CELLULAR INTERACTION OF THE SMUT FUNGUS USTACYSTIS-WALDSTEINIAE

The cellular interaction between the smut fungus Ustacystis waldsteini ae and its host Waldsteinia geoides was analyzed by serial-section ele ctron microscopy using chemically fixed and high-pressure frozen - fre eze-substituted samples. After penetration, each haustorium extends a short distance into the host cell where it often forms up to three sho rt lobes. The haustorium is wholly ensheathed by a prominent matrix. T he matrix is a complex structure, differing significantly from that kn own of other fungal plant parasites: it is filled with amorphous, elec tron-opaque material in which membrane-bounded, coralloid vesicles are embedded. During the contact phase of the hypha with the host cell wa ll, vesicles with electron-opaque contents accumulate in the contact a rea of the hypha where they appear to fuse with the fungal plasma memb rane and extrude their contents. Subsequently, the host cell wall incr eases in electron opacity and matrix material becomes deposited betwee n host plasma membrane and host cell wall exactly at the ends of the a ltered areas in the host cell wall. The coralloid vesicles within the matrix, however, are of host origin: exocytosis of Golgi products into the matrix results in the formation of coralloid vesicular buds in th e host plasma membrane. Subsequently, the buds seem to detach from the host plasma membrane to flow as coralloid vesicles into the matrix. M atrix development continues during penetration and after penetration a t the haustorial tips. After host wall penetration, the fungal cell wa ll comes in contact with the matrix. The fungal component of the matri x may play a key role in the inducement of these transfer cell-like co mpartments in host cells responding to infection.