In this thesis I describe the results of experiments aimed at characterizing different aspects of the immune system in plants, through the investigation of the Arabidopsis thaliana and Pseudomonas syringae interaction. First, we explored the ability of different microbe associated molecular patterns, MAMPS to induce certain defense marker genes. Additionally we explored specific single bacterial avirulence effectors for their ability to suppress MAMP induced defense signaling in R-gene-deficient plants (Chapter 1). Flagellin (flg22), cellulase, and salicylic acid induced the chosen marker genes in different ways, but plants seemed to respond to a lesser extent to the E.coli compound Kdo₂-lipid A, which induces defense in mammalian cells. The Pseudomonas effectors avrRpm1, avrRpt2, avrRps4, avrB, and avrPphB did not clearly suppress any of the marker-gene inductions. Moreover, some induction of markers was observed, suggesting recognition of the effectors by unknown (R-) proteins. We also explored the effects that light and temperature have on the hypersensitive response (HR) during the resistance (R) -gene defense response induced by expression of a single bacterial effector (Chapter 2). As expected deprivation of light delayed the HR. Surprisingly, and in contrast to other studies, HR was functional at high temperatures, and instead HR was delayed at low temperature (17⁰C). Since the function of many proteins in immune signaling is unknown, as well as potential protein-protein interactions, we explored the interactome of the protein NDR1. We identified 286 potential NDR1 interacting proteins with potential roles in plant immunity. Currently, candidate proteins are being tested for their role in defense.