Treatment with the Mycoparasite Pythium oligandrum Triggers Induction of Defense-Related Reactions in Tomato Roots When Challenged with Fusarium oxysporum f. sp. radicis-lycopersici

ABSTRACT The influence exerted by the mycoparasite Pythium oligandrum in triggering plant defense reactions was investigated using an experimental system in which tomato plants were infected with the crown and root rot pathogen Fusarium oxysporum f. sp. radicis-lycopersici. To assess the antagonistic potential of P. oligandrum against F. oxysporum f. sp. radicis-lycopersici, the interaction between the two fungi was studied by scanning and transmission electron microscopy (SEM and TEM, respectively). SEM investigations of the interaction region between the fungi demonstrated that collapse and loss of turgor of F. oxysporum f. sp. radicis-lycopersici hyphae began soon after close contact was established with P. oligandrum. Ultrastructural observations confirmed that intimate contact between hyphae of P. oligandrum and cells of the pathogen resulted in a series of disturbances, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and, finally, complete loss of the protoplasm. Cytochemical labeling of chitin with wheat germ agglutinin (WGA)/ovomucoid-gold complex showed that, except in the area of hyphal penetration, the chitin component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. Interestingly, the same antagonistic process was observed in planta. The specific labeling patterns obtained with the exoglucanase-gold and WGA-ovomucoid-gold complexes confirmed that P. oligandrum successfully penetrated invading cells of the pathogen without causing substantial cell wall alterations, shown by the intense labeling of chitin. Cytological investigations of samples from P. oligandrum-inoculated tomato roots revealed that the fungus was able to colonize root tissues without inducing extensive cell damage. However, there was a novel finding concerning the structural alteration of the invading hyphae, evidenced by the frequent occurrence of empty fungal shells in root tissues. Pythium ingress in root tissues was associated with host metabolic changes, culminating in the elaboration of structural barriers at sites of potential fungal penetration. Striking differences in the extent of F. oxysporum f. sp. radicis-lycopersici colonization were observed between P. oligandrum-inoculated and control tomato plants. In control roots, the pathogen multiplied abundantly through much of the tissues, whereas in P. oligandrum-colonized roots pathogen growth was restricted to the outermost root tissues. This restricted pattern of pathogen colonization was accompanied by deposition of newly formed barriers beyond the infection sites. These host reactions appeared to be amplified compared to those seen in nonchallenged P. oligandrum-infected plants. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. Wall appositions contained large amounts of callose, in addition to be infiltrated with phenolic compounds. The labeling pattern obtained with gold-complexed laccase showed that phenolics were widely distributed in Fusarium-challenged P. oligandrum-inoculated tomato roots. Such compounds accumulated in the host cell walls and intercellular spaces. The wall-bound chitin component in Fusarium hyphae colonizing P. oligandrum-inoculated roots was preserved at a time when hyphae had undergone substantial degradation. These observations provide the first convincing evidence that P. oligandrum has the potential to induce plant defense reactions in addition to acting as a mycoparasite.     

An Efficient Inoculation Method for Colonization of Chinese Cabbage Seedlings by the Root Endophytic Fungus Heteroconium chaetospira

To develop an efficient method to inoculate Chinese cabbage seedlings with the root endophytic fungus Heteroconium chaetospira, an appropriate nursery soil and glucose concentration for the nutrient medium for fungal colonization were determined. A grid-sheet method was established for estimating the degree of colonization of entire roots by the fungus. The fungus colonized at high frequencies when peat moss was used as the rooting medium. Colonization was highest (75%) when peat moss was amended with 0.1% glucose. Under these conditions, fungal hyphae developed intracellularly in root cortical cells. In contrast, under high glucose conditions, fungal colonization was restricted mostly to intercellular regions of epidermal or cortical root tissues. Here, hyphae formed inter- or intracellular microsclerotia. Received 19 August 2002/ Accepted in revised form 5 December 2002     

Bacterial-Mediated Induced Resistance in Cucumber: Beneficial Effect of the Endophytic Bacterium Serratia plymuthica on the Protection Against Infection by Pythium ultimum

ABSTRACT The potential of the endophytic bacterium Serratia plymuthica strain R1GC4 in stimulating defense reactions in cucumber (Cucumis sativus) seedlings inoculated with the soilborne pathogen Pythium ultimum was explored at the cellular level. Bacterial treatment prior to Pythium inoculation resulted in less seedling disease development as compared with that in nontreated control plants, in which typical root symptoms were visible by 3 days after inoculation with the pathogen. Histological investigations of root samples revealed striking differences in the extent of plant defense reactions between bacterized and nonbacterized plants. These observations were further confirmed at the ultrastructural level with the demonstration that restriction of fungal colonization to the outermost root tissues of bacterized seedlings correlated with the deposition of enlarged callose-enriched wall appositions at sites of potential pathogen penetration and the accumulation of an osmiophilic material in the colonized areas. Hyphae of the pathogen, surrounded by this electron-opaque material, exhibited considerable changes including cytoplasm disorganization and, in many cases, loss of the protoplasm. However, labeling with the beta-1,4-exoglucanase resulted in a regular labeling of Pythium cell walls, even at a time when these walls were entirely coated by the osmiophilic material. This material was also found to infiltrate into the invading hyphae to form either an internal coating of the cell wall or a network of polymorphic droplets in the area previously occupied by the cytoplasm. Cytochemical investigations revealed that callose, pectin, and cellulose appeared in the wall appositions. In addition, glucosides, lipids, and phenolics were detected in the electron-dense aggregates forming the core of most wall appositions. Finally, galactose residues were among the minor polysaccharidic compounds detected in the wall appositions. Evidence is provided in this study showing that treatment with S. plymuthica sensitizes susceptible cucumber plants to react more rapidly and more efficiently to Pythium attack through the formation of physical and chemical barriers at sites of potential fungal entry.     

Ultrastructural and Cytochemical Aspects of the Interaction Between the Mycoparasite Pythium oligandrum and Soilborne Plant Pathogens

ABSTRACT The interaction between the oomycete Pythium oligandrum and various soilborne oomycete and fungal plant pathogens (P. ultimum, P. aphanidermatum, Fusarium oxysporum f. sp. radicis-lycopersici, Verticillium albo-atrum, Rhizoctonia solani, and Phytophthora megasperma) was studied by light and electron microscopy in order to assess the relative contribution of mycoparasitism and antibiosis in the antagonistic process. Scanning electron microscope investigations of the interaction regions showed that structural alterations of all pathogenic fungi and oomycetes (except for Phytophthora megasperma) occurred soon after contact with the antagonist. Light and transmission electron microscope studies of the interaction region between the antagonist and P. ultimum revealed that intimate contact between both partners preceded a sequence of degradation events including aggregation of host cytoplasm and penetration of altered host hyphae. Localization of the host wall cellulose component showed that cellulose was altered at potential penetration sites. A similar scheme of events was observed during the interaction between P. oligandrum and F. oxysporum f. sp. radicis-lycopersici, with the exception that complete loss of host protoplasm was associated with antagonist invasion. The interaction between P. oligandrum and R. solani resulted in an abnormal deposition of a wall-like material at potential penetration sites for the antagonist. However, the antagonist displayed the ability to circumvent this barrier and penetrate host hyphae by locally altering the chitin component of the host hyphal wall. Interestingly, antagonist cells also showed extensive alteration as evidenced by the frequent occurrence of empty hyphal shells. In the case of Phytophthora megasperma, hyphal interactions did not occur, but hyphae of the plant pathogen were damaged severely. At least two distinct mechanisms appear to be involved in the process of oomycete and fungal attack by P. oligandrum: (i) mycoparasitism, mediated by intimate hyphal interactions, and (ii) antibiosis, with alteration of the host hyphae prior to contact with the antagonist. However, the possibility that the antagonistic process may rely on the dual action of antibiotics and hydrolytic enzymes is discussed.     

Ultrastructural study on interaction between a sterile,white endophytic fungus and eggplant roots

Eggplant roots colonized by a sterile, white mycelial endophyte (SWM) were previously found to become highly resistant to Verticillium wilt. SWM alone, however, caused no visible, disease symptoms, such as wilting or necrosis. The mechanism of the symptomless infection by SWM was investigated in this study. Electron microscopy revealed that hyphae of SWM were abundant on and inside the root epidermal cells 2 weeks after inoculation. Many terminal appressoria formed from apical tips of hyphae, and heavy degradation of the host cell walls was evident where hyphae accumulated. By 4 weeks following inoculation, penetration pegs easily breached epidermal cells, and the infection hyphae penetrated outer cortical cells. In response to the hyphal ingress, numerous tubule-like vesicles and membrane-bound, multivesicular bodies accumulated in cortical cytoplasm near the infection sites of the outer cortical cells, but no visible signs of the host reactions were seen in the epidermal cells. Papillae developed at the spaces between cell walls and plasma membranes at the infection sites. The penetration hyphae often grew out of the papillae, but further hyphal ingress was halted in the middle cortical cell layer. By 8 weeks following inoculation, papillae that developed in these cells contained larger amounts of highly electron-dense material and were reinforced by multilamellate, fibrous elements. Hyphae that entered such papillae were confined to them, and the hyphal cytoplasm degenerated. As the result of the activated resistance reactions, root vascular cylinders remained intact, and the host plants did not wilt.     

Compost and compost water extract-induced systemic acquired resistance in cucumber and Arabidopsis

ABSTRACT A biocontrol agent-fortified compost mix, suppressive to several diseases caused by soilborne plant pathogens, induced systemic acquired resistance (SAR) in cucumber against anthracnose caused by Colletotrichum orbiculare and in Arabidopsis against bacterial speck caused by Pseudomonas syringae pv. maculicola KD4326. A peat mix conducive to soilborne diseases did not induce SAR. The population size of P. syringae pv. maculicola KD4326 was significantly lower in leaves of Arabidopsis plants grown in the compost mix compared to those grown in the peat mix. Autoclaving destroyed the SAR-inducing effect of the compost mix, and inoculation of the autoclaved mix with nonautoclaved compost mix or Pantoea agglomerans 278A restored the effect, suggesting the SAR-inducing activity of the compost mix was biological in nature. Topical sprays with water extract prepared from the compost mix reduced symptoms of bacterial speck and the population size of pathogenic KD4326 in Arabidopsis grown in the peat mix but not in the compost mix. The peat mix water extract applied as a spray did not control bacterial speck on plants grown in either mix. Topical sprays with salicylic acid (SA) reduced the severity of bacterial speck on plants in the peat mix but did not further reduce the severity of symptoms on plants in the compost mix. The activity of the compost water extract was heat-stable and passed through a 0.2-mum membrane filter. beta-1,3-Glucanase activity was low in cucumber plants grown in either mix, but when infected with C. orbiculare, this activity was induced to significantly higher levels in plants grown in the compost mix than in plants grown in the peat mix. Similar results were obtained for beta-D-glucuronidase (GUS) activity driven by a PR2 (beta-1,3-glucanase) gene promoter in transgenic Arabidopsis plants grown in the compost or peat mix. GUS activity was induced with topical sprays of the compost water extract or SA in plants not inoculated with the pathogen, suggesting that compost-induced disease suppression more than likely involved the potentiation of resistance responses rather than their activation and that compost-induced SAR differed from SAR induced by pathogens, SA, or compost water extract.     

An ultrastructural study of tomato roots inoculated with pathogenic and non-pathogenic necrotrophic fungi and a saprophytic fungus

Tomato cultivar Moneymaker was independently inoculated with Alternaria alternata, Cunninghamella elegans, Fusarium culmorum, F. oxysporum f.sp. lycopersici, F. oxysporum f.sp. pisi and Stromatinia gladioli and analysed ultrastructurally. The extent and amount of superficial fungal growth on tomato roots was similar but C. elegans , a saprophyte, was exceptional in that hyphae were not closely appressed to plant surfaces and did not adhere to plant cell walls.
In general, the type of plant responses to fungal colonization and infection were similar in all of the interactions studied, with the exception of C. elegans which did not infect tomato root tissue. The failure to penetrate tomato roots by C. elegans may have been associated with the lack of hyphal adhesion to plant cell walls. Migration of cytoplasm and wall apposition/penetration papilla formation were regularly observed in tomato root tissue beneath appressed hyphae and at sites of fungal infection. Specific cellular reactions in the exodermis, namely the formation of wall 'inclusions' and appearance of 'sensitive' cells, indicated that exodermal cells were particularly responsive to fungal challenge.
Fusarium oxysporum f.sp. lycopersici , a pathogen of tomato, invaded tomato root tissue more extensively than the other fungi inoculated onto tomato roots. Infection of tomato by the other fungi studied was variable, and the extent and success of fungal invasion was tentatively associated with their necrotrophic capability and typical host range.     

Colonization of Pythium oligandrum in the tomato rhizosphere for biological control of bacterial wilt disease analyzed by real-time PCR and confocal laser-scanning microscopy

It recently has been reported that the non-plant-pathogenic oomycete Pythium oligandrum suppresses bacterial wilt caused by Ralstonia solanacearum in tomato. As one approach to determine disease-suppressive mechanisms of action, we analyzed the colonization of P. oligandrum in rhizospheres of tomato using real-time polymerase chain reaction (PCR) and confocal laser-scanning microscopy. The real-time PCR specifically quantified P. oligandrum in the tomato rhizosphere that is reliable over a range of 0.1 pg to 1 ng of P. oligandrum DNA from 25 mg dry weight of soil. Rhizosphere populations of P. oligandrum from tomato grown for 3 weeks in both unsterilized and sterilized field soils similarly increased with the initial application of at least 5 x 10(5) oospores per plant. Confocal microscopic observation also showed that hyphal development was frequent on the root surface and some hyphae penetrated into root epidermis. However, rhizosphere population dynamics after transplanting into sterilized soil showed that the P. oligandrum population decreased with time after transplanting, particularly at the root tips, indicating that this biocontrol fungus is rhizosphere competent but does not actively spread along roots. Protection over the long term from root-infecting pathogens does not seem to involve direct competition. However, sparse rhizosphere colonization of P. oligandrum reduced the bacterial wilt as well as more extensive colonization, which did not reduce the rhizosphere population of R. solanacearum. These results suggest that competition for infection sites and nutrients in rhizosphere is not the primary biocontrol mechanism of bacterial wilt by P. oligandrum.     

Parasitism of macroconidia, chlamydospores and hyphae of Fusarium culmorum by mycoparasitic Pythium species

Parasitism of macroconidia and endoconidial chlamydospores of Fusarium culmorum by Pythium oligandrum was studied on water agar (WA), corn-meal agar (CMA) and glass slides. Loss of cytoplasmic content in F. culmorum spores was followed by complete degradation, and P. oligandrum produced an abundance of oogonia on the parasitized macroconidia. A simple method for assessing the relative aggressiveness of isolates is presented, based on the percentage of macroconidial cells devoid of cytoplasm. Parasitism of macroconidia by P. acanthophoron , P. oligandrum and P. periplocum , but not by the plant pathogenic species, P. tracheiphilum , was demonstrated by this method. Interactions between hyphae of P. oligandrum and F. culmorum on WA resulted in an increase in the number of oogonia of P. oligandrum and a decrease in the sporulation of F. culmorum . The ability of isolates of P. oligandrum , P. periplocum , P. acanthophoron and P. mycoparasiticum to suppress disease symptoms caused by F. culmorum on barley seedlings was demonstrated in a greenhouse test.     

Interactions Between the Mycoparasite Pythium oligandrum and Sclerotia of the Plant Pathogen Sclerotinia sclerotiorum

Pythium oligandrum Drechsler is a mycoparasite which parasitizes hyphae of many fungal species. A detailed study of the interactions between P. oligandrum and the sclerotia of the plant pathogen Sclerotinia sclerotiorum (Lib.) de Bary is presented. Pythium oligandrum was present in Danish soils at concentrations between 4 and 26 cfu g^–1 soil. An increase in the natural population of P. oligandrum by addition of P. oligandrum zoospores to a soil reduced the ability of sclerotia of S. sclerotiorum to germinate myceliogenically and the sclerotia were colonized internally by P. oligandrum. This colonization and reduction of germination of sclerotia were also seen when sclerotia and P. oligandrum were incubated together in water. Small sclerotia were significantly more susceptible to parasitism by P. oligandrum than large sclerotia, and increasing the incubation time caused a further reduction in the germination ability of the sclerotia. P. oligandrum was able to pass through its entire life-cycle from zoospores to oogonia both with sclerotia as sole nutrient-source and in water containing exudates from the sclerotia. The cell wall degrading enzymes N-acetyl--D-glucosaminidase (NAGase), endo-chitinase, protease, -glucanase, -glucosidase and cellobiohydrolase were detected in culture filtrates of P. oligandrum cultivated with S. sclerotiorum. These findings suggest that P. oligandrum has a potential to reduce the survival of S. sclerotiorum sclerotia present naturally in soils, through mycoparasitic activity.     

玉米感染肿囊腐霉后寄主-病原物互作的超微结构研究

 本文利用透射电镜,首次对玉米不同抗性寄主与肿囊腐霉(Pythium inflatum)相互作用中的寄主反应及菌丝在寄主内的发展进行了系统研究。结果表明:玉米苗期根部接种后,孢子迅速萌发成菌丝在根表蔓延,随即穿透根表皮,进入表皮细胞、皮层甚至感病寄主的维管束组织。与此同时,寄主相应的反应迅速,寄主反应采取如下方式:细胞壁的沉积物及乳突在真菌的入侵处形成,各种无定形物质或纤丝构成的质网包围入侵菌丝。沉积在寄主细胞壁上并在受侵染细胞内聚集的化合物可能是酚类物质,这些嗜锇酸的电子致密物是用来机械地加强细胞壁的强度及产生抗真菌的环境。寄主反应的早晚及程度的强弱决定着菌丝在寄主体内发展繁殖的程度及寄主抗性的强弱。抗病玉米自交系的反应程度及速度显著强于感病玉米。     

Histological investigation of stripe rust (Puccinia striiformis f.sp. tritici) development in resistant and susceptible wheat cultivars

The wheat cultivar Kariega expresses complete adult plant resistance against stripe rust, whereas cv. Avocet S is susceptible. Using confocal laser scanning microscopy, initial fungal penetration into flag leaves was identical in both cultivars, with directional germ-tube growth towards stomata that were penetrated without the formation of an appressorium, followed by differentiation of a substomatal vesicle, infection hyphae, haustorial mother cells and haustoria. During the following 4 days, further fungal development occurred more quickly in the resistant than in the susceptible cultivar. However, by 7 days postinoculation (dpi) the situation changed, with exponential growth of the pathogen occurring only in the susceptible line. Induced cellular lignification, a typical defence reaction of cereals, was observed at 4 dpi in the resistant cultivar, and 2 days later lignified tissue completely surrounded the fungal colonies. In the susceptible cultivar, isolated lignified host cells occurred at 6 dpi, and long, unbranched fungal hyphae outgrowing the resistance reaction were observed.     

小麦与白粉病菌互作中β-1,3-葡聚糖的细胞定位

 利用免疫胶体金方法对小麦与白粉病菌互作过程中β-1,3-葡聚糖酶底物,即β-1,3-葡聚糖的合成与分布情况进行了研究。结果发现,白粉病菌的所有器官包括分生孢子、附着胞、入侵栓、吸器和菌丝中均未发现β-1,3-葡聚糖存在;相反,β-1,3-葡聚糖是小麦各种细胞壁的正常组分,在气孔保卫细胞、纤维细胞、导管分子等的细胞壁中含量非常丰富。病原菌的侵染导致植物细胞中β-1,3-葡聚糖的合成增加,但在乳突结构中不含β-1,3-葡聚糖。以上结果说明,往小麦中导入外源β-1,3-葡聚糖酶基因并高效表达,不会对病原菌结构产生直接的破坏和抑制作用;相反,有时可能会影响到某些植物细胞的正常发育。     

Effect of different organic amendments on lettuce fusarium wilt and on selected soilborne microorganisms

This study evaluated the effect of different organic amendments on lettuce fusarium wilt caused by Fusarium oxysporum f. sp. lactucae in pots under controlled conditions. Their effects on the density of the pathogen, on the total fungi and on fluorescent Pseudomonas spp. were also evaluated after two subsequent lettuce crops. A significant reduction in the severity of the symptoms of F. oxysporum f. sp. lactucae was found after the use of Brassica carinata pellets (52–79% reduction) and compost (49–67% reduction), while Brassica green manure and cattle and chicken manure only provided partial control of fusarium wilt. However, variations in effectiveness were observed for the same treatment in repeated trials. In general, an increase was observed in Pseudomonas and a decrease in fungal populations in the growing medium, which was obtained by mixing a blonde sphagnum peat and a sandy loam soil with B. carinata pellets and compost after two consecutive cropping cycles. Prolonging the Brassica and compost treatments from 30 to 60 days did not significantly affect disease severity, plant growth or the microbial population of the total fungi or Pseudomonas. The largest lettuce biomass was obtained in the non‐inoculated growing medium amended with brassica flour, chicken manure, B. carinata pellets and compost, as a consequence of fertilization. The treatment with B. juncea green manure, B. carinata (pellets and flour) and compost applied 30 days before planting led to promising results and merits further investigation for use under field conditions.     

Suppression of Fusarium wilt of spinach with compost amendments

The effect of different organic composts on the suppression of wilt disease of spinach caused by Fusarium oxysporum f. sp. spinaciae was evaluated in a continuous cropping system in both containers and in microplot field trials. Test soils infested with the pathogen were amended with wheatbran, wheatbran and sawdust, coffee grounds, chicken manure, or mixture of different composts with and without 5% (w/w) crab shell powder either once (5%, w/w) or continuously (2.5%) into the test soils infested with the pathogen. In the container trials, the soil amended with composts became suppressive to disease development on the second and third cropping. The suppressive effect was notable in the soil amended with the mixture of compost with and without crab shell powder. The coffee compost lowered soil pH but became suppressive to the disease after modifying the soil pH. In the field trial using the mixture of the different composts containing 5% crab shell powder, a combination of 5% before the first cropping and 2.5% every second cropping gave stable disease control and promoted plant growth. After compost amendment, populations of fungi, bacteria and actinomycetes as measured by dilution plate counting and the total microbial activity as evaluated by fluorescein diacetate hydrolysis increased and population of the pathogen gradually decreased. These phenomena were especially notable in soils amended with the mixture of different composts. These results indicate that diversity in the organic materials promotes higher microbial activity and population in the soil thereby enhancing disease suppressiveness.     

New insights into radiata pine seedling root infection by Fusarium circinatum

Pine root infection by Fusarium circinatum has been reported in the literature, but the underlying pathogenic interaction is poorly understood. A green fluorescent protein (GFP)‐tagged F. circinatum isolate, together with confocal microscopy, was used in order to monitor the events associated with root infection of Pinus radiata seedlings. It was found that in order to reach and successfully infect pine roots, F. circinatum employed features that are similar to those previously described for other root‐infecting pathogens, such as mycelial strands, single runner hyphae and simple hyphopodia as well as other features that are reminiscent of those that are known to be involved in biotrophic invasion, such as bulbous invasive hyphae and filamentous invasive hyphae. Abundant sporulation was observed at the root surface as well as inside tracheids both in roots and in the root collar region. The fungus can spread from the roots to the aerial parts of the plant, and once there, colonization appears to be similar to the process that occurs when the pathogen is inoculated in the stem. Wilting symptoms and plant demise may be the result of a reduction in water uptake by roots and of the blockage of the vascular system by fungal hyphae and resin.     

Cytological Characterization of Elicitin-Induced Protection in Tobacco Plants Infected by Phytophthora parasitica or Phytoplasma

ABSTRACT Elicitins, small proteins secreted by Phytophthora and Pythium spp., display the ability to induce plant resistance toward pathogens. Ultrastructural investigations of cryptogein-treated tobacco plants evidenced host defense responses such as (i) formation of a calcium pectate gel in intercellular spaces of parenchymas, (ii) impregnation of pectin by phenolic compounds in intercellular spaces of phloem bundles, and (iii) accumulation of phloem proteins (P proteins) in the lumen of leaf sieve elements. These cytological modifications lead to the enhancement of physical barriers that prevent pathogen ingress and restrict host tissue colonization when cryptogein-treated tobacco plants were challenged with the pathogen Phytophthora parasitica. Wall appositions also were observed at most sites of penetration of hyphae. Moreover, growing hyphae exhibited severe morphological damages, suggesting a modified toxic environment. The same induction of P proteins in mature sieve tubes of tobacco leaves was obtained with oligandrin treatment, another elicitin. Cryptogein or oligandrin treatment prevented symptom expression in phytoplasma-infected tobacco plants in contrast with nontreated tobacco plants. Moreover, P protein plugs and occlusion of pore sites by callose were evidenced in sieve elements of treated plants. Both these phloem modifications might prevent the in planta movement of phloem-restricted microorganisms.     

Elucidation of defence responses and signalling pathways induced in Arabidopsis thaliana following challenge with Phytophthora cinnamomi

Arabidopsis thaliana (Arabidopsis) Col-0 was inoculated with Phytophthora cinnamomi to assess the interaction and defence responses involved. Pathogen ingress and asexual reproduction occurred on root tissue but not leaf tissue. The colonisation of root tissue did not cause disease symptoms or plant death, indicating that Arabidopsis Col-0 was tolerant of the infection. The induction of several plant defence responses including the expression of defence-related genes were found, with differences displayed between inoculated root and leaf tissue. Arabidopsis defence-related gene mutant/over-expressing lines were also inoculated with P. cinnamomi but none of the lines tested exhibited a marked increase in susceptibility to the pathogen.     

Control of Fusarium wilt in melon by the fungal endophyte,Cadophora sp.

Two soil-borne fungal endophytes almost completely suppressed the effects of a post-inoculated and virulent strain of Fusarium oxysporum f. sp. melonis when inoculated to axenically reared melon seedlings in Petri dishes. They were identified as Cadophora sp. on the basis of ITS 1–5.8S rDNA–ITS 2 sequences and morphological characters and obtained from the roots of Chinese cabbage grown as bait plants in a mixed soil made up of samples from different forest soils from Alberta and British Columbia, Canada. Hyphae of Cadophora sp. grew along the surface of the root and colonized root cells of the cortex and reduced the ingress of the Fusarium pathogen into adjacent cells. Melon seedlings pre-inoculated with Cadophora sp. were also grown in soil amended with the different N sources, nitrate or the amino acids leucine and valine, and glucose (final C:N ratio?=?10:1). After 4 weeks, these seedlings were transplanted into the field and disease symptoms were assessed. Only the endophyte-inoculated seedlings treated with valine could effectively inhibit the development of Fusarium wilt in two plots and reduced disease symptom development by 43 and 62 %.