Rust Fungi

Rust fungi belong to the Basidiomycetes and are some of the most successful obligate biotrophs. A number have served as model systems for investigations of plant-pathogen interactions. For example, genetic studies of Melampsora lini (flax rust) formed the basis of the gene-for-gene hypothesis which states that every resistance gene in a plant has a corresponding avirulence (Avr) gene in the pathogen (Flor 1955). Molecular studies of Uromyces fabae (broad bean rust) have contributed to our understanding of nutrient acquisition during biotrophic growth. Due to their major contribution to disease development and propagation, dikaryotic asexual rust uredospores have been the focus of most studies of rust infection strategies (Fig. 1). When a uredospore lands on a plant, it adheres to the plant surface initially through passive, hydrophobic interactions before secreting material that forms an adhesive pad of glycoproteins and carbohydrates (Clement et al. 1993; Deising et al. 1996). Upon spore germination, the germ tube extends by tip growth across the leaf surface. The detection of chemical signals on the plant surface is no doubt a feature common to most, if not all fungi and oomycetes, but hyphae of rust fungi also sense and respond to topographical signals from the underlying surface. During this thigmotropic growth, hyphae of Puccinia species, for example, extend in a direction that is perpendicular to the long axis of the leaf epidermal cells (Read et al. 1992; Staples and Hoch 1997). When the hypha encounters a guard cell, the hyphal apex swells and differentiates into an appresso-rium over the stomatal pore (Hoch and Staples 1991). Both thigmotropic growth and appressorium formation can be induced on inert replicas of the leaf surface or artificial gratings providing clear evidence that the inductive signals are associated with topographical features of the underlying surface (Hoch et al. 1987). Entry of the fungus into the plant leaf then occurs through the stomatal cavity via a penetration hypha produced from the base of the appressorium.

On entering the leaf tissue, the penetration hypha expands to form a substomatal vesicle in the substomatal cavity. From this vesicle, one or more primary infection hyphae grow until they make contact with a host mesophyll cell whereupon they differentiate to form haustorial mother cells (Heath and Skalamera 1997). A penetration hypha is formed by the haustorial mother cell and grows through the plant cell wall, invaginating the host plasma membrane as it expands and differentiates into a haustorium (Harder and Chong 1991). Upon establishment of haustoria, the fungus is no longer metabolically independent and enters a "parasitic/

Fig. 1 Schematic representation of the rust fungal uredospore infection. Diagram design modified from Voegele (2006). U uredospore, Ad adhesive, GT germ tube, Ap appressorium, PH penetration hypha, SV substomatal vesicle, HMC haustorial mother cell, H haustorium, EHM extrahaustorial membrane, GC stomatal guard cell, IH infection hyphae, C cuticle, UP uredospore pustule. Early infection structures from the "penetration stage" are shown in dark brown, structures from the "parasitic/biotrophic stage" are shown in yellow and structures from the "sporulation stage" are shown in red

Fig. 1 Schematic representation of the rust fungal uredospore infection. Diagram design modified from Voegele (2006). U uredospore, Ad adhesive, GT germ tube, Ap appressorium, PH penetration hypha, SV substomatal vesicle, HMC haustorial mother cell, H haustorium, EHM extrahaustorial membrane, GC stomatal guard cell, IH infection hyphae, C cuticle, UP uredospore pustule. Early infection structures from the "penetration stage" are shown in dark brown, structures from the "parasitic/biotrophic stage" are shown in yellow and structures from the "sporulation stage" are shown in red biotrophic phase" during which it obtains nutrients from its host (Deising et al. 1996; Voegele 2006).

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