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Use of Heat Treatments in the Study of Acquired Resistance to Tobacco Mosaic Virus in Hypersensitive Tobacco. A. Frank Ross, Department of Plant Pathology and Laboratory of Cell Physiology, Growth and Development, Cornell University, Ithaca, New York 14850; H. W. Israel, Department of Plant Pathology and Laboratory of Cell Physiology, Growth and Development, Cornell University, Ithaca, New York 14850. Phytopathology 60:755-769. Accepted for publication 20 October 1969. DOI: 10.1094/Phyto-60-755.

Development of tobacco mosaic virus (TMV) lesions was markedly inhibited in Samsun NN tobacco leaves immersed for 40 sec in 50 C water during a period beginning 18 hr before inoculation and ending 36 hr after inoculation. Lesion inhibition was similar in leaves with induced systemic resistance and in nonresistant leaves. The same treatment applied after lesion appearance stopped virus multiplication, but resulted in an apparent enhancement of lesion development by collapsing a ring of tissue around each lesion. In resistant leaves, collapse followed treatments made 2-12 days after inoculation, with maximum effects at 3 days; the corresponding data for nonresistant leaves were 2-14 and 4 days, respectively. In other ways, lesions in resistant leaves responded much like older lesions in nonresistant leaves, an indication that the changes induced in advance of infection in the two types of leaves are similar, but develop earlier and more rapidly in the former. Electron microscope examination of 4-day lesions in nonresistant leaves before and after heat treatment showed that eventual heat collapse occurred in the region in which known structural changes characteristic of induced local immunity had developed. Immunity had also developed in this region and beyond; structural changes, but not heat sensitivity, later developed in the outer portion of the immune zone. No structural alterations were detected immediately after heat treatment, but after 24 hr the cells in a ring surrounding each lesion had collapsed, and resembled cells within the lesion except that they contained no TMV particles and often contained large paramural bodies. It is postulated that the changes induced in advance of infection are manifestations of a series of events that ultimately lead to cell collapse, and that this collapse may be instrumental in virus localization by virtue of its close proximity to infection.