Identification of an amino acid residue required for differential recognition of a viral movement protein by the Tomato mosaic virus resistance gene Tm-22

The Tm-2 gene of tomato and its allelic gene, Tm-22, confer resistance to Tomato mosaic virus (ToMV) and encode a member of the coiled-coil/nucleotide binding-ARC/leucine-rich repeat (LRR) protein class of plant resistance (R) genes. Despite exhibiting only four amino acid differences between the products of Tm-2 and Tm-22, Tm-22 confers resistance to ToMV mutant B7, whereas Tm-2 is broken by ToMV-B7. An Agrobacterium-mediated transient expression system was used to study the mechanism of differential recognition of the movement proteins (MPs), an avirulence factor for ToMV resistance, of ToMV-B7 by Tm-2 and Tm-22. Although resistance induced by Tm-2 and Tm-22 is not usually accompanied by hypersensitive response (HR), Tm-2 and Tm-22 induced HR-like cell death by co-expression with MP of a wild-type ToMV, a strain that causes resistance for these R genes, and Tm-22 but not Tm-2 induced cell death with B7-MP in this system. Site-directed amino acid mutagenesis revealed that Tyr-767 in the LRR of Tm-22 is required for the specific recognition of the B7-MP. These results suggest that the Tyr residue in LRR contributes to the recognition of B7-MP, and that Tm-2 and Tm-22 are involved in HR cell death.