A LAMELLATED STRIATED LAYER IN THE SPORE WALL OF THE SMUT FUNGUS ENTORRHIZA (USTILAGINALES)

The spore wall in the smut fungus Entorrhiza has been investigated, wi th particular reference to layer 3. The wall is stratified into four l ayers, numbered 1-4 from the outside to inside of the wall. Layer 3 ha s a lamellated or striated organization, depending on the type of spec imen preparation used for examination. In this study, thin sectioning and freeze-etching methods were used in transmission electron microsco py. Layer 3 is approximately 50 nm thick and is the narrowest layer of the wall. Thin sections viewed at high magnification show a lamellate d organization, consisting of alternate electron dense and translucent spaces. Usually between 16-20 lamellae form the layer, with a lamella having a thickness of about 1-2 nm. At high resolution, each electron dense lamella is resolved as a row of closely packed subunits, approx imately 1. 5 nm in diameter. The electron translucent lamellae probabl y represent mainly lipoidal material, which is extracted during specim en preparation. In freeze-etch preparations layer 3 is termed the ''st riated layer''. Fracturing exposes face views of the layer, which at l ow magnification has a wrinkled appearance. At high magnification, lay er 3 has a structure consisting of an irregular mosaic of striations. The striations in each area of the mosaic are arranged parallel, and r egularly spaced 11-13 nm apart. Each area of the mosaic is separated f rom an adjacent area by a small step; this represents where the plane of fracture changed to a different level within layer 3. Fracturing pr obably occurs in the lipoidal region of the layer, corresponding to th e electron translucent lamellae seen in thin section. At high magnific ation, the striations are resolved into subunit-like particles, approx imately 1-2 nm in diameter. Layer 3 is believed to form an impervious region in the spore wall. Layer 3 in Entorrhiza closely resembles the ''partition layer'' reported in spores of other Tilletiaceae. This dem onstration of a common wall layer strengthens the relationship between Entorrhiza and the rest of the Tilletiaceae. Entorrhiza is the only s mut that forms galls on host roots.