Pto allele from a L . hirsutum line that is resistant to bacterial speck disease encodes a protein that interacts with AvrPto Riely, B 1 and Martin, G . 2 1 Department of Agronomy, Purdue University, West Lafayette, IN   2 Boyce Thompson Institute and Department of Plant Pathology, Cornell University, Ithaca, NY In common with Lycopersicon pimpinellifolium , the source of the Pto resistance gene, some accessions of Lycopersicon hirsutum var . glabratum are resistant to strains of Pseudomonas syringae pv . tomato ( Pst ) which express the avirulence gene avrPto (Lawson and Summers 1984) .   We confirmed that bacterial speck disease resistance in L . hirsutum line PI134418 is avrPto - specific and introgressed it into the susceptible L . esculentum cultivar TA209 (Tanksley et al . , 1996) .   The introgression involved six backcrosses to the L . esculentum parent and a final selfing in order to create the homozygous resistant line 96T133 - 3 .   Throughout the backcrossing, Pst(avrPto) resistance segregated with an RFLP detected by the cloned Pto gene and mapped to the same location on chromosome five corresponding to the Pto locus in L . pimpinellifolium (Tanksley et al . , 1996) .   These data raised the likely possibility that a member of the Pto gene family is responsible for conferring Pst(avrPto) resistance in L . hirsutum PI134418 .   We constructed a cDNA library from 96T133 - 3, probed it with the Pto gene and isolated four classes of Pto - like genes .   The cDNAs were named hirPto1 , hirPto2 , hirPto3 , and hirPto4 and they encode predicted protein kinases that are 95%, 79%, 78%, and 77% identical to the L . pimpinellifolium Pto protein, respectively, at the amino acid level .    Genetic and molecular evidence indicates that physical interaction of Pto with the AvrPto protein is required for resistance to Pst ( avrPto) (Scofield et al . 1996; Tang et al . , 1996) .   In addition, previous studies indicated that a threonine residue at position 204 in the activation segment of Pto determines recognition specificity for AvrPto (Frederick et al . 1998) .   Thr - 204 is required for interaction with AvrPto in the yeast two - hybrid system and the residue is also required for Pto to elicit a hypersensitive response when co - expressed with avrPto in Nicotiana benthamiana .   An adjacent leucine residue at position 205 is not required, but it appears to enhance the ability of Pto to interact with AvrPto (Frederick et al . 1998) .   Of the four genes cloned from 96T133 - 3, only hirPto1 and hirPto2 encode proteins containing a threonine 204 in the activation segment and only hirPto1 contains a leucine at the 205 position (Figure 1) . We tested the ability of the L . hirsutum Pto - like proteins to interact with the avirulence protein AvrPto <