Acquired resistance in Arabidopsis

Interaction between a plasma membrane-localized ankyrin-repeat protein ITN1 and a nuclear protein RTV1

The increased tolerance to NaCl 1 (ITN1) protein is a plasma membrane (PM)-localized protein involved in responses to NaCl stress in Arabidopsis. The predicted structure of ITN1 is composed of multiple transmembrane regions and an ankyrin-repeat domain that is known to mediate protein-protein interactions. To elucidate the molecular functions of ITN1, we searched for interacting partners using a yeast two-hybrid assay, and a nuclear-localized DNA-binding protein, RTV1, was identified as a candidate. Bimolecular fluorescence complementation analysis revealed that RTV1 interacted with ITN1 at the PM and nuclei in vivo. RTV1 tagged with red fluorescent protein localized to nuclei and ITN1 tagged with green fluorescent protein localized to PM; however, both proteins localized to both nuclei and the PM when co-expressed. These findings suggest that RTV1 and ITN1 regulate the subcellular localization of each other.

A high-throughput virus-induced gene-silencing vector for screening transcription factors in virus-induced plant defense response in orchid

The large number of species and worldwide spread of species of Orchidaceae indicates their successful adaptation to environmental stresses. Thus, orchids provide rich resources to study how plants have evolved to cope with stresses. This report describes our improvement of our previously reported orchid virus-induced gene silencing vector, pCymMV-pro60, with a modified Gateway cloning system which requires only one recombination and can be inoculated by agroinfiltration. We cloned 1,700 DNA fragments, including 187 predicted transcription factors derived from an established expression sequence tag library of orchid, into pCymMV-Gateway. Phalaenopsis aphrodite was inoculated with these vectors that contained DNA fragments of the 187 predicted transcription factors. The viral vector initially triggered the expression of the salicylic acid (SA)-related plant defense responses and later induced silencing of the endogenous target transcription factor genes. By monitoring the expression of the SA-related plant defense marker PhaPR1 (homolog of PR1), we identified a gene, PhaTF15, involved in the expression of PhaPR1. Knockdown of PhaTF15 by virus-induced gene silencing and by transient delivery of double-stranded RNA (dsRNA) reduced expression of the orchid homolog of the conserved positive defense regulator NPR1, PhaNPR1. Cymbidium mosaic virus also accumulated to high levels with knockdown of PhaTF15 by transient delivery of dsRNA. We demonstrated efficient cloning and screening strategies for high-throughput analysis of orchid and identify a gene, PhaTF15, involved in regulation of SA-related plant defense.

Genome-Wide Characterization of ISR Induced in Arabidopsis thaliana by Trichoderma hamatum T382 Against Botrytis cinerea Infection

In this study, the molecular basis of the induced systemic resistance (ISR) in Arabidopsis thaliana by the biocontrol fungus Trichoderma hamatum T382 against the phytopathogen Botrytis cinerea B05-10 was unraveled by microarray analysis both before (ISR-prime) and after (ISR-boost) additional pathogen inoculation. The observed high numbers of differentially expressed genes allowed us to classify them according to the biological pathways in which they are involved. By focusing on pathways instead of genes, a holistic picture of the mechanisms underlying ISR emerged. In general, a close resemblance is observed between ISR-prime and systemic acquired resistance, the systemic defense response that is triggered in plants upon pathogen infection leading to increased resistance toward secondary infections. Treatment with T. hamatum T382 primes the plant (ISR-prime), resulting in an accelerated activation of the defense response against B. cinerea during ISR-boost and a subsequent moderation of the B. cinerea induced defense response. Microarray results were validated for representative genes by qRT-PCR. The involvement of various defense-related pathways was confirmed by phenotypic analysis of mutants affected in these pathways, thereby proving the validity of our approach. Combined with additional anthocyanin analysis data these results all point to the involvement of the phenylpropanoid pathway in T. hamatum T382-induced ISR.

Dufulin activates HrBP1 to produce antiviral responses in tobacco

BACKGROUND

Dufulin is a new antiviral agent that is highly effective against plant viruses and acts by activating systemic acquired resistance (SAR) in plants. In recent years, it has been used widely to prevent and control tobacco and rice viral diseases in China. However, its targets and mechanism of action are still poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

Here, differential in-gel electrophoresis (DIGE) and classical two-dimensional electrophoresis (2-DE) techniques were combined with mass spectrometry (MS) to identify the target of Dufulin. More than 40 proteins were found to be differentially expressed (≥1.5 fold or ≤1.5 fold) upon Dufulin treatment in Nicotiana tabacum K(326). Based on annotations in the Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, these proteins were found to be related to disease resistance. Directed acyclic graph (DAG) analysis of the various pathways demonstrated harpin binding protein-1 (HrBP1) as the target of action of Dufulin. Additionally, western blotting, semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), and real time PCR analyses were also conducted to identify the specific mechanism of action of Dufulin. Our results show that activation of HrBP1 triggers the salicylic acid (SA) signaling pathway and thereby produces antiviral responses in the plant host. A protective assay based on lesion counting further confirmed the antiviral activity of Dufulin.

CONCLUSION

This study identified HrBP1 as a target protein of Dufulin and that Dufulin can activate the SA signaling pathway to induce host plants to generate antiviral responses.

A PR-1-like protein of Fusarium oxysporum functions in virulence on mammalian hosts

The pathogenesis-related PR-1-like protein family comprises secreted proteins from the animal, plant, and fungal kingdoms whose biological function remains poorly understood. Here we have characterized a PR-1-like protein, Fpr1, from Fusarium oxysporum, an ubiquitous fungal pathogen that causes vascular wilt disease on a wide range of plant species and can produce life-threatening infections in immunocompromised humans. Fpr1 is secreted and proteolytically processed by the fungus. The fpr1 gene is required for virulence in a disseminated immunodepressed mouse model, and its function depends on the integrity of the proposed active site of PR-1-like proteins. Fpr1 belongs to a gene family that has expanded in plant pathogenic Sordariomycetes. These results suggest that secreted PR-1-like proteins play important roles in fungal pathogenicity.

Broad-spectrum acquired resistance in barley induced by the Pseudomonas pathosystem shares transcriptional components with Arabidopsis systemic acquired resistance

Inducible resistance responses play a central role in the defense of plants against pathogen attack. Acquired resistance (AR) is induced alongside defense toward primary attack, providing broad-spectrum protection against subsequent pathogen challenge. The localization and molecular basis of AR in cereals is poorly understood, in contrast with the well-characterized systemic acquired resistance (SAR) response in Arabidopsis. Here, we use Pseudomonas syringae as a biological inducer of AR in barley, providing a clear frame of reference to the Arabidopsis-P. syringae pathosystem. Inoculation of barley leaf tissue with the nonadapted P. syringae pv. tomato avrRpm1 (PstavrRpm1) induced an active local defense response. Furthermore, inoculation of barley with PstavrRpm1 resulted in the induction of broad-spectrum AR at a distance from the local lesion, "adjacent" AR, effective against compatible isolates of P. syringae and Magnaporthe oryzae. Global transcriptional profiling of this adjacent AR revealed similarities with the transcriptional profile of SAR in Arabidopsis, as well as transcripts previously associated with chemically induced AR in cereals, suggesting that AR in barley and SAR in Arabidopsis may be mediated by analogous pathways.

Biotin deficiency causes spontaneous cell death and activation of defense signaling

In addition to its essential metabolic functions, biotin has been suggested to play a critical role in regulating gene expression. The first committed enzyme in biotin biosynthesis in Arabidopsis, 7-keto-8-aminopelargonic acid synthase, is encoded by At5g04620 (BIO4). We isolated a T-DNA insertion mutant of BIO4 (bio4-1) with a spontaneous cell death phenotype, which was rescued both by exogenous biotin and genetic complementation. The bio4-1 plants exhibited massive accumulation of hydrogen peroxide and constitutive up-regulation of a number of genes that are diagnostic for defense and reactive oxygen species signaling. The cell-death phenotype was independent of salicylic acid and jasmonate signaling. Interestingly, the observed increase in defense gene expression was not accompanied by enhanced resistance to bacterial pathogens, which may be explained by uncoupling of defense gene transcription from accumulation of the corresponding protein. Characterization of biotinylated protein profiles showed a substantial reduction of both chloroplastic biotinylated proteins and a nuclear biotinylated polypeptide in the mutant. Our results suggest that biotin deficiency results in light-dependent spontaneous cell death and modulates defense gene expression. The isolation and molecular characterization of the bio4-1 mutant provides a valuable tool for elucidating new functions of biotin.

Type III secretion and effectors shape the survival and growth pattern of Pseudomonas syringae on leaf surfaces

The bacterium Pseudomonas syringae pv syringae B728a (PsyB728a) uses a type III secretion system (T3SS) to inject effector proteins into plant cells, a process that modulates the susceptibility of different plants to infection. Analysis of GREEN FLUORESCENT PROTEIN-expressing PsyB728a after spray inoculation without additives under moderate relative humidity conditions permitted (1) a detailed analysis of this strain's survival and growth pattern on host (Nicotiana benthamiana) and nonhost (tomato [Solanum lycopersicum]) leaf surfaces, (2) an assessment of the role of plant defenses in affecting PsyB728a leaf surface (epiphytic) growth, and (3) the contribution of the T3SS and specific effectors to PsyB728a epiphytic survival and growth. On host leaf surfaces, PsyB728a cells initially persist without growing, and show an increased population only after 48 h, unless plants are pretreated with the defense-inducing chemical benzothiazole. During the persistence period, some PsyB728a cells induce a T3SS reporter, whereas a T3SS-deficient mutant shows reduced survival. By 72 h, rare invasion by PsyB728a to the mesophyll region of host leaves occurs, but endophytic and epiphytic bacterial growths are not correlated. The effectors HopZ3 and HopAA1 delay the onset of epiphytic growth of PsyB728a on N. benthamiana, whereas they promote epiphytic survival/growth on tomato. These effectors localize to distinct sites in plant cells and likely have different mechanisms of action. HopZ3 may enzymatically modify host targets, as it requires residues important for the catalytic activity of other proteins in its family of proteases. Thus, the T3SS, HopAA1, HopZ3, and plant defenses strongly influence epiphytic survival and/or growth of PsyB728a.

Non-host defense response in a novel Arabidopsis-Xanthomonas citri subsp. citri pathosystem

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is one of the most destructive diseases of citrus. Progress of breeding citrus canker-resistant varieties is modest due to limited resistant germplasm resources and lack of candidate genes for genetic manipulation. The objective of this study is to establish a novel heterologous pathosystem between Xcc and the well-established model plant Arabidopsis thaliana for defense mechanism dissection and resistance gene identification. Our results indicate that Xcc bacteria neither grow nor decline in Arabidopsis, but induce multiple defense responses including callose deposition, reactive oxygen species and salicylic aicd (SA) production, and defense gene expression, indicating that Xcc activates non-host resistance in Arabidopsis. Moreover, Xcc-induced defense gene expression is suppressed or attenuated in several well-characterized SA signaling mutants including eds1, pad4, eds5, sid2, and npr1. Interestingly, resistance to Xcc is compromised only in eds1, pad4, and eds5, but not in sid2 and npr1. However, combining sid2 and npr1 in the sid2npr1 double mutant compromises resistance to Xcc, suggesting genetic interactions likely exist between SID2 and NPR1 in the non-host resistance against Xcc in Arabidopsis. These results demonstrate that the SA signaling pathway plays a critical role in regulating non-host defense against Xcc in Arabidopsis and suggest that the SA signaling pathway genes may hold great potential for breeding citrus canker-resistant varieties through modern gene transfer technology.

Antagonistic plant defense system regulated by phytohormones assists interactions among vector insect, thrips and a tospovirus

The western flower thrips (Frankliniella occidentalis) is a polyphagous herbivore that causes serious damage to many agricultural plants. In addition to causing feeding damage, it is also a vector insect that transmits tospoviruses such as Tomato spotted wilt virus (TSWV). We previously reported that thrips feeding on plants induces a jasmonate (JA)-regulated plant defense, which negatively affects both the performance and preference (i.e. host plant attractiveness) of the thrips. The antagonistic interaction between a JA-regulated plant defense and a salicylic acid (SA)-regulated plant defense is well known. Here we report that TSWV infection allows thrips to feed heavily and multiply on Arabidopsis plants. TSWV infection elevated SA contents and induced SA-regulated gene expression in the plants. On the other hand, TSWV infection decreased the level of JA-regulated gene expression induced by thrips feeding. Importantly, we also demonstrated that thrips significantly preferred TSWV-infected plants to uninfected plants. In JA-insensitive coi1-1 mutants, however, thrips did not show a preference for TSWV-infected plants. In addition, SA application to wild-type plants increased their attractiveness to thrips. Our results suggest the following mechanism: TSWV infection suppresses the anti-herbivore response in plants and attracts its vector, thrips, to virus-infected plants by exploiting the antagonistic SA-JA plant defense systems.

The Arabidopsis glucosyltransferase UGT76B1 conjugates isoleucic acid and modulates plant defense and senescence

Plants coordinate and tightly regulate pathogen defense by the mostly antagonistic salicylate (SA)- and jasmonate (JA)-mediated signaling pathways. Here, we show that the previously uncharacterized glucosyltransferase UGT76B1 is a novel player in this SA-JA signaling crosstalk. UGT76B1 was selected as the top stress-induced isoform among all 122 members of the Arabidopsis thaliana UGT family. Loss of UGT76B1 function leads to enhanced resistance to the biotrophic pathogen Pseudomonas syringae and accelerated senescence but increased susceptibility toward necrotrophic Alternaria brassicicola. This is accompanied by constitutively elevated SA levels and SA-related marker gene expression, whereas JA-dependent markers are repressed. Conversely, UGT76B1 overexpression has the opposite effect. Thus, UGT76B1 attenuates SA-dependent plant defense in the absence of infection, promotes the JA response, and delays senescence. The ugt76b1 phenotypes were SA dependent, whereas UGT76B1 overexpression indicated that this gene possibly also has a direct effect on the JA pathway. Nontargeted metabolomic analysis of UGT76B1 knockout and overexpression lines using ultra-high-resolution mass spectrometry and activity assays with the recombinant enzyme led to the ab initio identification of isoleucic acid (2-hydroxy-3-methyl-pentanoic acid) as a substrate of UGT76B1. Exogenously applied isoleucic acid increased resistance against P. syringae infection. These findings indicate a novel link between amino acid-related molecules and plant defense that is mediated by small-molecule glucosylation.

Quantitative genetic analysis of salicylic acid perception in Arabidopsis

Salicylic acid (SA) is a phytohormone required for a full resistance against some pathogens in Arabidopsis, and NPR1 (Non-Expressor of Pathogenesis Related Genes 1) is the only gene with a strong effect on resistance induced by SA which has been described. There can be additional components of SA perception that escape the traditional approach of mutagenesis. An alternative to that approach is searching in the natural variation of Arabidopsis. Different methods of analyzing the variation between ecotypes have been tried and it has been found that measuring the growth of a virulent isolate of Pseudomonas syringae after the exogenous application of SA is the most effective one. Two ecotypes, Edi-0 and Stw-0, have been crossed, and their F2 has been studied. There are two significant quantitative trait loci (QTLs) in this population, and there is one QTL in each one of the existing mapping populations Col-4 × Laer-0 and Laer-0 × No-0. They have different characteristics: while one QTL is only detectable at low concentrations of SA, the other acts after the point of crosstalk with methyl jasmonate signalling. Three of the QTLs have candidates described in SA perception as NPR1, its interactors, and a calmodulin binding protein.

ADS1 encodes a MATE-transporter that negatively regulates plant disease resistance

Multidrug and toxic compound extrusion (MATE) proteins comprise the most recently identified family of multidrug transporters. In plants, the numbers of MATE proteins has undergone a remarkable expansion, underscoring the importance of these transporters within this kingdom. Here, we describe the identification and characterization of Activated Disease Susceptibility 1 (ADS1) which encodes a putative MATE transport protein. An activation tagging screen uncovered the ads1-Dominant (ads1-D) mutant, which was subsequently characterized by molecular, genetic and biochemical approaches. The ads1-D mutant was compromised in both basal and nonhost resistance against microbial pathogens. Further, plant defence responses conferred by RPS4 were also disabled in ads1-D plants. By contrast, depletion of ADS1 transcripts by RNA-interference (RNAi) promoted basal disease resistance. Unexpectedly, ads1-D plants were found to constitutively accumulate reactive oxygen intermediates (ROIs). However, analysis of ads1-D Arabidopsis thaliana respiratory burst oxidase (atrboh) double and triple mutants indicated that an increase in ROIs did not impact ads1-D-mediated disease susceptibility. Our findings imply that ADS1 negatively regulates the accumulation of the plant immune activator salicylic acid (SA) and cognate Pathogenesis-Related 1 (PR1) gene expression. Collectively, these data highlight an important role for MATE proteins in the establishment of plant disease resistance.

Detection of histone modifications in plant leaves

Chromatin structure is important for the regulation of gene expression in eukaryotes. In this process, chromatin remodeling, DNA methylation, and covalent modifications on the amino-terminal tails of histones H3 and H4 play essential roles(1-2). H3 and H4 histone modifications include methylation of lysine and arginine, acetylation of lysine, and phosphorylation of serine residues(1-2). These modifications are associated either with gene activation, repression, or a primed state of gene that supports more rapid and robust activation of expression after perception of appropriate signals (microbe-associated molecular patterns, light, hormones, etc.)(3-7). Here, we present a method for the reliable and sensitive detection of specific chromatin modifications on selected plant genes. The technique is based on the crosslinking of (modified) histones and DNA with formaldehyde(8,9), extraction and sonication of chromatin, chromatin immunoprecipitation (ChIP) with modification-specific antibodies(9,10), de-crosslinking of histone-DNA complexes, and gene-specific real-time quantitative PCR. The approach has proven useful for detecting specific histone modifications associated with C(4;) photosynthesis in maize(5,11) and systemic immunity in Arabidopsis(3).

Evaluation of the use of the tobacco PR-1a promoter to monitor defense gene expression by the luciferase bioluminescence reporter system

Because of their marked responsiveness to induction signals, genes encoding pathogenesis-related proteins are used as markers to monitor defense gene expression in plants. To develop a non-invasive bioluminescence reporter assay system, we tested acidic PR-1 gene promoters from tobacco and Arabidopsis. These two promoters share common regulatory elements and are believed to show similar responsiveness to various stimuli but the results of transient expression assays by microprojectile bombardment of various plant cells and npr1 mutant Arabidopsis suggest that the tobacco PR-1a promoter is superior to its Arabidopsis counterpart in terms of responsiveness to salicylic acid treatment. Transgenic Arabidopsis seedlings harboring the tobacco PR-1a promoter fused to firefly luciferase showed marked induction in response to treatment with chemicals that induce defense gene expression in plants. These results suggest that the tobacco PR-1a promoter is applicable in monitoring defense-gene expression in various plant species.

[Construction and analysis of the SSH library with the resistant wheat near-isogenic line and its susceptible parent infected by Puccinia striiformis Westend. f. sp. tritici]

To analyze the differentially expressed genes between resistant and susceptible wheat near-isogenic lines infected by Puccinia striiformis Westend. f. sp. tritici, a subtractive library containing about 1300 clones was constructed using suppression subtractive hybridization (SSH) in which the cDNA from resistant Yr4/6 × Taichung 29 seedlings inoculated with race CY26 was used as the tester, and the corresponding cDNA from susceptible Taichung 29 as the driver. Six hundred clones from the library were analyzed with reverse Northern blot. The positive clones were further tested by Northern blotting analysis. Twelve clones were verified and showed significant difference. By means of sequencing and BlastX analysis, six function-known differentially expressed sequences were detected, and their putative products were leucine-rich repeat protein, catalase, thioredoxin H-type, RNA binding protein, ascorbate peroxidase, and heat shock protein, respectively. Among them, leucine-rich repeat protein belongs to signal transduction protein, and others belong to defense response protein.

Induction of systemic resistance in rice by leaf extracts of Zizyphus jujuba and Ipomoea carnea against Rhizoctonia solani

Plants accumulate a great diversity of natural products, many of which confer protective effects against phytopathogenic attack. Earlier we had demonstrated that the leaf extracts of Zizyphus jujuba and Ipomoea carnea inhibit the in vitro mycelial growth of Rhizoctonia solani, and effectively reduce the incidence of sheath blight disease in rice. Here we demonstrate that foliar application of the aqueous leaf extracts of Z. jujuba and I. carnea followed by challenge inoculation with R. solani induces systemic resistance in rice as evident from significantly increased accumulation of pathogenesis-related proteins such as chitinase, β-1,3-glucanase and peroxidase, as well as defense-related compounds such as phenylalanine ammonia-lyase and phenolic substances. Thin layer chromatographic separation of secondary metabolites revealed presence of alkaloid and terpenoid compounds in the leaf extracts of Z. jujuba that exhibited toxicity against R. solani under in vitro condition. Thus, the enhanced sheath blight resistance in rice seedlings treated with leaf extracts of Z. jujuba or I. carnea can be attributed to the direct inhibitory effects of these leaf extracts as well as their ability to elicit systemic resistance against R. solani.

DNA repair proteins are directly involved in regulation of gene expression during plant immune response

Systemic acquired resistance (SAR), an inducible plant-defense response to local infection, requires the signaling molecule salicylic acid (SA) and the transcriptional coactivator NPR1, with concerted activation of pathogenesis-related (PR) genes. Arabidopsis sni1 is an npr1 suppressor and derepression of defense genes in sni1 causes reduced growth and fertility and increased homologous recombination. Characterizing suppressors of sni1, we identify the DNA damage repair proteins SSN2 and RAD51D as genetic and physical interactors with SNI1. During plant defense, SSN2 and possibly RAD51D replace the transcription repressor SNI1 at pathogenesis-related gene promoters. In the presence of SNI1, NPR1 is also required for SSN2 binding. Thus, coordinated action of SNI1, SSN2-RAD51D, and NPR1 ensures the tight control of plant immune gene expression. Given that the SSN2-RAD51D complex is conserved in eukaryotes, their dual function in homologous recombination and transcription regulation of plant-defense genes suggests a general link between these two stress responses.

Analysis of differential transcriptional profiling in wheat infected by Blumeria graminis f. sp. tritici using GeneChip

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a devastating disease of wheat. The use of wheat cultivars resistant to powdery mildew provides an effective, economical, and environmentally friendly method to control the disease. Previously, we identified a dominant resistance gene, temporarily named Pmhym, from the wheat cultivar Hongyoumai. In order to screen differential transcripts related to Pmhym-mediated resistance, four F3 homozygous resistant and four susceptible progenies derived from the Hongyoumai/Yumai13 cross were selected to construct two different pools, respectively, representing an incompatible and compatible interaction with Bgt. Pre-inoculated control and the pathogen-inoculated treatments at 24 h post inoculation (hpi) were used. Three groups of differential genes were categorized from three comparisons as pre- and post-induced, respectively, in two interactions, and post-induced between incompatible and compatible interaction. It was found that salicylic acid (SA), jasmonate (JA), and ethylene (ET) signaling-related genes were differentially expressed, thus suggesting that they are involved in the defensive response against Bgt infection. In compatible interactions, the genes involved in the abscisic acid (ABA) signaling pathway might be inhibitory to the above-mentioned three pathways, resulting in a susceptible reaction. Genes involved in disease/defense, signal transduction, and reactive oxygen species (ROS) metabolism were up-regulated in incompatible interactions, implying a role in resistant response. The results of qRT-PCR analysis on several candidate genes were consistent in their expression patterns as revealed by microarray analysis. The differential expression analyses in the present study are good candidates for further elucidation of wheat defensive response to powdery mildew.

Nicotiana tabacum overexpressing γ-ECS exhibits biotic stress tolerance likely through NPR1-dependent salicylic acid-mediated pathway

The elaborate networks and the crosstalk of established signaling molecules like salicylic acid (SA), jasmonic acid (JA), ethylene (ET), abscisic acid (ABA), reactive oxygen species (ROS) and glutathione (GSH) play key role in plant defense response. To obtain further insight into the mechanism through which GSH is involved in this crosstalk to mitigate biotic stress, transgenic Nicotiana tabacum overexpressing Lycopersicon esculentum gamma-glutamylcysteine synthetase (LeECS) gene (NtGB lines) were generated with enhanced level of GSH in comparison with wild-type plants exhibiting resistance to pathogenesis as well. The expression levels of non-expressor of pathogenesis-related genes 1 (NPR1)-dependent genes like pathogenesis-related gene 1 (NtPR1), mitogen-activated protein kinase kinase (NtMAPKK), glutamine synthetase (NtGLS) were significantly enhanced along with NtNPR1. However, the expression levels of NPR1-independent genes like NtPR2, NtPR5 and short-chain dehydrogenase/reductase family protein (NtSDRLP) were either insignificant or were downregulated. Additionally, increase in expression of thioredoxin (NtTRXh), S-nitrosoglutathione reductase 1 (NtGSNOR1) and suppression of isochorismate synthase 1 (NtICS1) was noted. Comprehensive analysis of GSH-fed tobacco BY2 cell line in a time-dependent manner reciprocated the in planta results. Better tolerance of NtGB lines against biotrophic Pseudomonas syringae pv. tabaci was noted as compared to necrotrophic Alternaria alternata. Through two-dimensional gel electrophoresis (2-DE) and image analysis, 48 differentially expressed spots were identified and through identification as well as functional categorization, ten proteins were found to be SA-related. Collectively, our results suggest GSH to be a member in cross-communication with other signaling molecules in mitigating biotic stress likely through NPR1-dependent SA-mediated pathway.

Molecular characterization and genomic mapping of the pathogenesis-related protein 1 (PR-1) gene family in hexaploid wheat (Triticum aestivum L.)

The group 1 pathogenesis-related (PR-1) proteins, known as hallmarks of defense pathways, are encoded by multigene families in plants as evidenced by the presence of 22 and 32 PR-1 genes in the finished Arabidopsis and rice genomes, respectively. Here, we report the initial characterization and mapping of 23 PR-1-like (TaPr-1) genes in hexaploid wheat (Triticum aestivum L.), which possesses one of the largest (>16,000 megabases) genomes among monocot crop plants. Sequence analysis revealed that the 23 TaPr-1 genes all contain intron-free open reading frames that encode a signal peptide at the N-terminus and a conserved PR-1-like domain. Phylogenetic analysis indicated that TaPr-1 genes form three major monophyletic groups along with their counterparts in other monocots; each group consists of genes encoding basic, basic with a C-terminal extension, and acidic PR-1 proteins, respectively, suggesting diversity and conservation of PR-1 gene functions in monocot plants. Mapping analysis assisted by untranslated region-specified discrimination (USD) markers and various cytogenetic stocks located the 23 TaPr-1 genes to seven different chromosomes, with the majority mapping to chromosomes of homoeologous groups 5 and 7. Reverse transcriptase (RT)-PCR analysis revealed that 12 TaPr-1 genes were induced or up-regulated upon pathogen challenge. Together, this study provides insights to the origin, evolution, homoeologous relationships, and expression patterns of the TaPr-1 genes. The data presented provide critical information for further genome-wide characterization of the wheat PR-1 gene family and the USD markers developed will facilitate genetic and functional analysis of PR-1 genes associated with plant defense and/or other important traits.

PeaT1-induced systemic acquired resistance in tobacco follows salicylic acid-dependent pathway

Systemic acquired resistance (SAR) is an inducible defense mechanism which plays a central role in protecting plants from pathogen attack. A new elicitor, PeaT1 from Alternaria tenuissima, was expressed in Escherichia coil and characterized with systemic acquired resistance to tobacco mosaic virus (TMV). PeaT1-treated plants exhibited enhanced systemic resistance with a significant reduction in number and size of TMV lesions on wild tobacco leaves as compared with control. The quantitative analysis of TMV CP gene expression with real-time quantitative PCR showed there was reduction in TMV virus concentration after PeaT1 treatment. Similarly, peroxidase (POD) activity and lignin increased significantly after PeaT1 treatment. The real-time quantitative PCR revealed that PeaT1 also induced the systemic accumulation of pathogenesis-related gene, PR-1a and PR-1b which are the markers of systemic acquired resistance (SAR), NPR1 gene for salicylic acid (SA) signal transduction pathway and PAL gene for SA synthesis. The accumulation of SA and the failure in development of similar level of resistance as in wild type tobacco plants in PeaT1 treated nahG transgenic tobacco plants indicated that PeaT1-induced resistance depended on SA accumulation. The present work suggested that the molecular mechanism of PeaT1 inducing disease resistance in tobacco was likely through the systemic acquired resistance pathway mediated by salicylic acid and the NPR1 gene.

Priming for enhanced defence responses by specific inhibition of the Arabidopsis response to coronatine

The priming agent β-aminobutyric acid (BABA) is known to enhance Arabidopsis resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 by potentiating salicylic acid (SA) defence signalling, notably PR1 expression. The molecular mechanisms underlying this phenomenon remain unknown. A genome-wide microarray analysis of BABA priming during Pst DC3000 infection revealed direct and primed up-regulation of genes that are responsive to SA, the SA analogue benzothiadiazole and pathogens. In addition, BABA was found to inhibit the Arabidopsis response to the bacterial effector coronatine (COR). COR is known to promote bacterial virulence by inducing the jasmonic acid (JA) response to antagonize SA signalling activation. BABA specifically repressed the JA response induced by COR without affecting other plant JA responses. This repression was largely SA-independent, suggesting that it is not caused by negative cross-talk between SA and JA signalling cascades. Treatment with relatively high concentrations of purified COR counteracted BABA inhibition. Under these conditions, BABA failed to protect Arabidopsis against Pst DC3000. BABA did not induce priming and resistance in plants inoculated with a COR-deficient strain of Pst DC3000 or in the COR-insensitive mutant coi1-16. In addition, BABA blocked the COR-dependent re-opening of stomata during Pst DC3000 infection. Our data suggest that BABA primes for enhanced resistance to Pst DC3000 by interfering with the bacterial suppression of Arabidopsis SA-dependent defences. This study also suggests the existence of a signalling node that distinguishes COR from other JA responses.

Berry skin development in Norton grape: distinct patterns of transcriptional regulation and flavonoid biosynthesis

BACKGROUND

The complex and dynamic changes during grape berry development have been studied in Vitis vinifera, but little is known about these processes in other Vitis species. The grape variety 'Norton', with a major portion of its genome derived from Vitis aestivalis, maintains high levels of malic acid and phenolic acids in the ripening berries in comparison with V. vinifera varieties such as Cabernet Sauvignon. Furthermore, Norton berries develop a remarkably high level of resistance to most fungal pathogens while Cabernet Sauvignon berries remain susceptible to those pathogens. The distinct characteristics of Norton and Cabernet Sauvignon merit a comprehensive analysis of transcriptional regulation and metabolite pathways.

RESULTS

A microarray study was conducted on transcriptome changes of Norton berry skin during the period of 37 to 127 days after bloom, which represents berry developmental phases from herbaceous growth to full ripeness. Samples of six berry developmental stages were collected. Analysis of the microarray data revealed that a total of 3,352 probe sets exhibited significant differences at transcript levels, with two-fold changes between at least two developmental stages. Expression profiles of defense-related genes showed a dynamic modulation of nucleotide-binding site-leucine-rich repeat (NBS-LRR) resistance genes and pathogenesis-related (PR) genes during berry development. Transcript levels of PR-1 in Norton berry skin clearly increased during the ripening phase. As in other grapevines, genes of the phenylpropanoid pathway were up-regulated in Norton as the berry developed. The most noticeable was the steady increase of transcript levels of stilbene synthase genes. Transcriptional patterns of six MYB transcription factors and eleven structural genes of the flavonoid pathway and profiles of anthocyanins and proanthocyanidins (PAs) during berry skin development were analyzed comparatively in Norton and Cabernet Sauvignon. Transcriptional patterns of MYB5A and MYB5B were similar during berry development between the two varieties, but those of MYBPA1 and MYBPA2 were strikingly different, demonstrating that the general flavonoid pathways are regulated under different MYB factors. The data showed that there were higher transcript levels of the genes encoding flavonoid-3'-O-hydroxylase (F3'H), flavonoid-3',5'-hydroxylase (F3'5'H), leucoanthocyanidin dioxygenase (LDOX), UDP-glucose:flavonoid 3'-O-glucosyltransferase (UFGT), anthocyanidin reductase (ANR), leucoanthocyanidin reductase (LAR) 1 and LAR2 in berry skin of Norton than in those of Cabernet Sauvignon. It was also found that the total amount of anthocyanins was markedly higher in Norton than in Cabernet Sauvignon berry skin at harvest, and five anthocyanin derivatives and three PA compounds exhibited distinctive accumulation patterns in Norton berry skin.

CONCLUSIONS

This study provides an overview of the transcriptome changes and the flavonoid profiles in the berry skin of Norton, an important North American wine grape, during berry development. The steady increase of transcripts of PR-1 and stilbene synthase genes likely contributes to the developmentally regulated resistance during ripening of Norton berries. More studies are required to address the precise role of each stilbene synthase gene in berry development and disease resistance. Transcriptional regulation of MYBA1, MYBA2, MYB5A and MYBPA1 as well as expression levels of their putative targets F3'H, F3'5'H, LDOX, UFGT, ANR, LAR1, and LAR2 are highly correlated with the characteristic anthocyanin and PA profiles in Norton berry skin. These results reveal a unique pattern of the regulation of transcription and biosynthesis pathways underlying the viticultural and enological characteristics of Norton grape, and yield new insights into the understanding of the flavonoid pathway in non-vinifera grape varieties.

Chilling of dormant buds hyperinduces FLOWERING LOCUS T and recruits GA-inducible 1,3-beta-glucanases to reopen signal conduits and release dormancy in Populus

In trees, production of intercellular signals and accessibility of signal conduits jointly govern dormancy cycling at the shoot apex. We identified 10 putative cell wall 1,3-β-glucanase genes (glucan hydrolase family 17 [GH17]) in Populus that could turn over 1,3-β-glucan (callose) at pores and plasmodesmata (PD) and investigated their regulation in relation to FT and CENL1 expression. The 10 genes encode orthologs of Arabidopsis thaliana BG_ppap, a PD-associated glycosylphosphatidylinositol (GPI) lipid-anchored protein, the Arabidopsis PD callose binding protein PDCB, and a birch (Betula pendula) putative lipid body (LB) protein. We found that these genes were differentially regulated by photoperiod, by chilling (5°C), and by feeding of gibberellins GA(3) and GA(4). GA(3) feeding upregulated all LB-associated GH17s, whereas GA(4) upregulated most GH17s with a GPI anchor and/or callose binding motif, but only GA(4) induced true bud burst. Chilling upregulated a number of GA biosynthesis and signaling genes as well as FT, but not CENL1, while the reverse was true for both GA(3) and GA(4). Collectively, the results suggest a model for dormancy release in which chilling induces FT and both GPI lipid-anchored and GA(3)-inducible GH17s to reopen signaling conduits in the embryonic shoot. When temperatures rise, the reopened conduits enable movement of FT and CENL1 to their targets, where they drive bud burst, shoot elongation, and morphogenesis.

NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1) and some NPR1-related proteins are sensitive to salicylic acid

NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1; also known as NIM1) is a master regulator of systemic acquired resistance (SAR). SAR is induced by salicylic acid (SA), leading to the expression of PATHOGENESIS-RELATED (PR) genes. Current evidence suggests that NPR1 is part of a transcription complex tethered to activation sequence-1 (as-1)-like cis-acting elements in PR-1 gene promoters through TGA transcription factors, and that SA-dependent PR-1 gene expression is regulated by NIM1-INTERACTING (NIMIN) proteins. In Arabidopsis, NPR1 is active only after SA induction. Regulation of Arabidopsis NPR1 activity has been proposed to comprise cysteine-156 (Cys-156), mediating SA-induced cytoplasmic oligomer-nuclear monomer exchange, and Cys-521 and Cys-529, mediating SA-dependent transcriptional activation. Tobacco NPR1 does not harbour these residues. To understand the function of tobacco NPR1, we analysed its biochemical capabilities in a heterologous system: yeast. Tobacco NPR1 differs from Arabidopsis NPR1 in its subcellular localization and its transactivation potential. Yet, both tobacco and Arabidopsis NPR1, as well as tobacco NIM1-like1, alter some of their biochemical activities in response to SA. Whereas the addition of SA to yeast growth medium induces transcriptional activity in tobacco NPR1, its interaction with NIMIN2-type proteins is suppressed. The effects of SA are specific, sensitive and occur coordinately. They are abolished completely by mutation of the arginine residue within the invariable penta-amino acid motif LENRV, as present in the nonfunctional Arabidopsis nim1-4 allele. Furthermore, NPR1 proteins with the LENRV domain coincidently harbour a broad and strongly conserved NIMIN1/NIMIN2 binding site. Our data suggest that NPR1 and some NPR1-like proteins are sensitive to the plant hormone SA, altering some of their biochemical capabilities to enable stimulus-dependent gene expression. The sensitivity of NPR1 proteins to SA, together with their differential interaction with diverse NIMIN proteins, seems a plausible molecular basis for the timely and coordinated activation of PR genes during SAR.

Disruption of the vacuolar calcium-ATPases in Arabidopsis results in the activation of a salicylic acid-dependent programmed cell death pathway

Calcium (Ca(2+)) signals regulate many aspects of plant development, including a programmed cell death pathway that protects plants from pathogens (hypersensitive response). Cytosolic Ca(2+) signals result from a combined action of Ca(2+) influx through channels and Ca(2+) efflux through pumps and cotransporters. Plants utilize calmodulin-activated Ca(2+) pumps (autoinhibited Ca(2+)-ATPase [ACA]) at the plasma membrane, endoplasmic reticulum, and vacuole. Here, we show that a double knockout mutation of the vacuolar Ca(2+) pumps ACA4 and ACA11 in Arabidopsis (Arabidopsis thaliana) results in a high frequency of hypersensitive response-like lesions. The appearance of macrolesions could be suppressed by growing plants with increased levels (greater than 15 mm) of various anions, providing a method for conditional suppression. By removing plants from a conditional suppression, lesion initials were found to originate primarily in leaf mesophyll cells, as detected by aniline blue staining. Initiation and spread of lesions could also be suppressed by disrupting the production or accumulation of salicylic acid (SA), as shown by combining aca4/11 mutations with a sid 2 (for salicylic acid induction-deficient2) mutation or expression of the SA degradation enzyme NahG. This indicates that the loss of the vacuolar Ca(2+) pumps by itself does not cause a catastrophic defect in ion homeostasis but rather potentiates the activation of a SA-dependent programmed cell death pathway. Together, these results provide evidence linking the activity of the vacuolar Ca(2+) pumps to the control of a SA-dependent programmed cell death pathway in plants.

Global transcript profiling of transgenic plants constitutively overexpressing the RNA-binding protein AtGRP7

BACKGROUND

The clock-controlled RNA-binding protein AtGRP7 influences circadian oscillations of its own transcript at the post-transcriptional level. To identify additional targets that are regulated by AtGRP7, transcript profiles of transgenic plants constitutively overexpressing AtGRP7 (AtGRP7-ox) and wild type plants were compared.

RESULTS

Approximately 1.4% of the transcripts represented on the Affymetrix ATH1 microarray showed changes in steady-state abundance upon AtGRP7 overexpression. One third of the differentially expressed genes are controlled by the circadian clock, and they show a distinct bias of their phase: The up-regulated genes preferentially peak around dawn, roughly opposite to the AtGRP7 peak abundance whereas the down-regulated genes preferentially peak at the end of the day. Further, transcripts responsive to abiotic and biotic stimuli were enriched among AtGRP7 targets. Transcripts encoding the pathogenesis-related PR1 and PR2 proteins were elevated in AtGRP7-ox plants but not in plants overexpressing AtGRP7 with a point mutation in the RNA-binding domain, indicating that the regulation involves RNA binding activity of AtGRP7. Gene set enrichment analysis uncovered components involved in ribosome function and RNA metabolism among groups of genes upregulated in AtGRP7-ox plants, consistent with its role in post-transcriptional regulation.

CONCLUSION

Apart from regulating a suite of circadian transcripts in a time-of-day dependent manner AtGRP7, both directly and indirectly, affects other transcripts including transcripts responsive to abiotic and biotic stimuli. This suggests a regulatory role of AtGRP7 in the output of the endogenous clock and a complex network of transcripts responsive to external stimuli downstream of the AtGRP7 autoregulatory circuit.

Pseudomonas syringae virulence factor syringolin A counteracts stomatal immunity by proteasome inhibition

The peptide derivative syringolin A, a product of a mixed nonribosomal peptide and polyketide synthetase, is secreted by certain strains of the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Syringolin A was shown to be a virulence factor for P. syringae pv. syringae B728a because disease symptoms on its host Phaseolus vulgaris (bean) were greatly reduced upon inoculation with syringolin A-negative mutants. Syringolin A's mode of action was recently shown to be irreversible proteasome inhibition. Here, we report that syringolin A-producing bacteria are able to open stomata and, thus, counteract stomatal innate immunity in bean and Arabidopsis. Syringolin A-negative mutants, which induce stomatal closure, can be complemented by exogenous addition of not only syringolin A but also MG132, a well-characterized and structurally unrelated proteasome inhibitor. This demonstrates that proteasome activity is crucial for guard cell function. In Arabidopsis, stomatal immunity was salicylic acid (SA)-dependent and required NPR1, a key regulator of the SA-dependent defense pathway whose proteasome-dependent turnover has been reported to be essential for its function. Thus, elimination of NPR1 turnover through proteasome inhibition by syringolin A is an attractive hypothesis to explain the observed inhibition of stomatal immunity by syringolin A.

AtCPK1 calcium-dependent protein kinase mediates pathogen resistance in Arabidopsis

In mammals, lipid bodies play a key role during pathological and infectious diseases. However, our knowledge on the function of plant lipid bodies, apart from their role as the major site of lipid storage in seed tissues, remains limited. Here, we provide evidence that a calcium-dependent protein kinase (CPK) mediates pathogen resistance in Arabidopsis. AtCPK1 expression is rapidly induced by fungal elicitors. Loss-of-function mutants of AtCPK1 exhibit higher susceptibility to pathogen infection compared to wild-type plants. Conversely, over-expression of AtCPK1 leads to accumulation of salicylic acid (SA) and constitutive expression of SA-regulated defence and disease resistance genes, which, in turn, results in broad-spectrum protection against pathogen infection. Expression studies in mutants affected in SA-mediated defence responses revealed an interlocked feedback loop governing AtCPK1 expression and components of the SA-dependent signalling pathway. Moreover, we demonstrate the dual localization of AtCPK1 in lipid bodies and peroxisomes. Overall, our findings identify AtCPK1 as a component of the innate immune system of Arabidopsis plants.

Do ethylene response factorS9 and -14 repress PR gene expression in the interaction between Piriformospora indica and Arabidopsis?

The plant hormone ethylene (ET) plays a crucial role in the signalling network when plants have to respond to biotic stresses. We investigate the beneficial interaction between the model plant Arabidopsis thaliana and the endophytic fungus Piriformospora indica. Recently, we showed that ET signalling and ETHYLENE RESPONSE FACTOR (ERF)1 are important to balance beneficial and nonbeneficial traits in this symbiosis. 147 ERF genes in Arabidopsis encode transcriptional regulators with a variety of functions involved in development, physiological processes as well as plant/microbe interactions. In the beneficial symbiosis between Arabidopsis and P. indica, overexpression of ERF1 activates defence responses, strongly reduces root colonization and thus abolishes the benefits for the plants. Here we show that additional transcription factors of the ERF family, the ERF DOMAIN PROTEIN9 (ERF9) and the ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR14 (ERF14) are involved in the interaction between the two symbionts and are required for growth promotion of the host plant. Expression of these genes is upregulated in colonized wild-type roots. Insertional inactivation of ERF9 and ERF14 diminishes the P. indica-induced growth promotion and activates the expression of the PATHOGENESIS-RELATED (PR)-1 and PR-2 genes. We propose that ERF9 and ERF14 repress PR gene expression in colonized Arabidopsis roots and thus contribute to the establishment of a beneficial interaction.

Brassinosteroid-mediated stress tolerance in Arabidopsis shows interactions with abscisic acid, ethylene and salicylic acid pathways

BACKGROUND

Brassinosteroids (BRs) play crucial roles in plant development and also promote tolerance to a range of abiotic stresses. Although much has been learned about their roles in plant development, the mechanisms by which BRs control plant stress responses and regulate stress-responsive gene expression are not fully known. Since BR interacts with other plant hormones, it is likely that the stress tolerance conferring ability of BR lies in part in its interactions with other stress hormones.

RESULTS

Using a collection of Arabidopsis mutants that are either deficient in or insensitive to abscisic acid (ABA), ethylene (ET), jasmonic acid (JA) and salicylic acid (SA), we studied the effects of 24-epibrassinloide (EBR) on basic thermotolerance and salt tolerance of these mutants. The positive impact of EBR on thermotolerance in proportion to wild type was evident in all mutants studied, with the exception of the SA-insensitive npr1-1 mutant. EBR could rescue the ET-insensitive ein2 mutant from its hypersensitivity to salt stress-induced inhibition of seed germination, but remained ineffective in increasing the survival of eto1-1 (ET-overproducer) and npr1-1 seedlings on salt. The positive effect of EBR was significantly greater in the ABA-deficient aba1-1 mutant as compared to wild type, indicating that ABA masks BR effects in plant stress responses. Treatment with EBR increased expression of various hormone marker genes in both wild type and mutant seedlings, although to different levels.

CONCLUSIONS

These results together indicate that the redox-sensitive protein NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1), a master regulator of SA-mediated defense genes, is likely a critical component of EBR-mediated increase in thermotolerance and salt tolerance, but it is not required for EBR-mediated induction of PR-1 (PATHOGENESIS-RELATED1) gene expression; that BR exerts anti-stress effects independently as well as through interactions with other hormones; that ABA inhibits BR effects during stress; and that BR shares transcriptional targets with other hormones.

The pepper oxidoreductase CaOXR1 interacts with the transcription factor CaRAV1 and is required for salt and osmotic stress tolerance

RAV1 (Related to ABI3/VP1) proteins function as a transcription factor in signal transduction pathways in plants. The yeast-two-hybrid and in vivo coimmunoprecipitation assays identified the pepper (Capsicum annuum) oxidoreductase protein CaOXR1 that physically interacts with the pepper CaRAV1 transcription factor. The AP2 domain of CaRAV1 protein is essential for its direct interaction with CaOXR1. Both CaRAV1 and CaOXR1 proteins co-localize to the nuclei of plant cells. Virus-induced gene silencing of CaRAV1 and CaRAV1/CAOXR1 confers enhanced susceptibility to high salinity and osmotic stresses, which is accompanied by altered expression of the stress marker genes in pepper. Expression of CaAMP1 (pepper antimicrobial protein) and CaOSM1 (pepper osmotin) is suppressed by 1.2-6.6-fold in silenced leaves upon treatment with NaCl or mannitol. Overexpression of CaRAV1, CaOXR1 and CaOXR1/CaRAV1 in Arabidopsis also confers enhanced resistance to the biotrophic oomycete Hyaloperonospora arabidopsidis infection. In addition, CaRAV1- and CaOXR1/CaRAV1-overexpression (OX) Arabidopsis plants are highly tolerant to high salinity and osmotic stress. Together, these results suggest that CaOXR1 protein positively controls CaRAV1-mediated plant defense during biotic and abiotic stresses.

Nuclear targeted AtS40 modulates senescence associated gene expression in Arabidopsis thaliana during natural development and in darkness

The Arabidopsis thaliana AtS40-3 gene belongs to a group of genes sharing the conserved DUF548 sequence motif with up to now unknown function. One member of this group, the barley HvS40, was shown before to play a role in regulation of leaf senescence. Similar as the barley gene, AtS40-3 is induced during senescence and is also regulated in response to dark treatment, ABA, salicylic acid and pathogen attack. By localization of the GUS fusion protein, the AtS40-3 gene was shown to encode a nucleus targeted protein. The s40-3a mutant with a T-DNA insertion in the promoter region of the gene was observed to have a staygreen phenotype. By quantitative real-time PCR analyses expression of the AtS40-3 gene in this mutant was observed to be constitutive and not induced during senescence. This coincided with WRKY53 gene expression in nonsenescent leaves and lowered expression levels of WRKY53 and SAG12 at later stages of development. While in the wildtype expression of AtS40-3 was activated by darkness, in the s40-3a mutant the expression of AtS40-3 stayed at a low level. This coincided with lower expression of the SEN1 and SAG12 genes. In another promoter mutant with a T-DNA insertion further upstream of the coding sequence the levels of AtS40-3 and SAG12 transcripts increased in parallel both in a natural light-dark regime and in darkness. The data on gene expression in promoter T-DNA insertion mutants of the s40-3 gene indicate that AtS40 regulates senescence either by modulation of WRKY53 or by activation of SAG12 independent of WRKY53.

Accumulation of isochorismate-derived 2,3-dihydroxybenzoic 3-O-beta-D-xyloside in arabidopsis resistance to pathogens and ageing of leaves

An intricate network of hormone signals regulates plant development and responses to biotic and abiotic stress. Salicylic acid (SA), derived from the shikimate/isochorismate pathway, is a key hormone in resistance to biotrophic pathogens. Several SA derivatives and associated modifying enzymes have been identified and implicated in the storage and channeling of benzoic acid intermediates or as bioactive molecules. However, the range and modes of action of SA-related metabolites remain elusive. In Arabidopsis, Enhanced Disease Susceptibility 1 (EDS1) promotes SA-dependent and SA-independent responses in resistance against pathogens. Here, we used metabolite profiling of Arabidopsis wild type and eds1 mutant leaf extracts to identify molecules, other than SA, whose accumulation requires EDS1 signaling. Nuclear magnetic resonance and mass spectrometry of isolated and purified compounds revealed 2,3-dihydroxybenzoic acid (2,3-DHBA) as an isochorismate-derived secondary metabolite whose accumulation depends on EDS1 in resistance responses and during ageing of plants. 2,3-DHBA exists predominantly as a xylose-conjugated form (2-hydroxy-3-beta-O-D-xylopyranosyloxy benzoic acid) that is structurally distinct from known SA-glucose conjugates. Analysis of DHBA accumulation profiles in various Arabidopsis mutants suggests an enzymatic route to 2,3-DHBA synthesis that is under the control of EDS1. We propose that components of the EDS1 pathway direct the generation or stabilization of 2,3-DHBA, which as a potentially bioactive molecule is sequestered as a xylose conjugate.

Molecular cloning of a PR-5 like protein gene from cherry tomato and analysis of the response of this gene to abiotic stresses

LePR-5, a putative PR5 like protein gene was amplified from a cherry tomato (Lycopersicon esculentum), which encodes a precursor protein of 250 amino acid residues, and shares high degrees of homology with a number of other PR5 genes. Expression of LePR-5 in different tomato organs was analyzed with Semi-quantitative RT-PCR, showing that LePR-5 expressed at different levels in leaves, stems, roots, flowers and fruits. In addition, expression of LePR-5 under different abiotic stresses was carried out at different time points. Three of the four tested abiotic stimuli, ethophen, salicylic acid and methyl jasmonate, triggered a significant induction of LePR-5 after treatment. However, LePR-5 was weaker induced by abscisic acid than by others. The positive responses of LePR-5 to the three abiotic stimuli suggested that LePR-5 may play an important role in response to abiotic stresses, and it may also be involved in plant defense system against pathogens. In addition, different expression patterns between tomato fruit and seedling suggested that LePR-5 may play a distinctive role in the defensive system protecting tomato fruit and seedling.

Temperature modulates plant defense responses through NB-LRR proteins

An elevated growth temperature often inhibits plant defense responses and renders plants more susceptible to pathogens. However, the molecular mechanisms underlying this modulation are unknown. To genetically dissect this regulation, we isolated mutants that retain disease resistance at a higher growth temperature in Arabidopsis. One such heat-stable mutant results from a point mutation in SNC1, a NB-LRR encoding gene similar to disease resistance (R) genes. Similar mutations introduced into a tobacco R gene, N, confer defense responses at elevated temperature. Thus R genes or R-like genes involved in recognition of pathogen effectors are likely the causal temperature-sensitive component in defense responses. This is further supported by snc1 intragenic suppressors that regained temperature sensitivity in defense responses. In addition, the SNC1 and N proteins had a reduction of nuclear accumulation at elevated temperature, which likely contributes to the inhibition of defense responses. These findings identify a plant temperature sensitive component in disease resistance and provide a potential means to generate plants adapting to a broader temperature range.

The lesion-mimic mutant cpr22 shows alterations in abscisic acid signaling and abscisic acid insensitivity in a salicylic acid-dependent manner

A number of Arabidopsis (Arabidopsis thaliana) lesion-mimic mutants exhibit alterations in both abiotic stress responses and pathogen resistance. One of these mutants, constitutive expresser of PR genes22 (cpr22), which has a mutation in two cyclic nucleotide-gated ion channels, is a typical lesion-mimic mutant exhibiting elevated levels of salicylic acid (SA), spontaneous cell death, constitutive expression of defense-related genes, and enhanced resistance to various pathogens; the majority of its phenotypes are SA dependent. These defense responses in cpr22 are suppressed under high-humidity conditions and enhanced by low humidity. After shifting plants from high to low humidity, the cpr22 mutant, but not the wild type, showed a rapid increase in SA levels followed by an increase in abscisic acid (ABA) levels. Concomitantly, genes for ABA metabolism were up-regulated in the mutant. The expression of a subset of ABA-inducible genes, such as RD29A and KIN1/2, was down-regulated, but that of other genes, like ABI1 and HAB1, was up-regulated in cpr22 after the humidity shift. cpr22 showed reduced responsiveness to ABA not only in abiotic stress responses but also in germination and stomatal closure. Double mutant analysis with nahG plants that degrade SA indicated that these alterations in ABA signaling were attributable to elevated SA levels. Furthermore, cpr22 displayed suppressed drought responses by long-term drought stress. Taken together, these results suggest an effect of SA on ABA signaling/abiotic stress responses during the activation of defense responses in cpr22.

Arabidopsis auxin mutants are compromised in systemic acquired resistance and exhibit aberrant accumulation of various indolic compounds

Systemic acquired resistance is a widespread phenomenon in the plant kingdom that confers heightened and often enduring immunity to a range of diverse pathogens. Systemic immunity develops through activation of plant disease resistance protein signaling networks following local infection with an incompatible pathogen. The accumulation of the phytohormone salicylic acid in systemically responding tissues occurs within days after a local immunizing infection and is essential for systemic resistance. However, our knowledge of the signaling components underpinning signal perception and the establishment of systemic immunity are rudimentary. Previously, we showed that an early and transient increase in jasmonic acid in distal responding tissues was central to effective establishment of systemic immunity. Based upon predicted transcriptional networks induced in naive Arabidopsis (Arabidopsis thaliana) leaves following avirulent Pseudomonas syringae challenge, we show that a variety of auxin mutants compromise the establishment of systemic immunity. Linking together transcriptional and targeted metabolite studies, our data provide compelling evidence for a role of indole-derived compounds, but not auxin itself, in the establishment and maintenance of systemic immunity.

Resistance and biomass in Arabidopsis: a new model for salicylic acid perception

Salicylic acid (SA) is an essential hormone for plant defence and development. SA perception is usually measured by counting the number of pathogens that grow in planta upon an exogenous application of the hormone. A biological SA perception model based on plant fresh weight reduction caused by disease resistance in Arabidopsis thaliana is proposed. This effect is more noticeable when a chemical analogue of SA is used, like Benzothiadiazole (BTH). By spraying BTH several times, a substantial difference in plant biomass is observed when compared with the mock treatment. Such difference is dose-dependent and does not require pathogen inoculation. The model is robust and allows for the comparison of different Arabidopsis ecotypes, recombinant inbreed lines, and mutants. Our results show that two mutants, non-expresser of pathogenesis-related genes 1 (npr1) and auxin resistant 3 (axr3), fail to lose biomass when BTH is applied to them. Further experiments show that axr3 responds to SA and BTH in terms of defence induction. NPR1-related genotypes also confirm the pivotal role of NPR1 in SA perception, and suggest an active program of depletion of resources in the infected tissues.

A protein phosphatase 2C, responsive to the bacterial effector AvrRpm1 but not to the AvrB effector, regulates defense responses in Arabidopsis

Using a proteomics approach, a PP2C-type phosphatase (renamed PIA1, for PP2C induced by AvrRpm1) was identified that accumulates following infection by Pseudomonas syringae expressing the type III effector AvrRpm1, and subsequent activation of the corresponding plant NB-LRR disease resistance protein RPM1. No accumulation of PIA1 protein was seen following infection with P. syringae expressing AvrB, another type III effector that also activates RPM1, although PIA transcripts were observed. Accordingly, mutation of PIA1 resulted in enhanced RPM1 function in response to P. syringae pathover tomato (Pto) DC3000 (avrRpm1) but not to Pto DC3000 (avrB). Thus, PIA1 is a protein marker that distinguishes AvrRpm1- and AvrB-dependent activation of RPM1. AvrRpm1-induced expression of the pathogenesis-related genes PR1, PR2 and PR3, and salicylic acid accumulation were reduced in two pia1 mutants. By contrast, expression of other defense-related genes, including PR5 and PDF1.2 (plant defensin), was elevated in unchallenged pia1 mutants. Hence, PIA1 is required for AvrRpm1-induced responses, and confers dual (both positive and negative) regulation of defense gene expression.

A novel simple extracellular leucine-rich repeat (eLRR) domain protein from rice (OsLRR1) enters the endosomal pathway and interacts with the hypersensitive-induced reaction protein 1 (OsHIR1)

Receptor-like protein kinases (RLKs) containing an extracellular leucine-rich repeat (eLRR) domain, a transmembrane domain and a cytoplasmic kinase domain play important roles in plant disease resistance. Simple eLRR domain proteins structurally resembling the extracellular portion of the RLKs may also participate in signalling transduction and plant defence response. Yet the molecular mechanisms and subcellular localization in regulating plant disease resistance of these simple eLRR domain proteins are still largely unclear. We provided the first experimental evidence to demonstrate the subcellular localization and trafficking of a novel simple eLRR domain protein (OsLRR1) in the endosomal pathway, using both confocal and electron microscopy. Yeast two-hybrid and in vitro pull-down assays show that OsLRR1 interacts with the rice hypersensitive-induced response protein 1 (OsHIR1) which is localized on plasma membrane. The interaction between LRR1 and HIR1 homologs was shown to be highly conserved among different plant species, suggesting a close functional relationship between the two proteins. The function of OsLRR1 in plant defence response was examined by gain-of-function tests using transgenic Arabidopsis thaliana. The protective effects of OsLRR1 against bacterial pathogen infection were shown by the alleviating of disease symptoms, lowering of pathogen titres and higher expression of defence marker genes.

Extracellular ATP is a regulator of pathogen defence in plants

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

The salicylic acid-induced protection of non-climacteric unripe pepper fruit against Colletotrichum gloeosporioides is similar to the resistance of ripe fruit

The anthracnose fungus Colletotrichum gloeosporioides deleteriously affects unripe pepper fruit, but not ripe fruit. Here, we show that the induction of local acquired resistance (LAR) by salicylic acid (SA), 2,6-dichloroisonicotinic acid, or benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester pretreatment protects unripe pepper fruit against the fungus, while jasmonic acid (JA) does not. The SA-mediated LAR in the unripe fruit inhibited the fungal appressoria, resulting in protection against fungal infection. Microarray analysis revealed that 177 of 7,900 cDNA clones showed more than fourfold transcriptional accumulation in SA-treated unripe fruit. The reverse transcription-polymerase chain reaction showed that most of the SA-responsive genes (SRGs) were regulated by SA, but not by JA or ethylene-releasing ethephon. Furthermore, most of the SRGs were preferentially expressed in the ripe fruit. These results suggest that the SA-mediated transcriptional regulation of SRGs has a critical role in the resistance of ripe pepper fruit to fungal infection.

Agrobacterium tumefaciens promotes tumor induction by modulating pathogen defense in Arabidopsis thaliana

Agrobacterium tumefaciens causes crown gall disease by transferring and integrating bacterial DNA (T-DNA) into the plant genome. To examine the physiological changes and adaptations during Agrobacterium-induced tumor development, we compared the profiles of salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and auxin (indole-3-acetic acid [IAA]) with changes in the Arabidopsis thaliana transcriptome. Our data indicate that host responses were much stronger toward the oncogenic strain C58 than to the disarmed strain GV3101 and that auxin acts as a key modulator of the Arabidopsis-Agrobacterium interaction. At initiation of infection, elevated levels of IAA and ET were associated with the induction of host genes involved in IAA, but not ET signaling. After T-DNA integration, SA as well as IAA and ET accumulated, but JA did not. This did not correlate with SA-controlled pathogenesis-related gene expression in the host, although high SA levels in mutant plants prevented tumor development, while low levels promoted it. Our data are consistent with a scenario in which ET and later on SA control virulence of agrobacteria, whereas ET and auxin stimulate neovascularization during tumor formation. We suggest that crosstalk among IAA, ET, and SA balances pathogen defense launched by the host and tumor growth initiated by agrobacteria.

Systemic acquired resistance in soybean is regulated by two proteins, Orthologous to Arabidopsis NPR1

BACKGROUND

Systemic acquired resistance (SAR) is induced in non-inoculated leaves following infection with certain pathogenic strains. SAR is effective against many pathogens. Salicylic acid (SA) is a signaling molecule of the SAR pathway. The development of SAR is associated with the induction of pathogenesis related (PR) genes. Arabidopsis non-expressor of PR1 (NPR1) is a regulatory gene of the SA signal pathway 123. SAR in soybean was first reported following infection with Colletotrichum trancatum that causes anthracnose disease. We investigated if SAR in soybean is regulated by a pathway, similar to the one characterized in Arabidopsis.

RESULTS

Pathogenesis-related gene GmPR1 is induced following treatment of soybean plants with the SAR inducer, 2,6-dichloroisonicotinic acid (INA) or infection with the oomycete pathogen, Phytophthora sojae. In P. sojae-infected plants, SAR was induced against the bacterial pathogen, Pseudomonas syringae pv. glycinea. Soybean GmNPR1-1 and GmNPR1-2 genes showed high identities to Arabidopsis NPR1. They showed similar expression patterns among the organs, studied in this investigation. GmNPR1-1 and GmNPR1-2 are the only soybean homologues of NPR1and are located in homoeologous regions. In GmNPR1-1 and GmNPR1-2 transformed Arabidopsis npr1-1 mutant plants, SAR markers: (i) PR-1 was induced following INA treatment and (ii) BGL2 following infection with Pseudomonas syringae pv. tomato (Pst), and SAR was induced following Pst infection. Of the five cysteine residues, Cys82, Cys150, Cys155, Cys160, and Cys216 involved in oligomer-monomer transition in NPR1, Cys216 in GmNPR1-1 and GmNPR1-2 proteins was substituted to Ser and Leu, respectively.

CONCLUSION

Complementation analyses in Arabidopsis npr1-1 mutants revealed that homoeologous GmNPR1-1 and GmNPR1-2 genes are orthologous to Arabidopsis NPR1. Therefore, SAR pathway in soybean is most likely regulated by GmNPR1 genes. Substitution of Cys216 residue, essential for oligomer-monomer transition of Arabidopsis NPR1, with Ser and Leu residues in GmNPR1-1 and GmNPR1-2, respectively, suggested that there may be differences between the regulatory mechanisms of GmNPR1 and Arabidopsis NPR proteins.