Abstract
Primary leaves of 7-day-old seedlings of three oat cultivars were inoculated with uredospores of two races of Puccinia coronata avenae and were subjected to electron microscopic observations in order to elucidate changes in fine structure during the resistance expression. Shokan 1 responds to race 226 with hypersensitive necrosis and fungal growth ceases 35-40hr after inoculation. Cellular disorganization in mesophyll tissue was found by 28hr after inoculation, subsequent to the invasion of the first haustoria. No decompartmentalization of subcellular organelles could be detected in host cells around the infected sites at least within 20hr after inoculation. This fact supports our previous proposal that in this host-pathogen system the determination of resistance which occurs between 8 and 12hr after inoculation is not triggered by the collapse of host cells. During the prehaustorial stage of infection of Shokan 1 with race 226, the number of Golgi apparatus increased, comparing with that of non-inoculated control, and Golgi vesicles containing an electron dense material (ED), which had never been observed in healthy leaves, became abundant. ED was also observed as a deposite at intercellular space of mesophyll tissues and on inner surface of host cell walls where the host plasma membrane invaginated. Large ED particles free from enclosures were occasionaly observed in the host cytoplasm. Contrary to Shokan 1, in Hyuga-Kairyo-Kuro, resistant to race 226, the fungus continues to grow till 72-96hr after inoculation and no necrotic symptom appears. In this cultivar, the number of Golgi apparatus in mesophyll cells increased after infection, but ED was seldom observed in Golgi vesicles and no deposition of ED occurred. Neither production of ED-containing vesicles nor deposition of ED was observed in leaves of compatible combination, i.e. Shokan 1 and Hyuga-Kairyo-Kuro inoculated with race 203, and Victoria 226-S inoculated with race 226. These observations strongly indicate that production and excretion of ED are characteristic of Shokan 1 responding hypersensitively to race 226, though causal linkage of these events with the resistance expression has not yet been made clear.
