Introduction to Fungi, Third Edition

BLUMERIA GRAMINIS
397
leaf surface. The tip of the penetration peg then
enlarges to form the haustorium initial which
differentiates in the course of 2 3 days, invaginating
but not breaching the host plasmalemma.
As a nutrient supply is established, surface
hyphae grow from the appressorium or the
appressorial germ tube and further epidermis
cells are penetrated. It is also possible for the same
host cell to be penetrated repeatedly so that it
contains several haustoria. Haustorium formation
represents the end-point of in planta growth;
further penetration occurs from epidermal
hyphae.
13.3.2 Self-defence of the host plant
against infection
Good summaries of this vast topic may be found
in Carver et al. (1995), Giese et al. (1997) and Zeyen
et al. (2002). Although the ungerminated conidium
already emits signals perceived by the epidermis
cell, it is the contact of the primary germ
tube with the host cell surface which elicits initial
defence reactions. These are visible as a dramatic
re-organization of the cytoplasm of the attacked
epidermal cell, with dense cytoplasm aggregating
beneath the point of contact of the primary germ
tube with the leaf surface. This is followed by
a modification of the epidermal cell wall by
secreted substances. The altered cell wall region is
visible as a halo (Fig. 13.2a). Phenolic substances
and hydrolytic enzymes become incorporated
into the cell wall within the halo region. This is
a non-specific defence reaction because it is elicited
by both virulent and avirulent strains of the
pathogen. A further such halo is produced by the
epidermal cell when the appressorial penetration
peg attempts to penetrate. A papilla is then
formed around the penetration peg between the
host cell wall and the host plasma membrane, and
this may succeed in plugging the peg and preventing
infection. Papillae are easily seen with an
epifluorescence microscope because they show
strong autofluorescence, a general indicator of an
attempt by the host plant to resist infection.
Autofluorescence is due mainly to the accumulation
of phenolic substances which have antimicrobial
activity (von Röpenack et al., 1998).
Additionally, hydrolytic enzymes, callose and
silica may be deposited in the papilla. Papilla
formation is an important mechanism of general
resistance, although it is unknown why some
strains of B. graminis can penetrate the papillae
and others fail. Even if a susceptible host is
infected, only about 70% of the penetration
events succeed beyond the papilla stage. In certain
barley cultivars, particularly thick papillae
are formed because of mutations in which restrictions
of the resistance response are lifted; only
0.5% of infection pegs get through the epidermis
of these mlo mutants, and barley cultivars homozygous
for the mlo allele show broad-spectrum
resistance to all strains of B. graminis f. sp. hordei
(Jørgensen, 1994; Collins et al., 2002). It should be
noted here that papillae are not particularly
prominent in infections of dicotyledons by other
powdery mildews.
In cereals attacked by B. graminis, most strainspecific
resistance mechanisms are initiated later,
when the tip of the penetration peg enlarges and
begins to differentiate into the first haustorium.
At this point the infected epidermal cell of
a resistant cultivar displays an oxidative burst,
i.e. it releases H 2 O 2 and various enzymes into its
own cytoplasm and dies (Zhou et al., 1998). This
phenomenon is known as the hypersensitive
response. Since B. graminis is an obligate biotroph,
penetration ending in a haustorium inside a dead
cell is a wasted effort. The haustorium itself may
also be directly affected by the oxidative burst,
with first signs of degeneration appearing in the
mitochondria (Hippe-Sanwald et al., 1992). If sufficient
nutrient reserves are present in the appressorial
germ tube, further appressoria may be
formed, each resulting in failure to establish a
functional haustorium in resistant cultivars.
Successful infection results if the host cell
tolerates the establishment of the initial haustorium.
Curiously, therefore, it is sensitivity rather
than tolerance which leads to resistance. Numerous
genes are involved in the various lines of
defence which barley plants possess against
B. graminis f. sp. hordei (Collinge et al., 2002).
13.3.3 Genetics of plant resistance against
B. graminis
The fact that only certain strains of B. graminis
f. sp. hordei can elicit the hypersensitive response