玉米瘤黑粉菌的寄生策略及其调控机制

玉米瘤黑粉病是由担子菌Ustilago maydis对玉米的活体寄生所引起的真菌病害。该病原菌为双相型真菌,需要寄生于玉米植株来完成其有性生殖过程。综合相关研究报道,本文把U.maydis对寄主植物的寄生过程划分为7个阶段,包括形成致病性双核菌丝体、附着寄主植物表面、穿透寄主表皮、消减寄主防御反应、在寄主体内菌丝增殖、使寄主瘤变和生成厚垣孢子等。围绕寄生进程特点和关键基因,分别阐述了各个阶段的相关调控机制以及对寄主植物的致病性;展现了U.maydis为达到有性生殖目的而实施步步为营的寄生策略。本文对U.maydis寄生过程的阶段划分,有助于人们深入了解U.maydis与寄主植物之间互作机制、提供相关病害防控新思路。     

Production of Pectic Enzymes by Fusarium oxysporum f. sp. cepae and its Involvement in Onion Bulb Rot

Fusarium oxysporum f. sp. cepae produced an exo-polygalacturonase (exo-PG) and endopectin-trans-eliminase (endo-PTE) in a mineral medium supplemented with a restricted supply of either D-galacturonic acid or onion cell walls. These enzymes were also extracted from infected onion tissue, but only endo-PTE caused tissue maceration and cell death. The patterns of host tissue colonization and pectic enzyme production were followed during bulb rot development. Stem plates were invaded within two weeks of inoculation. The pathogen then remained confined to the stem plates for several weeks or months, before spreading to the outer fleshy scales to initiate a basal rot. In most cases the inner leaf sheaths containing the lateral bud remained healthy. Exo-PG activity m stem plate tissue was greatest at two weeks after inoculation, then it declined. Endo-PTE was not detected in newly invaded stem plate tissue, but was recovered from infected stem plates before decay and from the bases of bulb scales and leaf sheaths at the onset of bulb rot. There was no pectic enzyme activity in uninvaded onion tissue. Spread of the fungus and pectic enzyme production in two Caledon Globe genotypes susceptible or tolerant to F. oxysporum f. sp. cepae were similar, but the onset of bulb rot in tolerant genotypes was considerably delayed.     

Production of a toxic metabolite and pectolytic enzyme by Pythium butleri

Pythium butleri which causes root-rot of Belladonna produced a heat stable toxic metabolitein vitro. The culture filtrate induced wilting when applied to cut shoots. The pathogen produced significant amount of endo-polygalacturonase, endo-polymethylgalacturonase, exo-polygalacturonase, polygalacturonate transeliminase and pectin methyl-trans-eliminase in media containing glucose and pectin as carbon source. The fungus did not produce pectin methyl estrase and was very weakly cellulolytic. The extensive distintegration of host tissue during pathogenesis can be considered due to the pectolytic enzymes produced by the pathogen. The physiology of this fungus differs markedly from other species ofPythium and justifies its taxonomic position as a distinct species.     

Effect of the stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, on the weed Emex australis

The stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, are two potential biological control agents for the annual weed, Emex australis. Neither pathogen nor weevil affected the development of rosette (5-wk-old) E. australis, but both significantly reduced stem length and number of new fruits in mature plants (10-wk-old) in growth room experiments. Stems grew two-thirds less than controls in plants attacked by weevils, had no net increase when both weevils and fungus were present, and collapsed in the presence of the fungus alone. Attack by weevils elicited a host response that slowed the development of the disease. Phomopsis emicis and Perapion antiquum reduced the ability of E. australis to produce new fruits by 77% and 68%, respectively. Pathogen and weevil together reduced the number of new fruits by 83%. Perapion antiquum did not carry or provide infection sites for P. emicis. The lack of damage by pathogen and weevil to rosettes could compromise their effectiveness as biological control agents.     

Direct infection of embryogenic tissue cultures of haploidBrassica napus with resting spores ofPlasmodiophora brassicae

Summary A direct infection of cultured tissue with resting spores of the obligate endoparasitePlasmodiophora brassicae has been made possible by using stem embryo cultures of haploid rape. The fungus develops and completes its lifecycle in the cultured cells. The results are discussed in connection with the possibility of using this system to select for resistance to the pathogen.Supported by the Deutsche Forschungsgemeinschaft     

Cross Protection Between Strains of Fusarium oxysporum f. sp. vasinfectum and its Effect on Vascular Resistance Mechanisms

The severity of symptoms induced in cotton plants by stem inoculation with Fusarium oxysporum f. sp. vasinfectum was reduced by inoculation with a non-pathogenic strain (NPS) of the fungus 3 days previously. Symptom reduction was associated with reduced populations of the pathogen in the stele tissue of the stem. This cross-protective response became effective within 24 h of inoculation with the NPS. Although both vascular occlusion and terpenoid aldehyde synthesis were induced by theNPS, the short time required for cross protection to become effective and the duration of the effect, indicated that vascular occlusion rather than fungitoxicity of the terpenoid aldehyde compounds was the mechanism of protection.     

Pathogenesis of barley leaves by Helminthosporium teres I: Green island formation and the possible involvement of cytokinins

Infection sites/green islands were formed in host leaf tissue infected with drops of H. teres. They exhibited higher cytokinin-like activity, sugar and starch than their surrounding tissue and tissue under water drops. The cytokinin-like activity at the infection sites increased from 24 to 72 h of incubation. However, the cytokinin-like activity of the tissue surrounding the infection drops and the tissue under water drops fell from 24 to 72 h incubation. The culture filtrate extracts of the fungus also produced cytokinin-like activity which increased from 1 to 10 days incubation. Application of this culture filtrate extract evoked green island formation. Application of kinetin to host leaves duplicated the green island effect. Thin-layer chromatographic fractions of the tissue extracts and the culture filtrate extracts revealed that a major portion of cytokinin-like activity corresponded to zeatin and zeatin riboside. The presence of zeatin and zeatin riboside (both in tissue and culture filtrate extracts) was confirmed by high performance liquid chromatography. Increases in the amounts of cytokinin-like substances, particularly zeatin and zeatin riboside, attributed to pathogen influence are suggested to be involved in infection and pathogenicity of H. teres.     

Facultative parasites and host respiration III. Tissue which is actively parasitised

Summary Fusarium oxysporium f.lycopersici andVerticillium albo-atrum caused an increase in the rate of respiration of tomato stem tissue.V. albo-atrum caused an increase in the rate of respiration of potato haulm tissue. By comparison of the respiration of 1 g ofV. alboatrum mycelium with 1 g fresh weight infected tomato stem tissue it was concluded that the respiration of the pathogen in situ does not contribute markedly to the respiratory increase. It is suggested that the increase in respiration has its origin in the metabolism of the host, possibly due to stimulation by a fungal respiratory toxin but not due to the augmentative effect of the pathogen's respiration.This work formed part of a Ph. D. thesis submitted to the University of London, July 1961.     

Culture storage age and fungal re‐isolation from host‐tissue influence Colletotrichum spp. virulence to pepper fruits

Using resistant cultivars is the most sustainable and practical approach against plant diseases. Plant germplasm and breeding lines are selected and assayed against, usually, the most aggressive or virulent strains of a pathogen (e.g., fungus) that causes the disease. However, prolong storage of the pathogen in culture media could affect virulence that, consequently, also influence the outcome of the resistance assay. This study demonstrates that long‐term storage (at least a year) of Colletotrichum truncatum and C. scovillei, causal agents of pepper anthracnose, in potato dextrose agar (PDA) medium decreased the aggressiveness and virulence of the fungus in host‐pepper fruits. However, reintroduction of the pathogen to the host and isolation of the pathogen as the new inoculum, prior to inoculation assays, increased the virulence of the fungi. These findings suggest that re‐inoculation and re‐isolation of Colletotichum truncatum and C. scovillei that have been stored for at least 1 year in PDA medium are necessary when using fungal cultures in pathogenicity and plant resistance assays to achieve desirable, comparable and reliable results.     

Fungus gnats vector Fusarium oxysporum f.sp. radicislycopersici1

Fungus gnat adults transported Fusarium oxysporum f.sp. radicis-lycopersici from Petri dish culture and infected host plants to the roots and hypocotyls of healthy tomato and bean plants. The source of the fungus did not affect the ability of fungus gnats to transport the fungus to healthy hosts. The presence of fungus gnat larvae in media in which young tomato plants were grown did not increase the incidence of plant infection by the pathogen. Fungus gnat adults appear to aid in the dissemination of F. oxysporum f.sp. radicis-lycopersici.     

Phymatotrichum (cotton) root rot caused by Phymatotrichopsis omnivora: retrospects and prospects

Phymatotrichum (cotton or Texas) root rot is caused by the soil‐borne fungus Phymatotrichopsis omnivora (Duggar) Hennebert. The broad host range of the fungus includes numerous crop plants, such as alfalfa and cotton. Together with an overview of existing knowledge, this review is aimed at discussing the recent molecular and genomic approaches that have been undertaken to better understand the disease development at the molecular level with the ultimate goal of developing resistant germplasm. Taxonomy: Phymatotrichopsis omnivora (Duggar) Hennebert [synonym Phymatotrichum omnivorum (Shear) Duggar] is an asexual fungus with no known sexual stage. Mitosporic botryoblastospores occasionally form on epigeous spore mats in nature, but perform no known function and do not contribute to the disease cycle. The fungus has been affiliated erroneously with the polypore basidiomycete Sistotrema brinkmannii (Bres.) J. Erikss. Recent phylogenetic studies have placed this fungus in the ascomycete order Pezizales. Host range and disease symptoms: The fungus infects most dicotyledonous field crops, causing significant losses to cotton, alfalfa, grape, fruit and nut trees and ornamental shrubs in the south‐western USA, northern Mexico and possibly parts of central Asia. However, this fungus does not cause disease in monocotyledonous plants. Symptoms include an expanding tissue collapse (rot) of infected taproots. In above‐ground tissues, the root rot results in vascular discoloration of the stem and rapid wilting of the leaves without abscission, and eventually the death of the plant. Characteristic mycelial strands of the pathogen are typically present on the root's surface, aiding diagnosis. Pathogenicity: Confocal imaging of P. omnivora interactions with Medicago truncatula roots revealed that infecting hyphae do not form any specialized structures for penetration and mainly colonize cortical cells and eventually form a mycelial mantle covering the root's surfaces. Cell wall‐degrading enzymes have been implicated in penetration and symptom development. Global gene expression profiling of infected M. truncatula revealed roles for jasmonic acid, ethylene and the flavonoid pathway during disease development. Phymatotrichopsis omnivora apparently evades induced host defences and may suppress the host's phytochemical defences at later stages of infection to favour pathogenesis. Disease control: No consistently effective control measures are known. The long‐lived sclerotia and facultative saprotrophism of P. omnivora make crop rotation ineffective. Chemical fumigation methods are not cost‐effective for most crops. Interestingly, no genetic resistance has been reported in any of the susceptible crop species.     

Infection of maize leaves with Ustilago maydis prevents establishment of C4 photosynthesis

The Basidiomycete fungus Ustilago maydis is the common agent of corn smut and is capable of inducing gall growth on infected tissue of the C₄ plant maize (Zea mays). While U. maydis is very well characterized on the genetic level, the physiological changes in the host plant in response to U. maydis infection have not been studied in detail, yet.Therefore, we examined the influence of U. maydis infection on photosynthetic performance and carbon metabolism in maize leaf galls.At all stages of development, U. maydis-induced leaf galls exhibited carbon dioxide response curves, CO₂ compensation points and enzymatic activities that are characteristic of C₃ photosynthesis, demonstrating that the establishment of C₄ metabolism is prevented in infected tissue. Hexose contents and hexose/sucrose ratio of leaf galls remained high at 6 days post infection, while a shift in free sugar metabolism was observed in the uninfected controls at that time point. Concomitantly, transitory starch production and sucrose accumulation during the light period remained low in leaf galls. Given that U. maydis is infectious on young developing tissue, the observed changes in carbohydrate metabolism suggest that the pathogen manipulates the developing leaf tissue to arrest sink-to-source transition in favor of maintaining sink metabolism in the host cells.Furthermore, evidence is presented that carbohydrate supply during the biotrophic phase of the pathogen is assured by a fungal invertase.     

Importance of ambient saturation deficits in an epizootic of the fungus Neozygites floridana in cassava green mites (Mononychellus tanajoa)

The mite-pathogenic fungus Neozygites floridana Fisher (Entomophthorales: Neozygitaceae) is considered to have potential for the biological control of the cassava green mite, Mononychellus tanajoa (Bondar). However, its activity is sporadic and laboratory data suggest a strong dependence on night-time saturation deficits for transmission. We report on an epizootic of this fungus in a mite population in northeastern Brazil. During the epizootic, host populations appeared to be limited by a combination of the pathogen and a predatory mite Neoseiulus idaeus (Acari: Phytoseiidae). When temperatures increased, the epizootic finished and the host population began to grow. Abiotic conditions could not explain the variation in host mortality following pickup of infective propagules in this epizootic. However, night-time saturation did help to explain the variation in transmission from infective cadavers to newly killed hosts. This supports laboratory observations that horizontal transmission between hosts is determined mainly by saturation deficits, while the process of infection is little affected by abiotic conditions. A further field observation was the near-absence of resting spores in dead mites (ca. 0.1% of cadavers), suggesting that the pathogen population was unsuccessful in producing inoculum to infect future M. tanajoa populations. The implications are that this pathogen will only be effective as a biological control agent in periods of high relative humidity, and establishment in new areas may be limited by resting spore formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Toxicity of hydrolysis volatile products of Brassica plants to Sclerotinia sclerotiorum,in vitro

Oilseed rape stem rot disease caused by Sclerotinia sclerotiorum causes serious yield losses worldwide. Glucosinolates as specific secondary metabolites of Brassicaceae are produced in various parts of the host plants. Their enzymatic hydrolysis releases chemical components, particularly isothiocyanates, with fungitoxic activity and volatile characteristics. To investigate the effect of volatiles derived from Brassica tissues, the pathogen was exposed to hydrolysis products of Brassica shoot parts as sources of glucosinolates including oilseed rape varieties and two species, black and white mustard. The results showed significant differences in inhibition of S. sclerotiorum growth between varieties and species. All tissues of black mustard inhibited completely the exposed colonies of the pathogen and oilseed rape varieties Dunkeld, Oscar and Rainbow had significant inhibitory effect on the fungus. The genotypes demonstrated significant differences for the production of toxic volatiles, indicating that GSL contents in Brassica species and even cultivars have different potentials for toxic products.     

Diagnosis and management of Sclerotinia stem rot (white mould) of lentils in Greece

The paper describes the field-level symptoms, the identification and management of Sclerotinia stem rot of lentil caused by the soilborne plant pathogen Sclerotinia sclerotiorum in Greece. Regarding symptoms at a field level, initially plants before flowering turn yellow with roots and the base of the plants become brown; then rotten plants exhibit a dry stem and die. On the diseased tissue, at the base of the stem, the typical white mycelium and the resting bodies (sclerotia) were observed. According to our pathogenicity studies in vitro, on the infected plant tissues the fungus first develop a characteristic fluffy white mycelium which will give rise to large black sclerotia, the most obvious evidence of plants infected with S. sclerotiorum. Finally, concerning evaluation of fungicides, isolates of S. sclerotiorum were sensitive to thiophanate-methyl and to triazole fungicides. Thiophanate-methyl and triazole fungicides proved to be most effective in controlling the disease emerged from mycelium or sclerotia.     

Induced tolerance of Sclerotium cepivorum to antibiotics in the presence of onion exudates

The onion white rot pathogen Sclerotium cepivorum was cultured on agar media containing 2% malt extract and one of the antifungal antibiotics, endomycin, griseofulvin, venturicidin and cycloheximide at concentrations that reduced but did not prevent growth of mycelium. When onion seeds or agar discs impregnated with diffusates from onion bulbs were placed on the antibiotic media, radial growth of the fungus was greatly increased, and there was a profuse development of aerial mycelium. Gaseous diffusates from onion tissue and from impregnated agar discs were also effective. On the antibiotic media, tomato, cabbage and radish seeds did not stimulate the growth of S. cepivorum and the onion exudates did not stimulate the growth of four other fungi. This and other evidence is considered to show that the stimulation of growth of S. cepivorum was not caused by any direct effect on the antibiotics but by a tolerance of the fungus to them, which was specifically induced by an exudate from its host plant. The phenomenon may be related to the reported reversal by onion extracts of the inhibitory effects of soil mycostasis on germination of sclerotia of the fungus.     

Association between the potato cyst-nematode, Heterodera rostochiensis Woll., and Verticillium dahliae Kleb. in the early-dying disease of potatoes

Investigation of a possible association between Verticillium dahliae and H. rostochiensis (pathotype E, British notation) was based on field observations and an examination of disease development in single-stemmed potato plants grown in pots. An association was found in the distribution of the nematode and the fungus in the field, and the disease was far more severe with a combined infection than with either pathogen alone. Studies on leaf-area development and yield reduction suggest there is synergism between fungus and nematode, the reductions produced by the combined infection exceeding the sum of those produced by either pathogen alone. Fungal mycelium and the extent of host colonization by V. dahliae were greatly increased by the presence of the nematode. The possible benefits to V. dahliae in the fungus-nematode complex are discussed.