Resistance Mechanisms of Tobacco Mosaic Virus Strains in Tomato and Tobacco

Summary

Ls1 is a temperature-sensitive (ts) strain of tobacco mosaic virus (TMV) which was isolated from a culture of a wild-type strain, L. The temperature sensitivity of this mutant did not show any host specificity, when assayed in six systemic hosts and four local lesion hosts. In experiments using protoplasts and leaf discs of tomato the temperature sensitivity was shown to be associated with a defect at the non-permissive temperature in the ability of virus to move from cell to cell. Microscopic observation of Ls1-inoculated tomato leaves revealed that virus had multiplied mainly in separate single epidermal cells without spreading to neighbouring cells. This malfunction of virus movement may be due to one amino acid substitution in a TMV-encoded 30K protein induced by a mutation, as suggested by several molecular biological studies. Another mutant, L₁₁A, which produces small lesions on local lesion hosts, is characterized by a low level of production of 30K protein mRNA. These results indicate that the 30K protein is important for the spread of virus from cell to cell.

Resistance mechanisms against TMV in tomato, specified by Tm-2 and Tm-2² genes, were investigated by fluorescent-antibody staining of virus-inoculated epidermis. In the resistant tomato lines separate single cells were mainly infected when the epidermis was inoculated with L. This was in contrast to the systemic spread of virus from primary infected cells to neighbouring cells in the epidermis of the susceptible line inoculated with L or of the Tm-2 line inoculated with Ltb1 (a mutant which breaks resistance). In these studies it is possible that interactions between gene products of the virus strain and its host are directly or indirectly responsible for expression of a type of virus resistance of plants.