Introduction to Fungi, Third Edition

614 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
aeciospores can be displaced by teliospores. Such
aecia are sometimes termed aecidioid uredinia.
Another common variation is the microcyclic
one in which both aecia and uredinia are
lacking. Such rusts are, of course, autoecious.
Many microcyclic rusts do not undergo sexual
reproduction in the sense of sexual recombination,
i.e. spermogonia are absent and the only
reproductive unit is the teliospore with or
without basidiospores. Such species are either
homothallic or asexual. The wide array of
possible nuclear cycles has been summarized by
Ono (2002).
An example of a microcyclic rust fungus is
P. mesnieriana, which produces telia on Rhamnus
catharticus (buckthorn). The teliospores germinate
to produce metabasidia, but only two basidiospores
are formed per basidium (Anikster & Wahl,
1985). This species is homothallic because a single
basidiospore can infect R. catharticus, giving rise to
a telium-producing infection. The teliospores are
highly characteristic because they carry a ‘crown’
of spiny outgrowths. Very similar teliospores (see
Fig. 22.13a) are produced by the macrocyclic
‘crown rust’, Puccinia coronata, which also colonizes
Rhamnus and related plants as alternate
hosts, but infects grasses and cereals, especially
oat (Avena), as the principal (i.e. uredinial and
telial) host. Working in the late nineteenth and
early twentieth century when the elucidation of
rust life cycles was very much in vogue, the
Russian mycologist Vladimir Tranzschel generalized
that an unknown aecial stage of a macrocyclic
uredinial/telial rust should be sought on
plant species infected by microcyclic rusts with
morphologically similar teliospores. This rule,
known as Tranzschel’s Law, has been applied
several times, and DNA-based studies have
confirmed the close relationship, for example
between P. mesnieriana and P. coronata (Zambino &
Szabo, 1993; Shattock & Preece, 2000). In other
words, the telia of the microcyclic species mimic
the aecia of the macrocyclic ancestor. These
two species are then said to be correlated. The
specialization of a recently evolved microcyclic
rust onto the alternate host of the ancestor rust
species may have something to do with the
observation that the alternate hosts are almost
always perennial, whereas principal hosts may
be annual (Shattock & Preece, 2000). Examples of
the reverse case, i.e. the evolution of the microcyclic
species on the principal host of the
macrocyclic ancestral species, do not seem to be
known.
22.2.3 The infection process
Concise reviews of this vast topic have been
written by Heath and Skalamera (1997), Mendgen
(1997) and Hahn (2000). One of the differences
between germ tubes arising from basidiospores
and those arising from heterokaryotic spores
(aeciospores or urediniospores) is that the former
usually penetrate the cuticle directly without an
appressorium, whereas the latter usually form
an appressorium and preferentially penetrate
through stomata (Figs. 22.4b, 22.5; Mendgen,
1997). An exception to this generalization is
presented by germinating urediniospores of
Phakopsora spp., which show appressoriummediated
direct penetration of the cuticle
(Adendorff & Rijkenberg, 2000). There are also
differences in the carbohydrate polymers making
up the walls of monokaryotic and dikaryotic
stages of the same species of rust fungus, and at
different steps of the infection process (Freytag &
Mendgen, 1991). Relatively little is known about
infection from basidiospores, not least because
these are difficult to obtain in the quantity
required for experiments (see Gold & Mendgen,
1991). We shall therefore focus on the infection
process arising from germinating urediniospores,
since these are the most thoroughly
researched system and have a major impact on
agriculture as carriers of the repeated infection
cycle.
Before urediniospores can germinate, germination
autoinhibitors such as methyl-cis-3,
4-dimethoxycinnamate (e.g. in Uromyces appendiculatus)
must be diluted out or degraded. This
substance is active at concentrations in the
10 11 M range (Macko et al., 1970; Staples, 2000),
i.e. at a similarly low concentration as the sex
hormones of Achlya (see p. 86). This makes it one
of the most potent biological molecules. Wolf
(1982) has suggested that the autoinhibitor acts
by blocking the lysis of the germ pore. Lysis must