- Plant parasitism
- Cellular interaction with primary interactive vesicles
- Cell wall carbohydrate composition with dominance of glucose and absence of xylose
- 5S rRNA secondary structure of type B
- Septal pores without parenthesomes, but in most cases with distinctive tripartite membrane caps or discs
- Life cycle with a parasitic dikaryophase and a saprobic haplophase
In contrast with the Urediniomycetes and Hymenomycetes, the Ustilaginomycetes are ecologically well characterized by their parasitism of vascular plants. Two of the more than 1400 species live on lycophytes, one on ferns, two on conifers, whereas all other Ustilaginomycetes parasitize angiosperms with about 810 species on Poaceae and 170 on Cyperaceae. Interestingly, no species has been reported to parasitize Orchidaceae although this family, with about 20,000 species, is one of the largest groups of the angiosperms. With a few exceptions the teliospore-forming species of the Ustilaginomycetes parasitize nonwoody herbs, whereas those without teliospores prefer woody trees or bushes. However, almost all species sporulate on or in parenchymatic tissues of the hosts. Depending upon the species the sori appear in different organs of the hosts, e.g. in roots, stems, leaves, inflorescences, flowers, anthers, ovaries, seeds etc.
An important apomorphy for the Ustilaginomycetes is the presence of zones of host-parasite interaction with fungal deposits resulting from exocytosis of primary interactive vesicles (Bauer et al. 1995a, 1997).
Fig.1. Transmission electron micrograph showing primary interactive vesicles in Exobasidium pachysporum. Scale bar = 0.2 µm. © R. Bauer 1997
Fig. 2. Transmission electron micrograph showing a transfer stage between Mycosyrinx cissi (upper cell) and its host (lower cell). Note the infiltrated host cell wall (between the two arrows) and the deposit at the host cell. Scale bar = 1 µm. © R. Bauer 1997
The contents of these vesicles (Fig. 1) are transferred to the host plasma membrane (Fig. 2). Two major types are recognized. (i) Local interaction zones (Fig. 3): short-term production of primary interactive vesicles per interaction site results in local interaction zones, and (ii) enlarged interaction zones (Fig. 4): continuous production and exocytosis of primary interactive vesicles results in the continuous deposition of fungal material at the whole contact area with the host cell.
Fig. 3. Transmission electron micrograph showing a local interaction zone (arrows) between Exobasidium pachysporum (lower cell) and its host (upper cell). Note the interaction apparatus (arrowheads) and the deposit at the host cell. Scale bar = 0.5 µm. © R. Bauer 1997
Fig. 4. Transmission electron micrograph showing an enlarged interaction zone between Ustacystis waldsteiniae and its host. The haustorium (h) is encased by electron-opaque material. Scale bar = 2 µm. © R. Bauer 1997
Cell wall carbohydrate composition
Prillinger et al. (1993) distinguished between several types of cell wall carbohydrate composition within the Basidiomycota. The Ustilaginomycetes have a distinctive type with dominance of glucose and absence of xylose that separates them from the Urediniomycetes and Hymenomycetes.
In contrast with the Hymenomycetes, the septal pores of the Ustilaginomycetes are without multilayered parenthesomes. In contrast with the Urediniomycetes, in most Ustilaginomycetes the septal pores are enclosed by distinctive, tripartite membrane caps or discs (Bauer et al. 1995b, Bauer et al. 1997, Fig. 5).
Fig. 5. Transmission electron micrograph showing a typical septal pore apparatus of the Ustilaginomycetes (Entyloma callitrichis) with two membrane caps (arrows). Scale bar = 0.1 µm. © R. Bauer 1997
The Ustilaginomycetes present a rather uniform life cycle with a saprobic haploid phase and a parasitic dikaryophase (e.g. Sampson 1939; Fig. 6). The haploid phase usually commences with the formation of basidiospores after meiosis of the diploid nucleus in the basidium and ends with the conjugation of compatible haploid cells to produce dikaryotic, parasitic mycelia. The dikaryotic phase ends with the production of basidia. In the majority of the Ustilaginomycetes the young basidium becomes a thick-walled teliospore and separates at maturity from the sorus, thus functioning as a dispersal unit. Most of the Ustilaginomycetes are dimorphic, producing a yeast or yeast-like phase in the haploid state. Almost all Ustilaginomycetes multiply mitotically in the saprobic phase, either with yeasts or with ballistoconidia, or with both.
Fig. 6. Generalized life cycle of the Ustilaginomycetes. © R. Bauer and F. Oberwinkler 1997
The Ustilaginomycetes share most characteristics of the life cycle with the Microbotryales, which traditionally were considered belonging to the Ustilago-group. However, several independent characters show that the microbotryaceous species of the genera Aurantiosporium, Fulvisporium, Liroa, Microbotryum, Sphacelotheca, Ustilentyloma and Zundeliomyces are actually Urediniomycetes (Gottschalk and Blanz 1985, Prillinger et al. 1993, Swann and Taylor 1993, Bauer et al. 1997).
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