We show that oomycete-derived Nep1 (for necrosis and ethylene-inducing peptide1)-like proteins (NLPs) trigger a comprehensive immune response in Arabidopsis thaliana, comprising posttranslational activation of mitogen-activated protein kinase activity, deposition of callose, production of nitric oxide, reactive oxygen intermediates, ethylene, and the phytoalexin camalexin, as well as cell death. Transcript profiling experiments revealed that NLPs trigger extensive reprogramming of the Arabidopsis transcriptome closely resembling that evoked by bacteria-derived flagellin. NLP-induced cell death is an active, light-dependent process requiring HSP90 but not caspase activity, salicylic acid, jasmonic acid, ethylene, or functional SGT1a/SGT1b. Studies on animal, yeast, moss, and plant cells revealed that sensitivity to NLPs is not a general characteristic of phospholipid bilayer systems but appears to be restricted to dicot plants. NLP-induced cell death does not require an intact plant cell wall, and ectopic expression of NLP in dicot plants resulted in cell death only when the protein was delivered to the apoplast. Our findings strongly suggest that NLP-induced necrosis requires interaction with a target site that is unique to the extracytoplasmic side of dicot plant plasma membranes. We propose that NLPs play dual roles in plant pathogen interactions as toxin-like virulence factors and as triggers of plant innate immune responses.

The innate immune system is a host defense mechanism that is evolutionarily conserved from insects to human and is mainly involved in the recognition and control of the early stage of infection in all animals (1). Over the last decade, it has become increasingly evident that also plants have acquired the ability to recognize "non self" via sensitive perception systems for components of microorganisms called pathogen-associated molecular patterns (PAMPs) 2 (2-4). As classically defined, PAMPs are highly characteristic of potentially infectious microbes, but are not present in the host. In addition, such patterns are often vital for microbial survival and are therefore not subject to mutational variation. PAMPs that trigger innate immune responses in various vertebrate and non-vertebrate organisms include lipopolysaccharides (LPS) from Gram-negative bacteria, eubacterial flagellin, viral, and bacterial nucleic acids, fungal cell wall-derived glucans, chitins, mannans, or proteins and peptidoglycans (PGN) from Gram-positive bacteria (5-8). Peptidoglycan (PGN) is an essential and unique component of the bacterial envelope that provides rigidity and structure to the bacterial cell. Virtually all bacteria contain a layer of PGN, but the amount, location, and specific composition vary. PGN is a polymer of alternating N-acetylglucosamine (GlcNAc) and N-acetyl-muramic acid (MurNAc) residues in ␤-1-4 linkage which are cross-linked by short peptides (9, 10). The glycan chains display little variation among different bacterial species while the peptide subunit and the interpeptide bridge reveal species specific differences. PGN from Staphylococcus aureus belongs to the L-lysine (Lys)-type, which is primarily found in Gram-positive bacteria whereas meso-diaminopimelate (Dap)-type PGN is typical for many Gram-negative bacteria.As PGNs are located on most bacterial surfaces they constitute excellent targets for recognition by the innate immune system. Indeed, PGN is known for a long time to promote an innate immune response in vertebrates and insects (11-13), and a breakdown product of PGN, muramyl dipeptide (MurNAc-L-Ala-D-Glu; MDP) was found to be the minimal chemical structure required for PAMP activity in mammals (14). PGN is perceived in animals via various pattern recognition receptors (PRRs), including scavenger receptors, nucleotide-binding oligomerization domain-containing proteins (NODs), a family of peptidoglycan recognition proteins (PGRPs), PGN-lytic enzymes and Toll-like receptor TLR2 (15-19).Remarkable similarities have been uncovered in the molecular mode of PAMP perception in animals and plants (2,20,21). Perception of flagellin in Arabidopsis was shown to be depend-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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