Introduction to Fungi, Third Edition

398 HYMENOASCOMYCETES: ERYSIPHALES
in a given barley cultivar indicates that specific
recognition mechanisms between host and pathogen
must be involved. The molecular basis of
recognition is still obscure, but the genetics are
well understood. They are based on the genefor-gene
concept first formulated by Flor (1955)
for an interaction between the rust fungus
Melampsora lini and flax, but soon after also
demonstrated for B. graminis and cereal hosts (see
Moseman, 1966). Given that in strain-specific
interactions recognition leads to resistance via
the hypersensitive response, every avirulence
gene of the pathogen (e.g. encoding a surface
protein recognized by the host) is matched by
a specific resistance gene of the host (e.g. a
receptor). It follows that avirulence alleles should
be dominant to virulence alleles in diploid or
dikaryotic pathogens (not, of course, applicable
to the haploid B. graminis), whereas resistance
should be dominant to susceptibility in the host.
Successful infection occurs only if the avirulence
gene is modified so that the host can no longer
recognize the pathogen. Numerous resistance
genes have been identified especially in barley
and are being used for breeding programmes,
although it is relatively easy for the pathogen
to overcome such single-gene resistance (Brown
et al., 1993; Collins et al., 2002). It should be noted
that there are deviations from the classical genefor-gene
concept in the interaction between
B. graminis f. sp. hordei and barley. Further, interactions
between a given race of B. graminis and its
host can take various courses with intermediates
between complete resistance and full development
of symptoms, due to the influence of minor
genes depending on the host’s genetic make-up,
and also due to environmental parameters.
A good summary of this complicated topic has
been written by Brown (2002). In general terms,
research on the molecular biology of B. graminis
would benefit greatly from the availability of
a reliable DNA transformation method for this
important pathogen.
13.3.4 The haustorium of B. graminis
In compatible interactions, a functional haustorium
is formed by the enlarging tip of the
penetration peg. The papilla remains as a collar
around the peg at the point where it penetrated
the epidermis wall. The host plasmalemma is
invaginated around the haustorium, but it is not
in direct contact with the plasma membrane of
the haustorium. Instead, the two membranes are
separated by the haustorial wall, and surrounding
it by the extrahaustorial matrix which is of
host origin. It is a compartment with a gelatinous
texture, sealed by the host and pathogen
plasma membranes, and at the epidermal wall by
a collar (Manners, 1989). The host plasmalemma
is strongly modified and seems to lack H þ
ATPases, so that the host cell may not be able
to control leakage of solutes into the extrahaustorial
matrix (Gay et al., 1987). The escape of
solutes from the extrahaustorial matrix to the
cell surface is prevented by the collar seal. Haustoria
of the Erysiphales contain a full complement
of organelles including a single nucleus.
The haustorium is separated from the surface
hypha by a septum which is perforated, thus
permitting nutrient transfer. Ultrastructural
details are summarized in Fig. 13.5 and
have been described by Bracker (1968b) and
Hippe-Sanwald et al. (1992).
Whereas in other biotrophic plant pathogens
nutrients can, to a certain extent, be absorbed by
intercellular hyphae in addition to haustoria, in
the Erysiphales the haustorium seems to be the
sole means of nutrient uptake. Not surprisingly,
the haustorial membrane differs from that of
surface hyphae in terms of protein composition
and physiology (Manners, 1989; Mendgen &
Deising, 1993; Green et al., 2002). Nutrient
uptake has traditionally been studied with the
haustoria of E. pisi which can be isolated intact
from infected pea plants (Gil & Gay, 1977). Sutton
et al. (1999) have shown that glucose is the sugar
which is taken up by haustoria of B. graminis f. sp.
tritici, and that the plant hydrolyses the transport
sugar sucrose before this reaches the epidermal
cells and the haustoria contained within them.
Glucose probably diffuses passively into the
extrahaustorial matrix and is then taken up
across the haustorial membrane by a proton
uniport mechanism (Sutton et al., 1999). This is
in line with the situation in most other
fungi examined to date, which generally take
up glucose but not sucrose (Jennings, 1995).