Introduction to Fungi, Third Edition

22
Urediniomycetes: Uredinales (rust fungi)
22.1 Urediniomycetes
Following extensive re-arrangements, the class
Urediniomycetes (about 8000 species) is now
considered to be monophyletic, although the
naming of orders and families is still proving
difficult (Swann & Taylor, 1995; Kirk et al., 2001;
Swann et al., 2001). The order Uredinales (rust
fungi) is by far the largest (about 7000 species)
and the most important. The order Microbotryales,
although taxonomically part of the Urediniomycetes,
is a group of fungi causing smut
diseases and will be discussed in Chapter 23.
Many Urediniomycetes belonging to several
orders occur predominantly in the yeast state.
An important group, the Sporidiales, contains
the red yeasts Sporidiobolus and Rhodosporidium
(anamorphs Sporobolomyces and Rhodotorula, respectively),
and this order is considered in more
detail on pp. 666 670.
General information on the groups included
in the Urediniomycetes has been given by Swann
et al. (2001). They have defined Urediniomycetes
as fungi in which the processes of karyogamy
and meiosis occur in distinct parts of the basidium,
i.e. the probasidium and metabasidium,
respectively. The metabasidium is typically transversely
septate, with basidiospores produced
laterally (see Fig. 22.2d). Another useful character
is the structure of septa viewed by
transmission electron microscopy. Urediniomycete
septa are simple with a single pore which
may be open or plugged, but they typically
lack the dolipore arrangement found in other
basidiomycetes (see Fig. 18.9). Clamp connections
are absent.
22.2 Uredinales: the rust fungi
Rust fungi (Uredinales) are a fascinating group of
organisms. The life cycle of a typical rust species
is among the most complex found anywhere in
nature, consisting of five different spore stages
on two plant hosts which are taxonomically
entirely unrelated to each other. These pathogens
infect most groups of vascular plants,
including Pteridophytes (ferns), Gymnosperms,
and Angiosperms (both monocots and dicots).
Numerous fundamental questions about rust
fungi remain to be answered, e.g. how a biotrophic
organism manages to infect and parasitize
two unrelated hosts using different
mechanisms on either; how the five spore
stages with their numerous different dispersal
mechanisms could have evolved; how easily one
or more of them can become aborted in derived
(reduced) life cycles; how rust fungi survive in
situations where one of their two hosts is
unavailable; and how quickly new rust species
or races spread to new habitats and then come to
an equilibrium with their host plants. A protocol
to generate stable transformants of rust fungi
would greatly facilitate experimental work on
them, but unfortunately this is not yet available.
The species concept in rust fungi is also
challenging. Morphological species are readily
recognized, but these can show considerable