A temperature-sensitive () strain, Ls1, was isolated from a culture of L, a tomato strain of tobacco mosaic virus (TMV). Ls1 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 1 day) allowed L to produce necrotic lesions surrounded by a collapsed area, whereas Ls1 caused necrotic lesions without a collapsed surrounding area, suggesting that Ls1 did not spread outside the lesions at 32 °C. In tomato leaf discs at 22 °C, infectivity of Ls1 increased in parallel with that of L, but at 32 °C the increase in infectivity of Ls1 was negligible and contrasted with the large increase of infectivity of L. Few mesophyll cells from Ls1-inoculated discs incubated at 32 °C were stained by fluorescent antibody but many of those incubated at 22 °C were stained. In protoplasts at 32 °C, however, Ls1 and L infected and multiplied similarly. When leaves inoculated with Ls1 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 Ls1 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 Ls1 in leaf discs at 32 °C resembled the so-called ‘one-step growth curve’ of Ls1 or L in protoplasts, and not the growth curve of L in discs, in which the virus could spread from cell to cell. These results suggest that Ls1 is a strain that multiplies normally at the non-permissive temperature but has a malfunction in cell-to-cell movement.


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