Introduction to Fungi, Third Edition

438 HYMENOASCOMYCETES: HELOTIALES (INOPERCULATE DISCOMYCETES)
as a dessert wine. Probably the most famous
example is produced in the Sauternes region of
France, using the thin-skinned Sémillon grape
which is particularly susceptible to B. cinerea.
Botrytis cinerea can kill its host’s tissue
rapidly and then carries on growing on the
dead remains. It is thus a classical necrotrophic
pathogen. Several research groups have examined
factors which may be involved in the
pathogenicity of B. cinerea, but it is as yet
impossible to say which ones are the most
important. Quite possibly B. cinerea employs
different strategies for the colonization and
killing of different hosts. This subject has been
reviewed by Prins et al. (2000) and Kars and van
Kan (2004) and is summarized below.
Attachment
Macroconidia of B. cinerea have a hydrophobic
surface, but this is apparently not due to the
presence of hydrophobin-type proteins (Doss et al.,
1997). Initial attachment of the macroconidium
to the host surface is by weak hydrophobic interactions.
When the germ tube emerges, it secretes
a polysaccharide-based matrix which acts as
a much stronger glue (Doss et al., 1995). This
polysaccharide may be the same as cinerean, a
b-(1,3)-glucan with frequent b-(1,6) cross-linkages
which is produced by B. cinerea from excess
glucose in liquid culture and in infected grapes
(Dubourdieu et al., 1978b; Monschau et al., 1997).
When free glucose becomes scarce, cinerean is
hydrolysed again by extracellular glucanases
(Stahmann et al., 1993). On the plant surface,
the glucan matrix may thus serve in attachment,
as an external carbohydrate reservoir, and as
a matrix for hydrolytic enzymes (Doss, 1999).
Lytic enzymes
After a short period of growth, the germ
tube terminates in a slightly swollen infection
structure which may be considered a rudimentary
appressorium. This is non-melanized, and
thus penetration of the cuticle is probably
mediated mainly by lytic enzymes rather than
turgor pressure (see pp. 381 and 395).
Cutin-degrading enzymes are secreted by
B. cinerea during the initial infection stages
(Comménil et al., 1998), and proteases may also
play a role in pathogenesis (Movahedi & Heale,
1990). Later, a battery of cell wall-degrading
enzymes (especially pectinolytic enzymes) is
produced during the colonization of the host
tissue beyond the initial necrotic lesion. Pectin
seems to be a major carbohydrate source for
B. cinerea (Prins et al., 2000). The degradation of
pectin from the middle lamella may also be a
contributing factor to host cell death (Tribe, 1955)
and causes rapid and widespread maceration
of host tissue (Kapat et al., 1998; Kars & van Kan,
2004), which is typical of the necrotrophic
appearance of B. cinerea infections. Oxalic acid is
secreted by B. cinerea as it is by many other fungi,
and its presence is also correlated with tissue
necrosis. However, rather than acting directly as
a toxin, it is more likely to enhance the activity
of the pectinolytic enzymes which have an acidic
pH optimum, and to chelate Ca 2þ ions (Prins et al.,
2000). Substantial quantities of Ca 2þ ions can
be released during pectin degradation from the
carboxylic acid groups of the monomers, galacturonic
acid, which often form calcium salts.
Hypersensitive response
Biotrophic pathogens such as downy or powdery
mildews or rust fungi fail to infect incompatible
host plants because these recognize their presence.
One important mechanism of defence is the
hypersensitive response (see pp. 115 and 397) in
which epidermal cells in the vicinity of the
infection site undergo programmed cell death
(Mayer et al., 2001). The hypersensitive response is
accompanied by an ‘oxidative burst’ followed by
the synthesis of phytoalexins. With biotrophic
pathogens which require living host cells for
their nutrition, the hypersensitive response is
often sufficient to kill the infection unit. If the
necrotrophic B. cinerea attempts to infect a host
plant, the hypersensitive response also takes
place, but it fails to control the infection because
B. cinerea can exploit the dead cells for nutrition
and initial growth (Govrin & Levine, 2000).
The reactive oxygen intermediates (especially
superoxide and H 2 O 2 ) released during the oxidative
burst may be detoxified by the enzymes
superoxide dismutase and catalase, respectively,
which are secreted by B. cinerea and are probably
localized in the glucan matrix surrounding
the infection hypha (Gil-ad et al., 2001).