Systemic Resistance In Cucumber Research Articles

Induction of Systemic Resistance in Cucumber by Hypovirulent Binucleate Rhizoctonia against Anthracnose Caused by Colletotrichum orbiculare.

Treatment with hypovirulent binucleate Rhizoctonia (HBNR) isolates induced systemic resistance against anthracnose infected by Colletotrichum orbiculare in cucumber, as there were no direct interaction between HBNR and C. orbiculare. This is because of the different distances between HBNR and C. orbiculare, where the root was treated with HBNR isolate and C. orbiculare was challenged and inoculated in leaves or first true leaves were treated with HBNR isolate and C. orbiculare was challenged and inoculated in second true leaves. The use of barley grain inocula and culture filtrates of HBNR significantly reduced the lesion diameter compared to the control (p = 0.05). The total lesion diameter reduction by applying barley grain inoculum of HBNR L2, W1, W7, and Rhv7 was 28%, 44%, 39%, and 40%, respectively. Similar results was also observed in treatment using culture filtrate, and the reduction of total lesion diameter by culture filtrate of HBNR L2, W1, W7, and Rhv7 was 45%, 46%, 42%, and 48%, respectively. When cucumber root was treated with culture filtrates of HBNR, the lignin was enhanced at the pathogen penetration, which is spread along the epidermis tissue of cucumber hypocotyls. Peroxidase activity in hypocotyls in the treated cucumber plant with culture filtrates of HBNR significantly increased before and after inoculation of pathogens as compared to the control. Significant enhancement was also observed in the fast-moving anodic peroxidase isozymes in the treated plants with culture filtrates of HBNR. The results showed the elicitor(s) contained in culture filtrates in HBNR. The lignin deposition as well as the peroxidase activity is an important step to prevent systemically immunised plants from pathogen infection.

Induced systemic resistance in cucumber and Arabidopsis thaliana by the combination of Trichoderma harzianum Tr6 and Pseudomonas sp. Ps14

Trichoderma species and fluorescent Pseudomonas spp. have been reported to induce systemic resistance in plants. In this study the effectiveness of a combination of these biological control agents on the efficacy of induced resistance was investigated in cucumber and the model plant Arabidopsis thaliana. Trichoderma harzianum Tr6, and Pseudomonas sp. Ps14, both isolated from the rhizosphere of cucumber, were tested as a single application and in combination for their abilities to elicit induced resistance in cucumber against Fusarium oxysporum f. sp. radicis cucumerinum and in A. thaliana against Botrytis cinerea. The combination of Tr6 and Ps14 induced a significantly higher level of resistance in cucumber, which was associated with the primed expression of a set of defense-related genes upon challenge with Fusarium. In Arabidopsis both Ps14 and Tr6 triggered ISR against B. cinerea but their combination did not show enhanced effects. In the induced systemic resistance-defective Arabidopsis mutant myb72, none of the treatments protected against B. cinerea, whereas in the SA-impaired mutant sid2 all treatments were effective. Taken together, these results indicate that in Arabidopsis Ps14 and Tr6 activate the same signaling pathway and thus have no enhanced effect in combination. The enhanced protection in cucumber by the combination is most likely due to activation of different signaling pathways by the two biocontrol agents.

Induction of plant defense responses by Ocimum gratissimum L. (Lamiaceae) leaf extracts

Aqueous extracts of the leaves of Ocimum gratissimum at 10, 25, 40 and 50% (w/v) concentrations induced the production of phytoalexins in soybean cotyledons and sorghum mesocotyls. The aqueous extracts also induced systemic resistance in cucumber to Colletotrichum lagenarium, reflected by reduction in disease incidence and an increase in chitinase production. Modes of action and the existence of possible elicitors of defense response in O. gratissimum leaf extracts are discussed.

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.

Proteomics Approach for Investigating the Disease Resistance Using Cucumber as Model Plant

The profile of proteins induced in cucumber leaves pretreated with Acibenzolar-S-Methyl (ASM) and challenge inoculated with Colletotrichum orbiculare are presented. ASM was applied to the first leaf and the third leaf collected at different time intervals of 0-48 h and the total protein extracted using trichloroacetic acid acetone was analyzed by two-dimensional gel electrophoresis. Increased expressions of pyridoxine biosynthesis protein, carbonic anhydrase, proteasome subunit (beta type 1), phosphomannomutase and ascorbate peroxidase proteins were noticed in cucumber plants systemically induced resistance by ASM. All the above proteins are well known to play a major role in scavenging of active oxygen species and preventing the cell from oxidative damage. Our results suggest that production and scavenging of ROS are one of the major events that occur during ASM induced systemic resistance in cucumber.

Interactions Between Plant Growth Promoting Fungi and Arbuscular Mycorrhizal Fungus Glomus Mosseae and Induction of Systemic Resistance to Anthracnose Disease in Cucumber

Cucumber plants were treated with plant growth promoting fungi (PGPF), Phoma sp. (isolates GS8-2 and GS8-3) and Penicillium simplicissimum (isolate GP17-2) with or without the arbuscular mycorrhizal fungus (AMF) Glomus mosseae. Induction of systemic resistance in cucumber against the anthracnose disease caused by Colletotrichum orbiculare was tested to evaluate the nature of the interaction between the PGPF and AMF. Root colonizing ability of each fungal species as influenced by their interaction was also evaluated. Plant roots were pre-inoculated with each PGPF isolate and/or G. mosseae for four weeks and leaves were then challenge inoculated with the pathogen C. orbiculare. Plants treated with each PGPF isolate showed considerable protection against the disease, but the treatment of G. mosseae had no significant effect on disease development. However, combined inoculation of Phoma GS8-2 or GS8-3 with G. mosseae reduced the level of disease protection induced by single inoculation of each Phoma isolate. In contrast, the high levels of protection induced by the P. simplicissimum GP17-2 were not altered by combining it with G. mosseae. Root colonization of both Phoma sp. isolates was also suppressed by the presence of the G. mosseae, but such an effect was not found on the population development of P. simplicissimum. The percent cucumber root length colonized by G. mosseae was not affected by any of the PGPF isolates tested.

Trichoderma Mitogen-Activated Protein Kinase Signaling Is Involved in Induction of Plant Systemic Resistance

The role of a mitogen-activated protein kinase (MAPK) TmkA in inducing systemic resistance in cucumber against the bacterial pathogen Pseudomonas syringae pv. lacrymans was investigated by using tmkA loss-of-function mutants of Trichoderma virens. In an assay where Trichoderma spores were germinated in proximity to cucumber roots, the mutants were able to colonize the plant roots as effectively as the wild-type strain but failed to induce full systemic resistance against the leaf pathogen. Interactions with the plant roots enhanced the level of tmkA transcript in T. virens and its homologue in Trichoderma asperellum. At the protein level, we could detect the activation of two forms reacting to the phospho-p44/42 MAPK antibody. Biocontrol experiments demonstrated that the tmkA mutants retain their biocontrol potential against Rhizoctonia solani in soil but are not effective against Sclerotium rolfsii in reducing disease incidence. Our results show that, unlike in many plant-pathogen interactions, Trichoderma TmkA MAPK is not involved in limited root colonization. Trichoderma, however, needs MAPK signaling in order to induce full systemic resistance in the plant.

Gene expression analysis in cucumber leaves primed by root colonization with Pseudomonas chlororaphis O6 upon challenge-inoculation with Corynespora cassiicola.

Root colonization by Pseudomonas chlororaphis O6, a non-pathogenic rhizobacterium, induced systemic resistance in cucumber against target leaf spot caused by Corynespora cassiicola. A cDNA library was constructed using mRNA extracted from cucumber leaves 12 h after inoculation with C. cassiicola, using plants colonized by O6. To identify genes involved in O6-mediated induced systemic resistance (ISR), we employed a subtractive hybridization method using mRNAs extracted from pathogen-challenged cucumber leaves of plants lacking colonization. Differential screening of the cDNA library led to the isolation of six distinct genes encoding a GTP binding protein, a 60S ribosomal protein, a hypersensitive-induced reaction protein, a ubiquitin extension protein, a pyridine nucleotide-disulfide oxidoreductase, and a signal recognition particle receptor. Expression of these genes was not induced by O6 colonization alone. Rather, transcript accumulation of these genes increased significantly faster and stronger in the O6 colonized than in non-colonized plants after challenge infection. Therefore, O6-mediated ISR may be associated with an enhanced capacity for the rapid and effective activation of cellular defence responses after challenge inoculation.

Induction of Systemic Resistance in Cucumber against Several Diseases by Plant Growth-promoting Fungi: lignification and Superoxide Generation

Five fungal isolates (Trichoderma, Fusarium, Penicillium, Phoma and a sterile fungus) from zoysiagrass rhizosphere that promote plant growth were tested for their ability to induce systemic resistance in cucumber plants against Colletotrichum orbiculare. Roots of cucumber plants were treated with these fungal isolates using barley grain inocula (BGI), mycelial inocula (MI) or culture filtrate (CF). Most isolate/inoculum form combinations significantly reduced the disease except BGI of Trichoderma. These fungal isolates were also evaluated for induction of systemic resistance against bacterial angular leaf spot and Fusarium wilt by treatment with BGI. Penicillium, Phoma and the sterile fungus significantly reduced the disease incidence of bacterial angular leaf spot. Phoma and sterile fungus protected plants significantly against Fusarium wilt. Roots treated with CFs of these fungal isolates induced lignification at Colletotrichum penetration points indicating the presence of an elicitor in the CFs. The elicitor activity of CFs was evaluated by the chemiluminescence assay using tobacco callus and cucumber fruit disks. The CFs of all isolates elicited conspicuous superoxide generation. The chemiluminescence activity of the CF of Penicillium was extremely high, and its intensity was almost 100-fold higher than that of other isolates. The chemiluminescence activity was not lost following treatment with protease or autoclaving or after removal of lipid. The MW 12,000 dialyzed CF fraction was highly effective in eliciting chemiluminescence activity. Chemiluminescence emission from cucumber fruit disks treated with Penicillium was the same as that obtained from tobacco callus, except that the lipid fraction also showed a high activity. Both the MW 12,000 fraction and the lipid fraction induced lignification in the epidermal tissues of cucumber hypocotyls.

Biocontrol of Cucumber Diseases in the Field by Plant Growth-Promoting Rhizobacteria With and Without Methyl Bromide Fumigation.

Field trials were conducted in 1996 and 1997 to determine the effect of plant growth-promoting rhizobacteria (PGPR) strains, which previously were found to induce systemic resistance in cucumber, on cucumber plant growth and on naturally occurring cucumber diseases with and without methyl bromide fumigation. Seven PGPR seed treatments included single-strain treatments and mixtures of Bacillus pumilus strain INR7, Curtobacterium flaccumfaciens strain ME1, and Bacillus subtilis strain GB03. In both years, in the absence of methyl bromide, all seven PGPR treatments significantly promoted plant growth, compared to the non-treated control, while with methyl bromide fumigation, only 3 and 1 of the same PGPR treatments promoted growth significantly in 1996 and 1997, respectively. In 1996, main runner length of plants in all seven PGPR treatments without fumigation was statistically equivalent to the main runner length of the nontreated control with methyl bromide fumigation. Naturally occurring foliar diseases were angular leaf spot, caused by Pseudomonas syringae pv. lachrymans in 1996, and a mixed infestation of angular leaf spot and anthracnose, caused by Colletotrichum orbiculare in 1997. In both years, all PGPR treatments significantly reduced severity of foliar disease, compared to the nontreated control, with and without methyl bromide. Mixtures of PGPR strains showed a higher level of disease protection in both years with and without methyl bromide. The results indicate that attempts to develop PGPR-mediated induced systemic resistance into components of vegetable integrated pest management should not be negatively impacted by the planned withdraw of MeBr from standard vegetable production and that PGPR may help compensate for reduced plant growth often seen without methyl bromide fumigation.

Salicylic Acid Produced by Serratia marcescens 90-166 Is Not the Primary Determinant of Induced Systemic Resistance in Cucumber or Tobacco

The rhizobacterial strain Serratia marcescens 90–66 mediates induced systemic resistance (ISR) to fungal, bacterial, and viral pathogens. It was determined that strain 90–166 produced salicylic acid (SA), using the salicylateresponsive reporter plasmid pUTK21. High-pressure liquid chromatography analysis of culture extracts confirmedthe production of SA in broth culture. Mini-Tn5phoA mutants, which did not produce detectable amounts of SA, retained ISR activity in cucumber against the fungal pathogen Colletotrichum orbiculare. Strain 90–166 induced disease resistance to Pseudomonas syringae pv. tabaci in wild-type Xanthi-nc and transgenic NahG-10 tobacco expressing salicylate hydroxylase. Increasing ferric iron concentrations in vitro reduced SA production below detectable limits, and increasing ferric iron concentration in planta, applied as a root drench, significantly reduced the level of ISR observed in cucumber to C. orbiculare. An ISR¯ mutant (90-166-2882) still produced SA. The results of this study indicate that SA produced by 90–166 is not the primary bacterial determinant of ISR and that this bacterial-mediated ISR system is affected by iron concentration.

Induction of Systemic Resistance in Cucumber Against Cucumber Beetles (Coleoptera: Chrysomelidae) by Plant Growth-Promoting Rhizobacteria

Field studies were conducted in 1993 and 1994 to evaluate the effects of induced resistance in cucumber by plant growth-promoting rhizobacteria (PGPR) on numbers of the spotted cucumber beetle, Diabrotica undecimpunctata howardi Barber, and the striped cucumber beetle, Acalymma vittatum (F.). Cucumber plant growth and yields were significantly ( P <0.05) greater, and populations of cucumber beetles were significantly lower, on PGPR-treated cucumber than on nontreated cucumber. On dates when peak beetle populations were present, PGPR treatment resulted in significantly( P <0.05) greater cucumber beetle control than weekly applications of esfenvalerate insecticide. In no-choice greenhouse cage e:periments with 3 cucumber cultivars, beetles infected with the cucurbit wilt pathogen, Erwinia tracheiplila, were released and allowed to feed on PGPR-treated or nontreated cucumber plants. The incidence of cucurbit wilt disease was significantly( P <0.05) lower on PGPR-treated cucumber plants than on nontreated plants. These results indicate that PGPR-induced resistance may be more effective than insecticides for control of cucumber beetles and cucurbit wilt disease on cucumber. Possible mechanisms for PGPR-induced resistance against cucumber beetles are discussed.

Possible involvement of salicylic acid in systemic acquired resistance ofCucumis sativus againstSphaerotheca fuliginea

The possible involvement of salicylic acid in systemic acquired resistance ofCucumis sativus againstSphaerotheca fuliginea was studied. Cucumber plants were inoculated with tobacco necrosis virus on the cotyledons and the level of endogenous salicylic acid in the first true leaf was determined by gas chromatography. Salicylic acid increased continously from the second day after virus inoculation to the fifth day, when the same leaf was inoculated withSphaerotheca fuliginea. In healthy plants, the efficiency of exogenous salicylic acid in inducing resistance was assayed by applying aqueous solutions at different times beforeSphaerotheca fuliginea inoculation. To evaluate the level of induced resistance, the following parameters were examined by light microscopy: percentage of conidial germination, length of the hyphae derived from single conidia, number of haustoria, percentage of epidermal cells with lignified walls and of necrotic cells underlying fungal hyphae. In treated plants conidial germination was reduced, the total length of the hyphae was shorter, the number of haustoria was lower and the haustorium-containing epidermal cells had more frequently lignified walls. Moreover, an evident increase in callose deposition was observed leading to the formation of oversized papillae around the penetration pegs. These results indicate that the application of salicylic acid before inoculation withSphaerotheca fuliginea reduces the intensity of the infectious process and that salicylic acid is involved in the expression of systemic resistance in cucumber challenged by the biotrophic pathogenSphaerotheca fuliginea.

Induced Systemic Resistance in Cucumber and Tomato Against Cucumber Mosaic Cucumovirus Using Plant Growth-Promoting Rhizobacteria (PGPR)

Plant growth-promoting rhizobacteria (PGPR) strains 89B-27 (Pseudomonas fluorescens) and 90-166 (Serratia marcescens), which previously demonstrated induced systemic resistance in cucumber against some fungal and bacterial diseases, were tested for their capacity to protect Cucumis sativus L. cv. Straight 8 from disease development of cucumber mosaic cucumovirus (CMV). Seed treatment with both PGPR strains significantly and consistently reduced mean numbers of symptomatic plants when CMV was inoculated onto cotyledons. Plants treated with PGPR did not develop initial symptoms 14 days after CMV inoculation and remained symptomless throughout the experimental period. In a comprehensive study with cucumber, no viral antigen could be detected by enzyme-linked immunosorbent assay (ELISA) in any asymptomatic PGPR-treated plants ; whereas CMV was evident in every leaf of symptomatic plants. The same two PGPR strains were evaluated for effects on CMV symptom development in tomato in three experiments by measuring the disease severity at six observation dates. In all experiments, the area under the disease progress curve (AUDPC) was significantly lower with strain 89B-27 than in the nonbacterized control. The AUDPC with strain 90-166 was also significantly lower than with strain 89B-27. These results suggest that PGPR should be further evaluated for their potential to contribute toward management of viral plant diseases.

Persistence of induced systemic resistance in cucumber in relation to root colonization by plant growth promoting fungal isolates

Systemic resistance against Colletotrichum orbiculare was induced in cucumber plants by amending potting medium with the barley grain inocula of plant growth promoting fungal (PGPF) isolates obtained from zoysiagrass rhizosphere. Out of 16 isolates tested, 10 protected plants against a spore concentration of 10 4 spores ml −1 of C. orbiculare. However, as the spore concentration of the pathogen was increased, the protection afforded by PGPF isolates decreased. Among those isolates, five protected plants even at a high pathogen spore concentration of 10 6 spores ml −1. Three, among these five isolates, colonized roots; GS8-3 colonized roots to the maximum extent, while GS8-1 and GS8-2 were moderate and poor colonizers, respectively. These three root colonizers, along with one root non-colonizer (GU21-2), were tested for their induction ability over a period of 9 weeks. All four isolates protected plants but their ability to initiate resistance varied. Root colonization by GS8-1 and GS8-3 correlated with the protection offered. Their failure to induce resistance when provided as autoclaved barley grain inocula suggests that the initiation of resistance was attributed to invasion of roots. The other two isolates, on the other hand, induced systemic resistance to some extent when provided as autoclaved inocula suggesting that thermostable factors might be responsible. The four select isolates enhanced growth by increasing plant height and biomass significantly, which was quite evident after 5 weeks of growth. The plants associated with GS8-1 and GS8-2 were induced to produce a significant increase in the number of fruits. The dual nature of these soil-borne sterile fungi as inducers of resistance and as growth promoters over prolonged durations are discussed.

Induction of Systemic Resistance in Cucumber Against Bacterial Angular Leaf Spot by Plant Growth-Promoting Rhizobacteria

Two strains of plant growth-promoting rhizobacteria (PGPR) Pseudomonas putida 89B-27 and Serratia marcescens 90-166, that previously induced systemic resistance (ISR) in cucumber against cucumber anthracnose induced resistance against bacterial angular leaf spot caused by Pseudomonas syringae pv, lachrymans. Treatment of seeds or cotyledons with both strains resulted in significant decreases in lesion number and size compared to those of a noninduced disease control. Pathogen populations (log CFU per cm 2 of leaf) in inoculated leaves also were significantly decreased with PGPR treatments, declining from log 8.0 CFU per cm 2 in noninduced controls to log 5.5 CFU per cm 2 with strain 89B-27 and log 6.2 CFU per cm 2 with strain 90-166 when cotyledon injections were used. There was no significant difference in the level of induced resistance between seed inoculation and cotyledon injection with both PGPR strains. Classic ISR treatments (either induced by introducing Colletotrichum orbiculare or P. syringae pv. lachrymans on the first true leaves) also significantly decreased lesion number and size and population densities of the pathogen in inoculated leaves compared to those of the noninduced disease control. Neither lesion size nor pathogen population densities was significantly different between PGPR-mediated ISR and classic ISR. Local necrotic lesions on cotyledons were observed 5 days after inoculation with the pathogen, and necrosis was required in the classic ISR system. However, no visible lesions developed on cotyledons after injections of PGPR. Populations of strains 89B-27 and 90-166 increased rapidly in the cotyledons after injection and were maintained above log 9 CFU per cotyledon up to 14 days after inoculation. The results indicate that PGPR-mediated ISR is similar to classic ISR in that multiple pathogens may be controlled. However, unlike classic inducing agents, PGPR do not cause visible localized necrosis.

Induction of Systemic Resistance in Cucumber by Plant Growth-Promoting Rhizobacteria: Duration of Protection and Effect of Host Resistance on Protection and Root Colonization

Two plant growth-promoting rhizobacteria, Pseudomonas putida strain 89B-27 and Serratia marcescens strain 90-166, which previously induced systemic resistance (ISR) in cucumber 'Straight 8' against anthracnose caused by Colletotrichum orbiculare, were used to determine the duration of ISR in cucumber. ISR activity induced by strain 89B-27 occurred at the first-leaf stage, increased over time, and was maintained at least to the fifth-leaf stage. With strain 90-166, ISR was less stable, occurring at the second-, fourth-, and fifth-leaf stages. Cultivar specificity of ISR and colonization capacity of the two strains were studied in one resistant and three susceptible cucumber cultivars under greenhouse conditions. Strain 89B-27 induced systemic resistance in all three, whereas strain 90-166 induced systemic resistance in two of the three susceptible cultivars. Both strains failed to induce resistance in the resistant cultivar. Root colonization patterns of strains 89B-27 and 90-166 also differed. Populations of strain 89B-27 declined at a consistent rate over time, dropping from log 10 8.2 CFU/g fresh weight of roots 1 week after planting to log 10 3.2 CFU/g fresh weight of roots by 4 weeks after planting. In contrast, populations of strain 90-166 declined rapidly from log 10 7.4 CFU/g fresh weight of roots 2 weeks after inoculation and then decreased more slowly to log 10 3.2 CFU/g fresh weight of roots by 4 weeks after planting. There was no relationship between ISR activity and populations of the two strains on roots. ISR increased over time, while populations of the bacteria decreased.

Induction of Systemic Resistance in Cucumber Against Fusarium Wilt by Plant Growth-Promoting Rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) strains 89B-27 (Pseudomonas putida) and 90-166 (Serratia marcescens) were tested for their ability to induce systemic resistance against Fusarium wilt, a vascular disease of cucumber, using a split-root assay. PGPR strains and Fusarium oxysporum f. sp. cucumerinum were inoculated on separate halves of roots of cucumber seedlings at the same time and then planted in separate pots. Both PGPR strains induced systemic resistance against F. oxysporum f. sp. cucumerinum as expressed by delayed disease symptom development and reduced number of dead plants after PGPR treatments compared to the nonbacterized, E oxysporum f. sp. cucumerinum-inoculated controls 5 weeks after inoculation. E oxysporum f. sp. cucumerinum was recovered from lower stems 2 weeks after root inoculation and from the first, second, and third petioles 5 weeks after inoculation in the nonbacterized control. In contrast,.F oxysporum f. sp. cucumerinum was isolated from stems of plants treated with PGPR only 4 weeks after inoculation and from the first petiole 5 weeks after inoculation, indicating that PGPR treatment reduced spread of the pathogen. Movement of PGPR in cucumber split-root systems was monitored with a bioluminescent derivative of 89B-27, strain L211, that was detected with a charge-coupled device camera. Strain L211 provided protection against F. oxysporum f. sp. cucumerinum at levels similar to the wild-type PGPR strain. L211 colonized cucumber roots up to 5 weeks after root inoculation and was not detected inside stems or petioles. The bacterium showed only limited movement within inoculated pots and did not move to the pots in which the pathogen was inoculated, demonstrating that the PGPR and pathogen remained spatially separated. We conclude that the two PGPR strains induced resistance systemically in cucumber against Fusarium wilt.

Rapid induction of systemic resistance in cucumber by Pseudomonas syringae pv. syringae

Inoculation of the first leaf of cucumber with Pseudomonas syringae pv. syringae (wheat isolate) elicited a rapid hypersensitive response and also induced systemic resistance to Colletotrichum lagenarium. Systemic increases in peroxidase activity and resistance were observed within 1 day of inoculation with P. syringae pv. syringae. The induction of systemic resistance and peroxidase activity by P. syringae pv. syringae was at least 4 days faster than that observed for the cucumber pathogen Pseudomonas syringae pv. lachrymans. Detaching the leaf inoculated with P. syringae pv. syringae at intervals after inoculation demonstrated that the signal(s) involved in systemic induced resistance and increases in peroxidase was generated within 6 h after the inducing inoculation and before the development of visible necrosis. Tn5 mutants of P. syringae pv. syringae, selected for their inability to induce the hypersensitive response and systemic peroxidase activity in cucumber, had also lost their ability to elicit systemic disease resistance in cucumber and cause disease on wheat. A genomic clone was isolated from a library of P. syringae pv. syringae which restored the ability of both mutants to induce the hypersensitive response, systemic disease resistance and peroxidase activity. The rapidity and level of resistance induced by P. syringae pv. syringae suggest that this organism may be useful in future studies on the nature of systemic signaling and the regulation of induced resistance in cucurbits.

Cytology of induced systemic resistance of cucumber to Colletotrichum lagenarium

The infection of cucumber leaves by Colletotrichum lagenarium was studied using cytological methods. Its progress in untreated plants was compared with that in plants in which systemic resistance had been induced by pre-infecting the first true leaf with the same fungus. In induced plants, a reduction of fungal development was observed at the leaf surface, in the epidermis, and in the mesophyll. On the leaf surface, formation of appressoria was slightly reduced. In the epidermis, enhanced formation of papillae beneath appressoria, and possibly increased lignification of entire cells, correlated with reduced development of infection hyphae. Papillae contained callose, identified by staining with aniline-blue fluorochrome and digestion with β-1,3-glucanase, as a main structural component. In the mesophyll, reduced fungal development provided evidence for the existence of an additional induced defence reaction. The results imply that preinfection elicited a systemic, multicomponent defence reaction of the host plant against the fungus.