Uncoupling resistance from cell death in the hypersensitive response of Nicotiana species to cauliflower mosaic virus infection

Enhanced Resistance to Viruses in Nicotiana edwardsonii 'Columbia' Is Dependent on Salicylic Acid, Correlates with High Glutathione Levels, and Extends to Plant-Pathogenic Bacteria and Abiotic Stress

Our earlier research showed that an interspecific tobacco hybrid (Nicotiana edwardsonii 'Columbia' [NEC]) displays elevated levels of salicylic acid (SA) and enhanced resistance to localized necrotic symptoms (hypersensitive response [HR]) caused by tobacco mosaic virus (TMV) and tobacco necrosis virus (TNV), as compared with another interspecific hybrid (Nicotiana edwardsonii [NE]) derived from the same parents. In the present study, we investigated whether symptomatic resistance in NEC is indeed associated with the inhibition of TMV and TNV and whether SA plays a role in this process. We demonstrated that enhanced viral resistance in NEC is manifested as both milder local necrotic (HR) symptoms and reduced levels of TMV and TNV. The presence of an adequate amount of SA contributes to the enhanced defense response of NEC to TMV and TNV, as the absence of SA resulted in seriously impaired viral resistance. Elevated levels of subcellular tripeptide glutathione (GSH) in NEC plants in response to viral infection suggest that in addition to SA, GSH may also contribute to the elevated viral resistance of NEC. Furthermore, we found that NEC displays an enhanced resistance not only to viral pathogens but also to bacterial infections and abiotic oxidative stress induced by paraquat treatments. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Signaling and Resistosome Formation in Plant Innate Immunity to Viruses: Is There a Common Mechanism of Antiviral Resistance Conserved across Kingdoms?

Virus-specific proteins, including coat proteins, movement proteins, replication proteins, and suppressors of RNA interference are capable of triggering the hypersensitive response (HR), which is a type of cell death in plants. The main cell death signaling pathway involves direct interaction of HR-inducing proteins with nucleotide-binding leucine-rich repeats (NLR) proteins encoded by plant resistance genes. Singleton NLR proteins act as both sensor and helper. In other cases, NLR proteins form an activation network leading to their oligomerization and formation of membrane-associated resistosomes, similar to metazoan inflammasomes and apoptosomes. In resistosomes, coiled-coil domains of NLR proteins form Ca2+ channels, while toll-like/interleukin-1 receptor-type (TIR) domains form oligomers that display NAD+ glycohydrolase (NADase) activity. This review is intended to highlight the current knowledge on plant innate antiviral defense signaling pathways in an attempt to define common features of antiviral resistance across the kingdoms of life.

Cauliflower mosaic virus: Virus-host interactions and its uses in biotechnology and medicine

Cauliflower mosaic virus (CaMV) was the first discovered plant virus with genomic DNA that uses reverse transcriptase for replication. The CaMV 35S promoter is a constitutive promoter and thus, an attractive driver of gene expression in plant biotechnology. It is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. In the last century, producing food for the world's population while preserving the environment and human health is the main topic of agriculture. The damage caused by viral diseases has a significant negative economic impact on agriculture, and disease control is based on two strategies: immunization and prevention to contain virus spread, so correct identification of plant viruses is important for disease management. Here, we discuss CaMV from different aspects: taxonomy, structure and genome, host plants and symptoms, transmission and pathogenicity, prevention, control and application in biotechnology as well as in medicine. Also, we calculated the CAI index for three ORFs IV, V, and VI of the CaMV virus in host plants, the results of which can be used in the discussion of gene transfer or antibody production to identify the CaMV.

Severe Stunting Symptoms upon Nepovirus Infection Are Reminiscent of a Chronic Hypersensitive-like Response in a Perennial Woody Fruit Crop

Virus infection of plants can result in various degrees of detrimental impacts and disparate symptom types and severities. Although great strides have been made in our understanding of the virus-host interactions in herbaceous model plants, the mechanisms underlying symptom development are poorly understood in perennial fruit crops. Grapevine fanleaf virus (GFLV) causes variable symptoms in most vineyards worldwide. To better understand GFLV-grapevine interactions in relation to symptom development, field and greenhouse trials were conducted with a grapevine genotype that exhibits distinct symptoms in response to a severe and a mild strain of GFLV. After validation of the infection status of the experimental vines by high-throughput sequencing, the transcriptomic and metabolomic profiles in plants infected with the two viral strains were tested and compared by RNA-Seq and LC-MS, respectively, in the differentiating grapevine genotype. In vines infected with the severe GFLV strain, 1023 genes, among which some are implicated in the regulation of the hypersensitive-type response, were specifically deregulated, and a higher accumulation of resveratrol and phytohormones was observed. Interestingly, some experimental vines restricted the virus to the rootstock and remained symptomless. Our results suggest that GFLV induces a strain- and cultivar-specific defense reaction similar to a hypersensitive reaction. This type of defense leads to a severe stunting phenotype in some grapevines, whereas others are resistant. This work is the first evidence of a hypersensitive-like reaction in grapevine during virus infection.

Reactive Oxygen Species Contribute to Symptomless, Extreme Resistance to Potato virus X in Tobacco

Here we show that in tobacco (Nicotiana tabacum cultivar Samsun NN Rx1) the development of Rx1 gene-mediated, symptomless, extreme resistance to Potato virus X (PVX) is preceded by an early, intensive accumulation of the reactive oxygen species (ROS) superoxide (O2·-), evident between 1 and 6 h after inoculation and associated with increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activities. This suggests a direct contribution of this ROS to virus restriction during symptomless, extreme resistance. Superoxide inhibition in PVX-inoculated leaves by infiltration of antioxidants (superoxide dismutase [SOD] and catalase [CAT]) partially suppresses extreme resistance in parallel with the appearance of localized leaf necrosis resembling a hypersensitive resistance (HR) response. F1 progeny from crosses of Rx1 and ferritin overproducer (deficient in production of the ROS OH·) tobaccos also display a suppressed extreme resistance to PVX, because significantly increased virus levels are coupled to HR, suggesting a role of the hydroxyl radical (OH·) in this symptomless antiviral defense. In addition, treatment of PVX-susceptible tobacco with a superoxide-generating agent (riboflavin/methionine) results in HR-like symptoms and reduced PVX titers. Finally, by comparing defense responses during PVX-elicited symptomless, extreme resistance and HR-type resistance elicited by Tobacco mosaic virus, we conclude that defense reactions typical of an HR (e.g., induction of cell death/ROS-regulator genes and antioxidants) are early and transient in the course of extreme resistance. Our results demonstrate the contribution of early accumulation of ROS (superoxide, OH·) in limiting PVX replication during symptomless extreme resistance and support earlier findings that virus-elicited HR represents a delayed, slower resistance response than symptomless, extreme resistance.

From Player to Pawn: Viral Avirulence Factors Involved in Plant Immunity

In the plant immune system, according to the 'gene-for-gene' model, a resistance (R) gene product in the plant specifically surveils a corresponding effector protein functioning as an avirulence (Avr) gene product. This system differs from other plant-pathogen interaction systems, in which plant R genes recognize a single type of gene or gene family because almost all virus genes with distinct structures and functions can also interact with R genes as Avr determinants. Thus, research conducted on viral Avr-R systems can provide a novel understanding of Avr and R gene product interactions and identify mechanisms that enable rapid co-evolution of plants and phytopathogens. In this review, we intend to provide a brief overview of virus-encoded proteins and their roles in triggering plant resistance, and we also summarize current progress in understanding plant resistance against virus Avr genes. Moreover, we present applications of Avr gene-mediated phenotyping in R gene identification and screening of segregating populations during breeding processes.

R gene mediated defense against viruses

The relationship of Resistance (R) gene-mediated defense to other forms of resistance in plants is considered, and the natures of the products of dominant and recessive R genes are reviewed. Various factors involved in expressing R gene-mediated resistance are described. These include phytohormones and plant effector molecules: the former regulating different pathways for disease resistance and the latter having direct effects on viral genomes or encoded proteins. Finally, the status of our knowledge concerning the cell-death hypersensitive response and its relationship to the actual resistance response involved in inhibiting virus infection is examined.

The P1 gene of Cauliflower mosaic virus is responsible for breaking resistance in Arabidopsis thaliana ecotype Enkheim (En-2)

Arabidopsis thaliana ecotype En-2 is resistant to several strains of Cauliflower mosaic virus (CaMV), including strain W260, but is susceptible to strain NY8153. Resistance in En-2 is conditioned by a single, semi-dominant gene called CAR1. We constructed several recombinant infectious clones between W260 and NY8153 and evaluated their capability to infect En-2. This analysis showed that the capacity of NY8153 to break resistance in En-2 was conditioned by mutations within the CaMV gene 1, a gene that encodes a protein dedicated to cell-to-cell movement (P1), and conversely, that P1 of W260 is responsible for eliciting the plant defense response. A previous study had shown that P6 of W260 was responsible for overcoming resistance in Arabidopsis ecotype Tsu-0 and that P6 of CaMV strain CM1841 was responsible for triggering resistance. The present study now shows that a second gene of CaMV is targeted by Arabidopsis for plant immunity.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants

Plants respond to pathogens using an innate immune system that is broadly divided into PTI (pathogen-associated molecular pattern- or PAMP-triggered immunity) and ETI (effector-triggered immunity). PTI is activated upon perception of PAMPs, conserved motifs derived from pathogens, by surface membrane-anchored pattern recognition receptors (PRRs). To overcome this first line of defense, pathogens release into plant cells effectors that inhibit PTI and activate effector-triggered susceptibility (ETS). Counteracting this virulence strategy, plant cells synthesize intracellular resistance (R) proteins, which specifically recognize pathogen effectors or avirulence (Avr) factors and activate ETI. These coevolving pathogen virulence strategies and plant resistance mechanisms illustrate evolutionary arms race between pathogen and host, which is integrated into the zigzag model of plant innate immunity. Although antiviral immune concepts have been initially excluded from the zigzag model, recent studies have provided several lines of evidence substantiating the notion that plants deploy the innate immune system to fight viruses in a manner similar to that used for non-viral pathogens. First, most R proteins against viruses so far characterized share structural similarity with antibacterial and antifungal R gene products and elicit typical ETI-based immune responses. Second, virus-derived PAMPs may activate PTI-like responses through immune co-receptors of plant PTI. Finally, and even more compelling, a viral Avr factor that triggers ETI in resistant genotypes has recently been shown to act as a suppressor of PTI, integrating plant viruses into the co-evolutionary model of host-pathogen interactions, the zigzag model. In this review, we summarize these important progresses, focusing on the potential significance of antiviral immune receptors and co-receptors in plant antiviral innate immunity. In light of the innate immune system, we also discuss a newly uncovered layer of antiviral defense that is specific to plant DNA viruses and relies on transmembrane receptor-mediated translational suppression for defense.

A model for intracellular movement of Cauliflower mosaic virus: the concept of the mobile virion factory

The genomes of many plant viruses have a coding capacity limited to <10 proteins, yet it is becoming increasingly clear that individual plant virus proteins may interact with several targets in the host for establishment of infection. As new functions are uncovered for individual viral proteins, virologists have realized that the apparent simplicity of the virus genome is an illusion that belies the true impact that plant viruses have on host physiology. In this review, we discuss our evolving understanding of the function of the P6 protein of Cauliflower mosaic virus (CaMV), a process that was initiated nearly 35 years ago when the CaMV P6 protein was first described as the 'major inclusion body protein' (IB) present in infected plants. P6 is now referred to in most articles as the transactivator (TAV)/viroplasmin protein, because the first viral function to be characterized for the Caulimovirus P6 protein beyond its role as an inclusion body protein (the viroplasmin) was its role in translational transactivation (the TAV function). This review will discuss the currently accepted functions for P6 and then present the evidence for an entirely new function for P6 in intracellular movement.

Dominant resistance against plant viruses

To establish a successful infection plant viruses have to overcome a defense system composed of several layers. This review will overview the various strategies plants employ to combat viral infections with main emphasis on the current status of single dominant resistance (R) genes identified against plant viruses and the corresponding avirulence (Avr) genes identified so far. The most common models to explain the mode of action of dominant R genes will be presented. Finally, in brief the hypersensitive response (HR) and extreme resistance (ER), and the functional and structural similarity of R genes to sensors of innate immunity in mammalian cell systems will be described.

Up-regulation of antioxidants in tobacco by low concentrations of H₂O₂ suppresses necrotic disease symptoms

Pretreatment of tobacco leaves with low concentrations (5 to 10 mM) of H₂O₂ suppressed hypersensitive-type necrosis associated with resistance to Tobacco mosaic virus (TMV) or Pseudomonas syringae pv. phaseolicola. The same pretreatment resulted in suppression of normosensitive necrosis associated with susceptibility to Botrytis cinerea. This type of H₂O₂-mediated, induced disease symptom resistance correlated with enhanced host antioxidant capacity, i.e., elevated enzymatic activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POX) after viral and bacterial infections. Induction of genes that encode the antioxidants superoxide dismutase (SOD), CAT, and APX was also enhanced early after TMV infection. Artificial application of SOD and CAT suppressed necroses caused by viral, bacterial, or fungal pathogens similarly as H₂O₂ pretreatment, implying that H₂O₂-mediated symptom resistance operates through enhancement of plant antioxidant capacity. Pathogen multiplication was not significantly affected in H₂O₂-pretreated plants. Salicylic acid (SA), a central component of plant defense, does not seem to function in this type of H₂O₂-mediated symptom resistance, indicated by unchanged levels of free and bound SA and a lack of early up-regulation of an SA glucosyltransferase gene in TMV-infected H₂O₂-pretreated tobacco. Taken together, H₂O₂-mediated, induced resistance to necrotic symptoms in tobacco seems to depend on enhanced antioxidant capacity.

Salicylic acid-dependent restriction of Tomato ringspot virus spread in tobacco is accompanied by a hypersensitive response, local RNA silencing, and moderate systemic resistance

Tomato ringspot virus (ToRSV, a Nepovirus sp.) systemically infects many herbaceous plants. Viral RNA accumulates in symptomatic leaves and in young, asymptomatic leaves that emerge late in infection. Here, we show that systemic infection by ToRSV is restricted in tobacco. After an initial hypersensitive response in inoculated leaves, only a few plants showed limited systemic symptoms. Viral RNA did not usually accumulate to detectable levels in asymptomatic leaves. ToRSV-derived small-interfering RNAs and PR1a transcripts were only detected in tissues that contained viral RNA, indicating local induction of RNA silencing and salicylic acid (SA)-dependent defense responses. Lesion size and viral systemic spread were reduced with SA pretreatment but enhanced in NahG transgenic lines deficient in SA accumulation, suggesting that SA-dependent mechanisms play a key role in limiting ToRSV spread in tobacco. Restriction of virus infection was enhanced in transgenic lines expressing the P1-HC-Pro suppressor of silencing. Knocking down the SA-inducible RNA-dependent RNA polymerase 1 exacerbated the necrotic reaction but did not affect viral systemic spread. ToRSV-infected tobacco plants were susceptible to reinoculation by ToRSV or Tobacco mosaic virus, although a small reduction in lesion size was observed. This moderate systemic resistance suggests inefficient induction or spread of RNA silencing and systemic acquired resistance signal molecules.

The Arabidopsis resistance-like gene SNC1 is activated by mutations in SRFR1 and contributes to resistance to the bacterial effector AvrRps4

The SUPPRESSOR OF rps4-RLD1 (SRFR1) gene was identified based on enhanced AvrRps4-triggered resistance in the naturally susceptible Arabidopsis accession RLD. No other phenotypic effects were recorded, and the extent of SRFR1 involvement in regulating effector-triggered immunity was unknown. Here we show that mutations in SRFR1 in the accession Columbia-0 (Col-0) lead to severe stunting and constitutive expression of the defense gene PR1. These phenotypes were temperature-dependent. A cross between srfr1-1 (RLD background) and srfr1-4 (Col-0) showed that stunting was caused by a recessive locus in Col-0. Mapping and targeted crosses identified the Col-0-specific resistance gene SNC1 as the locus that causes stunting. SRFR1 was proposed to function as a transcriptional repressor, and SNC1 is indeed overexpressed in srfr1-4. Interestingly, co-regulated genes in the SNC1 cluster are also upregulated in the srfr1-4 snc1-11 double mutant, indicating that the overexpression of SNC1 is not a secondary effect of constitutive defense activation. In addition, a Col-0 RPS4 mutant showed full susceptibility to bacteria expressing avrRps4 at 24°C but not at 22°C, while RLD susceptibility was not temperature-dependent. The rps4-2 snc1-11 double mutant showed increased, but not full, susceptibility at 22°C, indicating that additional cross-talk between resistance pathways may exist. Intriguingly, when transiently expressed in Nicotiana benthamiana, SRFR1, RPS4 and SNC1 are in a common protein complex in a cytoplasmic microsomal compartment. Our results highlight SRFR1 as a convergence point in at least a subset of TIR-NBS-LRR protein-mediated immunity in Arabidopsis. Based on the cross-talk evident from our results, they also suggest that reports of constitutive resistance phenotypes in Col-0 need to consider the possible involvement of SNC1.

Systemic acquired resistance is induced by R gene-mediated responses independent of cell death

On infection by pathogens, plants initiate defence responses that are able to curtail infection locally. These responses are mediated either by receptor-like proteins that recognize pathogen-associated molecular patterns or by the protein products of disease resistance (R) genes. At the same time, primary defence responses often result in the generation of signals that induce what is known as systemic acquired resistance (SAR), such that defence responses are enhanced on secondary pathogen challenge in distal tissues. R protein-mediated SAR induction is normally accompanied by a type of programmed cell death known as the hypersensitive response (HR) and, in some instances, cell death alone has been implicated in the induction of SAR. This has raised the question of whether R protein-mediated signalling per se induces SAR or whether SAR is an indirect result of the induction of HR. Using the Rx gene of potato, which confers resistance to Potato Virus X in the absence of cell death, we have shown that the HR is dispensable for R protein-mediated induction of SAR and that Rx-induced SAR is mediated by the same salicylate-dependent pathway induced by other R proteins.

Dual roles of reactive oxygen species and NADPH oxidase RBOHD in an Arabidopsis-Alternaria pathosystem

Arabidopsis (Arabidopsis thaliana) NADPH oxidases have been reported to suppress the spread of pathogen- and salicylic acid-induced cell death. Here, we present dual roles of RBOHD (for respiratory burst oxidase homolog D) in an Arabidopsis-Alternaria pathosystem, suggesting either initiation or prevention of cell death dependent on the distance from pathogen attack. Our data demonstrate that a rbohD knockout mutant exhibits increased spread of cell death at the macroscopic level upon inoculation with the fungus Alternaria brassicicola. However, the cellular patterns of reactive oxygen species accumulation and cell death are fundamentally different in the AtrbohD mutant compared with the wild type. Functional RBOHD causes marked extracellular hydrogen peroxide accumulation as well as cell death in distinct, single cells of A. brassicicola-infected wild-type plants. This single cell response is missing in the AtrbohD mutant, where infection triggers spreading-type necrosis preceded by less distinct chloroplastic hydrogen peroxide accumulation in large clusters of cells. While the salicylic acid analog benzothiadiazole induces the action of RBOHD and the development of cell death in infected tissues, the ethylene inhibitor aminoethoxyvinylglycine inhibits cell death, indicating that both salicylic acid and ethylene positively regulate RBOHD and cell death. Moreover, A. brassicicola-infected AtrbohD plants hyperaccumulate ethylene and free salicylic acid compared with the wild type, suggesting negative feedback regulation of salicylic acid and ethylene by RBOHD. We propose that functional RBOHD triggers death in cells that are damaged by fungal infection but simultaneously inhibits death in neighboring cells through the suppression of free salicylic acid and ethylene levels.

Single amino acid substitution in the methyltransferase domain of Paprika mild mottle virus replicase proteins confers the ability to overcome the high temperature-dependent Hk gene-mediated resistance in Capsicum plants

Capsicum plants harboring the Hk gene (Hk) show resistance to Paprika mild mottle virus (PaMMV) at 32 degrees C but not 24 degrees C. To identify the viral elicitor that activates the Hk-mediated resistance, several chimeric viral genomes were constructed between PaMMV and Tobacco mosaic virus-L. Infection patterns of these chimeric viruses in Hk-harboring plants revealed responsibility of PaMMV replicase genes for activation of the Hk-mediated resistance. The comparison of nucleotide sequence of replicase genes between PaMMV and PaHk1, an Hk-resistance-breaking strain of PaMMV, revealed that the adenine-to-uracil substitution at the nucleotide position 721 causes an amino acid change from threonine to serine at the 241st residue in the methyltransferase domain. Introduction of the A721U mutation into the replicase genes of parental PaMMV overcame the Hk resistance at 32 degrees C. The results indicate that Hk-mediated resistance is induced by PaMMV replicase proteins and that methyltransferase domain has a role in this elicitation.

Soybean defense responses to the soybean aphid

Transcript profiles in aphid (Aphis glycines)-resistant (cv. Dowling) and -susceptible (cv. Williams 82) soybean (Glycine max) cultivars using soybean cDNA microarrays were investigated. Large-scale soybean cDNA microarrays representing approx. 18 000 genes or c. 30% of the soybean genome were compared at 6 and 12 h post-application of aphids. In a separate experiment utilizing clip cages, expression of three defense-related genes were examined at 6, 12, 24, 48, and 72 h in both cultivars by quantitative real-time PCR. One hundred and forty genes showed specific responses for resistance; these included genes related to cell wall, defense, DNA/RNA, secondary metabolism, signaling and other processes. When an extended time period of sampling was investigated, earlier and greater induction of three defense-related genes was observed in the resistant cultivar; however, the induction declined after 24 or 48 h in the resistant cultivar but continued to increase in the susceptible cultivar after 24 h. Aphid-challenged resistant plants showed rapid differential gene expression patterns similar to the incompatible response induced by avirulent Pseudomonas syringae. Five genes were identified as differentially expressed between the two genotypes in the absence of aphids.

Suppression of tobacco mosaic virus-induced hypersensitive-type necrotization in tobacco at high temperature is associated with downregulation of NADPH oxidase and superoxide and stimulation of dehydroascorbate reductase

Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 degrees C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H(2)O(2)) are able to induce HR-type necroses in TMV-inoculated Xanthi-nc tobacco even at high temperatures (e.g. 30 degrees C). The amount of superoxide (O(2)(*-)) decreased, while H(2)O(2) slightly increased in TMV- and mock-inoculated leaves at 30 degrees C, as compared with 20 degrees C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 degrees C, as compared with 20 degrees C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 degrees C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 degrees C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 degrees C.

Silencing of the N family of resistance genes in Nicotiana edwardsonii compromises the hypersensitive response to tombusviruses

The nontarget effects associated with silencing of the N gene in Nicotiana edwardsonii, an amphidiploid species derived from N. glutinosa and N. clevelandii, have been characterized in this study. The N protein confers resistance to Tobacco mosaic virus (TMV), and is representative of a family of nucleotide-binding site leucine-rich repeat proteins present in N. glutinosa. Previous studies have shown that silencing of the N gene or of other plant genes associated with N-mediated defenses abolishes host resistance to TMV, and this effect can be measured through enhancements in movement or replication of TMV in the N-silenced plants. However, the nontarget effects of gene silencing have not been investigated thoroughly. Notably, are the functions of other resistance (R) genes also affected in experiments designed to silence the N gene? To investigate whether heterologous sequences could silence the N gene, we selected an R gene homolog from N. glutinosa that differed from the N gene by approximately 17%, created a hairpin transgene, and developed transgenic N. edwardsonii plants. Expression of this hairpin in the transgenic N. edwardsonii plants compromised the hypersensitive response to TMV, demonstrating that a single hairpin transgene could silence a block of R genes related by sequence similarity. We then investigated whether the response of N-silenced plants to other viruses would be altered, and found that the hypersensitive response triggered against the tombusviruses Tomato bushy stunt virus and Cymbidium ringspot virus also was compromised. This study indicates that a Tombusvirus R gene shares some homology with the N gene, which could facilitate the cloning of this gene.

An NB-LRR protein required for HR signalling mediated by both extra- and intracellular resistance proteins

Tomato (Solanum lycopersicum) Cf resistance genes confer hypersensitive response (HR)-associated resistance to strains of the pathogenic fungus Cladosporium fulvum that express the matching avirulence (Avr) gene. Previously, we identified an Avr4-responsive tomato (ART) gene that is required for Cf-4/Avr4-induced HR in Nicotiana benthamiana as demonstrated by virus-induced gene silencing (VIGS). The gene encodes a CC-NB-LRR type resistance (R) protein analogue that we have designated NRC1 (NB-LRR protein required for HR-associated cell death 1). Here we describe that knock-down of NRC1 in tomato not only affects the Cf-4/Avr4-induced HR but also compromises Cf-4-mediated resistance to C. fulvum. In addition, VIGS using NRC1 in N. benthamiana revealed that this protein is also required for the HR induced by the R proteins Cf-9, LeEix, Pto, Rx and Mi. Transient expression of NRC1(D481V), which encodes a constitutively active NRC1 mutant protein, triggers an elicitor-independent HR. Subsequently, we transiently expressed this auto-activating protein in N. benthamiana silenced for genes known to be involved in HR signalling, thereby allowing NRC1 to be positioned in an HR signalling pathway. We found that NRC1 requires RAR1 and SGT1 to be functional, whereas it does not require NDR1 and EDS1. As the Cf-4 protein requires EDS1 for its function, we hypothesize that NRC1 functions downstream of EDS1. We also found that NRC1 acts upstream of a MAP kinase pathway. We conclude that Cf-mediated resistance signalling requires a downstream NB-LRR protein that also functions in cell death signalling pathways triggered by other R proteins.

Activation of defence reactions in Solanaceae: where is the specificity?

When a potential pathogen attempts to infect a plant, biochemical and molecular communication takes place and leads to the induction of plant defence mechanisms. In the case of efficient defence, visible symptoms are restricted and the pathogen does not multiply (incompatible interaction); when defence is inefficient, the plant becomes rapidly infected (compatible interaction). During the last 30 years, a growing body of knowledge on plant-pathogen interactions has been gathered, and a large number of studies investigate the induction of various plant defence reactions by pathogens or by pathogen-derived compounds. However, as most papers focus on incompatible interactions, there is still a lack of understanding about the similarities and differences between compatible and incompatible situations. This review targets the question of specificity in Solanaceae-pathogen interactions, by comparing defence patterns in plants challenged with virulent or avirulent pathogens (or with pathogen-associated molecular patterns from these). A special emphasis is made on analysing whether defence reactions in Solanaceae depend primarily on the type of elicitor, on the plant genotype/species, or on the type of interaction (compatible or incompatible).

Artificial evolution extends the spectrum of viruses that are targeted by a disease-resistance gene from potato

A major class of disease-resistance (R) genes in plants encode nucleotide-binding site/leucine-rich repeat (LRR) proteins. The LRR domains mediate recognition of pathogen-derived elicitors. Here we describe a random in vitro mutation analysis illustrating how mutations in an R protein (Rx) LRR domain generate disease-resistance specificity. The original Rx protein confers resistance only against a subset of potato virus X (PVX) strains, whereas selected mutants were effective against an additional strain of PVX and against the distantly related poplar mosaic virus. These effects of LRR mutations indicate that in vitro evolution of R genes could be exploited for enhancement of disease resistance in crop plants. Our results also illustrate how short-term evolution of disease resistance in wild populations might be toward broader spectrum resistance against multiple strains of the pathogen. The breadth of the disease-resistance phenotype from a natural R gene may be influenced by the tradeoff between the costs and benefits of broad-spectrum disease resistance.

Modeling-based characterization of the elicitor function of amino acid 461 of Cucumber mosaic virus 1a protein in the hypersensitive response

The Ns strain of Cucumber mosaic virus (CMV) induces hypersensitive response (HR) on Nicotiana tabacum cv. Xanthi-nc and on Nicotiana glutinosa. The genetic determinant of the HR induction was localized earlier to amino acid 461 of the 1a protein. The 3D structure of the 1a protein is still unknown and building a homology model is impossible. Nevertheless, on the basis of secondary structure predictions we have created partial protein models for the region surrounding residue 461 which can account structurally for the effect of aa 461 on elicitor function. Seven different amino acid mutations were designed and introduced to the position 461 of the 1a protein in RNA 1. Three of the mutations (proline, glutamic acid, asparagine) inhibited virus replication. Two of the mutants caused systemic symptom development (lysine and arginine). Two mutants (alanine and serine) resulted in localization of the virus, but strong necrosis similar to the original Ns-CMV strain was not observed. Inoculation of purified Ns-CMV virions at extremely high concentration provoked systemic symptoms.

Introgression of a Tombusvirus Resistance Locus from Nicotiana edwardsonii var. Columbia to N. clevelandii

ABSTRACT A new variety of Nicotiana, N. edwardsonii var. Columbia, was evaluated for its capacity to serve as a new source for virus resistance genes. Columbia was developed from a hybridization between N. glutinosa and N. clevelandii, the same parents used for the formation of the original N. edwardsonii. However, in contrast to the original N. edwardsonii, crosses between Columbia and either of its parents are fertile. Thus, the inheritance of virus resistance genes present in N. glutinosa could be characterized by using Columbia as a bridge plant in crosses with the susceptible parent, N. clevelandii. To determine how virus resistance genes would segregate in interspecific crosses between Columbia and N. clevelandii, we followed the fate of the N gene, a single dominant gene that specifies resistance to Tobacco mosaic virus (TMV). Our genetic evidence indicated that the entire chromosome containing the N gene was introgressed into N. clevelandii to create an addition line, designated N. clevelandii line 19. Although line 19 was homozygous for resistance to TMV, it remained susceptible to Tomato bushy stunt virus (TBSV) and Cauliflower mosaic virus (CaMV) strain W260, indicating that resistance to these viruses must reside on other N. glutinosa chromosomes. We also developed a second addition line, N. clevelandii line 36, which was homozygous for resistance to TBSV. Line 36 was susceptible to TMV and CaMV strain W260, but was resistant to other tombusviruses, including Cucumber necrosis virus, Cymbidium ringspot virus, Lettuce necrotic stunt virus, and Carnation Italian ringspot virus.

Natural variation in the Arabidopsis response to the avirulence gene hopPsyA uncouples the hypersensitive response from disease resistance

The plant hypersensitive response (HR) is tightly associated with gene-for-gene resistance and has been proposed to function in containing pathogens at the invasion site. This tight association has made it difficult to unequivocally evaluate the importance of HR for plant disease resistance. Here, hopPsyA from Pseudomonas syringae pv. syringae 61 is identified as a new avirulence gene for Arabidopsis that triggers resistance in the absence of macroscopic HR. Resistance to P. syringae pv. tomato DC3000 expressing hopPsyA was EDS1-dependent and NDR1-independent. Intriguingly, several Arabidopsis accessions were resistant to DC3000(hopPsyA) in the absence of HR. This is comparable to the Arabidopsis response to avrRps4, but it is shown that hopPsyA does not signal through RPS4. In a cross between two hopPsyA-resistant accessions that differ in their HR response, the HR segregated as a recessive phenotype regulated by a single locus. This locus, HED1 (HR regulator in EDS1 pathway), is proposed to encode a protein whose activity can cause suppression of the EDS1-dependent HR signaling pathway. HED1-regulated symptomless gene-for-gene resistance responses may explain some cases of Arabidopsis resistance to bacteria that are classified as nonhost resistance.

The plant gene CCD1 selectively blocks cell death during the hypersensitive response to Cauliflower mosaic virus infection

The P6 protein of Cauliflower mosaic virus (CaMV) W260 elicits a hypersensitive response (HR) on inoculated leaves of Nicotiana edwardsonii. This defense response, common to many plant pathogens, has two key characteristics, cell death within the initially infected tissues and restriction of the pathogen to this area. We present evidence that a plant gene designated CCD1, originally identified in N. bigelovii, can selectively block the cell death pathway during HR, whereas the resistance pathway against W260 remains intact. Suppression of cell death was evident not only macroscopically but also microscopically. The suppression of HR-mediated cell death was specific to CaMV, as Tobacco mosaic virus was able to elicit HR in the plants that contained CCD1. CCD1 also blocks the development of a systemic cell death symptom induced specifically by the P6 protein of W260 in N. clevelandii. Introgression of CCD1 from N. bigelovii into N. clevelandii blocked the development of systemic cell death in response to W260 infection but could not prevent systemic cell death induced by Tomato bushy stunt virus. Thus, CCD1 blocks both local and systemic cell death induced by P6 of W260 but does not act as a general suppressor of cell death induced by other plant viruses. Furthermore, experiments with CCD1 provide further evidence that cell death could be uncoupled from resistance in the HR of Nicotiana edwardsonii to CaMV W260.

The open reading frame VI product of Cauliflower mosaic virus is a nucleocytoplasmic protein: its N terminus mediates its nuclear export and formation of electron-dense viroplasms

The Cauliflower mosaic virus (CaMV) open reading frame VI product (P6) is essential for the viral infection cycle. It controls translation reinitiation of the viral polycistronic RNAs and forms cytoplasmic inclusion bodies (viroplasms) where virus replication and assembly occur. In this study, the mechanism involved in viroplasm formation was investigated by in vitro and in vivo experiments. Far protein gel blot assays using a collection of P6 deletion mutants demonstrated that the N-terminal alpha-helix of P6 mediates interaction between P6 molecules. Transient expression in tobacco (Nicotiana tabacum) BY-2 cells of full-length P6 and P6 mutants fused to enhanced green fluorescent protein revealed that viroplasms are formed at the periphery of the nucleus and that the N-terminal domain of P6 is an important determinant in this process. Finally, this study led to the unexpected finding that P6 is a nucleocytoplasmic shuttle protein and that its nuclear export is mediated by a Leu-rich sequence that is part of the alpha-helix domain implicated in viroplasm formation. The discovery that P6 can localize to the nucleus opens new prospects for understanding yet unknown roles of this viral protein in the course of the CaMV infection cycle.

Genetics of plant virus resistance

Genetic resistance to plant viruses has been used for at least 80 years to control agricultural losses to viral diseases. To date, hundreds of naturally occurring genes for resistance to plant viruses have been reported from studies of both monocot and dicot crops, their wild relatives, and the plant model, Arabidopsis. The isolation and characterization of a few of these genes in the past decade have resulted in detailed knowledge of some of the molecules that are critical in determining the outcome of plant viral infection. In this chapter, we have catalogued genes for resistance to plant viruses and have summarized current knowledge regarding their identity and inheritance. Insofar as information is available, the genetic context, genomic organization, mechanisms of resistance and agricultural deployment of plant virus resistance genes are also discussed.

Temporal expression of PR-1 and enhanced mature plant resistance to virus infection is controlled by a single dominant gene in a new Nicotiana hybrid

A new variety of Nicotiana edwardsonii, designated N. edwardsonii cv. Columbia, expresses pathogenesis-related (PR) proteins in a temporal manner 45 to 49 days postplanting and also exhibits enhanced resistance to Tobacco mosaic virus, Tobacco necrosis virus, and Tomato bushy stunt virus. In contrast, PR proteins were not expressed in the original N. edwardsonii variety at comparable ages but were induced after onset of a hypersensitive response to viral infection. The temporal induction of PR proteins in 'Columbia' was correlated with increases in salicylic acid and glycosylated salicylic acid. Earlier studies noted that some Nicotiana hybrids derived from interspecific crosses constitutively express PR proteins, but the genetic basis of this phenomenon had not been investigated, likely because many interspecific Nicotiana crosses are sterile. However, the close genetic relationship between N. edwardsonii and 'Columbia' indicated that a hybrid between these two plants might be fertile, and this proved to be true. Genetic crosses between 'Columbia' and N. edwardsonii demonstrated that a single, dominant gene conditioned temporal expression of PR proteins and enhanced resistance. This gene was designated TPR1 (for temporal expression of PR proteins).

Activation of multiple antiviral defence mechanisms by salicylic acid

SUMMARY The plant signal molecule salicylic acid (SA) can induce resistance to a wide range of pathogen types. In the case of viruses, SA can stimulate the inhibition of all three main stages in virus infection: replication, cell-to-cell movement and long-distance movement. Induction of resistance by SA appears to depend, in part, on downstream signalling via the mitochondrion. However, evidence has recently emerged that SA may stimulate a separate downstream pathway, leading to the induction of an additional mechanism of resistance based on RNA interference. In this review our aims are to document these recent advances and to suggest possible future avenues of research on SA-induced resistance to viruses.

High throughput virus-induced gene silencing implicates heat shock protein 90 in plant disease resistance

Virus-induced gene silencing was used to assess the function of random Nicotiana benthamiana cDNAs in disease resistance. Out of 4992 cDNAs tested from a normalized library, there were 79 that suppressed a hypersensitive response (HR) associated with Pto-mediated resistance against Pseudomonas syringae. However, only six of these clones blocked the Pto-mediated suppression of P.syringae growth. The three clones giving the strongest loss of Pto resistance had inserts corresponding to HSP90 and also caused loss of Rx-mediated resistance against potato virus X and N-mediated tobacco mosaic virus resistance. The role of HSP90 as a cofactor of disease resistance is associated with stabilization of Rx protein levels and could be accounted for in part by SGT1 and other cofactors of disease resistance acting as co-chaperones. This approach illustrates the potential benefits and limitations of RNA silencing in forward screens of gene function in plants.

In spite of induced multiple defense responses, tomato plants infected with Cucumber mosaic virus and D satellite RNA succumb to systemic necrosis

Cucumber mosaic virus (CMV) D satellite RNA (satRNA) attenuates the symptoms induced by CMV in most plants, but causes leaf epinasty and systemic necrosis in tomato plants, where programmed cell death (PCD) is involved. However, our understanding of the cellular and molecular responses to the infection of CMV D satRNA that result in this lethal disease remains limited. In this article, we show for the first time, by histochemical and molecular analysis, that multiple defense responses are specifically induced in CMV and D satRNA (CMV/D satRNA)-infected tomato plants but not in mock-inoculated or CMV-infected plants. These responses include callose deposition and hydrogen peroxide accumulation in infected plants. Furthermore, the transcription of several tomato defense-related genes (e.g., PR-1a1, PR-1b1, PR-2, and PR-10) were activated, and the expression of tomato PR-5 and some abiotic and biotic stress-responsive genes (e.g., catalase II and tomato analogs of Arabidopsis AtBI-1 and tobacco hsr203j) are enhanced. The activation and increase in expression of these genes is correlated with the appearance of leaf epinasty and the development of systemic necrosis in infected tomato plants, while increased expression of the hsr203j analog precedes the development of any disease symptoms. The spatial and temporal expression patterns of these genes as detected by RNA in situ hybridization point to the involvement of a complex developmental program that accompanies disease development resulting from CMV/D satRNA infection.

Expression of the baculovirus p35 protein in tobacco affects cell death progression and compromises N gene-mediated disease resistance response to Tobacco mosaic virus

The p35 protein from baculovirus is a broad-range caspase inhibitor and suppresses programmed cell death in animals. We report here the effects of transgenic expression in tobacco of the p35 protein during the hypersensitive response (HR). Expression of p35 causes partial inhibition of nonhost HR triggered by bacteria and gene-for-gene HR triggered by virus. Infection of p35-expressing tobacco plants with Tobacco mosaic virus (TMV) disrupts N-mediated disease resistance, causing systemic spreading of the virus within a resistant background. Mutant variants altered in aspartate residues within the loop region of p35 are inefficient substrates for caspases in vitro, and they do not suppress caspase proteolytic activity in animal systems. Tobacco plants expressing these mutant variants of the p35 protein do not show inhibition of HR cell death or enhanced virus systemic movement. Thus, HR inhibition and TMV systemic spreading phenotype in p35-expressing plants correlate with the ability of the p35 protein to suppress caspase activity in animal systems. In addition, a C-terminal truncated variant of p35 is unable to suppress cell death in animals as well as HR cell death in transgenic tobacco. Our results provide evidence for the participation of caspase-like proteases during the HR. In addition, they suggest that timely activation of cell death is necessary for effective TMV containment within the primary infection site.

Function of a mitogen-activated protein kinase pathway in N gene-mediated resistance in tobacco

The active defense of plants against pathogens often includes rapid and localized cell death known as hypersensitive response (HR). Protein phosphorylation and dephosphorylation are implicated in this event based on studies using protein kinase and phosphatase inhibitors. Recent transient gain-of-function studies demonstrated that the activation of salicylic acid-induced protein kinase (SIPK) and wounding-induced protein kinase (WIPK), two tobacco mitogen-activated protein kinases (MAPKs) by their upstream MAPK kinase (MAPKK), NtMEK2 leads to HR-like cell death. Here, we report that the conserved kinase interaction motif (KIM) in MAPKKs is required for NtMEK2 function. Mutation of the conserved basic amino acids in this motif, or the deletion of N-terminal 64 amino acids containing this motif significantly compromised or abolished the ability of NtMEK2DD to activate SIPK/WIPK in vivo. These mutants were also defective in interacting with SIPK and WIPK, suggesting protein-protein interaction is required for the functional integrity of this MAPK cascade. To eliminate Agrobacterium that is known to activate a number of defense responses in transient transformation experiments, we generated permanent transgenic plants. Induction of NtMEK2DD expression by dexamethasone induced HR-like cell death in both T1 and T2 plants. In addition, by using PVX-induced gene silencing, we demonstrated that the suppression of all three known components in the NtMEK2-SIPK/WIPK pathway attenuated N gene-mediated TMV resistance. Together with previous report that SIPK and WIPK are activated by TMV in a gene-for-gene-dependent manner, we conclude that NtMEK2-SIPK/WIPK pathway plays a positive role in N gene-mediated resistance, possibly through regulating HR cell death.

Plant disease resistance: commonality and novelty in multicellular innate immunity

Pathogen avirulence genes encode for effector molecules that play a crucial role in the process of pathogen colonization of plant tissue. Successful host defense requires rapid and efficient detection of the pathogen avirulence factors. In the last few years, much progress has been made in delineating the plant molecular sentinels that participate in pathogen identification. Because this ability is genetic information that is 'hard-wired' into the genome, it is called 'innate immunity' and it draws its origins from a phylogenetically ancient form of immunity common to plants and animals. Conservation is shown in many of the functional molecular motifs of innate genes such as the Toll/interleukin 1 receptor domains, nucleotide binding domains and structures that contain leucine rich repeats. Novel plant molecular surveillance domains also include pathogen pattern recognition by coiled-coil domains and specialized kinases. The rapid evolution of plant innate immunity genes is readily detected in their sequence polymorphism, by their massive amplification and appearance in the genome in a clustered organization. By comparative biology of highly diverged innate immunity systems we can enhance our appreciation of the truly basic forces that have shaped its evolution in mutlicellular organisms.

Multiple domains within the Cauliflower mosaic virus gene VI product interact with the full-length protein

The Cauliflower mosaic virus (CaMV) gene VI product (P6) is a multifunctional protein essential for viral propagation. It is likely that at least some of these functions require P6 self-association. The work described here was performed to confirm that P6 self-associates and to identify domains involved in this interaction. Yeast two-hybrid analyses indicated that full-length P6 self-associates and that this interaction is specific. Additional analyses indicated that at least four independent domains bind to full-length P6. When a central domain (termed domain D3) was removed, these interactions were abolished. However, this deleted P6 was able to bind to the full-length wild-type protein and to isolated domain D3. Viruses lacking domain D3 were incapable of producing a systemic infection. Isolated domain D3 was capable of binding to at least two of the other domains but was unable to self-associate. This suggests that domain D3 facilitates P6 self-association by binding to the other domains but not itself. The presence of multiple domains involved in P6 self-association may help explain the ability of this protein to form the intracellular inclusions characteristic of caulimoviruses.

A comparative analysis of the avirulence and translational transactivator functions of gene VI of Cauliflower mosaic virus

The primary function associated at present with the gene VI product of Cauliflower mosaic virus (CaMV) is that of a translational transactivator (TAV). In this capacity, it alters the host translational machinery to allow reinitiation of translation of other CaMV genes on the polycistronic 35S RNA of CaMV. In addition, the gene VI protein can elicit a specific type of plant defense response called the hypersensitive response (HR) in Nicotiana edwardsonii. In this study, we have adapted the agroinfiltration technique to compare the sequences of CaMV gene VI required for TAV function and elicitation of HR. To measure the activity of the TAV, we coagroinfiltrated gene VI of CaMV strain W260 with a bicistronic GUS reporter plasmid. TAV function could be assayed 4 days postinfiltration, before the onset of HR in N. edwardsonii. Through the use of the TAV and HR assays, we could show that the TAV functions of gene VI of CaMV strains W260 and D4 were equivalent, but only W260 gene VI elicited HR. A mutational analysis of W260 gene VI showed that the structural requirements for elicitation of HR were much more stringent than those for TAV function. Small deletions from either the 5' or 3' end of W260 gene VI abolished its ability to elicit HR, although the TAV function was retained in the mutant. The TAV function could also tolerate a small insertion within gene VI; this insertion abolished the elicitor function. This study provides direct evidence that the TAV function of gene VI is separate from its role as an elicitor of HR.

The 5' Third of Cauliflower mosaic virus Gene VI Conditions Resistance Breakage in Arabidopsis Ecotype Tsu-0

ABSTRACT Arabidopsis thaliana ecotypes vary in their responses to viruses. In this study, we analyzed the variation in response of A. thaliana ecotype Tsu-0 to Cauliflower mosaic virus (CaMV). This ecotype was previously reported to be resistant to two CaMV isolates (CM1841 and CM4-184), but susceptible to W260. In this study, we show that Tsu-0 is resistant to four additional CaMV isolates. CaMV propagated within the rosette leaves of Tsu-0 plants, but did not appear to spread systemically into the inflorescence. However, virus viability in rosette leaves of Tsu-0 plants apparently was not compromised because infectious CaMV could be recovered from these organs. W260 overcomes Tsu-0 resistance by a passive mechanism (i.e., this virus avoids activating plant defenses). The portion of the viral genome responsible for W260 resistance breakage was mapped to the 5' third of gene VI, which we have termed RBR-1. This region is also responsible for controlling the ability of CaMV to infect different types of solanaceous plants. Hence, the pathways by which plants of different families interact with CaMV may be conserved through evolution.

Chemically induced virus resistance in Arabidopsis thaliana is independent of pathogenesis-related protein expression and the NPR1 gene

Salicylic acid (SA) treatment triggers inhibition of replication or movement of several positive-sense RNA plant viruses in tobacco. This resistance can also be stimulated by nonlethal concentrations of cyanide and antimycin A (AA) without triggering induction of pathogenesis-related PR-1 protein genes. In two ecotypes of Arabidopsis thaliana (Columbia and Nössen), SA-induced resistance to a tobamovirus, Turnip vein clearing virus (TVCV), was also induced by nonlethal concentrations of cyanide and AA without concomitant induction of PR-1 gene expression. Furthermore, chemically induced resistance to TVCV, as well as the induction of the plant mitochondrial alternative oxidase (a potential target for the chemicals), was independent of NPR1, a gene that plays a key role downstream of SA in the induction of PR proteins. The chemically induced resistance to TVCV appeared to be due to inhibition of replication at the site of inoculation. Taken together, these results show that in Arabidopsis, as in tobacco, resistance to viruses can be induced via a distinct branch of the defensive signal transduction pathway. This suggests that the existence of this virus-specific branch may be widespread among plants.

Rds and Rih mediate hypersensitive cell death independent of gene-for-gene resistance to the oat crown rust pathogen Puccinia coronata f. sp. avenae

The Pca crown rust resistance cluster in the diploid Avena genus confers gene-for-gene specificity to numerous isolates of Puccinia coronata f. sp. avenae. Recombination breakpoint analysis indicates that specificities conferred by the Pca cluster are controlled by at least five distinct genes, designated Pc81, Pc82, Pc83, Pc84, and Pc85. Avena plants with the appropriate genotype frequently respond to P. coronata by undergoing hypersensitive cell death at the sites of fungal infection. Autofluorescence of host cells in response to P. coronata occurs in plants that develop visible necrotic lesions but not in plants that lack this phenotype. Two newly described, non-Pc loci were shown to control hypersensitive cell death. Rds (resistance-dependent suppressor of cell death) suppresses the hypersensitive response (HR), but not the resistance, mediated by the Pc82 resistance gene. In contrast, Rih (resistance-independent hypersensitive cell death) confers HR in both resistant and susceptible plants. Linkage analysis indicates that Rds is unlinked to the Pca cluster, whereas Rih is tightly linked to it. These results indicate that multiple synchronous pathways affect the development of hypersensitive cell death and that HR is not essential for resistance to crown rust. Further characterization of these genes will clarify the relationship between plant disease resistance and localized hypersensitive cell death.

Functional expression of Cf9 and Avr9 genes in Brassica napus induces enhanced resistance to Leptosphaeria maculans

The tomato Cf9 resistance gene induces an Avr9-dependent hypersensitive response (HR) in tomato and transgenic Solanaceae spp. We studied whether the Cf9 gene product responded functionally to the corresponding Avr9 gene product when introduced in a heterologous plant species. We successfully expressed the Cf9 gene under control of its own promoter and the Avr9 or Avr9R8K genes under control of the p35S1 promoter in transgenic oilseed rape. We demonstrated that the transgenic oilseed rape plants produced the Avr9 elicitor with the same specific necrosis-inducing activity as reported for Cladosporium fulvum. An Avr9-dependent HR was induced in Cf9 oilseed rape upon injection of intercellular fluid containing Avr9. We showed Avr9-specific induction of PR1, PR2, and Cxc750 defense genes in oilseed rape expressing CJ9. Cf9 x Avr9 oilseed rape did not result in seedling death of the F1 progeny, independent of the promoters used to express the genes. The F1 (Cf9 x Avr9) plants, however, were quantitatively more resistant to Leptosphaeria maculans. Phytopathological analyses revealed that disease development of L. maculans was delayed when the pathogen was applied on an Avr9-mediated HR site. We demonstrate that the CJ9 and Avr9 gene can be functionally expressed in a heterologous plant species and that the two components confer an increase in disease resistance.