Appressorium-like Structures Research Articles

Adhesion and pre-invasion behaviour of urediniospores of Uromyces viciae-fabae during germination on host and synthetic surfaces

Dry urediniospores of Uromyces viciae-fabae will not germinate but will adhere to substrata. High humidity or free water initiate germination. Spores imbibe free water, swell and germ pores are lysed. Extracellular matrix is produced and possibly released through the germ pore. Emerging germ tubes are enclosed in a sheath of matrix and adhere tightly to the substratum. This is a non- specific response. Several factors, either singly or in combination, may induce the formation of appressorium-like structures. These are discussed in relation to the availability of water and the topography of the substratum surface.

Ultrastructure of conidial germ-tube development in vitro by the insect pathogen Entomophaga aulicae

In an effort to understand the factors influencing the formation of infection structures by the insect pathogen Entomophaga aulicae, we examined the ultrastructural development of conidial germ tubes formed in vitro under conditions that resemble those producing appressoria and protoplasts during infection. Conidia germinated on formvar in a nonaqueous environment produced a single viable germ tube, which in turn produced appressorium-like structures or secondary conidia, structures similar to those formed on the host. The formation of appressorium-like structures indicates that stimuli for appressorium formation are relatively nonspecific, whereas development of the infection hypha requires different triggers. Conidia germinated in a liquid culture medium, which supports the growth of the protoplast stage of the fungus, produced a single viable germ tube that continued to grow apically. This apical growth was accompanied by the loss of the outer layer of the germ tube wall and the presence of electron-opaque granules in an extensive system of cytoplasmic membranes. Protoplasts did not form directly from these germ tubes and ultrastructural details of tip growth in this medium did not resemble those previously described in infection hyphae prior to protoplast formation in the host.

Penetrating Structures of Anaerobic Rumen Fungi in Cattle and Swamp Buffalo

Rhizoids of anaerobic fungi colonizing fragments of guinea grass and rice straw in the rumen of cattle and swamp buffalo produced appressorium-like structures for penetration of the plant cell walls. These structures were generally lobed vesicles with fine penetration pegs, which penetrated through the cell wall and then expanded and continued their growth in adjacent cells, forming normal rhizoids which would in turn produce more ‘appressoria’ for penetration of the next cell. Their size and form varied with the size of the cell in which they developed. Small, flat ‘appressoria’ were produced in cells with small lumina while bigger and longer ‘appressoria’ developed in larger cells. ‘Appressoria’ were clearly observed in rhizoids of fungi colonizing the plant fragments about 1 h after incubation in the rumen and were produced in rumen fungi with spherical, oval or ovoid sporangia.

Production of appressoria at the tips of hyphae of Pyricularia oryzae.

Appressoria-like structures (ALS) were formed at the tips of hyphae of the blast fungus in Petri dishes containing PDA or water agar. Some of them germinated in PDA media and produced again a relatively small appressorium at the tip of hypha germinated from the primary appressorium. The ALS were pigmented brown to olive, granular, smooth and thick walled. They were variable in size and shape, but mostly globose, ovoid or oblong, and 4-11μm (mean 10.1μm) in diameter. ALS were formed in all isolates used. More ALS were formed in plastic Fetri dishes than in glass ones. On the detached leaf sheaths of rice plants ALS adhered firmly to the host surface, from which the fungus penetrated into the host by piercing the outer walls of epidermal cells with a penetration peg. Susceptible blast lesions developed on the leaf blades of rice plants inoculated with mycelial agar blocks. Susceptible lesions also developed on the leaf sheaths of rice plants inoculated by injecting mycelial suspension. Micrographs of ultrathin sections revealed that ALS were covered with a mucilaginous substance and their walls were relatively thick and more electron-opaque than those of hyphae. From these observations of morphological and functional features, we concluded that ALS of hyphae are appressoria identical to the ones formed by germ tubes from conidia. This is the first report confirming that appressoria form at the hyphal tips of the blast fungus.

The development of infection structures of Plasmopara halstedii in suspensions of sunflower cells

Zoospores of Plasmopara halstedii germinate in water by a germ tube which elongates but does not differentiate. Zoospores in a suspension containing sunflower cells produced a short germ tube terminating in an appressorium flattened against a host cell. Zoospores which adhered to the Petri dish without contacting a host cell also produced appressorialike structures, which in some cases differentiated further to produce structures resembling infection vesicles in Plasmopara-infected sunflower cells. Initiation of infection structures may require diffusible products from host cells but apparently does not require contact with the cells.

Hyphal interactions between a mycoparasite, Pythium nunn, and several soil fungi

The modes of hyphal interaction between a mycoparasite, Pythium nunn, and several soil fungi were studied by both phase-contrast and scanning-electron microscopy. In the zone of interaction, Pyth. nunn massively coiled around and subsequently lysed hyphae of Pyth. ultimum and Pyth. vexans. In contrast, Pyth. nunn penetrated and eventually parasitized hyphae of Rhizoctonia solani, Pyth. aphanidermatum, Phytophthora parasitica, and Phyto. cinnamomi by forming appressoriumlike structures. However, Pyth. nunn was not mycoparasitic against Fusarium oxysporum f.sp. cucumerinum or Trichoderma koningnii and was destroyed by T. harzianum and T. viride. These observations demonstrated that Pyth. nunn was a necrotrophic mycoparasite, with a limited host range and differential modes of action among suscepts.

The penetration and growth of blue-stain fungi in the sapwood of lodgepole pine attacked by mountain pine beetle

The growth of blue-stain fungi was investigated in naturally blue-stained lodgepole pine (Pinus contorta var. latifolia Engelm.) sapwood. Events occurring at the leading edge of hyphal penetration were studied. Fungi are initially confined to the sapwood rays. Hyphae readily penetrate the primary cell walls of ray parenchyma cells and proliferate within. Hyphae also grow freely in the region of the middle lamella of the rays. Host cell walls are breeched mechanically by a penetration peg originating from an appressoriumlike structure. Eventually, hyphae enter tracheids by penetrating the primary cell walls of pinoid, half-bordered pit pairs. Within the tracheid, fungal hyphae grow in a longitudinal fashion, branching infrequently. Hyphae may pass from tracheid to tracheid via bordered pit pairs. Ensuing water stress and eventual tree death is discussed in light of histological evidence presented.

Ultrastructure of the interaction between Pyrenophora teres and a susceptible barley host

Host–-parasite interfaces in leaves of a susceptible barley cultivar inoculated with Pyrenophora teres were studied using light and electron microscopy. Entry into the host rarely occurred through stomata, normal entry was by penetration into epidermal cells. Disruption of cell contents was evident in all initial penetration sites. After penetrating the epidermal cell, the fungus produced a large infection vesicle which gave rise to one or sometimes two intracellular hyphae. Hyphae spread down through the leaf tissue intracellularly for one or two cell layers and intercellularly thereafter. A septum, coincident with the external epidermal cell wall, was present in the infection peg located between the appressorium and the vesicle. Prior to leaving the epidermal cell, hyphae swelled to produce appressorium-like structures. These hyphae were sometimes septate at the point of exit. At sites where the infection had penetrated only four or five cell layers deep, the cellular contents of the mesophyll cells were relatively unaffected despite the presence of intercellular hyphae. However, when hyphae occasionally penetrated a host cell, gross disruption of their contents was apparent.

Ultrastructure of penetration by Helminthosporium maydis

Penetration by Helminthosporium maydis race T into susceptible leaves from Texas male sterile and resistant ones from normal plants was studied with four different corn inbred lines. Six hours after inoculation, appressoria-like structures were present but were rare, and no evidence of penetration beneath such structures was found. All penetrations seen 6 h after inoculation took place between epidermal cells, and at least two-thirds of these occurred between cells adjacent to stomates. Breaks in the cuticle above subsidiary cells of the stomatal apparatus may account for the high frequency of penetration in this area. Eight to 10 h after inoculation, miniature infection cushions had formed at initial points of penetration, and from these, hyphae radiated out beneath the cuticle of epidermal cells. Haustoria-like branches produced by these subcuticular hyphae appeared to function in secondary infection. Hyphae of the fungus were seen occasionally within stomatal openings (apparent stomatal penetration), but these were consistently associated with penetrations which had occurred between nearby epidermal cells. No differences were observed at any stage of penetration or initial colonization between Texas male sterile and normal plants of the same inbred or among the four inbreds studied. These results suggest that resistance to this pathogen develops only after infection has occurred and that the selective pathotoxin produced by race T does not play an important rôle in the infection process.

Appressorium-like structures in insect-parasitizing deuteromycetes

Paecilomyces farinosus in slide cultures forms clavate branches of limited growth against the glass, but never, in either slide or plate cultures, on the agar medium itself. The possibilities that these branches represent abortive conidiophores, storage organs, and appressoria are considered, and evidence against the first two possibilities is presented. The possibility that they are appressoria is supported by the circumstances of their formation and their role in the penetration of artificial membranes of paraffin wax. They form on excised pieces of integument of the insect Tenebrio molitor, and the fungus is able to penetrate such pieces, but it has not yet been possible to establish their participation in penetration of integument in vitro or in vivo. Similar appressorium-like structures are formed in vitro by Beauveria bassiana and Metarrhizium anisopliae also. It is argued that because there are certain similarities in the structure and composition of insect cuticle and cuticularized plant epidermises, it is worth seriously considering the possibility that host-boundary penetration by insect-parasitizing fungi resembles that by many appressorium-forming plant pathogens in that it involves first a process of mechanical penetration and then one of enzymic digestion, possibly accompanied by mechanical pressure.

ANTAGONISME DES MICROORGANISMES DU SOL ENVERS FUSARIUM OXYSPORUM F. LINI, AGENT DE LA FLÉTRISSURE DU LIN

Nineteen bacteria and 11 actinomycetes isolated from various soils in which fiber flax had been grown in the Lower St. Lawrence district were found to be antagonistic to Fusarium oxysporum f. lini. Among the 11 organisms forming the largest inhibition zone on agar plates, six had a fungicidal action on the spores of F. oxysporum f. lini while the other five exerted a fungistatic action. Among the 19 antagonistic bacteria under trial, six induced a swelling of the germ tubes or the production of an appressoriumlike structure on agar. Green-house experiments have indicated that 15 bacteria and 11 actinomycetes offered a certain protection to seedlings some of which reached. 6, 12, and sometimes 24 in. in height before showing any signs of uniting. The organisms evidently delayed the onset of wilt symptoms, but failed to protect the plants through their entire growth.