Introduction to Fungi, Third Edition

EXOBASIDIALES (USTILAGINOMYCETES)
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or transcription factor acting on genes encoded
on the autosomes or the X chromosome, since
the expression patterns of some male-specific
genes in healthy male and infected female
flowers are similar (Scutt et al., 1997).
In perennial host species, M. violaceum can
colonize the entire host in the course of
1 2 years, overwintering systemically in the
root system (Alexander & Antonovics, 1988).
Infected plants acquire lifelong sterility, i.e.
they will continue to produce flowers but these
will all be male, developing anthers in which
pollen grains are replaced by teliospores.
The ecology of M. violaceum
The ecological effects of M. violaceum infections
have been considered by several workers.
Systemically infected host plants flower earlier
than healthy plants, and this may force pollinators
such as bumblebees to visit infected flowers
early in the season. Consequently, teliospore
dispersal peaks earlier than pollen dispersal
(Jennersten, 1988). In direct comparison, diseased
flowers are less attractive to bumblebees
(Shykoff & Bucheli, 1995), possibly because they
contain less nectar and are asymmetric and
smaller than healthy flowers (Shykoff & Kaltz,
1998). A pollinator preference for healthy flowers
may be advantageous to the pathogen because it
enhances the opportunity for spreading the
infection following an accidental visit to a
diseased plant (Shykoff & Bucheli, 1995).
Although systemically infected host plants
produce an increased number of flowers which
also remain open for longer, the drawback is
that the root biomass is decreased, thereby
potentially affecting the chance of the pathogen
to overwinter systemically. Since healthy
female flowers remain open for much longer
than healthy male flowers, they are at greater
risk of infection (Kaltz & Shykoff, 2001). As in
many venereal diseases, a more attractive display
draws a greater number of visitors, and one
possible adaptation to M. violaceum is predicted to
be a reduction in flower size. Another might be
the change from a perennial to an annual habit,
as this would prevent overwintering of the
pathogen (Kaltz & Shykoff, 2001). In situations
where perennial hosts are forced into an annual
habit, e.g. near field borders or railway lines
subject to regular clearing, infections by
M. violaceum are rare or absent (Alexander &
Antonovics, 1988). The interaction between
M. violaceum and its hosts may therefore have
different outcomes, ranging from local extinction
of the host to that of the pathogen.
23.4 Exobasidiales
(Ustilaginomycetes)
The order Exobasidiales (subclass Exobasidiomycetidae)
is a well-defined group (Begerow et al.,
1997, 2002) of ecologically obligate, biotrophic
plant pathogens containing less than 100 species.
Most species cause systemic or limited infections
of shoots or leaves, and these are often accompanied
by hypertrophy of the infected tissue.
The most important genus is Exobasidium. Fungi
grouped in the Exobasidiales firmly belong to
the Ustilaginomycetes and possess a haploid
saprotrophic yeast-like phase and a dikaryotic
biotrophic phase. However, they differ from the
‘true smuts’ (Ustilaginomycetidae) in several
aspects:
1. They do not produce teliospores, but
basidia are formed directly on the surface of
the infected host.
2. The basidium appears similar to that of
the Homobasidiomycetes, i.e. it looks like a
holobasidium, not divided into a pro- and metabasidium
as in smut fungi (but see below).
3. Basidiospores are violently discharged
using the surface-tension catapult mechanism,
and they often become septate after discharge.
4. The mycelium is usually intercellular,
with haustoria often present. These differ from
the intracellular hyphae of smut fungi in not
being completely surrounded by a sheath.
Instead, a complex interaction apparatus is
formed at the haustorial apex (Fig. 23.17).
23.4.1 Exobasidium
About 50 Exobasidium spp. are known. All of
them cause either local infections of individual
leaves or more widespread, sometimes systemic
infections of whole shoots or shoot tips