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.