The phytohormone auxin regulates almost every aspect of plant development. At the molecular level, auxin induces gene expression through direct physical interaction with the TIR1-like F box proteins, which in turn remove the Aux/IAA family of transcriptional repressors [1-4]. A growing body of evidence indicates that many plant pathogens can either produce auxin themselves or manipulate host auxin biosynthesis to interfere with the host's normal developmental processes [5-11]. In response, plants probably evolved mechanisms to repress auxin signaling during infection as a defense strategy. Plants overaccumulating the defense signal molecule salicylic acid (SA) frequently display morphological phenotypes that are reminiscent of auxin-deficient or auxin-insensitive mutants, indicating that SA might interfere with auxin responses. By using the Affymetrix ATH1 GeneChip for Arabidopsis thaliana, we performed a comprehensive study of the effects of SA on auxin signaling . We found that SA causes global repression of auxin-related genes, including the TIR1 receptor gene, resulting in stabilization of the Aux/IAA repressor proteins and inhibition of auxin responses. We demonstrate that this inhibitory effect on auxin signaling is a part of the SA-mediated disease-resistance mechanism.