Isolation of a mutant of Arabidopsis thaliana carrying two simultaneous mutations affecting tobacco mosaic virus multiplication within a single cell.

Tobacco mosaic virus strain Cg (TMV-Cg) infects A. thaliana systemically. In order to identify host factors involved in the multiplication of TMV-Cg, we isolated mutant of A. thaliana from an M2 population mutagenized by fast neutron irradiation, in which the accumulation of the coat protein in upper systemic leaves was reduced to low levels. The phenotype of the mutant, YS241, was controlled primarily by a single nuclear recessive mutation named tom2-1, which was distinct from tom1, a separate mutation which also affects TMV-Cg multiplication. The tom2-1 mutation affected the accumulation of TMV-related RNAs in protoplasts in a tobamovirus-specific manner, suggesting that the wild-type TOM2 gene product is necessary for efficient amplification of TMV-related RNAs within a single cell, through specific interaction with virus-coded factors. Furthermore, we found that YS241 contained a single dominant modifier named ttm1, which increased the efficiency of multiplication of TMV-Cg and a tomato strain of TMV in a tom2-1 genetic background, both in plants and in protoplasts. We propose that the ttm1 element might be a translocated form of the TOM2 gene.