Pronase E and tunicamycin, a putative inhibitor of protein
glycosylation, strongly reduced the frequency of germlings adhering to
smooth polystyrene and completely inhibited appressorium formation of
adhering germlings without inhibiting germ-tube growth of
the pathogen. Additionally, α-mannosidase or α-glucosidase,
but
not β-glucosidase or lipase, partly inhibited adhesion to smooth
polystyrene and substantially inhibited appressorium formation by germlings
adhering to this substrate. Infection structure formation
was also reduced by the addition of either lectins binding to mannose or
glucose residues (ConA and LCA), or by the addition of
IgG. The presence of ConA- and IgG-binding (glyco)proteins among proteins
obtained from the germination fluid, or from
differentiated germlings, has been confirmed on Western blots. Removal
of
proteinaceous surface material by pronase E inhibited
appressorium formation on smooth polystyrene. After the protease was removed,
newly formed ConA-binding glycoproteins have
been detected at the germ-tube apex with FITC-conjugated ConA. The
(re)appearance of these glycoproteins was correlated with the
time period when appressorium formation was observed. Interestingly,
removal of proteinaceous materials from the germ-tube
surface by pronase E, and also the addition of IgG had no inhibitory
effect on appressorium formation on furrows in polystyrene.
Our results suggest ConA-binding surface glycoproteins present at the
germ-tube apex as essential factors for infection structure
formation on smooth polystyrene. In contrast, these glycoproteins are
unlikely to be involved in appressorium induction over
grooves in the same substrate.