Introduction to Fungi, Third Edition

336 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.11 Xylaria hypoxylon. (a) Conidial
stroma.Conidia are borne on the white
tips of the branches. (b) Perithecial
stromata which have developed at the
base of the old conidial stroma.The
knobbly swellings are the perithecia.
distinct mycelia of H. fragiforme, indicating that
the infections are derived from a number of
separate ascospores. The fungus is present in
apparently healthy sapwood as a number of
inconspicuous pockets of ‘inoculum units’ from
which mycelial growth is held in check by the
high water content of the wood. On drying,
these infections can spread rapidly. This phenomenon
is another example of latent invasion
(Boddy & Rayner, 1983). Chapela (1989) used the
term ‘xylotropic endophyte’ for fungi such as
H. fragiforme which occur within living trees and
have the capacity to extend into secondary xylem
upon drying of the wood.
Ascospore eclosion
The results of experimental studies on ascospore
germination in H. fragiforme provide a partial
explanation of its host specificity. Although
known to fruit on several other woody hosts,
perithecial stromata are most regularly associated
with drying branches and trunks of
European beech (Fagus sylvatica) and American
beech (F. grandiflora). Ascospores suspended in
extracts from living F. sylvatica twigs react by a
dramatic germination process termed eclosion,
an entomological term for the emergence of
a pupa from its case or a larva from an egg
(Chapela et al., 1990, 1993). In H. fragiforme this
appears to be a host-specific recognition system.
Exposure to aqueous extracts of beech twigs for
a period of about 10 min activates the eclosion
mechanism, which is a two-stage process. In the
first stage the spore swells slightly and its germ
slit widens until the pigmented spore wall opens
abruptly along the germ slit, forcing the outer
transparent sheath (corresponding to the W1
layer of D. concentrica) to crack open transversely.
This stage is a millisecond event. The second
stage, lasting about 10 s, involves the widening
of the germ slit up to about 7 mm, and expansion
of the coloured wall of the ascospore. This
forces the transparent outer sheath away from
the rest of the spore, which then escapes and is
free to germinate by the formation of a germ
tube. Eclosion is readily observed with the light
microscope, as summarized in Fig. 12.14 (see
Webster & Weber, 2004).
The triggers which stimulate eclosion have
been identified as two monolignol glucosides,
Z-syringin and Z-isoconiferin, which are mobile
transport intermediates of lignin biosynthesis.
Both induce eclosion at micromolar concentrations.
However, the presence of these germination
activators in beech extracts cannot fully
explain the host specificity of H. fragiforme
because eclosion is induced by extracts from a
range of trees which do not normally support
fruiting of this fungus, such as Abies, Corylus and
Populus (Chapela et al., 1991).