Abscisic acid deficiency causes changes in cuticle permeability and pectin composition that influence tomato resistance to Botrytis cinerea

A mutant of tomato (Solanum lycopersicum) with reduced abscisic acid (ABA) production (sitiens) exhibits increased resistance to the necrotrophic fungus Botrytis cinerea. This resistance is correlated with a rapid and strong hydrogen peroxide-driven cell wall fortification response in epidermis cells that is absent in tomato with normal ABA production. Moreover, basal expression of defense genes is higher in the mutant compared with the wild-type tomato. Given the importance of this fast response in sitiens resistance, we investigated cell wall and cuticle properties of the mutant at the chemical, histological, and ultrastructural levels. We demonstrate that ABA deficiency in the mutant leads to increased cuticle permeability, which is positively correlated with disease resistance. Furthermore, perturbation of ABA levels affects pectin composition. sitiens plants have a relatively higher degree of pectin methylesterification and release different oligosaccharides upon inoculation with B. cinerea. These results show that endogenous plant ABA levels affect the composition of the tomato cuticle and cell wall and demonstrate the importance of cuticle and cell wall chemistry in shaping the outcome of this plant-fungus interaction.

CaMsrB2, pepper methionine sulfoxide reductase B2, is a novel defense regulator against oxidative stress and pathogen attack

Reactive oxygen species (ROS) are inevitably generated in aerobic organisms as by-products of normal metabolism or as the result of defense and development. ROS readily oxidize methionine (Met) residues in proteins/peptides to form Met-R-sulfoxide or Met-S-sulfoxide, causing inactivation or malfunction of the proteins. A pepper (Capsicum annuum) methionine sulfoxide reductase B2 gene (CaMsrB2) was isolated, and its roles in plant defense were studied. CaMsrB2 was down-regulated upon inoculation with either incompatible or compatible pathogens. The down-regulation, however, was restored to the original expression levels only in a compatible interaction. Gain-of-function studies using tomato (Solanum lycopersicum) plants transformed with CaMsrB2 resulted in enhanced resistance to Phytophthora capsici and Phytophthora infestans. Inversely, loss-of-function studies of CaMsrB2 using virus-induced gene silencing in pepper plants (cv Early Calwonder-30R) resulted in accelerated cell death from an incompatible bacterial pathogen, Xanthomonas axonopodis pv vesicatoria (Xav) race 1, and enhanced susceptibility to a compatible bacterial pathogen, virulent X. axonopodis pv vesicatoria race 3. Measurement of ROS levels in CaMsrB2-silenced pepper plants revealed that suppression of CaMsrB2 increased the production of ROS, which in turn resulted in the acceleration of cell death via accumulation of ROS. In contrast, the CaMsrB2-transgenic tomato plants showed reduced production of hydrogen peroxide. Taken together, our results suggest that the plant MsrBs have novel functions in active defense against pathogens via the regulation of cell redox status.

Conserved fungal LysM effector Ecp6 prevents chitin-triggered immunity in plants

Multicellular organisms activate immunity upon recognition of pathogen-associated molecular patterns (PAMPs). Chitin is the major component of fungal cell walls, and chitin oligosaccharides act as PAMPs in plant and mammalian cells. Microbial pathogens deliver effector proteins to suppress PAMP-triggered host immunity and to establish infection. Here, we show that the LysM domain-containing effector protein Ecp6 of the fungal plant pathogen Cladosporium fulvum mediates virulence through perturbation of chitin-triggered host immunity. During infection, Ecp6 sequesters chitin oligosaccharides that are released from the cell walls of invading hyphae to prevent elicitation of host immunity. This may represent a common strategy of host immune suppression by fungal pathogens, because LysM effectors are widely conserved in the fungal kingdom.

Methods to study PAMP-triggered immunity using tomato and Nicotiana benthamiana

Understanding the molecular basis of plant responses to pathogen-associated molecular patterns (PAMPs) is an active area of research in the field of plant-microbe interactions. A growing number of plant genes involved in various steps of PAMP-triggered immunity (PTI) pathways and microbial factors involved in the elicitation or suppression of PTI have been identified. These studies have largely relied on Arabidopsis thaliana and, therefore, most of the PTI assays have been developed and optimized for that model plant system. Although PTI is a conserved feature among plants, the response spectra vary across different species. Thus, there is a need for robust PTI assays in other pathosystems, such as those involving Solanaceae plant-pathogen interactions, which include many economically important plants and their diseases. We have optimized molecular, cellular, and whole-plant methods to measure PTI responses in two widely studied solanaceous species, tomato (Solanum lycopersicum) and Nicotiana benthamiana. Here, we provide detailed protocols for measuring various PTI-associated phenotypes, including bacterial populations after pretreatment of leaves with PAMPs, induction of reporter genes, callose deposition, activation of mitogen-activated protein kinases, and a luciferase-based reporter system. These methods will facilitate limited genetic screens and detailed characterization of potential PTI-related genes in model and economically important Solanaceae spp.-pathogen interactions.

Co-sensitisation (but co-recognition also) to novel banana and tomato allergens

An unusual case of both banana and tomato allergy is reported. In vitro tests showed that both co-sensitization to and co-recognition of allergen in the two fruits were present. Interestingly, the patients showed IgE reactivity to hitherto not described, high molecular weight allergens.

Xanthomonas campestris pv. vesicatoria effector AvrBsT induces cell death in pepper, but suppresses defense responses in tomato

A type III effector protein, AvrBsT, is secreted into plant cells from Xanthomonas campestris pv. vesicatoria Bv5-4a, which causes bacterial spot disease on pepper (Capsicum annuum) and tomato (Solanum lycopersicum). To define the function and recognition of AvrBsT in the two host plants, avrBsT was introduced into the virulent pepper strain X. campestris pv. vesicatoria Ds1. Expression of AvrBsT in Ds1 rendered the strain avirulent to pepper plants. Infection of pepper leaves with Ds1 (avrBsT) expressing AvrBsT but not with near-isogenic control strains triggered a hypersensitive response (HR) accompanied by strong H(2)O(2) generation, callose deposition, and defense-marker gene expressions. Mutation of avrBsT, however, compromised HR induction by X. campestris pv. vesicatoria Bv5-4a, suggesting its avirulence function in pepper plants. In contrast, AvrBsT acted as a virulence factor in tomato plants. Growth of strains Ds1 (avrBsT) and Bv5-4a DeltaavrBsT was significantly enhanced and reduced, respectively, in tomato leaves. X. campestris pv. vesicatoria-expressed AvrBsT also significantly compromised callose deposition and defense-marker gene expression in tomato plants. Together, these results suggest that the X. campestris pv. vesicatoria type III effector AvrBsT is differentially recognized by pepper and tomato plants.

Chitin synthase-deficient mutant of Fusarium oxysporum elicits tomato plant defence response and protects against wild-type infection

A mutant of the root pathogen Fusarium oxysporum f. sp. lycopersici, deficient in class V chitin synthase, has been shown previously to be nonvirulent. In this study, we tested the hypothesis that the cause of its avirulence could be the elicitation of the induced plant defence response, leading to the restriction of fungal infection. Co-inoculation of tomato plants with the wild-type strain and the DeltachsV mutant resulted in a significant reduction in symptom development, supporting a protective mechanism exerted by the mutant. The ability of the mutant to penetrate and colonize plant tissues was determined by scanning and transmission electron microscopy, as well as fluorescence microscopy using green fluorescent protein- or cherry fluorescent protein-labelled fungal strains. The extent of wild-type strain colonization in co-inoculated plants decreased steadily throughout the infection process, as shown by the quantification of fungal biomass using real-time polymerase chain reaction. The hypothesis that defence responses are activated by the DeltachsV mutant was confirmed by the analysis of plant pathogenesis-related genes using real-time reverse transcriptase-polymerase chain reaction. Tomato plants inoculated with the DeltachsV mutant showed a three fold increase in endochitinase activity in comparison with wild-type inoculated plants. Taken together, these results suggest that the perturbation of fungal cell wall biosynthesis results in elicitation of the plant defence response during the infection process.

WRKY72-type transcription factors contribute to basal immunity in tomato and Arabidopsis as well as gene-for-gene resistance mediated by the tomato R gene Mi-1

WRKY transcription factors play a central role in transcriptional reprogramming associated with plant immune responses. However, due to functional redundancy, typically the contribution of individual members of this family to immunity is only subtle. Using microarray analysis, we found that the paralogous tomato WRKY genes SlWRKY72a and b are transcriptionally up-regulated during disease resistance mediated by the R gene Mi-1. Virus-induced gene silencing of these two genes in tomato resulted in a clear reduction of Mi-1-mediated resistance as well as basal defense against root-knot nematodes (RKN) and potato aphids. Using Arabidopsis T-DNA insertion mutants, we found that their Arabidopsis ortholog, AtWRKY72, is also required for full basal defense against RKN as well as to the oomycete Hyaloperonospora arabidopsidis. Despite their similar roles in basal defense against RKN in both tested plant species, WRKY72-type transcription factors in tomato, but not in Arabidopsis, clearly contributed to basal defense against the bacterial pathogen Pseudomonas syringae. Of the five R genes that we tested in tomato and Arabidopsis, only Mi-1 appeared to be dependent on WRKY72-type transcription factors. Interestingly, AtWRKY72 target genes, identified by microarray analysis of H. arabidopsidis-triggered transcriptional changes, appear to be largely non-responsive to analogs of the defense hormone salicylic acid (SA). Thus, similarly to Mi-1, which in part acts independently of SA, AtWRKY72 appears to utilize SA-independent defense mechanisms. We propose that WRKY72-type transcription factors play a partially conserved role in basal defense in tomato and Arabidopsis, a function that has been recruited to serve Mi-1-dependent immunity.

Tomato cultivar tolerant to Tomato leaf curl New Delhi virus infection induces virus-specific short interfering RNA accumulation and defence-associated host gene expression

Tomato leaf curl New Delhi virus (ToLCNDV) infection causes significant yield loss in tomato. The availability of a conventional tolerance source against this virus is limited in tomato. To understand the molecular mechanism of virus tolerance in tomato, the abundance of viral genomic replicative intermediate molecules and virus-directed short interfering RNAs (siRNAs) by the host plant in a naturally tolerant cultivar H-88-78-1 and a susceptible cultivar Punjab Chhuhara at different time points after agroinfection was studied. We report that less abundance of viral replicative intermediate in the tolerant cultivar may have a correlation with a relatively higher accumulation of virus-specific siRNAs. To study defence-related host gene expression in response to ToLCNDV infection, the suppression subtractive hybridization technique was used. A library was prepared from tolerant cultivar H-88-78-1 between ToLCNDV-inoculated and Agrobacterium mock-inoculated plants of this cultivar at 21 days post-inoculation (dpi). A total of 106 nonredundant transcripts was identified and classified into 12 different categories according to their putative functions. By reverse Northern analysis and quantitative real-time polymerase chain reaction (qRT-PCR), we identified the differential expression pattern of 106 transcripts, 34 of which were up-regulated (>2.5-fold induction). Of these, eight transcripts showed more than four fold induction. qRT-PCR analysis was carried out to obtain comparative expression profiling of these eight transcripts between Punjab Chhuhara and H-88-78-1 on ToLCNDV infection. The expression patterns of these transcripts showed a significant increase in differential expression in the tolerant cultivar, mostly at 14 and 21 dpi, in comparison with that in the susceptible cultivar, as analysed by qRT-PCR. The probable direct and indirect relationship of siRNA accumulation and up-regulated transcripts with the ToLCNDV tolerance mechanism is discussed.

Resistance in tomato and wild relatives to crown and root rot caused by Phytophthora capsici

Phytophthora capsici causes root, crown, and fruit rot of tomato, a major vegetable crop grown worldwide. The objective of this study was to screen tomato cultivars and wild relatives of tomato for resistance to P. capsici. Four P. capsici isolates were individually used to inoculate 6-week-old seedlings (1 g of P. capsici-infested millet seed per 10 g of soilless medium) of 42 tomato cultivars and wild relatives of tomato in a greenhouse. Plants were evaluated daily for wilting and death. All P. capsici isolates tested caused disease in seedlings but some isolates were more pathogenic than others. A wild relative of cultivated tomato, Solanum habrochaites accession LA407, was resistant to all P. capsici isolates tested. Moderate resistance to all isolates was identified in the host genotypes Ha7998, Fla7600, Jolly Elf, and Talladega. P. capsici was frequently recovered from root and crown tissue of symptomatic inoculated seedlings but not from leaf tissue or asymptomatic or control plants. The phenotype of the recovered isolate matched the phenotype of the inoculum. Pathogen presence was confirmed in resistant and moderately resistant tomato genotypes by species-specific polymerase chain reaction of DNA from infected crown and root tissue. Amplified fragment length polymorphisms of tomato genotypes showed a lack of correlation between genetic clusters and susceptibility to P. capsici, indicating that resistance is distributed in several tomato lineages. The results of this study create a baseline for future development of tomato cultivars resistant to P. capsici.

Resistance to Tomato chlorosis virus in wild tomato species that impair virus accumulation and disease symptom expression

Tomato chlorosis virus (ToCV) (genus Crinivirus, family Closteroviridae) is an emerging threat to tomato crops worldwide. Although symptoms on fruits are not obvious, yield losses occur through decreased fruit size and number. Control of ToCV epidemics is difficult because the virus is transmitted by several whitefly vector species and its relatively wide host range facilitates establishment in local wild reservoirs. Therefore, breeding for ToCV resistance offers the best control alternative. However, no sources for resistance are available thus far. Here, a screen of tomatoes and wild species relatives was performed in search of ToCV resistance. Two sources of resistance to ToCV were identified in this work, lines '802-11-1' and '821-13-1', each derived by two self-pollinations from ToCV asymptomatic plants of the population 'IAC CN RT' (derived from an interspecific hybrid Solanum lycopersicum x S. peruvianum accession LA0444) and accession LA1028 (S. chmielewskii), respectively. The resistance was expressed by impairing virus accumulation and disease symptom expression, both under natural infection and after challenging with ToCV in controlled inoculations. Genetic control of resistance to ToCV infection in '821-13-1' was conferred by a major locus with mainly additive effects but also partial dominance for higher susceptibility. Also, an additive x dominance epistatic interaction with at least one additional gene was evident.

Hormonal and transcriptional profiles highlight common and differential host responses to arbuscular mycorrhizal fungi and the regulation of the oxylipin pathway

Arbuscular mycorrhizal (AM) symbioses are mutualistic associations between soil fungi and most vascular plants. The symbiosis significantly affects the host physiology in terms of nutrition and stress resistance. Despite the lack of host range specificity of the interaction, functional diversity between AM fungal species exists. The interaction is finely regulated according to plant and fungal characters, and plant hormones are believed to orchestrate the modifications in the host plant. Using tomato as a model, an integrative analysis of the host response to different mycorrhizal fungi was performed combining multiple hormone determination and transcriptional profiling. Analysis of ethylene-, abscisic acid-, salicylic acid-, and jasmonate-related compounds evidenced common and divergent responses of tomato roots to Glomus mosseae and Glomus intraradices, two fungi differing in their colonization abilities and impact on the host. Both hormonal and transcriptional analyses revealed, among others, regulation of the oxylipin pathway during the AM symbiosis and point to a key regulatory role for jasmonates. In addition, the results suggest that specific responses to particular fungi underlie the differential impact of individual AM fungi on plant physiology, and particularly on its ability to cope with biotic stresses.

Variations in the mycorrhization characteristics in roots of wild-type and ABA-deficient tomato are accompanied by specific transcriptomic alterations

Abscissic acid (ABA) determines mycorrhiza functionality and arbuscule development. In this study, we performed transcriptome analysis in response to different mycorrhization status according to the ABA content in the root to identify genes that may play a role in arbuscule functionality. Affymetrix Tomato GeneChip (approximately 10,000 probes) allowed us to detect and compare the transcriptional root profiling of tomato (Solanum lycopersicum) wild-type and ABA-deficient sitiens plants colonized by Glomus intraradices. A number of identified genes in tomato belong to a category of genes already described as "mycorrhizal core-set" in other host plants. The impairment in arbuscular mycorrhiza (AM) formation in ABA-deficient mutants was associated with upregulation of genes related to defense and cell wall modification, whereas functional mycorrhization in wild-type plants was associated with activation of genes related to isoprenoid metabolism. The oxylipin pathway was activated in tomato mycorrhizal roots at late stages of interaction, and was related to the control of fungal spread in roots, not with the establishment of the symbiosis. Induction of selected genes, representing a range of biological functions and representative of the three sets of genes specifically upregulated in the different plant phenotype, was confirmed by quantitative reverse-transcription polymerase chain reaction, and their response to phythohormone treatment was tested, showing that ethylene and jasmonic acid are key regulators of gene expression during AM development. Comparative analysis of mycorrhiza upregulated functional categories revealed significant changes in gene expression associated with the different mycorrhization status according to the ABA content in the roots.

Inducible positive mutant screening system to unveil the signaling pathway of late blight resistance

Late blight is the most devastating potato disease and it also causes serious yield loss in tomato. Several disease resistance genes (R genes) to late blight have been cloned from potato in the past decade. However, the resistance mechanisms remain elusive. Tomato and potato belong to the botanical family Solanaceace and share remarkably conserved genome structure. Since tomato is a model system in genetic and plant pathology research, we used tomato to develop a powerful mutant screening system that will greatly facilitate the analysis of the signaling pathway of resistance to Phytophthora infestans. First we proved that the R3a transgenic tomatoes developed specific hypersensitive cell death response (HR) to P. infestans strains carrying the corresponding avirulence gene Avr3a, indicating that the signaling pathway from the R3a-Avr3a recognition to HR is conserved between potato and tomato. Second, we generated transgenic tomatoes carrying both R3a and Avr3a genes, with the latter under the control of a glucocorticiod-inducible promoter. Dexamethasone induced expression of Avr3a and resulted in localized HR. This versatile system can be used to construct a mutant library to screen surviving mutants whose resistance signal transduction was interrupted, providing the basis to identify key genes involved in the resistance to late blight in Solanaceae.

Identification of tomato phosphatidylinositol-specific phospholipase-C (PI-PLC) family members and the role of PLC4 and PLC6 in HR and disease resistance

The perception of pathogen-derived elicitors by plants has been suggested to involve phosphatidylinositol-specific phospholipase-C (PI-PLC) signalling. Here we show that PLC isoforms are required for the hypersensitive response (HR) and disease resistance. We characterised the tomato [Solanum lycopersicum (Sl)] PLC gene family. Six Sl PLC-encoding cDNAs were isolated and their expression in response to infection with the pathogenic fungus Cladosporium fulvum was studied. We found significant regulation at the transcriptional level of the various SlPLCs, and SlPLC4 and SlPLC6 showed distinct expression patterns in C. fulvum-resistant Cf-4 tomato. We produced the encoded proteins in Escherichia coli and found that both genes encode catalytically active PI-PLCs. To test the requirement of these Sl PLCs for full Cf-4-mediated recognition of the effector Avr4, we knocked down the expression of the encoding genes by virus-induced gene silencing. Silencing of SlPLC4 impaired the Avr4/Cf-4-induced HR and resulted in increased colonisation of Cf-4 plants by C. fulvum expressing Avr4. Furthermore, expression of the gene in Nicotiana benthamiana enhanced the Avr4/Cf-4-induced HR. Silencing of SlPLC6 did not affect HR, whereas it caused increased colonisation of Cf-4 plants by the fungus. Interestingly, Sl PLC6, but not Sl PLC4, was also required for resistance to Verticillium dahliae, mediated by the transmembrane Ve1 resistance protein, and to Pseudomonas syringae, mediated by the intracellular Pto/Prf resistance protein couple. We conclude that there is a differential requirement of PLC isoforms for the plant immune response and that Sl PLC4 is specifically required for Cf-4 function, while Sl PLC6 may be a more general component of resistance protein signalling.

The Pseudomonas syringae type III effector HopG1 targets mitochondria, alters plant development and suppresses plant innate immunity

The bacterial plant pathogen Pseudomonas syringae uses a type III protein secretion system to inject type III effectors into plant cells. Primary targets of these effectors appear to be effector-triggered immunity (ETI) and pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). The type III effector HopG1 is a suppressor of ETI that is broadly conserved in bacterial plant pathogens. Here we show that HopG1 from P. syringae pv. tomato DC3000 also suppresses PTI. Interestingly, HopG1 localizes to plant mitochondria, suggesting that its suppression of innate immunity may be linked to a perturbation of mitochondrial function. While HopG1 possesses no obvious mitochondrial signal peptide, its N-terminal two-thirds was sufficient for mitochondrial localization. A HopG1-GFP fusion lacking HopG1's N-terminal 13 amino acids was not localized to the mitochondria reflecting the importance of the N-terminus for targeting. Constitutive expression of HopG1 in Arabidopsis thaliana, Nicotiana tabacum (tobacco) and Lycopersicon esculentum (tomato) dramatically alters plant development resulting in dwarfism, increased branching and infertility. Constitutive expression of HopG1 in planta leads to reduced respiration rates and an increased basal level of reactive oxygen species. These findings suggest that HopG1's target is mitochondrial and that effector/target interaction promotes disease by disrupting mitochondrial functions.

Naringenin chalcone suppresses allergic asthma by inhibiting the type-2 function of CD4 T cells

BACKGROUND

Some polyphenols possess anti-allergic activities. Naringenin chalcone is one of the polyphenols that is present in the skin of red tomatoes. In this study, we investigated the effect of naringenin chalcone in allergic responses in vivo using an experimental mouse model system of allergic asthma.

METHODS

Allergic airway inflammation was induced in mice by sensitization and challenge with ovalbumin. Naringenin chalcone was orally administrated every day during the course of the experiment. Airway hyperreactivity, the eosinophilic infiltration in the bronchioalveolar lavage fluid and Th2 cytokine production from splenic CD4 T cells were assessed.

RESULTS

Eosinophilic airway inflammation, airway hyperreactivity and Th2 cytokine production from CD4 T cells were significantly suppressed in mice that were treated with naringenin chalcone. Hyperproduction of mucus was slightly reduced.

CONCLUSIONS

The results of this study suggest that naringenin chalcone suppresses asthmatic symptoms by inhibiting Th2 cytokine production from CD4 T cells. Thus, naringenin chalcone may be a useful supplement for the suppression of allergic symptoms in humans.

Impact of Piriformospora indica on tomato growth and on interaction with fungal and viral pathogens

Piriformospora indica is a root endophytic fungus with plant-promoting properties in numerous plant species and induces resistance against root and shoot pathogens in barley, wheat, and Arabidopsis. A study over several years showed that the endophyte P. indica colonised the roots of the most consumed vegetable crop tomato. P. indica improved the growth of tomato resulting in increased biomass of leaves by up to 20%. Limitation of disease severity caused by Verticillium dahliae by more than 30% was observed on tomato plants colonised by the endophyte. Further experiments were carried out in hydroponic cultures which are commonly used for the indoor production of tomatoes in central Europe. After adaptation of inoculation techniques (inoculum density, plant stage), it was shown that P. indica influences the concentration of Pepino mosaic virus in tomato shoots. The outcome of the interaction seems to be affected by light intensity. Most importantly, the endophyte increases tomato fruit biomass in hydroponic culture concerning fresh weight (up to 100%) and dry matter content (up to 20%). Hence, P. indica represents a suitable growth promoting endophyte for tomato which can be applied in production systems of this important vegetable plant not only in soil, but also in hydroponic cultures.

Development and evaluation of robust molecular markers linked to disease resistance in tomato for distinctness, uniformity and stability testing

Molecular markers linked to phenotypically important traits are of great interest especially when traits are difficult and/or costly to be observed. In tomato where a strong focus on resistance breeding has led to the introgression of several resistance genes, resistance traits have become important characteristics in distinctness, uniformity and stability (DUS) testing for Plant Breeders Rights (PBR) applications. Evaluation of disease traits in biological assays is not always straightforward because assays are often influenced by environmental factors, and difficulties in scoring exist. In this study, we describe the development and/or evaluation of molecular marker assays for the Verticillium genes Ve1 and Ve2, the tomato mosaic virus Tm1 (linked marker), the tomato mosaic virus Tm2 and Tm2 ( 2 ) genes, the Meloidogyne incognita Mi1-2 gene, the Fusarium I (linked marker) and I2 loci, which are obligatory traits in PBR testing. The marker assays were evaluated for their robustness in a ring test and then evaluated in a set of varieties. Although in general, results between biological assays and marker assays gave highly correlated results, marker assays showed an advantage over biological tests in that the results were clearer, i.e., homozygote/heterozygote presence of the resistance gene can be detected and heterogeneity in seed lots can be identified readily. Within the UPOV framework for granting of PBR, the markers have the potential to fulfil the requirements needed for implementation in DUS testing of candidate varieties and could complement or may be an alternative to the pathogenesis tests that are carried out at present.

Prf immune complexes of tomato are oligomeric and contain multiple Pto-like kinases that diversify effector recognition

Cytoplasmic recognition of pathogen virulence effectors by plant NB-LRR proteins leads to strong induction of defence responses termed effector triggered immunity (ETI). In tomato, a protein complex containing the NB-LRR protein Prf and the protein kinase Pto confers recognition of the Pseudomonas syringae effectors AvrPto and AvrPtoB. Although structurally unrelated, AvrPto and AvrPtoB interact with similar residues in the Pto catalytic cleft to activate ETI via an unknown mechanism. Here we show that the Prf complex is oligomeric, containing at least two molecules of Prf. Within the complex, Prf can associate with Pto or one of several Pto family members including Fen, Pth2, Pth3, or Pth5. The dimerization surface for Prf is the novel N-terminal domain, which also coordinates an intramolecular interaction with the remainder of the molecule, and binds Pto kinase or a family member. Thus, association of two Prf N-terminal domains brings the associated kinases into close promixity. Tomato lines containing Prf complexed with Pth proteins but not Pto possessed greater immunity against P. syringae than tomatoes lacking Prf. This demonstrates that incorporation of non-Pto kinases into the Prf complex extends the number of effector proteins that can be recognized.

Phosphorylation of the Pseudomonas syringae effector AvrPto is required for FLS2/BAK1-independent virulence activity and recognition by tobacco

The type III effector protein AvrPto from Pseudomonas syringae pv. tomato is secreted into plant cells where it promotes bacterial growth and enhances symptoms of speck disease on susceptible tomato plants. The virulence activity of AvrPto is due, in part, to its interaction with components of host pattern recognition receptor complexes, which disrupts pathogen-associated molecular pattern-triggered immunity. This disruption mechanism requires a structural element of the AvrPto protein, the CD loop, which is also required for triggering Pto/Prf-mediated resistance in tomato. We have shown previously that the carboxyl-terminal domain (CTD) of AvrPto is phosphorylated and also contributes to bacterial virulence. Here we report that phosphorylation of the CTD on S147 and S149 promotes bacterial virulence in an FLS2/BAK1-independent manner, which is mechanistically distinct from the CD loop. In a striking corollary with Pto recognition of the CD loop in tomato, the tobacco species Nicotiana sylvestris and Nicotiana tabacum have a recognition mechanism that specifically detects the phosphorylation status of the CTD. Thus different species in the Solanaceae family have evolved distinct recognition mechanisms to monitor the same type III effector.

Tomato allergy: detection of IgE-binding lipid transfer proteins in tomato derivatives and in fresh tomato peel, pulp, and seeds

There is an increasing consumption of tomatoes worldwide: fresh in salads, cooked in household sauces, or industrially processed. Although many tomato allergens have been identified, there is no information in the literature on the allergenic components found in commercial tomato products. The primary aim of the study was to evaluate the allergenic profile of commercial tomato products by skin prick tests (SPTs) and IgE/immunoblotting in tomato-allergic subjects. The secondary end point was the study of the IgE-binding profile of tomato peel, pulp, and seeds. Forty tomato-allergic patients, reporting oral allergy syndrome (OAS) at different grades of severity for fresh and, in some cases, also for cooked tomato, were selected on the basis of positive tomato allergy history or open food challenge (OFC). They were evaluated by SPTs with different experimental tomato extracts. SDS-PAGE/immunoblotting was performed to detect tomato allergens, which were then identified by Edman degradation. Twenty-three patients (57.5%) presented first-grade OAS at the OFC, whereas 17 (42.5%) reported severe symptoms. Ten of these 17 patients (25%) reported allergic reactions to cooked tomatoes; in immunoblotting tests, their sera reacted only to lipid transfer protein (LTP). In commercial products, LTP was the only detectable allergen. In contrast to other LTP-containing fruits, in tomato, an IgE-binding LTP was identified not only in the peel but also in the pulp and seeds. This study demonstrates that, in fresh tomato, different LTP isoforms are present and allergenic. Industrial tomato derivatives still contain LTP, thus presenting a problem for LTP-allergic patients.

Virulence development and genetic polymorphism in Meloidogyne incognita (Kofoid & White) Chitwood after prolonged exposure to sublethal concentrations of nematicides and continuous growing of resistant tomato cultivars

BACKGROUND

The root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, is an important plant pathogen damaging to tomato. Continuous use of resistant tomato cultivars and nematicides for its effective management might lead to resistance break-up or nematicide failure. Genetic variability and virulence in M. incognita on susceptible Pusa Ruby tomato were analysed by bioassay, esterase and DNA polymorphism after a 5 year weekly exposure to carbofuran, carbosulfan, cadusafos and triazophos at 0.0125, 0.0250 and 0.0500 microg g(-1). Virulence in M. incognita after a 5 year multiplication on resistant tomatoes was assessed.

RESULTS

The nematicidal treatments resulted in the development of virulent M. incognita populations. Their invasion potential increased significantly after continuous exposure to low concentrations of the nematicides. Also, growing resistant tomato cultivars for ten successive seasons resulted in a 6.6% increase in the invasion potential. These virulent populations exhibited 1-3 additional esterase and DNA bands compared with untreated populations.

CONCLUSION

A 5 year exposure of M. incognita to sublethal concentrations of nematicides or resistant tomato cultivars exerted enough selection pressure to cause genomic alterations for virulence development. Isozyme markers can be used for rapid and precise diagnostics of field populations by advisory services, enabling judicious remedial management decisions.

Comparative proteomic analysis of bacterial wilt susceptible and resistant tomato cultivars

To investigate the molecular mechanisms of bacterial resistance in susceptible and resistant cultivars of tomato, a proteomic approach was adopted. Four cultivars of tomato were selected on the basis of their response to bacterial (Pseudomonas solanacearum) inoculation wherein cultivar Roma and Riogarande, and cultivar Pusa Ruby and Pant Bahr were considered as resistant and susceptible cultivars, respectively. Proteins were extracted from leaves of 3-week-old seedlings of the four cultivars and separated by 2-DE. A total of nine proteins were found to be differentially expressed between the susceptible and resistant cultivars. Amino acid sequences of these proteins were determined with a protein sequencer. The identified proteins belongs to the categories of energy, protein destination and storage, and defense. Of these proteins, a 60kDa chaperonin and an apical membrane antigen were significantly upregulated in resistant cultivars compared with susceptible cultivars. Application of jasmonic acid and salicylic acid resulted in significant changes in levels of apical membrane antigen and protein disulfide-isomerase. Taken together, these results suggest that apical membrane antigen might be involved in bacterial resistance process through salicylic acid induced defense mechanism signaling in tomato plants.

Heat treatment in combination with antagonistic yeast reduces diseases and elicits the active defense responses in harvested cherry tomato fruit

This study investigated the effects of heat treatment (hot air at 38 degrees C) and antagonistic yeast (Pichia guilliermondii) alone or in combination against postharvest diseases (Botrytis cinerea, Alternaria alternata and Rhizopus nigricans) on cherry tomato fruit, and evaluated the elicitation of active defense responses. Results showed that heat treatment at 38 degrees C for 24 h in combination with P. guilliermondii at 1 x 10(8) CFU mL(-1) was the most effective approach to reduce various infections on cherry tomato fruit's wounds. Moreover, the combined heat and P. guilliermondii treatment stimulated a rapid increase of H(2)O(2) and higher lignin deposition in cherry tomato fruit showing that the oxidative burst and biological synthesis of lignin might play important roles in the fruit's active defense responses. In addition, the reduction of the fruit's susceptibility to pathogens by the combined treatment was positively correlated with higher activities of phenylalanine ammonia-lyase (PAL) and beta-1,3-glucanase in cherry tomato fruits, both of which are associated with plant defense responses.