Plant pattern-recognition receptors

Wheat receptor-kinase-like protein Stb6 controls gene-for-gene resistance to fungal pathogen Zymoseptoria tritici

Deployment of fast-evolving disease-resistance genes is one of the most successful strategies used by plants to fend off pathogens^1,2. In gene-for-gene relationships, most cloned disease-resistance genes encode intracellular nucleotide-binding leucine-rich-repeat proteins (NLRs) recognizing pathogen-secreted isolate-specific avirulence (Avr) effectors delivered to the host cytoplasm^3,4. This process often triggers a localized hypersensitive response, which halts further disease development ⁵ . Here we report the map-based cloning of the wheat Stb6 gene and demonstrate that it encodes a conserved wall-associated receptor kinase (WAK)-like protein, which detects the presence of a matching apoplastic effector^6-8 and confers pathogen resistance without a hypersensitive response ⁹ . This report demonstrates gene-for-gene disease resistance controlled by this class of proteins in plants. Moreover, Stb6 is, to our knowledge, the first cloned gene specifying resistance to Zymoseptoria tritici, an important foliar fungal pathogen affecting wheat and causing economically damaging septoria tritici blotch (STB) disease^10-12.

Plant-microbe interactions: organelles and the cytoskeleton in action

Contents Summary 1012 I. Introduction 1012 II. The endomembrane system in plant-microbe interactions 1013 III. The cytoskeleton in plant-microbe interactions 1017 IV. Organelles in plant-microbe interactions 1019 V. Inter-organellar communication in plant-microbe interactions 1022 VI. Conclusions and prospects 1023 Acknowledgements 1024 References 1024 SUMMARY: Plants have evolved a multilayered immune system with well-orchestrated defense strategies against pathogen attack. Multiple immune signaling pathways, coordinated by several subcellular compartments and interactions between these compartments, play important roles in a successful immune response. Pathogens use various strategies to either directly attack the plant's immune system or to indirectly manipulate the physiological status of the plant to inhibit an immune response. Microscopy-based approaches have allowed the direct visualization of membrane trafficking events, cytoskeleton reorganization, subcellular dynamics and inter-organellar communication during the immune response. Here, we discuss the contributions of organelles and the cytoskeleton to the plant's defense response against microbial pathogens, as well as the mechanisms used by pathogens to target these compartments to overcome the plant's defense barrier.

A Verticillium dahliae Extracellular Cutinase Modulates Plant Immune Responses

Cutinases have been implicated as important enzymes during the process of fungal infection of aerial plant organs. The function of cutinases in the disease cycle of fungal pathogens that invade plants through the roots has been less studied. Here, functional analysis of 13 cutinase (carbohydrate esterase family 5 domain-containing) genes (VdCUTs) in the highly virulent vascular wilt pathogen Verticillium dahliae Vd991 was performed. Significant sequence divergence in cutinase family members was observed in the genome of V. dahliae Vd991. Functional analyses demonstrated that only VdCUT11, as purified protein, induced cell death and triggered defense responses in Nicotiana benthamiana, cotton, and tomato plants. Virus-induced gene silencing showed that VdCUT11 induces plant defense responses in Nicotiana benthamania in a BAK1 and SOBIR-dependent manner. Furthermore, coinfiltration assays revealed that the carbohydrate-binding module family 1 protein (VdCBM1) suppressed VdCUT11-induced cell death and other defense responses in N. benthamiana. Targeted deletion of VdCUT11 in V. dahliae significantly compromised virulence on cotton plants. The cutinase VdCUT11 is an important secreted enzyme and virulence factor that elicits plant defense responses in the absence of VdCBM1.

Plant cell wall-mediated immunity: cell wall changes trigger disease resistance responses

Plants have evolved a repertoire of monitoring systems to sense plant morphogenesis and to face environmental changes and threats caused by different attackers. These systems integrate different signals into overreaching triggering pathways which coordinate developmental and defence-associated responses. The plant cell wall, a dynamic and complex structure surrounding every plant cell, has emerged recently as an essential component of plant monitoring systems, thus expanding its function as a passive defensive barrier. Plants have a dedicated mechanism for maintaining cell wall integrity (CWI) which comprises a diverse set of plasma membrane-resident sensors and pattern recognition receptors (PRRs). The PRRs perceive plant-derived ligands, such as peptides or wall glycans, known as damage-associated molecular patterns (DAMPs). These DAMPs function as 'danger' alert signals activating DAMP-triggered immunity (DTI), which shares signalling components and responses with the immune pathways triggered by non-self microbe-associated molecular patterns that mediate disease resistance. Alteration of CWI by impairment of the expression or activity of proteins involved in cell wall biosynthesis and/or remodelling, as occurs in some plant cell wall mutants, or by wall damage due to colonization by pathogens/pests, activates specific defensive and growth responses. Our current understanding of how these alterations of CWI are perceived by the wall monitoring systems is scarce and few plant sensors/PRRs and DAMPs have been characterized. The identification of these CWI sensors and PRR-DAMP pairs will help us to understand the immune functions of the wall monitoring system, and might allow the breeding of crop varieties and the design of agricultural strategies that would enhance crop disease resistance.

Behind the lines-actions of bacterial type III effector proteins in plant cells

Pathogenicity of most Gram-negative plant-pathogenic bacteria depends on the type III secretion (T3S) system, which translocates bacterial effector proteins into plant cells. Type III effectors modulate plant cellular pathways to the benefit of the pathogen and promote bacterial multiplication. One major virulence function of type III effectors is the suppression of plant innate immunity, which is triggered upon recognition of pathogen-derived molecular patterns by plant receptor proteins. Type III effectors also interfere with additional plant cellular processes including proteasome-dependent protein degradation, phytohormone signaling, the formation of the cytoskeleton, vesicle transport and gene expression. This review summarizes our current knowledge on the molecular functions of type III effector proteins with known plant target molecules. Furthermore, plant defense strategies for the detection of effector protein activities or effector-triggered alterations in plant targets are discussed.

The Xanthomonas euvesicatoria type III effector XopAU is an active protein kinase that manipulates plant MAP kinase signaling

The Gram-negative bacterium Xanthomonas euvesicatoria (Xe) is the causal agent of bacterial spot disease of pepper and tomato. Xe delivers effector proteins into host cells through the type III secretion system to promote disease. Here, we show that the Xe effector XopAU, which is conserved in numerous Xanthomonas species, is a catalytically active protein kinase and contributes to the development of disease symptoms in pepper plants. Agrobacterium-mediated expression of XopAU in host and non-host plants activated typical defense responses, including MAP kinase phosphorylation, accumulation of pathogenesis-related (PR) proteins and elicitation of cell death, that were dependent on the kinase activity of the effector. XopAU-mediated cell death was not dependent on early signaling components of effector-triggered immunity and was also observed when the effector was delivered into pepper leaves by Xanthomonas campestris pv. campestris, but not by Xe. Protein-protein interaction studies in yeast and in planta revealed that XopAU physically interacts with components of plant immunity-associated MAP kinase cascades. Remarkably, XopAU directly phosphorylated MKK2 in vitro and enhanced its phosphorylation at multiple sites in planta. Consistent with the notion that MKK2 is a target of XopAU, silencing of the MKK2 homolog or overexpression of the catalytically inactive mutant MKK2K99R in N. benthamiana plants reduced XopAU-mediated cell death and MAPK phosphorylation. Furthermore, yeast co-expressing XopAU and MKK2 displayed reduced growth and this phenotype was dependent on the kinase activity of both proteins. Together, our results support the conclusion that XopAU contributes to Xe disease symptoms in pepper plants and manipulates host MAPK signaling through phosphorylation and activation of MKK2.

MiR858-Mediated Regulation of Flavonoid-Specific MYB Transcription Factor Genes Controls Resistance to Pathogen Infection in Arabidopsis

MicroRNAs (miRNAs) are a class of short endogenous non-coding small RNAs that direct post-transcriptional gene silencing in eukaryotes. In plants, the expression of a large number of miRNAs has been shown to be regulated during pathogen infection. However, the functional role of the majority of these pathogen-regulated miRNAs has not been elucidated. In this work, we investigated the role of Arabidopsis miR858 in the defense response of Arabidopsis plants to infection by fungal pathogens with necrotrophic (Plectosphaerella cucumerina) or hemibiotrophic (Fusarium oxysporum and Colletotrichum higginsianum) lifestyles. Whereas overexpression of MIR858 enhances susceptibility to pathogen infection, interference with miR858 activity by target mimics (MIM858 plants) results in disease resistance. Upon pathogen challenge, stronger activation of the defense genes PDF1.2 and PR4 occurs in MIM858 plants than in wild-type plants, whereas pathogen infection induced weaker activation of these genes in MIR858 overexpressor plants. Reduced miR858 activity, and concomitant up-regulation of miR858 target genes, in MIM858 plants, also leads to accumulation of flavonoids in Arabidopsis leaves. The antifungal activity of phenylpropanoid compounds, including flavonoids, is presented. Furthermore, pathogen infection or treatment with fungal elicitors is accompanied by a gradual decrease in MIR858 expression in wild-type plants, suggesting that miR858 plays a role in PAMP (pathogen-associated molecular pattern)-triggered immunity. These data support that miR858 is a negative regulator of Arabidopsis immunity and provide new insights into the relevant role of miR858-mediated regulation of the phenylpropanoid biosynthetic pathway in controlling Arabidopsis immunity.

Plasmodesmal regulation during plant-pathogen interactions

Contents Summary 62 I. Introduction 62 II. Plasmodesmal regulation is an innate defence response 63 III. Reactive oxygen species regulate plasmodesmal function 63 IV. Plasmodesmal regulation by and of defence-associated small molecules 64 V. Plasmodesmata facilitate systemic defence signalling 64 VI. Virulent pathogens exploit plasmodesmata 66 VII. Outlook 66 Acknowledgements 66 References 66 SUMMARY: Plasmodesmata (PD) are plasma membrane-lined pores that connect neighbouring plant cells, bridging the cell wall and establishing cytoplasmic and membrane continuity between cells. PD are dynamic structures regulated by callose deposition in a variety of stress and developmental contexts. This process crudely controls the aperture of the pore and thus the flux of molecules between cells. During pathogen infection, plant cells initiate a range of immune responses and it was recently identified that, following perception of fungal and bacterial pathogens, plant cells initially close their PD. Systemic defence responses depend on the spread of signals between cells, raising questions about whether PD are in different functional states during different immune responses. It is well established that viral pathogens exploit PD to spread between cells, but it has more recently been identified that protein effectors secreted by fungal pathogens can spread between host cells via PD. It is possible that many classes of pathogens specifically target PD to aid infection, which would infer antagonistic regulation of PD by host and pathogen. How PD regulation benefits both host immune responses and pathogen infection is an important question and demands that we examine the multicellular nature of plant-pathogen interactions.

A Pathogen-Responsive Leucine Rich Receptor Like Kinase Contributes to Fusarium Resistance in Cereals

Receptor-like kinases form the largest family of receptors in plants and play an important role in recognizing pathogen-associated molecular patterns and modulating the plant immune responses to invasive fungi, including cereal defenses against fungal diseases. But hitherto, none have been shown to modulate the wheat response to the economically important Fusarium head blight (FHB) disease of small-grain cereals. Homologous genes were identified on barley chromosome 6H (HvLRRK-6H) and wheat chromosome 6DL (TaLRRK-6D), which encode the characteristic domains of surface-localized receptor like kinases. Gene expression studies validated that the wheat TaLRRK-6D is highly induced in heads as an early response to both the causal pathogen of FHB disease, Fusarium graminearum, and its' mycotoxic virulence factor deoxynivalenol. The transcription of other wheat homeologs of this gene, located on chromosomes 6A and 6B, was also up-regulated in response to F. graminearum. Virus-induced gene silencing (VIGS) of the barley HvLRRK-6H compromised leaf defense against F. graminearum. VIGS of TaLRRK-6D in two wheat cultivars, CM82036 (resistant to FHB disease) and cv. Remus (susceptible to FHB), confirmed that TaLRRK-6D contributes to basal resistance to FHB disease in both genotypes. Although the effect of VIGS did not generally reduce grain losses due to FHB, this experiment did reveal that TaLRRK-6D positively contributes to grain development. Further gene expression studies in wheat cv. Remus indicated that VIGS of TaLRRK-6D suppressed the expression of genes involved in salicylic acid signaling, which is a key hormonal pathway involved in defense. Thus, this study provides the first evidence of receptor like kinases as an important component of cereal defense against Fusarium and highlights this gene as a target for enhancing cereal resistance to FHB disease.

Transcriptional analysis and histochemistry reveal that hypersensitive cell death and H2O2 have crucial roles in the resistance of tea plant (Camellia sinensis (L.) O. Kuntze) to anthracnose

Anthracnose causes severe losses of tea production in China. Although genes and biological processes involved in anthracnose resistance have been reported in other plants, the molecular response to anthracnose in tea plant is unknown. We used the susceptible tea cultivar Longjing 43 and the resistant cultivar Zhongcha 108 as materials and compared transcriptome changes in the leaves of both cultivars following Colletotrichum fructicola inoculation. In all, 9015 and 8624 genes were differentially expressed between the resistant and susceptible cultivars and their controls (0 h), respectively. In both cultivars, the differentially expressed genes (DEGs) were enriched in 215 pathways, including responses to sugar metabolism, phytohormones, reactive oxygen species (ROS), biotic stimuli and signalling, transmembrane transporter activity, protease activity and signalling receptor activity, but DEG expression levels were higher in Zhongcha 108 than in Longjing 43. Moreover, functional enrichment analysis of the DEGs showed that hydrogen peroxide (H₂O₂) metabolism, cell death, secondary metabolism, and carbohydrate metabolism are involved in the defence of Zhongcha 108, and 88 key genes were identified. Protein-protein interaction (PPI) network demonstrated that putative mitogen-activated protein kinase (MAPK) cascades are activated by resistance (R) genes and mediate downstream defence responses. Histochemical analysis subsequently validated the strong hypersensitive response (HR) and H₂O₂ accumulation that occurred around the hyphal infection sites in Zhongcha 108. Overall, our results indicate that the HR and H₂O₂ are critical mechanisms in tea plant defence against anthracnose and may be activated by R genes via MAPK cascades.

The Bacterial Effector AvrPto Targets the Regulatory Coreceptor SOBIR1 and Suppresses Defense Signaling Mediated by the Receptor-Like Protein Cf-4

Receptor-like proteins (RLPs) and receptor-like kinases (RLKs) are cell-surface receptors that are essential for detecting invading pathogens and subsequent activation of plant defense responses. RLPs lack a cytoplasmic kinase domain to trigger downstream signaling leading to host resistance. The RLK SOBIR1 constitutively interacts with the tomato RLP Cf-4, thereby providing Cf-4 with a kinase domain. SOBIR1 is required for Cf-4-mediated resistance to strains of the fungal tomato pathogen Cladosporium fulvum that secrete the effector Avr4. Upon perception of this effector by the Cf-4/SOBIR1 complex, the central regulatory RLK SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3a (SERK3a) is recruited to the complex and defense signaling is triggered. SOBIR1 is also required for RLP-mediated resistance to bacterial, fungal ,and oomycete pathogens, and we hypothesized that SOBIR1 is targeted by effectors of such pathogens to suppress host defense responses. In this study, we show that Pseudomonas syringae pv. tomato DC3000 effector AvrPto interacts with Arabidopsis SOBIR1 and its orthologs of tomato and Nicotiana benthamiana, independent of SOBIR1 kinase activity. Interestingly, AvrPto suppresses Arabidopsis SOBIR1-induced cell death in N. benthamiana. Furthermore, AvrPto compromises Avr4-triggered cell death in Cf-4-transgenic N. benthamiana, without affecting Cf-4/SOBIR1/SERK3a complex formation. Our study shows that the RLP coreceptor SOBIR1 is targeted by a bacterial effector, which results in compromised defense responses.

The Ralstonia solanacearum type III effector RipAY targets plant redox regulators to suppress immune responses

The subversion of plant cellular functions is essential for bacterial pathogens to proliferate in host plants and cause disease. Most bacterial plant pathogens employ a type III secretion system to inject type III effector (T3E) proteins inside plant cells, where they contribute to the pathogen-induced alteration of plant physiology. In this work, we found that the Ralstonia solanacearum T3E RipAY suppresses plant immune responses triggered by bacterial elicitors and by the phytohormone salicylic acid. Further biochemical analysis indicated that RipAY associates in planta with thioredoxins from Nicotiana benthamiana and Arabidopsis. Interestingly, RipAY displays γ-glutamyl cyclotransferase (GGCT) activity to degrade glutathione in plant cells, which is required for the reported suppression of immune responses. Given the importance of thioredoxins and glutathione as major redox regulators in eukaryotic cells, RipAY activity may constitute a novel and powerful virulence strategy employed by R. solanacearum to suppress immune responses and potentially alter general redox signalling in host cells.

Molecular Profiling of Pierce's Disease Outlines the Response Circuitry of Vitis vinifera to Xylella fastidiosa Infection

Pierce's disease is a major threat to grapevines caused by the bacterium Xylella fastidiosa. Although devoid of a type 3 secretion system commonly employed by bacterial pathogens to deliver effectors inside host cells, this pathogen is able to influence host parenchymal cells from the xylem lumen by secreting a battery of hydrolytic enzymes. Defining the cellular and biochemical changes induced during disease can foster the development of novel therapeutic strategies aimed at reducing the pathogen fitness and increasing plant health. To this end, we investigated the transcriptional, proteomic, and metabolomic responses of diseased Vitis vinifera compared to healthy plants. We found that several antioxidant strategies were induced, including the accumulation of gamma-aminobutyric acid (GABA) and polyamine metabolism, as well as iron and copper chelation, but these were insufficient to protect the plant from chronic oxidative stress and disease symptom development. Notable upregulation of phytoalexins, pathogenesis-related proteins, and various aromatic acid metabolites was part of the host responses observed. Moreover, upregulation of various cell wall modification enzymes followed the proliferation of the pathogen within xylem vessels, consistent with the intensive thickening of vessels' secondary walls observed by magnetic resonance imaging. By interpreting the molecular profile changes taking place in symptomatic tissues, we report a set of molecular markers that can be further explored to aid in disease detection, breeding for resistance, and developing therapeutics.

Unraveling the molecules hidden in the gray shadows of quantitative disease resistance to pathogens

Una de las preguntas más desafiantes del fitomejoramiento y de la fitopatología molecular es ¿cuáles son las bases genéticas y moleculares de la resistencia cuantitativa a enfermedades?. El escaso conocimiento de cómo este tipo de resistencia funciona ha obstaculizado que los fitomejoradores la aprovecharlo plenamente. Para superar estos obstáculos se han desarrollado nuevas metodologías para el estudio de rasgos cuantitativos. Los enfoques como el mapeo genético, la identificación de loci de rasgos cuantitativos (QTL) y el mapeo por asociaciones, incluyendo el enfoque de genes candidatos y los estudios de asociación amplia del genoma, se han llevado a cabo históricamente para describir rasgos cuantitativos y por lo tanto para estudiar QDR. Además, se han proporcionado grandes avances en la obtención de datos fenotípicos cuantitativos para mejorar estos análisis. Recientemente, algunos genes asociados a QDR han sido clonados, lo que conduce a nuevas hipótesis sobre las bases moleculares de este tipo de resistencia. En esta revisión presentamos los avances más recientes sobre QDR y la correspondiente aplicación, que han permitido postular nuevas ideas que pueden ayudar a construir nuevos modelos. Algunas de las hipótesis presentadas aquí como posibles explicaciones para QDR están relacionadas con el nivel de expresión y el splicing alternativo de algunos genes relacionados con la defensa, la acción de "alelos débiles" de genes R, la presencia de variantes alélicas en los genes implicados en la respuesta de defensa y un papel central de quinasas o pseudoqinasas. Con la información recapitulada en esta revisión es posible concluir que la distinción conceptual entre resistencia cualitativa y cuantitativa puede ser cuestionada ya que ambos comparten importantes componentes.

An oomycete plant pathogen reprograms host pre-mRNA splicing to subvert immunity

The process of RNA splicing influences many physiological processes, including plant immunity. However, how plant parasites manipulate host RNA splicing process remains unknown. Here we demonstrate that PsAvr3c, an avirulence effector from oomycete plant pathogen Phytophthora sojae, physically binds to and stabilizes soybean serine/lysine/arginine-rich proteins GmSKRPs. The SKRPs are novel proteins that associate with a complex that contains plant spliceosome components, and are negative regulators of plant immunity. Analysis by RNA-seq data indicates that alternative splicing of pre-mRNAs from 401 soybean genes, including defense-related genes, is altered in GmSKRP1 and PsAvr3c overexpressing lines compared to control plants. Representative splicing events mediated by GmSKRP1 and PsAvr3c are tested by infection assays or by transient expression in soybean plants. Our results show that plant pathogen effectors can reprogram host pre-mRNA splicing to promote disease, and we propose that pathogens evolved such strategies to defeat host immune systems.

Overexpression of OsHSP18.0-CI Enhances Resistance to Bacterial Leaf Streak in Rice

BACKGROUND

The small heat shock proteins represent a large family of proteins that respond to a wide range of abiotic and biotic stresses. OsHsp18.0-CI confers tolerance to salt and cadmium and interacts with viral RNA-dependent RNA polymerase (RdRp). However, the direct function of OsHsp18.0-CI in resistance against biotic stresses remains unclear in rice.

RESULTS

Here, we report that the expression of OsHsp18.0-CI was up-regulated upon inoculation with RS105, a strain of Xanthomonas oryzae pv. oryzicola (Xoc) that causes bacterial leaf streak in rice. In comparison with wild-type, OsHsp18.0-CI overexpression (OE) lines exhibited enhanced resistance to RS105, whereas repression lines exhibited compromised resistance to RS105. In addition, the transcriptional profiles of wild type and OE lines were compared with and without inoculation with RS105. After inoculation with RS105, most of the genes with up-regulated expression were commonly stimulated in the wild type and OE lines, with stronger induction in the OE lines than in wild type.

CONCLUSION

Our study reveals that OsHsp18.0-CI positively regulates resistance to Xoc by mediating an enhanced version of the basal defense response in rice.

The intimate talk between plants and microorganisms at the leaf surface

The plant epidermis or cuticle is constantly exposed to external and internal environmental factors, including an enriched and diverse community of bacteria, yeast, fungi, viruses, and mites. It is not only where the plant has its first physical barrier, but also where organisms can be recognized and potentially where the plant defense responses can be triggered. The plant cuticle is a polymeric composite formed by an array of structurally and chemically heterogeneous compounds, including cutin and wax. A few studies have shown that cuticular components are essential and important drivers of the structure and size of the bacterial community. On the other hand, cuticular components are also important for both pathogens and plants, to initiate the pre-invasion and infection process and to activate the innate immune response, respectively. In this review, we explore current knowledge on the role of the cuticle during the intimate interactions between plants and microorganisms, in particular pathogenic and non-pathogenic bacteria and fungi. Finally, we propose new perspectives on the potential use of this information for agriculture.

RNA sequencing of Brassica napus reveals cellular redox control of Sclerotinia infection

Brassica napus is one of the world's most valuable oilseeds and is under constant pressure by the necrotrophic fungal pathogen, Sclerotinia sclerotiorum, the causal agent of white stem rot. Despite our growing understanding of host pathogen interactions at the molecular level, we have yet to fully understand the biological processes and underlying gene regulatory networks responsible for determining disease outcomes. Using global RNA sequencing, we profiled gene activity at the first point of infection on the leaf surface 24 hours after pathogen exposure in susceptible (B. napus cv. Westar) and tolerant (B. napus cv. Zhongyou 821) plants. We identified a family of ethylene response factors that may contribute to host tolerance to S. sclerotiorum by activating genes associated with fungal recognition, subcellular organization, and redox homeostasis. Physiological investigation of redox homeostasis was further studied by quantifying cellular levels of the glutathione and ascorbate redox pathway and the cycling enzymes associated with host tolerance to S. sclerotiorum. Functional characterization of an Arabidopsis redox mutant challenged with the fungus provides compelling evidence into the role of the ascorbate-glutathione redox hub in the maintenance and enhancement of plant tolerance against fungal pathogens.

Hop/Sti1 - A Two-Faced Cochaperone Involved in Pattern Recognition Receptor Maturation and Viral Infection

Perception of pathogens by host pattern recognition receptors (PRRs) or R proteins is a prerequisite to promote successful immune responses. The Hsp70/Hsp90 organizing protein Hop/Sti1, a multifunctional cochaperone, has been implicated in the maturation of a receptor-like kinase (RLK) necessary for chitin sensing. However, it remains unknown whether Hop/Sti1 is generally participating in PRR genesis. Using RNA-interference (RNAi), we silenced Hop/Sti1 expression in Nicotiana tabacum to gain further insight into the role of the cochaperone in plant defense responses. As expected, transgenic plants do not respond to chitin treatment anymore. In contrast to this, trafficking and functionality of the flagellin PRR FLS2 were unaltered, suggesting a selective involvement of Hop/Sti1 during PRR maturation. Furthermore, Hop/Sti1 was identified as a cellular determinant of Potato virus Y (PVY) symptom development in tobacco, since PVY was able to accumulate to near wild-type level without provoking the usual veinal necrosis phenotype. In addition, typical antiviral host defense responses were suppressed in the transgenic plants. These data suggest that perception of PVY is dependent on Hop/Sti1-mediated receptor maturation, while viral symptoms represent a failing attempt to restrict PVY spread. In addition, Hop/Sti1 colocalized with virus-induced membrane aggregates in wild-type plants. The retention of Hop/Sti1 in potential viral replication complexes suggests a role during viral translation/replication, explaining why RNAi-lines do not exhibit increased susceptibility to PVY. This study provides evidence for a dual role of Hop/Sti1 in PRR maturation and pathogen perception as well as in promoting viral proliferation.

Plant response to biotic stress: Is there a common epigenetic response during plant-pathogenic and symbiotic interactions?

Plants constantly interact with pathogenic and symbiotic microorganisms. Recent studies have revealed several regulatory mechanisms controlling these interactions. Among them, the plant defense system is activated not only in response to pathogenic, but also in response to symbiotic microbes. Interestingly, shortly after symbiotic microbial recognition, the plant defense system is suppressed to promote plant infection by symbionts. Research studies have demonstrated the influence of the plant epigenome in modulating both pathogenic and symbiotic plant-microbe interactions, thereby influencing plant survival, adaptation and evolution of the plant response to microbial infections. It is however unclear if plant pathogenic and symbiotic responses share similar epigenomic profiles or if epigenomic changes differentially regulate plant-microbe symbiosis and pathogenesis. In this mini-review, we provide an update of the current knowledge of epigenomic control on plant immune responses and symbiosis, with a special attention being paid to knowledge gap and potential strategies to fill-in the missing links.

Emerging Insights into the Functions of Pathogenesis-Related Protein 1

The members of the pathogenesis-related protein 1 (PR-1) family are among the most abundantly produced proteins in plants on pathogen attack, and PR-1 gene expression has long been used as a marker for salicylic acid-mediated disease resistance. However, despite considerable interest over several decades, their requirement and role in plant defence remains poorly understood. Recent reports have emerged demonstrating that PR-1 proteins possess sterol-binding activity, harbour an embedded defence signalling peptide, and are targeted by plant pathogens during host infection. These studies have re-energised the field and provided long-awaited insights into a possible PR-1 function. Here we review the current status of PR-1 proteins and discuss how these recent advances shed light on putative roles for these enigmatic proteins.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

Enhanced Bacterial Wilt Resistance in Potato Through Expression of Arabidopsis EFR and Introgression of Quantitative Resistance from Solanum commersonii

Bacterial wilt (BW) caused by Ralstonia solanacearum is responsible for substantial losses in cultivated potato (Solanum tuberosum) crops worldwide. Resistance genes have been identified in wild species; however, introduction of these through classical breeding has achieved only partial resistance, which has been linked to poor agronomic performance. The Arabidopsis thaliana (At) pattern recognition receptor elongation factor-Tu (EF-Tu) receptor (EFR) recognizes the bacterial pathogen-associated molecular pattern EF-Tu (and its derived peptide elf18) to confer anti-bacterial immunity. Previous work has shown that transfer of AtEFR into tomato confers increased resistance to R. solanacearum. Here, we evaluated whether the transgenic expression of AtEFR would similarly increase BW resistance in a commercial potato line (INIA Iporá), as well as in a breeding potato line (09509.6) in which quantitative resistance has been introgressed from the wild potato relative Solanum commersonii. Resistance to R. solanacearum was evaluated by damaged root inoculation under controlled conditions. Both INIA Iporá and 09509.6 potato lines expressing AtEFR showed greater resistance to R. solanacearum, with no detectable bacteria in tubers evaluated by multiplex-PCR and plate counting. Notably, AtEFR expression and the introgression of quantitative resistance from S. commersonii had a significant additive effect in 09509.6-AtEFR lines. These results show that the combination of heterologous expression of AtEFR with quantitative resistance introgressed from wild relatives is a promising strategy to develop BW resistance in potato.

A Covert Operation by a Plant Pathogen

Phytopathogens have mastered the ability to evade plant innate immunity. In this issue of Cell Host & Microbe, Zhou and colleagues (Li et al., 2016) uncover a strategy whereby a bacterial pathogen disables the plant immune system with such precision as to avoid triggering alarms that could potentially reveal its presence.

Redox-mediated quorum sensing in plants

The rhizosphere, the narrow zone of soil around plant roots, is a complex network of interactions between plants, bacteria, and a variety of other organisms. The absolute dependence on host-derived signals, or xenognosins, to regulate critical developmental checkpoints for host commitment in the obligate parasitic plants provides a window into the rhizosphere's chemical dynamics. These sessile intruders use H2O2 in a process known as semagenesis to chemically modify the mature root surfaces of proximal host plants and generate p-benzoquinones (BQs). The resulting redox-active signaling network regulates the spatial and temporal commitments necessary for host attachment. Recent evidence from non-parasites, including Arabidopsis thaliana, establishes that reactive oxygen species (ROS) production regulates similar redox circuits related to root recognition, broadening xenognosins' role beyond the parasites. Here we compare responses to the xenognosin dimethoxybenzoquinone (DMBQ) between the parasitic plant Striga asiatica and the non-parasitic A. thaliana. Exposure to DMBQ simulates the proximity of a mature root surface, stimulating an increase in cytoplasmic Ca2+ concentration in both plants, but leads to remarkably different phenotypic responses in the parasite and non-parasite. In S. asiatica, DMBQ induces development of the host attachment organ, the haustorium, and decreases ROS production at the root tip, while in A. thaliana, ROS production increases and further growth of the root tip is arrested. Obstruction of Ca2+ channels and the addition of antioxidants both lead to a decrease in the DMBQ response in both parasitic and non-parasitic plants. These results are consistent with Ca2+ regulating the activity of NADPH oxidases, which in turn sustain the autocatalytic production of ROS via an external quinone/hydroquinone redox cycle. Mechanistically, this chemistry is similar to black and white photography with the emerging dynamic reaction-diffusion network laying the foundation for the precise temporal and spatial control underlying rhizosphere architecture.

Genome-wide analysis of cytosine DNA methylation revealed salicylic acid promotes defense pathways over seedling development in pearl millet

Cytosine DNA methylation is an epigenetic regulatory system used by plants to control gene expression. Methylation pattern always changes after abiotic stresses, pathogens and pest infections or after a treatment with salicylic acid (SA). The latter is a key player in plant development and defense against insect herbivores, pathogens, and abiotic stresses. The roles of SA on the methylation patterns and the plant development were performed in 4 pearl millet (Pennisetum glaucum) varieties. Seedlings of 4 early-flowering photosensitive genotypes (PMS3, PMI8, PMG, and PMT2) were grown on MS medium supplemented with null or different doses of SA. Root growth was used as a parameter to evaluate the effects of SA at early stage development. DNA from these seedlings was extracted and Methylation-Sensitive Amplified Polymorphism (MSAP) was measured to assess the effects of SA on methylome. The methylation analysis revealed that SA treatment decreased the methylation, while inhibiting the root growth for all varieties tested, except in PMG at 0.5 mM, indicating a dose and a genotype response-dependence. The methylation level was positively correlated with the root growth. This suggests that SA influences both the methylome by demethylation activities and the root growth by interfering with the root development-responsive genes. The demethylation process, induced by the REPRESSOR OF SILCENCING 1 (ROS1) may activate R genes, or GH3.5 and downregulate the hormonal pathway under root development. These findings showed the pearl millet metabolism prioritized and promoted the defense pathways over vegetative development during stress.

The Tomato Kinase Pti1 Contributes to Production of Reactive Oxygen Species in Response to Two Flagellin-Derived Peptides and Promotes Resistance to Pseudomonas syringae Infection

The Pti1 kinase was identified from a reverse genetic screen as contributing to pattern-triggered immunity (PTI) against Pseudomonas syringae pv. tomato (Pst). The tomato genome has two Pti1 genes, referred to as Pti1a and Pti1b. A hairpin-Pti1 (hpPti1) construct was developed and was used to generate two independent stable transgenic tomato lines that had reduced transcript abundance of both genes. In response to P. syringae pv. tomato inoculation, these hpPti1 plants developed more severe disease symptoms, supported higher bacterial populations, and had reduced transcript accumulation of PTI-associated genes, as compared with wild-type plants. In response to two flagellin-derived peptides, the hpPti1 plants produced lesser amounts of reactive oxygen species (ROS) but showed no difference in mitogen-activated protein kinase (MAPK). Synthetic Pti1a and Pti1b genes designed to avoid silencing were transiently expressed in the hpPti1 plants and restored the ability of the plants to produce wild-type levels of ROS. Our results identify a new component of PTI in tomato that, because it affects ROS production but not MAPK signaling, appears to act early in the immune response.

Identification and analysis of brown planthopper-responsive microRNAs in resistant and susceptible rice plants

The brown planthopper (BPH) is the most devastating insect pest of rice. The rice gene BPH15 confers resistance to BPH. MicroRNAs (miRNAs) regulate a spectrum of development and defense response processes in plants. In this study, we analyzed six miRNA profiles of a BPH15 introgression line (P15) and a susceptible recipient line (PC) at three time points (0 h, 6 h and 48 h) after BPH attack, and identified 464 known miRNAs and 183 potential novel miRNAs. Before the BPH feeding, we identified 23 miRNAs differentially expressed in P15 and PC. We speculated that the resistant plant is in a priming state by the regulation of miRNAs. After the BPH feeding, 104 miRNAs were found to be expressed differentially in P15 (68 in P15-6/P15-0, 36 in P15-48/P15-0), and 80 miRNAs were found expressed differentially in PC (32 in PC-6/PC-0, 48 in PC-48/PC-0), which illustrated that miRNA expression is activated upon attack. These miRNAs regulate different pathways that contribute to the basal defense and specific resistance of rice to the BPH. Our study provides additional data for scientists to further explore the mechanism of plant defense against insect attack and to find a way for efficient insect control.

Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance

Plants are constantly exposed to would-be pathogens and pests, and thus have a sophisticated immune system to ward off these threats, which otherwise can have devastating ecological and economic consequences on ecosystems and agriculture. Plants employ receptor kinases (RKs) and receptor-like proteins (RLPs) as pattern recognition receptors (PRRs) to monitor their apoplastic environment and detect non-self and damaged-self patterns as signs of potential danger. Plant PRRs contribute to both basal and non-host resistances, and treatment with pathogen-/microbe-associated molecular patterns (PAMPs/MAMPs) or damage-associated molecular patterns (DAMPs) recognized by plant PRRs induces both local and systemic immunity. Here, we comprehensively review known PAMPs/DAMPs recognized by plants as well as the plant PRRs described to date. In particular, we describe the different methods that can be used to identify PAMPs/DAMPs and PRRs. Finally, we emphasize the emerging biotechnological potential use of PRRs to improve broad-spectrum, and potentially durable, disease resistance in crops.

Plant signalling in symbiosis and immunity

Plants encounter a myriad of microorganisms, particularly at the root-soil interface, that can invade with detrimental or beneficial outcomes. Prevalent beneficial associations between plants and microorganisms include those that promote plant growth by facilitating the acquisition of limiting nutrients such as nitrogen and phosphorus. But while promoting such symbiotic relationships, plants must restrict the formation of pathogenic associations. Achieving this balance requires the perception of potential invading microorganisms through the signals that they produce, followed by the activation of either symbiotic responses that promote microbial colonization or immune responses that limit it.

Immunity and starvation: new opportunities to elevate disease resistance in crops

Plants use multiple mechanisms to defend themselves against invading microbes. Besides using their immune system to surveil and eliminate pathogens, plants actively block the pathogens' access to nutrients as an alternative way to prevent colonization. In this review, we focus on immunity and starvation as major obstacles for pathogens' adaptation. We summarize the key mechanisms employed by pathogens to modulate host immunity and to guarantee sugar uptake. In contrast to genes that deal with the immune system and show high levels of plasticity, pathogen genes involved in sugar acquisition are highly conserved, and may not have adapted to co-evolving interactions with the host. We propose a model to assess the durability of different control strategies based on the ability of pathogens to deal with host immunity or starvation. This analysis opens new opportunities to elevate disease resistance in crops by reducing the likelihood of pathogen adaptation.

Calcium signatures and signaling events orchestrate plant-microbe interactions

Calcium (Ca²⁺) acts as an essential second messenger connecting the perception of microbe signals to the establishment of appropriate immune and symbiotic responses in plants. Accumulating evidence suggests that plants distinguish different microorganisms through plasma membrane-localized pattern recognition receptors. The particular recognition events are encoded into Ca²⁺ signatures, which are sensed by diverse intracellular Ca²⁺ binding proteins. The Ca²⁺ signatures are eventually decoded to distinct downstream responses through transcriptional reprogramming of the defense or symbiosis-related genes. Recent observations further reveal that Ca²⁺-mediated signaling is also involved in negative regulation of plant immunity. This review is intended as an overview of Ca²⁺ signaling during immunity and symbiosis, including Ca²⁺ responses in the nucleus and cytosol.

Plum pox virus capsid protein suppresses plant pathogen-associated molecular pattern (PAMP)-triggered immunity

The perception of pathogen-associated molecular patterns (PAMPs) by immune receptors launches defence mechanisms referred to as PAMP-triggered immunity (PTI). Successful pathogens must suppress PTI pathways via the action of effectors to efficiently colonize their hosts. So far, plant PTI has been reported to be active against most classes of pathogens, except viruses, although this defence layer has been hypothesized recently as an active part of antiviral immunity which needs to be suppressed by viruses for infection success. Here, we report that Arabidopsis PTI genes are regulated upon infection by viruses and contribute to plant resistance to Plum pox virus (PPV). Our experiments further show that PPV suppresses two early PTI responses, the oxidative burst and marker gene expression, during Arabidopsis infection. In planta expression of PPV capsid protein (CP) was found to strongly impair these responses in Nicotiana benthamiana and Arabidopsis, revealing its PTI suppressor activity. In summary, we provide the first clear evidence that plant viruses acquired the ability to suppress PTI mechanisms via the action of effectors, highlighting a novel strategy employed by viruses to escape plant defences.

Sensing of molecular patterns through cell surface immune receptors

In plants, sensing of Pathogen/Microbe-Associated Molecular Patterns (PAMPs/MAMPs) and host-derived Damage-Associated Molecular Patterns (DAMPs) by host cell surface Pattern Recognition Receptors (PRRs) activates Pattern-Triggered Immunity (PTI). The identification of an increasing number of immunogenic patterns and PRRs illustrates that PTI is a universal defence mechanism against pathogens, pests, and parasitic plants, and that evolutionary selective pressure drives diversification of molecular patterns and diversity of PRRs. Further advances unravelled how some prototypical PRRs get activated to initiate metabolic adaptation and defence responses that stop invaders. Deeper insights into the repertoire of PRRs will reveal how plants manage to mount appropriate defence against diverse kinds of invaders and how we can biotechnologically exploit nature's design for sustainable agriculture.

Adaptation, specialization, and coevolution within phytobiomes

Growth patterns of individual plants and evolutionary trajectories of plant communities are intimately linked with and are critically affected by host-associated microbiomes. Research across systems has begun to shed light on how these phytobiomes are established under laboratory and natural conditions, and have cultivated hope that a better understanding of the governing principles for host-microbe interactions can guide attempts to engineer microbiomes to boost agricultural yields. One important, yet relatively understudied, parameter in regards to phytobiome membership is the degree to which specialization and coevolution between plant species and microbes provides structure to these communities. In this article, I provide an overview of mechanisms enabling adaptation and specialization of phytobiome communities to host plants as well as the potential for plants themselves to recruit and cultivate beneficial interactions. I further explore the possibility of host-beneficial microbe coevolution and suggest particular situations that could promote the evolution of such close-knit partnerships. It is my hope that this overview will encourage future experiments that can begin to fill in this black box of ecological and evolutionary interactions across phytobiomes.

Mechanism of host substrate acetylation by a YopJ family effector

The Yersinia outer protein J (YopJ) family of bacterial effectors depends on a novel acetyltransferase domain to acetylate signalling proteins from plant and animal hosts. However, the underlying mechanism is unclear. Here, we report the crystal structures of PopP2, a YopJ effector produced by the plant pathogen Ralstonia solanacearum, in complex with inositol hexaphosphate (InsP₆), acetyl-coenzyme A (AcCoA) and/or substrate Resistance to Ralstonia solanacearum 1 (RRS1-R)_WRKY. PopP2 recognizes the WRKYGQK motif of RRS1-R_WRKY to position a targeted lysine in the active site for acetylation. Importantly, the PopP2-RRS1-R_WRKY association is allosterically regulated by InsP₆ binding, suggesting a previously unidentified role of the eukaryote-specific cofactor in substrate interaction. Furthermore, we provide evidence for the reaction intermediate of PopP2-mediated acetylation, an acetyl-cysteine covalent adduct, lending direct support to the 'ping-pong'-like catalytic mechanism proposed for YopJ effectors. Our study provides critical mechanistic insights into the virulence activity of YopJ class of acetyltransferases.

Plant Pathogenic Fungi

Fungi are among the dominant causal agents of plant diseases. To colonize plants and cause disease, pathogenic fungi use diverse strategies. Some fungi kill their hosts and feed on dead material (necrotrophs), while others colonize the living tissue (biotrophs). For successful invasion of plant organs, pathogenic development is tightly regulated and specialized infection structures are formed. To further colonize hosts and establish disease, fungal pathogens deploy a plethora of virulence factors. Depending on the infection strategy, virulence factors perform different functions. While basically all pathogens interfere with primary plant defense, necrotrophs secrete toxins to kill plant tissue. In contrast, biotrophs utilize effector molecules to suppress plant cell death and manipulate plant metabolism in favor of the pathogen. This article provides an overview of plant pathogenic fungal species and the strategies they use to cause disease.

Desirable Traits of a Good Biocontrol Agent against Verticillium Wilt

The soil-borne fungus Verticillium causes serious vascular disease in a wide variety of annual crops and woody perennials. Verticillium wilt is notoriously difficult to control by conventional methods, so there is great potential for biocontrol to manage this disease. In this study we aimed to review the research about Verticillium biocontrol to get a better understanding of characteristics that are desirable in a biocontrol agent (BCA) against Verticillium wilt. We only considered studies in which the BCAs were tested on plants. Most biocontrol studies were focused on plants of the Solanaceae, Malvaceae, and Brassicaceae and within these families eggplant, cotton, and oilseed rape were the most studied crops. The list of bacterial BCAs with potential against Verticillium was dominated by endophytic Bacillus and Pseudomonas isolates, while non-pathogenic xylem-colonizing Verticillium and Fusarium isolates topped the fungal list. Predominant modes of action involved in biocontrol were inhibition of primary inoculum germination, plant growth promotion, competition and induced resistance. Many BCAs showed in vitro antibiosis and mycoparasitism but these traits were not correlated with activity in vivo and there is no evidence that they play a role in planta. Good BCAs were obtained from soils suppressive to Verticillium wilt, disease suppressive composts, and healthy plants in infested fields. Desirable characteristics in a BCA against Verticillium are the ability to (1) affect the survival or germination of microsclerotia, (2) colonize the xylem and/or cortex and compete with the pathogen for nutrients and/or space, (3) induce resistance responses in the plant and/or (4) promote plant growth. Potential BCAs should be screened in conditions that resemble the field situation to increase the chance of successful use in practice. Furthermore, issues such as large scale production, formulation, preservation conditions, shelf life, and application methods should be considered early in the process of selecting BCAs against Verticillium.

Communication in the Phytobiome

The phytobiome is composed of plants, their environment, and diverse interacting microscopic and macroscopic organisms, which together influence plant health and productivity. These organisms form complex networks that are established and regulated through nutrient cycling, competition, antagonism, and chemical communication mediated by a diverse array of signaling molecules. Integration of knowledge of signaling mechanisms with that of phytobiome members and their networks will lead to a new understanding of the fate and significance of these signals at the ecosystem level. Such an understanding could lead to new biological, chemical, and breeding strategies to improve crop health and productivity.

Growing evidence for facultative biotrophy in saprotrophic fungi: data from microcosm tests with 201 species of wood-decay basidiomycetes

Ectomycorrhizal (ECM) symbioses have evolved a minimum of 78 times independently from saprotrophic lineages, indicating the potential for functional overlap between ECM and saprotrophic fungi. ECM fungi have the capacity to decompose organic matter, and although there is increasing evidence that some saprotrophic fungi exhibit the capacity to enter into facultative biotrophic relationships with plant roots without causing disease symptoms, this subject is still not well studied. In order to determine the extent of biotrophic capacity in saprotrophic wood-decay fungi and which systems may be useful models, we investigated the colonization of conifer seedling roots in vitro using an array of 201 basidiomycete wood-decay fungi. Microtome sectioning, differential staining and fluorescence microscopy were used to visualize patterns of root colonization in microcosm systems containing Picea abies or Pinus sylvestris seedlings and each saprotrophic fungus. Thirty-four (16.9%) of the tested fungal species colonized the roots of at least one tree species. Two fungal species showed formation of a mantle and one showed Hartig net-like structures. These features suggest the possibility of an active functional symbiosis between fungus and plant. The data indicate that the capacity for facultative biotrophic relationships in free-living saprotrophic basidiomycetes may be greater than previously supposed.

Bacterial phytopathogen infection disrupts belowground plant indirect defense mediated by tritrophic cascade

Plants can defend themselves against herbivores through activation of defensive pathways and attraction of third-trophic-level predators and parasites. Trophic cascades that mediate interactions in the phytobiome are part of a larger dynamic including the pathogens of the plant itself, which are known to greatly influence plant defenses. As such, we investigated the impact of a phloem-limited bacterial pathogen, Candidatus Liberibacter asiaticus (CLas), in cultivated citrus rootstock on a well-studied belowground tritrophic interaction involving the attraction of an entomopathogenic nematode (EPN), Steinernema diaprepesi, to their root-feeding insect hosts, Diaprepes abbreviatus larvae. Using belowground olfactometers, we show how CLas infection interferes with this belowground interaction by similarly inducing the release of a C12 terpene, pregeijerene, and disconnecting the association of the terpene with insect presence. D. abbreviatus larvae that were not feeding but in the presence of a CLas-infected plant were more likely to be infected by EPN than those near uninfected plants. Furthermore, nonfeeding larvae associated with CLas-infected plants were just as likely to be infected by EPN as those near noninfected plants with D. abbreviatus larval damage. Larvae of two weevil species, D. abbreviatus and Pachnaeus litus, were also more attracted to plants with infection than to uninfected plants. D. abbreviatus larvae were most active when exposed to pregeijerene at a concentration of 0.1 μg/μl. We attribute this attraction to CLas-infected plants to the same signal previously thought to be a herbivore-induced plant volatile specifically induced by root-feeding insects, pregeijerene, by assessing volatiles collected from the roots of infected plants and uninfected plants with and without feeding D. abbreviatus. Synthesis. Phytopathogens can influence the structuring of soil communities extending to the third trophic level. Field populations of EPN may be less effective at host-finding using pregeijerene as a cue in citrus grove agroecosystems with high presence of CLas infection.

The Nep1-like protein family of Magnaporthe oryzae is dispensable for the infection of rice plants

The necrosis- and ethylene-inducing protein 1 (Nep1)-like proteins (NLPs) are a class of microbe-associated molecular patterns widely distributed across diverse groups of plant-associated microorganisms. In spite of the cytotoxic activity in dicot plants, the role of most NLPs in the virulence of plant pathogens is still largely unknown. We showed that the MoNLP family of rice blast fungus varied very little in amino acid sequence, transient expression of three MoNLPs induced cell death and the production of reactive oxygen species in Nicotiana benthamiana, and the expression of MoNLPs was induced during infection of susceptible rice plants. To further investigate the biological role of the MoNLP family, a marker-free gene replacement vector was developed and used to knock out the whole family in Magnaporthe oryzae. Results showed no significant difference in disease levels caused by wild type and the quadruple ΔMoNLP mutant strains. Likewise, the sporulation and radial growth of the two strains were similar under various unfavorable cultural conditions including malnutrition and abiotic stresses. These observations demonstrated that the MoNLP family is dispensable for the fungal tolerance to the tested adverse cultural conditions, and more importantly, for the virulence of blast fungus on susceptible rice plants.

Prediction and analysis of three gene families related to leaf rust (Puccinia triticina) resistance in wheat (Triticum aestivum L.)

BACKGROUND

The resistance to leaf rust (Lr) caused by Puccinia triticina in wheat (Triticum aestivum L.) has been well studied over the past decades with over 70 Lr genes being mapped on different chromosomes and numerous QTLs (quantitative trait loci) being detected or mapped using DNA markers. Such resistance is often divided into race-specific and race-nonspecific resistance. The race-nonspecific resistance can be further divided into resistance to most or all races of the same pathogen and resistance to multiple pathogens. At the molecular level, these three types of resistance may cover across the whole spectrum of pathogen specificities that are controlled by genes encoding different protein families in wheat. The objective of this study is to predict and analyze genes in three such families: NBS-LRR (nucleotide-binding sites and leucine-rich repeats or NLR), START (Steroidogenic Acute Regulatory protein [STaR] related lipid-transfer) and ABC (ATP-Binding Cassette) transporter. The focus of the analysis is on the patterns of relationships between these protein-coding genes within the gene families and QTLs detected for leaf rust resistance.

RESULTS

We predicted 526 ABC, 1117 NLR and 144 START genes in the hexaploid wheat genome through a domain analysis of wheat proteome. Of the 1809 SNPs from leaf rust resistance QTLs in seedling and adult stages of wheat, 126 SNPs were found within coding regions of these genes or their neighborhood (5 Kb upstream from transcription start site [TSS] or downstream from transcription termination site [TTS] of the genes). Forty-three of these SNPs for adult resistance and 18 SNPs for seedling resistance reside within coding or neighboring regions of the ABC genes whereas 14 SNPs for adult resistance and 29 SNPs for seedling resistance reside within coding or neighboring regions of the NLR gene. Moreover, we found 17 nonsynonymous SNPs for adult resistance and five SNPs for seedling resistance in the ABC genes, and five nonsynonymous SNPs for adult resistance and six SNPs for seedling resistance in the NLR genes. Most of these coding SNPs were predicted to alter encoded amino acids and such information may serve as a starting point towards more thorough molecular and functional characterization of the designated Lr genes. Using the primer sequences of 99 known non-SNP markers from leaf rust resistance QTLs, we found candidate genes closely linked to these markers, including Lr34 with distances to its two gene-specific markers being 1212 bases (to cssfr1) and 2189 bases (to cssfr2).

CONCLUSION

This study represents a comprehensive analysis of ABC, NLR and START genes in the hexaploid wheat genome and their physical relationships with QTLs for leaf rust resistance at seedling and adult stages. Our analysis suggests that the ABC (and START) genes are more likely to be co-located with QTLs for race-nonspecific, adult resistance whereas the NLR genes are more likely to be co-located with QTLs for race-specific resistance that would be often expressed at the seedling stage. Though our analysis was hampered by inaccurate or unknown physical positions of numerous QTLs due to the incomplete assembly of the complex hexaploid wheat genome that is currently available, the observed associations between (i) QTLs for race-specific resistance and NLR genes and (ii) QTLs for nonspecific resistance and ABC genes will help discover SNP variants for leaf rust resistance at seedling and adult stages. The genes containing nonsynonymous SNPs are promising candidates that can be investigated in future studies as potential new sources of leaf rust resistance in wheat breeding.

The Bacterial Effector AvrB-Induced RIN4 Hyperphosphorylation Is Mediated by a Receptor-Like Cytoplasmic Kinase Complex in Arabidopsis

Bacterial pathogen Pseudomonas syringae delivers diverse type III effectors into host cells to interfere with their immune responses. One of the effectors, AvrB, targets a host guardee protein RIN4 and induces RIN4 phosphorylation in Arabidopsis. Phosphorylated RIN4 activates the immune receptor RPM1 to mount defense. AvrB-induced RIN4 phosphorylation depends on RIPK, a receptor-like cytoplasmic kinase (RLCK). In this study, we found several other RLCKs that were also able to phosphorylate RIN4. We demonstrated that these RLCKs formed a complex with RIPK and were functionally redundant to RIPK. We also found that unphosphorylated RIN4 was epistatic to phosphorylated RIN4 in terms of RPM1 activation. AvrB-induced RLCK gene expression and phosphorylated RIN4-triggered RPM1 activation required RAR1, a central regulator in plant innate immunity. Our results unravel a mechanism in which plants employ multiple kinases to hyperphosphorylate the guardee protein RIN4 to ensure immune activation during pathogen invasion.

Immune responses induced by oligogalacturonides are differentially affected by AvrPto and loss of BAK1/BKK1 and PEPR1/PEPR2

Plants possess an innate immune system capable of restricting invasion by most potential pathogens. At the cell surface, the recognition of microbe-associated molecular patterns (MAMPs) and/or damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs) represents the first event for the prompt mounting of an effective immune response. Pathogens have evolved effectors that block MAMP-triggered immunity. The Pseudomonas syringae effector AvrPto abolishes immunity triggered by the peptide MAMPs flg22 and elf18, derived from the bacterial flagellin and elongation factor Tu, respectively, by inhibiting the kinase function of the corresponding receptors FLS2 and EFR, as well as their co-receptors BAK1 and BKK1. Oligogalacturonides (OGs), a well-known class of DAMPs, are oligomers of α-1,4-linked galacturonosyl residues, released on partial degradation of the plant cell wall homogalacturonan. We show here that AvrPto affects only a subset of the OG-triggered immune responses and that, among these responses, only a subset is affected by the concomitant loss of BAK1 and BKK1. However, the antagonistic effect on auxin-related responses is not affected by either AvrPto or the loss of BAK1/BKK1. These observations reveal an unprecedented complexity among the MAMP/DAMP response cascades. We also show that the signalling system mediated by Peps, another class of DAMPs, and their receptors PEPRs, contributes to OG-activated immunity. We hypothesize that OGs are sensed through multiple and partially redundant perception/transduction complexes, some targeted by AvrPto, but not necessarily comprising BAK1 and BKK1.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

The Role of Plant Innate Immunity in the Legume-Rhizobium Symbiosis

A classic view of the evolution of mutualism is that it derives from a pathogenic relationship that attenuated over time to a situation in which both partners can benefit. If this is the case for rhizobia, then one might uncover features of the symbiosis that reflect this earlier pathogenic state. For example, as with plant pathogens, it is now generally assumed that rhizobia actively suppress the host immune response to allow infection and symbiosis establishment. Likewise, the host has retained mechanisms to control the nutrient supply to the symbionts and the number of nodules so that they do not become too burdensome. The open question is whether such events are strictly ancillary to the central symbiotic nodulation factor signaling pathway or are essential for rhizobial host infection. Subsequent to these early infection events, plant immune responses can also be induced inside nodules and likely play a role in, for example, nodule senescence. Thus, a balanced regulation of innate immunity is likely required throughout rhizobial infection, symbiotic establishment, and maintenance. In this review, we discuss the significance of plant immune responses in the regulation of symbiotic associations with rhizobia, as well as rhizobial evasion of the host immune system.