Infection Structures

Nearly all fungal plant pathogens gain entry into their host via a structure called the appres-sorium. The firm attachment of the appressorium to the hydrophobic host surface prevents lift-off as the infection (penetration) peg from the underside forces entry through the narrow stomata pore or pierces through the cuticle and cell wall. Mutants of Magnaporthe grisea that are deficient in hydrophobin show reduced pathogenesis (Talbot et al., 1996). Howard et al. (1991) used solutions of polyethylene glycol to plasmolyse and by this technique estimated the force generated in appressorium produced on Mylar membrane (composed of polyethylene terephthalate) was in excess of 8.0 megapascal. The fungus apparently converts the lipid and glycogen reserves into glycerol that causes water uptake and the build-up of turgor pressure. The appressorium cell wall contains melanin, which reduces the porosity of the appressorium cell to solute flux but not water flux and contributes to osmotically generated increase of turgor pressure (Howard and Ferrari, 1989).

The signal for appressorium differentiation induces division of nuclei and a sequence of cellular development that results in the formation of a substomatal vesicle, formation of infection hyphae (Maheshwari et al., 1967c) that develops a projection called the haustorium that penetrates into the host cell, surrounded by an invagination of the host plasma membrane (Figure 4.6). Haustoria are especially prominent in obligate parasites: downy mildew (Straminipila, order Peronosporales), powdery mildew (Asco-mycotina, order Erysiphales) and rust fungi (Basidiomycotina, order Uredinales). In powdery mildew, all the hyphae are on the surface of the host and only haustoria penetrate the epidermal cell and are therefore the site of the uptake of nutrients. The formation of

Figure 4.5 Diagram of infection structures developed from germinating urediospore of the rust fungus, Puccinia antirrhini. (a) Appressorium over stomata in snapdragon leaf. (b) Substomatal vesicle on the underside of leaf epidermis. (c) Appressorium on isolated leaf cuticle. (d) Substomatal vesicle and infection hyphae on isolated cuticle. (From Maheshwari et al. (1967). With permission of American Phytopathological Society.)

Figure 4.5 Diagram of infection structures developed from germinating urediospore of the rust fungus, Puccinia antirrhini. (a) Appressorium over stomata in snapdragon leaf. (b) Substomatal vesicle on the underside of leaf epidermis. (c) Appressorium on isolated leaf cuticle. (d) Substomatal vesicle and infection hyphae on isolated cuticle. (From Maheshwari et al. (1967). With permission of American Phytopathological Society.)

the haustorium occurs under the control of species- or even variety-specific signals and is also the site for the exchange of information between the host and the parasite. Mendgen and coworkers succeeded in isolating haustoria from rust fungus-infected broad bean leaves by separating them from tissue homogenates by affinity chromatography with the lectin, concanavalin A (Hahn and Mendgen, 1997; Voegele et al., 2001). The mRNA prepared from haustoria was used to construct a cDNA library of infection structures formed in vitro (Figure 4.7), which was screened by Northern (RNA) hybridization for genes specifically expressed in haustorium. Sequence analysis identified genes involved in nutrient uptake and vitamin biosynthesis. Using antibodies against the yeast hexose transporter as a heterologous probe and against a putative plant-induced amino acid transporter, nutrient transporters were immunochemically localized exclusively in haus-torial plasma membrane of the bean rust fungus Uromyces fabae (Basidiomycotina), suggesting the uptake of sugar, and possibly of other nutrients (amino acids), occurs only through the haustorium. We need to understand how the fungal haustorium acts as a sink that effectively competes with the normal sink-source translocation pattern in plants. The specific localization of nutrient transporters in haustorium (Figure 4.8) suggests that the identification of host stimuli for haustoria differentiation will be crucial in designing conditions for the in vitro culture of obligately parasitic fungi on artificial media. Obligate parasites such as the white rust fungus Albugo (Straminipila), the powdery mildew fungus

Albugo Diagram

Fungal cell-wall - Intercellular hypha

Fungal cell membrane

//I Callose collar

Host cell membrane

Fungal cell-wall - Intercellular hypha

Fungal cell membrane

Figure 4.6 Diagram of a haustorium. The haustorium invaginates the host cell membrane. (From Ingold and Hudson, The Biology of Fungi (1973), Chapman and Hall. With permission of Kluwer Academic Publishers.)

Erysiphe (Ascomycotina) and the rust fungi Uromyces and Puccinia (Basidiomycotina) are among important pathogens of crop plants.

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