Pattern-triggered Immunity Signaling Research Articles

A receptor for dual ligands governs plant immunity and hormone response and is targeted by a nematode effector

In this study, we show that the potato (Solanum tuberosum) pattern recognition receptor (PRR) NEMATODE-INDUCED LEUCINE-RICH REPEAT (LRR)-RLK1 (StNILR1) functions as a dual receptor, recognizing both nematode-associated molecular pattern ascaroside #18 (Ascr18) and plant hormone brassinosteroid (BR) to activate two different physiological outputs: pattern-triggered immunity (PTI) and BR response. Ascr18/BR-StNILR1 signaling requires the coreceptor potato BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (StBAK1) and perception of either ligand strengthens StNILR1 interaction with StBAK1 in plant cells. Significantly, the parasitically successful potato cyst nematode (Globodera pallida) utilizes the effector RHA1B, which is a functional ubiquitin ligase, to target StNILR1 for ubiquitination-mediated proteasome-dependent degradation, thereby countering Ascr18/BR-StNILR1-mediated PTI in potato and facilitating nematode parasitism. These findings broaden our understanding of PRR specificity and reveal a nematode parasitic mechanism that targets a PTI signaling pathway.

Rice NLR protein XinN1, induced by a pattern recognition receptor XA21, confers enhanced resistance to bacterial blight.

Rice CC-type NLR XinN1, specifically induced by a PRR XA21, activates defense pathways against Xoo. Plants have evolved two layers of immune systems regulated by two different types of immune receptors, cell surface located pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat-containing receptors (NLRs). Plant PRRs recognize conserved molecular patterns from diverse pathogens, resulting in pattern-triggered immunity (PTI), whereas NLRs are activated by effectors secreted by pathogens into plant cells, inducing effector-triggered immunity (ETI). Rice PRR, XA21, recognizes a tyrosine-sulfated RaxX peptide (required for activation of XA21-mediated immunity X) as a molecular pattern secreted by Xanthomonas oryzae pv. oryzae (Xoo). Here, we identified a rice NLR gene, XinN1, that is specifically induced during the XA21-mediated immune response against Xoo. Transgenic rice plants overexpressing XinN1 displayed increased resistance to infection by Xoo with reduced lesion length and bacterial growth. Overexpression of autoactive mutant of XinN1 (XinN1D543V) also displayed increased resistance to Xoo, accompanied with severe growth retardation and cell death. In rice protoplast system, overexpression of XinN1 or XinN1D543V significantly elevated reactive oxygen species (ROS) production and cytosolic-free calcium (Ca2+) accumulations. In addition, XinN1 overexpression additionally elevated the ROS burst caused by the interaction between XA21 and RaxX-sY and induced the transcription of PTI signaling components, including somatic embryogenesis receptor kinases (OsSERKs) and receptor-like cytoplasmic kinases (OsRLCKs). Our results suggest that XinN1 induced by the PRR XA21 activates defense pathways and provides enhanced resistance to Xoo in rice.

Integrated proteomic analysis reveals interactions between phosphorylation and ubiquitination in rose response to Botrytis infection.

As two of the most abundant post-translational modifications, phosphorylation and ubiquitination play a significant role in modulating plant-pathogen interactions and increasing evidence indicates their crosstalk in plant immunity. Rose (Rosa sp.) is one of the most important ornamental plants and can be seriously infected by Botrytis cinerea. Here, integrated proteomics analysis was performed to detect global proteome, phosphorylation, and ubiquitination changes in rose upon B. cinerea infection and investigate the possible phosphorylation and ubiquitination crosstalk. A total of 6165 proteins, 11 774 phosphorylation and 10 582 ubiquitination sites, and 77 phosphorylation and 13 ubiquitination motifs were identified. Botrytis cinerea infection resulted in 169 up-regulated and 122 down-regulated proteins, 291 up-regulated and 404 down-regulated phosphorylation sites, and 250 up-regulated and 634 down-regulated ubiquitination sites. There were 12 up-regulated PR10 proteins and half of them also showed reduced ubiquitination. A lot of kinases probably involved in plant pattern-triggered immunity signaling were up-regulated phosphoproteins. Noticeably, numerous kinases and ubiquitination-related proteins also showed a significant change in ubiquitination and phosphorylation, respectively. A cross-comparison of phosphoproteome and ubiquitylome indicated that both of two post-translational modifications of 104 proteins were dynamically regulated, and many putative pattern-triggered immunity signaling components in the plant plasma membrane were co-regulated. Moreover, five selected proteins, including four PR10 proteins and a plasma membrane aquaporin, were proven to be involved in rose resistance to B. cinerea. Our study provides insights into the molecular mechanisms underlying rose resistance to B. cinerea and also increases the database of phosphorylation and ubiquitination sites in plants.

Bringing Plant Immunity to Light: A Genetically Encoded, Bioluminescent Reporter of Pattern-Triggered Immunity in Nicotiana benthamiana.

Plants rely on innate immune systems to defend against a wide variety of biotic attackers. Key components of innate immunity include cell-surface pattern-recognition receptors (PRRs), which recognize pest- and pathogen-associated molecular patterns (PAMPs). Unlike other classes of receptors that often have visible cell-death immune outputs upon activation, PRRs generally lack rapid methods for assessing function. Here, we describe a genetically encoded bioluminescent reporter of immune activation by heterologously expressed PRRs in the model organism Nicotiana benthamiana. We characterized N. benthamiana transcriptome changes in response to Agrobacterium tumefaciens and subsequent PAMP treatment to identify pattern-triggered immunity (PTI)-associated marker genes, which were then used to generate promoter-luciferase fusion fungal bioluminescence pathway (FBP) constructs. A reporter construct termed pFBP_2xNbLYS1::LUZ allows for robust detection of PTI activation by heterologously expressed PRRs. Consistent with known PTI signaling pathways, reporter activation by receptor-like protein (RLP) PRRs is dependent on the known adaptor of RLP PRRs, i.e., SOBIR1. The FBP reporter minimizes the amount of labor, reagents, and time needed to assay function of PRRs and displays robust sensitivity at biologically relevant PAMP concentrations, making it ideal for high throughput screens. The tools described in this paper will be powerful for investigations of PRR function and characterization of the structure-function of plant cell-surface receptors. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2023.

Involvement of a receptor-like kinase complex of FvFLS2 and FvBAK1 in brassinosteroids-induced immunity in postharvest strawberry fruit

Conserved pathogen-derived features of microbial invaders are sensed by plants via pattern-recognition receptors (PRRs) and induce both local and systemic immune responses. The plant PRR flagellin sensitive 2 (FLS2), a leucine-rich repeat-receptor kinase, recognizes pathogen-derived molecules and initiates pattern-triggered immunity (PTI) responses by associating with other potential kinases to propagate downstream signalling cascades. Here, we identified a plasma membrane-localized receptor-like kinase BAK1 from strawberry (FvBAK1) that is rapidly induced upon exposure to the plant-specific hormone brassinosteroid (BR), which provides strawberry fruit with immunity to the necrotrophic fungus Botrytis cinerea. FvBAK1 reciprocally interacts with FvFLS2 and the receptor-like cytoplasmic kinase FvBIK1 in yeast cells in vivo. Subsequently, FvBIK1 can directly interact with respiratory burst oxidase homologue D (FvRBOHD), contributing to FvRBOHD-dependent ROS accumulation to amplify the SA signalling pathway in the BR-stimulated immunity of postharvest strawberry fruit to B. cinerea infection. Importantly, overexpression of bak1 promotes ROS accumulation and BR-induced defence responses, whereas bak1 loss-of-function mutants have compromised BR-induced SA signalling and immunity to necrotrophic fungal pathogens. In such a situation, FvBAK1 is an essential component of BR-mediated PTI signalling, which links the pathogen-derived receptor complex to downstream intracellular signalling pathways. Taken together, the findings of the current study highlight the importance of FvBAK1 as an upstream regulator of FvBIK1 and as a coreceptor of FvFLS2 in establishing BR-induced PTI signalling in postharvest strawberry fruit.

Tomato receptor-like cytoplasmic kinase Fir1 interacts with a negative regulator of jasmonic acid signaling.

Plant cells detect potential pathogens through plasma membrane-localized pattern recognition receptors (PRRs) that recognize microbe-associated molecular patterns (MAMPs) and activate pattern-triggered immunity (PTI). PRR-mediated MAMP perception is linked to PTI signaling by receptor-like cytoplasmic kinases (RLCKs). In tomato, Flagellin-sensing 2 (Fls2)/Fls3 interacting RLCK 1 (Fir1) is involved in PTI triggered by flagellin perception. Fir1 is necessary for regulation of jasmonic acid (JA) signaling and is involved in pre-invasion immunity. We show that Fir1 physically interacts with JASMONATE-ZIM-DOMAIN PROTEIN 3 (JAZ3), a negative regulator of JA signaling. This finding suggests that Fir1 modulates JA signaling by regulating JAZ3.

PABP/purine-rich motif as an initiation module for cap-independent translation in pattern-triggered immunity

Upon stress, eukaryotes typically reprogram their translatome through GCN2-mediated phosphorylation of the eukaryotic translation initiation factor, eIF2α, to inhibit general translation initiation while selectively translating essential stress regulators. Unexpectedly, in plants, pattern-triggered immunity (PTI) and response to other environmental stresses occur independently of the GCN2/eIF2α pathway. Here, we show that while PTI induces mRNA decapping to inhibit general translation, defense mRNAs with a purine-rich element ("R-motif") are selectively translated using R-motif as an internal ribosome entry site (IRES). R-motif-dependent translation is executed by poly(A)-binding proteins (PABPs) through preferential association with the PTI-activating eIFiso4G over the repressive eIF4G. Phosphorylation by PTI regulators mitogen-activated protein kinase 3 and 6 (MPK3/6) inhibits eIF4G's activity while enhancing PABP binding to the R-motif and promoting eIFiso4G-mediated defense mRNA translation, establishing a link between PTI signaling and protein synthesis. Given its prevalence in both plants and animals, the PABP/R-motif translation initiation module may have a broader role in reprogramming the stress translatome.

Recognition of Microbe- and Damage-Associated Molecular Patterns by Leucine-Rich Repeat Pattern Recognition Receptor Kinases Confers Salt Tolerance in Plants.

In plants, a first layer of inducible immunity is conferred by pattern recognition receptors (PRRs) that bind microbe- and damage-associated molecular patterns to activate pattern-triggered immunity (PTI). PTI is strengthened or followed by another potent form of immunity when intracellular receptors recognize pathogen effectors, termed effector-triggered immunity. Immunity signaling regulators have been reported to influence abiotic stress responses as well, yet the governing principles and mechanisms remain ambiguous. Here, we report that PRRs of a leucine-rich repeat ectodomain also confer salt tolerance in Arabidopsis thaliana, following recognition of cognate ligands such as bacterial flagellin (flg22 epitope) and elongation factor Tu (elf18 epitope), and the endogenous Pep peptides. Pattern-triggered salt tolerance (PTST) requires authentic PTI signaling components; namely, the PRR-associated kinases BAK1 and BIK1 and the NADPH oxidase RBOHD. Exposure to salt stress induces the release of Pep precursors, pointing to the involvement of the endogenous immunogenic peptides in developing plant tolerance to high salinity. Transcriptome profiling reveals an inventory of PTST target genes, which increase or acquire salt responsiveness following a preexposure to immunogenic patterns. In good accordance, plants challenged with nonpathogenic bacteria also acquired salt tolerance in a manner dependent on PRRs. Our findings provide insight into signaling plasticity underlying biotic or abiotic stress cross-tolerance in plants conferred by PRRs.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Lipid Signaling Requires ROS Production to Elicit Actin Cytoskeleton Remodeling during Plant Innate Immunity.

In terrestrial plants a basal innate immune system, pattern-triggered immunity (PTI), has evolved to limit infection by diverse microbes. The remodeling of actin cytoskeletal arrays is now recognized as a key hallmark event during the rapid host cellular responses to pathogen attack. Several actin binding proteins have been demonstrated to fine tune the dynamics of actin filaments during this process. However, the upstream signals that stimulate actin remodeling during PTI signaling remain poorly characterized. Two second messengers, reactive oxygen species (ROS) and phosphatidic acid (PA), are elevated following pathogen perception or microbe-associated molecular pattern (MAMP) treatment, and the timing of signaling fluxes roughly correlates with actin cytoskeletal rearrangements. Here, we combined genetic analysis, chemical complementation experiments, and quantitative live-cell imaging experiments to test the role of these second messengers in actin remodeling and to order the signaling events during plant immunity. We demonstrated that PHOSPHOLIPASE Dβ (PLDβ) isoforms are necessary to elicit actin accumulation in response to flg22-associated PTI. Further, bacterial growth experiments and MAMP-induced apoplastic ROS production measurements revealed that PLDβ-generated PA acts upstream of ROS signaling to trigger actin remodeling through inhibition of CAPPING PROTEIN (CP) activity. Collectively, our results provide compelling evidence that PLDβ/PA functions upstream of RBOHD-mediated ROS production to elicit actin rearrangements during the innate immune response in Arabidopsis.

Research on ADR1s helps understanding the plant immune network

Plant innate immunity begins with the recognition of pathogens by plasma membrane localized pattern-recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NLRs), which lead to pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively. For a long time, PTI and ETI have been regarded as two independent processes although they share multiple components and signal outputs. Increasing evidence shows an intimate link between PTI and ETI. PTI and ETI mutually potentiate each other, and this is essential for robust disease resistance during pathogen infection. An ancient class of NLRs called RNLs, so named because they carry a Resistance to Powdery Mildew 8 (RPW8)-like coiled-coil (CC) domain in the N terminus, has emerged as a key node connecting PTI and ETI. RNLs not only act as helper NLRs that signal downstream of sensor NLRs, they also directly mediate PTI signaling by associating with PRR complexes. Here, we focus on Activated Disease Resistance 1 (ADR1), a subclass of RNLs, and discuss its role and mechanism in plant immunity.

Arabidopsis PUB2 and PUB4 connect signaling components of pattern-triggered immunity.

Plants use pattern recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs) and activate pattern-triggered immunity (PTI). Precise regulation of information from PRRs to downstream signaling components is vital to mounting an appropriate immune response and requires dynamic interactions of these PTI components. We used transcriptome profiling, phenotypic analysis, molecular genetics, and protein-protein interaction analysis to understand the roles of the Arabidopsis plant U-box (PUB) proteins PUB2 and PUB4 in disease resistance and PTI signaling. Loss of function of both PUB2 and PUB4 diminishes the PAMP-triggered oxidative bursts and dampens mitogen-activated protein kinase signaling, resulting in a severe compromise in resistance to not only pathogenic but also nonpathogenic strains of Pseudomonas syringae. Within PUB4, the E3 ligase activity is dispensable, but the armadillo repeat region is essential and sufficient for its function in immunity. PUB2 and PUB4 interact with PTI signaling components, including FLS2, BIK1, PBL27, and RbohD, and enhance FLS2-BIK1 and BIK1-RbohD interactions. Our study reveals that PUB2 and PUB4 are critical components of plant immunity and connect PTI components to positively regulate defense responses.

Formin nanoclustering-mediated actin assembly during plant flagellin and DSF signaling

Plants respond to bacterial infection acutely with actin remodeling during plant immune responses. The mechanisms by which bacterial virulence factors (VFs) modulate plant actin polymerization remain enigmatic. Here, we show that plant-type-I formin serves as the molecular sensor for actin remodeling in response to two bacterial VFs: Xanthomonas campestris pv. campestris (Xcc) diffusible signal factor (DSF), and pathogen-associated molecular pattern (PAMP) flagellin in pattern-triggered immunity (PTI). Both VFs regulate actin assembly by tuning the clustering and nucleation activity of formin on the plasma membrane (PM) at the nano-sized scale. By being integrated within the cell-wall-PM-actin cytoskeleton (CW-PM-AC) continuum, the dynamic behavior and function of formins are highly dependent on each scaffold layer's composition within the CW-PM-AC continuum during both DSF and PTI signaling. Our results reveal a central mechanism for rapid actin remodeling during plant-bacteria interactions, in which bacterial signaling molecules fine tune plant formin nanoclustering in a host mechanical-structure-dependent manner.

Multiple Xanthomonas campestris pv. campestris 8004 type III effectors inhibit immunity induced by flg22.

Xanthomonas campestris pv. campestris 8004 secretes several effector proteins that interfere with plant phosphorylation. Xanthomonas campestris pv. campestris (Xcc) can infect cruciferous plants and cause black rot. The strain Xcc8004 secretes effector proteins that interfere with plant cellular processes into host cells using a type III secretion (T3S) system. Several of the 24 predicted T3S effectors in the Xcc8004 genome have been implicated in the suppression of the Arabidopsis thaliana pattern-triggered immunity (PTI) response. We used an A. thaliana mesophyll protoplast-based assay to identify Xcc8004 T3S effectors that effectively interfere with PTI signalling induced by the bacterial peptide flg22. 11 of the 24 tested effector proteins (XopK, XopQ, HrpW, XopN, XopAC, XopD, XopZ1, XopAG, AvrBs2, XopL and XopX-1) inhibited expression of the flg22-inducible gene FRK1, and five effectors (XopK, XopG, XopQ, XopL and XopX-1) inhibited the expression of the flg22-inducible gene WRKY33. Therefore, there are 12 effector proteins that can inhibit the expression of relevant flg22-inducible genes. It was further investigated whether the 12 effector proteins affect the phosphorylation activation of mitogen-activated protein (MAP) kinases MPK3/MPK6, and four effector proteins (XopK, XopQ, XopZ1 and XopX-1) were found to markedly inhibit MPK3/MPK6 activation. Moreover, a subcellular localisation analysis revealed that the tested effectors were localised within various subcellular compartments. These results indicate that multiple T3S effectors in the Xcc8004 genome interfere with flg22-induced PTI signalling via various molecular mechanisms.

Structural biology of plant defence.

Plants employ the innate immune system to discriminate between self and invaders through two types of immune receptors, one on the plasma membrane and the other in the intracellular space. The immune receptors on the plasma membrane are pattern recognition receptors (PRRs) that can perceive pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) leading to pattern-triggered immunity (PTI). Particular pathogens are capable of overcoming PTI by secreting specific effectors into plant cells to perturb different components of PTI signalling through various mechanisms. Most of the immune receptors from the intracellular space are the nucleotide-binding leucine-rich repeat receptors (NLRs), which specifically recognize pathogen-secreted effectors to mediate effector-triggered immunity (ETI). In this review, we will summarize recent progress in structural studies of PRRs, NLRs, and effectors, and discuss how these studies shed light on ligand recognition and activation mechanisms of the two types of immune receptors and the diversified mechanisms used by effectors to manipulate plant immune signalling.

A malectin-like receptor kinase regulates cell death and pattern-triggered immunity in soybean.

Plant cells can sense conserved molecular patterns through pattern recognition receptors (PRRs) and initiate pattern‐triggered immunity (PTI). Details of the PTI signaling network are starting to be uncovered in Arabidopsis, but are still poorly understood in other species, including soybean (Glycine max). In this study, we perform a forward genetic screen for autoimmunity‐related lesion mimic mutants (lmms) in soybean and identify two allelic mutants, which carry mutations in Glyma.13G054400, encoding a malectin‐like receptor kinase (RK). The mutants exhibit enhanced resistance to both bacterial and oomycete pathogens, as well as elevated ROS production upon treatment with the bacterial pattern flg22. Overexpression of GmLMM1 gene in Nicotiana benthamiana severely suppresses flg22‐triggered ROS production and oomycete pattern XEG1‐induced cell death. We further show that GmLMM1 interacts with the flg22 receptor FLS2 and its co‐receptor BAK1 to negatively regulate flg22‐induced complex formation between them. Our study identifies an important component in PTI regulation and reveals that GmLMM1 acts as a molecular switch to control an appropriate immune activation, which may also be adapted to other PRR‐mediated immune signaling in soybean.

The root-invading pathogen Fusarium oxysporum targets pattern-triggered immunity using both cytoplasmic and apoplastic effectors.

Summary Plant pathogens use effector proteins to promote host colonisation. The mode of action of effectors from root‐invading pathogens, such as Fusarium oxysporum (Fo), is poorly understood. Here, we investigated whether Fo effectors suppress pattern‐triggered immunity (PTI), and whether they enter host cells during infection.Eight candidate effectors of an Arabidopsis‐infecting Fo strain were expressed with and without signal peptide for secretion in Nicotiana benthamiana and their effect on flg22‐triggered and chitin‐triggered reactive oxidative species (ROS) burst was monitored. To detect uptake, effector biotinylation by an intracellular Arabidopsis‐produced biotin ligase was examined following root infection.Four effectors suppressed PTI signalling; two acted intracellularly and two apoplastically. Heterologous expression of a PTI‐suppressing effector in Arabidopsis enhanced bacterial susceptibility. Consistent with an intracellular activity, host cell uptake of five effectors, but not of the apoplastically acting ones, was detected in Fo‐infected Arabidopsis roots.Multiple Fo effectors targeted PTI signalling, uncovering a surprising overlap in infection strategies between foliar and root pathogens. Extracellular targeting of flg22 signalling by a microbial effector provides a new mechanism on how plant pathogens manipulate their host. Effector translocation appears independent of protein size, charge, presence of conserved motifs or the promoter driving its expression.

Perception of double-stranded RNA in plant antiviral immunity.

SummaryRNA silencing and antiviral pattern‐triggered immunity (PTI) both rely on recognition of double‐stranded (ds)RNAs as defence‐inducing signals. While dsRNA recognition by dicer‐like proteins during antiviral RNA silencing is thoroughly investigated, the molecular mechanisms involved in dsRNA perception leading to antiviral PTI are just about to be untangled. Parallels to antimicrobial PTI thereby only partially facilitate our view on antiviral PTI. PTI against microbial pathogens involves plasma membrane bound receptors; however, dsRNAs produced during virus infection occur intracellularly. Hence, how dsRNA may be perceived during this immune response is still an open question. In this short review, we describe recent discoveries in PTI signalling upon sensing of microbial patterns and endogenous ‘danger’ molecules with emphasis on immune signalling‐associated subcellular trafficking processes in plants. Based on these studies, we develop different scenarios how dsRNAs could be sensed during antiviral PTI.

Disruption of the MAMP-Induced MEKK1-MKK1/MKK2-MPK4 Pathway Activates the TNL Immune Receptor SMN1/RPS6.

Mitogen-activated protein kinase (MAPK) pathways have a pivotal role in innate immunity signaling in plants. In Arabidopsis, the MAPK pathway that consists of MEKK1, MKK1/MKK2 and MPK4 is involved in pattern-triggered immunity signaling upstream of defense gene expression. This pathway is partly guarded by SUMM2, a nucleotide-binding domain leucine-rich repeat (NLR) protein, which is activated by disruption of the MAPK pathway. To identify other components required for the guard mechanism, here we developed a new mutant screening system utilizing a dwarf autoimmune line that overexpressed the N-terminal regulatory domain of MEKK1. Mutants with suppression of the dwarf, autoimmune phenotypes were identified, and one locus responsible for the phenotype was designated as suppressor of MEKK1N overexpression-induced dwarf 1 (SMN1). MutMap analysis revealed that SMN1 encodes the Toll/Interleukin-1 receptor (TIR)-class NLR protein RPS6, a previously identified resistant protein against bacterial pathogen Pseudomonas syringae pv. tomato expressing the HopA1 effector. Importantly, mutations in SMN1/RPS6 also partially suppressed the dwarf, autoimmune phenotypes of mekk1 and mpk4 plants. Our results suggest that the two structurally distinct NLR proteins, SMN1/RPS6 and SUMM2, monitor integrity of the MEKK1-MKK1/MKK2-MPK4 pathway.

The plant cell wall integrity maintenance and immune signaling systems cooperate to control stress responses in Arabidopsis thaliana.

Cell walls surround all plant cells, and their composition and structure are modified in a tightly controlled, adaptive manner to meet sometimes opposing functional requirements during growth and development. The plant cell wall integrity (CWI) maintenance mechanism controls these functional modifications, as well as responses to cell wall damage (CWD). We investigated how the CWI system mediates responses to CWD in Arabidopsis thaliana CWD induced by cell wall-degrading enzymes or an inhibitor of cellulose biosynthesis elicited similar, turgor-sensitive stress responses. Phenotypic clustering with 27 genotypes identified a core group of receptor-like kinases (RLKs) and ion channels required for the activation of CWD responses. A genetic analysis showed that the RLK FEI2 and the plasma membrane-localized mechanosensitive Ca2+ channel MCA1 functioned downstream of the RLK THE1 in CWD perception. In contrast, pattern-triggered immunity (PTI) signaling components, including the receptors for plant elicitor peptides (AtPeps) PEPR1 and PEPR2, repressed responses to CWD. CWD induced the expression of PROPEP1 and PROPEP3, which encode the precursors of AtPep1 and AtPep3, and the release of PROPEP3 into the growth medium. Application of AtPep1 and AtPep3 repressed CWD-induced phytohormone accumulation in a concentration-dependent manner. These results suggest that AtPep-mediated signaling suppresses CWD-induced defense responses controlled by the CWI mechanism. This suppression was alleviated when PTI signaling downstream of PEPR1 and PEPR2 was impaired. Defense responses controlled by the CWI maintenance mechanism might thus compensate to some extent for the loss of PTI signaling elements.

Pattern-Triggered Immunity Alters the Transcriptional Regulation of Virulence-Associated Genes and Induces the Sulfur Starvation Response in Pseudomonas syringae pv. tomato DC3000

Pattern-triggered immunity (PTI) can confer broad defense against diverse microbes and pathogens with disparate lifestyles through the detection of microbial extracellular signatures by surface-exposed pattern recognition receptors. However, unlike recognition of pathogen effectors by cytosolic resistance proteins, PTI is typically not associated with a host-cell programmed cell death response. Although host PTI signaling has been extensively studied, the mechanisms by which it restricts microbial colonization are poorly understood. We sought to gain insight into the mechanisms of PTI action by using bacterial transcriptomics analysis during exposure to PTI. Here, we describe a method for bacterial cell extraction from inoculated leaves that was used to analyze a time course of genome-wide transcriptional responses in the pathogen Pseudomonas syringae pv. tomato DC3000 during early naïve host infection and exposure to pre-induced PTI in Arabidopsis thaliana. Our analysis revealed early transcriptional regulation of important bacterial metabolic processes and host interaction pathways. We observed peak induction of P. syringae virulence genes at 3 h postinoculation and that exposure to PTI was associated with significant reductions in the expression of virulence genes. We also observed the induction of P. syringae sulfur starvation response genes such as sulfate and sulfonate importers only during exposure to PTI.