ULTRASTRUCTURE OF THE LEAF STRIPE SMUT FUNGUS USTILAGO-STRIIFORMIS - HOST-PATHOGEN RELATIONSHIP AND TELIOSPORE DEVELOPMENT

Ustilago striiformis, which causes stripe smut disease of grasses, was investigated on the host Poa pratensis with light and electron microscopy. Fungal hyphae were present immediately below apical meristems and in leaf primordia and maturing leaves. Hyphal growth in leaves was both intracellular and intercellular. In the case of intracellular hyphae, an encasement consisting of two distinct layers was observed between the host plasma membrane and the fungal wall. Host cells initially did not appear to be adversely affected by the presence of hyphae, and invaded cells were capable of division. Hyphal compartments appeared to be binucleate, and structures suggestive of clamp connections were present at septa. Hyphal aggregations became extensive in developing leaves, and they coalesced into stripes that often extended nearly the entire length of a leaf blade. Sporulation occurred in leaf parenchyma cells when hyphal aggregations were converted into spores. Sporulation involved gelatinization of hyphal walls and subsequent deposition of ornamented teliospore walls. Leaf cells were replaced by the teliospores, and eventually masses of spores were released through ruptures in the leaf epidermis.