Behaviour of a Temperature Sensitive Strain of Tobacco Mosaic Virus in Tomato Leaves and Protoplasts

A temperature-sensitive (ts) strain, Lsi, was isolated from a culture of L, a tomato strain of tobacco mosaic virus (TMV). LsI caused smaller necrotic local lesions than L in leaves of hypersensitive tobacco plants. A temperature shift treatment (22 °C for 3 days, 32 °C for 2 days and 22 °C for I day) allowed L to produce necrotic lesions surrounded by a collapsed area, whereas Lst caused necrotic lesions without a collapsed surrounding area, suggesting that LsI did not spread outside the l sions at 32 °C. In tomato leaf discs at 22 °C, infectivity of LsI increased in parallel with that of L, but at 32 °C the increase in infectivity of LsI was negligible and contrasted with the large increase of infectivity of L. Few mesophyll cells from LsI-inoculated iscs incubated at 32 °C were stained by fluorescent antibody but many of those incubated at 22 °C were stained. In proto-plasts at 32 °C, however, Lsi and L infected and multiplied similarly. When leaves inoculated with Lsi were kept at 20 to 25 °C for 24 h, and discs were then prepared and cultured for 12 h at 32 or 22 °C, the proportion of mesophyll cells containing virus antigen did not increase at 32 °C but infectivity increased greatly. This suggests that Lsi multiplied readily at 32 °C in already infected cells. At 22 °C the proportion of infected cells and the infectivity of leaf extracts increased rapidly. The growth curve of Lsi in leaf discs at 32 °C resembled the so-called ' one-step growth curve ' of LsI or L in protoplasts, and not he growth curve of L in discs, in which the virus could spread from cell to cell. These results suggest that LsI is a ts strain that multiplies normally at the non-permissive t mperature but has a malfunction in cell-to-cell movement