The products of the broken Tm-2 and the durable Tm-22 resistance genes from tomato differ in four amino acids

To gain an insight into the processes underlying disease resistance and its durability, the durable Tm-2(2) resistance gene was compared with the broken Tm-2 resistance gene. The Tm-2 gene of tomato could be isolated via PCR with primers based on the Tm-2(2) sequence. The Tm-2 gene, like the Tm-2(2) gene, encodes an 861 amino acid polypeptide, which belongs to the coiled coil/nucleotide binding site/leucine-rich repeat class of resistance proteins. The functionality and the nature of the isolated Tm-2 gene were confirmed by introducing the gene under the control of the 35S promoter into tomato mosaic virus-susceptible tobacco. This transgenic tobacco was crossed with transgenic tobacco plants producing the movement protein (MP)-authenticated MP as the Avr protein of the Tm-2 resistance. The Tm-2(2) and Tm-2 open reading frames only differ in seven nucleotides, which on a protein level results in four amino acid differences, of which two are located in the nucleotide binding site and two are located in the leucine-rich repeat domain. The small difference between the two proteins suggests a highly similar interaction of these proteins with the MP, which has major implications for the concept of durability. Comparison of the two resistance-conferring alleles (Tm-2 and Tm-2(2)) with two susceptible alleles (tm-2 and lptm-2) allowed discussion of the structure-function relationship in the Tm-2 proteins. It is proposed that the Tm-2 proteins display a partitioning of the leucine-rich repeat domain, in which the N-terminal and C-terminal parts function in signal transduction and MP recognition, respectively.