Plants have evolved a multilayered immune system to counter pathogen attacks. EDS1 (Enhanced Disease Susceptibility 1) and PAD4 (Phytoalexin Deficient 4) are two plant- specific lipase-like proteins that function as essential regulators of plant innate immunity. They are crucial for basal defence that restricts growth of virulent pathogens and for race-specific resistance to avirulent pathogens triggered by TIR (Toll-Interleukin 1) type NBS-LRR (Nucleotide Binding Site � Leucine Rich Repeats) immune receptors. Moreover, EDS1 and PAD4 generate and perceive (a) signal(s) needed to induce systemic immunity. These regulators stimulate accumulation of the phenolic defence signaling molecule salicylic acid (SA) and SA, in turn, induces their expression creating a positive feedback loop in defence potentiation. EDS1 and PAD4 transcript and correspondent protein levels increase upon pathogen challenge. However, earlier changes in expression of a set of distinct genes which are EDS1- and PAD4-dependent imply the activation of pre-existing EDS1/PAD4 complexes through post-translational mechanism(s). In this work I investigated the relative importance of transcriptional regulation and post-transcriptional processes for EDS1 and PAD4 protein functions. I characterized Arabidopsis thaliana transgenic lines overexpressing either EDS1, PAD4 or both. Only lines cooverexpressing EDS1 and PAD exhibited growth retardation associated with constitutive activation of the SA pathway and increased resistance to virulent pathogens resulting from a faster SA pathway activation. These lines exhibit also increased tolerance to chemically induced oxidative stress consistent with a known role of EDS1 and PAD4 in processing reactive oxygen species (ROS) - derived signals. The insufficiency of EDS1-PAD4 cooverexpression to fully recapitulate defence activation implies the existence of post-translational mechanisms of regulation. The existence of regulatory post-translational modifications of the EDS1 protein was investigated and lines expressing constitutively or conditionally activated functional epitope-tagged EDS1 were generated. The data presented here demonstrate that EDS1 and PAD4 operate as a signaling unit. The basis of the observed dramatic biotic and abiotic stress phenotypes will be further investigated as it should provide important insight into EDS1 and PAD4 functions
