Cloning and characterization of the durable tomato mosaic virus resistance gene Tm-22 from Lycopersicon esculentum

In tomato, infections by tomato mosaic virus are controlled by durable Tm-22 resistance. In order to gain insight into the processes underlying disease resistance and its durability, we cloned and analysed the Tm-22 resistance gene and the susceptible allele, tm-2. The Tm-22 gene was isolated by transposon tagging using a screen in which plants with a destroyed Tm-22 gene survive. The Tm-22 locus consists of a single gene that encodes an 861 amino acid polypeptide, which belongs to the CC-NBS-LRR class of resistance proteins. The putative tm-2 allele was cloned from susceptible tomato lines via PCR with primers based on the Tm-22 sequence. Interestingly, the tm-2 gene has an open reading frame that is comparable to the Tm-22 allele. Between the tm-2 and the Tm-22 polypeptide 38 amino acid differences are present of which 26 are located in the second half of the LRR-domain. Susceptible tomato plants, which were transformed with the Tm-22 gene, displayed resistance against ToMV infection. In addition, virus specificity, displayed by the Tm-22 resistance was conserved in these transgenic lines. To explain the durability of this resistance, it is proposed that the Tm-22-encoded resistance is aimed at the Achilles' heel of the virus.