Arch Virol (1991) 120:97-t07 _Archives Virology © Springer-Verlag 199 I Printed in Austria Subeellular localization of acidic and basic PR proteins in tobacco mosaic virus-infected tobacco Isabelle Dore 1, M. Legrand 2, B. J. C. Cornelissen 3, and J. F. Bol 1 Gorlaeus Laboratories, Leiden University, Leiden, The Netherlands 2 Institut de Biologic Moleculaire des Plantes, Centre National de ta Recherche Scientifique, Strasbourg, France 3 Mogen Int., Leiden, The Netherlands Accepted January 8, 1991 Summary. Infection of Samsum NN tobacco with tobacco mosaic virus (TMV) results in the induction of the synthesis of acidic and basic isoforms of many pathogenesis-related (PR) proteins. By immunogold-electronmicroscopy we have shown that PR proteins accumulate mainly in cells around the necrotic spots of TMV-induced lesions. The acidic chitinases, 13-(1,3)-glucanases and thaumatin-like proteins were found to accumulate in extracellular "pocket-like" vesicles while the basic chitinases were found in electron dense inclusion bodies in the vacuoles. These structures were no1: detectable in PR-containing leaves devoid of virus nor in healthy plants. Introduction Infection of Samsun NN tobacco (Nicotiana tabacum) with tobacco mosaic virus (TMV) leads to the production of necrotic lesions on the inoculated leaves. The virus does not spread systemically in the plant but remains localized in necrotic spots. The ultrastructure of these lesions has been described by Israel and Ross [11]. These authors distinguished three zones: the necrotic central part of the lesion constituted by dead ceils, a surrounding dark-coloured Z zone of tissue consisting of metabolically active cells with enlarged cytoplasm, and healthy cells around this Z zone. The hypersensitive response of Samsun NN tobacco to TMV infection results in the de novo production of at least five different groups of pathogenesis- related (PR) proteins [13, 24]. Concomitantly with the synthesis of these pro- teins, the plant develops a resistance to further infection with a wide variety of pathogens. Initially, the synthesis of PR proteins and the induction of resistance