Pseudomonas syringae represents the primary model organism with which the molecular basis of plant-pathogen interactions and co-evolution of host-specificity have been explored. Its virulence is dependent upon the needle-like Type-III secretion system (T3SS) for translocation of effector proteins that interfere with plant defence signalling into the host cell cytoplasm. The sigma factor (σ) HrpL co-ordinates the concomitant transcription of genes encoding the T3SS complex and its associated substrates. In P. syringae, the paralogous activator proteins HrpR and HrpS are both required for σ54-dependent transcription of hrpL. Furthermore, HrpS is subject to allosteric inhibition by HrpV, which is in turn relieved by HrpG. A holistic view of how signalling and regulatory processes control the transition into hrpL-dependent pathogenicity in response to the in planta environment is lacking. In this thesis, I first present evidence for negative autogenous control of the hrpL gene by its σ factor product in the P. syringae pv. tomato DC3000 strain. This novel mechanism is dependent upon HrpL recruiting the bacterial RNA polymerase to the adjacent and divergently transcribed hrpJ promoter. Preliminary in vitro studies suggest that the physical proximity of the RNAP-HrpL holoenzyme complex inhibits at a distance one or more regulatory events required for transcription initiation at hrpL. With respect to its physiological significance, functional analyses infer that the fine control of HrpL expression afforded by negative autogenous control of HrpL expression contributes to correctly ordered T3SS secretion. Secondly, I show that by modulating the activity of the global Gac-Rsm signalling cascade the sensor histidine kinase RetS positively regulates T3SS gene expression as well as affecting other aspects of lifestyle switching. Control of hrpL expression by Gac-Rsm is exerted in part via post-transcriptional regulation of HrpV which, indicative of divergent target specificities, is achieved to varying extents by the three CsrA paralogues in P. syringae.