Elicitation Of Resistance Research Articles

Minor effects of two elicitors of insect and pathogen resistance on volatile emissions and parasitism of Spodoptera frugiperda in Mexican maize fields

Synthetic elicitors can be used to induce resistance in plants against pathogens and arthropod herbivores. Such compounds may also change the emission of herbivore-induced plant volatiles, which serve as important cues for parasitic wasps to locate their hosts. Therefore, the use of elicitors in the field may affect biological control of insect pests. To test this, we treated maize seedlings growing in a subtropical field in Mexico with methyl jasmonate (MeJA), an elicitor of defense responses against many insects, and benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), an elicitor of resistance against certain pathogens. Volatile emission, herbivore infestation, pathogen infection, and plant performance (growth and grain yield) of treated and untreated maize plants were measured. Application of BTH slightly reduced volatile emission in maize, while MeJA increased the emission compared to control treatments. Despite the apparent changes in volatile emissions, the elicitor application did not consistently affect infestation by Spodoptera frugiperda larvae, the main insect pest found on the maize seedlings, and had only marginal effects on parasitism rates. Similarly, there were no treatment effects on infestation by other herbivores and pathogens. Results for the six replications that stretched over one summer and one winter season were highly variable, with parasitism rates and the species composition of the parasitoids differing significantly between seasons. This variability, as well as the severe biotic and abiotic stresses on young seedlings might explain why we measured only slight effects of elicitor application on pest incidence and biological control in this specific field study. Indeed, an additional field experiment under milder and more standardized conditions revealed that BTH induced significant resistance against Bipolaris maydis, a major pathogen in the experimental maize fields. Similar affects can be expected for herbivory and parasitism rates.

Involvement of salicylic acid in induction of nematode resistance in plants

The role of salicylic acid (SA) as a possible signaling component in the case of the infection of plants with nematodes has been studied using a model system consisting of the tomato (Lycopersicon esculentum (Mill.) and race 1 of the gall eelworm Meloidogyne incognita (Kofoid and White, 1919; Chitwood, 1949). The pre-planting SA treatment of tomato seeds results in an increased nematode resistance of susceptible tomato cultivars; the protective effect is higher in the case of SA combined with chitosan, a biogenic elicitor of plant resistance. The studied preparations stimulate the growth and development of the plants. The increase in the resistance of tomato plants is related to the increased activity of phenylalanine ammonia-lyase and an increased SA content in plant tissues infected with nematodes; both these factors significantly influence nematode development.

Possible trade-off associated with the use of a combination of resistance elicitors

A combination of three resistance elicitors, acibenzolar-S-methyl (ASM), β-aminobutyric acid (BABA) and cis-jasmone (CJ) was shown to control powdery mildew ( Blumeria graminis f.sp. hordei) and leaf scald (caused by Rhynchosporium secalis) in two spring barley varieties under field conditions. However, over the two consecutive years of field experiments, levels of Ramularia leaf spot (RLS), caused by Ramularia collo-cygni, an important, emerging pathogen of barley in temperate areas, were increased significantly in both varieties treated with the elicitor combination. Interestingly, treatment of leaves three and four of greenhouse-grown barley, led to up-regulation of the systemic acquired resistance (SAR) marker gene PR1-b in those leaves, but to a substantial down-regulation of LOX2, a gene involved in biosynthesis of jasmonic acid (JA). This suggests that the elicitor combination activates SAR in barley, but suppresses the JA signalling pathway. It is suggested that the increase in levels of RLS in field-grown barley represents a trade-off associated with the use of the elicitor combination, and might be related to the down-regulation of LOX2 observed in glasshouse-grown plants.

Antioxidant Profile and Polyphenol Oxidase Activities in Apple Leaves after Erwinia amylovora Infection and Pretreatment with a Benzothiadiazole‐type Resistance Inducer (BTH)

AbstractThis study focused on the biochemical effects of benzo‐(1,2,3)‐thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH), an active compound of the commercial preparation Bion, as an elicitor of resistance to fire blight (Erwinia amylovora) in apple. We determined activities of main antioxidant enzymes: ascorbate peroxidase (APX), catalase, glutathione peroxidase (GSH‐Px) and glutathione transferase (GST), enzymes associated with phenolic metabolism: phenylalanine ammonia‐lyase and polyphenol oxidases (PPOs), levels of low molecular antioxidants [ascorbate, glutathione, tocopherol (TOC)], phenolic acids and flavonoids as well as markers of oxidative processes: superoxide anion radical (O2·−) and thiobarbituric acid reactive substances in apple leaf tissues pretreated and non‐pretreated with BTH before inoculation with E. amylovora. The results indicate that successful invasion of pathogen might be associated with a significant decrease in the ascorbate pool in an early phase of plant–pathogen interaction. Generation of O2·− and changes in some antioxidant parameters (APX, GSH‐Px, GST, TOC) were less intensive and/or occurred earlier in the BTH‐pretreated apple leaves than in non‐pretreated ones. In the BTH‐pretreated group, PPOs activities were higher than in the control throughout the experiment, whereas in the non‐pretreated group, the increase started from the 7th day after inoculation. Infection strongly enhanced chlorogenic and o‐coumaric acids accumulation, and BTH weakened this effect. The opposite was observed with respect to phloretin and phloridzin.

BIOLOGICAL ELICITATION OF RESISTANCE AGAINST ANTHRACNOSE IN AUBERGINE

The information on the extent of postharvest losses of aubergine and the causes of these losses are meager. Fruit rot and wilt are the most serious diseases of aubergine in the tropics and among the most common postharvest diseases of the aubergine are fruit rots and anthracnose. Induction of natural disease resistance (NDR) in harvested horticultural crops using physical, biological and/or chemical elicitors has received increasing attention over recent years, it being considered a preferred strategy for disease management. This study reviews the enhancement of inducible antifungal compounds and suppression of anthracnose diseases in aubergine through bio-elicitors. The results revealed that the anthracnose, caused by Colletotrichum capsici, is one of the major postharvest diseases in aubergine and that pre-inoculation with Fusarium solani, found to be a weaker pathogen in aubergine, delayed the anthracnose disease development by 4 days. Thin layer chromatography bioassays showed that the pre-inoculation with F. solani resulted in greater phytoalexin accumulation and thus, F. solani appears to be an effective elicitor of host natural resistance in aubergine. Purification of the phytoalexin using flash chromatography showed that the compound was lubimin, a sesquiterpenoid of aubergine.

Sugarcane metabolites produced in CO2-rich temporary immersion bioreactors (TIBs) induce tomato (Solanum lycopersicum) resistance against bacterial wilt (Ralstonia solanacearum)

The production of phenolic metabolites has been optimized in parallel to sugarcane micropropagation in temporary immersion bioreactors (TIBs). Culturing micropropagated plants in 0.4% CO2-rich air induced their photosynthetic activity by enhancing the change from a heterotrophic to a photomixotrophic metabolic stage. Under 0.4% CO2 enrichment, the transcript levels of both phenylalanine ammonia-lyase (PAL EF189195) and ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco CF576750) increased and were correlated to a 4.6- and 6.3-fold increase in the phenolic levels when plants were multiplied in 20 or 30 g/l sucrose, respectively. A novel application of plant phenolic metabolites as elicitors of resistance to tomato bacterial wilt in the Solanum lycopersicum–Ralstonia solanacearum pathosystem has been identified. The culture media was collected, and the phenolics were sprayed onto tomato plants infected with R. solanacearum, eliciting and/or maintaining an early defense signaling mechanism that resulted in the protection of the plant against the tomato bacterial wilt disease. RT-PCR analyses confirmed that selected genes from defense-related pathways were differentially expressed between plants treated with sugarcane metabolites, non-treated pathogen-free plants, and non-treated plants inoculated with R. solanacearum.

Induction of Systemic Resistance in Plants by Biochar, a Soil-Applied Carbon Sequestering Agent

Biochar is the solid coproduct of biomass pyrolysis, a technique used for carbon-negative production of second-generation biofuels. The biochar can be applied as a soil amendment, where it permanently sequesters carbon from the atmosphere as well as improves soil tilth, nutrient retention, and crop productivity. In addition to its other benefits in soil, we found that soil-applied biochar induces systemic resistance to the foliar fungal pathogens Botrytis cinerea (gray mold) and Leveillula taurica (powdery mildew) on pepper and tomato and to the broad mite pest (Polyphagotarsonemus latus Banks) on pepper. Levels of 1 to 5% biochar in a soil and a coconut fiber-tuff potting medium were found to be significantly effective at suppressing both diseases in leaves of different ages. In long-term tests (105 days), pepper powdery mildew was significantly less severe in the biochar-treated plants than in the plants from the unamended controls although, during the final 25 days, the rate of disease development in the treatments and controls was similar. Possible biochar-related elicitors of systemic induced resistance are discussed.

Induction of resistance to root-knot nematodes by SAR elicitors in tomato

Different concentrations of the systemic acquired resistance (SAR) elicitors, salicylic acid (SA), methyl-salicylic acid (MetSA), acibenzolar-S-methyl (ASM) and 2,6-dicholoroisonicotinic acid (INA), were provided to tomato seedlings as root-dip or soil-drench one day before inoculation with the root-knot nematode, Meloidogyne incognita. Nematode infestation was evaluated by counting egg masses/plant (EM), eggs/plant (Pf), and sedentary forms/plant (SF) in treated and untreated plants seven weeks after inoculation. An index of plant fitness (PF) was also calculated to assess the costs of chemically-induced resistance and the possible phytotoxicity of the treatments. SA and ASM were found to be effective elicitors of resistance when applied at suitable concentrations and method of application. Soil-drench with SA and root-dip in ASM were the most effective treatments as they markedly reduced both nematode reproduction (less than 50% that of untreated plants) and infestation (50–70% EM reduction). MetSA was less effective than SA in eliciting resistance because of its negative effects on plant fitness when it was provided as soil-drench. INA did not reduce nematode infestation at any of the non-phytotoxic rates. The reduction of nematode infestation and reproduction by SA applied as soil-drench was potentiated when the soil was enriched with humic acids. Soil-drenching with SA and MetSA were the only treatments that caused a long-lasting induction of plant defences as they inhibited the infestation by the second generation of the nematode.

Activity of benzothiadiazole on chrysanthemum yellows phytoplasma (‘ Candidatus Phytoplasma asteris’) infection in daisy plants

A commercial preparation of the plant resistance elicitor benzothiadiazole (BTH) (Bion, Syngenta Crop Protection) was tested for its capacity to induce systemic resistance against chrysanthemum yellow phytoplasma (CYP) infection in the Chrysanthemum carinatum plant. Following one 2.4 mM BTH application, plants were exposed to CYP-infective Macrosteles quadripunctulatus leafhoppers. Symptom development and phytoplasma multiplication in the test plants were measured over time. BTH application delayed symptom development and phytoplasma multiplication in treated plants compared with the control ones. CYP titre and symptom severity were significantly lower for the first two weeks post-inoculation in treated plants compared with the control ones, suggesting that systemic acquired resistance (SAR) induced by BTH in C. carinatum is temporary. Higher concentrations of BTH resulted in phytotoxic effects involving the whitening of apical leaves. BTH application was ineffective in reducing the transmission efficiency of CYP by its leafhopper vector. Actually, in both single and group transmission tests, the proportion of infected plants was similar among BTH-treated and untreated plants. The survival of M. quadripunctulatus was unaffected by feeding on BTH-treated daisy plants. Moreover, when leafhoppers were allowed to choose between treated and untreated plants, they showed no preference. We conclude that SAR induced in daisy plants by BTH has no detrimental effects on the vector leafhopper. If the activity of BTH against phytoplasmas is confirmed also on other phytoplasma/host–plant associations, BTH applications might be included in new, more environmentally friendly, integrated management strategies of phytoplasmoses.

BTH-mediated antioxidant system responses in apple leaf tissues

BTH (S-methylbenzo-1,2,3-thiadiazole-7-carbothiate), an active compound of the commercial preparation Bion, has been studied as an elicitor of resistance to fire blight ( Erwinia amylovora) in apple. However, the biochemical mechanisms of its action are not fully elucidated. Our study indicated that BTH at the best time of its protection activity (2–14 days after application) induced changes in prooxidant–antioxidant balance in the leaves of apple trees, but in different ways in the enzymatic and nonenzymatic antioxidants. Glutathione as low molecular antioxidant as well as superoxide anion radical and lipid peroxides as oxidants exhibited changes at the early phase of BTH action. Glutathione-dependent enzymes were strongly affected by the elicitor used. On the 2nd day glutathione transferase (GST) and glutathione peroxidase (GSH-Px) activities increased by about 70% and 30% above the control, respectively. GST activity normalized about the 14th day but GSH-Px at the same time showed 27% of the control value. Among enzymes utilising hydrogen peroxide only catalase showed increase (37%) at the early phase of experiment. Compared with the control, BTH-treated plants did not show changes in ascorbate peroxidase and phenylalanine ammonia-lyase activities. Tocopherol (TOC) level diminished starting from the 7th day after BTH treatment and on the 14th day it was only 28% of the control. It is proposed that extinguishing of BTH-mediated signal resulted from TOC and glutathione action. The diminished ascorbate level at all examined times may play a crucial role in BTH-mediated cell growth regulation. The direct influence of BTH on lipid metabolism should be also taken into consideration.

Indução de resistência do mamoeiro à podridão radicular por indutores bióticos e abióticos

O objetivo deste trabalho foi avaliar o potencial do uso de indutores de resistência bióticos e abióticos na redução da podridão radicular em mamoeiro. Mudas de mamoeiro foram pulverizadas com os fungicidas fosetil-Al, metalaxil e Mancozeb (2 g L-1), com os indutores abióticos fosfito de potássio (2,5 e 5 mL L-1), ácido salicílico 0,15 e 0,30%, Reforce (indutor comercial) + ácido salicílico a 5%, acibenzolar-S-metil (ASM) (0,15 e 0,30 g L-1), e com o indutor biótico Saccharomyces cerevisiae (3 e 6 mL L-1), três e seis dias antes da pulverização de 1 mL de suspensão de 10(5) zoósporos mL-1 de Phytophthora palmivora. Todos os tratamentos tiveram efeito no controle da podridão de raízes em relação à testemunha, com exceção do Reforce + ácido salicílico a 5% (3 mL L-1), seis dias antes da inoculação. Os tratamentos com ASM, com exceção da dosagem 0,15 g L-1 seis dias antes da inoculação, apresentaram resultados similares aos dos fungicidas metalaxil e Mancozeb. Plantas pulverizadas com ASM apresentaram aumento de atividade da peroxidase e beta-1,3-glucanase e maior concentração de lignina que a testemunha. No entanto, esses tratamentos não tiverem efeito sobre a atividade da quitinase. O ASM é um potencial indutor de resistência a P. palmivora em mamoeiro.

The type III effectors of Xanthomonas

A review of type III effectors (T3 effectors) from strains of Xanthomonas reveals a growing list of candidate and known effectors based on functional assays and sequence and structural similarity searches of genomic data. We propose that the effectors and suspected effectors should be distributed into 39 so-called Xop groups reflecting sequence similarity. Some groups have structural motifs for putative enzymatic functions, and recent studies have provided considerable insight into the interaction with host factors in their function as mediators of virulence and elicitors of resistance for a few specific T3 effectors. Many groups are related to T3 effectors of plant and animal pathogenic bacteria, and several groups appear to have been exploited primarily by Xanthomonas species based on available data. At the same time, a relatively large number of candidate effectors remain to be examined in more detail with regard to their function within host cells.

Strain-Specific Cylindrical Inclusion Protein of Soybean mosaic virus Elicits Extreme Resistance and a Lethal Systemic Hypersensitive Response in Two Resistant Soybean Cultivars

In the Soybean mosaic virus (SMV)-soybean pathosystem, three independent genes (Rsv1, Rsv3, and Rsv4) conferring resistance to SMV have been identified. Recently, we constructed infectious cDNA clones of SMV G7H and G5H strains and found that these two strains differ in their ability to infect soybean genotypes possessing different SMV resistance genes despite a difference of only 33 amino acids. In particular, pSMV-G7H induced mosaic symptoms systemically in L29 (Rsv3) and provoked a lethal systemic hypersensitive response (LSHR) in Jinpumkong-2, whereas pSMV-G5H could not infect these soybean genotypes. To identify the responsible pathogenic determinants of SMV, we exploited the differential responses of pSMV-G7H- and pSMV-G5H-derived chimeric viruses and amino acid substitution mutant viruses in several soybean genotypes and demonstrated that cylindrical inclusion (CI) protein is the elicitor of Rsv3-mediated extreme resistance and a pathogenic determinant provoking LSHR in Jinpumkong-2. A single amino acid substitution in CI was found to be responsible for gain or loss of elicitor function of CI. Our finding provides a role for CI as a pathogenic determinant in the SMV-soybean pathosystem, and increases the understanding of the basis of the different disease responses of SMV strains.

L-Alanine augments rhizobacteria-induced systemic resistance in cucumber

Bacillus vallismortis strain EXTN-1 is a proven biotic elicitor of systemic resistance in many crops against various pathogens. L: -Alanine (Ala) was tested in cucumber as a chemical elicitor of induced systemic resistance (ISR) against Colletotrichum orbiculare. In the greenhouse, both Ala and EXTN-1 induced significant levels of disease suppression in cucumber against anthracnose. When cucumber plants were treated with EXTN-1 and Ala together, augmentative disease suppression was observed. Experiments with transgenic tobacco plants carrying pathogenesis-related genes fused with the beta-glucuronidase (GUS) reported gene (PR-1a::GUS & PDF 1.2::GUS) showed an enhanced activation of both PR-1a and PDF 1.2 genes upon combined treatment with Ala and EXTN-1. RT-PCR analysis with transgenic (PR-1a or PDF 1.2 over expressing) Arabidopsis plant showed more enhanced expression of resistance genes PR-1a and PDF 1.2 upon combined treatment with Ala and EXTN-1 than either alone. An augmentative ISR effect, when the bacterial elicitor and chemical elicitor were combined together, was confirmed.

Antimicrobial property of chitosan and induction of systemic acquired resistance for the control of rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Swings et al.)

Chitosan has been reported to have antimicrobial property to some pathogen species as well as an elicitor of resistance in plants , particularly Systemic Acquired Resistance (SAR). A bioassay of chitosan against Xanthomonas oryzae pv. oryzae (Xoo) was conducted to determine its antimicrobial property against bacterial blight pathogen and the optimum concentration that is most inhibitory to the pathogen. Chitosan was also tested as foliar spray to rice plants to evaluate its potential to induce SAR against bacterial blight disease. Chitosan treatments (300 ppm, 400 ppm, and 500 ppm) possessed antimicrobial property against Xoo in vitro, producing zones of inhibition which were generally significantly bigger than those of the control (streptomycin, acetic acid, and sterile distilled water) at 2, 4, and 6 days after inoculation. Chitosan-sprayed plants showed significantly shorter bacterial blight lesions which were comparable to the plants sprayed with streptomycin, and Boost, a commercial plant defense activator and a known inducer of SAR. Chitosan is found effective in reducing bacterial blight lesions in rice plants due to its antimicrobial property and also most likely due to the induction of SAR.

USE OF BENZOTHIADIAZOLE (BTH) FOR INDUCING SYSTEMIC RESISTANCE IN COTTON SEEDLINGS AGAINST SOME SOIL-BORNE PATHOGENIC FUNGI

The resistance elicitor Benzothiadiazole (BTH) has no significant inhibitory effects on seed germination of cotton cultivars Giza 86 and Giza 90 or on mycelial growth of the tested fungi, i.e. Fusarium solani, F. oxysporum, F. moniliforme, Sclerotium rolfsii, Macrophomina phaseolina, Pythium sp. and Rhizoctonia solani. BTH seed treatment (seed soaking) resulted in resistant cotton plants against infection by all pathogenic fungi tested. Efficiency of BTH to induce resistance in cotton seedlings varied with BTH concentration and fungi tested. Increasing BTH concentration enhanced resistance capacity of cotton cultivars. In addition. When the concentration was increased to 100 ppm, BTH significantly increased plant height from 8.36 to 26.07 % and from 8.93 to 28.82 % for cvs. Giza 86 and Giza 90, respectively. Also, it increased dry weight from 2.87 to 53.26 % and from 9.85 to 31.16 % for cvs. Giza 86 and Giza 90, respectively. The highest efficiency value (60%) was achieved by BTH against F. oxysporum on Giza 86 and F. moniliforme on Giza 90 while the least efficiency was observed by BTH seed treated against Rhizoctonia solani infection. Since 50 ppm of BTH seed treatment caused 18.18 and 21.49% protection in cotton seedlings of cvs. Giza 86 and Giza 90, respectively. Using 100ppm of BTH gave cotton seedlings with 31.81 and 25.62 % protection for Giza 86 and Giza 90, respectively.

Identification of Puccinia pimpinellae on Anise Plant in Egypt and Its Control

An emerging problem for the wider adoption of anise plantation in Egypt is the damage caused by the rust fungus. The detailed description and taxonomic studies (using light and scanning electron microscopy) show that such an obligate parasite fungus (Puccinia pimpinellae) is autoecious microcyclic (uredinial-telial stage only). Among tested Apiaceae plants, the host range test proved the specificity of the rust fungus to anise. To the researcher's knowledge, this is the first investigated record of a rust fungus on Pimpinella anisum plants in Egypt. The effectiveness of some plant resistance elicitors and two active chitinase producers; Bacillus subtilis Bio4 and isolated Trichoderma harizianum CH4 (both of them recorded the highest clear zone/colony size ratio on chitin agar plates) in controlling anise rust disease and on growth and yield of anise were evaluated in two successive growing seasons. Spraying chitosan at 1000 ppm was the most potent in reducing Disease Severity (DS) and Incidence (DI) as well as improving plant height, chlorophyll content, inflorescence No. plant-1 (74.2 and 76), 1000-fruit weight (2.94 and 2.83 g) and anise yield (646.8 and 670.0 kg fed-1), during both seasons. B. subtilis Bio4 and T. harizianum CH4 showed moderate effect on the tested parameters.

Induce Systemic Resistance in Lupine Against Root Rot Diseases

Root rot caused by soil borne pathogenic fungi is the most sever disease attacks lupine plants. Isolation trials from diseased plants in some areas of Dakahlia Province (Egypt) was carried out. Rhizoctonia solani and Fusarium solani proved to be the most dominant isolates. Meanwhile, Fusarium oxysporum and Sclerotium rolfsii were less frequent. Efficacies of some plant resistance elicitors viz.: chitosan (CHI), Salicylic Acid (SA) and hydroquinone (HQ) in comparing to the fungicide Rhizolex T-50 as seed treatments showed significant reduction in the fungal growth in vitro. Chitosan at 8 g L(-1) and fungicide completely inhibited the growth of all isolated fungi, while SA at 1.4 g L(-1) and HQ at 1.2 g L(-1) inhibited the growth of Fusarium solani and F. oxysporum, respectively. The greenhouse experiments showed that S. rolfesii (No. 6) and R. solani (No. 2) followed by F. solani (No. 5) and F. oxysporum (No. 9) were the most aggressive root rot fungi. Soaking susceptible lupine seeds (Giza 1) in each one of the three selected elicitors showed a significant reduction in seedlings mortality. CHI at 8 g L(-1) was superior in increasing the percentage of healthy plants to record 72.5, 80.9, 62.7and 64.3%, when seeds were grown in soil infested with of F. solani, F. oxysporum, R. solani and S. rolfesii, respectively. These results were confirmed under field conditions in two different locations i.e., Tag El-Ezz and El-Serow Research Stations. CHI 8 g L(-1) proved to be the best elicitor after fungicide, in reducing lupine root rot disease. It showed 41 and 60% reduction in the plants mortality comparing to 56.37 and 69.13% in case of Rhizolex-T in Tag El-Ezz and El-Serow locations, respectively. The treatments were accompanied with a significant increase in lupine growth parameters, yield components and physiological aspects. Application of CHI at 8 g L(-1) or HQ at 1.2 g L(-1) was the most potent in this respect as compared to check treatment.

SRFR1, a suppressor of effector‐triggered immunity, encodes a conserved tetratricopeptide repeat protein with similarity to transcriptional repressors

Effector-triggered immunity provides plants with strong protection from pathogens. However, this response has the potential to be highly deleterious to the host and needs to be tightly controlled. The molecular mechanisms in the plant that regulate the balance between activation and suppression of resistance are not fully understood. Previously, we identified Arabidopsis suppressor of rps4-RLD 1 (srfr1) mutants with enhanced resistance to the bacterial effector AvrRps4. These mutants were recessive and retained full susceptibility to virulent bacteria, suggesting that SRFR1 functions as a negative regulator and that AvrRps4-triggered immunity was specifically enhanced in the mutants. Consistent with this, we show here that the response to flagellin, an elicitor of basal resistance, is unaltered in srfr1-1. In contrast, resistance to AvrRps4 in srfr1-1 requires EDS1, a central regulator of effector-triggered immunity via multiple resistance genes. SRFR1 is a single-copy gene encoding a pioneer tetratricopeptide repeat protein conserved between plants and animals. The SRFR1 tetratricopeptide repeat domain shows sequence similarity to those of transcriptional repressors in Saccharomyces cerevisiae and Caenorhabditis elegans. Indeed, a sub-pool of SRFR1 transiently expressed in Nicotiana benthamiana leaf cells localizes to the nucleus. Identification of SRFR1 may therefore provide insight into the regulation of the transcriptional reprogramming that is activated by effector-triggered immunity.

Early Responses of Tobacco Suspension Cells to Rhizobacterial Elicitors of Induced Systemic Resistance

Colonization of roots by selected strains of fluorescent Pseudomonas spp. can trigger induced systemic resistance (ISR) against foliar pathogens in a plant species-specific manner. It has been suggested that early responses in cell suspension cultures in response to rhizobacterial elicitors, such as generation of active oxygen species (AOS) and extracellular medium alkalinization (MA), are linked to the development of ISR in whole plants. Perception of flagellin was demonstrated to elicit ISR in Arabidopsis, and bacterial lipopolysaccharides (LPS) have been shown to elicit several defense responses and to act as bacterial determinants of ISR in various plant species. In the present study, the LPS-containing cell walls, the pyoverdine siderophores, and the flagella of Pseudomonas putida WCS358, P. fluorescens WCS374, and P. fluorescens WCS417, which are all known to act as elicitors of ISR in selected plant species, were tested for their effects on the production of AOS, MA, elevation of cytoplasmic Ca(2+) ([Ca(2+)](cyt)), and defense-related gene expression in tobacco suspension cells. The LPS of all three strains, the siderophore of WCS374, and the flagella of WCS358 induced a single, transient, early burst of AOS, whereas the siderophores of WCS358 and WCS417 and the flagella of WCS374 and WCS417 did not. None of the compounds caused cell death. Once stimulated by the active compounds, the cells became refractory to further stimulation by any of the active elicitors, but not to the elicitor cryptogein from the oomycete Phytophthora cryptogea, indicating that signaling upon perception of the different rhizobacterial compounds rapidly converges into a common response pathway. Of all compounds tested, only the siderophores of WCS358 and WCS417 did not induce MA; the flagella of WCS374 and WCS417, although not active as elicitors of AOS, did induce MA. These results were corroborated by using preparations from relevant bacterial mutants. The active rhizobacterial elicitors led to a rapid increase in [Ca(2+)](cyt), peaking at 6 min, whereas the inactive siderophores of WCS358 and WCS417 elicited a single spike at 1 min. Elicitation of the cells by cell-wall LPS of WCS358 or the siderophore of WCS374 induced a weak, transient expression of several defense-related genes, including PAL and GST. The spectrum of early responses of the suspension cells was not matched by the expression of ISR in whole tobacco plants against Erwinia carotovora pv. carotovora. Of the live bacterial strains, only WCS358 elicited significant ISR, but application of the LPS or the siderophore of all three strains also elicited ISR. Notably, the absence of elicitation of AOS and MA in suspension-cultured cells but induction of ISR in whole plants by the siderophore of WCS358, which was lost upon treatment with the siderophore-minus mutant of WCS358, indicates that the early responses in suspension cells are not predictive of the ability to induce ISR in whole plants. Possible explanations for these discrepancies are discussed.