Intergeneric transfer and functional expression of the tomato disease resistance gene Pto.

Plant disease resistance loci have been used successfully in breeding programs to transfer traits from resistant germplasm to susceptible plant cultivars. The molecular cloning of plant disease resistance genes now permits the transfer of such traits across species boundaries by genetic transformation of recipient hosts. The tomato disease resistance gene Pto confers resistance to strains of the bacterial pathogen Pseudomonas syringae pv tomato expressing the avirulence gene avrPto. Transformation of Nicotiana benthamiana with Pto results in specific resistance to P. s. pv tabaci strains carrying avrPto. The resistant phenotype is manifested by a strong inhibition of bacterial growth and the ability to exhibit a hypersensitive response. Resistance cosegregates with the Pto gene in transgene selfings and testcrosses. Our results demonstrate the conservation of disease resistance functions across genus boundaries and suggest that the utility of host-specific resistance genes can be extended by intergeneric transfer.     

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.     

Expression of the Tomato Pto Gene in Tobacco Enhances Resistance to Pseudomonas syringae pv tabaci Expressing avrPto

The Pto gene encodes a serine-threonine kinase that confers resistance in tomato to Pseudomonas syringae pv tomato strains expressing the avirulence gene avrPto. We examined the ability of Pto to function in tobacco, a species that is sexually incompatible with tomato. Evidence that a heterologous Pto-like signal transduction pathway is present in tobacco was suggested by the fact that tobacco line Wisconsin-38 exhibits a hypersensitive response after infection with P. syringae pv tabaci expressing avrPto. We introduced a Pto transgene into cultivar Wisconsin-38 and assessed the ability of transformed plants to further inhibit growth of the P. s. tabaci strain expressing avrPto. The Pto-transformed tobacco plants exhibited a significant increase in resistance to the avirulent P. s. tabaci strain compared with wild-type tobacco as indicated by (1) more rapid development of a hypersensitive resistance response at high inoculum concentrations (108 colony-forming units per mL); (2) lessened severity of disease symptoms at moderate inoculum concentrations (106 and 107 colony-forming units per mL); and (3) reduced growth of avirulent P. s. tabaci in inoculated leaves. The results indicate that essential components of a Pto-mediated signal transduction pathway are conserved in tobacco and should prompt examination of resistance gene function across even broader taxonomic distances.     

A cluster of mutations disrupt the avirulence but not the virulence function of AvrPto

avrPto in Pseudomonas syringae pv. tomato encodes an avirulence protein that triggers race-specific resistance in tomato plants carrying Pto. The AvrPto protein is secreted from P. syringae pv. tomato to plant cells through the type III secretion pathway and activates race-specific resistance by a direct interaction with the Pto protein. Here we report that avrPto enhances the virulence of P. syringae pv. tomato in a strain-dependent manner in tomato plants lacking Pto. To determine whether the virulence function can be structurally separated from the avirulence function, we examined the virulence activity of a group of AvrPto mutants that carry single amino acid substitutions and lack the avirulence activity on tomato plants. Three mutants that were clustered in the center of AvrPto exhibited virulence activity in tomato plants with or without Pto. The rest of the mutations abolished the virulence. The identification of these mutants suggested that the avirulence function of AvrPto can be structurally separated from the virulence function.     

Pto mutants differentially activate Prf-dependent,avrPto-independent resistance and gene-for-gene resistance

Pto confers disease resistance to Pseudomonas syringae pv tomato carrying the cognate avrPto gene. Overexpression of Pto under the cauliflower mosaic virus 35S promoter activates spontaneous lesions and confers disease resistance in tomato (Lycopersicon esculentum) plants in the absence of avrPto. Here, we show that these AvrPto-independent defenses require a functional Prf gene. Several Pto-interacting (Pti) proteins are thought to play a role in Pto-mediated defense pathways. To test if interactions with Pti proteins are required for the AvrPto-independent defense responses by Pto overexpression, we isolated several Pto mutants that were unable to interact with one or more Pti proteins, but retained normal interaction with AvrPto. Overexpression of two mutants, Pto(G50S) and Pto(R150S), failed to activate AvrPto-independent defense responses or confer enhanced resistance to the virulent P. s. pv tomato. When introduced into plants carrying 35S::Pto, 35S::Pto(G50S) dominantly suppressed the AvrPto-independent resistance caused by former transgene. 35S::Pto(G50S) also blocked the induction of a number of defense genes by the wild-type 35S::Pto. However, 35S::Pto(G50S) and 35S::Pto(R150S) plants were completely resistant to P. s. pv tomato (avrPto), indicating a normal gene-for-gene resistance. Furthermore, 35S::Pto(G50S) plants exhibited normal induction of defense genes in recognition of avrPto. Thus, the AvrPto-independent defense activation and gene-for-gene resistance mediated by Pto are functionally separable.     

Overexpression of Pto activates defense responses and confers broad resistance.

The tomato disease resistance (R) gene Pto specifies race-specific resistance to the bacterial pathogen Pseudomonas syringae pv tomato carrying the avrPto gene. Pto encodes a serine/threonine protein kinase that is postulated to be activated by a physical interaction with the AvrPto protein. Here, we report that overexpression of Pto in tomato activates defense responses in the absence of the Pto-AvrPto interaction. Leaves of three transgenic tomato lines carrying the cauliflower mosaic virus 35S::Pto transgene exhibited microscopic cell death, salicylic acid accumulation, and increased expression of pathogenesis-related genes. Cell death in these plants was limited to palisade mesophyll cells and required light for induction. Mesophyll cells of 35S::Pto plants showed the accumulation of autofluorescent compounds, callose deposition, and lignification. When inoculated with P. s. tomato without avrPto, all three 35S::Pto lines displayed significant resistance and supported less bacterial growth than did nontransgenic lines. Similarly, the 35S::Pto lines also were more resistant to Xanthomonas campestris pv vesicatoria and Cladosporium fulvum. These results demonstrate that defense responses and general resistance can be activated by the overexpression of an R gene.     

Tomato mutants altered in bacterial disease resistance provide evidence for a new locus controlling pathogen recognition.

We have employed a genetic approach to study the resistance of tomato to the phytopathogenic bacterium Pseudomonas syringae pv tomato. Resistance to P. s. tomato depends upon expression of the Pto locus in tomato, which encodes a protein with similarity to serine/threonine protein kinases and recognizes pathogen strains expressing the avirulence gene avrPto. Eleven tomato mutants were isolated with altered resistance to P. s. tomato strains expressing avrPto. We identified mutations both in the Pto resistance locus and in a new locus designated Prf (for Pseudomonas resistance and fenthion sensitivity). The genetic approach allowed us to dissect the roles of these loci in signal transduction in response to pathogen attack. Lines carrying mutations in the Pto locus vary 200-fold in the degree to which they are susceptible to P. s. tomato strains expressing avrPto. The pto mutants retain sensitivity to the organophosphate insecticide fenthion; this trait segregates with Pto in genetic crosses. This result suggested that contrary to previous hypotheses, the Pto locus controls pathogen recognition but not fenthion sensitivity. Interestingly, mutations in the prf locus result in both complete susceptibility to P. s. tomato and insensitivity to fenthion, suggesting that Prf plays a role in tomato signaling in response to both pathogen elicitors and fenthion. Because pto and prf mutations do not alter recognition of Xanthomonas campestris strains expressing avrBsP, an avirulence gene recognized by all tested tomato cultivars, Prf does not play a general role in disease resistance but possibly functions specifically in resistance against P. s. tomato. Genetic analysis of F2 populations from crosses of pto and prf homozygotes indicated that the Pto and Prf loci are tightly linked.     

The pseudomonas AvrPto protein is differentially recognized by tomato and tobacco and is localized to the plant plasma membrane

The avrPto gene of Pseudomonas syringae pv tomato triggers race-specific resistance in tomato plants carrying Pto, a resistance gene encoding a protein kinase. When introduced into P. s. tabaci, avrPto triggers resistance in tobacco W38 plants that carry the corresponding R gene. The AvrPto protein is believed to be secreted into host cells through the bacterial type III secretion pathway, where it activates disease resistance in tomato by interacting with Pto. We report here the identification of two distinct regions in AvrPto that determine the recognition specificity of this protein in tomato and tobacco. Point mutations in the central region disrupted the avirulence activity in tomato but not in tobacco. Conversely, point mutations in the C-terminal region abolished the avirulence in tobacco but not in tomato. We further report that AvrPto was localized to the plasma membrane of plant cells. Disrupting the membrane association by mutating a putative myristoylation motif of AvrPto abolished the avirulence activity in both tomato and tobacco. These findings demonstrate that AvrPto is recognized differently by the R genes in tomato and tobacco and that the recognition of AvrPto probably is associated with the plasma membrane.     

Identification of a disease resistance locus in Arabidopsis that is functionally homologous to the RPG1 locus of soybean

A new disease resistance locus in Arabidopsis, RPS3 , was identified using a previously cloned avirulence gene from a non- Arabidopsis pathogen. The avrB avirulence gene from the soybean pathogen Pseudomonas syringae pv. glycinea was transferred into a P. syringae pv. tomato strain that is virulent on Arabidopsis , and conversion to avirulence was assayed on Arabidopsis plants. The avrB gene had avirulence activity on most, but not all, Arabidopsis ecotypes. Of 53 ecotypes examined, 45 were resistant to a P. syringae pv. tomato strain carrying avrB , and eight were susceptible. The inheritance of this resistance was examined using crosses between the resistant ecotype Col-0 and the susceptible ecotype Bla-2. In F₂ plants from this cross, the ratio of resistant:susceptible plants was approximately 3:1, indicating that resistance to P. syringae expressing avrB is determined by a single dominant locus in ecotype Col-0, which we have designated RPS3 . Using RFLP analysis, RPS3 was mapped to chromosome 3, adjacent to markers M583 and G4523, and ≤ 1 cM from another disease resistance locus, RPM1 . In soybean, resistance to P. syringae strains that carry avrB is controlled by the locus RPG1 . Thus, RPG1 and RPS3 both confer avrB -specific disease resistance, suggesting that these genes may be homologs.     

Pseudomonas syringae effector avrB confers soybean cultivar-specific avirulence on Soybean mosaic virus adapted for transgene expression but effector avrPto does not

Soybean mosaic virus (SMV) was adapted for transgene expression in soybean and used to examine the function of avirulence genes avrB and avrPto of Pseudomonas syringae pvs. glycinea and tomato, respectively. A cloning site was introduced between the P1 and HC-Pro genes in 35S-driven infectious cDNAs of strains SMV-N and SMV-G7. Insertion of the uidA gene or the green fluorescent protein gene into either modified cDNA and bombardment into primary leaves resulted in systemic expression that reflected the pattern of viral movement into uninoculated leaves. Insertion of avrB blocked symptom development and detectable viral movement in cv. Harosoy, which carries the Rpg1-b resistance gene corresponding to avrB, but not in cvs. Keburi or Hurrelbrink, which lack Rpg1-b. In Keburi and Hurrelbrink, symptoms caused by SMV carrying avrB appeared more quickly and were more severe than those caused by the virus without avrB. Insertion of avrPto enhanced symptoms in Harosoy, Hurrelbrink, and Keburi. This result was unexpected because avrPto was reported to confer avirulence on P. syringae pv. glycinea inoculated to Harosoy. We inoculated Harosoy with P syringae pv. glycinea expressing avrPto, but observed no hypersensitive reaction, avrPto-dependent induction of pathogenesis-related protein la, or limitation of bacterial population growth. In Hurrelbrink, avrPto enhanced bacterial multiplication and exacerbated symptoms. Our results establish SMV as an expression vector for soybean. They demonstrate that resistance triggered by avrB is effective against SMV, and that avrB and avrPto have general virulence effects in soybean. The results also led to a reevaluation of the reported avirulence activity of avrPto in this plant.     

Identification of Pseudomonas syringae pathogens of Arabidopsis and a bacterial locus determining avirulence on both Arabidopsis and soybean.

To develop a model system for molecular genetic analysis of plant-pathogen interactions, we studied the interaction between Arabidopsis thaliana and the bacterial pathogen Pseudomonas syringae pv tomato (Pst). Pst strains were found to be virulent or avirulent on specific Arabidopsis ecotypes, and single ecotypes were resistant to some Pst strains and susceptible to others. In many plant-pathogen interactions, disease resistance is controlled by the simultaneous presence of single plant resistance genes and single pathogen avirulence genes. Therefore, we tested whether avirulence genes in Pst controlled induction of resistance in Arabidopsis. Cosmids that determine avirulence were isolated from Pst genomic libraries, and the Pst avirulence locus avrRpt2 was defined. This allowed us to construct pathogens that differed only by the presence or absence of a single putative avirulence gene. We found that Arabidopsis ecotype Col-0 was susceptible to Pst strain DC3000 but resistant to the same strain carrying avrRpt2, suggesting that a single locus in Col-0 determines resistance. As a first step toward genetically mapping the postulated resistance locus, an ecotype susceptible to infection by DC3000 carrying avrRpt2 was identified. The avrRpt2 locus from Pst was also moved into virulent strains of the soybean pathogen P. syringae pv glycinea to test whether this locus could determine avirulence on soybean. The resulting strains induced a resistant response in a cultivar-specific manner, suggesting that similar resistance mechanisms may function in Arabidopsis and soybean.     

Occurrence of Pseudomonas syringae pv. Tomato race 1 in Italy on Pto Gene-Bearing Tomato Plants

Bacterial speck symptoms on leaves of the tomato cultivar Erminia Fl (Petoseed), heterozygous for the Pto resistant gene, were observed in June 1995 in Northern Italy. Using individual-lesion isolation, 8 bacterial isolates were obtained from the affected leaves, which were identified as Pseudomonas syringae pv. tomato by biochemical, physiological, nutritional and pathogenicity tests. When the differential cultivar Ontario 7710, homozygous for the Pto gene, was inoculated with the bacterial isolates, 4 of them provoked the typical bacterial speck symptoms and therefore belonged to race 1. This is the first occurrence of P. syringae pv. tomato race 1 in Italy and the first time this race has been isolated on a Pto gene-bearing tomato cultivar grown in open field.     

Molecular characterization of cloned avirulence genes from race 0 and race 1 of Pseudomonas syringae pv. glycinea.

A wide-host-range cosmid cloning vector, pLAFR3, was constructed and used to make cosmid libraries of partially digested Sau3A DNA from race 0 and race 1 of Pseudomonas syringae pv. glycinea. Two avirulence genes, avrB0 and avrC, cloned from race 0, elicited the hypersensitivity reaction (HR) on specific cultivars of soybean. Race 4 transconjugants containing avrB0 induced a dark brown necrotic HR within 24 h on the soybean cultivars Harosoy and Norchief, whereas race 4 transconjugants containing avrC induced a light brown necrotic HR within 48 h on the soybean cultivars Acme, Peking, Norchief, and Flambeau. An additional avirulence gene, avrB1, cloned from race 1, appeared to be identical to avrB0 from race 0. The avrB0 and avrC genes from race 0 were characterized by restriction enzyme mapping, Southern blot analysis, Tn5 transposon mutagenesis, and site-directed gene replacements. The effects of these three genes on the in planta bacterial growth of race 4 transconjugants have also been examined. The identification and cloning of avrB1 provides genetic evidence for a gene-for-gene interaction in the bacterial blight disease of soybean, as avrB1 from race 1 interacts with the soybean disease resistance locus, Rpg1.     

Overexpression of the disease resistance gene Pto in tomato induces gene expression changes similar to immune responses in human and fruitfly

The Pto gene encodes a serine/threonine protein kinase that confers resistance in tomato (Lycopersicon esculentum) to Pseudomonas syringae pv tomato strains that express the type III effector protein AvrPto. Constitutive overexpression of Pto in tomato, in the absence of AvrPto, activates defense responses and confers resistance to several diverse bacterial and fungal plant pathogens. We have used a series of gene discovery and expression profiling methods to examine the effect of Pto overexpression in tomato leaves. Analysis of the tomato expressed sequence tag database and suppression subtractive hybridization identified 600 genes that were potentially differentially expressed in Pto-overexpressing tomato plants compared with a sibling line lacking Pto. By using cDNA microarrays, we verified changes in expression of many of these genes at various time points after inoculation with P. syringae pv tomato (avrPto) of the resistant Pto-overexpressing line and the susceptible sibling line. The combination of these three approaches led to the identification of 223 POR (Pto overexpression responsive) genes. Strikingly, 40% of the genes induced in the Pto-overexpressing plants previously have been shown to be differentially expressed during the human (Homo sapiens) and/or fruitfly (Drosophila melanogaster) immune responses.     

hrp gene-dependent induction of hin1: a plant gene activated rapidly by both harpins and the avrPto gene-mediated signal

Two classes of bacterial genes are involved in the elicitation of the plant hypersensitive response (HR) in resistant plants: hrp genes and avr genes. hrp genes have been shown to be involved in the production and secretion of a new class of bacterial virulence/avirulence proteins, including harpin of Erwinia amylovora and harpin_Pss of Pseudomonas syringae . The ability of avr genes in the elicitation of the HR/resistance is dependent on functional hrp genes. The relationships between harpins and avr gene products are not known. This study investigates the plant genes induced by harpins and the effect of avr genes on the expression of such plant genes. A tobacco gene highly induced by harpins was isolated by a subtractive hybridization method. Induction of hin1 by P.s. pv. syringae 61 (Pss61) was found to be dependent on functional bacterial hrp genes. P. fluorescens (a saprophyte) or hrp mutants defective in the Hrp secretion pathway did not induce hin1 significantly. A hin1 -related gene in tomato cv. Rio Grande-PtoR was found to be rapidly induced by P. s. pv. tomato T1 (a virulent bacterium on Rio Grande-PtoR) containing the avrPto gene, which mediates the elicitation of the HR/resistance in a Pto plant resistance gene-dependent manner. The induction of hin1 by bacteria correlates with production of harpins in planta . The putative open reading frame of hin1 encodes a novel protein of 221 amino acids. The data suggest that harpins and the avrPto -mediated signal induce a common plant gene in the elicitation of the HR.     

Natural variation in the Pto pathogen resistance gene within species of wild tomato (Lycopersicon). I. Functional analysis of Pto alleles

Disease resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) in the cultivated tomato, Lycopersicon esculentum, and the closely related L. pimpinellifolium is triggered by the physical interaction between plant disease resistance protein, Pto, and the pathogen avirulence protein, AvrPto. To investigate the extent to which variation in the Pto gene is responsible for naturally occurring variation in resistance to Pst, we determined the resistance phenotype of 51 accessions from seven species of Lycopersicon to isogenic strains of Pst differing in the presence of avrPto. One-third of the plants displayed resistance specifically when the pathogen expressed AvrPto, consistent with a gene-for-gene interaction. To test whether this resistance in these species was conferred specifically by the Pto gene, alleles of Pto were amplified and sequenced from 49 individuals and a subset (16) of these alleles was tested in planta using Agrobacterium-mediated transient assays. Eleven alleles conferred a hypersensitive resistance response (HR) in the presence of AvrPto, while 5 did not. Ten amino acid substitutions associated with the absence of AvrPto recognition and HR were identified, none of which had been identified in previous structure-function studies. Additionally, 3 alleles encoding putative pseudogenes of Pto were isolated from two species of Lycopersicon. Therefore, a large proportion, but not all, of the natural variation in the reaction to strains of Pst expressing AvrPto can be attributed to sequence variation in the Pto gene.     

Molecular analysis of the avirulence gene avr9 of the fungal tomato pathogen Cladosporium fulvum fully supports the gene-for-gene hypothesis

The interaction between the fungal pathogen Cladosporium fulvum and tomato is supposed to have a gene-for-gene basis. Races of C. fulvum which have 'overcome' the resistance gene Cf9 of tomato, lack the avirulence gene avr9 which encodes a race-specific peptide elicitor. Races avirulent on tomato genotypes carrying the resistance gene Cf9 produce the race-specific peptide elicitor, which induces the hypersensitive response (HR) on those genotypes. The causal relationship between the presence of a functional avr9 gene and avirulence on tomato genotype Cf9 was demonstrated by cloning of the avr9 gene and subsequent transformation of C. fulvum. A race virulent on tomato genotype Cf9 was shown to become avirulent by transformation with the cloned avr9 gene. These results clearly demonstrate that the avr9 gene is responsible for cultivar specificity on tomato genotype Cf9 and fully support the gene-for-gene hypothesis. The avr9 gene is the first fungal avirulence gene to be cloned.     

Genetic and molecular requirements for function of the Pto/Prf effector recognition complex in tomato and Nicotiana benthamiana

The Pto gene of tomato (Solanum lycopersicum) confers specific recognition of the unrelated bacterial effector proteins AvrPto and AvrPtoB. Pto resides in a constitutive molecular complex with the nucleotide binding site-leucine rich repeats protein Prf. Prf is absolutely required for specific recognition of both effectors. Here, using stable transgenic lines, we show that expression of Pto from its genomic promoter in susceptible tomatoes was sufficient to complement recognition of Pseudomonas syringae pv. tomato (Pst) bacteria expressing either avrPto or avrPtoB. Pto kinase activity was absolutely required for specific immunity. Expression of the Pto N-myristoylation mutant, pto(G2A), conferred recognition of Pst (avrPtoB), but not Pst (avrPto), although bacterial growth in these lines was intermediate between resistant and susceptible lines. Overexpression of pto(G2A) complemented recognition of avrPto. Transgenic tomato plants overexpressing wild-type Pto exhibited constitutive growth phenotypes, but these were absent in lines overexpressing pto(G2A). Therefore, Pto myristoylation is a quantitative factor for effector recognition in tomato, but is absolutely required for overexpression phenotypes. Native expression of Pto in the heterologous species Nicotiana benthamiana did not confer resistance to P. syringae pv. tabaci (Pta) expressing avrPto or avrPtoB, but recognition of both effectors was complemented by Prf co-expression. Thus, specific resistance conferred solely by Pto in N. benthamiana is an artefact of overexpression. Finally, pto(G2A) did not confer recognition of either avrPto or avrPtoB in N. benthamiana, regardless of the presence of Prf. Thus, co-expression of Prf in N. benthamiana complements many but not all aspects of normal Pto function.     

The avirulence gene avrBs1 from Xanthomonas campestris pv. vesicatoria encodes a 50-kD protein

A gene cloned from Xanthomonas campestris pv. vesicatoria race 2, avrBs1, specified avirulence on pepper cultivars containing the resistance gene Bs1. A series of exonuclease III deletions were made on a 3.2-kbp DNA fragment that determined full avirulence activity, observed as hypersensitive response (HR) induction. The deletion products were subcloned into the broad host range cloning vector pLAFR3, conjugated into a virulent X. c. pv. vesicatoria race 1 strain, 82-8, and scored for their ability to induce a HR on a pepper cultivar (ECW10R) containing the resistance gene Bs1. A span of approximately 1.8 kbp of DNA was necessary for full induction of the HR. The nucleotide sequence revealed two open reading frames (ORFs) capable of encoding proteins of 12.3 and 49.8 kD, designated ORF1 and ORF2, respectively. Deletions into ORF1 altered the HR-inducing activity to give an intermediate phenotype. Deletions into ORF2 completely destroyed activity. When the ORF2 coding region was driven by the lacZ promoter on plasmid pLAFR3 (placD), full avirulence activity was restored, indicating that ORF2 alone can induce the HR. Antisera raised to a beta-galactosidase-ORF2 fusion protein reacted with a 50-kD protein in X. c. pv. vesicatoria race 1 (placD) transconjugants. The deduced amino acid sequence of ORF2 had approximately 47% overall homology to the carboxyl terminus of the avirulence gene, avrA, isolated from Pseudomonas syringae pv. glycinea race 6, and 86% homology over a region of 49 amino acids. P. s. pv. glycinea, however, did not induce an HR on ECW10R plants.(ABSTRACT TRUNCATED AT 250 WORDS)