The N gene of tobacco confers resistance to tobacco mosaic virus in transgenic tomato

Accumulation of the two transcripts of the N gene, conferring resistance to tobacco mosaic virus, is probably important for N gene-dependent hypersensitive cell death

The N gene is a Toll/interleukin-1 receptor (TIR)-nucleotide-binding site (NBS)-leucine-rich repeat (LRR)-type resistance (R) gene that generates two alternative transcripts, N(S) and N(L). N(S) encodes the full-length N protein while N(L) is predicted to encode a truncated form of the protein lacking most of the LRR region. We found that the two transcripts were accumulated at 20 degrees C, a permissive temperature, but not at 30 degrees C, a non-permissive temperature for the N gene, in tobacco mosaic virus (TMV)-inoculated leaves. When N gene-dependent cell death was triggered by transient 20 degrees C treatment for 2-6 h, considerable levels of the transcripts were accumulated just before cell death, although the levels of N(S) were always higher. The accumulation was induced by transient expression of the 50 kDa helicase domain (p50) of TMV replicase which is the Avr component of N, but not by transient expression of NtMEK2 (DD)-mediated cell death or N gene-independent hypersensitive cell death. These results suggest that the accumulation of N(S) and N(L) is associated with the function of N and, above a certain threshold, triggers N-mediated hypersensitive cell death.

Organization and evolution of resistance gene analogs in peanut

The scarcity of genetic polymorphism in Arachis hypogaea (peanut), as in other monophyletic polyploid species, makes it especially vulnerable to nematode, bacterial, fungal, and viral pathogens. Although no disease resistance genes have been cloned from peanut itself, the conserved motifs in cloned resistance genes from other plant species provide a means to isolate and analyze similar genes from peanut. To survey the number, diversity, evolutionary history, and genomic organization of resistance gene-like sequences in peanut, we isolated 234 resistance gene analogs (RGAs) by using primers designed from conserved regions of different classes of resistance genes including NBS-LRR, and LRR-TM classes. Phylogenetic and sequence analyses were performed to explore evolutionary relationships both among peanut RGAs and with orthologous genes from other plant taxa. Fifty-six overgos designed from the RGA sequences on the basis of their phyletic association were applied to a peanut BAC library; 736 hybridizing BAC clones were fingerprinted and contigs were formed in order to gain insights into the genomic organization of these genes. All the fingerprinting gels were blotted and screened with the respective overgos in order to verify the authenticity of the hits from initial screens, and to explore the physical organization of these genes in terms of both copy number and distribution in the genome. As a result, we identified 250 putative resistance gene loci. A correlation was found between the phyletic positions of the sequences and their physical locations. The BACs isolated here will serve as a valuable resource for future applications, such as map-based cloning, and will help improve our understanding of the evolution and organization of these genes in the peanut genome.

Plant rhabdoviruses

This chapter provides an overview of plant rhabdovirus structure and taxonomy, genome structure, protein function, and insect and plant infection. It is focused on recent research and unique aspects of rhabdovirus biology. Plant rhabdoviruses are transmitted by aphid, leafhopper or planthopper vectors, and the viruses replicate in both their insect and plant hosts. The two plant rhabdovirus genera, Nucleorhabdovirus and Cytorhabdovirus, can be distinguished on the basis of their intracellular site of morphogenesis in plant cells. All plant rhabdoviruses carry analogs of the five core genes: the nucleocapsid (N), phosphoprotein (P), matrix (M), glycoprotein (G) and large or polymerase (L). However, compared to vesiculoviruses that are composed of the five core genes, all plant rhabdoviruses encode more than these five genes, at least one of which is inserted between the P and M genes in the rhabdoviral genome. Interestingly, while these extra genes are not similar among plant rhabdoviruses, two encode proteins with similarity to the 30K superfamily of plant virus movement proteins. Analysis of nucleorhabdoviral protein sequences revealed nuclear localization signals for the N, P, M and L proteins, consistent with virus replication and morphogenesis of these viruses in the nucleus. Plant and insect factors that limit virus infection and transmission are discussed.

Transgenic tomato plants expressing the Arabidopsis NPR1 gene display enhanced resistance to a spectrum of fungal and bacterial diseases

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.

EDS1 in tomato is required for resistance mediated by TIR-class R genes and the receptor-like R gene Ve

In tobacco and other Solanaceae species, the tobacco N gene confers resistance to tobacco mosaic virus (TMV), and leads to induction of standard defense and resistance responses. Here, we report the use of N-transgenic tomato to identify a fast-neutron mutant, sun1-1 (suppressor of N), that is defective in N-mediated resistance. Induction of salicylic acid (SA) and expression of pathogenesis-related (PR) genes, each signatures of systemic acquired resistance, are both dramatically suppressed in sun1-1 plants after TMV treatment compared to wild-type plants. Application of exogenous SA restores PR gene expression, indicating that SUN1 acts upstream of SA. Upon challenge with additional pathogens, we found that the sun1-1 mutation impairs resistance mediated by certain resistance (R) genes, (Bs4, I, and Ve), but not others (Mi-1). In addition, sun1-1 plants exhibit enhanced susceptibility to TMV, as well as to virulent pathogens. sun1-1 has been identified as an EDS1 homolog present on chromosome 6 of tomato. The discovery of enhanced susceptibility in the sun1-1 (Le_eds1-1) mutant plant, which contrasts to reports in Nicotiana benthamiana using virus-induced gene silencing, provides evidence that the intersection of R gene-mediated pathways with general resistance pathways is conserved in a Solanaceous species. In tomato, EDS1 is important for mediating resistance to a broad range of pathogens (viral, bacterial, and fungal pathogens), yet shows specificity in the class of R genes that it affects (TIR-NBS-LRR as opposed to CC-NBS-LRR). In addition, a requirement for EDS1 for Ve-mediated resistance in tomato exposes that the receptor-like R gene class may also require EDS1.

Plant defence responses: conservation between models and crops

Diseases of plants are a major problem for agriculture world wide. Understanding the mechanisms employed by plants to defend themselves against pathogens may lead to novel strategies to enhance disease resistance in crop plants. Much of the research in this area has been conducted with Arabidopsis as a model system, and this review focuses on how relevant the knowledge generated from this model system will be for increasing resistance in crop plants. In addition, the progress made using other model plant species is discussed. While there appears to be substantial similarity between the defence responses of Arabidopsis and other plants, there are also areas where significant differences are evident. For this reason it is also necessary to increase our understanding of the specific aspects of the defence response that cannot be studied using Arabidopsis as a model.

Molecular genetics of disease resistance in cereals

AIMS

This Botanical Briefing attempts to summarize what is currently known about the molecular bases of disease resistance in cereal species and suggests future research directions.

SCOPE

An increasing number of resistance (R) genes have been isolated from rice, maize, wheat and barley that encode both structurally related and unique proteins. This R protein diversity may be attributable to the different modus operandi employed by pathogen species in some cases, but it is also a consequence of multiple defence strategies being employed against phytopathogens. Mutational analysis of barley has identified additional genes required for activation of an R gene-mediated defence response upon pathogen infection. In some instances very closely related barley R proteins require different proteins for defence activation, demonstrating that, within a single plant species, multiple resistance signalling pathways and different resistance strategies have evolved to confer protection against a single pathogen species. Despite the apparent diversity of cereal resistance mechanisms, some of the additional molecules required for R protein function are conserved amongst cereal and dicotyledonous species and even other eukaryotic species. Thus the derivation of functional homologues and interacting partner proteins from other species is contributing to the understanding of resistance signalling in cereals. The potential and limit of utilizing the rice genome sequence for further R gene isolation from cereal species is also considered, as are the new biotechnological possibilities for disease control arising from R gene isolation.

CONCLUSIONS

Molecular analyses in cereals have further highlighted the complexity of plant-pathogen co-evolution and have shown that numerous active and passive defence strategies are employed by plants against phytopathogens. Many advances in understanding the molecular basis of disease resistance in cereals have focused on monogenic resistance traits. Future research targets are likely to include less experimentally tractable, durable polygenic resistances and nonhost resistance mechanisms.

Aberrant mRNA processing of the maize Rp1-D rust resistance gene in wheat and barley

The maize Rp1-D gene confers race-specific resistance against Puccinia sorghi (common leaf rust) isolates containing a corresponding avrRp1-D avirulence gene. An Rp1-D genomic clone and a similar Rp1-D transgene regulated by the maize ubiquitin promoter were transformed independently into susceptible maize lines and shown to confer Rp1-D resistance, demonstrating that this resistance can be transferred as a single gene. Transfer of these functional transgenes into wheat and barley did not result in novel resistances when these plants were challenged with isolates of wheat stem rust (P. graminis), wheat leaf rust (P. triticina), or barley leaf rust (P. hordei). Regardless of the promoter employed, low levels of gene expression were observed. When constitutive promoters were used for transgene expression, a majority of Rp1-D transcripts were truncated in the nucleotide binding site-encoding region by premature polyadenylation. This aberrant mRNA processing was unrelated to gene function because an inactive version of the gene also generated such transcripts. These data demonstrate that resistance gene transfer between species may not be limited only by divergence of signaling effector molecules and pathogen avirulence ligands, but potentially also by more fundamental gene expression and transcript processing limitations.

RNA silencing: no mercy for viruses?

'RNA silencing' is a highly conserved mechanism leading to suppression of gene expression through nucleotide sequence-specific interactions that are mediated by 21-24 nucleotide-long RNAs. This process was first discovered as an unexpected consequence of transgenesis in plants, and similarly, it was subsequently identified in animals as an anomaly of antisense gene inhibition. We summarize the progressive steps that paved the way to our current understanding of the molecular basis and fundamental biological roles of RNA silencing in both plants and animals. In particular, we describe the general antiviral function of this mechanism in higher plants where it forms the basis of a highly elaborate immune system. All defense systems show some level of fallibility, and RNA silencing is no exception to this rule, as plant viruses have developed sophisticated ways to counteract various steps of the process. Recent work indicates that viruses are also engaged into a similar arms race in insects, but it remains unclear if RNA silencing plays a defensive role against virus infection of higher vertebrates. We also discuss some biotechnological applications of RNA silencing in mammalian cells that have fueled optimism that this mechanism may hold a promising future in antiviral human therapy.

The Melampsora lini AvrL567 avirulence genes are expressed in haustoria and their products are recognized inside plant cells

The Linum usitatissimum (flax) L gene alleles, which encode nucleotide binding site-Leu rich repeat class intracellular receptor proteins, confer resistance against the Melampsora lini (flax rust) fungus. At least 11 different L resistance specificities are known, and the corresponding avirulence genes in M. lini map to eight independent loci, some of which are complex and encode multiple specificities. We identified an M. lini cDNA marker that cosegregates in an F2 rust family with a complex locus determining avirulence on the L5, L6, and L7 resistance genes. Two related avirulence gene candidates, designated AvrL567-A and AvrL567-B, were identified in a genomic DNA contig from the avirulence allele, whereas the corresponding virulence allele contained a single copy of a related gene, AvrL567-C. Agrobacterium tumefaciens-mediated transient expression of the mature AvrL567-A or AvrL567-B (but not AvrL567-C) proteins as intracellular products in L. usitatissimum and Nicotiana tabacum (tobacco) induced a hypersensitive response-like necrosis that was dependent on coexpression of the L5, L6, or L7 resistance gene. An F1 seedling lethal or stunted growth phenotype also was observed when transgenic L. usitatissimum plants expressing AvrL567-A or AvrL567-B (but not AvrL567-C) were crossed to resistant lines containing L5, L6, or L7. The AvrL567 genes are expressed in rust haustoria and encode 127 amino acid secreted proteins. Intracellular recognition of these rust avirulence proteins implies that they are delivered into host cells across the plant membrane. Differences in the three AvrL567 protein sequences result from diversifying selection, which is consistent with a coevolutionary arms race.

Tobacco transgenic for the flax rust resistance gene L expresses allele-specific activation of defense responses

Tobacco was transformed with three different alleles (L2, L6, and L10) of the flax rust resistance gene L, a member of the toll interleukin-1 receptor, nucleotide-binding site, leucine-rich repeat (TIR-NBS-LRR) class of plant disease resistance genes. L6 transgenics had a stunted phenotype, expressed several defense response genes constitutively, and had increased resistance to the fungus Cercospora nicotianae and the oomycete Phytophthora parasitica pv. nicotianae. L2 and L10 transgenics, with one exception for L10, did not express these phenotypes, indicating that the activation of tobacco defense responses is L6 allele-specific. The phenotype of the exceptional L10 transgenic plant was associated with the presence of a truncated L10 gene resulting from an aberrant T-DNA integration. The truncated gene consisted of the promoter, the complete TIR region, and 39 codons of the NBS domain fused inframe to a tobacco retrotransposon-like sequence. A similar truncated L10 gene, constructed in vitro, was transiently expressed in tobacco leaves and gave rise to a strong localized necrotic reaction. Together, these results suggest that defense signaling properties of resistance genes can be expressed in an allele-specific and pathogen-independent manner when transferred between plant genera.

Tobacco mosaic virus: a model system for plant biology

Tobacco mosaic virus (TMV) has had an illustrious history for more than 100 years, dating to Beijerinck's description of the mosaic disease of tobacco as a contagium vivum fluidum and the modern usage of the word "virus." Since then, TMV has been acknowledged as a preferred didactic model and a symbolic model to illuminate the essential features that define a virus. TMV additionally emerged as a prototypic model to investigate the biology of host plants, namely tobacco. TMV also exemplifies how a model system furthers novel, and often unexpected, developments in biology and virology. Today, TMV is used as a tool to study host-pathogen interactions and cellular trafficking, and as a technology to express valuable pharmaceutical proteins in tobacco. The history of TMV illustrates how pragmatic strategies to control an economically important disease of tobacco have had unexpected and transforming effects across platforms that impinge on plant health and public health.

Potential Sources of Resistance for Melon to Nonpersistently Aphid-borne Viruses

Two hundred and sixty-eight Cucumis melo and wild relative accessions were evaluated for resistance to Cucumber mosaic virus (CMV), Papaya ringspot virus strain W (PRSV-W), Watermelon mosaic virus (WMV), and Zucchini yellow mosaic virus (ZYMV). Symptom development and systemic infection based on double antibody sandwich enzyme-linked immunosorbent assays were monitored. Sources of resistance were scarce. However, a number of them were found among the C. melo accessions tested. Thus, the accession C-189 behaved similarly to PI 161375 and showed resistance to "common" CMV strains although it was infected by a "song" type CMV. For WMV, the accessions C-768 and C-425, although infected, exhibited very mild symptoms, and recovery from infection occurred in some occasions in the former. A similar trait was also found in C-105, although in this case symptoms rarely appear and part of the plants are not infected. In addition, C-105 was highly resistant to virus transmission by Aphis gossypii, similarly to PI 161375. Accessions C-885 and C-769 exhibited resistance to PRSV-W, WMV, and ZYMV; therefore they are potential sources of multiple resistance. Resistance traits were also found in wild relatives that could be exploited when interspecific barriers with C. melo can be circumvented.

The Arabidopsis genes RPW8.1 and RPW8.2 confer induced resistance to powdery mildew diseases in tobacco

Plant disease resistance (R) gene products recognize pathogen avirulence (Avr) gene products and induce defense responses. It is not known if an R gene can function in different plant families, however. The Arabidopsis thaliana R genes RPW8.1 and RPW8.2 confer resistance to the powdery mildew pathogens Erysiphe orontii, E. cichoracearum, and Oidium lycopersici, which also infect plants from other families. We produced transgenic Nicotiana tabacum, N. benthamiana, and Lycopersicon esculentum plants containing RPW8.1 and RPW8.2. Transgenic N. tabacum plants had increased resistance to E. orontii and O. lycopersici, transgenic N. benthamiana plants had increased resistance to E. cichoracearum, but transgenic L. esculentum plants remained susceptible to these pathogens. The defense responses induced in transgenic N. tabacum and N. benthamiana were similar to those mediated by RPW8.1 and RPW8.2 in Arabidopsis. Apparently, RPW8.1 and RPW8.2 could be used to control powdery mildew diseases of plants from other families.

Engineering pathogen resistance in crop plants

As the world population continues to increase, food supplies must also grow to meet nutritional requirements. One means of ensuring the stability and plentitude of the food supply is to mitigate crop loss caused by plant pathogens. Strategies for combating disease include traditional technologies such as plant breeding and chemical applications; current technologies such as generating transgenic plants that express components of known defense signaling pathways; and the adaptation of newer technologies such as RNA silencing of pathogen and plant transcripts. Breeding has been used to pyramid resistance (R) genes into many different plants including rice. Chemical strategies include application of salicylic acid (SA) analogs to stimulate systemic acquired resistance (SAR) responses. Genetic screens in Arabidopsis have identified genes controlling SAR and these genes have been manipulated and used to engineer crop plants. The diseases caused by plant viruses are being thwarted through the initiation of endogenous RNA silencing mechanisms. Many of these strategies show great promise, some limitations, and exciting opportunities to develop many new tools for combating plant pests.

NPK1, an MEKK1-like mitogen-activated protein kinase kinase kinase, regulates innate immunity and development in plants

Mitogen-activated protein kinase (MAPK) cascades are rapidly activated upon plant recognition of invading pathogens. Here, we describe the use of virus-induced gene silencing (VIGS) to study the role of candidate plant MAP kinase kinase kinase (MAPKKK) homologs of human MEKK1 in pathogen-resistance pathways. We demonstrate that silencing expression of a tobacco MAPKKK, Nicotiana Protein Kinase 1 (NPK1), interferes with the function of the disease-resistance genes N, Bs2, and Rx, but does not affect Pto- and Cf4-mediated resistance. Further, NPK1-silenced plants also exhibit reduced cell size, defective cytokinesis, and an overall dwarf phenotype. Our results provide evidence that NPK1 functions in the regulation of N-, Bs2-, and Rx-mediated resistance responses and may play a role in one or more MAPK cascades, regulating multiple cellular processes.

Role of SCF ubiquitin-ligase and the COP9 signalosome in the N gene-mediated resistance response to Tobacco mosaic virus

The tobacco N gene confers resistance to Tobacco mosaic virus (TMV) and encodes a toll-interleukin-1 receptor/nucleotide binding/Leu-rich repeat class protein. Recent evidence indicates that the Nicotiana benthamiana Rar1 gene (NbRar1), which encodes a protein with a zinc finger motif called CHORD (Cys- and His-rich domain), is required for the function of N. To investigate the role of NbRar1 in plant defense, we identified its interaction partners. We show that the NbRar1 protein interacts with NbSGT1, a highly conserved component of the SCF (Skp1/Cullin/F-box protein)-type E3 ubiquitin ligase complex involved in protein degradation. In addition, we show that NbSGT1 interacts with NbSKP1. Suppression of NbSGT1 and NbSKP1 shows that these genes play an important role in the N-mediated resistance response to TMV. Both NbRar1 and NbSGT1 associate with the COP9 signalosome, another multiprotein complex involved in protein degradation via the ubiquitin-proteasome pathway. Silencing of the NbCOP9 signalosome also compromises N-mediated resistance to TMV. Our results reveal new roles for SCF and the COP9 signalosome in plant defense signaling.

Potato gene Y-1 is an N gene homolog that confers cell death upon infection with potato virus Y

ADG2 is a DNA sequence mapped to a resistance (R) gene-rich region at the distal end of chromosome XI in potato (Solanum tuberosum subsp. andigena). The gene, in which ADG2 represents the predicted nucleotide-binding domain (NBS), was cloned and characterized. The coding region of the gene (designated as Y-1) is 6,187 bp long and structurally similar to gene N that confers hypersensitive resistance to Tobacco mosaic virus in Nicotiana spp. Both belong to the TIR-NBS-LRR class of genes and show 57% identity at the amino acid sequence level. The introns of Y-1 were spliced as predicted from the sequence. Y-1 cosegregated with Ry(adg), a gene for extreme resistance to Potato virus Y (PVY) on chromosome XI, as tested in a potato-mapping population and with independent potato cultivars. Leaves of the transgenic potato plants expressing Y-1 under the control of Cauliflower mosaic virus 35S promoter developed necrotic lesions upon infection with PVY, but no significant resistance was observed, and plants were systemically infected with PVY.

The tobacco mosaic virus resistance gene, N

Summary In this mini review we discuss recent advances in the understanding of the N gene-mediated resistance to tobacco mosaic virus (TMV). The tobacco N gene belongs to toll-interleukin-1 receptor homology/nucleotide binding/leucine rich repeat (TIR-NB-LRR) class of resistance genes. It encodes two transcripts, N(S) and N(L), by alternative splicing, both of which are required to confer resistance to TMV. The structure-function analysis of the N gene indicates that the TIR, NB and LRR domains are indispensable for its function. The N gene response is elicited by the C-terminal helicase domain of the 126 kDa TMV replicase protein. Tobacco N gene can also confer resistance to TMV in heterologous plants like tomato and Nicotiana benthamiana. Recent studies on N-mediated signalling suggest that EDS1, Rar1 and NPR1 genes play an important role in TMV resistance. Finally, we discuss current status of the N-mediated signal transduction and speculate directions for future work to understand N-TMV interaction.

Tobacco Rar1, EDS1 and NPR1/NIM1 like genes are required for N-mediated resistance to tobacco mosaic virus

The tobacco N gene confers resistance to tobacco mosaic virus (TMV) and encodes a Toll-interleukin-1 receptor/nucleotide binding site/leucine-rich repeat (TIR-NBS-LRR) class protein. We have developed and used a tobacco rattle virus (TRV) based virus induced gene silencing (VIGS) system to investigate the role of tobacco candidate genes in the N-mediated signalling pathway. To accomplish this we generated transgenic Nicotiana benthamiana containing the tobacco N gene. The transgenic lines exhibit hypersensitive response (HR) to TMV and restrict virus spread to the inoculated site. This demonstrates that the tobacco N gene can confer resistance to TMV in heterologous N. benthamiana. We have used this line to study the role of tobacco Rar1-, EDS1-, and NPR1/NIM1- like genes in N-mediated resistance to TMV using a TRV based VIGS approach. Our VIGS analysis suggests that these genes are required for N function. EDS1-like gene requirement for the N function suggests that EDS1 could be a common component of bacterial, fungal and viral resistance signalling mediated by the TIR-NBS-LRR class of resistance proteins. Requirement of Rar1- like gene for N-mediated resistance to TMV and some powdery mildew resistance genes in barley provide the first example of converging points in the disease resistance signalling pathways mediated by TIR-NBS-LRR and CC-NBS-LRR proteins. The TRV based VIGS approach as described here to study N-mediated resistance signalling will be useful for the analysis of not only disease resistance signalling pathways but also of other signalling pathways in genetically intractable plant systems.

An EDS1 orthologue is required for N-mediated resistance against tobacco mosaic virus

In Arabidopsis, EDS1 is essential for disease resistance conferred by a structural subset of resistance (R) proteins containing a nucleotide-binding site, leucine-rich-repeats and amino-terminal similarity to animal Toll and Interleukin-1 (so-called TIR-NBS-LRR proteins). EDS1 is not required by NBS-LRR proteins that possess an amino-terminal coiled-coil motif (CC-NBS-LRR proteins). Using virus-induced gene silencing (VIGS) of a Nicotiana benthaminana EDS1 orthologue, we investigated the role of EDS1 in resistance specified by structurally distinct R genes in transgenic N. benthamiana. Resistance against tobacco mosaic virus mediated by tobacco N, a TIR-NBS-LRR protein, was EDS1-dependent. Two other R proteins, Pto (a protein kinase), and Rx (a CC-NBS-LRR protein) recognizing, respectively, a bacterial and viral pathogen did not require EDS1. These data, together with the finding that expression of N. benthamiana and Arabidopsis EDS1 mRNAs are similarly regulated, lead us to conclude that recruitment of EDS1 by TIR-NBS-LRR proteins is evolutionarily conserved between dicotyledenous plant species in resistance against bacterial, oomycete and viral pathogens. We further demonstrate that VIGS is a useful approach to dissect resistance signaling pathways in a genetically intractable plant species.

Antibody-based resistance to plant pathogens

Plant diseases are a major threat to the world food supply, as up to 15% of production is lost to pathogens. In the past, disease control and the generation of resistant plant lines protected against viral, bacterial or fungal pathogens, was achieved using conventional breeding based on crossings, mutant screenings and backcrossing. Many approaches in this field have failed or the resistance obtained has been rapidly broken by the pathogens. Recent advances in molecular biotechnology have made it possible to obtain and to modify genes that are useful for generating disease resistant crops. Several strategies, including expression of pathogen-derived sequences or anti-pathogenic agents, have been developed to engineer improved pathogen resistance in transgenic plants. Antibody-based resistance is a novel strategy for generating transgenic plants resistant to pathogens. Decades ago it was shown that polyclonal and monoclonal antibodies can neutralize viruses, bacteria and selected fungi. This approach has been improved recently by the development of recombinant antibodies (rAbs). Crop resistance can be engineered by the expression of pathogen-specific antibodies, antibody fragments or antibody fusion proteins. The advantages of this approach are that rAbs can be engineered against almost any target molecule, and it has been demonstrated that expression of functional pathogen-specific rAbs in plants confers effective pathogen protection. The efficacy of antibody-based resistance was first shown for plant viruses and its application to other plant pathogens is becoming more established. However, successful use of antibodies to generate plant pathogen resistance relies on appropriate target selection, careful antibody design, efficient antibody expression, stability and targeting to appropriate cellular compartments.

Tomato Ve disease resistance genes encode cell surface-like receptors

In tomato, Ve is implicated in race-specific resistance to infection by Verticillium species causing crop disease. Characterization of the Ve locus involved positional cloning and isolation of two closely linked inverted genes. Expression of individual Ve genes in susceptible potato plants conferred resistance to an aggressive race 1 isolate of Verticillium albo-atrum. The deduced primary structure of Ve1 and Ve2 included a hydrophobic N-terminal signal peptide, leucine-rich repeats containing 28 or 35 potential glycosylation sites, a hydrophobic membrane-spanning domain, and a C-terminal domain with the mammalian E/DXXXLphi or YXXphi endocytosis signals (phi is an amino acid with a hydrophobic side chain). A leucine zipper-like sequence occurs in the hydrophobic N-terminal signal peptide of Ve1 and a Pro-Glu-Ser-Thr (PEST)-like sequence resides in the C-terminal domain of Ve2. These structures suggest that the Ve genes encode a class of cell-surface glycoproteins with receptor-mediated endocytosis-like signals and leucine zipper or PEST sequences.

Candidate disease resistance genes in sunflower cloned using conserved nucleotide-binding site motifs: genetic mapping and linkage to the downy mildew resistance gene Pl1

Disease resistance gene candidates (RGCs) belonging to the nucleotide-binding site (NBS) superfamily have been cloned from numerous crop plants using highly conserved DNA sequence motifs. The aims of this research were to (i) isolate genomic DNA clones for RGCs in cultivated sunflower (Helianthus annuus L.) and (ii) map RGC markers and Pl1, a gene for resistance to downy mildew (Plasmopara halstedii (Farl.) Berl. & de Toni) race 1. Degenerate oligonucleotide primers targeted to conserved NBS DNA sequence motifs were used to amplify RGC fragments from sunflower genomic DNA. PCR products were cloned, sequenced, and assigned to 11 groups. RFLP analyses mapped six RGC loci to three linkage groups. One of the RGCs (Ha-4W2) was linked to Pl1, a downy mildew resistance gene. A cleaved amplified polymorphic sequence (CAPS) marker was developed for Ha-4W2 using gene-specific oligonucleotide primers. Downy mildew susceptible lines (HA89 and HA372) lacked a 276-bp Tsp5091 restriction fragment that was present in downy mildew resistant lines (HA370, 335, 336, 337, 338, and 339). HA370 x HA372 F2 progeny were genotyped for the Ha-4W2 CAPS marker and phenotyped for resistance to downy mildew race 1. The CAPS marker was linked to but did not completely cosegregate with Pl1 on linkage group 8. Ha-4W2 was found to comprise a gene family with at least five members. Although genetic markers for Ha-4W2 have utility for marker-assisted selection, the RGC detected by the CAPS marker has been ruled out as a candidate gene for Pl1. Three of the RGC probes were monomorphic between HA370 and HA372 and still need to be mapped and screened for linkage to disease resistance loci.

The MLA6 coiled-coil, NBS-LRR protein confers AvrMla6-dependent resistance specificity to Blumeria graminis f. sp. hordei in barley and wheat

The barley Mla locus confers multiple resistance specificities to the obligate fungal biotroph, Blumeria (= Erysiphe) graminis f. sp. hordei. Interspersed within the 240 kb Mla complex are three families of resistance gene homologs (RGHs). Probes from the Mla-RGH1 family were used to identify three classes of cDNAs. The first class is predicted to encode a full-length CC-NBS-LRR protein and the other two classes contain alternatively spliced, truncated variants. Utilizing a cosmid that contains a gene corresponding to the full-length candidate cDNA, two single-cell expression assays were used to demonstrate complementation of AvrMla6-dependent, resistance specificity to B. graminis in barley and wheat. The first of these assays was also used to substantiate previous genetic data that the Mla6 allele requires the signaling pathway component, Rar1, for function. Computational analysis of MLA6 and the Rar1-independent, MLA1 protein reveals 91.2% identity and shows that the LRR domain is subject to diversifying selection. Our findings demonstrate that highly related CC-NBS-LRR proteins encoded by alleles of the Mla locus can dictate similar powdery mildew resistance phenotypes yet still require distinct downstream signaling components.

Cell-autonomous expression of barley Mla1 confers race-specific resistance to the powdery mildew fungus via a Rar1-independent signaling pathway

The barley Mla locus encodes 28 characterized resistance specificities to the biotrophic fungal pathogen barley powdery mildew. We describe a single-cell transient expression assay using entire cosmid DNAs to pinpoint Mla1 within the complex 240-kb Mla locus. The MLA1 cDNA encodes a 108-kD protein containing an N-terminal coiled-coil structure, a central nucleotide binding domain, and a C-terminal leucine-rich repeat region; it also contains a second short open reading frame at the 5' end that has a possible regulatory function. Although most Mla-encoded resistance specificities require Rar1 for their function, we used the single-cell expression system to demonstrate that Mla1 triggers full resistance in the presence of the severely defective rar1-2 mutant allele. Wheat contains an ortholog of barley Mla, designated TaMla, that is tightly linked to (0.7 centimorgan) but distinct from a tested resistance specificity at the complex Pm3 locus to wheat powdery mildew. Thus, the most polymorphic powdery mildew resistance loci in barley and wheat may have evolved in parallel at two closely linked homeoloci. Barley Mla1 expressed in wheat using the single-cell transformation system failed to trigger a response to any of the wheat powdery mildew Avr genes tested, indicating that AvrMla1 is not genetically fixed in wheat mildew strains.

Cell-autonomous expression of barley Mla1 confers race-specific resistance to the powdery mildew fungus via a Rar1-independent signaling pathway

The barley Mla locus encodes 28 characterized resistance specificities to the biotrophic fungal pathogen barley powdery mildew. We describe a single-cell transient expression assay using entire cosmid DNAs to pinpoint Mla1 within the complex 240-kb Mla locus. The MLA1 cDNA encodes a 108-kD protein containing an N-terminal coiled-coil structure, a central nucleotide binding domain, and a C-terminal leucine-rich repeat region; it also contains a second short open reading frame at the 5' end that has a possible regulatory function. Although most Mla-encoded resistance specificities require Rar1 for their function, we used the single-cell expression system to demonstrate that Mla1 triggers full resistance in the presence of the severely defective rar1-2 mutant allele. Wheat contains an ortholog of barley Mla, designated TaMla, that is tightly linked to (0.7 centimorgan) but distinct from a tested resistance specificity at the complex Pm3 locus to wheat powdery mildew. Thus, the most polymorphic powdery mildew resistance loci in barley and wheat may have evolved in parallel at two closely linked homeoloci. Barley Mla1 expressed in wheat using the single-cell transformation system failed to trigger a response to any of the wheat powdery mildew Avr genes tested, indicating that AvrMla1 is not genetically fixed in wheat mildew strains.

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

Cauliflower mosaic virus strain W260 elicits a hypersensitive response (HR) in leaves of Nicotiana edwardsonii, an interspecific hybrid derived from a cross between N. glutinosa and N. clevelandii. Interestingly, we found that N. glutinosa is resistant to W260, but responds with local chlorotic lesions rather than necrotic lesions. In contrast, N. clevelandii responds to W260 with systemic cell death. The reactions of the progenitors of N. edwardsonii to W260 infection indicated that each contributed a factor toward the development of HR. In this study, we present two lines of evidence to show that the resistance and cell death that comprise the HR elicited by W260 can indeed be uncoupled. First, we showed that the non-necrotic resistance response of N. glutinosa could be converted to HR when these plants were crossed with N. clevelandii. Second, we found that cell death and resistance segregated independently in the F2 population of a cross between N. edwardsonii and N. clevelandii. We concluded that the resistance of N. edwardsonii to W260 infection was conditioned by a gene derived from N. glutinosa, whereas cell death was conditioned by a gene derived from N. clevelandii. An analysis of pathogenesis-related (PR) protein expression in response to W260 infection revealed that elicitation of PR proteins was associated with resistance rather than with the onset of cell death.

Resistance gene complexes: evolution and utilization

More than 30 genes have been characterized from different plant species that provide resistance to a variety of different pathogen and pest species. The structures of most are consistent with a role in pathogen recognition and defense response signaling. Resistance genes are very abundant in plant genomes and most belong to tightly linked gene families. Evolution of R genes is driven by selection on allelic variation created by mutation and re-assorted by recombination between alleles and sometimes between different gene family members. Selection favors genes that can recognize pathogen avr gene products that are present in pathogen populations. Selection at linked gene families favors haplotypes with useful combinations of genes but a limited physiological cost to the plant. Future utilization of R genes will include transfer between related genera and identification or construction of genes that condition durable resistance to variable pathogens. Genes with durable resistance may interact with conserved pathogen elicitors or condition resistance responses that are independent of specific Avr gene interactions.

Isolation and characterization of disease resistance gene homologues from rice cultivar IR64

We initiated a search for disease resistance (R) gene homologues in rice cultivar IR64, one of the most agronomically important rice varieties in the world, with the assumption that some of these homologues would correspond to previously identified disease resistance loci. A family of rice R gene homologues was identified using the Arabidopsis NBS-LRR disease resistance gene RPS2 as a hybridization probe. Because member genes of this rice R gene family exhibit features characteristic of the NBS-LRR class of resistance genes, the family was given the name NRH (for NBS-LRR resistance gene homologues). Three members of the NRH family, NRH1, NRH2, and NRH3, were cloned and studied in detail. In IR64, NRH1 and NRH2 appear to encode full-length polypeptides, whereas NRH3 is prematurely truncated with a stop codon generated by a frameshift. NRH1 maps on chromosome 5, and NRH2 and NRH3 are less than 48kb apart on chromosome 11. Although NRH1, NRH2, and NRH3 map to regions of the rice genome where disease resistance loci to Xanthomonas oryzae pv. oryzae (Xoo) have been identified, susceptible rice varieties transformed with either NRH1 or NRH2 failed to exhibit increased resistance to a set of well-characterized Xoo strains.

The generation of plant disease resistance gene specificities

We are gaining an understanding of the molecular basis of resistance specificity and of the natural processes that generate different specificities. This is a prerequisite for the genetic engineering of new plant disease-resistance genes to control diseases for which naturally occurring resistance is inadequate. DNA sequence analysis indicates that point mutation, recombination and selection can generate and maintain the high levels of polymorphism observed in resistance genes. Comparisons of closely related resistance proteins indicate that specificity can be determined by variation in at least two regions. One of these contains leucine-rich repeats, which are a common feature of most resistance proteins.

A Century of Plant Pathology: A Retrospective View on Understanding Host-Parasite Interactions

▪ Abstract  The twentieth century has been productive for the science of plant pathology and the field of host-parasite interactions-both in understanding how pathogens and plant defense work and in developing more effective means of disease control. Early in the twentieth century, plant pathology adopted a philosophy that encouraged basic scientific investigation of pathogens and disease defense. That philosophy led to the strategy of developing disease-resistant plants as a prima facie disease-control measure-and in the process saved billions of dollars and avoided the use of tons of pesticides. Plant pathology rapidly adopted molecular cloning and its spin-off technologies, and these have fueled major advances in our basic understanding of plant diseases. This knowledge and the development of efficient technologies for producing transgenic plants convey optimism that plant diseases will be more efficiently controlled in the twenty-first century.

Genetic mapping of the Tsw locus for resistance to the Tospovirus Tomato spotted wilt virus in Capsicum spp. and its relationship to the Sw-5 gene for resistance to the same pathogen in tomato

The Tsw gene conferring dominant resistance to the Tospovirus Tomato spotted wilt virus (TSWV) in Capsicum spp. has been tagged with a random amplified polymorphic DNA marker and mapped to the distal portion of chromosome 10. No mapped homologues of Sw-5, a phenotypically similar dominant TSWV resistance gene in tomato, map to this region in C. annuum, although a number of Sw-5 homologues are found at corresponding positions in pepper and tomato. The relationship between Tsw and Sw-5 was also examined through genetic studies of TSWV. The capacity of TSWV-A to overcome the Tsw gene in pepper and the Sw-5 gene in tomato maps to different TSWV genome segments. Therefore, despite phenotypic and genetic similarities of resistance in tomato and pepper, we infer that distinct viral gene products control the outcome of infection in plants carrying Sw-5 and Tsw, and that these loci do not appear to share a recent common evolutionary ancestor.

Agroinfiltration is a versatile tool that facilitates comparative analyses of Avr9/Cf-9-induced and Avr4/Cf-4-induced necrosis

The avirulence genes Avr9 and Avr4 from the fungal tomato pathogen Cladosporium fulvum encode extracellular proteins that elicit a hypersensitive response when injected into leaves of tomato plants carrying the matching resistance genes, Cf-9 and Cf-4, respectively. We successfully expressed both Avr9 and Avr4 genes in tobacco with the Agrobacterium tumefaciens transient transformation assay (agroinfiltration). In addition, we expressed the matching resistance genes, Cf-9 and Cf-4, through agroinfiltration. By combining transient Cf gene expression with either transgenic plants expressing one of the gene partners, Potato virus X (PVX)-mediated Avr gene expression, or elicitor injections, we demonstrated that agroinfiltration is a reliable and versatile tool to study Avr/Cf-mediated recognition. Significantly, agroinfiltration can be used to quantify and compare Avr/Cf-induced responses. Comparison of different Avr/Cf-interactions within one tobacco leaf showed that Avr9/Cf-9-induced necrosis developed slower than necrosis induced by Avr4/Cf-4. Quantitative analysis demonstrated that this temporal difference was due to a difference in Avr gene activities. Transient expression of matching Avr/Cf gene pairs in a number of plant families indicated that the signal transduction pathway required for Avr/Cf-induced responses is conserved within solanaceous species. Most non-solanaceous species did not develop specific Avr/Cf-induced responses. However, co-expression of the Avr4/Cf-4 gene pair in lettuce resulted in necrosis, providing the first proof that a resistance (R) gene can function in a different plant family.

Isolation and characterization of two pathogen- and salicylic acid-induced genes encoding WRKY DNA-binding proteins from tobacco

A pathogen- and salicylic acid (SA)-induced DNA-binding activity has been recently identified in tobacco that is related to a previously identified class of WRKY DNA-binding proteins. To identify members of the WRKY gene family associated with this DNA-binding activity, we have attempted to isolate those WRKY genes that are induced by pathogen infection. Using a domain-specific differential display procedure, we have isolated two tobacco WRKY genes, tWRKY3 and tWRKY4, that are rapidly induced in resistant tobacco plants after infection by tobacco mosaic virus (TMV). Both tWRK3 and tWRKY4 encode proteins with a single WRKY domain that contain the conserved WRKYGQK sequence. Unlike other isolated WRKY proteins that contain the Cys2His2 zinc motif, tWRKY3 and tWRKY4 appear to contain the Cys2HisCys zinc motif. Nonetheless, both tWRKY3 and tWRKY4 are capable of binding DNA molecules with the W-box (TTGAC) element recognized by other WRKY proteins. Expression of the tWRKY3 and tWRKY4 genes could be rapidly induced not only by TMV infection but also by SA or its biologically active analogues that are capable of inducing pathogenesis-related genes and enhanced resistance. Interestingly, induction of both genes by TMV infection was still observed in resistant tobacco plants expressing the bacterial salicylate hydroxylase gene (nahG), although the levels of induction appeared to be reduced. Identification of pathogen- and SA-induced genes encoding WRKY DNA-binding proteins should facilitate future studies on the regulation and functions of this novel group of DNA-binding proteins.

NRSA-1: a resistance gene homolog expressed in roots of non-host plants following parasitism by Striga asiatica (witchweed)

Studies of the initial interactions of Striga asiatica with the non-host plant species Tagetes erecta (marigold) established that parasite penetration through the root is arrested most frequently in the cortex. The arrest of parasite ingress is associated with browning and necrosis of root cortical cells flanking the invading endophyte and with increased intracellular wall appositions on the root cell walls directly adjacent to the plant-parasite interface. Using a polymerase chain reaction-based differential cDNA amplification strategy followed by 5'-RACE, we have identified several gene products whose expression is induced in marigold roots during attempted parasitism by Striga. Among these was a 917 bp cDNA encoding a 221 amino acid protein with significant homology to proteins encoded by disease resistance genes from other plant species, including N, RPP5, L6 and M. This cDNA was subsequently used to isolate a nuclear gene, designated NRSA-1, for non-host resistance to Striga asiatica. NRSA-1 is a member of a small gene family in marigold consisting of two to four members. RNA gel blot analysis showed that NRSA-1 transcripts accumulate to high levels in roots near the site of Striga invasion within 120 h after parasite attachment, and appear at lower levels throughout the rest of the plant under Striga parasitism. NRSA-1 expression is rapidly induced by treatment with jasmonic acid (JA), but not by mechanical wounding, treatment with salicylic acid, paraquat or ABA. A possible role for NRSA-1 in the non-host resistance mechanism is discussed.

Identification of an ancestral resistance gene cluster involved in the coevolution process between Phaseolus vulgaris and its fungal pathogen Colletotrichum lindemuthianum

The recent cloning of plant resistance (R) genes and the sequencing of resistance gene clusters have shed light on the molecular evolution of R genes. However, up to now, no attempt has been made to correlate this molecular evolution with the host-pathogen coevolution process at the population level. Cross-inoculations were carried out between 26 strains of the fungal pathogen Colletotrichum lindemuthianum and 48 Phaseolus vulgaris plants collected in the three centers of diversity of the host species. A high level of diversity for resistance against the pathogen was revealed. Most of the resistance specificities were overcome in sympatric situations, indicating an adaptation of the pathogen to the local host. In contrast, plants were generally resistant to allopatric strains, suggesting that R genes that were efficient against exotic strains but had been overcome locally were maintained in the plant genome. These results indicated that coevolution processes between the two protagonists led to a differentiation for resistance in the three centers of diversity of the host. To improve our understanding of the molecular evolution of these different specificities, a recombinant inbred (RI) population derived from two representative genotypes of the Andean (JaloEEP558) and Mesoamerican (BAT93) gene pools was used to map anthracnose specificities. A gene cluster comprising both Andean (Co-y; Co-z) and Mesoamerican (Co-9) host resistance specificities was identified, suggesting that this locus existed prior to the separation of the two major gene pools of P. vulgaris. Molecular analysis revealed a high level of complexity at this locus. It harbors 11 restriction fragment length polymorphisms when R gene analog (RGA) clones are used. The relationship between the coevolution process and diversification of resistance specificities at resistance gene clusters is discussed.

A cDNA from tobacco codes for an inhibitor of virus replication (IVR)-like protein

We have shown previously that localization of tobacco mosaic virus (TMV) in tobacco is associated with a ca. 23 kDa protein that inhibits replication of several plant viruses. This protein, named 'inhibitor of virus replication' (IVR), was purified from the medium of TMV-inoculated protoplasts derived from Nicotiana tabacum cv. Samsun NN. IVR was shown to be present also in induced-resistant leaf tissue of N. tabacum cv. Samsun NN. We prepared an expression cDNA library from such induced-resistant tissue and screened it with a polyclonal antibody raised against the IVR protein. A 1016 bp clone (named NC330) containing a 597 bp open reading frame, coding for a 21.6 kDa polypeptide, was isolated. The NC330 clone hybridized with RNA from induced-resistant tissue from N. tabacum cv. Samsun NN but not with RNA from non-induced tissue. Likewise, it did not hybridize with RNA from infected or uninfected tissue of N. tabacum cv. Samsun nn. Similarly, the NC330 cloned probe hybridized with the RT-PCR products from RNA of the induced-resistant tissue only. In Southern blot hybridization the NC330 DNA probe detected several genomic DNA fragments in both N. tabacum cv. Samsun NN and Samsun nn. The size of the DNA fragments differed in Samsun NN and Samsun nn. We suggest that DNA encoding the IVR-like protein is present in resistant and susceptible N. tabacum genotypes, but is expressed only in NN. We have inserted the NC330 into the expression vector pET22b and a 21.6 kDa protein was produced in Escherichia coli that reacted in immunoblots with the IVR antibody. This protein greatly reduced replication of TMV in N. tabacum cv. Samsun nn leaf disk assays.

Bax-induced cell death in tobacco is similar to the hypersensitive response

Bax, a death-promoting member of the Bcl-2 family of proteins, triggered cell death when expressed in plants from a tobacco mosaic virus vector. Analysis of Bax deletion mutants demonstrated a requirement for the BH1 and BH3 domains in promoting rapid cell death, whereas deletion of the carboxyl-terminal transmembrane domain completely abolished the lethality of Bax in plants. The phenotype of cell death induced by Bax closely resembled the hypersensitive response induced by wild-type tobacco mosaic virus in tobacco plants carrying the N gene. The cell death-promoting function of Bax in plants correlated with accumulation of the defense-related protein PR1, suggesting Bax activated an endogenous cell-death program in plants. In support of this view, both N gene- and Bax-mediated cell death was blocked by okadaic acid, an inhibitor of protein phosphatase activity. The ability of Bax to induce cell death and a defense reaction in plants suggests that some features of animal and plant cell death processes may be shared.

Resistance to oomycetes: a general role for the hypersensitive response?

Oomycete plant pathogens, such as Phytophthora, downy mildews and Pythium, have devastating disease effects on numerous crop and ornamental plants. Various types of genetic resistance to oomycetes occur in plants, and can be determined at the subspecific or varietal level (race or cultivar-specific resistance), or at the species or genus level (nonhost resistance). In addition, resistance might be a quantitative phenotype (partial resistance). Resistance reactions are often associated with the hypersensitive response - a programed cell death pathway. Recent advances in the genetic, biochemical and cytological characterization of disease resistance suggests that the hypersensitive response is associated with all forms of resistance to Phytophthora and downy mildews. Identification of the resistance genes involved in nonhost and partial resistance to oomycetes remains an important challenge.

The helicase domain of the TMV replicase proteins induces the N-mediated defence response in tobacco

Tobacco mosaic virus (TMV) induces the hypersensitive response (HR) in tobacco plants containing the N gene. This defence response is characterized by cell death at the site of virus infection and inhibition of viral replication and movement. A previous study indicated that a portion of the TMV replicase containing a putative helicase domain is involved in HR induction. Here, this observation is confirmed and extended by showing that non-viral expression of a 50 kDa TMV helicase fragment (p50) is sufficient to induce the N-mediated HR in tobacco. Like the HR elicited by TMV infection, transgenic expression of p50 induces a temperature-sensitive defence response. We demonstrate that recombinant p50 protein has ATPase activity, as suggested by the presence of conserved sequence motifs found in ATPase/helicase enzymes. A point mutation that alters one of these motifs abolishes ATPase activity in vitro but does not affect HR induction. These results suggest that features of the TMV helicase domain, independent of its enzymatic activity, are recognized by N-containing tobacco to induce TMV resistance.

Interactions between tobacco mosaic virus and the tobacco N gene

The interaction between tobacco mosaic virus (TMV) and tobacco harbouring the N gene is a classical system for studying gene-for-gene interactions in disease resistance. The N gene confers resistance to TMV by mediating defence responses that function to limit viral replication and movement. We isolated the N gene and determined that N belongs to the nucleotide-binding-site-leucine-rich-repeat (NBS-LRR) class of plant disease resistance genes, and encodes both full-length and truncated proteins. Sequence homologies and mutagenesis studies indicated a signalling role for the N protein similar to that seen for proteins involved in defence responses in insects and mammals. The N gene confers resistance to TMV in transgenic tomato, demonstrating the use of the NBS-LRR class of disease resistance genes in engineering crop resistance. From the pathogen side of this interaction, the TMV 126 kDa replicase protein has been implicated as the avirulence factor that triggers N-mediated defence responses. We employed Agrobacterium-mediated expression strategies to demonstrate that expression of the putative helicase region of the replicase protein is sufficient to elicit N-mediated defences. The thermosensitivity of the N-mediated response to TMV is retained when induced by expression of this replicase fragment. Thus, both components of this gene-for-gene interaction are now available for studies that address the molecular mechanisms involved in N-mediated TMV resistance.

Plants expressing the Pto disease resistance gene confer resistance to recombinant PVX containing the avirulence gene AvrPto

Elicitation of hypersensitive cell death and induction of plant disease resistance by Pseudomonas syringae pv. tomato (Pst) is dependent on activity of the Pst Hrp secretion system and the gene-for-gene interaction between the tomato resistance gene Pto and the bacterial avirulence gene avrPto. AvrPto was expressed transiently in resistant or susceptible plant lines via a potato virus X (PVX) vector. We found that while PVX is normally virulent on tomato, a PVX derivative expressing avrPto was only capable of infecting plants lacking a functional Pto resistance pathway. Mutations in either the Pto or Prf genes allowed systemic spread of the recombinant virus. These results indicate that recognition of AvrPto by Pto in resistant plant lines triggers a plant defense response that can confer resistance to a viral as well as a bacterial pathogen.

THE TOMATO-CLADOSPORIUM FULVUM INTERACTION: A Versatile Experimental System to Study Plant-Pathogen Interactions

Over the past 20 years, the interaction between the biotrophic fungal pathogen Cladosporium fulvum and tomato has developed into a versatile experimental system for molecular plant pathology and resistance breeding. This interaction provided the resources for cloning of fungal avirulence genes for the first time and interesting clues on recognition of their extracellular products by tomato, as well as mechanisms employed by the fungus to circumvent this recognition. A wealth of information has become available on the structure and genomic organization of Cf resistance genes. The occurrence of many clustered Cf homologues allows the generation of new genes with additional recognitional specificities by reshuffling. It is anticipated that potentially all proteins secreted by C. fulvum are recognized by one or more individuals in a population of tomato genotypes, a hypothesis that has been experimentally confirmed. The future challenge will be to elucidate the mechanisms of perception of avirulence factors and the subsequent signaling eventually leading to activation of host defense responses.

An oligo selection procedure for identification of sequence-specific DNA-binding activities associated with the plant defence response

Sequence-specific DNA-binding (SSDB) factors play central roles in transcription, DNA replication, recombination and repair. This report describes a simple procedure for high-throughput identification of SSDB activities without prior knowledge of their target genes or binding sequences. The procedure starts with a population of completely random oligo(nucleotide) sequences and selects for those oligo molecules that specifically bind to cellular SSDB factors by use of common gel-retardation assays and PCR. Amplification and subsequent cloning of these selected oligo molecules result in the establishment of oligo DNA libraries enriched in DNA molecules containing specific sequences recognized by SSDB factors. These oligo libraries can be rapidly screened to identify a large number of SSDB activities, including those that are differentially regulated by developmental and environmental signals. With identified oligo DNA as probes, the corresponding SSDB factors can be isolated and analysed with respect to their structures, regulation and functions. Using this procedure, we have identified approximately 100 SSDB activities from tobacco leaves, including seven that are differentially regulated during the tobacco mosaic virus-induced hypersensitive response in resistant tobacco plants.

The tomato Cf-9 disease resistance gene functions in tobacco and potato to confer responsiveness to the fungal avirulence gene product avr 9

The Cf-9 gene encodes an extracytoplasmic leucine-rich repeat protein that confers resistance in tomato to races of the fungus Cladosporium fulvum that express the corresponding avirulence gene Avr 9. We investigated whether the genomic Cf-9 gene functions in potato and tobacco. Transgenic tobacco and potato plants carrying Cf-9 exhibit a rapid hypersensitive cell death response (HR) to Avr 9 peptide injection. Cf 9 tobacco plants were reciprocally crossed to Avr 9-producing tobacco. A developmentally regulated seedling lethal phenotype occurred in F1 progeny when Cf9 was used as the male parent and Avr 9 as the female parent. However, when Cf9 was inherited in the maternal tissue and a heterozygous Avr 9 plant was used as the pollen donor, a much earlier reaction was caused, leading to no germination of any F1 seed. Detailed analysis of the Avr 9-induced responses in Cf 9 tobacco leaves revealed that (1) most mesophyll cells died within 3 hr (compared with 12 to 16 hr in tomato); (2) the macroscopic HR was visible at an Avr 9 titer five times lower than that which caused visible symptoms in tomato; (3) the HR invariably extended into noninjected panels of the tobacco leaf; (4) no HR occurred in leaves of young tobacco plants; (5) in older plants, the HR was dramatically enhanced by sequential Avr 9 challenges; and (6) coexpression of a salicylate hydroxylase transgene (nahG) from Pseudomonas putida reduced the severity of the macroscopic leaf HR and also restored germination to Cf 9 x 35S:Avr 9 F1 seedlings. Simultaneous introduction of Cf-9 homologs (Hcr 9-9 genes A and B or D) along with the native Cf-9 gene did not alter the responses that were specifically induced by Avr 9. Various ways to use the Cf-9-Avr 9 gene combination to engineer broad-spectrum disease resistance in several solanaceous species are discussed.