PAMP-triggered Immunity Research Articles

Genome and Transcriptome Sequence Resources and Effector Repertoire of Pythium myriotylum Drechsler.

Genome and Transcriptome Sequence Resources and Effector Repertoire of Pythium myriotylum Drechsler.

RppM, Encoding a Typical CC-NBS-LRR Protein, Confers Resistance to Southern Corn Rust in Maize.

Southern corn rust (SCR) caused by Puccinia polysora Underw. poses a major threat to maize production worldwide. The utilization of host SCR-resistance genes and the cultivation of resistant cultivars are the most effective, economical strategies for controlling SCR. Here, we identified and cloned a new SCR resistance gene, RppM, from the elite maize inbred line Jing2416K. RppM was found to encode a typical CC-NBS-LRR protein localized in both the nucleus and cytoplasm. This gene was constitutively expressed at all developmental stages and in all tissues examined, with the strongest expression detected in leaves at the mature stage. A transcriptome analysis provided further evidence that multiple defense systems were initiated in Jing2416K, including pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, reinforcement of cell walls, accumulation of antimicrobial compounds, and activation of phytohormone signaling pathways. Finally, we developed functional Kompetitive allele-specific PCR markers for RppM using two conserved SNP sites and successfully applied these functional markers for the detection of RppM and the cultivation of resistant maize cultivars, demonstrating their great potential utility in maize breeding.

Role of PsnWRKY70 in Regulatory Network Response to Infection with Alternaria alternata (Fr.) Keissl in Populus.

WRKY is an important complex family of transcription factors involved in plant immune responses. Among them, WRKY70 plays an important role in the process of the plant defense response to the invasion of pathogens. However, the defense mechanism of PsnWRKY70 is not clear in Populus nigra. In this study, we showed that PsnWRKY70-overexpression lines (OE) had fewer leaf blight symptoms than PsnWRKY70-repressing lines (RE). PsnWRKY70 activated MAP kinase cascade genes (PsnM2K4, PsnMPK3, PsnM3K18), calcium channel proteins-related genes (PsnCNG3, PsnCNGC1, PsnCNG4), and calcium-dependent protein kinases genes (PsnCDPKL, PsnCDPKW, PsnCDPKS, PsnCDPKQ). Furthermore, 129 genes of PsnWRKY70 putative genome-wide direct targets (DTGs) were identified by using transcriptome (RNA-seq) and DNA affinity purification sequencing (DAP-seq). PsnWRKY70 directly binds to the promoters of homologous genes and LRR domain proteins to promote the expression of WRKY6, WRKY18, WRKY22, and WRKY22–1, LRR domain proteins LRR8, LRR-RLK, ADR1-like 2, NB-ARC, etc. Our study suggests that PsnWRKY70 enhances the resistance of A. alternata in poplar by activating genes in both pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI).

Functional Diversification Analysis of Soybean Malectin/Malectin-Like Domain-Containing Receptor-Like Kinases in Immunity by Transient Expression Assays

Plants have responded to microbial pathogens by evolving a two-tiered immune system, involving pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). Malectin/malectin-like domain-containing receptor-like kinases (MRLKs) have been reported to participate in many biological functions in plant including immunity and resistance. However, little is known regarding the role of MRLKs in soybean immunity. This is a crucial question to address because soybean is an important source of oil and plant proteins, and its production is threatened by various pathogens. Here, we systematically identified 72 Glycine max MRLKs (GmMRLKs) and demonstrated that many of them are transcriptionally induced or suppressed in response to infection with microbial pathogens. Next, we successfully cloned 60 GmMRLKs and subsequently characterized their roles in plant immunity by transiently expressing them in Nicotiana benthamiana, a model plant widely used to study host-pathogen interactions. Specifically, we examined the effect of GmMRLKs on PTI responses and noticed that a number of GmMRLKs negatively regulated the reactive oxygen species burst induced by flg22 and chitin, and cell death triggered by XEG1 and INF1. We also analyzed the microbial effectors AvrB- and XopQ-induced hypersensitivity response and identified several GmMRLKs that suppressed ETI activation. We further showed that GmMRLKs regulate immunity probably by coupling to the immune receptor complexes. Furthermore, transient expression of several selected GmMRLKs in soybean hairy roots conferred reduced resistance to soybean pathogen Phytophthora sojae. In summary, we revealed the common and specific roles of GmMRLKs in soybean immunity and identified a number of GmMRLKs as candidate susceptible genes that may be useful for improving soybean resistance.

Phosphorylation-mediated inactivation of C3H14 by MPK4 enhances bacterial-triggered immunity in Arabidopsis.

Perception of pathogen-associated molecular patterns (PAMPs) triggers mitogen-activated protein (MAP) kinase 4 (MPK4)-mediated phosphorylation and induces downstream transcriptional reprogramming, but the mechanisms of the MPK4 defense pathway are poorly understood. Here, we showed that phosphorylation-mediated inactivation of the CCCH protein C3H14 by MPK4 positively regulates the immune response in Arabidopsis (Arabidopsis thaliana). Compared with wild-type plants, loss-of-function mutations in C3H14 and its paralog C3H15 resulted in enhanced defense against Pst DC3000 in infected leaves and the development of systemic acquired resistance (SAR), whereas C3H14 or C3H15 overexpression enhanced susceptibility to this pathogen and failed to induce SAR. The functions of C3H14 in PAMP-triggered immunity (PTI) and SAR were dependent on MPK4-mediated phosphorylation. Challenge with Pst DC3000 or the flagellin peptide flg22 enhanced the phosphorylation of C3H14 by MPK4 in the cytoplasm, relieving C3H14-inhibited expression of PTI-related genes and attenuating C3H14-activated expression of its targets NIM1-INTERACTING1 (NIMIN1) and NIMIN2, two negative regulators of SAR. Salicylic acid (SA) affected the MPK4-C3H14-NIMIN1/2 cascades in immunity, but SA signaling mediated by the C3H14-NIMIN1/2 cascades was independent of MPK4 phosphorylation. Our study suggests that C3H14 might be a negative component of the MPK4 defense signaling pathway.

The simultaneous perception of self- and non-self-danger signals potentiates plant innate immunity responses

Main conclusionThe simultaneous perception of endogenous and exogenous danger signals potentiates PAMP-triggered immunity in tomato and other downstream defence responses depending on the origin of the signal. Plant cells perceive a pathogen invasion by recognising endogenous or exogenous extracellular signals such as Damage-Associated Molecular Patterns (DAMPs) or Pathogen-Associated Molecular Patterns (PAMPs). In particular, DAMPs are intracellular molecules or cell wall fragments passive or actively released to the apoplast, whose extracellular recognition by intact cells triggers specific immune signalling, the so-called DAMP-triggered immunity. The extracellular recognition of DAMPs and PAMPs leads to a very similar intracellular signalling, and this similarity has generated a biological need to know why plants perceive molecules with such different origins and with overlapped innate immunity responses. Here, we report that the simultaneous perception of DAMPs and a PAMP strengthens early and late plant defence responses. To this aim, we studied classical PTI responses such as the generation of ROS and MAPK phosphorylation, but we also monitored the biosynthesis of phytocytokines and performed a non-targeted metabolomic analysis. We demonstrate that co-application of the bacterial peptide flagellin with the DAMPs cyclic AMP or cellobiose amplifies PAMP-triggered immunity responses. Both co-applications enhanced the synthesis of phytocytokines, but only simultaneous treatments with cAMP strengthened the flagellin-dependent metabolomic responses. In addition, cAMP and cellobiose treatments induced resistance against the hemibiotrophic bacteria Pseudomonas syringae pv. tomato DC3000. Overall, these results indicate that the complex mixture of DAMPs and PAMPs carries specific information that potentiates plant defence responses. However, downstream responses seem more specific depending on the composition of the mixture.

The Pattern Recognition Receptor FLS2 Can Shape the Arabidopsis Rhizosphere Microbiome β-Diversity but Not EFR1 and CERK1

Pathogen associated molecular pattern (PAMP) triggered immunity (PTI) is the first line of plant defense. We hypothesized that the absence of pattern recognition receptors (PRRs) in plants could influence the rhizosphere microbiome. Here, we report sequencing of the 16S ribosomal RNA gene and the fungal ribosomal RNA internal transcribed spacer region of rhizosphere DNA from three Arabidopsis PRR mutants involved in plant innate immunity (efr1, fls2, and cerk1). We conducted experiments in a growth chamber using native soil from the Red River Farm (Terral, OK, USA) to detect microbial community shifts in the rhizosphere that may occur in the absence of PRR receptors compared to wild-type (WT; Col-0) plants. No difference in the α-diversity of the rhizosphere microbial population was observed between the PRR mutants tested and the WT. Plant host genotype had a significant impact in bacterial β-diversity only between the fls2 mutant and the WT. Surprisingly, no significant changes in fungal β-diversity were observed between the PRR mutants and WT, although we observed an increase in relative abundance for the cup fungi (Pezizaceae) in the cerk1 mutant. This finding suggests that the FLS2 receptor can modulate the rhizosphere-associated microbiome β-diversity and expands the list of current known genotypes that can modulate the rhizosphere microbiota.

Photoperiod Stress in Arabidopsis thaliana Induces a Transcriptional Response Resembling That of Pathogen Infection.

Plants are exposed to regular diurnal rhythms of light and dark. Changes in the photoperiod by the prolongation of the light period cause photoperiod stress in short day-adapted Arabidopsis thaliana. Here, we report on the transcriptional response to photoperiod stress of wild-type A. thaliana and photoperiod stress-sensitive cytokinin signaling and clock mutants and identify a core set of photoperiod stress-responsive genes. Photoperiod stress caused altered expression of numerous reactive oxygen species (ROS)-related genes. Photoperiod stress-sensitive mutants displayed similar, but stronger transcriptomic changes than wild-type plants. The alterations showed a strong overlap with those occurring in response to ozone stress, pathogen attack and flagellin peptide (flg22)-induced PAMP triggered immunity (PTI), which have in common the induction of an apoplastic oxidative burst. Interestingly, photoperiod stress triggers transcriptional changes in jasmonic acid (JA) and salicylic acid (SA) biosynthesis and signaling and results in increased JA, SA and camalexin levels. These responses are typically observed after pathogen infections. Consequently, photoperiod stress increased the resistance of Arabidopsis plants to a subsequent infection by Pseudomonas syringae pv. tomato DC3000. In summary, we show that photoperiod stress causes transcriptional reprogramming resembling plant pathogen defense responses and induces systemic acquired resistance (SAR) in the absence of a pathogen.

Understanding the mechanisms of resistance to tomato leaf mold: A review

Tomato leaf mold is a common disease in tomato production and severely impacts the growth, fruit quality and yield of tomato plants. Research on tomato leaf mold has a long history and has focused mainly on the differentiation of pathogen races, the structure and function of pathogen Avr gene products, the cloning of Cf resistance genes and the analysis of disease resistance mechanisms. Interactions between Cf and Avr are in accordance with the "gene-for-gene" hypothesis and typical Cf/Avr interactions are part of effector-triggered immunity (ETI). However, an increasing number of studies have proven that pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is involved in the disease resistance response system mediated by Cf genes. In addition, different Cf genes have both similar and different roles in the disease resistance response, indicating that the disease resistance mechanism of Cf genes is complex. In this paper, progress in tomato leaf mold research was reviewed, and the regulatory mechanism underlying the Cf-mediated disease resistance response was thoroughly explored. We hope this summary will lay a foundation for research on tomato leaf mold disease resistance mechanisms and is applicable to breeding for disease resistance.

Transcriptome Analysis of Fusarium Root-Rot-Resistant and -Susceptible Alfalfa (Medicago sativa L.) Plants during Plant-Pathogen Interactions.

Alfalfa (Medicago sativa L.) is a perennial leguminous forage cultivated globally. Fusarium spp.-induced root rot is a chronic and devastating disease affecting alfalfa that occurs in most production fields. Studying the disease resistance regulatory network and investigating the key genes involved in plant–pathogen resistance can provide vital information for breeding alfalfa that are resistant to Fusarium spp. In this study, a resistant and susceptible clonal line of alfalfa was inoculated with Fusarium proliferatum L1 and sampled at 24 h, 48 h, 72 h, and 7 d post-inoculation for RNA-seq analysis. Among the differentially expressed genes (DEGs) detected between the two clonal lines at the four time points after inoculation, approximately 81.8% were detected at 24 h and 7 d after inoculation. Many DEGs in the two inoculated clonal lines participated in PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI) mechanisms. In addition, transcription factor families such as bHLH, SBP, AP2, WRKY, and MYB were detected in response to infection. These results are an important supplement to the few existing studies on the resistance regulatory network of alfalfa against Fusarium root rot and will help to understand the evolution of host–pathogen interactions.

From Functional Characterization to the Application of SWEET Sugar Transporters in Plant Resistance Breeding.

The occurrence of plant diseases severely affects the quality and quantity of plant production. Plants adapt to the constant invasion of pathogens and gradually form a series of defense mechanisms, such as pathogen-associated molecular pattern-triggered immunity and microbial effector-triggered immunity. Moreover, many pathogens have evolved to inhibit the immune defense system and acquire plant nutrients as a result of their coevolution with plants. The sugars will eventually be exported transporters (SWEETs) are a novel family of sugar transporters that function as uniporters. They provide a channel for pathogens, including bacteria, fungi, and viruses, to hijack sugar from the host. In this review, we summarize the functions of SWEETs in nectar secretion, grain loading, senescence, and long-distance transport. We also focus on the interaction between the SWEET genes and pathogens. In addition, we provide insight into the potential application of SWEET genes to enhance disease resistance through the use of genome editing tools. The summary and perspective of this review will deepen our understanding of the role of SWEETs during the process of pathogen infection and provide insights into resistance breeding.

ANGUSTIFOLIA negatively regulates resistance to Sclerotinia sclerotiorum via modulation of PTI and JA signalling pathways in Arabidopsis thaliana

Sclerotinia sclerotiorum is a devastating pathogen that infects a broad range of host plants. The mechanism underlying plant defence against fungal invasion is still not well characterized. Here, we report that ANGUSTIFOLIA (AN), a CtBP family member, plays a role in the defence against S. sclerotiorum attack. Arabidopsis an mutants exhibited stronger resistance to S. sclerotiorum at the early stage of infection than wild‐type plants. Accordingly, an mutants exhibited stronger activation of pathogen‐associated molecular pattern (PAMP)‐triggered immunity (PTI) responses, including mitogen‐activated protein kinase activation, reactive oxygen species accumulation, callose deposition, and the expression of PTI‐responsive genes, upon treatment with PAMPs/microbe‐associated molecular patterns. Moreover, Arabidopsis lines overexpressing AN were more susceptible to S. sclerotiorum and showed defective PTI responses. Our luminometry, bimolecular fluorescence complementation, coimmunoprecipitation, and in vitro pull‐down assays indicate that AN interacts with allene oxide cyclases (AOC), essential enzymes involved in jasmonic acid (JA) biosynthesis, negatively regulating JA biosynthesis in response to S. sclerotiorum infection. This work reveals AN is a negative regulator of the AOC‐mediated JA signalling pathway and PTI activation.

PAMP-triggered genetic reprogramming involves widespread alternative transcription initiation and an immediate transcription factor wave.

Immune responses triggered by pathogen-associated molecular patterns (PAMPs) are key to pathogen defense, but drivers and stabilizers of the growth-to-defense genetic reprogramming remain incompletely understood in plants. Here, we report a time-course study of the establishment of PAMP-triggered immunity (PTI) using cap analysis of gene expression. We show that around 15% of all transcription start sites (TSSs) rapidly induced during PTI define alternative transcription initiation events. From these, we identify clear examples of regulatory TSS change via alternative inclusion of target peptides or domains in encoded proteins, or of upstream open reading frames in mRNA leader sequences. We also find that 60% of PAMP response genes respond earlier than previously thought. In particular, a cluster of rapidly and transiently PAMP-induced genes is enriched in transcription factors (TFs) whose functions, previously associated with biological processes as diverse as abiotic stress adaptation and stem cell activity, appear to converge on growth restriction. Furthermore, examples of known potentiators of PTI, in one case under direct mitogen-activated protein kinase control, support the notion that the rapidly induced TFs could constitute direct links to PTI signaling pathways and drive gene expression changes underlying establishment of the immune state.

Arabidopsis Plasma Membrane ATPase AHA5 Is Negatively Involved in PAMP-Triggered Immunity

Plants evolve a prompt and robust immune system to defend themselves against pathogen infections. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is the first battle layer activated upon the PAMP’s perception, which leads to multiple defense responses. The plasma membrane (PM) H+-ATPases are the primary ion pumps to create and maintain the cellular membrane potential that is critical for various essential biological processes, including plant growth, development, and defense. This study discovered that the PM H+-ATPase AHA5 is negatively involved in Arabidopsis PTI against the virulent pathogen Pseudomonas syringae pvr. tomato (Pto) DC3000 infection. The aha5 mutant plants caused the reduced stomata opening upon the Pto infection, which was associated with the salicylic acid (SA) pathway. In addition, the aha5 mutant plants caused the increased levels of callose deposition, defense-related gene expression, and SA accumulation. Our results also indicate that the PM H+-ATPase activity of AHA5 probably mediates the coupling of H2O2 generation and the apoplast alkalization in PTI responses. Moreover, AHA5 was found to interact with a vital defense regulator, RPM1-interacting protein 4 (RIN4), in vitro and in vivo, which might also be critical for its function in PTI. In summary, our studies show that AHA5 functions as a novel and critical component that is negatively involved in PTI by coordinating different defense responses during the Arabidopsis–Pto DC3000 interaction.

RIPK: a crucial ROS signaling component in plants.

Reactive oxygen species (ROS) are involved in plant development and stress acclimation. Recently, Li et al. reported that ROS production is controlled by receptor-like cytoplasmic kinase (RLCK)-mediated respiratory burst oxidase homolog D (RBOHD) phosphorylation, which subsequently regulates pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), damage-associated molecular pattern (DAMP)-triggered immunity (DTI), effector-triggered immunity (ETI), and systemic acquired resistance (SAR) in plants, thus enhancing our understanding of biotic stress tolerance.

XopZ and ORP1C cooperate to regulate the virulence of Xanthomonas oryzae pv. oryzae on Nipponbare

ABSTRACT Bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae (Xoo) has always been considered to be one of the most severe worldwide diseases in rice. Xoo strains usually use the highly conserved type III secretion system (T3SS) to deliver virulence effectors into rice cells and further suppress the host’s immunity. Previous studies reported that different Xanthomonas outer protein (Xop) effectors include XopZ from one strain appear to share functional redundancies on suppressing rice PAMP-triggered immunity (PTI). But only xopZ, except other xop genes, could significantly impaire Xoo virulence when individually deleting in PXO99 strains. Thus, the XopZ effector should not only suppress rice PTI pathway, but also has other unknown indispensable pathological functions in PXO99–rice interactions. Here, we also found that ∆xopZ mutant strains displayed lower virulence on Nipponbare leaves compared with PXO99 strains. We identified an oxysterol-binding related protein, ORP1C, as a XopZ-interacting protein in rice. Further studies found that rice ORP1C preliminarily played a positive role in regulating the resistance to PXO99 strains, and XopZ–ORP1C interactions cooperated to regulate the compatible interactions of PXO99-Nipponbare rice. The reactive oxygen species (ROS) burst and PTI marker gene expression data indicated that ORP1C were not directly relevant to the PTI pathway in rice. The deeper mechanisms underlying XopZ–ORP1C interaction and how XopZ and ORP1C cooperate for regulating the PXO99–rice interactions require further exploration.

Plant immunity suppression via PHR1-RALF-FERONIA shapes the root microbiome to alleviate phosphate starvation.

The microbiome plays an important role in shaping plant growth and immunity, but few plant genes and pathways impacting plant microbiome composition have been reported. In Arabidopsis thaliana, the phosphate starvation response (PSR) was recently found to modulate the root microbiome upon phosphate (Pi) starvation through the transcriptional regulator PHR1. Here, we report that A.thaliana PHR1 directly binds to the promoters of rapid alkalinization factor (RALF) genes, and activates their expression under phosphate-starvation conditions. RALFs in turn suppress complex formation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) receptor through FERONIA, a previously-identified PTI modulator that increases resistance to certain detrimental microorganisms. Suppression of immunity via the PHR1-RALF-FERONIA axis allows colonization by specialized root microbiota that help to alleviate phosphate starvation by upregulating the expression of PSR genes. These findings provide a new paradigm for coordination of host-microbe homeostasis through modulating plant innate immunity after environmental perturbations.

PTI and ETI: convergent pathways with diverse elicitors.

Plants rely on PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI) to detect invading pathogens and subsequently activate defense mechanisms. Recently, four Nature papers (Yuan et al., Ngou et al., Pruitt et al., and Tian et al.)demonstrated that important components in PTI and ETI are required for both PTI and ETI, and PTI and ETI potentiate each other to achieve stronger plant defenses.

MPK3 and MPK6 control salicylic acid signaling by up-regulatingNLRreceptors during pattern- and effector-triggered immunity

AbstractArabidopsis thaliana mitogen-activated protein kinases 3 and 6 (MPK3/6) are activated transiently during pathogen-associated molecular pattern-triggered immunity (PTI) and durably during effector-triggered immunity (ETI). The functional differences between these two kinds of activation kinetics and how they coordinate the two layers of plant immunity remain poorly understood. Here, by suppressor analyses, we demonstrate that ETI-mediating nucleotide-binding domain leucine-rich repeat receptors (NLRs) and the NLR signaling components NDR1 and EDS1 can promote the salicylic acid sector of defense downstream of MPK3 activity. Moreover, we provide evidence that both sustained and transient MPK3/6 activities positively control the expression of several NLR genes, including AT3G04220 and AT4G11170. We further show that NDR1 and EDS1 contribute to the up-regulation of these two NLRs in both an ETI and a PTI context. Remarkably, whereas in ETI MPK3/6 activities are dependent on NDR1 and EDS1, they are not in PTI, suggesting crucial differences in the two signaling pathways. Finally, we demonstrate that expression of the NLR AT3G04220 is sufficient to induce expression of defense genes from the salicylic acid branch. Overall, this study expands our knowledge of MPK3/6 functions during immunity and provides new insights into the intricate interplay of PTI and ETI.

Regulation of heat shock proteins 70 and their role in plant immunity.

Heat shock proteins 70 (HSP70s) are steadily gaining more attention in the field of plant biotic interactions. Though their regulation and activity in plants are much less well characterized than are those of their counterparts in mammals, accumulating evidence indicates that the role of HSP70-mediated defense mechanisms in plant cells is indispensable. In this review, we summarize current knowledge of HSP70 post-translational control in plants. We comment on the phytohormonal regulation of HSP70 expression and protein abundance, and identify a prominent role for cytokinin in HSP70 control. We outline HSP70s' subcellular localizations, chaperone activity, and chaperone-mediated protein degradation. We focus on the role of HSP70s in plant pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and discuss the contribution of different HSP70 subfamilies to plant defense against pathogens.