SummaryThe perception of pathogen or microbe-associated molecular pattern molecules by plants triggers a basal defense response analogous to animal innate immunity, and is defined in part by the deposition of the glucan polymer callose at the cell wall at the site of pathogen contact. Transcriptional and metabolic profiling in Arabidopsis mutants, coupled with the monitoring of pathogen triggered callose deposition, have identified major roles in pathogen response for the plant hormone ethylene and the secondary metabolite 4-methoxy-indol-3-ylmethylglucosinolate. Two genes, PEN2 and PEN3, are also necessary for resistance to pathogens and are required for both callose deposition and glucosinolate activation, suggesting that the pathogen triggered callose response is required for resistance to microbial pathogens. Our study shows that well-studied plant metabolites, previously identified as important in avoiding damage by herbivores, are also required as a component of the plant defense response against microbial pathogens.Although plants are in continual contact with potential pathogens, a successful infection is rare. The ability of a particular plant species to prevent the successful colonization by a given pathogen species is referred to as non-host resistance (1-3). The molecular basis of non-host resistance is poorly understood, but presumably relies on both constitutive barriers and inducible responses. These layers are comprised of a constitutive defense layer and multiple pattern recognition receptors that respond to highly conserved microbe-associated molecular pattern (MAMP) molecules such as bacterial flagellin or peptidoglycan (4,5). MAMP recognition triggers the activation of serine/threonine-specific protein kinases (MAPKs) and various hormone signaling pathways (6). This signaling starts a cascade that activates a variety of defense responses including callose deposition, programmed cell death, production and accumulation of antimicrobial reactive oxygen species, and induction of phytoalexins and other secondary metabolites such as the indolic antimicrobial compound camalexin (3-thiazol-2′yl-indole) and glucosinolates (1-thio-beta-D-glucosides)(3). Interestingly, plants also constitutively synthesize and store glucosinolates, which are converted by endogenous S-* This manuscript has been accepted for publication in Science. This version has notundergone final editing. Please refer to the complete version of record athttp://www.sciencemag.org/. The manuscript may not be reproduced or used in anymanner that does not fall within the fair use provisions of the Copyright Act without theprior, written permission of AAAS. †To whom correspondence should be addressed: ausubel@molbio.mgh.harvard.edu. Despite detailed characterization of MAMP recognition and various hormone-mediated signaling pathways in plant defense response, relatively little is known about the host mechanisms that connect the perception of a particular pathogen to the downstream signaling pathways that lead to activation of specific immu...

We carried out transcriptional profiling analysis in 10-d-old Arabidopsis thaliana seedlings treated with oligogalacturonides (OGs), oligosaccharides derived from the plant cell wall, or the bacterial flagellin peptide Flg22, general elicitors of the basal defense response in plants. Although detected by different receptors, both OGs and Flg22 trigger a fast and transient response that is both similar and comprehensive, and characterized by activation of early stages of multiple defense signaling pathways, particularly JA-associated processes. However, the response to Flg22 is stronger in both the number of genes differentially expressed and the amplitude of change. The magnitude of induction of individual genes is in both cases dose-dependent, but, even at very high concentrations, OGs do not induce a response that is as comprehensive as that seen with Flg22. While high doses of either microbe-associated molecular pattern (MAMP) elicit a late response that includes activation of senescence processes, SA-dependent secretory pathway genes and PR1 expression are substantially induced only by Flg22. These results suggest a lower threshold for activation of early responses than for sustained or SA-mediated late defenses. Expression patterns of amino-cyclopropane-carboxylate synthase genes also implicate ethylene biosynthesis in regulation of the late innate immune response.