Downy mildew pathogens
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Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension | Downy mildew pathogens | Scoop.it

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis resulted in an epidemic that stunned the pickling cucumber (Cucumis sativus L.) industry along the East Coast of the United States. The scenario was repeated in 2005 and 2006 in the Midwestern United States. In severely affected areas yields were dramatically reduced, and some fields were abandoned without harvesting. The epidemic of 2004 established the pathogen’s potential for long-distance dispersal, since, wherever it occurred, it was virulent on previously resistant cucumber cultivars and resistant to mefenoxam- and strobilurin-based fungicides wherever it occurred. The resurgence of CDM reignited efforts to address basic questions on the epidemiology of the disease, particularly the long-distance dispersal from overwintering sources in the southern United States. In addition, recent population genetics studies and virulence tests on differential host cultivars demonstrated that considerable diversity exists within the species. Currently, an integrated management strategy that combines CDM alert systems, cultural practices, and fungicides is the best approach to minimizing losses to the U.S. cucurbit industry.

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BMC Genomics: Genome analyses of the sunflower pathogen Plasmopara halstedii provide insights into effector evolution in downy mildews and Phytophthora

BMC Genomics: Genome analyses of the sunflower pathogen Plasmopara halstedii provide insights into effector evolution in downy mildews and Phytophthora | Downy mildew pathogens | Scoop.it
Downy mildews are the most speciose group of oomycetes and affect crops of great economic importance. So far, there is only a single deeply-sequenced downy mildew genome available, from Hyaloperonospora arabidopsidis. Further genomic resources for downy mildews are required to study their evolution, including pathogenicity effector proteins, such as RxLR effectors. Plasmopara halstedii is a devastating pathogen of sunflower and a potential pathosystem model to study downy mildews, as several Avr-genes and R-genes have been predicted and unlike Arabidopsis downy mildew, large quantities of almost contamination-free material can be obtained easily.
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Precision QTL mapping of downy mildew resistance in hop (Humulus lupulus L.)

Precision QTL mapping of downy mildew resistance in hop (Humulus lupulus L.) | Downy mildew pathogens | Scoop.it
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UEA E-Thesis Service - Investigating the role of downy mildew effectors in host resistance and susceptibility. S Piquerez. 2011.

UEA E-Thesis Service - Investigating the role of downy mildew effectors in host resistance and susceptibility. S Piquerez. 2011. | Downy mildew pathogens | Scoop.it

Through specialised structures called haustoria, filamentous eukaryotic plant pathogens such as rusts and mildews can deliver “effector” proteins directly into plant cells in order to manipulate specific defence responses and processes. Hyaloperonospora arabidopsidis (Hpa) is a haustorium-producing biotrophic oomycete pathogen that causes downy mildew on Arabidopsis. Over 100 candidate effectors (called “HaRxLs”), carrying a characteristic RxLR protein motif, have been predicted from the Hpa genome sequence. This PhD work focused on the characterisation of some of these effectors and their interaction with plant defences. First, the Pseudomonas syringae pv. tomato (Pst) strain DC3000 type III secretion system was used to examine the effect of single oomycete effectors in planta. One of these, ATR13 from Hpa isolate Emco5 (ATR13Emco5) has been shown to be recognised in some Arabidopsis accessions by the RPP13 protein, preventing Hpa Emco5 from completing its life cycle on such resistant plants. Hpa Emco5 can however complete its life cycle on Ws-0 but paradoxically, ATR13Emco5 is also recognised in this accession. Here, I show that ATR13Emco5 is weakly recognised in Ws-0 by a single RPP13-independent, EDS1-independent dominant gene called RHA13, shown by rough mapping to position on Arabidopsis chromosome 4. Second, I contributed to two independent functional screens performed in our laboratory to experimentally characterise the Hpa effectorome. The first screen focused on HaRxLs effects on bacteria virulence using the EDV system. The second screen was based on HaRxLs subcellular localisation using Agrobacterium-mediated transient expression. As presented in this work, I showed that both screens identified putative interesting effectors which increased plant susceptibility to Pst and Hpa and localised to various plant subcellular compartments. One particular effector candidate, HaRxL79, localised to microtubules in planta and interacted with two microtubule-associated proteins in a yeast-two-hybrid assay. HaRxL79 was also found to interact with Arabidopsis histone chaperones (AtNAP1s) in the plant cytoplasm, which were observed to play a role in plant susceptibility to Hpa and partially to the necrotroph Botrytis cinerea, but not to the hemibiotrophs Pst and Phytophthora parasitica. AtNAP1s also interact with HaRxL67, a vacuole-associated Hpa effector. From this, I propose that AtNAP1s are susceptibility factors for Hpa, specifically targeted by Hpa effectors HaRxL67 and HaRxL79 in order to promote susceptibility and maintain biotrophy. - See more at: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.569388#sthash.615Rd4W7.dpuf

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Computational Prediction and Molecular Characterization of an Oomycete Effector and the Cognate Arabidopsis Resistance Gene

Computational Prediction and Molecular Characterization of an Oomycete Effector and the Cognate Arabidopsis Resistance Gene | Downy mildew pathogens | Scoop.it

Interactions between Arabidopsis thaliana and its native obligate oomycete pathogenHyaloperonospora arabidopsidis (Hpa) represent a model system to study evolution of natural variation in a host/pathogen interaction. Both Arabidopsis and Hpa genomes are sequenced and collections of different sub-species are available. We analyzed ~400 interactions between different Arabidopsis accessions and five strains of Hpa. We examined the pathogen's overall ability to reproduce on a given host, and performed detailed cytological staining to assay for pathogen growth and hypersensitive cell death response in the host. We demonstrate that intermediate levels of resistance are prevalent among Arabidopsis populations and correlate strongly with host developmental stage. In addition to looking at plant responses to challenge by whole pathogen inoculations, we investigated the Arabidopsis resistance attributed to recognition of the individual Hpa effectors, ATR1 and ATR13. Our results suggest that recognition of these effectors is evolutionarily dynamic and does not form a single clade in overall Arabidopsis phylogeny for either effector. Furthermore, we show that the ultimate outcome of the interactions can be modified by the pathogen, despite a defined gene-for-gene resistance in the host. These data indicate that the outcome of disease and disease resistance depends on genome-for-genome interactions between the host and its pathogen, rather than single gene pairs as thought previously.

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The Downy Mildew Effector Proteins ATR1 and ATR13 Promote Disease Susceptibility in Arabidopsis thaliana

The Downy Mildew Effector Proteins ATR1 and ATR13 Promote Disease Susceptibility in Arabidopsis thaliana | Downy mildew pathogens | Scoop.it

The downy mildew (Hyaloperonospora parasitica) effector proteins ATR1 and ATR13 trigger RPP1-Nd/WsB– and RPP13-Nd–dependent resistance, respectively, in Arabidopsis thaliana. To better understand the functions of these effectors during compatible and incompatible interactions of H. parasitica isolates onArabidopsis accessions, we developed a novel delivery system usingPseudomonas syringae type III secretion via fusions of ATRs to the N terminus of the P. syringae effector protein, AvrRPS4. ATR1 and ATR13 both triggered the hypersensitive response (HR) and resistance to bacterial pathogens in Arabidopsiscarrying RPP1-Nd/WsB or RPP13-Nd, respectively, when delivered from P. syringae pv tomato (Pst) DC3000. In addition, multiple alleles of ATR1 and ATR13 confer enhanced virulence to Pst DC3000 on susceptible Arabidopsis accessions. We conclude that ATR1 and ATR13 positively contribute to pathogen virulence inside host cells. Two ATR13 alleles suppressed bacterial PAMP (for Pathogen-Associated Molecular Patterns)-triggered callose deposition in susceptibleArabidopsis when delivered by DC3000 ΔCEL mutants. Furthermore, expression of another allele of ATR13 in plant cells suppressed PAMP-triggered reactive oxygen species production in addition to callose deposition. Intriguingly, although Wassilewskija (Ws-0) is highly susceptible to H. parasitica isolate Emco5, ATR13Emco5 when delivered by Pst DC3000 triggered localized immunity, including HR, on Ws-0. We suggest that an additional H. parasitica.

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454 Genome sequencing of Pseudoperonospora cubensis reveals effector proteins with a QXLR translocation motif

454 Genome sequencing of Pseudoperonospora cubensis reveals effector proteins with a QXLR translocation motif | Downy mildew pathogens | Scoop.it

Pseudoperonospora cubensis is a biotrophic oomycete pathogen that causes downy mildew of cucurbits, a devastating foliar disease threatening cucurbit production worldwide. We sequenced P. cubensisgenomic DNA using 454 pyrosequencing and obtained random genomic sequences covering approximately 14% of the genome, thus providing the first set of useful genomic sequence information for P. cubensis. Using bioinformatics approaches, we identified 32 putative RXLR effector proteins. Interestingly, we also identified 29 secreted peptides with high similarity to RXLR effectors at the N-terminal translocation domain, yet containing an R-to-Q substitution in the first residue of the translocation motif. Among these, a family of QXLR-containing proteins, designated as PcQNE, was confirmed to have a functional signal peptide and was further characterized as being localized in the plant nucleus. Internalization of secreted PcQNE into plant cells requires the QXLR-EER motif. This family has a large number of near-identical copies within the P. cubensis genome, is under diversifying selection at the C-terminal domain, and is upregulated during infection of plants, all of which are common characteristics of characterized oomycete effectors. Taken together, the data suggest that PcQNE are bona fide effector proteins with a QXLR translocation motif, and QXLR effectors are prevalent in P. cubensis. Furthermore, the massive duplication ofPcQNE suggests that they might play pivotal roles in pathogen fitness and pathogenicity.

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mRNA-Seq Analysis of the Pseudoperonospora cubensis Transcriptome During Cucumber (Cucumis sativus L.) Infection

mRNA-Seq Analysis of the Pseudoperonospora cubensis Transcriptome During Cucumber (Cucumis sativus L.) Infection | Downy mildew pathogens | Scoop.it
PLOS ONE: an inclusive, peer-reviewed, open-access resource from the PUBLIC LIBRARY OF SCIENCE. Reports of well-performed scientific studies from all disciplines freely available to the whole world.
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Homologous RXLR effectors from Hyaloperonospora arabidopsidis and Phytophthora sojae suppress immunity in distantly related plants

Homologous RXLR effectors from Hyaloperonospora arabidopsidis and Phytophthora sojae suppress immunity in distantly related plants | Downy mildew pathogens | Scoop.it
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Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension | Downy mildew pathogens | Scoop.it

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis resulted in an epidemic that stunned the pickling cucumber (Cucumis sativus L.) industry along the East Coast of the United States. The scenario was repeated in 2005 and 2006 in the Midwestern United States. In severely affected areas yields were dramatically reduced, and some fields were abandoned without harvesting. The epidemic of 2004 established the pathogen’s potential for long-distance dispersal, since, wherever it occurred, it was virulent on previously resistant cucumber cultivars and resistant to mefenoxam- and strobilurin-based fungicides wherever it occurred. The resurgence of CDM reignited efforts to address basic questions on the epidemiology of the disease, particularly the long-distance dispersal from overwintering sources in the southern United States. In addition, recent population genetics studies and virulence tests on differential host cultivars demonstrated that considerable diversity exists within the species. Currently, an integrated management strategy that combines CDM alert systems, cultural practices, and fungicides is the best approach to minimizing losses to the U.S. cucurbit industry.

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Mechanisms of Nuclear Suppression of Host Immunity by Effectors from the Arabidopsis Downy Mildew Pathogen Hyaloperonospora arabidopsidis (Hpa)

Mechanisms of Nuclear Suppression of Host Immunity by Effectors from the Arabidopsis Downy Mildew Pathogen Hyaloperonospora arabidopsidis (Hpa) | Downy mildew pathogens | Scoop.it

Filamentous phytopathogens form sophisticated intracellular feeding structures called haustoria in plant cells. Pathogen effectors are likely to play a role in the establishment and maintenance of haustoria additional to their more characterized role of suppressing plant defense. Recent studies suggest that effectors may manipulate host transcription or other nuclear regulatory components for the benefit of pathogen development. However, the specific mechanisms by which these effectors promote susceptibility remain unclear. Of two recent screenings, we identified 15 nuclear-localized Hpa effectors (HaRxLs) that interact directly or indirectly with host nuclear components. When stably expressed in planta, nuclear HaRxLs cause diverse developmental phenotypes highlighting that nuclear effectors might interfere with fundamental plant regulatory mechanisms. Here, we report recent advances in understanding how a pathogen can manipulate nuclear processes in order to cause disease.

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Effector identification in the lettuce downy mildew Bremia lactucae by massively parallel transcriptome sequencing

Effector identification in the lettuce downy mildew Bremia lactucae by massively parallel transcriptome sequencing | Downy mildew pathogens | Scoop.it

Lettuce downy mildew (Bremia lactucae) is a rapidly adapting oomycete pathogen affecting commercial lettuce cultivation. Oomycetes are known to use a diverse arsenal of secreted proteins (effectors) to manipulate their hosts. Two classes of effector are known to be translocated by the host: the RXLRs and Crinklers. To gain insight into the repertoire of effectors used by B. lactucae to manipulate its host, we performed massively parallel sequencing of cDNA derived from B. lactucae spores and infected lettuce (Lactuca sativa) seedlings. From over 2.3 million 454 GS FLX reads, 59 618 contigs were assembled representing both plant and pathogen transcripts. Of these, 19 663 contigs were determined to be of B. lactucae origin as they matched pathogen genome sequences (SOLiD) that were obtained from >270 million reads of spore-derived genomic DNA. After correction of cDNA sequencing errors with SOLiD data, translation into protein models and filtering, 16 372 protein models remained, 1023 of which were predicted to be secreted. This secretome included elicitins, necrosis and ethylene-inducing peptide 1-like proteins, glucanase inhibitors and lectins, and was enriched in cysteine-rich proteins. Candidate host-translocated effectors included 78 protein models with RXLR effector features. In addition, we found indications for an unknown number of Crinkler-like sequences. Similarity clustering of secreted proteins revealed additional effector candidates. We provide a first look at the transcriptome of B. lactucae and its encoded effector arsenal.

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A Downy Mildew Effector Attenuates Salicylic Acid–Triggered Immunity in Arabidopsis by Interacting with the Host Mediator Complex

A Downy Mildew Effector Attenuates Salicylic Acid–Triggered Immunity in Arabidopsis by Interacting with the Host Mediator Complex | Downy mildew pathogens | Scoop.it
PLOS Biology is an open-access, peer-reviewed journal that features works of exceptional significance in all areas of biological science, from molecules to ecosystems, including works at the interface with other disciplines.
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The cucurbit downy mildew pathogen Pseudoperonospora cubensis

The cucurbit downy mildew pathogen Pseudoperonospora cubensis | Downy mildew pathogens | Scoop.it

Pseudoperonospora cubensis[(Berkeley & M. A. Curtis) Rostovzev], the causal agent of cucurbit downy mildew, is responsible for devastating losses worldwide of cucumber, cantaloupe, pumpkin, watermelon and squash. Although downy mildew has been a major issue in Europe since the mid-1980s, in the USA, downy mildew on cucumber has been successfully controlled for many years through host resistance. However, since the 2004 growing season, host resistance has been effective no longer and, as a result, the control of downy mildew on cucurbits now requires an intensive fungicide programme. Chemical control is not always feasible because of the high costs associated with fungicides and their application. Moreover, the presence of pathogen populations resistant to commonly used fungicides limits the long-term viability of chemical control. This review summarizes the current knowledge of taxonomy, disease development, virulence, pathogenicity and control of Ps. cubensis. In addition, topics for future research that aim to develop both short- and long-term control measures of cucurbit downy mildew are discussed.

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