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Ultrastructural Changes in Pepper Cells in an Incompatible Interaction with Phytophthora infestans. D. R. Jones, Postdoctorate Research Fellow, Agriculture Canada, Research Institute, University Sub Post Office, London, Ontario, Canada N6A 5B7; W. G. Graham(2), and E. W. B. Ward(3). (2)(3)Electron Microscopist, and Plant Pathologist, respectively, Agriculture Canada, Research Institute, University Sub Post Office, London, Ontario, Canada N6A 5B7. Phytopathology 65:1274-1285. Accepted for publication 10 June 1975. DOI: 10.1094/Phyto-65-1274.

Hypersensitive death of the first infected host cells was detected 4 hours after suspensions of Phytophthora infestans zoospores were injected into cavities of ripening fruit of sweet pepper (Capsicum frutescens). After 24 hours, many invaded host cells bordering the fruit cavity and some in the second layer, appeared to have reacted hypersensitively, while other invaded and adjacent uninvaded cells in the first and second layers were responding more slowly. In these cells, cell border lesions were observed between the host cell wall and invaginated plasmalemma prior to invasion and were associated with penetrating haustorial pegs during invasion. The reactivated cytoplasm of these cells was predominantly composed of smooth endoplasmic reticulum, polyribosomes, rough ER, ribosomes, and mitochondria. Haustorial contact with this cytoplasm was brief and a separation zone, containing vesicles and cytoplasmic fragments, was formed between the extrahaustorial matrix and the retreating extrahaustorial membrane. This zone resembled the cell border lesion matrix and was considered to be a similar cellular response. Disruption of the tonoplast membrane and fragmentation of the remaining cytoplasm eventually led to cell death. The first two-to-three cell layers bordering the fruit cavity were invaded during the 36 hours following zoospore inoculation. Most hyphae began degenerating between 36 and 48 hours and were dead after 7 days. Cells around the infected tissue slowly degenerated and possessed swollen nuclei, numerous lipid bodies, and chloroplasts with lipid-like granal deposits.