Molecular dissection of the functions of the plant resistance protein, Pto

Introducing disease resistance into crops is limited by multiple factors. One constraint is the limited genetic diversity within genepools of sexually compatible species. Another is the restricted taxonomic function of resistance (R) genes, like Pto, that limits the potential for transgenic horizontal transfer of disease resistance genes among species. A lack of conservation in downstream signaling components among genera could explain restricted taxonomic function of R genes.;A comparative analysis of AvrPto recognition in the Solanaceous species tomato and in lettuce, a member of the Compositae, was used to examine the differences in the ability of the AvrPto to elicit a resistance response in different species. Lettuce is capable of recognizing AvrPto; however, a constitutively active form of Pto from tomato does not elicit an HR in lettuce. Pto-like sequences from lettuce were identified from EST libraries and assayed for Pto phenotypes. Genetic mapping of the determinants of AvrPto-induced HR to linkage group 9 in lettuce provided additional candidates for the functional homolog of Pto in lettuce. An AvrPto recognition determinant was not confirmed among the candidate genes. In addition, a yeast two-hybrid screen of a lettuce cDNA library with AvrPto as bait identified proteins with sequence similarities to a transmembrane receptor kinase and to katanins; however, it did not identify any Pto-like genes. Two lettuce GTPases were also identified as AvrPto-interactors with homology to two previously-identified AvrPto-interacting tomato GTPases indicating that there may be conservation of some activities of AvrPto across these two plant families.;This project investigated which features of the serine-threonine kinase-encoding resistance gene, Pto, are involved in the initiation of disease resistance responses downstream of AvrPto detection. A DNA-shuffling approach utilized the natural variation found in the Pto gene family and identified multiple residues that are correlated with the functions of Pto-related proteins in Solanaceous spp., such as the ability to signal downstream to a hypersensitive response (HR) and interaction with plant signaling proteins and bacterial effectors. The distribution of residues significant for downstream phenotypes and effector binding overlapped.