Resistance Of Cucumber Plants Research Articles

Attenuation of Zucchini mosaic virus disease in cucumber plants by mycorrhizal symbiosis

Key messageArbuscular mycorrhizal fungi generated systemic acquired resistance in cucumber to Zucchini yellow mosaic virus, indicating their prospective application in the soil as a sustainable, environmentally friendly approach to inhibit the spread of pathogens.The wide spread of plant pathogens affects the whole world, causing several plant diseases and threatening national food security as it disrupts the quantity and quality of economically important crops. Recently, environmentally acceptable mitigating practices have been required for sustainable agriculture, restricting the use of chemical fertilizers in agricultural areas. Herein, the biological control of Zucchini yellow mosaic virus (ZYMV) in cucumber (Cucumis sativus L.) plants using arbuscular mycorrhizal (AM) fungi was investigated. Compared to control plants, ZYMV-infected plants displayed high disease incidence (DI) and severity (DS) with various symptoms, including severe yellow mosaic, mottling and green blisters of leaves. However, AM fungal inoculation exhibited 50% inhibition for these symptoms and limited DS to 26% as compared to non-colonized ones. The detection of ZYMV by the Enzyme-Linked Immunosorbent Assay technique exhibited a significant reduction in AM-inoculated plants (5.23-fold) compared with non-colonized ones. Besides, mycorrhizal root colonization (F%) was slightly reduced by ZYMV infection. ZYMV infection decreased all growth parameters and pigment fractions and increased the malondialdehyde (MDA) content, however, these parameters were significantly enhanced and the MDA content was decreased by AM fungal colonization. Also, the protein, proline and antioxidant enzymes (POX and CAT) were increased with ZYMV infection with more enhancements due to AM root colonization. Remarkably, defence pathogenesis-related (PR) genes such as PR-a, PR-b, and PR-10 were quickly expressed in response to AM treatment. Our findings demonstrated the beneficial function of AM fungi in triggering the plant defence against ZYMV as they caused systemic acquired resistance in cucumber plants and supported their potential use in the soil as an environment-friendly method of hindering the spread of pathogenic microorganisms sustainably.

Evaluation of genetic and induced resistance phenomena in cucumbers against the root-knot nematode (Meloidogyne incognita)

Meloidogyne spp. is an important pest of cucurbits in tunnel farming of vegetables in Pakistan. A cucumber germplasm was evaluated for resistance against the root-knot nematode (RKN, Meloidogyne incognita) based on the number of galls and egg masses recorded under glass house conditions. All the cucumber accessions showed varying responses towards the RKN inoculation. Out of the fifteen cucumber lines, two were found to be moderately susceptible to M. incognita, ten were susceptible while the other three were highly susceptible. For the management of the nematodes, resistance was induced in a highly susceptible cucumber accession (28294) by the application (both drench and foliar) of elicitors, i.e., salicylic acid (SA) and benzothiadiazole (BTH). The application of SA and BTH significantly enhanced the defence mechanism of the cucumber plants when compared to the control. Increased enzymatic activities in the cucumber plants as the result of the elicitor applications were determined through spectrophotometer to correlate the induced resistance. There was a significant increase in the enzymatic activities when compared to the control in the cucumber plants, which ultimately enhanced the resistance as there was a decrease in the number of galls and egg masses per plant. The enzymatic antioxidant activity was also found to increase in response to the nematode infection. Both SA and BTH were reported to play pivotal roles in inducing resistance in cucumber plants against M. incognita.

Proteomic insights of chitosan mediated inhibition of Fusarium oxysporum f. sp. cucumerinum

Fusarium oxysporum f. sp. cucumerinum (FOC) infects cucumber plants, causing significant yield losses. Chitosan is a natural biodegradable compound that has antifungal properties. To understand the inhibitory mechanism of chitosan against FOC, a comprehensive proteomic study was carried out for the identification of chitosan responsive proteins (CRPs) from the mycelia of chitosan-treated FOC. Two-dimensional gel electrophoresis (2-DE) coupled with LC-MS/MS analysis led to the identification of 62 differentially abundant CRPs. Functional classification of these CRPs revealed that most proteins were involved in metabolism and defense. Gene Ontology analysis revealed that the majority of the proteins were assigned in proteolysis and hydrolase activity. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that among the biologically active pathways in chitosan-treated FOC mycelia, ‘carbohydrate metabolism’ was enriched for most of the proteins. This study gives a snapshot of the molecular basis of fungal inhibition by chitosan resulting in disease resistance in cucumber plants after inoculation with chitosan-treated FOC by explaining how chitosan restricted disease severity (i.e., down-regulating the plant cell wall degrading enzymes, FOC self-attack, hindering FOC structural and functional protein biosynthesis and DNA biosynthesis and affecting FOC transporter proteins). This study contributes to putting more weight on using the bioactive natural compound chitosan as an antifungal material instead of applying chemical fungicides in agriculture. SignificanceChitosan has been used as one of the safe and effective alternatives to fungicides in controlling cucumber vascular wilt disease caused by Fusarium oxysporum f. sp. cucumerinum (FOC) that is responsible for severe production losses. Chitosan application showed a significant decrease in wilt disease severity compared to chitosan untreated FOC and showed an efficiency of 91.7% in reducing pathogenicity. A comprehensive proteomic investigation of chitosan-responsive proteins (CRPs) from the mycelia of chitosan-treated FOC was carried out in order to better understand the inhibitory mechanism of chitosan against FOC which led us to identify 62 differentially expressed CRPs. Our proteomic study revealed CRPs in FOC involved in a variety of functions, including disease inhibition in cucumber. This study depicts what happens inside the fungus following treatment with chitosan and how chitosan played the role of the maestro in influencing the synthesis of proteins responsible for the virulence of FOC and their respective pathways, rendering FOC unable to infect the cucumber plant and lose its pathogenic potential to cause wilt disease. The efficiency of chitosan in inhibiting certain proteins or specific pathways of FOC gives a golden opportunity in controlling vascular wilt, so we highly recommend applying chitosan in disease management under greenhouse conditions or in the open field

Pseudomonas fluorescens DN16 Enhances Cucumber Defense Responses Against the Necrotrophic Pathogen Botrytis cinerea by Regulating Thermospermine Catabolism.

Plants can naturally interact with beneficial rhizobacteria to mediate defense responses against foliar pathogen infection. However, the mechanisms of rhizobacteria-mediated defense enhancement remain rarely clear. In this study, beneficial rhizobacterial strain Pseudomonas fluorescens DN16 greatly increased the resistance of cucumber plants against Botrytis cinerea infection. RNA-sequencing analyses showed that several polyamine-associated genes including a thermospermine (TSpm) synthase gene (CsACL5) and polyamine catabolic genes (CsPAO1, CsPAO5, and CsCuAO1) were notably induced by DN16. The associations of TSpm metabolic pathways with the DN16-mediated cucumber defense responses were further investigated. The inoculated plants exhibited the increased leaf TSpm levels compared with the controls. Accordantly, overexpression of CsACL5 in cucumber plants markedly increased leaf TSpm levels and enhanced defense against B. cinerea infection. The functions of TSpm catabolism in the DN16-mediated defense responses of cucumber plants to B. cinerea were further investigated by pharmacological approaches. Upon exposure to pathogen infection, the changes of leaf TSpm levels were positively related to the enhanced activities of polyamine catabolic enzymes including polyamine oxidases (PAOs) and copper amine oxidases (CuAOs), which paralleled the transcription of several defense-related genes such as pathogenesis-related protein 1 (CsPR1) and defensin-like protein 1 (CsDLP1). However, the inhibited activities of polyamine catabolic enzymes abolished the DN16-induced cucumber defense against B. cinerea infection. This was in line with the impaired expression of defense-related genes in the inoculated plants challenged by B. cinerea. Collectively, our findings unraveled a pivotal role of TSpm catabolism in the regulation of the rhizobacteria-primed defense states by mediating the immune responses in cucumber plants after B. cinerea infection.

Inducing resistance of cucumber plants to diseases by using plant growth regulators

Goal. To investigate the influence of plant growth regulators (PGR) and microfertilizer as plant resistance inducers for reducing the development of cucumber diseases in film greenhouses.
 Methods. Field and laboratory.
 Results. Analysis of the activity of total peroxidase showed a positive dynamics of activation of this enzyme - an increase of 1,2 – 2,2 times, which is 14,3 – 54,6%. It was the highest in the version with the use of Epin extra and salicylic acid. For the third day after plant treatment, the enzyme activity increased to 74.7-75.1 mg-ekv/g·s, Due to increased plant resistance, a decrease in the level of infestation of cucumber plants is noted. Depending on the drugs, the intensity of development of root rot in the average years of research was kept within 12.3-17.7 % at 30.5 % in the control. The smallest development of root rot was observed after application of microfertilizer CompleMet – 12.3 %. Treatment of plants by growth regulators led to a decrease in the level of damage of cucumber by downy mildew. The smallest development of the disease was noted in variants with Vimpel Maxi, Epin extra and CompleMet – 8.5% at 13.7% in control.
 Conclusions. The tendency to increase the level of activity of peroxidase in cucumber plants after treatment with plant growth regulators is revealed, which indicates an increase in the activity of one of the key non-specific protective reactions and, consequently, unspecific plant resistance. The effectiveness of PGR on the basis of humic acids, biologically active substances and microfertilizer CompleMet as an inducer of plant resistance to diseases has been established. The most effective against root rot is microfertilizer CompleMet – 60 % among growth regulators: Vimpel Maxi, salicylic acid, Epin extra (biological efficiency – 56, 55, 52 % respectively). Influence from the action of PGR against downy mildewis observed at the initial stages of disease development. A noticeable effect is observed when applying Epin extra, Vimpel Maxi and CompleMet (biological efficiency – 38%).

Effect of some biological agents and chemical fungicides to induce systemic resistance in cucumber plants against Fusarium oxysporum f.sp. cucumerinum

The aim of the study was to evaluate the efficiency of some biological agents and chemical fungicides to reduce disease percentage and disease severity of Fusarium wilt disease in cucumber, which caused by Fusarium oxysporum f.sp. cucumerinum under plastic house conditions, and detection of the effectiveness of factors induction through the investigation for activity peroxidase enzyme , the amount of phenols as well as the amount of total chlorophyll in cucumber plants leaves . Results showed superiority treatments of biological agents Trichoderma viride , Trichoderma harzianum , bio-preparation Biohealth and preparation of bacterial Fluramel (alone or in mixture). Treatments reduced disease percentage and disease severity to 0 and 0 % respectively, compared to control treatment (pathogen) was 86.67 and 68% respectively. Results showed that after 7and 14 days of the addition pathogenic fungus to cucumber plants , was superior treatment of interaction between Biohealth with chemical fungicide (Uniform) in the presence of pathogen in the increase activity of peroxidase enzyme where amounted to average absorption 7.17 and 5.89 min/gfw respectively . Also the same treatment superior in recording highest phenols in leaves cucumber plants after 7and 14 days of the addition treatment pathogenic fungus to cucumber plants to 7.77 and 7.38 mg/gfw respectively, while where amounted to treatment of pathogen 2.63 and 2.48 mg/gfw respectively , and superior treatment of Biohealth on the other treatments in increase total chlorophyll after 7and 14 days of treatment with pathogenic fungus to cucumber plants to 38.3 and 36.5 Spad respectively, while in treatment of pathogenic fungus recorded 27.7 and 24.4 Spad respectively.

Acibenzolar‐S‐methyl‐Induced Systemic Resistance against Anthracnose and Powdery Mildew Diseases on Cucumber Plants without Accumulation of Phytoalexins

AbstractIn a study to elucidate the possible involvement of phytoalexins in acibenzolar‐S‐methyl (ASM)‐induced systemic resistance in cucumber plants (Cucumis sativus L.), the phenolic compounds were extracted from ASM‐treated and inoculated plants and compared with those from Milsana®‐treated plants previously reported to accumulate phytoalexins in cucumber. The glycoside‐linked phenolic compounds from cucumber leaves were tested for their antifungal activity to the growth of pathogens which were most effective against the cucumber anthracnose fungus Colletotrichum orbiculare, followed by the scab fungus Cladosporium cucumerinum but ineffective against the Corynespora leaf spot fungus Corynespora cassiicola. Nevertheless, the accumulations of active compounds appeared to increase with the growth stages of cucumber plants irrespective of ASM treatment. In ASM‐pretreated cucumber plants either inoculated with anthracnose or the powdery mildew fungus, there was no increase in phytoalexin‐like phenolics.

EFFECT OF ABIOTIC AND BIOTIC TREATMENTS ON BACTERIAL ANGULAR LEAF SPOT AND PHENOLIC COMPOUNDS OF CUCUMBER

Bacterial angular leaf spot disease caused by Pseudomonas syringae pv. lachrymans is one of the most important foliage disease of cucumber. The present work was planned to control the dis-ease using six inducers i.e. salicylic acid, cobalt sulphate, di-basic potassium phosphate, lithium chloride, potassium silicate and tri-potassium phosphate, in addition two bioagents named Pseu-domonas fluorescens and Bacillus subtilis were applied as foliar treatments to induce systemic resistance in cucumber plants against bacterial angular leaf spot disease. The most effective in-ducers were salicylic acid, dibasic potassium phosphate and lithium chloride, respectively. On the other hand, Bacillus subtilis was least effective one followed by potassium silicate and Ps. fluo-rescens, respectively. While, tri-potassium phos-phate, cobalt sulphate were moderate in this re-spect. Efficacy of used inducers prolonged about till six weeks after treatment.The inducers were shown to increase phenolic compounds of cucumber leaves, whereas after two weeks of treatment up to six weeks, free and total phenol in the treated plants showed higher amounts than those in the untreated ones. Another trend was obtained with conjugated phenol. A positive correlation had been noticed between ef-ficacy of inducers and accumulated amounts of free and total phenol in cucumber leaves, i.e. the higher inducer efficacy the higher free and total phenol accumulation.

Rhizobacteria-mediated Induced Systemic Resistance in Cucumber Plants against Anthracnose Disease Caused by Colletotrichum orbiculare

Bacterial isolates TRL2-3 and TRK2-2 showing anti-fungal activity in vitro test against some plant pathogens were identified as Pseudomonas putida and Micrococcus luteus, respectively. Pre-treatment with both bacterial isolates at the concentration 1.0<TEX>$\times$</TEX> <TEX>$10^7$</TEX> and <TEX>$10^6$</TEX>cfu/ml in the rhizosphere could trigger induced systemic resistance in the aerial part of cucumber plants against anthracnose caused by Colletotrichum orbiculare. However, the pre-treatment with the higher concentration at 1.0 <TEX>$\times$</TEX> <TEX>$10^8$</TEX> cfu/ml of both isolates could not induce resistance after challenge inoculation with C. orbiculare. As a positive control, the treatment with DL-3 amino butyric acid caused a remarkable reduction of disease severity whereas the lesions on the leaves of untreated plants developed apparently after the fungal inoculation. From these results, it was recomended that disease control using both bacterial isolates inducing systemic resistance in the field where chemical application is forbid.

Fungitoxicidade, atividade elicitora de fitoalexinas e proteção de pepino contra Colletotrichum lagenarium, pelo extrato aquoso de Eucalyptus citriodora

Compostos secundários presentes em plantas medicinais podem desempenhar funções importantes em interações planta-patógeno, através de ação antimicrobiana direta ou ativando mecanismos de defesa de outras plantas que venham a ser tratadas com esses compostos. Com o objetivo de verificar o potencial de eucalipto (Eucalyptus citriodora) no controle alternativo de antracnose em pepino (Cucumus sativus), extrato aquoso (EA) desta essência florestal, autoclavado ou não autoclavado, nas concentrações de 0,1, 1, 5, 10, 15, 20 e 25% foi utilizado nos seguintes ensaios: indução de fitoalexinas em mesocótilos estiolados de sorgo (Sorghum bicolor) e em cotilédones de soja (Glycine max); fungitoxicidade in vitro sobre conídios de Colletotricum lagenarium; indução de resistência local ou sistêmica em plantas de pepino. Água e Bion foram utilizados como tratamentos controle. Os resultados indicaram que os EAs autoclavados e não autoclavados induziram a síntese de fitoalexinas em sorgo a partir da concentração de 1%. Em soja ocorreu síntese de gliceolina a partir de 10% e 15% dos EAs autoclavados e não autoclavados, respectivamente. Houve inibição total na germinação de esporos e formação de apressórios de C. lagenarium em concentrações de 20% e 1% do EA autoclavado, respectivamente. Para o extrato não autoclavado houve 75% de inibição da germinação de esporos em 25% do EA e inibição total da formação de apressórios em 15% do EA. Baseado no parâmetro tamanho de lesão, o extrato aquoso de E. citriodora, não autoclavado, apresenta potencial para induzir resistência local em pepino contra C. lagenarium.

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.

Lack of Specificity in Plant Extracts and Chemicals as Inducers of Systemic Resistance in Cucumber Plants to Anthracnose

AbstractCanoia, lettuce, green bean, dwarf pea, tobacco, tomato, poplar, corn, wheat, and cucumber leaves were extracted by boiling with 95% ethanol. The ethanol was removed from the ethanol‐water extract, and the water‐soluble fraction was centrifuged and filtered to remove compounds with a molecular weight &gt; 1 000 D. The undersides of the first true leaf of a cucumber plant (leaf 1) and the leaf above (leaf 2) were sprayed with each plant extract (induction), and 7 days later leaf 3 was inoculated with a conidial suspension of Colletotrichym lagenarium (challenge). Extracts from all plants tested induced systemic resistance in leaf 3 to C. lagenarium. Commercially available compounds were screened to determine structural similarities among the compounds which can induce systemic resistance. All compounds which induced resistance in leaf 3 caused some damage to leaves 1 and 2. but not all compounds which damaged leaves 1 and 2 induced resistance in leaf 3. Galacturonic acid, glucuronic acid, salicyhc acid, m‐hydroxybenzoic acid, p‐hydroxybenzoic acid, y‐resorcylic acid, protocatecbuic acid, gallic acid, hemimelletic acid, trimelletic acid, trimesic acid, phloroglucinol, and glycine induced systemic resistance, whereas catechol, resorcinol, hydroquinone, pyrogaLol, hydroxyhydroquinone, benzoic acid, phthalic acid, α‐resorcylic acid, β‐resorcylic acid, gentisic acid. 2, 3‐dihydroxybenzoic acid, benzaldehyde, 2,3‐dihydroxybenzaldehyde, β‐resorcaldehyde, protocatechuic aldehyde, benzyl alcohol, salicyl alcohol, myo‐inositol, phytic acid, pinitol, and quebrachitol did not. Structural similarities were not evident among either compounds which induced systemic resistance or those that did not.

Plant Growth Promoting Fungi from Zoysiagrass Rhizosphere as Potential Inducers of Systemic Resistance in Cucumbers

Sixteen fungal isolates from zoysiagrass rhizospheres that promoted the growth of a variety of crop plants were tested for their ability to induce systemic resistance in cucumber plants against Colletotrichum orbiculare, the anthracnose pathogen. Roots of cucumber plants were treated for different durations with these fungal isolates as barley grain inoculum, mycelial inoculum, or culture filtrate. The induced plants were challenge inoculated with C. orbiculare at three different spore concentrations: 10 4 , 10 5 , or 10 6 spores per milliliter. Among the 16 isolates, only 10 reduced the disease significantly and the protection obtained by these isolates varied among the methods of inducer application. However, isolates GU21-2, GU23-3, and GU24-3 produced the same effect when treated in all three forms and challenged with 10 4 spores per milliliter of the pathogen. Some isolates protected plants against high inoculum densities of the pathogen, while most of them were less effective under such pressure. The reduction in total lesion area and lesion number on leaves of plants induced by isolates GS6-1 and GU21-2 was similar to that induced by C. orbiculare. Among isolates that induced protection against anthracnose, isolates GS8-1, GS8-2, and GS8-3 colonized roots and significantly reduced the disease compared with the protection caused by noninduced control. The other isolates that were unable to colonize roots also offered protection. The degree of protection was highly dependent on the pathogen spore concentration and the inducer treatment period. A 24-h period was sufficient to initiate resistance; however, a 72-h duration proved to be more effective. The induction of systemic resistance in cucumber plants against C. orbiculare might be due not only to root colonization, but also to triggering of the host defense mechanism(s) by certain factors produced by fungi and their metabolites. When provided as barley grain inocula, most of the isolates increased the plant height and biomass significantly. Treatment of cucumber seeds with the mycelial inocula and culture filtrates of certain isolates also caused promotion of height and biomass of plants

Plant chitinases

Plant chitinases

Silicon induced resistance in cucumber plants against Pythium ultimum

The effect of the amendment of nutrient solutions with soluble potassium silicate on the response of cucumber (cv. Corona) root and hypocotyl tissues infected by Pythium ultimum was examined by light and electron microscopy, and by energy dispersive X-ray analysis (EDX). Plants were grown in 0 or 1·7 m m Si-amended nutrient solutions, and root and hypocotyl samples were collected at different times after inoculation with P. ultimum. By 48 h after infection, striking differences in the expression of defence reactions were observed between Si-amended and Si-free cucumber plants. Treatment of plants with Si markedly stimulated the accumulation of an electron-dense, phenolic-like material in infected host tissues, and significantly increased the percentage of cells filled with this material. Fungal hyphae colonizing occluded host cells were seriously damaged, and were often reduced to empty hyphal shells. Additionally, Si-treated cucumber plants responded to P. ultimum infection by forming electron-dense layers along primary and secondary cell walls, as well as over pit membranes of xylem vessels. EDX analysis failed to reveal the presence of silica deposits in P. ultimum-infected plants grown in Si-supplemented media. Our results suggest that a relationship exists between Si treatment, resistance to P. ultimum attack, and expression of plant defence mechanisms.

Local and systemic induction of peroxidase, chitinase and resistance in cucumber plants by K 2HPO 4

Spraying the lower surface of the first true leaf (leaf 1) of cucumber plants with 50 m m K 2HPO 4 induced systemic resistance to anthracnose caused by Colletotrichum lagenarium, 2 to 7 days later. Within 16 h of application, the activities of peroxidase and chitinase increased in the induced leaf, and they continued to increase over the next 7 days reaching levels at least ten-fold higher than in leaf 1 of plants treated with water. During this period, the activities of both enzymes were usually two-fold or higher in leaf 2 of plants induced with K 2HPO 4 than in plants treated with water. If the induced plants were not further treated or mock-challenged with water on leaf 2, the enzyme levels increased slightly and levelled out at between two or four-fold higher than in the corresponding control plants. If plants were challenged with C. lagenarium on leaf 2, the enzyme activity at the site of challenge increased in the induced plants prior to rising in the control plants. Large increases in enzyme activity were only evident after the lesions became visible (3–4 days after challenge) in the control plants and this only occurred at the site of challenge. Seven days after challenge, the enzyme activities in the leaf tissue adjacent to the site of inoculation were similar in both the control and induced plants. Correlations were made between peroxidase and chitinase activities induced by several treatments on leaf 1 and the level of protection observed in leaf 2 after challenge with C. lagenarium.

Elicitors of the production of lignin-like compounds in cucumber hypocotyls

Lignification appears to be a component of the mechanism of disease resistance in cucumber plants to the fungus Cladosporium cucumerinum . A bioassay for elicitors of lignification in cucumber hypocotyls is described which is based upon the detection of lignin-like compounds on the wound surface of treated slices of cucumber hypocotyls using phloroglucinol/HCl. Elicitors of lignification were detected in extracts of cucumber cell walls arid mycelia of C . cucumerinum , and in culture filtrates of the fungus. Of a range of autoclaved polysaccharides and polyanionic compounds only pectin, polygalacturonic acid and chitosan had elicitor activity. Elicitors were released from cucumber cell walls by autoclaving and after partial purification from autoclaved cucumber cell walls were found to be oligomers of galacturonic acid. One of the most active of the cell wall elicitors was enriched in uronic acid (93·5% by weight) and contained low amounts of neutral sugars. It still showed elicitor activity when diluted down to a concentration of 12·5 μg uronic acid units ml −1 . Similar but more active elicitors were obtained by autoclaving polygalacturonic acid. These were found to be oligosaccharides of about 11 galactur-onosyl units which still showed activity when diluted down to a concentration of 0·8 μg uronic acid units ml −1 .