Resistance to Tomato mosaic virus in Tomato Conferred by Tm22

Tm22, the second tobamovirus R gene isolated, is one of the three R genes, Tm1, Tm2, and Tm22, used widely in tomato breeding to control Tomato mosaic virus (ToMV). The Tm1 gene from S. hirsutum confers extreme resistance and was mapped to chromosome 2. Tm2 and Tm22, considered to be alleles from S. peruvianum, are located close to the centromere of chromosome 7. Tm22, considered the more durable of the two alleles, was isolated by transposon tagging and encodes an 861 amino acid CC-NBS-LRR protein. The predicted protein from the susceptible allele tm2 also encodes a CC-NBS-LRR protein that appears comparable in most respects to the protein encoded by the resistance allele. Analysis of the nucleotide sequence of resistance-breaking virus isolates indicated that the MP protein is the avirulence factor in this resistance system. However, different mutations are required to overcome Tm2 and Tm22.