulation of opening and closing of stomatal aperture, flowering, seedling germination, thermotolerance, photosynthesis, and drought tolerance. Recent identification of NPR3, NPR4 and NPR1 as potential SA receptors and α-ketoglutarate dehydrogenase (KGDHE2), several glutathione S transferases (GSTF) such as SA binding proteins have generated more interest in this field. Some of these SA binding proteins may have direct/indirect role in plant processes other than pathogen defense signaling. Development and use of new techniques with higher specificity to identify SA-interacting proteins have shown great promise and have resulted in the identification of several new SA interactors. This review focuses on SA interaction/binding proteins identified so far and their likely role in mediating plant defenses.

Role of NPR1 and KYP in long-lasting induced resistance by β-aminobutyric acid

Priming of defense increases the responsiveness of the plant immune system and can provide broad-spectrum protection against disease. Recent evidence suggests that priming of defense can be inherited epigenetically to following generations. However, the mechanisms of long-lasting defense priming within one generation remains poorly understood. Here, we have investigated the mechanistic basis of long-lasting induced resistance after treatment with β -aminobutyric acid (BABA), an agent that mimics biologically induced resistance phenomena. BABA-induced resistance (BABA-IR) is based on priming of salicylic acid (SA)-dependent and SA-independent defenses. BABA-IR could be detected up to 28 days after treatment of wild-type Arabidopsis. This long-lasting component of the induced resistance response requires the regulatory protein NPR1 and is associated with priming of SA-inducible genes. In contrast, NPR1-independent resistance by BABA was transient and had disappeared by 14 days after treatment. Chromatin immunoprecipitation (ChIP) assays revealed no increased acetylation of histone H3K9 at promoters regions of priming-responsive genes, indicating that this post-translational histone modification is not critical for long-term transcriptional priming. Interestingly, the kyp-6 mutant, which is affected in methyltransferase activity of H3K9, was blocked in long-lasting BABA-IR, indicating a critical requirement of this post-translational histone modification in long-lasting BABA-IR. Considering that KYP suppresses gene transcription through methylation of H3K9 and CpHpG DNA methylation, we propose that KYP enables long-term defense gene priming by silencing suppressor genes of SA/NPR1-dependent genes.

Salicylic acid-dependent and -independent impact of an RNA-binding protein on plant immunity

Plants overexpressing the RNA-binding protein AtGRP7 (AtGRP7-ox plants) constitutively express the PR-1 (PATHOGENESIS-RELATED-1), PR-2 and PR-5 transcripts associated with salicylic acid (SA)-mediated immunity and show enhanced resistance against Pseudomonas syringae pv. tomato (Pto) DC3000. Here, we investigated whether the function of AtGRP7 in plant immunity depends on SA. Endogenous SA was elevated fivefold in AtGRP7-ox plants. The elevated PR-1, PR-2 and PR-5 levels were eliminated upon expression of the salicylate hydroxylase nahG in AtGRP7-ox plants and elevated PR-1 levels were suppressed by sid (salicylic acid deficient) 2-1 that is impaired in SA biosynthesis. RNA immunoprecipitation showed that AtGRP7 does not bind the PR-1 transcript in vivo, whereas it binds PDF1.2. Constitutive or inducible AtGRP7 overexpression increases PR-1 promoter activity, indicating that AtGRP7 affects PR-1 transcription. In line with this, the effect of AtGRP7 on PR-1 is suppressed by npr (non-expressor of PR genes) 1. Whereas AtGRP7-ox plants restricted growth of Pto DC3000 compared with wild type (wt), sid2-1 AtGRP7-ox plants allowed more growth than AtGRP7-ox plants. Furthermore, we show an enhanced hypersensitive response triggered by avirulent Pto DC3000 (AvrRpt2) in AtGRP7-ox compared with wt. In sid2-1 AtGRP7-ox, an intermediate phenotype was observed. Thus, AtGRP7 has both SA-dependent and SA-independent effects on plant immunity.

Development of disease-resistant rice using regulatory components of induced disease resistance

Infectious diseases cause huge crop losses annually. In response to pathogen attacks, plants activate defense systems that are mediated through various signaling pathways. The salicylic acid (SA) signaling pathway is the most powerful of these pathways. Several regulatory components of the SA signaling pathway have been identified, and are potential targets for genetic manipulation of plants' disease resistance. However, the resistance associated with these regulatory components is often accompanied by fitness costs; that is, negative effects on plant growth and crop yield. Chemical defense inducers, such as benzothiadiazole and probenazole, act on the SA pathway and induce strong resistance to various pathogens without major fitness costs, owing to their 'priming effect.' Studies on how benzothiadiazole induces disease resistance in rice have identified WRKY45, a key transcription factor in the branched SA pathway, and OsNPR1/NH1. Rice plants overexpressing WRKY45 were extremely resistant to rice blast disease caused by the fungus Magnaporthe oryzae and bacterial leaf blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo), the two major rice diseases. Disease resistance is often accompanied by fitness costs; however, WRKY45 overexpression imposed relatively small fitness costs on rice because of its priming effect. This priming effect was similar to that of chemical defense inducers, although the fitness costs were amplified by some environmental factors. WRKY45 is degraded by the ubiquitin-proteasome system, and the dual role of this degradation partly explains the priming effect. The synergistic interaction between SA and cytokinin signaling that activates WRKY45 also likely contributes to the priming effect. With a main focus on these studies, I review the current knowledge of SA-pathway-dependent defense in rice by comparing it with that in Arabidopsis, and discuss potential strategies to develop disease-resistant rice using signaling components.

How does SA signaling link the Flg22 responses?

Salicylic acid (SA) has a central role in activating plant resistance to pathogens. SA levels increase in plant tissue following pathogen infection and exogenous SA enhances resistance to a broad range of pathogens. To study the relevance of the SA signaling in the flg22 response, we investigated the responses of SA-related mutants to flg22, a 22-amino acid peptide of the flagellin bacterial protein. We identified SA as an important component of the flg22-triggered oxidative burst, a very early event after flg22 detection, and gene induction, an early event. SA acted partially by enhancing accumulation of FLS2 mRNA. We also provide new evidence that NPR1 play a role in SA-induced priming event that enhances the flg22-triggered oxidative burst, which is correlated with enhancement of the flg22-induced callose deposition. Based on these observations, we conclude that SA signaling is required for early as well as late flg22 responses.

A large-scale genetic screen for mutants with altered salicylic acid accumulation in Arabidopsis

Salicylic acid (SA) is a key defense signal molecule against biotrophic and hemibiotrophic pathogens in plants, but how SA is synthesized in plant cells still remains elusive. Identification of new components involved in pathogen-induced SA accumulation would help address this question. To this end, we performed a large-scale genetic screen for mutants with altered SA accumulation during pathogen infection in Arabidopsis using a bacterial biosensor Acinetobacter sp. ADPWH_lux-based SA quantification method. A total of 35,000 M2 plants in the npr1-3 mutant background have been individually analyzed for the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) ES4326-induced SA accumulation. Among the mutants isolated, 19 had SA levels lower than npr1 (sln) and two exhibited increased SA accumulation in npr1 (isn). Complementation tests revealed that seven of the sln mutants are new alleles of eds5/sid1, two are sid2/eds16 alleles, one is allelic to pad4, and the remaining seven sln and two isn mutants are new non-allelic SA accumulation mutants. Interestingly, a large group of mutants (in the npr1-3 background), in which Psm ES4326-induced SA levels were similar to those in the wild-type Columbia plants, were identified, suggesting that the signaling network fine-tuning pathogen-induced SA accumulation is complex. We further characterized the sln1 single mutant and found that Psm ES4326-induced defense responses were compromised in this mutant. These defense response defects could be rescued by exogenous SA, suggesting that SLN1 functions upstream of SA. The sln1 mutation was mapped to a region on the north arm of chromosome I, which contains no known genes regulating pathogen-induced SA accumulation, indicating that SLN1 likely encodes a new regulator of SA biosynthesis. Thus, the new sln and isn mutants identified in this genetic screen are valuable for dissecting the molecular mechanisms underlying pathogen-induced SA accumulation in plants.

Salicylic acid signal transduction: the initiation of biosynthesis, perception and transcriptional reprogramming

The phytohormone salicylic acid (SA) is a small phenolic compound that regulates diverse physiological processes, in particular plant resistance against pathogens. Understanding SA-mediated signaling has been a major focus of plant research. Pathogen-induced SA is mainly synthesized via the isochorismate pathway in chloroplasts, with ICS1 (ISOCHORISMATE SYNTHASE 1) being a critical enzyme. Calcium signaling regulates activities of a subset of transcription factors thereby activating nuclear ICS1 expression. The produced SA triggers extensive transcriptional reprogramming in which NPR1 (NON-EXPRESSOR of PATHOGENESIS-RELATED GENES 1) functions as the central coactivator of TGA transcription factors. Recently, two alternative but not exclusive models for SA perception mechanisms were proposed. The first model is that NPR1 homologs, NPR3 and NPR4, perceive SA thereby regulating NPR1 protein accumulation. The second model describes that NPR1 itself perceives SA, triggering an NPR1 conformational change thereby activating SA-mediated transcription. Besides the direct SA binding, NPR1 is also regulated by SA-mediated redox changes and phosphorylation. Emerging evidence show that pathogen virulence effectors target SA signaling, further strengthening the importance of SA-mediated immunity.

Arabidopsis cysteine-rich receptor-like kinase 45 positively regulates disease resistance to Pseudomonas syringae

Arabidopsis cysteine-rich receptor-like protein kinase 45 (CRK45) was found to be involved in ABA signaling in Arabidopsis thaliana previously. Here, we reported that it also positively regulates disease resistance. The CRK45 overexpression plants increased expression of the defense genes, and enhanced resistance to Pseudomonas syringae whereas the crk45 mutant were more sensitive to P. syringae and weakened expression of the defense genes, compared to the wild type. We also found that treatment with P. syringae leads to a declined expression of CRK45 in the npr1 mutant and the NahG transgenic plants. At the same time, significantly decreased expression of CRK45 transcript in the wrky70 mutant than that in the wild type was also detected. Our results suggested that CRK45 acted as a positive regulator in Arabidopsis disease resistance, and was regulated downstream of NPR1 and WRKY70 at the transcriptional level.

The NPR1 ortholog PhaNPR1 is required for the induction of PhaPR1 in Phalaenopsis aphrodite

BACKGROUND

Systematic acquired resistance (SAR) is an effective broad-spectrum defense mechanism that confers long-lasting protection against biotrophic pathogens trough defense related salicylic acid (SA) signaling. Gene(s) involved in SAR have been extensively studied in dicot plants; however, remains largely unresolved in monocot plants. NPR1, an evolutionary conserved gene, plays a central role in SAR, and PR-1 is widely used as a marker for effective SA signaling.

RESULTS

We identified NPR1 and PR-1 homologous genes, PhaNPR1 and PhaPR1, from an economically important orchid, Phalaenopsis aphrodite, and characterized their roles in SA signaling and Cymbidium mosaic virus (CymMV) resistance. A phylogenetic analysis of NPR1 homologs showed that these genes appear to have evolved before angiospermy. Similar to Arabidopsis NPR1, PhaNPR1 was only moderately induced upon SA treatment and CymMV infection. Although PhaPR1 shows only 36% identity with AtPR1, its promoter shared conserved elements with those of other PR-1 genes, and it was induced upon SA treatment and CymMV infection. After CymMV infection, silencing on PhaNPR1 also reduced PhaPR1 expression; however, CymMV accumulation was not affected.

CONCLUSIONS

In conclusion, after virus infection, PhaNPR1 is required for PhaPR1 induction, but plays little role in defense against CymMV.

The Arabidopsis Mediator complex subunits MED14/SWP and MED16/SFR6/IEN1 differentially regulate defense gene expression in plant immune responses

Pathogen infection in plants triggers large-scale transcriptional changes, both locally and systemically. Emerging evidence suggests that the Arabidopsis Mediator complex plays a crucial role in these transcriptional changes. Mediator is highly conserved in eukaryotes, and its core comprises more than 20 subunits organized into three modules named head, middle and tail. The head and middle modules interact with general transcription factors and RNA polymerase II, whereas the tail module associates with activators, and signals through the head and middle modules to the basal transcription machinery. In Arabidopsis, three tail module subunits, MED14, MED15 and MED16, have been identified. Both MED15 and MED16 have been implicated in plant immunity, but the role of MED14 has not been established. Here, we report the characterization of an Arabidopsis T-DNA insertion mutant of the MED14 gene. Similarly to the med15 and/or med16 mutations, the med14 mutation significantly suppresses salicylic acid-induced defense responses, alters transcriptional changes induced by the avirulent bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000/avrRpt2, and renders plants susceptible to both Pst DC3000/avrRpt2 and Pst DC3000. The med14 mutation also completely compromises biological induction of systemic acquired resistance (SAR), indicating that the tail module as a whole is essential for SAR. Interestingly, unlike the med16 mutation, which differentially affects expression of several SAR positive and negative regulators, med14 inhibits induction of a large group of defense genes, including both SAR positive and negative regulators, suggesting that individual subunits of the Mediator tail module employ distinct mechanisms to regulate plant immune responses.

Gene methylation in gastric cancer

Gastric cancer is one of the most common malignancies and remains the second leading cause of cancer-related death worldwide. Over 70% of new cases and deaths occur in developing countries. In the early years of the molecular biology revolution, cancer research mainly focuses on genetic alterations, including gastric cancer. Epigenetic mechanisms are essential for normal development and maintenance of tissue-specific gene expression patterns in mammals. Disruption of epigenetic processes can lead to altered gene function and malignant cellular transformation. Recent advancements in the rapidly evolving field of cancer epigenetics have shown extensive reprogramming of every component of the epigenetic machinery in cancer, including DNA methylation, histone modifications, nucleosome positioning, noncoding RNAs, and microRNAs. Aberrant DNA methylation in the promoter regions of gene, which leads to inactivation of tumor suppressor and other cancer-related genes in cancer cells, is the most well-defined epigenetic hallmark in gastric cancer. The advantages of gene methylation as a target for detection and diagnosis of cancer in biopsy specimens and non-invasive body fluids such as serum and gastric washes have led to many studies of application in gastric cancer. This review focuses on the most common and important phenomenon of epigenetics, DNA methylation, in gastric cancer and illustrates the impact epigenetics has had on this field.