Small oligomers of galacturonic acid are endogenous suppressors of disease resistance reactions in wheat leaves

Plants infected by a phytopathogenic fungus appear to recognize the presence of the pathogen by the molecular recognition of fungal cell wall fragments, termed 'elicitors', or of breakdown products of their own cell walls, termed 'endogenous elicitors'. Successful pathogens are thought to counteract this elicitation of active resistance reactions by the production of 'suppressors'. Evidence is presented here that fragments of the host cell wall, presumably produced enzymatically during fungal penetration, may act as 'endogenous suppressors' of resistance reactions in wheat. Pectic fractions were extracted from wheat cell walls by a variety of methods: Ca²⁺-chelators (CDTA and imidazole), a commercial mixture of pectic enzymes (Pectolyase Y23), a highly purified recombinant endopolygalacturonase (EPG), and solvolyses of the cell walls in anhydrous hydrogen fluoride at low temperatures followed by imidazole extraction. All of these fractions suppressed elicitor-induced activities of phenylalanine ammonia-lyase and peroxidases when co-injected with a glycoproteogalactan-elicitor, isolated from germ tubes of the wheat stem rust fungus, into the intercellular spaces of wheat leaves. Suppressor activity was correlated with the content of galacturonic acid in the extracts. Of the oligogalacturonides tested (monomer to hexamer), the dimer and trimer proved to be most active. This was not only true for suppression of elicitor-induced responses, but also for suppression of the hypersensitive resistance reaction in infected, genetically resistant host plants. As a consequence of reduced host cell necrosis in suppressor-treated leaves, the fungus developed larger colonies than in water-treated control leaves. Small oligomers of galacturonic acid, thus, are endogenous suppressors of resistance reactions in wheat leaves.     

Induction of systemic resistance to Pythium damping-off in cucumber plants by benzothiadiazole: ultrastructure and cytochemistry of the host response

Benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH, CGA 245704), a non-toxic, synthetic chemical, was applied as a foliar spray to cucumber plants and evaluated for its potential to induce defense mechanisms in root tissues infected by the soilborne pathogen, Pythium ultimum Trow. In non-treated cucumber plants, fungal colonization was intense and paralleled marked host tissue damage, whereas in BTH-treated plants, pathogen ingress towards the vascular stele was apparently halted by the massive deposition of a phenolic-enriched material which occluded a large number of cortical and vascular parenchyma cells. This considerable increase in the accumulation of phenolics was accompanied by cytological disorders of the invading pathogen at a time when the wall-bound cellulose component was preserved. In addition to phenolic compounds, the occluding material contained large amounts of beta-glucoside residues. These residues gradually decreased in the areas neighboring fungal cells whereas phenolic deposition appeared to be more uniformly distributed throughout the occluded host cells. Pathogen penetration in non-occluded cucumber root cells coincided with other changes, mainly characterized by both the deposition onto the inner surface of the cell walls of some heterogeneous wall appositions and the coating of some intercellular spaces with an electron-opaque material. Evidence is provided in this study that BTH has the ability to induce SAR in cucumber. Exogenous, foliar applications of the chemical sensitize susceptible cucumber plants to react more rapidly and more efficiently to P. ultimum attack, mainly through the massive accumulation of phenolic compounds at sites of attempted pathogen penetration.     

Benzothiadiazole-Mediated Induced Resistance to Fusarium oxysporum f. sp. radicis-lycopersici in Tomato

Benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a synthetic chemical, was applied as a foliar spray to tomato (Lycopersicon esculentum) plants and evaluated for its potential to confer increased resistance against the soil-borne pathogen Fusarium oxysporum f. sp. radicis-lycopersici (FORL). In nontreated tomato plants all root tissues were massively colonized by FORL hyphae. Pathogen ingress toward the vascular stele was accompanied by severe host cell alterations, including cell wall breakdown. In BTH-treated plants striking differences in the rate and extent of fungal colonization were observed. Pathogen growth was restricted to the epidermis and the outer cortex, and fungal ingress was apparently halted by the formation of callose-enriched wall appositions at sites of fungal penetration. In addition, aggregated deposits, which frequently established close contact with the invading hyphae, accumulated in densely colonized epidermal cells and filled most intercellular spaces. Upon incubation of sections with gold-complexed laccase for localization of phenolic-like compounds, a slight deposition of gold particles was observed over both the host cell walls and the wall appositions. Labeling was also detected over the walls of fungal cells showing signs of obvious alteration ranging from cytoplasm disorganization to protoplasm retraction. We provide evidence that foliar applications of BTH sensitize susceptible tomato plants to react more rapidly and more efficiently to FORL attack through the formation of protective layers at sites of potential fungal entry.     

Cytological effects of cellulases in the parasitism of Phytophthora parasitica by Pythium oligandrum

The ubiquitous oomycete Pythium oligandrum is a potential biocontrol agent for use against a wide range of pathogenic fungi and an inducer of plant disease resistance. The ability of P. oligandrum to compete with root pathogens for saprophytic colonization of substrates may be critical for pathogen increase in soil, but other mechanisms, including antibiosis and enzyme production, also may play a role in the antagonistic process. We used transmission electron microscopy and gold cytochemistry to analyze the intercellular interaction between P. oligandrum and Phytophthora parasitica. Growth of P. oligandrum towards Phytophthora cells correlated with changes in the host, including retraction of the plasma membrane and cytoplasmic disorganization. These changes were associated with the deposition onto the inner host cell surface of a cellulose-enriched material. P. oligandrum hyphae could penetrate the thickened host cell wall and the cellulose-enriched material, suggesting that large amounts of cellulolytic enzymes were produced. Labeling of cellulose with gold-complexed exoglucanase showed that the integrity of the cellulose was greatly affected both along the channel of fungal penetration and also at a distance from it. We measured cellulolytic activity of P. oligandrum in substrate-free liquid medium. The enzymes present were almost as effective as those from Trichoderma viride in degrading both carboxymethyl cellulose and Phytophthora wall-bound cellulose. P. oligandrum and its cellulolytic enzymes may be useful for biological control of oomycete pathogens, including Phytophthora and Pythium spp., which are frequently encountered in field and greenhouse production.     

Ability of nonpathogenic Fusarium oxysporum strain Fo47 to induce resistance against Pythium ultimum infection in cucumber

The influence exerted by nonpathogenic Fusarium oxysporum strain Fo47 in triggering cucumber protection against infection by Pythium ultimum was investigated ultrastructurally. Macroscopic and microscopic observations of the pathogen colony in dual cultures revealed that reduction of Pythium growth was associated with marked disorders, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and complete loss of the protoplasm. Cytochemical labeling of cellulose with an exoglucanase-gold complex showed that the cellulose component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. A similar antagonistic process was observed at the root cell surface. Most striking and interesting was the finding that mycoparasitism, as evidenced by the frequent occurrence of Fo47 hyphae within nearly empty cells of the pathogen, occurred not only at the root surface but also within the invaded root tissues. The specific labeling pattern obtained with the exoglucanase-gold complex confirmed that Fo47 successfully penetrated cells of the pathogen, both in the rhizosphere and inside the root tissues. Pythium cells that could evade the first defensive line in the rhizosphere could penetrate the root epidermis, but their growth was restricted to the outermost tissues. Positive correlations between Fo47 treatment and induced resistance to infection by P. ultimum in cucumber were confirmed by (i) the reduction of pathogen viability; (ii) the elaboration of newly formed barriers, a phenomenon which was not seen in Fo47-free plants, where the pathogen proliferated in all root tissues within a few days; and (iii) the occlusion of intercellular spaces with a dense material likely enriched in phenolics. Taken together, our observations provide the first convincing evidence that Fo47 exerts a direct inhibitory effect on P. ultimum through a combination of antibiosis and mycoparasitism, in addition to being a strong inducer of plant defense reactions.     

Induction of resistance against Fusarium wilt of tomato by combination of chitosan with an endophytic bacterial strain: ultrastructure and cytochemistry of the host response

The potential of Bacillus pumilus (PGPR strain SE 34), either alone or in combination with chitosan, for inducing defense reactions in tomato (Lycopersicon esculentum Mill.) plants inoculated with the vascular fungus, Fusarium oxysporum f. sp. radicis-lycopersici, was studied by light and transmission electron microscopy and further investigated by gold cytochemistry. The key importance of fungal challenge in the elaboration of defense mechanisms is discussed in relation to the possibility that an alarm signal provided by the pathogen itself is required for the expression of resistance in plants previously sensitized by biotic agents. Ultrastructural investigations of the infected root tissues from water-treated (control) plants showed a rapid colonization of all tissues including the vascular stele. In root tissues from bacterized tomato plants grown in the absence of chitosan, the limited fungal development coincided with marked changes in the host physiology. The main facets of the altered host metabolism concerned the induction of a structural response at sites of fungal entry and the abnormal accumulation of electron-dense substances in the colonized areas. A substantial increase in the extent and magnitude of the cellular changes induced by B. pumilus was observed when chitosan was supplied to bacterized tomato plants. These changes were characterized by a considerable enlargement of the callose-enriched wall appositions deposited onto the inner cell wall surface in the epidermis and the outer cortex. The use of the wheat germ agglutinin-ovomucoid-gold complex provided evidence that the wall-bound chitin component in Fusarium cells colonizing bacterized tomato roots was not substantially altered. One of the most-typical fungal cell reactions, observed only when bacterized tomato plants were grown in the presence of chitosan, was the formation of abnormal chitin-enriched deposits between the retracted plasma membrane and the cell wall. Results of the present study provide the first evidence that combination of biocontrol approaches is a promising step towards elaborating integrated pest management programmes. Received: 6 June 1997 / Accepted: 8 July 1997     

Accumulation of beta-Fructosidase in the Cell Walls of Tomato Roots following Infection by a Fungal Wilt Pathogen

Active defense in plants is associated with marked metabolic alterations, but little is known about the exact role of the reported changes in specific activity of several enzymes in infected plant tissues. β-Fructosidase (invertase), the enzyme that converts sucrose into glucose and fructose, increases upon infection by fungi and bacteria. To understand the relationship between fungal growth and β-fructosidase accumulation, we used an antiserum raised against a purified deglycosylated carrot cell wall β-fructosidase to study by immunogold labeling the spatial and temporal distribution of the enzyme in susceptible and resistant tomato (Lycopersicon esculentum) root tissues infected with the necrotrophic fungus, Fusarium oxysporum f. sp. racidis-lycopersici. In susceptible plants, the enzyme started to accumulate in host cell walls about 72 hours after inoculation. Accumulation occurred only in colonized cells and was mainly restricted to areas where the walls of both partners contacted each other. In resistant plants, accumulation of β-fructosidase was noticeable as soon as 48 hours after inoculation and appeared to reach an optimum by 72 hours after inoculation. Increase in wall-bound β-fructosidase was not restricted to infected cells but occurred also, to a large extent, in tissues that remained uncolonized during the infection process. The enzyme also accumulated in wall appositions (papillae) and intercellular spaces. This pattern of enzyme distribution suggests that induction of β-fructosidase upon fungal infection is part of the plant's defense response. The possible physiological role(s) of this enzyme in infected tomato plants is discussed in relation to the high demand in energy and carbon sources during pathogenesis.     

Subcellular Localization of Chitinase and of Its Potential Substrate in Tomato Root Tissues Infected by Fusarium oxysporum f. sp. radicis-lycopersici

Antiserum raised against a tomato (Lycopersicon esculentum Mill.) chitinase (molecular mass of 26 kilodaltons) was used as a probe to study the subcellular localization of this enzyme in tomato root tissues infected with Fusarium oxysporum f. sp. radicis-lycopersici. A time-course experiment revealed that chitinase accumulated earlier in the incompatible interaction than in the compatible one. However, in both systems, chitinase deposition was largely correlated with pathogen distribution. The enzyme was found to accumulate in areas where host walls were in close contact with fungal cells. In contrast, the enzyme could not be detected in vacuoles and intracellular spaces. The substantial amount of chitinase found at the fungus cell surface supports the view of an antifungal activity. However, the preferential association of the enzyme with altered fungal wall areas indicates that chitinase activity is either preceded by the hydrolytic action of other enzymes such as β-1,3-glucanases or coincides with these enzymes. The possibility that fungal glucans released through the action of β-1,3-glucanases may act as elicitors of chitinase production is discussed.     

Analysis of the distribution of copper amine oxidase in cell walls of legume seedlings

Copper-containing amine oxidase (CuAO) has been proposed to play a role in H2O2 production in plant cell walls during cell development and in response to pathogen attack. We have compared the localisation of CuAO in pea (Pisum sativum L.), lentil (Lens culinaris M.) and chick pea (Cicer arietinum L.) grown under different light conditions, using both immuno- and histochemical techniques. The enzyme was detected by indirect immunofluorescence in the cell walls of parenchyma tissues of etiolated pea and lentil plants and was particularly abundant at intercellular spaces. Upon de-etiolation, CuAO largely disappeared from cortical cell walls except in the region of intercellular spaces. In the apical internode of light-grown seedlings, CuAO occurred mainly in cortical cell walls and, to some extent, in cell walls of xylem vessels. In both the elongation zone and mature regions of roots, CuAO was restricted to cortical cell walls and some cell junctions close to the meristem. Extensin epitopes co-localised to intercellular spaces of the cortex in de-etiolated pea, indicating that CuAO may have a role in cell wall strengthening at intercellular spaces. In chick pea, the localisation of the enzyme varied between different cultivars that have differing susceptibility to the fungus Ascochyta rabiei. In a susceptible cultivar Calia, immunogold labelling localised CuAO to cell walls of the cortex, as in lentil and pea, while in a resistant cultivar Sultano, it was most abundant in xylem vessels and, in light-grown plants, in the epidermis. These expression patterns are discussed with regard to the possible functions of amine oxidase in cell growth, cell differentiation and pathogen resistance.     

Time-course study of the accumulation of hydroxyproline-rich glycoproteins in root cells of susceptible and resistant tomato plants infected byFusarium oxysporum f. sp.radicis-lycopersici

The accumulation of hydroxyproline-rich glycoproteins (HRGPs) in cell walls of dicotyledonous plants is thought to be involved in the defense response to pathogens. An antiserum raised against deglycosylated HRGPs from melon was used for studying the subcellular localization of these glycoproteins in susceptible and resistant tomato (Lycopersicon esculentum Mill.) root tissues infected by Fusarium oxysporum f.sp. radicis-lycopersici. A time-course of HRGP accumulation revealed that these glycoproteins increased earlier and to a higher extent in resistant than in susceptible cultivars. In the compatible interaction, increase in HRGPs was largely correlated with pathogen invasion and appeared to occur as a result of wall damage. In the incompatible interaction, HRGPs accumulated in the walls of uninvaded cells, thus indicating a possible role in the protection against fungal penetration. The occurrence of substantial amounts of HRGPs in papillae, known to be physical barriers formed in response to infection, and in intercellular spaces provides additional support to the concept that such glycoproteins play an important role in disease resistance.     

In situ localization of PR-1 mRNA and PR-1 protein in compatible and incompatible interactions of pepper stems withPhytophthora capsici

Summary In situ hybridization and immunogold labeling were performed to examine the temporal and spatial expression pattern of pathogenesis-related protein 1 (CABPR1) mRNA and PR-1 protein in pepper (Capsicum annuum L.) stem tissues infected by virulent and avirulent isolates ofPhytophthora capsici. CABPR1 mRNA accumulation was confirmed in the infected pepper stem tissue by Northern blot analysis and in situ hybridization. Northern blot analysis showed that the temporal expression ofCABPR1 mRNA varied greatly between compatible and incompatible interactions. An earlier expression of theCABPR1 gene, 6 h after inoculation, was observed in the incompatible interaction. In situ hybridization results revealed thatCABPR1 mRNA was expressed in the phloem areas of vascular bundles in infected pepper stem tissues, but especially strongly in the incompatible interaction. PR-1 protein was predominantly found in the intercellular spaces of pepper stem cells in the compatible and incompatible interactions 24 h after inoculation. Strikingly, the immunogold labeling was associated with fibrillar and electron-dense material localized in the intercellular space. Dense labeling of PR-1 protein was also seen at the interface of the pathogen and the host cell wall, whereas few gold particles were detected over the host cytoplasm. However, PR-1 protein was not detected over the fungal cell wall in either interaction.     

Ultrastructure at the Host-Parasite Interface of Phytophthora capsici in Roots and Stems of Capsicum annuum

The infecting hyphae of Phytophthora capsici grew intercellularly in infected tissues of roots and stems of pepper (Capsicum annuum). The vascular tissues were not markedly disorganized even when heavily infected. Intercellularly growing hyphae penetrated the host cells by forming haustorium-like bodies. The consistent features of ultrastructural changes in infected tissues of pepper roots and stems were degeneration of cell organelles and dissolution of host cell walls. The cytoplasm detached from the cell wall aggregated abundantly around some haustorium-like bodies or the penetration sites of fungal hyphae. The host cell walls were palely stained, thinned and swollen, possibly being biochemically altered by the action of fungal macerating enzymes. Electron-dense, wall-like material was apposed on the outer wall of xylem vessel contacted by fungal hyphae. The infecting hyphae were also surrounded by granular, dark-staining cytoplasm. Characteristics of host cell responses to the invading P. capsici were the deposition of papilla-like material on host cell walls next to hyphae and the encasement of haustorium-like bodies with wall appositions.     

Involvement of Suppressor-Glucans and Plant Epidermal Cells in Host-Selective Pathogenesis of Phytophthora capsici

Water-soluble glucans (WSG) from a virulent isolate of Phytophthora capsici (PCAP-3) which were released during germination of cystospore markedly suppressed the elicitor-induced death of suspension-cultured cells of susceptible sweet pepper and tomato but not that of resistant pepper and tobacco. PCAP-3, its polygalacturonase (PGase)-deficient mutant (PCAP3-M16), and galacturonic acid non-utilizable mutant carrying the PGase (PCAP-1) activity could penetrate in epidermal cells of host leaves, but similarly caused a hypersensitive response (HR) on non-injured leaves of resistant host (sweet pepper). In the case of inoculation on press-injured leaves, however, both of the resistant and nonhost plant leaves became quite susceptible to PCAP-3similar to susceptible hosts, but not to PCAP3-M16 and PCAP-1. The results suggested that host-selectivity of P. capsici may be determined in the leaf epidermal cells where the suppressor glucans released during infection effectively suppressed the occurrence of hypersensitive reaction. Furthermore, during growth of the fungus in intercellular spaces of leaf tissues, PGase may contribute not only to the virulence experession but also the supply of initial nutrition for fungal growth in the intercellular space of host tissues.     

Implication of Pectic Components in Cell Surface Interactions between Tomato Root Cells and Fusarium oxysporum f. sp. radicis-lycopersici: A Cytochemical Study by Means of a Lectin with Polygalacturonic Acid-Binding Specificity

Aplysia gonad lectin, a polygalacturonic acid-binding lectin isolated from the sea mollusc Aplysia depilans, was complexed to colloidal gold and used for localizing polygalacturonic-acid-containing molecules in tomato root tissues infected with Fusarium oxysporum f. sp. radicis-lycopersici (FORL). Colonization of host tissues by FORL was associated with striking wall modifications including disruption and even loss of middle lamellae. According to the labeling pattern observed in host wall areas adjacent to fungal penetration channels, it is likely that FORL pectolytic enzymes act through localized wall degradation. The release of polygalacturonic acid-rich wall fragments and the accumulation of polygalacturonic acid-containing molecules in some altered phloem cells were frequently observed and considered to be specific host reactions to fungal attack. The heavy deposition of such molecules at strategic sites such as wall oppositions and intercellular spaces provides support to their implication in the plant defense system. The possible interrelation between polygalacturonic acid-containing molecules and other polymers such as lignin and phenolic compounds remains to be investigated further. The role of these molecules in host-pathogen interactions is discussed in relation to plant defense.     

Temporal and subcellular localization of PR-1 proteins in tomato stem tissues infected by virulent and avirulent isolates of Phytophthora capsici

Summary. Immunoblot analysis and immunogold labeling of PR-1 protein (pathogenesis-related protein 1) in tomato (Lycopersicon esculentum Mill.) were performed to examine the temporal and spatial expression patterns of PR-1 protein induced by Phytophthora capsici infection. Soluble proteins with molecular masses of 10, 17, 25, 27 and 75 kDa were induced and accumulated in P. capsici-infected stem tissues during the compatible and incompatible interactions. Western blot analysis revealed that expression of PR-1 protein (17 kDa), at 12 to 24 h after inoculation, occurred earlier in the incompatible than in the compatible interaction. Immunogold labeling of PR-1 proteins occurred over cell walls and cytoplasm of the host and the oomycete pathogen and at the interface between host and oomycete cell walls at 24 h after inoculation in the compatible interaction. In the incompatible interaction, numerous PR-1 proteins accumulated predominantly over oomycete cell walls and at the interface between host and oomycete cell walls. The quantity of PR-1 proteins deposited in both host and oomycete cells was much less in the compatible than the incompatible interaction. Healthy tomato stem tissue was nearly free of immunogold labeling of PR-1 proteins. Received October 9, 2001 Accepted January 18, 2002     

Immunogold localization of beta-1,3-glucanases in two plants infected by vascular wilt fungi.

An antiserum raised against a purified tobacco beta-1,3-glucanase (PR-N) was used to study the subcellular localization of enzyme in fungus-infected plant tissues by means of post-embedding immunogold labeling. In susceptible tomato plants, the enzyme accumulation was found to occur as a result of successful tissue colonization, whereas it appeared to be an early event associated with limited spread of the fungus in resistant tissues. Although marked differences between susceptible and resistant tomato cultivars were observed in the rate of production of beta-1,3-glucanase, the pattern of enzyme distribution was similar. The enzyme was found to accumulate predominantly in host cell walls and secondary thickenings of xylem vessels. By contrast, a very low amount of enzyme was associated with compound middle lamellae. The occurrence of beta-1,3-glucanase at the cell surface of invading fungi was an indication of their possible antifungal activity. A low enzyme concentration was detected in vacuoles of both healthy and infected tissues. In infected eggplant tissue, the pattern of beta-1,3-glucanase distribution was similar to that observed with tomato. Whether these hydrolases accumulate first in vacuoles and are subsequently conveyed toward the outside to participate in fungal wall lysis remains to be determined.     

Ethylene insensitivity impairs resistance to soilborne pathogens in tobacco and Arabidopsis thaliana

Transgenic ethylene-insensitive tobacco (Tetr) plants spontaneously develop symptoms of wilting and stem necrosis when grown in nonautoclaved soil. Fusarium oxysporum, F. solani, Thielaviopsis basicola, Rhizopus stolonifer, and two Pythium spp. were isolated from these diseased Tetr plants and demonstrated to be causal agents of the disease symptoms. Pathogenicity of the two Pythium isolates and four additional Pythium spp. was tested on ethylene-insensitive tobacco and Arabidopsis seedlings. In both plant species, ethylene insensitivity enhanced susceptibility to the Pythium spp., as evidenced by both a higher disease index and a higher percentage of diseased plants. Based on the use of a DNA probe specific for Pythium spp., Tetr plants exhibited more pathogen growth in stem and leaf tissue than similarly diseased control plants. These results demonstrate that ethylene signaling is required for resistance to different root pathogens and contributes to limiting growth and systemic spread of the pathogen.     

Ability of Nonpathogenic Fusarium oxysporum Strain Fo47 To Induce Resistance against Pythium ultimum Infection in Cucumber

The influence exerted by nonpathogenic Fusarium oxysporum strain Fo47 in triggering cucumber protection against infection by Pythium ultimum was investigated ultrastructurally. Macroscopic and microscopic observations of the pathogen colony in dual cultures revealed that reduction of Pythium growth was associated with marked disorders, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and complete loss of the protoplasm. Cytochemical labeling of cellulose with an exoglucanase-gold complex showed that the cellulose component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. A similar antagonistic process was observed at the root cell surface. Most striking and interesting was the finding that mycoparasitism, as evidenced by the frequent occurrence of Fo47 hyphae within nearly empty cells of the pathogen, occurred not only at the root surface but also within the invaded root tissues. The specific labeling pattern obtained with the exoglucanase-gold complex confirmed that Fo47 successfully penetrated cells of the pathogen, both in the rhizosphere and inside the root tissues. Pythium cells that could evade the first defensive line in the rhizosphere could penetrate the root epidermis, but their growth was restricted to the outermost tissues. Positive correlations between Fo47 treatment and induced resistance to infection by P. ultimum in cucumber were confirmed by (i) the reduction of pathogen viability; (ii) the elaboration of newly formed barriers, a phenomenon which was not seen in Fo47-free plants, where the pathogen proliferated in all root tissues within a few days; and (iii) the occlusion of intercellular spaces with a dense material likely enriched in phenolics. Taken together, our observations provide the first convincing evidence that Fo47 exerts a direct inhibitory effect on P. ultimum through a combination of antibiosis and mycoparasitism, in addition to being a strong inducer of plant defense reactions.     

Function of Host and Fungal Metabolites in Resistance Response of Banana and Plantain in the Black Sigatoka Disease Pathosystem (Musa spp. –Mycosphaerella fijiensis)

The host–pathogen interactions of Musa spp. and Mycosphaerella fijiensis were investigated in order to determine the function of secondary metabolites within the pathosystem of the Black Sigatoka disease. The pentaketide metabolites flaviolin, 2-hydroxyjuglone, juglone and 2,4,8-trihydroxytetralone (2,4,8-THT) of the pathogen were identified. The concentration of 2,4,8-THT was significantly increased by application of the synthetic compound tricyclazole and by natural activators extracted from the intercellular space of leaf tissue of resistant Musa cultivars. When inoculated host plants were treated with tricyclazole, extensive necrosis of both susceptible and resistant Musa cultivar leaves were observed. Plant defence mechanisms of resistant Musa cultivars were first detected as an activation of phenylalanine–ammonia lyase and the subsequent accumulation of post-infectional substances which blocked fungal growth. These results indicated the bivalent importance of 2,4,8-THT for host-specific reactions, depending on its concentration at different stages of pathogenesis. Early activation of fungal 2,4,8-THT metabolism by resistant Musa cultivars caused necrotic micro-lesions and elicitation of post-infectional defence reactions leading to incompatibility between pathogen and host plant; growth of the fungus on susceptible cultivars caused necrotizing doses of 2,4,8-THT only after the establishment of a compatible interaction and development of typical symptoms at late stages of pathogenesis.     

A Potential Defense Mechanism of Tomato Against the Late Blight Disease is Suppressed by Germinating Sporangia-derived Substances from Phytophthora infestans

Hypersensitive browning, host cell death and phytoalexins (rishitin) synthesis in both callus cultures and cotyledons have been observed in a susceptible tomato cultivar following treatment with high molecular weight cell wall components from Phytophthora infestans race 1B. The growth of the pathogen was restricted on both elicited differentiated and undifferentiated tomato tissues. Subtance(s) released by germinating sporangia, but not by the mycelium, suppressed the hypersensitive reaction.