Introduction to Fungi, Third Edition

EXOBASIDIALES (USTILAGINOMYCETES)
657
(Fig. 23.16a). Basidiospores are initially aseptate
(Fig. 23.16b), but in many species they become
septate after their discharge, and both cells can
germinate by emitting germ tubes (Fig. 23.16c).
Elongated haploid yeast cells are produced from
these germ tubes or directly from the basidiospore
cell (Fig. 23.16d). Several Exobasidium spp.
grow as yeast cells in culture. Infection of host
plants may occur from basidiospores or yeast
cells and gives rise to a dikaryotic intercellular
mycelium (Mims & Nickerson, 1986). Exobasidium
spp. may overwinter systemically in their host,
or as spores on the outside, e.g. in bud scales
or bark.
Haustoria are formed by the intercellular
hyphae, and these show unique characteristics
in being lobed and producing an electron-dense
apical ring at the localized point of contact
with the host plasmalemma. This then becomes
elaborated into an apical cap which is associated
with the host wall (Bauer et al., 1997) or
directly with the host plasmalemma (Mims,
1982). The ring and cap material is secreted
by the haustorium from an elaborate tubular
membrane system (Fig. 23.17). This cap may be
homologous to the thick sheath which surrounds
the intracellular hyphae of smut fungi (see
Fig. 23.6).
Although the basidia of Exobasidium look
like typical club-shaped holobasidia, their
development differs from that in the homobasidiomycetes
(Mims et al., 1987). Nuclear fusion
occurs in a subterminal cell (strictly speaking
the probasidium), and then the nucleus migrates
into the basidium proper where meiosis is
completed. The basidium of Exobasidium is thus
a metabasidium in disguise, and the pattern of
basidial development is equivalent to that in the
rust and smut fungi. A further unusual feature
is that the hilar appendices of basidiospores on
the basidium point outwards (Fig. 23.16a), not
inwards as in Homobasidiomycetes.