Key words: TS WV/ultrastructure/defective form/sap transmissibility
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A Sap-transmissible, Defective Form of Tomato Spotted Wilt Virus (Accepted 3 December 1981) SUMMARY After multiple sap transmissions two isolates of tomato spotted wilt virus (TSWV) lost the characteristic particles, and amorphous, more or less diffuse masses were observed in the cytoplasm of cells. The amorphous material, which had previously been described associated with normal virus particles, could also be found in pellets from extracts of infected plants. Isolates of the 'amorphous' type had the same thermal inactivation point as those of the normal 'particulate' type. It is suggested that the 'amorphous' type is a defective form of TSWV.Tomato spotted wilt virus (TSWV) is a single-stranded RNA virus with spherical particles that have a unit-membrane envelope with knob-like glycoprotein structures on its outer surface (Mohamed et al., 1973; Tas et al., 1977). The virus RNA is present in segments (Van den Hurk et al., 1977; Verkleij et al., 1982). The virus can be transmitted by sap inoculation.In plant cells TSWV particles occur in clusters in the cytoplasm within dilated cisternae of the rough endoplasmic reticulum (Ie, 1964;Kitajima, 1965; Milne, 1970; Francki & Grivell, 1970;Mohamed et aL, 1973; Paliwal, 1974). In addition, dark, amorphous, more or less diffuse masses have been observed in the cytoplasm. These masses, showing local small electron-dense structures with a lattice periodicity of about 5 nm, are not bounded by a membrane but are scattered in the cytoplasm among the ribosomes (Ie, 1971). Similar structures were reported and called 'dense strands' by Kitajima (1965) or 'amorphous darkly stained material' by Milne (1970). These masses are Pronase-sensitive (Ie, 1971) but although RNase did not affect their appearance in sections prepared for electron microscopy (T. S. Ie, unpublished results) the question of whether or not they contain RNA remains open.Isolates of TSWV have been maintained by mechanical transmission for several years in our laboratory. One isolate (designated S-NN) originated from an infected Campanula isophylla plant collected in 1962, and another (designated P/S-NN) was obtained from Nicotiana tabacum leaves sent in 1963 from Poland. These two isolates were compared with two recently obtained isolates of TSWV. The new isolates, designated PD and X, originated from an amaryllis (Amaryllis sp.) plant obtained in 1973, and from an amarine (Amaryllis × Nerine) plant, obtained in 1974 respectively.Small pieces of leaf were prefixed in ice-cold 3.5% glutaraldehyde in 0.05 M-cacodylate buffer pH 7.4 containing 0.5 % CaCI 2. 2H20, for 1 h at room temperature in a vacuum of 30 Torr. The first post-fixation was in cold 1% OsO 4 in 0.1 M-cacodylate buffer pH 7.4 containing 0.5 % CaCI v 2H20 for 1 h at room temperature. After the specimens were washed in caeodylate buffer (5 min) and Veronal-acetate buffer (5 min) they underwent a second post-fixation in cold 1 to 2% uranyl acetate in 0.014 M-Veronal-acetate bu...
