Arabidopsis plants containing the ndr1-1 mutation are incapable of mounting a hypersensitive response to bacteria carrying avrRpt2, but show an exaggerated cell death response to bacteria carrying avrB (Century et al., 1995). We show here that ndr1-1 plants are severely impaired in induction of systemic acquired resistance and PR1-driven transcription of a reporter gene in response to Pseudomonas syringae strains carrying avrRpt2 but not in response to P. syringae carrying avrB. The ndr1-1 mutation also impaired salicylic acid (SA) accumulation in response to treatments that produced reactive oxygen species (ROS) and impaired induction of systemic acquired resistance in response to in situ production of ROS. Hydrogen peroxide accumulated in wild-type Arabidopsis leaves beginning 4 to 7 h postinoculation with P. syringae carrying either avrRpt2 or avrB. In ndr1-1 plants, P. syringae carrying avrRpt2 elicited no detectable hydrogen peroxide production. Hydrogen peroxide production in response to bacteria carrying avrB was similar to that of Columbia in kinetics but of lesser intensity at early time points. These data are interpreted to indicate that NDR1 links ROS generation to SA production and that the phenotypic consequences of the ndr1-1 mutation are caused by a reduced ability to accumulate SA upon pathogen infection.Exquisite specificity is a hallmark of gene-for-gene disease resistance. Individual plant lines carry a specific complement of disease resistance (R) genes. Plants resist infection only if the pathogen carries a specific avirulence (avr) gene that is the matched cognate of one of these plant R genes. Mutant plants with nonfunctional alleles of a particular R gene fail to recognize a pathogen carrying the corresponding avr gene, and disease ensues (Parker et al., 2000;Staskawicz, 2001). With bacterial pathogens, this specificity of molecular recognition in at least some cases is associated with direct binding of the plant R gene product to the bacterial avr gene product (Scofield et al., 1996; Tang et al., 1996). Recognition takes place inside the plant cell (Gopalan et al., 1996;Leister et al., 1996) following export of the avr gene product from the bacteria via a type III secretion system (Pirhonen et al., 1996;Mudgett and Staskawicz, 1998).In contrast to the specificity of upstream molecular recognition processes, downstream plant responses to pathogen infection often bear strong similarities despite being elicited by vastly different types of pathogen. Gene-for-gene disease resistance is usually accompanied by rapid cell death (the hypersensitive response [HR]; Klement, 1982) in plant cells that are in direct contact with pathogen (Turner and Novacky, 1974). Uninoculated regions of the plant are induced to display an immunity to further pathogen challenge termed systemic acquired resistance (SAR). SAR protects plants from a broad spectrum of pathogens including those very different from the original (Ryals et al., 1994). A set of genes termed "pathogenesis related" (PR) are induced both locall...