Introduction to Fungi, Third Edition

CERATOBASIDIALES
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Fig 21.2 Rhizoctonia cerealis and its mycorrhiza with the heath spotted orchid (Dactylorhiza maculata ssp. ericetorum)inthe
laboratory. (a) Vegetative hypha showing typical branching and dolipore septa (arrowheads). (b) Seeds of D. maculata on agar after
ten weeks without Rhizoctonia. Protocorms with a few epidermal hairs have formed. (c) Seeds after ten weeks but with R. cerealis
spreading from a food base.The protocorms have grown and are differentiating shoot tips (arrows). Same scale as (b). (d) The
split-plate experiment. Rhizoctonia cerealis has been inoculated onto a food base (tissue paper, top half) which is separated from
the orchid seeds by a partition.The fungus has overgrown this barrier, and the orchid protocorms are using the translocated sugars
derived from the degraded cellulose; 13 weeks after inoculation. (e) Penetration of an epidermal hair by R. cerealis. (f) Penetration of
R. cerealis hyphae through an epidermal hair into the cortex of a protocorm where pelotons have formed. (b) and (d f) reprinted
from Weber and Webster (2001b), with permission from Elsevier.
the seedling. Penetration of the wall of a cell
in the protocorm cortex invaginates the plasmalemma
and results in the formation of a peloton,
i.e. a dense mass of coiled hyphae (Fig. 21.2f).
Initially, each hypha is ensheathed by the host
plasmalemma, which is called the perifungal
membrane and is functionally modified from the
plasmalemma of uninfected regions (Peterson &
Currah, 1990; Peterson et al., 1996). An interfacial
matrix of unknown composition is located
between the perifungal membrane and the
fungal cell wall. Within 24 h of formation, a
peloton may begin to be degraded and is ultimately
left behind as an amorphous clump
of lysed hyphae surrounded by one continuous
perifungal membrane (Hadley & Williamson,
1971; Peterson & Currah, 1990). Any one
orchid cell can become repeately re-infected