Studying the Mechanism of Plasmopara viticola RxLR Effectors on Suppressing Plant Immunity

ADP-ribosylation factor 1 (ARF1) protein interacts with elicitor PvNLP7 from Plasmopara viticola to mediate PvNLP7-triggered immunity

Revealing the effector-host molecular interactions is crucial for understanding the host immunity against Plasmopara viticola and devising innovative disease management strategies. As a pathogenic oomycete causing grapevine downy mildew, Plasmopara viticola employs various effectors to manipulate the defense systems of host plants. One of these P. viticola derived effectors is necrosis- and ethylene-inducing peptide 1 (Nep1) -like protein (PvNLP7), which has been known to elicit cell death and immune responses in plants. However, the underlying molecular mechanisms remain obscure, prompting the focus of this study. Through yeast two-hybrid screening, we have identified the Vitis rotundifolia ADP-ribosylation factor (VrARF1) as a host interactor of PvNLP7. This interaction is corroborated through bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP) assays. Heterologous expression of VrARF1 in Nicotiana benthamiana verifies its accumulation in both the cytoplasm and nucleus, and induction of cell death. Moreover, the VrARF1 gene is strongly induced during early P. viticola infection and upon PvNLP7 transient expression. Overexpression of the VrARF1 gene in grapevine and N. benthamiana enhances resistance to P. viticola and Phytophthora capsici, respectively, via induction of defense related genes PR1 and PR2. Conversely, virus-induced gene silencing (VIGS) of NbARF1 in N. benthamiana, homologous to VrARF1, markedly attenuates PvNLP7-triggered cell death and reduces the expression of four PTI marker genes (PTI5, Acre31, WRKY7 and Cyp71D20) and two defense related genes (PR1 and PR2), rendering plants transiently transformed with PvNLP7 more susceptible to oomycete P. capsici. These findings highlight the role of ARF1 in mediating PvNLP7-induced immunity and indicate its potential as a target for engineering disease-resistant transgenic plants against oomycete pathogens.

The pathogenicity of Plasmopara viticola: a review of evolutionary dynamics, infection strategies and effector molecules

Oomycetes are filamentous organisms that resemble fungi in terms of morphology and life cycle, primarily due to convergent evolution. The success of pathogenic oomycetes lies in their ability to adapt and overcome host resistance, occasionally transitioning to new hosts. During plant infection, these organisms secrete effector proteins and other compounds during plant infection, as a molecular arsenal that contributes to their pathogenic success. Genomic sequencing, transcriptomic analysis, and proteomic studies have revealed highly diverse effector repertoires among different oomycete pathogens, highlighting their adaptability and evolution potential.The obligate biotrophic oomycete Plasmopara viticola affects grapevine plants (Vitis vinifera L.) causing the downy mildew disease, with significant economic impact. This disease is devastating in Europe, leading to substantial production losses. Even though Plasmopara viticola is a well-known pathogen, to date there are scarce reviews summarising pathogenicity, virulence, the genetics and molecular mechanisms of interaction with grapevine.This review aims to explore the current knowledge of the infection strategy, lifecycle, effector molecules, and pathogenicity of Plasmopara viticola. The recent sequencing of the Plasmopara viticola genome has provided new insights into understanding the infection strategies employed by this pathogen. Additionally, we will highlight the contributions of omics technologies in unravelling the ongoing evolution of this oomycete, including the first in-plant proteome analysis of the pathogen.

Advances in understanding grapevine downy mildew: From pathogen infection to disease management

Plasmopara viticola is geographically widespread in grapevine-growing regions. Grapevine downy mildew disease, caused by this biotrophic pathogen, leads to considerable yield losses in viticulture annually. Because of the great significance of grapevine production and wine quality, research on this disease has been widely performed since its emergence in the 19th century. Here, we review and discuss recent understanding of this pathogen from multiple aspects, including its infection cycle, disease symptoms, genome decoding, effector biology, and management and control strategies. We highlight the identification and characterization of effector proteins with their biological roles in host-pathogen interaction, with a focus on sustainable control methods against P. viticola, especially the use of biocontrol agents and environmentally friendly compounds.

Functional analysis of the Nep1-like proteins from Plasmopara viticola

Necrosis and ethylene-inducing peptide 1 (Nep1) -like proteins (NLP) are secreted by multiple taxonomically unrelated plant pathogens (bacteria, fungi, and oomycete) and are best known for inducing cell death and immune responses in dicotyledonous plants. A group of putative NLP genes from obligate biotrophic oomycete Plasmopara viticola were predicted by RNA-Seq in our previous study, but their activity has not been established. Therefore, we analyzed the P. viticola NLP (PvNLP) family and identified seven PvNLP genes. They all belong to type 1 NLP genes and form a P. viticola-specific cluster when compared with other pathogen NLP genes. The expression of PvNLPs was induced during early infection process and the expression patterns could be categorized into two groups. Agrobacterium tumefaciens-mediated transient expression assays revealed that only PvNLP7 was cytotoxic and could induce Phytophthora capsici resistance in Nicotiana benthamiana. Functional analysis showed that PvNLP4, PvNLP5, PvNLP7, and PvNLP10 significantly improved disease resistance of Arabidopsis thaliana to Hyaloperonospora arabidopsidis. Moreover, the four genes caused an inhibition of plant growth which is typically associated with enhanced immunity when over-expressed in Arabidopsis. Further research found that PvNLP7 could activate the expression of defense-related genes and its conserved NPP1 domain was critical for cell death- and immunity-inducing activity. This record of NLP genes from P. viticola showed a functional diversification, laying a foundation for further study on pathogenic mechanism of the devastating pathogen.

Candidate effector proteins from the oomycetes Plasmopara viticola and Phytophthora parasitica share similar predicted structures and induce cell death in Nicotiana species

Effector proteins secreted by plant pathogens are essential for infection. Cytoplasmic RXLR effectors from oomycetes are characterized by the presence of RXLR and EER motifs that are frequently linked to WY- and/or LWY-domains, folds that are exclusive to this effector family. A related family of secreted candidate effector proteins, carrying WY-domains and the EER motif but lacking the canonical RXLR motif, has recently been described in oomycetes and is mainly found in downy mildew pathogens. Plasmopara viticola is an obligate biotrophic oomycete causing grapevine downy mildew. Here we describe a conserved Pl. viticola secreted candidate non-RXLR effector protein with cell death-inducing activity in Nicotiana species. A similar RXLR effector candidate from the broad host range oomycete pathogen Phytophthora parasitica also induces cell death in Nicotiana. Through comparative tertiary structure modelling, we reveal that both proteins are predicted to carry WY- and LWY-domains. Our work supports the presence of LWY-domains in non-RXLR effectors and suggests that effector candidates with similar domain architecture may exert similar activities.

An in-planta comparative study of Plasmopara viticola proteome reveals different infection strategies towards susceptible and Rpv3-mediated resistance hosts

Plasmopara viticola, an obligate biotrophic oomycete, is the causal agent of one of the most harmful grapevine diseases, downy mildew. Within this pathosystem, much information is gathered on the host, as characterization of pathogenicity and infection strategy of a biotrophic pathogen is quite challenging. Molecular insights into P. viticola development and pathogenicity are just beginning to be uncovered, mainly by transcriptomic studies. Plasmopara viticola proteome and secretome were only predicted based on transcriptome data. In this study, we have identified the in-planta proteome of P. viticola during infection of a susceptible ('Trincadeira') and a Rpv3-mediated resistance ('Regent') grapevine cultivar. Four hundred and twenty P. viticola proteins were identified on a label-free mass spectrometry-based approach of the apoplastic fluid of grapevine leaves. Overall, our study suggests that, in the compatible interaction, P. viticola manipulates salicylic-acid pathway and isoprenoid biosynthesis to enhance plant colonization. Furthermore, during the incompatible interaction, development-associated proteins increased while oxidoreductases protect P. viticola from ROS-associated plant defence mechanism. Up to our knowledge this is the first in-planta proteome characterization of this biotrophic pathogen, thus this study will open new insights into our understanding of this pathogen colonization strategy of both susceptible and Rpv3-mediated resistance grapevine genotypes.

Systematic identification and functional characterization of the CFEM proteins in poplar fungus Marssonina brunnea

Proteins containing Common in Fungal Extracellular Membrane (CFEM) domains uniquely exist in fungi and play significant roles in their whole life history. In this study, a total of 11 MbCFEM proteins were identified from Marssonina brunnea f. sp. multigermtubi (MULT), a hemibiotrophic pathogenic fungus on poplars that causes severe leaf diseases. Phylogenic analysis showed that the 11 proteins (MbCFEM1-11) were divided into three clades based on the trans-membrane domain and the CFEM domain. Sequence alignment and WebLogo analysis of CFEM domains verified the amino acids conservatism therein. All of them possess eight cysteines except MbCFEM4 and MbCFEM11, which lack two cysteines each. Six MbCFEM proteins with a signal peptide and without trans-membrane domain were considered as candidate effectors for further functional analysis. Three-dimensional (3D) models of their CFEM domains presented a helical-basket structure homologous to the crucial virulence factor Csa2 of Candida albicans. Afterward, four (MbCFEM1, 6, 8, and 9) out of six candidate effectors were successfully cloned and a yeast signal sequence trap (YSST) assay confirmed their secretion activity. Pathogen challenge assays demonstrated that the transient expression of four candidate MbCFEM effectors in Nicotiana benthamiana promoted Fusarium proliferatum infection, respectively. In an N. benthamiana heterogeneous expression system, MbCFEM1, MbCFEM6, and MbCFEM9 appeared to suppress both BAX/INF1-triggered PCD, whereas MbCFEM8 could only defeat BAX-triggered PCD. Additionally, subcellular localization analysis indicated that the four candidate MbCFEM effectors accumulate in the cell membrane, nucleus, chloroplast, and cytosolic bodies. These results demonstrate that MbCFEM1, MbCFEM6, MbCFEM8, and MbCFEM9 are effectors of M. brunnea and provide valuable targets for further dissection of the molecular mechanisms underlying the poplar-M. brunnea interaction.

Ectopic Expression of HbRPW8-a from Hevea brasiliensis Improves Arabidopsis thaliana Resistance to Powdery Mildew Fungi (Erysiphe cichoracearum UCSC1)

The RPW8s (Resistance to Powdery Mildew 8) are atypical broad-spectrum resistance genes that provide resistance to the powdery mildew fungi. Powdery mildew of rubber tree is one of the serious fungal diseases that affect tree growth and latex production. However, the RPW8 homologs in rubber tree and their role of resistance to powdery mildew remain unclear. In this study, four RPW8 genes, HbRPW8-a, b, c, d, were identified in rubber tree, and phylogenetic analysis showed that HbRPW8-a was clustered with AtRPW8.1 and AtRPW8.2 of Arabidopsis. The HbRPW8-a protein was localized on the plasma membrane and its expression in rubber tree was significantly induced upon powdery mildew infection. Transient expression of HbRPW8-a in tobacco leaves induced plant immune responses, including the accumulation of reactive oxygen species and the deposition of callose in plant cells, which was similar to that induced by AtRPW8.2. Consistently, overexpression of HbRPW8-a in Arabidopsis thaliana enhanced plant resistance to Erysiphe cichoracearum UCSC1 and Pseudomonas syringae pv. tomato DC30000 (PstDC3000). Moreover, such HbRPW8-a mediated resistance to powdery mildew was in a salicylic acid (SA) dependent manner. Taken together, we demonstrated a new RPW8 member in rubber tree, HbRPW8-a, which could potentially contribute the resistance to powdery mildew.

Pathogen Adaptation to American (Rpv3-1) and Eurasian (Rpv29) Grapevine Loci Conferring Resistance to Downy Mildew

Durable resistance is a key objective in genetic improvement for disease resistance in grapevines, which must survive for years in the field in the presence of adaptable pathogen populations. In this study, the adaptation of 72 Northern Italian isolates of Plasmopara viticola, the downy mildew agent, has been investigated into Bianca, possessing Rpv3-1, the most frequently exploited resistance locus for genetic improvement, and Mgaloblishvili, a Vitis vinifera variety possessing the newly discovered Rpv29 locus. Infection parameters (latency period, infection frequency, and disease severity) and oospore production and viability were evaluated and compared to those of Pinot noir, the susceptible reference. The expected levels of disease control were achieved by both resistant cultivars (>90% on Bianca; >25% on Mgaloblishvili), despite the high frequency of isolates able to grow on one (28%) or both (46%) accessions. The disease incidence and severity were limited by both resistant cultivars and the strains able to grow on resistant accessions showed signatures of fitness penalties (reduced virulence, infection frequency, and oospore density). Together, these results indicate an adequate pathogen control but suitable practices must be adopted in the field to prevent the diffusion of the partially adapted P. viticola strains to protect resistance genes from erosion.

An RxLR effector from Plasmopara viticola suppresses plant immunity in grapevine by targeting and stabilizing VpBPA1

Grapevine downy mildew, caused by Plasmopara viticola, is one of the most devastating diseases in viticulture. Plasmopara viticola secretes RxLR effectors to modulate immune responses in grapevine. Here, we report an RxLR effector RxLR50253 from P. viticola that can interfere with plant immune response and thus promote pathogen colonization. RxLR50253 was induced at an early stage of P. viticola infection and could suppress elicitor (INF1 and Bax)-triggered cell death. RxLR50253 promote pathogen colonization in both tobacco and grapevine leaves. VpBPA1 was found to be the host target of RxLR50253 by yeast two-hybrid screening, and interaction between RxLR50253 and VpBPA1 was confirmed by multiple in vivo and in vitro assays. Further analysis revealed that VpBPA1 promoted pathogen colonization and decreased H₂ O₂ accumulation in transgenic tobacco and grapevine, while there was enhanced resistance and H₂ O₂ accumulation in NbBPA1-silenced Nicotiana benthamiana leaves. Moreover, transient expression of VpBPA1 in NbBPA1-silenced N. benthamiana leaves could reduce the accumulation of H₂ O₂ . Experiments in vivo demonstrated that RxLR50253 inhibits degradation of VpBPA1. Taken together, our findings showed that RxLR50253 targets and stabilizes VpBPA1 to attenuate plant immunity through decreasing H₂ O₂ accumulation during pathogen infection.

Identification and characterization of putative effectors from Plasmodiophora brassicae that suppress or induce cell death in Nicotiana benthamiana

Clubroot, caused by Plasmodiophora brassicae, is a major disease of crucifers. Effector proteins are important virulence factors in host recognition of pathogens and the interactions between pathogens and hosts. Secretory proteins, as effector candidates, have been studied in the interaction between Plasmodiophora brassicae and its hosts. In this study, 518 secretary proteins were screened from the Plasmodiophora brassicae genome. A total of 63 candidate effectors that induce or suppress cell death were identified using agroinfiltration-mediated transient expression in Nicothiana benthamiana. The candidate effectors, Pb4_102097 and Pb4_108104 showed high expressing level in the stage of rest spore maturity, could induce cell death and were associated with H₂O₂ accumulation in N. benthamiana leaves. In addition, 55 candidate effectors that could suppress BAX (Bcl-2-associated X protein) induced cell death, and 21 out of which could suppress the immunity caused by bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 expressing avrRps4 in Arabidopsis. Based on the expression pattern in different stages, 28 candidate effectors showed high expression levels during the primary and secondary infection stage. Five candidate effectors containing the RXLR motif functioned in the cytoplasm and cell membrane.

The Single-Stranded DNA-Binding Gene Whirly (Why1) with a Strong Pathogen-Induced Promoter from Vitis pseudoreticulata Enhances Resistance to Phytophthora capsici

Vitis vinifera plants are disease-susceptible while Vitis pseudoreticulata plants are disease-resistant; however, the molecular mechanism remains unclear. In this study, the single-stranded DNA- and RNA-binding protein gene Whirly (VvWhy1 and VpWhy1) were cloned from V. vinifera "Cabernet Sauvignon" and V. pseudoreticulata "HD1". VvWhy1 and VpWhy1 promoter sequences (pVv and pVp) were also isolated; however, the identity of the promoter sequences was far lower than that between the Why1 coding sequences (CDSs). Both Why1 gene sequences had seven exons and six introns, and they had a C-terminal Whirly conserved domain and N-terminal chloroplast transit peptide, which was then verified to be chloroplast localization. Transcriptional expression showed that VpWhy1 was strongly induced by Plasmopara viticola, while VvWhy1 showed a low expression level. Further, the GUS activity indicated pVp had high activity involved in response to Phytophthora capsici infection. In addition, Nicotiana benthamiana transiently expressing pVp::VvWhy1 and pVp::VpWhy1 enhanced the P. capsici resistance. Moreover, Why1, PR1 and PR10 were upregulated in pVp transgenic N. benthamiana leaves. This research presented a novel insight into disease resistance mechanism that pVp promoted the transcription of Why1, which subsequently regulated the expression of PR1 and PR10, further enhancing the resistance to P. capsici.

Novel sustainable strategies to control Plasmopara viticola in grapevine unveil new insights on priming responses and arthropods ecology

BACKGROUND

Reduction of fungicide consumption in agriculture is globally recognized as a priority. Government authorities are fostering research to achieve a reduction of risks associated with conventional pesticides and promoting the development of sustainable alternatives. To address these issues, in the present study, alternative protocols for the control of downy mildew infection in grapevine were compared to the standard protocol. In the first protocol, only resistance inducers were used, comprising a single formulation with Acibenzolar S-methyl, laminarin and disodium-phosphonate. The second and third protocols followed the standard protocol but substituted phosphonates with phosphorus pentoxide and Ecklonia maxima extract.

RESULTS

The results showed that at veraison downy mildew incidence and severity in all tested protocols were significantly reduced compared to nontreated controls on both canopy and bunches. Expression analysis of key genes involved in plant stress response, indicated that the two protocols for phosphites substitution induced a remodulation of salicylic acid (SA) and jasmonic acid (JA), with positive impact on yields. Analysis of the first protocol revealed that the primed state induced a short delay in bunch ripening, with a shift of carbohydrate metabolism to boost the plant defences, involving an upregulation of defence related-gene, SAR response and a decreased ROS detoxification. Additionally, analysis on the arthropods populations, in parallel with the positive results achieved using alternatives to conventional fungicides, were enriched by those showing the potential of naturally occurring predators of spider mites.

CONCLUSION

This study provides practical solutions to reduce the environmental impact of treatments for the control downy mildew in viticulture. © 2022 Society of Chemical Industry.

Sequence and Gene Expression Analysis of Recently Identified NLP from Plasmopara viticola

Grapevine downy mildew, evoked by the obligate biotrophic oomycete Plasmopara viticola, is one of the most challenging diseases in viticulture. P. viticola establishes an infection by circumvention of plant immunity, which is achieved by the secretion of effector molecules. One family of potential effectors are the necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP). NLP are most abundant in plant pathogenic microorganisms and exist in cytotoxic and non-cyctotoxic forms. Cytotoxic NLP often act as virulence factors and are synthesized in necrotrophic or hemibiotrophic pathogens during the transition from biotrophic to necrotrophic growth. In addition to these cytotoxic NLP, many non-cytotoxic NLP have been identified; their function in biotrophic pathogens is still unknown. In 2020, eight different NLP coding genes were identified in P. viticola and named PvNLP1 to PvNLP8 (Plasmopara viticolaNLP 1–8). In the present study, PvNLP4 to PvNLP8 were characterized by using qPCR analysis and transient expression in the model plant Nicotiana benthamiana. Gene expression analysis showed high PvNLP expression during the early stages of infection. Necrosis-inducing activity of PvNLP was not observed in the nonhost N. benthamiana.

Two sides of the same story in grapevine-pathogen interactions

Proteases are an integral part of plant defence systems, and their role in plant-pathogen interactions is unequivocal. Emerging evidence suggests that different protease families contribute to the establishment not only of hypersensitive response, priming, and signalling, but also of recognition events through complex proteolytic cascades. Moreover, they play a crucial role in pathogen/microbe-associated molecular pattern (PAMP/MAMP)-triggered immunity as well as in effector-triggered immunity. However, despite important advances in our understanding of the role of proteases in plant defence, the contribution of proteases to pathogen defence in grapevine remains poorly understood. In this review, we summarize current knowledge of the main grapevine pathosystems and explore the role of serine, cysteine, and aspartic proteases from both the host and pathogen point of views.

Plasmopara viticola effector PvRXLR53 suppresses innate immunity in Nicotiana benthamiana

Plasmopara viticola, the casual oomycete of grapevine downy mildew, could cause yield loss and compromise berry quantity. Previously, we have identified several PvRXLR effectors that could suppress plant immunity to promote infection and disease development. In this study, the role of effector, PvRXLR53, in plant-microbe interaction was investigated. PvRXLR53 has several orthologs in other oomycetes and contains a functional signal peptide. Expression level of PvRXLR53 was already detected upon inoculation, further induced in the early stage after P. viticola inoculation and decreased to low level in the late infection stage in grapevine (Vitis vinifera 'Cabernet Sauvignon'). PvRXLR53 is localized in both nucleus and cytoplasm. When transiently expressed in Nicotiana benthamiana, PvRXLR53 suppressed oomycete elicitor INF1-triggered programmed cell death and defense gene expression, and Phytophthora capsici-induced reactive oxygen species production (ROS) and eventually resistance to P. capsici. In summary, these findings suggest that P. viticola secretes PvRXLR53 to suppress host immunity from the very early stage of infection.

First Insights into the Effect of Mycorrhizae on the Expression of Pathogen Effectors during the Infection of Grapevine with Plasmopara viticola

Grapevine (Vitis vinifera L.), widely used for berry and wine production, is highly susceptible to the pathogenic oomycete Plasmopara viticola, the etiological agent of grapevine downy mildew disease. The method commonly used to prevent and control P. viticola infection relies on multiple applications of chemical fungicides. However, with European Union goals to lower the usage of such chemicals in viticulture there is a need to develop new and more sustainable strategies. The use of beneficial microorganisms with biocontrol capabilities, such as the arbuscular mycorrhizal fungi (AMF), has been pointed out as a viable alternative. With this study, we intended to investigate the effect of AMF colonization on the expression of P. viticola effectors during infection of grapevine. Grapevine plants were inoculated with the AMF Rhizophagus irregularis and, after mycorrhizae development, plants were infected with P. viticola. The expression of P. viticola RxLR effectors was analyzed by real-time PCR (qPCR) during the first hours of interaction. Results show that pre-mycorrhizal inoculation of grapevine alters the expression of several P. viticola effectors; namely, PvRxLR28, which presented decreased expression in mycorrhizal plants at the two time points post-infection tested. These results suggest that the pre-inoculation of grapevine with AMF could interfere with the pathogen’s ability to infect grapevine by modulation of pathogenicity effectors expression, supporting the hypothesis that AMF can be used to increase plant resistance to pathogens and promote more sustainable agriculture practices, particularly in viticulture.

Candidate Effectors from Botryosphaeria dothidea Suppress Plant Immunity and Contribute to Virulence

Fungal effectors play important roles in host-pathogen interactions. Botryosphaeria dothidea is an ascomycetous fungus that is responsible for the diseases of hundreds of woody plant species, including apple ring rot, which seriously affects apples worldwide. However, little is known about the effectors of B. dothidea. In this study, we analyzed the B. dothidea genome and predicted 320 candidate effector genes, 124 of which were successfully amplified and cloned. We investigated the effects of these genes on plant cell death in Nicotiana benthamiana while using a transient expression system. Twenty-four hours after initial inoculation with Agrobacterium tumefaciens cells carrying candidate effectors, the infiltrated leaves were challenged with A. tumefaciens cells carrying the BAX gene. In total, 116 candidate effectors completely inhibited, while one partially inhibited, the programmed cell death (PCD) of N. benthamiana induced by BAX, whereas seven candidate effectors had no effect. We then further tested seven candidate effectors able to suppress BAX-triggered PCD (BT-PCD) and found that they all completely inhibited PCD triggered by the elicitors INF1, MKK1, and NPK1. This result suggests that these effectors were activated in order to suppress pathogen-associated molecular pattern-triggered immunity. The signal peptides of these candidate effectors exhibited secretory activity in yeast (pSUC2 vector). Moreover, the respective deletion of Bdo_11198 and Bdo_12090 significantly reduced the virulence of B. dothidea. These results suggest that these effectors play important roles in the interaction of B. dothidea with its hosts.

A Sec-Dependent Secretory Protein of the Huanglongbing-Associated Pathogen Suppresses Hypersensitive Cell Death in Nicotiana benthamiana

"Candidatus Liberibacter asiaticus" (CLas) is a phloem-restricted Gram-negative bacterium that is the causal agent of citrus huanglongbing (HLB). In this study, we identified a CLas-encoded Sec-dependent secretory protein CLIBASIA_04405 that could contribute to the pathogenicity of this bacterium. The gene expression level of CLIBASIA_04405 was significantly higher in citrus than in psyllids. Transient overexpression of the mature CLIBASIA_04405 protein (m4405) in Nicotiana benthamiana leaves could suppress hypersensitive response (HR)-based cell death and H₂O₂ accumulation triggered by the mouse BAX and the Phytophthora infestans INF1. An alanine-substitution mutagenesis assay revealed the essential of amino acid clusters EKR^45-47 and DE^64-65 in cell death suppression. Challenge inoculation of the transgenic N. benthamiana-expressing m4405 with Pseudomonas syringae DC3000ΔhopQ1-1 demonstrated the greatly reduced bacterial proliferation. Remarkably, transcriptome profiling and RT-qPCR analysis disclosed that the gene expression of six small heat shock proteins (sHSPs), a set of plant defense regulators, were significantly elevated in the transgenic m4405 lines compared with those in wild-type N. benthamiana. In addition, the transgenic m4405 lines displayed phenotypes of dwarfism and leaf deformation. Altogether, these data indicated that m4405 was a virulence factor of CLas.

Insight Into Function and Subcellular Localization of Plasmopara viticola Putative RxLR Effectors

Grapevine downy mildew, caused by oomycete fungus Plasmopara viticola, is one of the most devastating diseases of grapes across the major production regions of the world. Although many putative effector molecules have been identified from this pathogen, the functions of the majority of these are still unknown. In this study, we analyzed the potential function of 26 P. viticola effectors from the highly virulent strain YL. Using transient expression in leaf cells of the tobacco Nicotiana benthamiana, we found that the majority of the effectors could suppress cell death triggered by BAX and INF1, while seven could induce cell death. The subcellular localization of effectors in N. benthamiana was consistent with their localization in cells of Vitis vinifera. Those effectors that localized to the nucleus (17/26) showed a variety of subnuclear localization. Ten of the effectors localized predominantly to the nucleolus, whereas the remaining seven localized to nucleoplasm. Interestingly, five of the effectors were strongly related in sequence and showed identical subcellular localization, but had different functions in N. benthamiana leaves and expression patterns in grapevine in response to P. viticola. This study highlights the potential functional diversity of P. viticola effectors.

Lso-HPE1, an Effector of 'Candidatus Liberibacter solanacearum', Can Repress Plant Immune Response

'Candidatus Liberibacter solanacearum' is a plant pathogen affecting the families Solanaceae and Apiaceae in different parts of the world. 'Ca. L. solanacearum' is a Gram-negative, fastidious α-proteobacterium that is vectored by different psyllid species. Plant-pathogenic bacteria are known for interfering with the host physiology or defense mechanisms, often by secreting bacterial effectors. Effector proteins are critical for virulence; therefore, the identification of effectors could help with disease management. In this study, we characterized the Sec-translocon-dependent 'Ca. L. solanacearum'-hypothetical protein effector 1 (Lso-HPE1). We compared this protein sequence in the different 'Ca. L. solanacearum' haplotypes. We predicted the signal peptide and validated its function using Escherichia coli's alkaline phosphatase fusion assay. Agrobacterium tumefaciens-mediated transient expression in Nicotiana benthamiana demonstrated that Lso-HPE1 from 'Ca. L. solanacearum' haplotypes A and B were able to inhibit the induction of cell death in plants. We also compared gene expression of the Lso-HPE1- transcripts in 'Ca. L. solanacearum' haplotypes A and B in tomato and in the vector Bactericera cockerelli. This work validates the identification of a Sec-translocon-dependent 'Ca. L. solanacearum' protein possibly involved in suppression of plant cell death.

An RXLR effector PlAvh142 from Peronophythora litchii triggers plant cell death and contributes to virulence

Litchi downy blight, caused by the phytopathogenic oomycete Peronophythora litchii, results in tremendous economic loss in litchi production every year. To successfully colonize the host cell, Phytophthora species secret hundreds of RXLR effectors that interfere with plant immunity and facilitate the infection process. Previous work has already predicted 245 candidate RXLR effector-encoding genes in P. litchii, 212 of which have been cloned and tested for plant cell death-inducing activity in this study. We found three such RXLR effectors could trigger plant cell death through transient expression in Nicotiana benthamiana. Further experiments demonstrated that PlAvh142 could induce cell death and immune responses in several plants. We also found that PlAvh142 localized in both the cytoplasm and nucleus of plant cells. The cytoplasmic localization was critical for its cell death-inducing activity. Moreover, deletion either of the two internal repeats in PlAvh142 abolished the cell death-inducing activity. Virus-induced gene silencing assays showed that cell death triggered by PlAvh142 was dependent on the plant transduction components RAR1 (require for Mla12 resistance), SGT1 (suppressor of the G2 allele of skp1) and HSP90 (heat shock protein 90). Finally, knockout of PlAvh142 resulted in significantly attenuated P. litchii virulence on litchi plants, whereas the PlAvh142-overexpressed mutants were more aggressive. These data indicated that PlAvh142 could be recognized in plant cytoplasm and is an important virulence RXLR effector of P. litchii.

Novel Aspects on The Interaction Between Grapevine and Plasmopara viticola: Dual-RNA-Seq Analysis Highlights Gene Expression Dynamics in The Pathogen and The Plant During The Battle For Infection

Mgaloblishvili, a Vitis vinifera cultivar, exhibits unique resistance traits against Plasmopara viticola, the downy mildew agent. This offers the unique opportunity of exploring the molecular responses in compatible and incompatible plant-pathogen interaction. In this study, whole transcriptomes of Mgaloblishvili, Pinot noir (a V. vinifera susceptible cultivar), and Bianca (a resistant hybrid) leaves, inoculated and non-inoculated with the pathogen, were used to identify P. viticola effector-encoding genes and plant susceptibility/resistance genes. Multiple effector-encoding genes were identified in P. viticola transcriptome, with remarkable expression differences in relation to the inoculated grapevine cultivar. Intriguingly, five apoplastic effectors specifically associated with resistance in V. vinifera. Gene coexpression network analysis identified specific modules and metabolic changes occurring during infection in the three grapevine cultivars. Analysis of these data allowed, for the first time, the detection in V. vinifera of a putative P. viticola susceptibility gene, encoding a LOB domain-containing protein. Finally, the de novo assembly of Mgaloblishvili, Pinot noir, and Bianca transcriptomes and their comparison highlighted novel candidate genes that might be at the basis of the resistant phenotype. These results open the way to functional analysis studies and to new perspectives in molecular breeding of grapevine for resistance to P. viticola.

Plasmopara viticola effector PvRXLR159 suppresses immune responses in Nicotiana benthamiana

Plasmopara viticola, the causal oomycete of grapevine downy mildew disease, secrets a series of RXLR effectors to manipulate host immunity. In this study, we characterized the role of a RXLR effector of P. viticola, PvRXLR159, in plant-microbe interaction. Transcription of PvRXLR159 in P. viticola was induced in the early stage of infection in grapevine (Vitis vinifera 'Thomson Seedless'). Further results revealed that PvRXLR159 contains a functional signal peptide and its C terminus was essential to inhibit cell death by elicitors, INF1 and BAX, in Nicotiana benthamiana. Transient expression of PvRXLR159 suppressed N. benthamiana resistance to a pathogenic oomycete, Phytophthora capsici. Taken together, we propose that PvRXLR159 is induced and secreted by P. viticola to suppress host resistance.

Identification and Characterization of Plasmodiophora brassicae Primary Infection Effector Candidates that Suppress or Induce Cell Death in Host and Nonhost Plants

Clubroot caused by Plasmodiophora brassicaeis one of the most important diseases in cruciferous crops. The recognition of P. brassicae by host plants is thought to occur at the primary infection stage, but the underlying mechanism remains unclear. Secretory proteins as effector candidates play critical roles in the recognition of pathogens and the interactions between pathogens and hosts. In this study, 33 P. brassicae secretory proteins expressed during primary infection were identified through transcriptome, secretory protein prediction, and yeast signal sequence trap analyses. Furthermore, the proteins that could suppress or induce cell death were screened through an Agrobacterium-mediated plant virus transient expression system and a protoplast transient expression system. Two secretory proteins, PBCN_002550 and PBCN_005499, were found to be capable of inducing cell death associated with H2O2 accumulation and electrolyte leakage in Nicotiana benthamiana. Moreover, PBCN_002550 could also induce cell death in Chinese cabbage. In addition, 24 of the remaining 31 tested secretory proteins could suppress mouse Bcl-2-associated X protein-induced cell death, and 28 proteins could suppress PBCN_002550-induced cell death.

A secreted WY-domain-containing protein present in European isolates of the oomycete Plasmopara viticola induces cell death in grapevine and tobacco species

Plasmopara viticola is a biotrophic oomycete pathogen causing grapevine downy mildew. We characterized the repertoire of P. viticola effector proteins which may be translocated into plants to support the disease. We found several secreted proteins that contain canonical dEER motifs and conserved WY-domains but lack the characteristic RXLR motif reported previously from oomycete effectors. We cloned four candidates and showed that one of them, Pv33, induces plant cell death in grapevine and Nicotiana species. This activity is dependent on the nuclear localization of the protein. Sequence similar effectors were present in seven European, but in none of the tested American isolates. Together our work contributes a new type of conserved P. viticola effector candidates.

The Nuclear-Localized RxLR Effector PvAvh74 From Plasmopara viticola Induces Cell Death and Immunity Responses in Nicotiana benthamiana

Downy mildew is one of the most serious diseases of grapevine (Vitis spp). The causal agent of grapevine downy mildew, Plasmopara viticola, is an obligate biotrophic oomycete. Although oomycete pathogens such as P. viticola are known to secrete RxLR effectors to manipulate host immunity, there have been few studies of the associated mechanisms by which these may act. Here, we show that a candidate P. viticola RxLR effector, PvAvh74, induces cell death in Nicotiana benthamiana leaves. Using agroinfiltration, we found that nuclear localization, two putative N-glycosylation sites, and 427 amino acids of the PvAvh74 carboxyl terminus were necessary for cell-death-inducing activity. Using virus-induced gene silencing (VIGS), we found that PvAvh74-induced cell death in N. benthamiana requires EDS1, NDR1, SGT1, RAR1, and HSP90, but not BAK1. The MAPK cascade components MEK2, WIPK, and SIPK were also involved in PvAvh74-induced cell death in N. benthamiana. Transient expression of PvAvh74 could suppress Phytophthora capsici colonization of N. benthamiana, which suggests that PvAvh74 elicits plant immune responses. Suppression of P. capsici colonization also was dependent on nuclear localization of PvAvh74. Additionally, PvAvh74-triggered cell death could be suppressed by another effector, PvAvh8, from the same isolate. This work provides a framework to further investigate the interactions of PvAvh74 and other RxLR effectors with host immunity.

A High-Quality Grapevine Downy Mildew Genome Assembly Reveals Rapidly Evolving and Lineage-Specific Putative Host Adaptation Genes

Downy mildews are obligate biotrophic oomycete pathogens that cause devastating plant diseases on economically important crops. Plasmopara viticola is the causal agent of grapevine downy mildew, a major disease in vineyards worldwide. We sequenced the genome of Pl. viticola with PacBio long reads and obtained a new 92.94 Mb assembly with high contiguity (359 scaffolds for a N50 of 706.5 kb) due to a better resolution of repeat regions. This assembly presented a high level of gene completeness, recovering 1,592 genes encoding secreted proteins involved in plant–pathogen interactions. Plasmopara viticola had a two-speed genome architecture, with secreted protein-encoding genes preferentially located in gene-sparse, repeat-rich regions and evolving rapidly, as indicated by pairwise dN/dS values. We also used short reads to assemble the genome of Plasmopara muralis, a closely related species infecting grape ivy (Parthenocissus tricuspidata). The lineage-specific proteins identified by comparative genomics analysis included a large proportion of RxLR cytoplasmic effectors and, more generally, genes with high dN/dS values. We identified 270 candidate genes under positive selection, including several genes encoding transporters and components of the RNA machinery potentially involved in host specialization. Finally, the Pl. viticola genome assembly generated here will allow the development of robust population genomics approaches for investigating the mechanisms involved in adaptation to biotic and abiotic selective pressures in this species.

Plasmopara viticola effector PvRXLR131 suppresses plant immunity by targeting plant receptor-like kinase inhibitor BKI1

The grapevine downy mildew pathogen Plasmopara viticola secretes a set of RXLR effectors (PvRXLRs) to overcome host immunity and facilitate infection, but how these effectors function is unclear. Here, the biological function of PvRXLR131 was investigated via heterologous expression. Constitutive expression of PvRXLR131 in Colletotrichum gloeosporioides significantly enhanced its pathogenicity on grapevine leaves. Constitutive expression of PvRXLR131 in Arabidopsis promoted Pseudomonas syringae DC3000 and P. syringae DC3000 (hrcC^- ) growth as well as suppressed defence-related callose deposition. Transient expression of PvRXLR131 in Nicotiana benthamiana leaves could also suppress different elicitor-triggered cell death and inhibit plant resistance to Phytophthora capsici. Further analysis revealed that PvRXLR131 interacted with host Vitis vinifera BRI1 kinase inhibitor 1 (VvBKI1), and its homologues in N. benthamiana (NbBKI1) and Arabidopsis (AtBKI1). Moreover, bimolecular fluorescence complementation analysis revealed that PvRXLR131 interacted with VvBKI1 in the plasma membrane. Deletion assays showed that the C-terminus of PvRXLR131 was responsible for the interaction and mutation assays showed that phosphorylation of a conserved tyrosine residue in BKI1s disrupted the interaction. BKI1 was a receptor inhibitor of growth- and defence-related brassinosteroid (BR) and ERECTA (ER) signalling. When silencing of NbBKI1 in N. benthamiana, the virulence function of PvRXLR131 was eliminated, demonstrating that the effector activity is mediated by BKI1. Moreover, PvRXLR131-transgenic plants displayed BKI1-overexpression dwarf phenotypes and suppressed BR and ER signalling. These physiological and genetic data clearly demonstrate that BKI1 is a virulence target of PvRXLR131. We propose that P. viticola secretes PvRXLR131 to target BKI1 as a strategy for promoting infection.

A High-Quality Grapevine Downy Mildew Genome Assembly Reveals Rapidly Evolving and Lineage-Specific Putative Host Adaptation Genes

Downy mildews are obligate biotrophic oomycete pathogens that cause devastating plant diseases on economically important crops. Plasmopara viticola is the causal agent of grapevine downy mildew, a major disease in vineyards worldwide. We sequenced the genome of Pl. viticola with PacBio long reads and obtained a new 92.94 Mb assembly with high contiguity (359 scaffolds for a N50 of 706.5 kb) due to a better resolution of repeat regions. This assembly presented a high level of gene completeness, recovering 1,592 genes encoding secreted proteins involved in plant-pathogen interactions. Plasmopara viticola had a two-speed genome architecture, with secreted protein-encoding genes preferentially located in gene-sparse, repeat-rich regions and evolving rapidly, as indicated by pairwise dN/dS values. We also used short reads to assemble the genome of Plasmopara muralis, a closely related species infecting grape ivy (Parthenocissus tricuspidata). The lineage-specific proteins identified by comparative genomics analysis included a large proportion of RxLR cytoplasmic effectors and, more generally, genes with high dN/dS values. We identified 270 candidate genes under positive selection, including several genes encoding transporters and components of the RNA machinery potentially involved in host specialization. Finally, the Pl. viticola genome assembly generated here will allow the development of robust population genomics approaches for investigating the mechanisms involved in adaptation to biotic and abiotic selective pressures in this species.

Up-regulated RxLR effector genes of Plasmopara viticola in synchronized host-free stages and infected leaves of hosts with different susceptibility

Fast recognition of host signals and early activation of infection mechanisms in Plasmopara viticola are decisive for successful infestation of Vitis vinifera. To better understand interactive processes at the first front line of combat between the pathogen and its host, a specific pre-infective stage was generated in a host-free system. Zoospore encystment was triggered within minutes after treatment with CaCl₂. Subsequently, high rates of germ tube formation occurred in a synchronized manner. This method was employed to compare development-related gene expression in strains of different virulence. Soon after germination, spores showed strong up-regulation of two effector genes, PvRxLR18 and PvRxLR28, particularly in the high virulence strain. On infected grapevine leaf-discs of cultivars with different susceptibility, a similar up-regulation was found at 6 hours post inoculation (hpi). This effect was much more evident in the high virulence than in the low virulence strain and was significantly higher on leaves of the tolerant cultivar Regent than on Müller-Thurgau. In addition, PvRxLR67 was up-regulated 24 hpi in the high virulence strain indicating that different effectors are active in later infection stages. Differences in the expression pattern of RxLR effector genes between the two strains corroborated with infection symptoms visible by sporulation.

Cloning, Characterization, and Functional Investigation of VaHAESA from Vitis amurensis Inoculated with Plasmopara viticola

Plant pattern recognition receptors (PRRs) are essential for immune responses and establishing symbiosis. Plants detect invaders via the recognition of pathogen-associated molecular patterns (PAMPs) by PRRs. This phenomenon is termed PAMP-triggered immunity (PTI). We investigated disease resistance in Vitis amurensis to identify PRRs that are important for resistance against downy mildew, analyzed the PRRs that were upregulated by incompatible Plasmopara viticola infection, and cloned the full-length cDNA of the VaHAESA gene. We then analyzed the structure, subcellular localization, and relative disease resistance of VaHAESA. VaHAESA and PRR-receptor-like kinase 5 (RLK5) are highly similar, belonging to the leucine-rich repeat (LRR)-RLK family and localizing to the plasma membrane. The expression of PRR genes changed after the inoculation of V. amurensis with compatible and incompatible P. viticola; during early disease development, transiently transformed V. vinifera plants expressing VaHAESA were more resistant to pathogens than those transformed with the empty vector and untransformed controls, potentially due to increased H₂O₂, NO, and callose levels in the transformants. Furthermore, transgenic Arabidopsis thaliana showed upregulated expression of genes related to the PTI pathway and improved disease resistance. These results show that VaHAESA is a positive regulator of resistance against downy mildew in grapevines.

In Planta Functional Analysis and Subcellular Localization of the Oomycete Pathogen Plasmopara viticola Candidate RXLR Effector Repertoire

Downy mildew is one of the most destructive diseases of grapevine, causing tremendous economic loss in the grape and wine industry. The disease agent Plasmopara viticola is an obligate biotrophic oomycete, from which over 100 candidate RXLR effectors have been identified. In this study, 83 candidate RXLR effector genes (PvRXLRs) were cloned from the P. viticola isolate "JL-7-2" genome. The results of the yeast signal sequence trap assay indicated that most of the candidate effectors are secretory proteins. The biological activities and subcellular localizations of all the 83 effectors were analyzed via a heterologous Agrobacterium-mediated Nicotiana benthamiana expression system. Results showed that 52 effectors could completely suppress cell death triggered by elicitin, 10 effectors could partially suppress cell death, 11 effectors were unable to suppress cell death, and 10 effectors themselves triggered cell death. Live-cell imaging showed that the majority of the effectors (76 of 83) could be observed with informative fluorescence signals in plant cells, among which 34 effectors were found to be targeted to both the nucleus and cytosol, 29 effectors were specifically localized in the nucleus, and 9 effectors were targeted to plant membrane system. Interestingly, three effectors PvRXLR61, 86 and 161 were targeted to chloroplasts, and one effector PvRXLR54 was dually targeted to chloroplasts and mitochondria. However, western blot analysis suggested that only PvRXLR86 carried a cleavable N-terminal transit peptide and underwent processing in planta. Many effectors have previously been predicted to target organelles, however, to the best of our knowledge, this is the first study to provide experimental evidence of oomycete effectors targeted to chloroplasts and mitochondria.

Molecular Characterization and Overexpression of VpRPW8s from Vitis pseudoreticulata Enhances Resistance to Phytophthora capsici in Nicotiana benthamiana

RPW8 genes are atypical broad-spectrum genes that provide resistance to powdery mildew, downy mildew, the cauliflower mosaic virus in Arabidopsis thaliana, and powdery mildew in tobacco. They play important roles in basal plant pathogen defense. They also provide insights into a novel disease resistance mechanism. In this study, we report on homologous RPW8 genes in Vitis pseudoreticulata. Five VpRPW8 genes were cloned; their Open Reading Frame (ORF) sequences ranged from 1994 base pairs to 2478 base pairs. They were comprised of five exons and four introns and shared 78.66% identity. Their proteins had typical conserved RPW8 and NB-LRR (the nucleotide-binding site and the leucine-rich repeats) domains (except VpRPW8-d, which lacked LRR domains). Prokaryotic expression results were consistent with predicted molecular weights. All five RPW8 genes were located in the cytoplasm. Quantitative real-time PCR (qRT-PCR) analysis showed that VpRPW8s in V. pseudoreticulata were induced by Plasmopara viticola, but nearly only VvRPW8-d genes were induced in Vitis vinifera. Furthermore, a VpRPW8 transgenic tobacco system was established. Overexpressed VpRPW8s enhanced resistance to Phytophthora capsici and VpRPW8s conferred varying degrees of resistance to Ph. capsici in Nicotiana benthamiana. Our study presents novel members of the plant RPW8 family and suggests that VpRPW8s are involved in enhanced resistance to P. viticola and Ph. capsici.

A multi-omics study of the grapevine-downy mildew (Plasmopara viticola) pathosystem unveils a complex protein coding- and noncoding-based arms race during infection

Fungicides are applied intensively to prevent downy mildew infections of grapevines (Vitis vinifera) with high impact on the environment. In order to develop alternative strategies we sequenced the genome of the oomycete pathogen Plasmopara viticola causing this disease. We show that it derives from a Phytophthora-like ancestor that switched to obligate biotrophy by losing genes involved in nitrogen metabolism and γ-Aminobutyric acid catabolism. By combining multiple omics approaches we characterized the pathosystem and identified a RxLR effector that trigger an immune response in the wild species V. riparia. This effector is an ideal marker to screen novel grape resistant varieties. Our study reveals an unprecedented bidirectional noncoding RNA-based mechanism that, in one direction might be fundamental for P. viticola to proficiently infect its host, and in the other might reduce the effects of the infection on the plant.

Genome sequence of Plasmopara viticola and insight into the pathogenic mechanism

Plasmopara viticola causes downy mildew disease of grapevine which is one of the most devastating diseases of viticulture worldwide. Here we report a 101.3 Mb whole genome sequence of P. viticola isolate 'JL-7-2' obtained by a combination of Illumina and PacBio sequencing technologies. The P. viticola genome contains 17,014 putative protein-coding genes and has ~26% repetitive sequences. A total of 1,301 putative secreted proteins, including 100 putative RXLR effectors and 90 CRN effectors were identified in this genome. In the secretome, 261 potential pathogenicity genes and 95 carbohydrate-active enzymes were predicted. Transcriptional analysis revealed that most of the RXLR effectors, pathogenicity genes and carbohydrate-active enzymes were significantly up-regulated during infection. Comparative genomic analysis revealed that P. viticola evolved independently from the Arabidopsis downy mildew pathogen Hyaloperonospora arabidopsidis. The availability of the P. viticola genome provides a valuable resource not only for comparative genomic analysis and evolutionary studies among oomycetes, but also enhance our knowledge on the mechanism of interactions between this biotrophic pathogen and its host.

Genome sequence of Plasmopara viticola and insight into the pathogenic mechanism

Plasmopara viticola causes downy mildew disease of grapevine which is one of the most devastating diseases of viticulture worldwide. Here we report a 101.3 Mb whole genome sequence of P. viticola isolate 'JL-7-2' obtained by a combination of Illumina and PacBio sequencing technologies. The P. viticola genome contains 17,014 putative protein-coding genes and has ~26% repetitive sequences. A total of 1,301 putative secreted proteins, including 100 putative RXLR effectors and 90 CRN effectors were identified in this genome. In the secretome, 261 potential pathogenicity genes and 95 carbohydrate-active enzymes were predicted. Transcriptional analysis revealed that most of the RXLR effectors, pathogenicity genes and carbohydrate-active enzymes were significantly up-regulated during infection. Comparative genomic analysis revealed that P. viticola evolved independently from the Arabidopsis downy mildew pathogen Hyaloperonospora arabidopsidis. The availability of the P. viticola genome provides a valuable resource not only for comparative genomic analysis and evolutionary studies among oomycetes, but also enhance our knowledge on the mechanism of interactions between this biotrophic pathogen and its host.

A candidate RxLR effector from Plasmopara viticola can elicit immune responses in Nicotiana benthamiana

BACKGROUND

Diverse plant pathogens deliver effectors into plant cells to alter host processes. Oomycete pathogen encodes a large number of putative RxLR effectors which are likely to play a role in manipulating plant defense responses. The secretome of Plasmopara viticola (downy mildew of grapevine) contains at least 162 candidate RxLR effectors discovered in our recent studies, but their roles in infection and pathogenicity remain to be determined. Here, we characterize in depth one of the putative RxLR effectors, PvRxLR16, which has been reported to induce cell death in Nicotiana benthamiana in our previous study.

RESULTS

The nuclear localization, W/Y/L motifs, and a putative N-glycosylation site in C-terminal of PvRxLR16 were essential for cell death-inducing activity. Suppressor of G-two allele of Skp1 (SGT1), heat shock protein 90 (HSP90) and required for Mla12 resistance (RAR1), but not somatic embryogenesis receptor-like kinase (SERK3), were required for the cell death response triggered by PvRxLR16 in N. benthamiana. Some mitogen-activated protein kinases and transcription factors were also involved in the perception of PvRxLR16 by N. benthamiana. PvRxLR16 could also significantly enhance plant resistance to Phytophthora capsici and the nuclear localization was required for this ability. However, some other PvRxLR effectors could suppress defense responses and disease resistance induced by PvRxLR16, suggesting that it may not trigger host cell death or immune responses during physiological infection under natural conditions.

CONCLUSION

These data demonstrate that PvRxLR16 may be recognized by endogenous proteins in nucleus to trigger immune responses in N. benthamiana, which in turn can be suppressed by other PvRxLR effectors.

Comparative analysis of pre- and post-parasitic transcriptomes and mining pioneer effectors of Heterodera avenae

BACKGROUND

The cereal cyst nematode (CCN, Heterodera avenae) is a devastating pathogen of wheat and barley crops in many countries. We aimed to prioritize genetic and molecular targets for H. avenae control via the powerful and integrative bioinformatics platform.

RESULTS

Here, we sequenced mRNA isolated from Chinese H. avenae at pre-parasitic (consisting of egg, J1 and hatched-J2) stages and post-parasitic (consisting of parasitic-J2, J3, J4 and adults) stages. Total 1,066,719 reads of whole life cycle transcriptomes were assembled into 10,811 contigs with N50 length of 1754 bp and 71,401 singletons. Comparative analyses of orthologous among H. avenae and 7 other nematodes with various life-styles revealed the significance and peculiarity of neurological system for sedentary phytonematode. KEGG pathway enrichment demonstrated active crosstalk events of nervous system at pre-parasitic stages, and 6 FMRFamide-like neuropeptides were verified to display an expression peak at the hatched-J2 stage in H. avenae. Furthermore, multiple approaches were undertaken to mine putative effectors and parasitism-specific genes. Notably, H. avenae might represent the first phytonematode reported to possess the pioneer effectors with RxLR motif and potential effectors with homologies to Ant-5/Ant-34.

CONCLUSION

Our work provides valuable resources for in-depth understanding the parasitism and pathogenicity of H. avenae, as well as developing new targets-oriented strategies on effective managements.

Overexpression of a Grapevine Sucrose Transporter (VvSUC27) in Tobacco Improves Plant Growth Rate in the Presence of Sucrose In vitro

The import of sugar from source leaves and it further accumulation in grape berries are considerably high during ripening, and this process is mediated via sucrose transporters. In this study, a grape sucrose transporter (SUT) gene, VvSUC27, located at the plasma membrane, was transferred to tobacco (Nicotiana tabacum). The transformants were more sensitive to sucrose and showed more rapid development, especially roots, when cultured on MS agar medium containing sucrose, considering that the shoot/root dry weight ratio was only half that of the control. Moreover, all transformed plants exhibited light-colored leaves throughout their development, which indicated chlorosis and an associated reduction in photosynthesis. The total sugar content in the roots and stems of transformants was higher than that in control plants. No significant difference was observed in the leaves between the transformants and control plants. The levels of growth-promoting hormones were increased, and those of stress-mediating hormones were reduced in transgenic tobacco plants. The qRT-PCR analysis revealed that the expression of VvSUC27 was 1,000 times higher than that of the autologous tobacco sucrose transporter, which suggested that the markedly increased growth rate of transformants was because of the heterogeneously expressed gene. The transgenic tobacco plants showed resistance to abiotic stresses. Strikingly, the overexpression of VvSUC27 leaded to the up regulation of most reactive oxygen species scavengers and abscisic acid-related genes that might enable transgenic plants to overcome abiotic stress. Taken together, these results revealed an important role of VvSUC27 in plant growth and response to abiotic stresses, especially in the presence of sucrose in vitro.

RXLR and CRN Effectors from the Sunflower Downy Mildew Pathogen Plasmopara halstedii Induce Hypersensitive-Like Responses in Resistant Sunflower Lines

Plasmopara halstedii is an obligate biotrophic oomycete causing downy mildew disease on sunflower, Helianthus annuus, an economically important oil crop. Severe symptoms of the disease (e.g., plant dwarfism, leaf bleaching, sporulation and production of infertile flower) strongly impair seed yield. Pl resistance genes conferring resistance to specific P. halstedii pathotypes were located on sunflower genetic map but yet not cloned. They are present in cultivated lines to protect them against downy mildew disease. Among the 16 different P. halstedii pathotypes recorded in France, pathotype 710 is frequently found, and therefore continuously controlled in sunflower by different Pl genes. High-throughput sequencing of cDNA from P. halstedii led us to identify potential effectors with the characteristic RXLR or CRN motifs described in other oomycetes. Expression of six P. halstedii putative effectors, five RXLR and one CRN, was analyzed by qRT-PCR in pathogen spores and in the pathogen infecting sunflower leaves and selected for functional analyses. We developed a new method for transient expression in sunflower plant leaves and showed for the first time subcellular localization of P. halstedii effectors fused to a fluorescent protein in sunflower leaf cells. Overexpression of the CRN and of 3 RXLR effectors induced hypersensitive-like cell death reactions in some sunflower near-isogenic lines resistant to pathotype 710 and not in susceptible corresponding lines, suggesting they could be involved in Pl loci-mediated resistances.