Introduction to Fungi, Third Edition

UREDINALES: THE RUST FUNGI
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Fig 22.3 Attachment of a hydrated urediniospore of
Uromyces viciae-fabae to the surface of a broad-bean leaf1h
after contact. (a) Spore removed with sticky tape.The germ
pore has partially lysed. (b) The adhesion pad on the leaf from
which the spore was removed. Some of the germ pore
material has become incorporated into the pad which has
made firm contact with the host cuticle.From Clement et al.
(1997), with permission from Elsevier.Original image kindly
provided by J. A.Clement.
occur before the germ tube can emerge (see
Fig. 22.3).
Attachment of urediniospores to surfaces is
a multi-step process. Initial attachment is probably
purely physical and based on hydrophobic
interactions since it is stronger on hydrophobic
than hydrophilic surfaces (Terhune & Hoch,
1993). As soon as the urediniospore becomes
fully hydrated upon contact with water, there is
evidence of the formation of an adhesion pad
of unknown composition (Fig. 22.3; Clement
et al., 1997). Further, cutinases and other esterases
are released, and their activity is thought
to modify the surface properties of the host
cuticle, cementing the attachment pad to the
host surface (Deising et al., 1992). The germ tube
is also tightly attached to the host surface by
means of a glue which probably consists of
glucans and proteins (Epstein et al., 1985;
Chaubal et al., 1991).
The process of appressorium differentiation by
dikaryotic stages of rusts may be unique among
fungi in being triggered by thigmotropism. A
ridge about 0.5 mm high is required and sufficient
to induce appressorium differentiation even on
chemically inert surfaces (Fig. 22.4a; Hoch et al.,
1987; Allen et al., 1991). In nature, the relevant
topographic feature is the stomatal lip, i.e. the
point where the cuticle broke during the developmental
expansion and opening of the stoma
(Terhune et al., 1991). Firm attachment of the
germ tube to the surface is required for the
perception of the signal for appressorium induction
in urediniospore germ tubes of U. appendiculatus,
and there is evidence that the reception
of the physical signal involves microtubules and
integrin-like molecules (Corrêa et al., 1996) and
is transmitted via stretch-activated Ca 2þ channels
(Zhou et al., 1991). We have already come across
stretch-activated Ca 2þ channels and integrin as
possible regulators of the rate of hyphal tip
extension (see p. 8). Once the signal has been
perceived, an appressorium differentiates in as
little as 60 min.
Following the differentiation of an appressorium
over a stoma, a thin penetration hypha
develops which swells beneath the guard cells
to form the substomatal vesicle (Fig. 22.5).
From this, intercellular hyphae grow and form
appressorium-like structures called haustorial
mother cells on the surface of leaf mesophyll
cells. The haustorial mother cell co-ordinates
penetration of the plant cell, leading to the
formation of a haustorium. Morphologically
recognizable haustoria are not normally formed
by monokaryotic rust stages; instead, the
hyphae appear to grow through mesophyll cells
whose plasmalemma invaginates around them