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

Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 (Johal and Briggs, 1992). Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.