Recovery of Virulence in a Mutant of Tobacco Mosaic Virus, LN27

A tomato strain of tobacco mosaic virus, TMV-L, was treated with nitrous acid in the extract of infected tobacco leaf, and a mutant was isolated by transferring individual local lesions on Nicotiana glutinosa to tomato seedlings. An isolate, designated as LN27, caused no symptoms on tomato, but when the extract of such symptomless leaves was inoculated on N. glutinosa, and extracts of single-lesions were further inoculated on tobacco plants, a portion of the tobacco plants developed mosaic symptoms after varying periods. Some of the plants which remained symptomless contained virus systemically, while others only in inoculated leaves, or in no parts of the plants. The mosaic symptoms developed on these plants were apparently similar to those caused by the parent virus TMV-L.Lesion productivities of the viruses contained in individual lesions of N. glutinosa were different. The viruses which produced many lesions caused mosaic symptoms on tobacco about 5 days after inoculation. Some of the viruses which produced only a few lesions (2-10 lesions per leaf) caused mosaic symptoms 12-41 days after inoculation, while others caused no symptoms at least during this period.When Xanthi nc tobacco was inoculated with the LN27-infected tomato extract, the leaves of the tobacco showed both distinctly large and small lesions. The viruses from the large lesions caused mosaic symptoms on tomato, while the viruses from the small lesions caused no symptoms for two weeks or more after inoculation, developing symptoms afterwards in a half to a third of them. Inoculation from the tomato which remained symptomless for some time and showed symptoms later gave rise to an increased number of large lesions.It was considered that a part of LN27 somehow recovered its virulence to tobacco and tomato during multiplication.