The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi

Abstract

The powdery mildew diseases, caused by fungal species of the Erysiphales, have an important economic impact on a variety of plant species and have driven basic and applied research efforts in the field of phytopathology for many years. Although the first taxonomic reports on the Erysiphales date back to the 1850's, advances into the molecular biology of these fungal species have been hampered by their obligate biotrophic nature and difficulties associated with their cultivation and genetic manipulation in the laboratory. The discovery in the 1990's of a few species of powdery mildew fungi that cause disease on Arabidopsis has opened a new chapter in this research field. The great advantages of working with a model plant species have translated into remarkable progress in our understanding of these complex pathogens and their interaction with the plant host. Herein we summarize advances in the study of Arabidopsis-powdery mildew interactions and discuss their implications for the general field of plant pathology. We provide an overview of the life cycle of the pathogens on Arabidopsis and describe the structural and functional changes that occur during infection in the host and fungus in compatible and incompatible interactions, with special emphasis on defense signaling, resistance pathways, and compatibility factors. Finally, we discuss the future of powdery mildew research in anticipation of the sequencing of multiple powdery mildew genomes. The cumulative body of knowledge on powdery mildews of Arabidopsis provides a valuable tool for the study and understanding of disease associated with many other obligate biotrophic pathogen species.

Figure 1.

Macroscopic infection phenotypes of susceptible and resistant Arabidopsis lines. Rosette leaves of 5–6 week old A. thaliana ecotypes Col-0 (A), Do-0 (B), and Sorbo (C) as well as the powdery mildew resistant mutant pmr6-3 (D) at 13 days post-inoculation with G. orontii. Completion of the asexual powdery mildew life cycle is evidenced by the occurrence of abundant sporulation (white powder) on inoculated rosette leaves of the susceptible accession, Col-0. Younger leaves without disease symptoms emerged after inoculation with fungal conidiospores. Note the difference in appearance of infected leaves of resistant accessions Do-0 and Sorbo. Resistance in both accessions is assumed to be governed by RPW8 (Göllner et al., 2008).

Figure 2.

Host and non-host powdery mildew interactions in Arabidopsis. Scanning electron micrographs of Arabidopsis leaf surface carrying germinated spores of adapted G. orontii (large image) and non-adapted B. graminis f. sp. hordei at 48 hours post-inoculation. Note that Bgh forms a primary (PGT) and a secondary germ tube (SGT), the latter of which differentiates into an appressorium (APP), while G. orontii produces only one germ tube (GT). In the case of Bgh, infection is arrested at this stage in approximately 95% of the cases. In contrast, G. orontii has already formed secondary hyphae (SH) indicating successful host cell penetration and haustorium formation (Photo courtesy of D. Meyer). Scale bar: 20 μm.

Figure 3.

O. neolycopersici growing on Arabidopsis Col-0 at four days after inoculation. Notice the lobate appressoria (arrowheads) that form at regular intervals along the secondary hyphae. The disease index on Col-0 is usually 2.6 (approximately 30% leaf coverage at 15 days post-inoculation; Bai et al., 2008). Scale bar: 10 μm (inset), 100 μm (large picture).

Figure 4.

Microscopic analysis of the development of a powdery mildew colony. The micrographs show the expansion of a G. orontii colony on the surface of a Col-0 rosette leaf. The series of events starts with a germinated spore at 24 hours post-inoculation (A) (see also Figure 2 for further details on spore germination) and continues with initial hyphal elongation (following successful establishment of the first haustorium inside a host cell) at 48 hours post-inoculation (B). Subsequently, a multi-branched mycelium develops (C; photo taken at 63 hours post-inoculation) and the appearance of numerous conidiophores (arrowheads) from a fully expanded fungal colony from 5 days post-inoculation onwards completes the asexual life cycle (D). Fungal structures were highlighted by Coomassie Blue staining of cleared leaf samples. Scale bar: 100 μm (A–C), 200 μm (D).

Figure 5.

Haustorial complexes of G. orontii. Phase contrast (A) and epifluorescence (B) micrographs of G. orontii haustoria isolated from Arabidopsis leaves. Notice the highly convoluted and complex folding of the haustorial cell surface providing a large area for nutrient uptake from and effector delivery into the host. Haustoria were labeled with wheat germ agglutinin-FITC. EHM: extrahaustorial membrane, E: encasement, N: haustorial neck, NB: neck band. Scale bars: 20 μm.

Figure 6.

Schematic diagram illustrating genetically-anchored components in Arabidopsis powdery mildew susceptibility/resistance. The figure depicts a section of a host cell attacked by a powdery mildew germling. Components coded by shape and color are explained in the legend below the scheme. app: appressorium, pp: penetration peg, Si: silicon. Question marks indicate presumed links/activities. For further details, see main text

Figure 7.

Typical cytological features of RPW8-mediated powdery mildew resistance in ecotype Ms-0. Whole-cell hydrogen peroxide (H₂O₂) accumulation in a G. orontii-attacked leaf epidermal cell of the Ms-0 ecotype (A), which is known to express RPW8-based powdery mildew resistance (Xiao et al., 2001). H₂O₂ accumulation is highlighted as a brownish 3,3-diaminobenzidine (DAB) precipitate (Thordal-Christensen et al., 1997) inside the challenged cell (arrowhead). Trailing necrosis of G. orontii-challenged leaf epidermal cells highlighted by Trypan-blue staining (B). Exemplary dead cells are highlighted by arrowheads. Scale bars: 100 μm

Figure 8.

Resistance in Arabidopsis mlo2 mlo6 mlo12 triple mutants is characterized by complete failure of successful host cell invasion. The micrograph shows attempted penetration of numerous G. orontii sporelings at 48 hours post-inoculation on a highly resistant mlo2 mlo6 mlo12 triple mutant in the genetic background of otherwise susceptible Col-0 (Consonni et al., 2006). Note the aborted fungal entry evidenced by a lack of secondary hyphae compared to the situation in a Col-0 wild type plant (Figure 4B). Scale bar: 100 μm.