Abstract:
The cell wall composition of Choanephora cucur -
bitarum and the host-parasite interface, after infection
with Piptocephalis virginiana , were examined in detail.
The cell walls of C_. cucurbitarum were determined to be
composed of chitin (17%), chitosan (28.4%), neutral sugars
(7.2%),uronic acid (2.4%), proteins (8.2%) and lipids
(13.8%). The structure of hyphal walls investigated by
electron microscopy of shadowed replicas before and after
alkali-acid hydrolysis, showed two distinct regions: microfibrillar
and amorphous. The microfibrils which were composed
of mainly chitin, were organized into two distinct
layers: an outer, thicker layer of randomly orientated
microfibrils and an inner, thin layer of parallel microfibrils.Electronmicrographs of the host-parasite interface of C_. cucurbitarum and the mycoparasite , P_. virginiana ,
30 h following inoculation, showed that the sheath zone
has a similar electron density to that of the host cell
wall. The sheath was not present around the young (18 h old)
haustorium. High-resolution autoradiographs of infected
host hyphae showed that radioactive N-acetyl-D-glucosamine
,
a precursor of chitin, was incorporated preferentially in
the host cell wall and sheath zone. Cell fractionation of
label fed hyphae showed that 84% of the label was present
in the cell wall and specifically in the chitin portion of
the wall. The antifungal antibiotic, Polyoxin D, a specific
inhibitor of the enzyme, chitin synthetase, suppressed the
incorporation of the label in the cell wall and sheath zone
and resulted in a decrease in electron density of the
developing sheath. The significance of these results is
discussed in the light of host resistance.
