The development of appressoria, penetration pegs, and infection hyphalike structures ofMagnaporthe griseaon a cellophane substratum were studied using light and electron microscopy of freeze-substituted specimens. Developing, hyaline appressoria contained numerous membrane cisternae and cytoplasmic vesicles. These vesicles were concentrated near the substratum where the cell wall was very thin and then absent. Vesicles and cisternae were not present in appressoria during melanization. The discrete melanin layer contained a heretofore undescribed microfibrillar component that did not label with wheat germ agglutinin. A bilayered pore wall overlay was deposited over the appressorium pore just prior to formation of a penetration peg. The innermost of these layers and the wall of the peg both labeled with wheat germ agglutinin and concanavalin A, and appeared continuous. The mature penetration peg was uniform in width, about 7ensp;mu;m in length, and contained predominantly putative cytoskeletal elements. Within the cellophane substratum, lateral branching from the distal end of the peg gave rise to a group of cells that resembled infection hyphae.
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