Syringae Pv Tomato Research Articles

Root exudates drive the soil-borne legacy of aboveground pathogen infection

BackgroundPlants are capable of building up beneficial rhizosphere communities as is evidenced by disease-suppressive soils. However, it is not known how and why soil bacterial communities are impacted by plant exposure to foliar pathogens and if such responses might improve plant performance in the presence of the pathogen. Here, we conditioned soil by growing multiple generations (five) of Arabidopsis thaliana inoculated aboveground with Pseudomonas syringae pv tomato (Pst) in the same soil. We then examined rhizosphere communities and plant performance in a subsequent generation (sixth) grown in pathogen-conditioned versus control-conditioned soil. Moreover, we assessed the role of altered root exudation profiles in shaping the root microbiome of infected plants.ResultsPlants grown in conditioned soil showed increased levels of jasmonic acid and improved disease resistance. Illumina Miseq 16S rRNA gene tag sequencing revealed that both rhizosphere and bulk soil bacterial communities were altered by Pst infection. Infected plants exhibited significantly higher exudation of amino acids, nucleotides, and long-chain organic acids (LCOAs) (C > 6) and lower exudation levels for sugars, alcohols, and short-chain organic acids (SCOAs) (C ≤ 6). Interestingly, addition of exogenous amino acids and LCOA also elicited a disease-suppressive response.ConclusionCollectively, our data suggest that plants can recruit beneficial rhizosphere communities via modification of plant exudation patterns in response to exposure to aboveground pathogens to the benefit of subsequent plant generations.

Integrated Phloem Sap mRNA and Protein Expression Analysis Reveals Phytoplasma-infection Responses in Mulberry

To gain insight into the response of mulberry to phytoplasma-infection, the expression profiles of mRNAs and proteins in mulberry phloem sap were examined. A total of 955 unigenes and 136 proteins were found to be differentially expressed between the healthy and infected phloem sap. These differentially expressed mRNAs and proteins are involved in signaling, hormone metabolism, stress responses, etc. Interestingly, we found that both the mRNA and protein levels of the major latex protein-like 329 (MuMLPL329) gene were increased in the infected phloem saps. Expression of the MuMLPL329 gene was induced by pathogen inoculation and was responsive to jasmonic acid. Ectopic expression of MuMLPL329 in Arabidopsis enhances transgenic plant resistance to Botrytis cinerea, Pseudomonas syringae pv tomato DC3000 (Pst. DC3000) and phytoplasma. Further analysis revealed that MuMLPL329 can enhance the expression of some defense genes and might be involved in altering flavonoid content resulting in increased resistance of plants to pathogen infection. Finally, the roles of the differentially expressed mRNAs and proteins and the potential molecular mechanisms of their changes were discussed. It was likely that the phytoplasma-responsive mRNAs and proteins in the phloem saps were involved in multiple pathways of mulberry responses to phytoplasma-infection, and their changes may be partially responsible for some symptoms in the phytoplasma infected plants.

Danger-Associated Peptides Close Stomata by OST1-Independent Activation of Anion Channels in Guard Cells.

The plant elicitor peptides (Peps), a family of damage/danger-associated molecular patterns (DAMPs), are perceived by two receptors, PEPR1 and PEPR2, and contribute to plant defense against pathogen attack and abiotic stress. Here, we show that the Peps-PEPR signaling pathway functions in stomatal immunity by activating guard cell anion channels in Arabidopsis thaliana The mutant plants lacking both PEPR1 and PEPR2 (pepr1 pepr2) displayed enhanced bacterial growth after being sprayed with Pseudomonas syringae pv tomato (Pst) DC3000, but not after pathogen infiltration into leaves, implicating PEPR function in stomatal immunity. Indeed, synthetic Arabidopsis Peps (AtPeps) effectively induced stomatal closure in wild-type but not pepr1 pepr2 mutant leaves, suggesting that the AtPeps-PEPR signaling pathway triggers stomatal closure. Consistent with this finding, patch-clamp recording revealed AtPep1-induced activation of anion channels in the guard cells of wild-type but not pepr1 pepr2 mutant plants. We further identified two guard cell-expressed anion channels, SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAH3, as functionally overlapping components responsible for AtPep1-induced stomatal closure. The slac1 slah3 double mutant, but not slac1 or slah3 single mutants, failed to respond to AtPep1 in stomatal closure assays. Interestingly, disruption of OPEN STOMATA1 (OST1), an essential gene for abscisic acid-triggered stomatal closure, did not affect the AtPep1-induced anion channel activity and stomatal response. Together, these results illustrate a DAMP-triggered signaling pathway that, unlike the flagellin22-FLAGELLIN-SENSITIVE2 pathway, triggers stomata immunity through an OST1-independent mechanism.

Arabidopsis nonresponding to oxylipins locus NOXY7 encodes a yeast GCN1 homolog that mediates noncanonical translation regulation and stress adaptation

Stress adaptation and translational regulation was studied using noxy7 (nonresponding to oxylipins7) from a series of Arabidopsis thaliana mutants. We identified the noxy7 mutation in At1g64790, which encodes a homolog of the yeast translational regulator General Control Nonderepressible1 (GCN1) that activates the GCN2 kinase; GCN2 in turn phosphorylates the α subunit of the translation initiation factor eIF2. This regulatory circuit is conserved in yeast and mammals, in which phosphorylated eIF2α (P-eIF2α) facilitates stress adaptation by inhibiting protein synthesis. In phenotypic and de novo protein synthesis studies with Arabidopsis mutants, we found that NOXY7/GCN1 and GCN2 mediate P-eIF2α formation and adaptation to amino acid deprivation; however, P-eIF2α formation is not linked to general protein synthesis arrest. Additional evidence suggested that NOXY7/GCN1 but not GCN2 regulates adaptation to mitochondrial dysfunction, high boron concentration, and activation of plant immunity to infection by Pseudomonas syringae pv tomato (Pst). In these responses, NOXY7/GCN1 acts with GCN20 to regulate translation in a noncanonical pathway independently of GCN2 and P-eIF2α. These results show the lesser functional relevance of GCN2 and P-eIF2α in plants relative to other eukaryotes and highlight the prominent role of NOXY7/GCN1 and GCN20 in regulation of translation and stress adaptation in plants.

Bacteria Exploit Autophagy for Proteasome Degradation and Enhanced Virulence in Plants.

Autophagy and the ubiquitin-proteasome system (UPS) are two major protein degradation pathways implicated in the response to microbial infections in eukaryotes. In animals, the contribution of autophagy and the UPS to antibacterial immunity is well documented and several bacteria have evolved measures to target and exploit these systems to the benefit of infection. In plants, the UPS has been established as a hub for immune responses and is targeted by bacteria to enhance virulence. However, the role of autophagy during plant-bacterial interactions is less understood. Here, we have identified both pro- and antibacterial functions of autophagy mechanisms upon infection of Arabidopsis thaliana with virulent Pseudomonas syringae pv tomato DC3000 (Pst). We show that Pst activates autophagy in a type III effector (T3E)-dependent manner and stimulates the autophagic removal of proteasomes (proteaphagy) to support bacterial proliferation. We further identify the T3E Hrp outer protein M1 (HopM1) as a principle mediator of autophagy-inducing activities during infection. In contrast to the probacterial effects of Pst-induced proteaphagy, NEIGHBOR OF BRCA1-dependent selective autophagy counteracts disease progression and limits the formation of HopM1-mediated water-soaked lesions. Together, we demonstrate that distinct autophagy pathways contribute to host immunity and bacterial pathogenesis during Pst infection and provide evidence for an intimate crosstalk between proteasome and autophagy in plant-bacterial interactions.

Alternative oxidase functions in the excess excitation energy-induced resistance to pathogen infection

Although excess excitation energy (EEE) can damage the photosynthetic apparatus and deregulate many cellular processes, some studies have reported that EEE can be used by plants to optimize the resistance to pathogen infection. Here, we investigated whether the EEE-induced resistance to pathogen infection might be mediated by mitochondrial alternative oxidase (AOX). The present work showed that exposure of Arabidopsis (Arabidopsis thaliana) leaves to short-term excess light or treatment of Arabidopsis leaves with the electron transport inhibitor, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), restricted the growth of virulent biotrophic bacteria, Pseudomonas syringae pv tomato (Pst) DC3000. Exposure to short-term excess light or DBMIB treatment also enhanced the level of the capacity of the cyanide-resistant respiration of the infected leaves. The lack of AOX1a gene in the AtAOX1a anti-sense line attenuated the excess light- or DBMIB-induced resistance toward Pst DC3000. These results indicate that AOX may play a role in regulating the EEE-induced resistance to virulent pathogen infection.

BRASSINOSTEROID-SIGNALING KINASE1 Phosphorylates MAPKKK5 to Regulate Immunity in Arabidopsis.

Arabidopsis (Arabidopsis thaliana) immune receptor FLAGELLIN SENSING2 (FLS2) rapidly forms a complex to activate pathogen-associated molecular pattern-triggered immunity (PTI) upon perception of the bacterial protein flagellin. The receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALINGKINASE1 (BSK1) interacts with FLS2 and is critical for the activation of PTI. However, it is unknown how BSK1 transduces signals to activate downstream immune responses. We identified MEK Kinase5 (MAPKKK5) as a potential substrate of BSK1 by whole-genome phosphorylation analysis. In addition, we demonstrated that BSK1 interacts with and phosphorylates MAPKKK5. In the bsk1-1 mutant, the Ser-289 residue of MAPKKK5 was not phosphorylated as it was in the wild type. Similar to the bsk1 mutant, the mapkkk5 mutant displayed enhanced susceptibility to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae pv tomato DC3000, and to the fungal powdery mildew pathogen Golovinomyces cichoracearum Phosphorylation of the Ser-289 residue is not involved in MAPKKK5-triggered cell death but is critical for MAPKKK5-mediated resistance to both bacterial and fungal pathogens. Furthermore, MAPKKK5 interacts with multiple MAPK kinases, including MKK1, MKK2, MKK4, MKK5, and MKK6. Overall, these results indicate that BSK1 regulates plant immunity by phosphorylating MAPKKK5 and suggest a direct regulatory mode of signaling from the immune complex to the MAPK cascade.

Phosphorylation of Arabidopsis MAP Kinase Phosphatase 1 (MKP1) Is Required for PAMP Responses and Resistance against Bacteria.

Plants perceive potential pathogens via the recognition of pathogen-associated molecular patterns (PAMPs) by surface-localized pattern recognition receptors, which initiates a series of intracellular responses that ultimately limit bacterial growth. PAMP responses include changes in intracellular protein phosphorylation, including the activation of mitogen-activated protein kinase (MAPK) cascades. MAP kinase phosphatases (MKPs), such as Arabidopsis (Arabidopsis thaliana) MKP1, are important negative regulators of MAPKs and play a crucial role in controlling the intensity and duration of MAPK activation during innate immune signaling. As such, the mkp1 mutant lacking MKP1 displays enhanced PAMP responses and resistance against the virulent bacterium Pseudomonas syringae pv tomato DC3000. Previous in vitro studies showed that MKP1 can be phosphorylated and activated by MPK6, suggesting that phosphorylation may be an important mechanism for regulating MKP1. We found that MKP1 was phosphorylated during PAMP elicitation and that phosphorylation stabilized the protein, resulting in protein accumulation after elicitation. MKP1 also can be stabilized by the proteasome inhibitor MG132, suggesting that MKP1 is constitutively degraded through the proteasome in the resting state. In addition, we investigated the role of MKP1 posttranslational regulation in plant defense by testing whether phenotypes of the mkp1 Arabidopsis mutant could be complemented by expressing phosphorylation site mutations of MKP1. The phosphorylation of MKP1 was found to be required for some, but not all, of MKP1's functions in PAMP responses and defense against bacteria. Together, our results provide insight into the roles of phosphorylation in the regulation of MKP1 during PAMP signaling and resistance to bacteria.

CAMTA-Mediated Regulation of Salicylic Acid Immunity Pathway Genes in Arabidopsis Exposed to Low Temperature and Pathogen Infection.

Arabidopsis thaliana calmodulin binding transcription activator (CAMTA) factors repress the expression of genes involved in salicylic acid (SA) biosynthesis and SA-mediated immunity in healthy plants grown at warm temperature (22°C). This repression is overcome in plants exposed to low temperature (4°C) for more than a week and in plants infected by biotrophic and hemibiotrophic pathogens. Here, we present evidence that CAMTA3-mediated repression of SA pathway genes in nonstressed plants involves the action of an N-terminal repression module (NRM) that acts independently of calmodulin (CaM) binding to the IQ and CaM binding (CaMB) domains, a finding that is contrary to current thinking that CAMTA3 repression activity requires binding of CaM to the CaMB domain. Induction of SA pathway genes in response to low temperature did not occur in plants expressing only the CAMTA3-NRM region of the protein. Mutational analysis provided evidence that the repression activity of the NRM was suppressed by action of the IQ and CaMB domains responding to signals generated in response to low temperature. Plants expressing the CAMTA3-NRM region were also impaired in defense against the bacterial hemibiotrophic pathogen Pseudomonas syringae pv tomato DC3000. Our results indicate that the regulation of CAMTA3 repression activity by low temperature and pathogen infection involves related mechanisms, but with distinct differences.

S5H/DMR6 Encodes a Salicylic Acid 5-Hydroxylase That Fine-Tunes Salicylic Acid Homeostasis.

The phytohormone salicylic acid (SA) plays essential roles in biotic and abiotic responses, plant development, and leaf senescence. 2,5-Dihydroxybenzoic acid (2,5-DHBA or gentisic acid) is one of the most commonly occurring aromatic acids in green plants and is assumed to be generated from SA, but the enzymes involved in its production remain obscure. DMR6 (Downy Mildew Resistant6; At5g24530) has been proven essential in plant immunity of Arabidopsis (Arabidopsis thaliana), but its biochemical properties are not well understood. Here, we report the discovery and functional characterization of DMR6 as a salicylic acid 5-hydroxylase (S5H) that catalyzes the formation of 2,5-DHBA by hydroxylating SA at the C5 position of its phenyl ring in Arabidopsis. S5H/DMR6 specifically converts SA to 2,5-DHBA in vitro and displays higher catalytic efficiency (Kcat/Km = 4.96 × 104 m-1 s-1) than the previously reported S3H (Kcat/Km = 6.09 × 103 m-1 s-1) for SA. Interestingly, S5H/DMR6 displays a substrate inhibition property that may enable automatic control of its enzyme activities. The s5h mutant and s5hs3h double mutant overaccumulate SA and display phenotypes such as a smaller growth size, early senescence, and a loss of susceptibility to Pseudomonas syringae pv tomato DC3000. S5H/DMR6 is sensitively induced by SA/pathogen treatment and is expressed widely from young seedlings to senescing plants, whereas S3H is more specifically expressed at the mature and senescing stages. Collectively, our results disclose the identity of the enzyme required for 2,5-DHBA formation and reveal a mechanism by which plants fine-tune SA homeostasis by mediating SA 5-hydroxylation.

WRKY70 and its homolog WRKY54 negatively modulate the cell wall-associated defenses to necrotrophic pathogens in Arabidopsis.

Previous studies have identified the Arabidopsis thaliana transcription factor WRKY70 as a node of convergence for salicylic acid (SA) and jasmonic acid (JA)-mediated defense signal pathways and, together with its closest homolog WRKY54, as a negative regulator of SA biosynthesis. Here, we demonstrate that WRKY70 together with WRKY54 negatively affect the response of Arabidopsis to the necrotrophic pathogens Pectobacterium carotovorum and Botrytis cinerea, but not to the hemibiotroph Pseudomonas syringae pv tomato (Pst) DC3000, as revealed by mutants studies. Unstressed wrky54wrky70 double mutants exhibited increased levels of SA, accumulation of hydrogen peroxide (H2O2) and up-regulated expression of both SA and JA/ethylene (ET) responsive defense related genes. Additionally, protein cross-linking in cell wall was promoted by endogenous SA, suggesting involvement of wall-associated defenses against necrotrophs. This response to necrotrophs was compromised by introducing the sid2-1 allele impairing SA biosynthesis and leading to reduction of H2O2 content and of defense gene expression. The data suggest that the elevated SA level in the wrky54wrky70 double mutant results in moderate accumulation of H2O2, in promoting cell wall fortification and consequently enhanced resistance to necrotrophs but is not sufficient to trigger hypersensitive reaction (HR)-like cell death and resistance to biotrophs/hemibiotrophs like Pst DC3000.

Holaphyllamine, a steroid, is able to induce defense responses in Arabidopsis thaliana and increases resistance against bacterial infection.

A chemical screen of plant-derived compounds identified holaphyllamine, a steroid, able to trigger defense responses in Arabidopsis thaliana and improve resistance against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. A chemical screen of 1600 plant-derived compounds was conducted and allowed the identification of a steroid able to activate defense responses in A. thaliana at a concentration of 1µM without altering growth. The identified compound is holaphyllamine (HPA) whose chemical structure is similar to steroid pregnanes of mammals. Our data show that HPA, which is not constitutively present in A. thaliana, is able to trigger the formation of reactive oxygen species, deposition of callose and expression of several pathogenesis-related genes of the salicylic and jasmonic acid pathways. In addition, the results show that pre-treatment of A. thaliana seedlings with HPA before infection with the pathogenic bacterium Pseudomonas syringae pv tomato DC3000 results in a significant reduction of symptoms (i.e., reduction of bacterial colonies). Using A. thaliana mutants, we have found that the activation of defense responses by HPA does not depend on BRI1/BAK1 receptor kinases. Finally, a structure/function study reveals that the minimal structure required for activity is a 5-pregnen-20-one steroid with an equatorial nucleophilic group in C-3. Together, these findings demonstrate that HPA can activate defense responses that lead to improved resistance against bacterial infection in A. thaliana.

A Lectin Receptor-Like Kinase Mediates Pattern-Triggered Salicylic Acid Signaling.

Plant surface-localized pathogen recognition receptors (PRRs) perceive conserved microbial features, termed pathogen-associated molecular patterns (PAMPs), resulting in disease resistance. PAMP perception leads to calcium influx, MAPK activation, a burst of reactive oxygen species (ROS) mediated by RbohD, accumulation of the defense hormone salicylic acid (SA), and callose deposition. Lectin receptor-like kinases (LecRKs) belong to a specific PRR family and are important players in plant innate immunity. Here, we report that LecRK-IX.2 is a positive regulator of PRR-triggered immunity. Pathogen infection activated the transcription of Arabidopsis (Arabidopsis thaliana) LecRK-IX.2, and the LecRK-IX.2 knockout lines exhibited enhanced susceptibility to virulent Pseudomonas syringae pv tomato DC3000. In addition, LecRK-IX.2 is capable of inducing RbohD phosphorylation, likely by recruiting calcium-dependent protein kinases to trigger ROS production in Arabidopsis. Overexpression of LecRK-IX.2 resulted in elevated ROS and SA and enhanced systemic acquired resistance to P. syringae pv tomato DC3000. Our data highlight the importance of LecRKs in plant immune signaling and SA accumulation.

Pseudomonas syringae pv. tomato DC3000 growth in multiple gene knockouts predicts interactions among hormonal, biotic and abiotic stress responses

Among other environmental factors, phytopathogens have key roles in limiting global crop production. While an in-depth understanding of plant-pathogen interactions is therefore necessary to develop sustainable crop protection strategies, the genetic basis of plant-pathogen interactions is poorly understood. In this study, we tested the growth and resistance of an important phytopathogen, Pseudomonas syringae pv tomato DC3000 (Pst DC3000), in 80 gene knockouts, identified previously through genome-wide association mapping of global Arabidopsis thaliana populations. Amongst these, most of the genes are previously known to regulate multiple functions in plants including cellular mechanisms, growth, reproduction, hormonal signaling, and tolerance to both biotic and abiotic stresses (extreme temperature, drought, salinity, pollutants, and nutrient stresses, etc.). The Pst DC3000 demonstrated two distinct growth patterns in all gene knockouts, showing maximum growth after 4 or 7 days of inoculation (DOI). Mostly, the wild-type (Col-0) demonstrated higher phytopathogen growth than almost all gene knockouts after 4 or 7 DOI. The differential abundance of Pst DC3000 in the multiple gene knockouts reveals a number of new genes important for plant defense. Meanwhile, our results and a review of previous literature suggest interaction of plant defense with plant growth, reproduction, hormonal and abiotic stress tolerance. Thus in future studies, a mechanistic understanding of the genetic, ecological and physiological basis of plant-pathogen interactions, and pleiotropic interactions may prove instrumental in developing sustainable plant protection practices.

A Subset of Ubiquitin-Conjugating Enzymes Is Essential for Plant Immunity.

Of the three classes of enzymes involved in ubiquitination, ubiquitin-conjugating enzymes (E2) have been often incorrectly considered to play merely an auxiliary role in the process, and few E2 enzymes have been investigated in plants. To reveal the role of E2 in plant innate immunity, we identified and cloned 40 tomato genes encoding ubiquitin E2 proteins. Thioester assays indicated that the majority of the genes encode enzymatically active E2. Phylogenetic analysis classified the 40 tomato E2 enzymes into 13 groups, of which members of group III were found to interact and act specifically with AvrPtoB, a Pseudomonas syringae pv tomato effector that uses its ubiquitin ligase (E3) activity to suppress host immunity. Knocking down the expression of group III E2 genes in Nicotiana benthamiana diminished the AvrPtoB-promoted degradation of the Fen kinase and the AvrPtoB suppression of host immunity-associated programmed cell death. Importantly, silencing group III E2 genes also resulted in reduced pattern-triggered immunity (PTI). By contrast, programmed cell death induced by several effector-triggered immunity elicitors was not affected on group III-silenced plants. Functional characterization suggested redundancy among group III members for their role in the suppression of plant immunity by AvrPtoB and in PTI and identified UBIQUITIN-CONJUGATING11 (UBC11), UBC28, UBC29, UBC39, and UBC40 as playing a more significant role in PTI than other group III members. Our work builds a foundation for the further characterization of E2s in plant immunity and reveals that AvrPtoB has evolved a strategy for suppressing host immunity that is difficult for the plant to thwart.

NAD Acts as an Integral Regulator of Multiple Defense Layers.

Pyridine nucleotides, such as NAD, are crucial redox carriers and have emerged as important signaling molecules in stress responses. Previously, we have demonstrated in Arabidopsis (Arabidopsis thaliana) that the inducible NAD-overproducing nadC lines are more resistant to an avirulent strain of Pseudomonas syringae pv tomato (Pst-AvrRpm1), which was associated with salicylic acid-dependent defense. Here, we have further characterized the NAD-dependent immune response in Arabidopsis. Quinolinate-induced stimulation of intracellular NAD in transgenic nadC plants enhanced resistance against a diverse range of (a)virulent pathogens, including Pst-AvrRpt2, Dickeya dadantii, and Botrytis cinerea Characterization of the redox status demonstrated that elevated NAD levels induce reactive oxygen species (ROS) production and the expression of redox marker genes of the cytosol and mitochondrion. Using pharmacological and reverse genetics approaches, we show that NAD-induced ROS production functions independently of NADPH oxidase activity and light metabolism but depends on mitochondrial respiration, which was increased at higher NAD. We further demonstrate that NAD primes pathogen-induced callose deposition and cell death. Mass spectrometry analysis reveals that NAD simultaneously induces different defense hormones and that the NAD-induced metabolic profiles are similar to those of defense-expressing plants after treatment with pathogen-associated molecular patterns. We thus conclude that NAD triggers metabolic profiles rather similar to that of pathogen-associated molecular patterns and discuss how signaling cross talk between defense hormones, ROS, and NAD explains the observed resistance to pathogens.

The Proteasome Acts as a Hub for Plant Immunity and Is Targeted by Pseudomonas Type III Effectors.

Recent evidence suggests that the ubiquitin-proteasome system is involved in several aspects of plant immunity and that a range of plant pathogens subvert the ubiquitin-proteasome system to enhance their virulence. Here, we show that proteasome activity is strongly induced during basal defense in Arabidopsis (Arabidopsis thaliana). Mutant lines of the proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae pv tomato DC3000 (Pst) and Pseudomonas syringae pv maculicola ES4326. Both proteasome subunits are required for pathogen-associated molecular pattern-triggered immunity responses. Analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome also plays an important role in defense priming and SAR In addition, we show that Pst inhibits proteasome activity in a type III secretion-dependent manner. A screen for type III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1, and HopG1 as putative proteasome inhibitors. Biochemical characterization of HopM1 by mass spectrometry indicates that HopM1 interacts with several E3 ubiquitin ligases and proteasome subunits. This supports the hypothesis that HopM1 associates with the proteasome, leading to its inhibition. Thus, the proteasome is an essential component of pathogen-associated molecular pattern-triggered immunity and SAR, which is targeted by multiple bacterial effectors.

Global Transcriptional Analysis Reveals Unique and Shared Responses in Arabidopsis thaliana Exposed to Combined Drought and Pathogen Stress.

With frequent fluctuations in global climate, plants are exposed to co-occurring drought and pathogen infection and this combination adversely affects plant survival. In the past, some studies indicated that morpho-physiological responses of plants to the combined stress are different from the individual stressed plants. However, interaction of drought stressed plants with pathogen has not been widely studied at molecular level. Such studies are important to understand the defense pathways that operate as part of combined stress tolerance mechanism. In this study, Arabidopsis thaliana was exposed to individual drought stress, Pseudomonas syringae pv tomato DC3000 (Pst DC3000) infection and their combination. Using Affymetrix WT gene 1.0 ST array, global transcriptome profiling of leaves under individual drought stress and pathogen infection was compared with their combination. The results obtained from pathway mapping (KAAS and MAPMAN) demonstrated the modulation in defense pathways in A. thaliana under drought and host pathogen Pst DC3000 infection. Further, our study revealed “tailored” responses under combined stress and the time of occurrence of each stress during their concurrence has shown differences in transcriptome profile. Our results from microarray and RT-qPCR revealed regulation of 20 novel genes uniquely during the stress interaction. This study indicates that plants exposed to concurrent drought and pathogen stress experience a new state of stress. Thus, under frequently changing climatic conditions, time of occurrence of each stress in the interaction defines the plant responses and should thus be studied explicitly.

Alexander M. Jones

Alexander M. Jones

Transcriptional Dynamics Driving MAMP-Triggered Immunity and Pathogen Effector-Mediated Immunosuppression in Arabidopsis Leaves Following Infection with Pseudomonas syringae pv tomato DC3000.

Transcriptional reprogramming is integral to effective plant defense. Pathogen effectors act transcriptionally and posttranscriptionally to suppress defense responses. A major challenge to understanding disease and defense responses is discriminating between transcriptional reprogramming associated with microbial-associated molecular pattern (MAMP)-triggered immunity (MTI) and that orchestrated by effectors. A high-resolution time course of genome-wide expression changes following challenge with Pseudomonas syringae pv tomato DC3000 and the nonpathogenic mutant strain DC3000hrpA- allowed us to establish causal links between the activities of pathogen effectors and suppression of MTI and infer with high confidence a range of processes specifically targeted by effectors. Analysis of this information-rich data set with a range of computational tools provided insights into the earliest transcriptional events triggered by effector delivery, regulatory mechanisms recruited, and biological processes targeted. We show that the majority of genes contributing to disease or defense are induced within 6 h postinfection, significantly before pathogen multiplication. Suppression of chloroplast-associated genes is a rapid MAMP-triggered defense response, and suppression of genes involved in chromatin assembly and induction of ubiquitin-related genes coincide with pathogen-induced abscisic acid accumulation. Specific combinations of promoter motifs are engaged in fine-tuning the MTI response and active transcriptional suppression at specific promoter configurations by P. syringae.