Resistance of nicotiana benthamiana to phytophthora infestans is mediated by the recognition of the elicitor protein INF1

MAP kinases function downstream of HSP90 and upstream of mitochondria in TMV resistance gene N-mediated hypersensitive cell death

Although the involvement of heat shock protein 90 (HSP90), mitogen-activated protein kinase (MAPK) cascades and organelle dysfunction in plant hypersensitive cell death has been suggested, the mutual relationship among them has not been elucidated. Here, we show the molecular network of HSP90, the wound-induced protein kinase (WIPK)/salicylic acid-induced protein kinase (SIPK)-mediated MAPK cascade and mitochondrial dysfunction in tobacco mosaic virus (TMV) resistance gene N-dependent cell death. p50, the Avr component for N, NtMEK2(DD), a constitutively active form of a MAPK kinase of WIPK/SIPK, and a mammalian pro-apoptotic factor Bax were used for cell death induction. Suppression of HSP90 and treatment with geldanamycin, a specific inhibitor of HSP90, compromised p50- but not NtMEK2(DD)- or Bax-mediated cell death accompanying the reduction of NtMEK2, WIPK and SIPK activation. In WIPK/SIPK-double knockdown plants, p50- and NtMEK2(DD)- but not Bax-mediated cell death was suppressed. All three types of cell death induced mitochondrial dysfunction, but they were similarly suppressed by Bcl-xL, which is a mammalian anti-apoptotic factor, and prevents mitochondrial dysfunction in plants as it does in animals in the cell death signal pathway. Taken together with the expression profile of hypersensitive reaction marker genes, it was indicated that the MAPK cascade functions downstream of HSP90 and transduces the cell death signal to mitochondria for N gene-dependent cell death. Furthermore, we found that WIPK and SIPK are functionally redundant in cell death signaling using WIPK/SIPK single or double knockdown plants.

A high-throughput screen of cell-death-inducing factors in Nicotiana benthamiana identifies a novel MAPKK that mediates INF1-induced cell death signaling and non-host resistance to Pseudomonas cichorii

A high-throughput overexpression screen of Nicotiana benthamiana cDNAs identified a gene for a mitogen-activated protein kinase kinase (MAPKK) as a potent inducer of the hypersensitive response (HR)-like cell death. NbMKK1 protein is localized to the nucleus, and the N-terminal putative MAPK docking site of NbMKK1 is required for its function as a cell-death inducer. NbMKK1-mediated leaf-cell death was compromised in leaves where NbSIPK expression was silenced by virus-induced gene silencing. A yeast two-hybrid assay showed that NbMKK1 and NbSIPK physically interact, suggesting that NbSIPK is one of the downstream targets of NbMKK1. Phytophthora infestans INF1 elicitor-mediated HR was delayed in NbMKK1-silenced plants, indicating that NbMKK1 is involved in this HR pathway. Furthermore, the resistance of N. benthamiana to a non-host pathogen Pseudomonas cichorii was compromised in NbMKK1-silenced plants. These results demonstrate that MAPK cascades involving NbMKK1 control non-host resistance including HR cell death.

Early events in the pathogenicity of Pseudomonas syringae on Nicotiana benthamiana

Conserved microbial molecules known as PAMPs (pathogen-associated molecular patterns) elicit defence responses in plants through extracellular receptor proteins. One important PAMP is the flagellin protein derived from motile bacteria. We show here that the solanaceous species Nicotiana benthamiana perceives the flagellin proteins of both pathogenic and non-host species of Pseudomonas syringae. The response to flagellin required a gene closely related to that encoding the Arabidopsis thaliana flagellin receptor that we designated NbFls2. In addition, silencing of NbFls2 led to increased growth of compatible, non-host and non-pathogenic strains of P. syringae. Thus, flagellin perception restricts growth of P. syringae strains on N. benthamiana. Pathogenic bacteria secrete effector proteins into the plant cell to enhance virulence. We tested the ability of several unrelated effectors to suppress PAMP-mediated defences. The effector proteins AvrPto and AvrPtoB, but not AvrRps4, suppressed all responses tested including the hypersensitive response induced by non-host flagellins and the oomycete elicitor INF1. Strikingly, transient expression of avrPto or avrPtoB stimulated the growth of non-pathogenic Agrobacterium tumefaciensin planta, suggesting that multiplication of this species is also restricted by PAMP perception. Unexpectedly, AvrPtoB but not AvrPto required the defence-associated genes Rar1, Sgt1 and Eds1 for suppression. This observation separates the respective mechanisms of the two effectors, and suggests that AvrPtoB may target the defence machinery directly for its suppressive effect.

Genomics of the plant pathogenic oomycete Phytophthora: insights into biology and evolution

The genus Phytophthora includes many destructive pathogens of plants. Although having "fungus-like" appearances, Phytophthora species reside in a eukaryotic kingdom separate from that of true fungi. Distinct strategies are therefore required to study and defend against Phytophthora. Large sequence databases have recently been developed for several species, and tools for functional genomics have been enhanced. This chapter will review current progress in understanding the genome and transcriptome of Phytophthora, and provide examples of how genomics resources are advancing molecular studies of pathogenesis, development, transcription, and evolution. A better understanding of these remarkable pathogens should lead to new approaches for managing their diseases.

Phytophthora sojae: root rot pathogen of soybean and model oomycete

SUMMARY Phytophthora sojae is an oomycete pathogen of soybean, classified in the kingdom Stramenopiles. It causes 'damping off' of seedlings and root rot of older plants, with an annual cost worldwide of $1-2 billion. Owing to its economic importance, this species, along with P. infestans, has been developed as a model species for the study of oomycete plant pathogens. It is readily transformed with DNA enabling over-expression and silencing of selected genes, genetic maps have been constructed and large expressed sequence tag sequence libraries have been developed. A draft genome sequence has recently been completed. This review briefly summarizes current information about the pathogenicity, evolution, molecular biology and genomics of P. sojae.

TAXONOMY

Phytophthora sojae (Kaufman & Gerdman): superkingdom Eukaryota; kingdom Stramenopila; phylum Oomycota; class Peronosporomycetidae; order Pythiales; family Pythiaceae; genus Phytophthora.

HOST RANGE

Soybean is the only economically important host. Several species of lupins have also been reported as hosts. Disease symptoms and signs: All parts of the soybean plant are susceptible to infection by P. sojae, from germinating seedlings to mature plants. In the field, P. sojae causes damping off of soybean seedlings and a root and stem rot of established plants. Leaves can be infected in the field as a result of rain splash or by deliberate inoculation in the laboratory. Damping off can affect germinating seeds or emerged seedlings and is most severe when the spring is very wet and warm (25-30 degrees C). Established plants can become infected when the soil is wet for extended periods, especially if the soil is poorly drained. Both the cortex and the vascular tissue are colonized by P. sojae, and the infection can spread rapidly along the vascular tissues in susceptible cultivars.

USEFUL WEBSITES

http://pmgn.vbi.vt.edu, http://phytophthora.vbi.vt.edu, http://www.jgi.doe.gov/Psojae, http://www.jgi.doe.gov/Pramorum, http://www.pfgd.org, http://pamgo.vbi.vt.edu, http://soy.vbi.vt.edu, https://www.vbi.vt.edu/article/articleview/78, http://plantpath.osu.edu/faculty/dorrance.php.

The pathogen-host interactions database (PHI-base) provides insights into generic and novel themes of pathogenicity

Fungal and oomycete pathogens of plants and animals are a major global problem. In the last 15 years, many genes required for pathogenesis have been determined for over 50 different species. Other studies have characterized effector genes (previously termed avirulence genes) required to activate host responses. By studying these types of pathogen genes, novel targets for control can be revealed. In this report, we describe the Pathogen-Host Interactions database (PHI-base), which systematically compiles such pathogenicity genes involved in pathogen-host interactions. Here, we focus on the biology that underlies this computational resource: the nature of pathogen-host interactions, the experimental methods that exist for the characterization of such pathogen-host interactions as well as the available computational resources. Based on the data, we review and analyze the specific functions of pathogenicity genes, the host-specific nature of pathogenicity and virulence genes, and the generic mechanisms of effectors that trigger plant responses. We further discuss the utilization of PHI-base for the computational identification of pathogenicity genes through comparative genomics. In this context, the importance of standardizing pathogenicity assays as well as integrating databases to aid comparative genomics is discussed.

Targeted gene mutation in Phytophthora spp

The genus Phytophthora belongs to the oomycetes and is composed of plant pathogens. Currently, there are no strategies to mutate specific genes for members of this genus. Whole genome sequences are available or being prepared for Phytophthora sojae, P. ramorum, P. infestans, and P. capsici and the development of molecular biological techniques for functional genomics is encouraged. This article describes the adaptation of the reverse-genetic strategy of targeting induced local lesions in genomes (TILLING) to isolate gene-specific mutants in Phytophthora spp. A genomic library of 2,400 ethylnitrosourea (ENU) mutants of P. sojae was created and screened for induced point mutations in the genes encoding a necrosisinducing protein (PsojNIP) and a Phytophthora-specific phospholipase D (PsPXTM-PLD). Mutations were detected in single individuals and included silent, missense, and nonsense changes. Homozygous mutant isolates carrying a potentially deleterious missense mutation in PsojNIP and a premature stop codon in PsPXTM-PLD were identified. No phenotypic effect has yet been found for the homozygous mutant of PsojNIP. For those of PsPXTM-PLD, a reduction in growth rate and an appressed mycelial growth was observed. This demonstrates the feasibility of target-selected gene disruption for Phytophthora spp. and adds an important tool for functional genomic investigation.

Comparative analysis of Phytophthora genes encoding secreted proteins reveals conserved synteny and lineage-specific gene duplications and deletions

Comparative analysis of two Phytophthora genomes revealed overall colinearity in four genomic regions consisting of a 1.5-Mb sequence of Phytophthora sojae and a 0.9-Mb sequence of P. ramorum. In these regions with conserved synteny, the gene order is largely similar; however, genome rearrangements also have occurred. Deletions and duplications often were found in association with genes encoding secreted proteins, including effectors that are important for interaction with host plants. Among secreted protein genes, different evolutionary patterns were found. Elicitin genes that code for a complex family of highly conserved Phytophthora-specific elicitors show conservation in gene number and order, and often are clustered. In contrast, the race-specific elicitor gene Avrlb-1 appeared to be missing from the region with conserved synteny, as were its five homologs that are scattered over the four genomic regions. Some gene families encoding secreted proteins were found to be expanded in one species compared with the other. This could be the result of either repeated gene duplications in one species or specific deletions in the other. These different evolutionary patterns may shed light on the functions of these secreted proteins in the biology and pathology of the two Phytophthora spp.

The plant immune system

Many plant-associated microbes are pathogens that impair plant growth and reproduction. Plants respond to infection using a two-branched innate immune system. The first branch recognizes and responds to molecules common to many classes of microbes, including non-pathogens. The second responds to pathogen virulence factors, either directly or through their effects on host targets. These plant immune systems, and the pathogen molecules to which they respond, provide extraordinary insights into molecular recognition, cell biology and evolution across biological kingdoms. A detailed understanding of plant immune function will underpin crop improvement for food, fibre and biofuels production.

Agroinfection-based high-throughput screening reveals specific recognition of INF elicitins in Solanum

SUMMARY We adapted and optimized the use of the Agrobacterium tumefaciens binary PVX expression system (PVX agroinfection) to screen Solanum plants for response to pathogen elicitors and applied the assay to identify a total of 11 clones of Solanum huancabambense and Solanum microdontum, out of 31 species tested, that respond to the elicitins INF1, INF2A and INF2B of Phytophthora infestans. Prior to this study, response to INF elicitins was only known in Nicotiana spp. within the Solanaceae. The identified S. huancabambense and S. microdontum clones also exhibited hypersensitivity-like cell death following infiltration with purified recombinant INF1, INF2A and INF2B, thereby validating the screening protocol. Comparison of INF elicitin activity revealed that Nicotiana plants responded to significantly lower concentrations than Solanum, suggesting variable levels of sensitivity to INF elicitins. We exploited natural variation in response to INF elicitins in the identified Solanum accessions to evaluate the relationship between INF recognition and late blight resistance. Interestingly, several INF-responsive Solanum plants were susceptible to P. infestans. Also, an S. microdontum xSolanum tuberosum (potato) population that segregates for INF response was generated but failed to identify a measurable contribution of INF response to resistance. These results suggest that in Solanum, INF elicitins are recognized as general elicitors and do not have a measurable contribution to disease resistance.

Induction of resistance against rice blast fungus in rice plants treated with a potent elicitor, N-acetylchitooligosaccharide

The mode of action of a potent elicitor, N-acetylchitooligosaccharide, in rice plants was examined. In intact seedlings, no significant uptake of the elicitor via the roots was observed within 3 h, whereas rapid uptake was observed in excised leaves. Rapid and transient expression of an elicitor-responsive gene, EL2, was induced in the leaves of intact seedlings sprayed with the elicitor or in the roots and leaves of intact seedlings by immersing roots in the elicitor solution. Histochemical analysis indicated that EL2 was expressed in cells exposed to the elicitor of root and leaves. In seedlings treated with the elicitor for 1 d or longer, hyphal growth of rice blast fungus was significantly delayed, and an accumulation of auto-fluorescence around the infection site was observed. Two defense-related genes, PR-1 and PR-10 (PBZ1), were induced in a systemic and local manner by elicitor treatment, in correlation with the induction of resistance against rice blast fungus. N-Acetylchitoheptaose did not inhibit the hyphal growth of the fungi. These results indicate the occurrence of systemic signal transmission from N-acetylchitooligosaccharide in rice plants.

The C-terminal half of Phytophthora infestans RXLR effector AVR3a is sufficient to trigger R3a-mediated hypersensitivity and suppress INF1-induced cell death in Nicotiana benthamiana

The RXLR cytoplasmic effector AVR3a of Phytophthora infestans confers avirulence on potato plants carrying the R3a gene. Two alleles of Avr3a encode secreted proteins that differ in only three amino acid residues, two of which are in the mature protein. Avirulent isolates carry the Avr3a allele, which encodes AVR3aKI (containing amino acids C19, K80 and I103), whereas virulent isolates express only the virulence allele avr3a, encoding AVR3aEM (S19, E80 and M103). Only the AVR3aKI protein is recognized inside the plant cytoplasm where it triggers R3a-mediated hypersensitivity. Similar to other oomycete avirulence proteins, AVR3aKI carries a signal peptide followed by a conserved motif centered on the consensus RXLR sequence that is functionally similar to a host cell-targeting signal of malaria parasites. The interaction between Avr3a and R3a can be reconstructed by their transient co-expression in Nicotiana benthamiana. We exploited the N. benthamiana experimental system to further characterize the Avr3a-R3a interaction. R3a activation by AVR3aKI is dependent on the ubiquitin ligase-associated protein SGT1 and heat-shock protein HSP90. The AVR3aKI and AVR3aEM proteins are equally stable in planta, suggesting that the difference in R3a-mediated death cannot be attributed to AVR3aEM protein instability. AVR3aKI is able to suppress cell death induced by the elicitin INF1 of P. infestans, suggesting a possible virulence function for this protein. Structure-function experiments indicated that the 75-amino acid C-terminal half of AVR3aKI, which excludes the RXLR region, is sufficient for avirulence and suppression functions, consistent with the view that the N-terminal region of AVR3aKI and other RXLR effectors is involved in secretion and targeting but is not required for effector activity. We also found that both polymorphic amino acids, K80 and I103, of mature AVR3a contribute to the effector functions.

Synergistic interactions of the plant cell death pathways induced by Phytophthora infestans Nepl-like protein PiNPP1.1 and INF1 elicitin

Cell death plays a ubiquitous role in plant-microbe interactions, given that it is associated with both susceptible and resistance interactions. A class of cell death-inducing proteins, termed Nepl-like proteins (NLPs), has been reported in bacteria, fungi, and oomycetes. These proteins induce nonspecific necrosis in a variety of dicotyledonous plants. Here, we describe three members of the NLP family from the oomycete Phytophthora infestans (PiNPP1.1, PiNPP1.2, and PiNPP1.3). Using agroinfection with a binary Potato virus X vector, we showed that PiNPP1.1 induces cell death in Nicotiana benthamiana and the host plant tomato. Expression analyses indicated that PiNPP1.1 is up-regulated during late stages of infection of tomato by P. infestans. We compared PiNPP1.1 necrosis-inducing activity to INF1 elicitin, a well-studied protein that triggers the hypersensitive response in Nicotiana spp. Using virus-induced gene silencing, we showed that the cell death induced by PiNPP1.1 is dependent on the ubiquitin ligase-associated protein SGT1 and the heat-shock protein HSP90. In addition, cell death triggered by PiNPP1.1 but not that by INF1 was dependent on the defense-signaling proteins COI1, MEK2, NPR1, and TGA2.2, suggesting distinct signaling requirements. Combined expression of PiNPP1.1 and INF1 in N. benthamiana resulted in enhanced cell death, suggesting synergistic interplay between the two cell-death responses. Altogether, these results point to potentially distinct but interacting cell-death pathways induced by PiNPP1.1 and INF1 in plants.

CDNA-AFLP combined with functional analysis reveals novel genes involved in the hypersensitive response

To identify genes required for the hypersensitive response (HR), we performed expression profiling of tomato plants mounting a synchronized HR, followed by functional analysis of differentially expressed genes. By cDNA-AFLP analysis, the expression profile of tomato plants containing both the Cf-4 resistance gene against Cladosporium fulvum and the matching Avr4 avirulence gene of this fungus was compared with that of control plants. About 1% of the transcript-derived fragments (442 out of 50,000) were derived from a differentially expressed gene. Based on their sequence and expression, 192 fragments, referred to as Avr4-responsive tomato (ART) fragments, were selected for VIGS (virus-induced gene silencing) in Cf-4-transgenic Nicotiana benthamiana. Inoculated plants were analyzed for compromised HR by agroinfiltration of either the C. fulvum Avr4 gene or the Inf1 gene of Phytophthora infestans, which invokes a HR in wild-type N. benthamiana. VIGS using 15 of the ART fragments resulted in a compromised HR, whereas VIGS with fragments of ART genes encoding HSP90, a nuclear GTPase, an L19 ribosomal protein, and most interestingly, a nucleotide binding-leucine rich repeat (NB-LRR)-type protein severely suppressed the HR induced both by Avr4 and Inf1. Requirement of an NB-LRR protein (designated NRC1, for NB-LRR protein required for HR-associated cell death 1) for Cf resistance protein function as well as Inf1-mediated HR suggests a convergence of signaling pathways and supports the recent observation that NB-LRR proteins play a role in signal transduction cascades downstream of resistance proteins.

Insight into Types I and II nonhost resistance using expression patterns of defense-related genes in tobacco

Plants protect themselves against pathogens using a range of response mechanisms. There are two categories of nonhost resistance: Type I, which does not result in visible cell death; and Type II, which entails localized programmed cell death (or hypersensitive response) in response to nonhost pathogens. The genes responsible for these two systems have not yet been intensively investigated at the molecular level. Using tobacco plants (Nicotiana tabacum), we compared expression of 12 defense-related genes between a Type I (Xanthomonas axonopodis pv. glycines 8ra) nonhost interaction, and two Type II (Pseudomonas syringae pv. syringae 61 and P. syringae pv. phaseolicola NPS3121) nonhost interactions, as well as those expressed during R gene-mediated resistance to Tobacco mosaic virus. In general, expression of most defense-related genes during R gene-mediated resistance was activated 48 h after challenge by TMV; the same genes were upregulated as early as 9 h after infiltration by nonhost pathogens. Surprisingly, X. axonopodis pv. glycines (Type I) elicited the same set of defense-related genes as did two pathovars of P. syringae, despite the absence of visible cell death. In two examples of Type II nonhost interactions, P. syringae pv. phaseolicola NPS3121 produced an expression profile more closely resembling that of X. axonopodis pv. glycines 8ra, than that of P. syringae pv. syringae 61. These results suggest that Type I nonhost resistance may act as a mechanism providing a more specific and active defense response against a broad range of potential pathogens.

The U-box protein CMPG1 is required for efficient activation of defense mechanisms triggered by multiple resistance genes in tobacco and tomato

We previously identified three Avr9/Cf-9 Rapidly Elicited (ACRE) genes essential for Cf-9- and Cf-4-dependent hypersensitive response (HR) production in Nicotiana benthamiana. Two of them encode putative E3 ubiquitin ligase components. This led us to investigate other ACRE genes associated with the ubiquitination pathway. ACRE74 encodes a U-box E3 ligase homolog, highly related to parsley (Petroselinum crispum) CMPG1 and Arabidopsis thaliana PLANT U-BOX20 (PUB20) and PUB21 proteins, and was called Nt CMPG1. Transcript levels of Nt CMPG1 and the homologous tomato (Solanum lycopersicum) Cmpg1 are induced in Cf9 tobacco (Nicotiana tabacum) and Cf9 tomato after Avr9 elicitation. Tobacco CMPG1 possesses in vitro E3 ligase activity. N. benthamiana plants silenced for Nt CMPG1 show reduced HR after Cf-9/Avr9 elicitation, while overexpression of Nt CMPG1 induces a stronger HR in Cf9 tobacco plants after Avr9 infiltration. In tomato, silencing of Cmpg1 decreased resistance to Cladosporium fulvum. Overexpression of epitope-tagged tobacco CMPG1 mutated in the U-box domain confers a dominant-negative phenotype. We also show that Nt CMPG1 is involved in the Pto/AvrPto and Inf1 responses. In summary, we show that the E3 ligase Nt CMPG1 is essential for plant defense and disease resistance.

Rewiring mitogen-activated protein kinase cascade by positive feedback confers potato blight resistance

Late blight, caused by the notorious pathogen Phytophthora infestans, is a devastating disease of potato (Solanum tuberosum) and tomato (Solanum lycopersicum), and during the 1840s caused the Irish potato famine and over one million fatalities. Currently, grown potato cultivars lack adequate blight tolerance. Earlier cultivars bred for resistance used disease resistance genes that confer immunity only to some strains of the pathogen harboring corresponding avirulence gene. Specific resistance gene-mediated immunity and chemical controls are rapidly overcome in the field when new pathogen races arise through mutation, recombination, or migration from elsewhere. A mitogen-activated protein kinase (MAPK) cascade plays a pivotal role in plant innate immunity. Here we show that the transgenic potato plants that carry a constitutively active form of MAPK kinase driven by a pathogen-inducible promoter of potato showed high resistance to early blight pathogen Alternaria solani as well as P. infestans. The pathogen attack provoked defense-related MAPK activation followed by induction of NADPH oxidase gene expression, which is implicated in reactive oxygen species production, and resulted in hypersensitive response-like phenotype. We propose that enhancing disease resistance through altered regulation of plant defense mechanisms should be more durable and publicly acceptable than engineering overexpression of antimicrobial proteins.

A catalogue of the effector secretome of plant pathogenic oomycetes

The oomycetes form a phylogenetically distinct group of eukaryotic microorganisms that includes some of the most notorious pathogens of plants. Oomycetes accomplish parasitic colonization of plants by modulating host cell defenses through an array of disease effector proteins. The biology of effectors is poorly understood but tremendous progress has been made in recent years. This review classifies and catalogues the effector secretome of oomycetes. Two classes of effectors target distinct sites in the host plant: Apoplastic effectors are secreted into the plant extracellular space, and cytoplasmic effectors are translocated inside the plant cell, where they target different subcellular compartments. Considering that five species are undergoing genome sequencing and annotation, we are rapidly moving toward genome-wide catalogues of oomycete effectors. Already, it is evident that the effector secretome of pathogenic oomycetes is more complex than expected, with perhaps several hundred proteins dedicated to manipulating host cell structure and function.

Host-Pathogen Interactions Between Phytophthora infestans and the Solanaceous Hosts Calibrachoa × hybridus, Petunia × hybrida, and Nicotiana benthamiana

Late blight, caused by the pathogen Phytophthora infestans, is a devastating disease of potato and tomato, but can also damage other solanaceous hosts. To gain a better understanding of the interaction between P. infestans and these other hosts, the susceptibility of species in three solanaceous genera was investigated. Of the 10 Calibrachoa × hybridus cultivars tested, four were susceptible and six were resistant to the pathogen; susceptible cultivars supported only very limited growth of P. infestans. The majority of the Petunia × hybrida (petunia) cultivars were susceptible, although less so than susceptible potatoes or tomatoes. Two petunia cultivars displayed differential resistance, suggesting the presence of R genes against P. infestans. The hypersensitive response was present in susceptible, partially resistant, and resistant petunia-P. infestans interactions, but was predominant in the resistant interaction. Young petunias (3 weeks) were more susceptible than older petunias (7 weeks). Nicotiana benthamiana was susceptible to all four P. infestans isolates tested in the lab and became infected during a field epidemic. Several of these isolates were tested for the presence of the inf1 gene, and were found to have and express the gene in vitro. In addition, culture filtrate from these isolates contained 10-kDa proteins and also elicited the hypersensitive response in Nicotiana tabacum and N. benthamiana.

A maize resistance gene functions against bacterial streak disease in rice

Although cereal crops all belong to the grass family (Poacea), most of their diseases are specific to a particular species. Thus, a given cereal species is typically resistant to diseases of other grasses, and this nonhost resistance is generally stable. To determine the feasibility of transferring nonhost resistance genes (R genes) between distantly related grasses to control specific diseases, we identified a maize R gene that recognizes a rice pathogen, Xanthomonas oryzae pv. oryzicola, which causes bacterial streak disease. Bacterial streak is an important disease of rice in Asia, and no simply inherited sources of resistance have been identified in rice. Although X. o. pv. oryzicola does not cause disease on maize, we identified a maize gene, Rxo1, that conditions a resistance reaction to a diverse collection of pathogen strains. Surprisingly, Rxo1 also controls resistance to the unrelated pathogen Burkholderia andropogonis, which causes bacterial stripe of sorghum and maize. The same gene thus controls resistance reactions to both pathogens and nonpathogens of maize. Rxo1 has a nucleotide-binding site-leucine-rich repeat structure, similar to many previously identified R genes. Most importantly, Rxo1 functions after transfer as a transgene to rice, demonstrating the feasibility of nonhost R gene transfer between cereals and providing a valuable tool for controlling bacterial streak disease.

Ancient Origin of Elicitin Gene Clusters in Phytophthora Genomes

The genus Phytophthora belongs to the oomycetes in the eukaryotic stramenopile lineage and is comprised of over 65 species that are all destructive plant pathogens on a wide range of dicotyledons. Phytophthora produces elicitins (ELIs), a group of extracellular elicitor proteins that cause a hypersensitive response in tobacco. Database mining revealed several new classes of elicitin-like (ELL) sequences with diverse elicitin domains in Phytophthora infestans, Phytophthora sojae, Phytophthora brassicae, and Phytophthora ramorum. ELIs and ELLs were shown to be unique to Phytophthora and Pythium species. They are ubiquitous among Phytophthora species and belong to one of the most highly conserved and complex protein families in the Phytophthora genus. Phylogeny construction with elicitin domains derived from 156 ELIs and ELLs showed that most of the diversified family members existed prior to divergence of Phytophthora species from a common ancestor. Analysis to discriminate diversifying and purifying selection showed that all 17 ELI and ELL clades are under purifying selection. Within highly similar ELI groups there was no evidence for positively selected amino acids suggesting that purifying selection contributes to the continued existence of this diverse protein family. Characteristic cysteine spacing patterns were found for each phylogenetic clade. Except for the canonical clade ELI-1, ELIs and ELLs possess C-terminal domains of variable length, many of which have a high threonine, serine, or proline content suggesting an association with the cell wall. In addition, some ELIs and ELLs have a predicted glycosylphosphatidylinositol site suggesting anchoring of the C-terminal domain to the cell membrane. The eli and ell genes belonging to different clades are clustered in the genomes. Overall, eli and ell genes are expressed at different levels and in different life cycle stages but those sharing the same phylogenetic clade appear to have similar expression patterns.

High-throughput in planta expression screening identifies a class II ethylene-responsive element binding factor-like protein that regulates plant cell death and non-host resistance

We performed high-throughput screening using the potato virus X (PVX) system to overexpress Nicotiana benthamiana genes in planta and identify positive regulators of cell death. This screening identified NbCD1, a novel class II ethylene-responsive element binding factor (ERF), as a potent inducer of the hypersensitive response (HR)-like cell death. NbCD1 expression was induced by treatments with INF1 elicitor and a non-host pathogen Pseudomonas cichorii. NbCD1 exhibited transcriptional repressor activity through its EAR motif, and this motif was necessary for NbCD1 to cause cell death. We identified 58 genes that displayed altered transcription following NbCD1 overexpression. NbCD1 overexpression downregulated the expression of HSR203, a negative regulator of hypersensitive death. Conditional expression of NbCD1 in Arabidopsis also caused cell death, indicating that NbCD1 downstream cascades are conserved in dicot plants. To further confirm the role of NbCD1 in defense, we used virus-induced gene silencing to demonstrate that NbCD1 is required for non-host resistance of N. benthamiana to the bacterial pathogen P. cichorii. Our data point to a model of transcriptional regulatory cascades. NbCD1 positively regulates cell death and contributes to non-host resistance, possibly by downregulating the expression of other defense response genes.

A Second Kazal-like protease inhibitor from Phytophthora infestans inhibits and interacts with the apoplastic pathogenesis-related protease P69B of tomato

The plant apoplast forms a protease-rich environment in which proteases are integral components of the plant defense response. Plant pathogenic oomycetes, such as the potato (Solanum tuberosum) and tomato (Lycopersicon esculentum) pathogen Phytophthora infestans, secrete a diverse family of serine protease inhibitors of the Kazal family. Among these, the two-domain EPI1 protein was shown to inhibit and interact with the pathogenesis-related protein P69B subtilase of tomato and was implicated in counter-defense. Here, we describe and functionally characterize a second extracellular protease inhibitor, EPI10, from P. infestans. EPI10 contains three Kazal-like domains, one of which was predicted to be an efficient inhibitor of subtilisin A by an additivity-based sequence to reactivity algorithm (Laskowski algorithm). The epi10 gene was up-regulated during infection of tomato, suggesting a potential role during pathogenesis. Recombinant EPI10 specifically inhibited subtilisin A among the major serine proteases, and inhibited and interacted with P69B subtilase of tomato. The finding that P. infestans evolved two distinct and structurally divergent protease inhibitors to target the same plant protease suggests that inhibition of P69B could be an important infection mechanism for this pathogen.

Differences in cell death induction by Phytophthora Elicitins are determined by signal components downstream of MAP kinase kinase in different species of Nicotiana and cultivars of Brassica rapa and Raphanus sativus

Elicitins are small, secreted proteins produced by species of the plant-pathogenic oomycete Phytophthora. They induce hypersensitive cell death in most Nicotiana species and in some cultivars of Brassica rapa and Raphanus sativus. In this study, two true-breeding Fast Cycling B. rapa lines were established that showed severe necrosis (line 7-R) or no visible response (line 18-NR) after treatment with elicitin. Unexpectedly, microscopic examination revealed localized cell death in line 18-NR plants, and expression levels of various defense-marker genes were comparable in both lines. These results suggested that both "responsive" and "nonresponsive" plants responded to elicitin but differed in the extent of the cell death response. Expression of a constitutively active form of Arabidopsis (Arabidopsis thaliana) MAP kinase kinase 4 (AtMEK4(DD)) also induced rapid development of confluent cell death in line 7-R, whereas line 18-NR showed no visible cell death. Similarly, elicitin-responsive Nicotiana species and R. sativus cultivars showed significantly stronger cell death responses following expression of AtMEK4(DD) compared with nonresponsive species/cultivars. Line 7-R also showed higher sensitivity to toxin-containing culture filtrates produced by Alternaria brassicicola, and toxin sensitivity cosegregated with elicitin responsiveness, suggesting that the downstream responses induced by elicitin and Alternaria toxin share factors that control the extent of cell death. Interestingly, elicitin responsiveness was shown to correlate with greater susceptibility to A. brassicicola (a necrotroph) in B. rapa but less susceptibility to Phytophthora nicotianae (a hemibiotroph) in Nicotiana, suggesting a more extensive cell death response could cause opposite effects on the outcomes of biotrophic versus necrotrophic plant-pathogen interactions.

NRG1, a CC-NB-LRR protein, together with N, a TIR-NB-LRR protein, mediates resistance against tobacco mosaic virus

In animals and plants, innate immunity is regulated by nucleotide binding domain and leucine-rich repeat (NB-LRR) proteins that mediate pathogen recognition and that activate host-cell defense responses. Plant NB-LRR proteins, referred to as R proteins, have amino-terminal domains that contain a coiled coil (CC) or that share similarity with animal Toll and interleukin 1 receptors (TIR). To investigate R protein function, we are using the TIR-NB-LRR protein N that mediates resistance against tobacco mosaic virus (TMV) through recognition of the TMV p50 protein. Here, we describe N requirement gene 1 (NRG1), a novel N-resistance component that was identified by a virus-induced gene silencing (VIGS) screen of a cDNA library. Surprisingly, NRG1 encodes an NB-LRR type R protein that, in contrast to N, contains a CC rather than a TIR domain. Our findings support emerging evidence that many disease-resistance pathways each recruit more than a single NB-LRR protein. The results also indicate that, in addition to the previously recognized role in elicitor recognition, NB-LRR proteins may also function in downstream signaling pathways.

Identification of tobacco ESTs with a hypersensitive response (HR)-specific pattern of expression and likely involved in the induction of the HR and/or localized acquired resistance (LAR)

Plant defense responses against pathogens often involve the restriction of the pathogen to its site of penetration. Restriction is achieved through the combined effects of the hypersensitive response (HR) and its tightly connected localized acquired resistance (LAR). As LAR is induced by unknown signals released by the cells undergoing the HR, LAR inducing/regulating genes must show a HR-specific pattern of expression. Here, we describe a differential display reverse-transcript polymerase chain reaction (DDRT-PCR) strategy to isolate tobacco expressed sequence tags (ESTs) characterized by such an expression profile, which also characterizes genes involved in the induction/execution of the HR. We compared the DDRT-PCR profile of tobacco cell suspensions treated with beta-megaspermin inducing the HR with that of untreated cells and cells treated with alpha-megaspermin inducing a Defense No Death (DND) phenotype. The expression profile of the selected ESTs was analyzed in tobacco plants expressing a beta-megaspermin-induced HR or a DND phenotype, including LAR, induced by three different elicitors. This comprehensive analysis allowed to identify 24 HR-specific ESTs, half of them shows no or non-significant homology with ESTs and genes in the databases. The other half exhibits homology with genes encoding a receptor-like kinase protein, proteins involved in the regulation of plasma membrane structure, proteins of the ubiquitin/26S proteasome proteolytic system, RNA binding proteins, and a protein hypothesized to be a true regulator of the HR.

A method for double-stranded RNA-mediated transient gene silencing in Phytophthora infestans

SUMMARY Gene silencing, triggered by double-stranded RNA (dsRNA), has proved to be a valuable tool for determining and confirming the function of genes in many organisms. For Phytophthora infestans, the cause of late blight on potato and tomato, gene silencing strategies have relied on stable transformation followed by spontaneous silencing of both the endogenous gene and the transgene. Here we describe the first application of transient gene silencing in P. infestans, by delivering in vitro synthesized dsRNA into protoplasts to trigger silencing. A marker gene, gfp, and two P. infestans genes, inf1 and cdc14, both of which have been silenced previously, were selected to test this strategy. Green fluorescent protein (GFP) fluorescence was reduced in regenerating protoplasts up to 4 days after exposure to gfp dsRNA. A secondary reduction in expression of all genes tested was not fully activated until 12-17 days after introduction of the respective homologous dsRNAs. At this time after exposure to dsRNA, reduced GFP fluorescence in gfp dsRNA-treated lines, and reduced INF1 production in inf1 dsRNA-treated lines, was observed. Introduction of dsRNA for the stage-specific gene, cdc14, yielded the expected phenotype of reduced numbers of sporangia when cdc14 expression was significantly reduced compared with control lines. Silencing was shown to be sequence-specific, because analysis of inf1 expression in gfp-silenced lines yielded wild-type levels of gene expression. This report shows that transient gene silencing can be used to generate detectable phenotypes in P. infestans and should provide a high-throughput tool for P. infestans functional genomics.

Differences in intensity and specificity of hypersensitive response induction in Nicotiana spp. by INF1, INF2A, and INF2B of Phytophthora infestans

Elicitins form a family of structurally related proteins that induce the hypersensitive response (HR) in plants, particularly Nicotiana spp. The elicitin family is composed of several classes. Most species of the plant-pathogenic oomycete genus Phytophthora produce the well-characterized 10-kDa canonical elicitins (class I), such as INF1 of the potato and tomato pathogen Phytophthora infestans. Two genes, inf2A and inf2B, encoding a distinct class (class III) of elicitin-like proteins, also occur in P. infestans. Unlike secreted class I elicitins, class III elicitins are thought to be cell-surface-anchored polypeptides. Molecular characterization of the inf2 genes indicated that they are widespread in Phytophthora spp. and occur as a small gene family. In addition, Southern blot and Northern blot hybridizations using gene-specific probes showed that inf2A and inf2B genes and transcripts can be detected in 17 different P. infestans isolates. Functional secreted expression in plant cells of the elicitin domain of the infl and inf2 genes was conducted using a binary Potato virus X (PVX) vector (agroinfection) and Agrobacterium tumefaciens transient transformation assays (agroinfiltration), and resulted in HR-like necrotic symptoms and induction of defense response genes in tobacco. However, comparative analyses of elicitor activity of INF1, INF2A, and INF2B revealed significant differences in intensity, specificity, and consistency of HR induction. Whereas INF1 induced the HR in Nicotiana benthamiana, INF2A induced weak symptoms and INF2B induced no symptoms on this plant. Nonetheless, similar to INF1, HR induction by INF2A in N. benthamiana required the ubiquitin ligase-associated protein SGT1. Overall, these results suggest that variation in the resistance of Nicotiana spp. to P. infestans is shadowed by variation in the response to INF elicitins. The ability of tobacco, but not N. benthamiana, to respond to INF2B could explain differences in resistance to P. infestans observed for these two species.

Large-scale gene discovery in the oomycete Phytophthora infestans reveals likely components of phytopathogenicity shared with true fungi

To overview the gene content of the important pathogen Phytophthora infestans, large-scale cDNA and genomic sequencing was performed. A set of 75,757 high-quality expressed sequence tags (ESTs) from P. infestans was obtained from 20 cDNA libraries representing a broad range of growth conditions, stress responses, and developmental stages. These included libraries from P. infestans-potato and -tomato interactions, from which 963 pathogen ESTs were identified. To complement the ESTs, onefold coveragethe P. infestans genome was obtained and regions of coding potential identified. A unigene set of 18,256 sequences was derived from the EST and genomic data and characterized for potential functions, stage-specific patterns of expression, and codon bias. Cluster analysis of ESTs revealed major differences between the expressed gene content of mycelial and spore-related stages, and affinities between some growth conditions. Comparisons with databases of fungal pathogenicity genes revealed conserved elements of pathogenicity, such as class III pectate lyases, despite the considerable evolutionary distance between oomycetes and fungi. Thirty-seven genes encoding components of flagella also were identified. Several genes not anticipated to occur in oomycetes were detected, including chitin synthases, phosphagen kinases, and a bacterial-type FtsZ cell-division protein. The sequence data described are available in a searchable public database.

Elicitin genes in Phytophthora infestans are clustered and interspersed with various transposon-like elements

Sequencing and annotation of a contiguous stretch of genomic DNA (112.3 kb) from the oomycete plant pathogen Phytophthora infestans revealed the order, spacing and genomic context of four members of the elicitin (inf) gene family. Analysis of the GC content at the third codon position (GC3) of six genes encoded in the region, and a set of randomly selected coding regions as well as random genomic regions, showed that a high GC3 value is a general feature of Phytophthora genes that can be exploited to optimize gene prediction programs for Phytophthora species. At least one-third of the annotated 112.3-kb P. infestans sequence consisted of transposons or transposon-like elements. The most prominent were four Tc3/gypsy and Tc1/copia type retrotransposons and three DNA transposons that belong to the Tc1/mariner, Pogo and PiggyBac groups, respectively. Comparative analysis of other available genomic sequences suggests that transposable elements are highly heterogeneous and ubiquitous in the P. infestans genome.

The spores of Phytophthora: weapons of the plant destroyer

Members of the genus Phytophthora are among the most serious threats to agriculture and food production, causing devastating diseases in hundreds of plant hosts. These fungus-like eukaryotes, which are taxonomically classified as oomycetes, generate asexual and sexual spores with characteristics that greatly contribute to their pathogenic success. The spores include survival and dispersal structures, and potent infectious propagules capable of actively locating hosts. Genetic tools and genomic resources developed over the past decade are now allowing detailed analysis of these important stages in the Phytophthora life cycle.

Pseudomonas Type III effector AvrPto suppresses the programmed cell death induced by two nonhost pathogens in Nicotiana benthamiana and tomato

Many gram-negative bacterial pathogens rely on a type III secretion system to deliver a number of effector proteins into the host cell. Though a number of these effectors have been shown to contribute to bacterial pathogenicity, their functions remain elusive. Here we report that AvrPto, an effector known for its ability to interact with Pto and induce Pto-mediated disease resistance, inhibited the hypersensitive response (HR) induced by nonhost pathogen interactions. Pseudomonas syringae pv. tomato T1 causes an HR-like cell death on Nicotiana benthamiana. This rapid cell death was delayed significantly in plants inoculated with P. syringae pv. tomato expressing avrPto. In addition, P. syringae pv. tabaci expressing avrPto suppressed nonhost HR on tomato prf3 and ptoS lines. Transient expression of avrPto in both N. benthamiana and tomato prf3 plants also was able to suppress nonhost HR. Interestingly, AvrPto failed to suppress cell death caused by other elicitors and nonhost pathogens. AvrPto also failed to suppress cell death caused by certain gene-for-gene disease resistance interactions. Experiments with avrPto mutants revealed several residues important for the suppression effects. AvrPto mutants G2A, G99V, P146L, and a 12-amino-acid C-terminal deletion mutant partially lost the suppression ability, whereas S94P and 196T enhanced suppression of cell death in N. benthamiana. These results, together with other discoveries, demonstrated that suppression of host-programmed cell death may serve as one of the strategies bacterial pathoens use for successful invasion.

Marker-based cloning of the region containing the UhAvr1 avirulence gene from the basidiomycete barley pathogen Ustilago hordei

Race-cultivar specialization during the interaction of the basidiomycete smut pathogen Ustilago hordei with its barley host was described in the 1940s. Subsequent genetic analyses revealed the presence of dominant avirulence genes in the pathogen that conform to the gene-for-gene theory. This pathosystem therefore presents an opportunity for the molecular genetic characterization of fungal genes controlling avirulence. We performed a cross between U. hordei strains to obtain 54 progeny segregating for three dominant avirulence genes on three differential barley cultivars. Bulked segregant analysis was used to identify RAPD and AFLP markers tightly linked to the avirulence gene UhAvr1. The UhAvr1 gene is located in an area containing repetitive DNA and this region is undetectable in cosmid libraries prepared from the avirulent parental strain. PCR and hybridization probes developed from the linked markers were therefore used to identify cosmid clones from the virulent (Uhavr1) parent. By walking on Uhavr1-linked cosmid clones, a nonrepetitive, nearby probe was found that recognized five overlapping BAC clones spanning 170 kb from the UhAvr1 parent. A contig of the clones in the UhAvr1 region was constructed and selected probes were used for RFLP analysis of the segregating population. This approach genetically defined an approximately 80-kb region that carries the UhAvr1 gene and provided cloned sequences for subsequent genetic analysis. UhAvr1 represents the first avirulence gene cloned from a basidiomycete plant pathogen.

Recognition of Cladosporium fulvum Ecp2 elicitor by non-host Nicotiana spp. is mediated by a single dominant gene that is not homologous to known Cf-genes

SUMMARY Cladosporium fulvum is a fungal pathogen of tomato that grows exclusively in the intercellular spaces of leaves. Ecp2 is one of the elicitor proteins that is secreted by C. fulvum and is specifically recognized by tomato plants containing the resistance gene Cf-Ecp2. Recognition is followed by a hypersensitive response (HR) resulting in resistance. HR-associated recognition of Ecp2 has been observed in Nicotiana paniculata, N. sylvestris, N. tabacum and N. undulata that are non-host plants of C. fulvum. Absence of Ecp2-recognition did not lead to growth of C. fulvum on Nicotiana plants. We show that HR-associated recognition of Ecp2 is mediated by a single dominant gene in N. paniculata. However, based on PCR and hybridization analysis this gene is not homologous to known Cf-genes.

The Nep1-like proteins-a growing family of microbial elicitors of plant necrosis

SUMMARY A novel family of microbial elicitors of plant necrosis has been identified. Designated Nep1-like proteins (NLPs), after the first family member isolated, they range from 24 to 26 kDa and are found in a variety of taxonomically unrelated micro-organisms. These include several fungi and oomycetes, as well as Gram-positive and Gram-negative bacteria. Some NLPs induce a hypersensitive-like response in plants, although the basis for initiation of this response remains unclear. Similarly, the cellular role of such highly conserved proteins is undetermined. It is not clear whether the NLPs are dedicated elicitors of plant defences or whether this induction occurs as a result of another activity.

The Rxo1/ Rba1 locus of maize controls resistance reactions to pathogenic and non-host bacteria

Infiltration of different maize lines with a variety of bacterial pathogens of maize, rice and sorghum identified qualitative differences in resistant reactions. Isolates from two bacterial species induced rapid hypersensitive reactions (HR) in some maize lines, but not others. All isolates of the non-host pathogen Xanthomonas oryzae pv. oryzicola (bacterial leaf streak disease of rice) and some isolates of the pathogenic bacterium Burkholderia andropogonis induced HR when infiltrated into maize line B73, but not Mo17. Genetic control of the HR to both bacteria segregated as a single dominant gene. Surprisingly, both phenotypes mapped to the same locus, indicating they are either tightly linked or controlled by the same gene. The locus maps on the short arm of maize chromosome six near several other disease-resistance genes. Results indicate the same type of genes may contribute to both non-host resistance and resistance to pathogens.

Interactions between auxin transport and the actin cytoskeleton in developmental polarity of Fucus distichus embryos in response to light and gravity

Land plants orient their growth relative to light and gravity through complex mechanisms that require auxin redistribution. Embryos of brown algae use similar environmental stimuli to orient their developmental polarity. These studies of the brown algae Fucus distichus examined whether auxin and auxin transport are also required during polarization in early embryos and to orient growth in already developed tissues. These embryos polarize with the gravity vector in the absence of a light cue. The auxin, indole-3-acetic acid (IAA), and auxin efflux inhibitors, such as naphthylphthalamic acid (NPA), reduced environmental polarization in response to gravity and light vectors. Young rhizoids are negatively phototropic, and NPA also inhibits rhizoid phototropism. The effect of IAA and NPA on gravity and photopolarization is maximal within 2.5 to 4.5 h after fertilization (AF). Over the first 6 h AF, auxin transport is relatively constant, suggesting that developmentally controlled sensitivity to auxin determines the narrow window during which NPA and IAA reduce environmental polarization. Actin patches were formed during the first hour AF and began to photolocalize within 3 h, coinciding with the time of NPA and IAA action. Treatment with NPA reduced the polar localization of actin patches but not patch formation. Latrunculin B prevented environmental polarization in a time frame that overlaps the formation of actin patches and IAA and NPA action. Latrunculin B also altered auxin transport. Together, these results indicate a role for auxin in the orientation of developmental polarity and suggest interactions between the actin cytoskeleton and auxin transport in F. distichus embryos.

A Galpha subunit controls zoospore motility and virulence in the potato late blight pathogen Phytophthora infestans

The heterotrimeric G-protein pathway is a ubiquitous eukaryotic signalling module that is known to regulate growth and differentiation in many plant pathogens. We previously identified Pigpa1, a gene encoding a G-protein alpha subunit from the potato late blight pathogen Phytophthora infestans. P. infestans belongs to the class oomycetes, a group of organisms in which signal transduction processes have not yet been studied at the molecular level. To elucidate the function of Pigpa1, PiGPA1-deficient mutants were obtained by homology-dependent gene silencing. The Pigpa1-silenced mutants produced zoospores that turned six to eight times more frequently, causing them to swim only short distances compared with wild type. Attraction to the surface, a phenomenon known as negative geotaxis, was impaired in the mutant zoospores, as well as autoaggregation and chemotaxis towards glutamic and aspartic acid. Zoospore production was reduced by 20-45% in different Pigpa1-silenced mutants. Transformants expressing constitutively active forms of PiGPA1, containing amino acid substitutions (R177H and Q203L), showed no obvious phenotypic differences from the wild-type strain. Infection efficiencies on potato leaves ranged from 3% to 14% in the Pigpa1-silenced mutants, compared with 77% in wild type, showing that virulence is severely impaired. The results prove that PiGPA1 is crucial for zoospore motility and for pathogenicity in an important oomycete plant pathogen.

A Kazal-like extracellular serine protease inhibitor from Phytophthora infestans targets the tomato pathogenesis-related protease P69B

The oomycetes form one of several lineages within the eukaryotes that independently evolved a parasitic lifestyle and consequently are thought to have developed alternative mechanisms of pathogenicity. The oomycete Phytophthora infestans causes late blight, a ravaging disease of potato and tomato. Little is known about processes associated with P. infestans pathogenesis, particularly the suppression of host defense responses. We describe and functionally characterize an extracellular protease inhibitor, EPI1, from P. infestans. EPI1 contains two domains with significant similarity to the Kazal family of serine protease inhibitors. Database searches suggested that Kazal-like proteins are mainly restricted to animals and apicomplexan parasites but appear to be widespread and diverse in the oomycetes. Recombinant EPI1 specifically inhibited subtilisin A among major serine proteases and inhibited and interacted with the pathogenesis-related P69B subtilisin-like serine protease of tomato in intercellular fluids. The epi1 and P69B genes were coordinately expressed and up-regulated during infection of tomato by P. infestans. Inhibition of tomato proteases by EPI1 could form a novel type of defense-counterdefense mechanism between plants and microbial pathogens. In addition, this study points to a common virulence strategy between the oomycete plant pathogen P. infestans and several mammalian parasites, such as the apicomplexan Toxoplasma gondii.

The Avr1b locus of Phytophthora sojae encodes an elicitor and a regulator required for avirulence on soybean plants carrying resistance gene Rps1b

We have used map-based approaches to clone a locus containing two genes, Avr1b-1 and Avr1b-2, required for avirulence of the oomycete pathogen Phytophthora sojae (Kaufmann & Gerdemann) on soybean plants carrying resistance gene Rps1b. Avr1b-1 was localized to a single 60-kb bacterial artificial chromosome (BAC) clone by fine-structure genetic mapping. Avr1b-1 was localized within the 60-kb region by identification of an mRNA that is expressed in a race-specific and infection-specific manner and that encodes a small secreted protein. When the Avr1b-1 protein was synthesized in the yeast Pichia pastoris and the secreted protein infiltrated into soybean leaves, it triggered a hypersensitive response specifically in host plants carrying the Rps1b resistance gene. This response eventually spread to the entire inoculated plant. In some isolates of P. sojae virulent on Rps1b-containing cultivars, such as P7081 (race 25) and P7076 (race 19), the Avr1b-1 gene had numerous substitution mutations indicative of strong divergent selection. In other isolates, such as P6497 (race 2) and P9073 (race 25), there were no substitutions in Avr1b-1, but Avr1b-1 mRNA did not accumulate. Genetic complementation experiments with P6497 revealed the presence of a second gene, Avr1b-2, required for the accumulation of Avr1b-1 mRNA. Avr1b-2 was genetically mapped to the same BAC contig as Avr1b-1, using a cross between P7064 (race 7) and P6497. The Avr1k gene, required for avirulence on soybean cultivars containing Rps1k, was mapped to the same interval as Avr1b-1.

Nonhost resistance: how much do we know?

Nonhost disease resistance is the most common form of disease resistance exhibited by plants against the majority of potentially pathogenic microorganisms. Recently, several components of nonhost disease resistance have been identified. Nonhost resistance exhibited against bacteria, fungi and oomycetes can be of two types. Type I nonhost resistance does not produce any visible symptoms whereas type II nonhost resistance results in a rapid hypersensitive response with cell death. Strong similarities exist between nonhost and gene-for-gene resistance responses but it is still not clear if the same mechanism is involved in producing these resistance responses.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

Gene expression analysis of plant host-pathogen interactions by SuperSAGE

The type III restriction endonuclease EcoP15I was used in isolating fragments of 26 bp from defined positions of cDNAs. We call this substantially improved variant to the conventional serial analysis of gene expression (SAGE) procedure "SuperSAGE." By applying SuperSAGE to Magnaporthe grisea (blast)-infected rice leaves, gene expression profiles of both the rice host and blast fungus were simultaneously monitored by making use of the fully sequenced genomes of both organisms, revealing that the hydrophobin gene is the most actively transcribed M. grisea gene in blast-infected rice leaves. Moreover, SuperSAGE was applied to study gene expression changes before the so-called hypersensitive response in INF1 elicitor-treated Nicotiana benthamiana, a "nonmodel" organism for which no DNA database is available. Again, SuperSAGE allowed rapid identification of genes up- or down-regulated by the elicitor. Surprisingly, many of the down-regulated genes coded for proteins involved in photosynthesis. SuperSAGE will be especially useful for transcriptome profiling of two or more interacting organisms like hosts and pathogens, and of organisms, for which no DNA database is available.

Active defence responses associated with non-host resistance of Arabidopsis thaliana to the oomycete pathogen Phytophthora infestans

SUMMARY The molecular basis of non-host resistance, or species-specific resistance, remains one of the major unknowns in the study of plant-microbe interactions. In this paper, we describe the characterization of a non-host pathosystem involving the model plant Arabidopsis thaliana and the economically important and destructive oomycete pathogen Phytophthora infestans. Cytological investigations into the early stages of this interaction revealed the germination of P. infestans cysts on Arabidopsis leaves, direct penetration of epidermal cells, formation of infection vesicles and occasionally secondary hyphae, followed by a typical hypersensitive response. P. infestans biomass dynamics during infection of Arabidopsis was monitored using kinetic PCR, revealing an increase in biomass during the first 24 h after inoculation, followed by a decrease in the later stages. Transgenic reporter lines and RNA blot analyses were used to characterize the defence responses induced following P. infestans infection. Significant induction of PDF1.2 was observed at 48 h after inoculation, whereas elevated levels of PR gene expression were detected three days after inoculation. To further characterize this defence response, DNA microarray analyses were carried out to determine the expression profiles for c. 11 000 Arabidopsis cDNAs 16 h after infection. These analyses revealed a significant overlap between Arabidopsis non-host response and other defence-related treatments described in the literature. In particular, non-host response to P. infestans was clearly associated with activation of the jasmonate pathway. The described Arabidopsis-P. infestans pathosystem offers excellent prospects for improving our understanding of non-host resistance.

Cytosolic HSP90 and HSP70 are essential components of INF1-mediated hypersensitive response and non-host resistance to Pseudomonas cichorii in Nicotiana benthamiana

SUMMARY Mitogen-activated protein kinases (MAPKs) play pivotal roles in the signal transduction pathway of plant defence responses against pathogens. A search for MAPK-interacting proteins revealed an interaction between a Nicotiana benthamiana MAPK, SIPK (NbSIPK) and cytosolic Hsp90 (NbHsp90c-1) in yeast two-hybrid assay. To study the function of Hsp90 in disease resistance, we silenced NbHsp90c-1 in N. benthamiana by virus-induced gene silencing (VIGS) with Potato virus X (PVX). NbHsp90c-1 silenced plants exhibited: (1) a stunted phenotype, (2) no hypersensitive response (HR) development after infiltration with the Phytophthora infestans protein INF1 and a non-host pathogen Pseudomonas cichorii that normally triggers HR in N. benthamiana, (3) compromised non-host resistance to P. cichorii, and (4) consistently reduced transcription levels of PR (pathogenesis related) protein genes. Similar phenotypes were observed also for plants in which a cytosolic Hsp70 (NbHsp70c-1), a gene for another class of molecular chaperon, was silenced. Hsp90 was isolated as a MAPK-interacting protein in yeast two-hybrid assay, therefore we tested the effect of NbHsp90c-1 silencing as well as NbHsp70c-1 silencing on the HR development caused by infiltration of a hyperactive potato MAPKK (StMEK1(DD)). No difference in the timing or extent of HR was found among NbHsp90c-1 silenced, NbHsp70c-1 silenced and control plants. This result indicates that observed impairment of INF1- and P. cichorii-mediated HR development in NbHsp90c-1 silenced and NbHsp70c-1 silenced plants was not caused by the abrogation in MAPK function downstream of active MAPKK that leads to HR. These findings suggest essential roles of Hsp90 and Hsp70 in plant defence signal transduction pathway upstream or independent of the MAPK cascade.

Virus-induced silencing of WIPK and SIPK genes reduces resistance to a bacterial pathogen, but has no effect on the INF1-induced hypersensitive response (HR) in Nicotiana benthamiana

Activation of two mitogen-activated protein kinases (MAPKs), wound-induced protein kinase (WIPK) and salicylic acid-induced protein kinase (SIPK), is one of the earliest responses that occur in tobacco plants that have been wounded, treated with pathogen-derived elicitors or challenged with avirulent pathogens. We isolated cDNAs for these MAPKs (NbWIPKand NbSIPK) from Nicotiana benthamiana. The function of NbWIPK and NbSIPK in mediating the hypersensitive response (HR) triggered by infiltration with INF1 protein (the major elicitin secreted by Phytophthora infestans), and the defense response to an incompatible bacterial pathogen (Pseudomonas cichorii), was investigated by employing virus-induced gene silencing (VIGS) to inhibit expression of the WIPK and SIPK genes in N. benthamiana. Silencing of WIPK or SIPK, or both genes simultaneously, resulted in reduced resistance to P. cichorii, but no change was observed in the timing or extent of HR development after treatment with INF1.