Acts as a transcriptional activator. Binds to the GCC-box pathogenesis-related promoter element. Activates plants defense genes. Can confer resistance to Pseudomonas syringae tomato by potentiating the transcription of a set of pathogenesis related genes, downstream to a defense signaling pathway involving PTO and PRF, but probably independent of ethylene, jasmonate and salicylic acid. Interacts with the Pto kinase. The stronger expression mediated by Pseudomonas syringae tomato during an incompatible interaction (avirulent T1 avrPto strain) than during a compatible one (virulent T1 strain) requires the resistance PTO-R as well as the Pseudomonas resistance and fenthion sensitivity (PRF) gene products. This induction is independent of ethylene (ET), jasmonate (JA) and salicylic acid (SA), and subsequent transcription of PTI5 does not require de novo protein synthesis. Also induced by Xanthomonas oryzae strain PXOA avrXa10, by Pseudomonas fluorescens strain 2-79, by PTO and by cycloheximide. Seems to not be influenced by abiotic stresses (ET, SA, JA and wounding).