The plant immune system involves cell-surface receptors that detect intercellular pathogenderived molecules, and intracellular receptors that activate immunity upon detection of pathogen-secreted effectors that act inside the plant cell. Surface receptor-mediated immunity has been extensively studied 1 , but intracellular receptor-mediated immunity has rarely been investigated in the absence of surface receptor-mediated immunity. Furthermore, interactions between these two immune pathways are poorly understood. By activating intracellular receptors in the absence of surface receptor-mediated immunity, we dissected interactions between the two distinct immune systems. Recognition by surface receptors activates multiple protein kinases and NADPH oxidases; we find intracellular receptors primarily potentiate the activation of these proteins by elevating their abundance via multiple mechanisms. Reciprocally, the intracellular receptor-dependent hypersensitive response is strongly enhanced by activation of surface receptors. Activation of either immune system alone is insufficient to provide effective resistance against the bacterial pathogen Pseudomonas syringae. Thus, immune pathways activated by cell-surface and intracellular receptors mutually potentiate to activate strong defense that thwarts pathogens. These

Salicylic acid (SA) is a defense hormone required for both local and systemic acquired resistance (SAR) in plants. Pathogen infections induce SA synthesis through up-regulating the expression of Isochorismate Synthase 1 (ICS1), which encodes a key enzyme in SA production. Here we report that both SAR Deficient 1 (SARD1) and CBP60g are key regulators for ICS1 induction and SA synthesis. Whereas knocking out SARD1 compromises basal resistance and SAR, overexpression of SARD1 constitutively activates defense responses. In the sard1-1 cbp60g-1 double mutant, pathogen-induced ICS1 upregulation and SA synthesis are blocked in both local and systemic leaves, resulting in compromised basal resistance and loss of SAR. Electrophoretic mobility shift assays showed that SARD1 and CBP60g represent a plant-specific family of DNA-binding proteins. Both proteins are recruited to the promoter of ICS1 in response to pathogen infections, suggesting that they control SA synthesis by regulating ICS1 at the transcriptional level.