[Abstract unavailable

Plants and animals employ innate immune systems to cope with microbial infection. Pattern-triggered immunity relies on the recognition of microbe-derived patterns by pattern recognition receptors (PRRs). Necrosis and ethylene-inducing peptide 1-like proteins (NLPs) constitute plant immunogenic patterns that are unique, as these proteins are produced by multiple prokaryotic (bacterial) and eukaryotic (fungal, oomycete) species. Here we show that the leucine-rich repeat receptor protein (LRR-RP) RLP23 binds in vivo to a conserved 20-amino-acid fragment found in most NLPs (nlp20), thereby mediating immune activation in Arabidopsis thaliana. RLP23 forms a constitutive, ligand-independent complex with the LRR receptor kinase (LRR-RK) SOBIR1 (Suppressor of Brassinosteroid insensitive 1 (BRI1)-associated kinase (BAK1)-interacting receptor kinase 1), and recruits a second LRR-RK, BAK1, into a tripartite complex upon ligand binding. Stable, ectopic expression of RLP23 in potato (Solanum tuberosum) confers nlp20 pattern recognition and enhanced immunity to destructive oomycete and fungal plant pathogens, such as Phytophthora infestans and Sclerotinia sclerotiorum. PRRs that recognize widespread microbial patterns might be particularly suited for engineering immunity in crop plants.

Recognition of microbial patterns by host pattern recognition receptors is a key step in immune activation in multicellular eukaryotes. Peptidoglycans (PGNs) are major components of bacterial cell walls that possess immunity-stimulating activities in metazoans and plants. Here we show that PGN sensing and immunity to bacterial infection in Arabidopsis thaliana requires three lysin-motif (LysM) domain proteins. LYM1 and LYM3 are plasma membrane proteins that physically interact with PGNs and mediate Arabidopsis sensitivity to structurally different PGNs from Gram-negative and Gram-positive bacteria. lym1 and lym3 mutants lack PGN-induced changes in transcriptome activity patterns, but respond to fungus-derived chitin, a pattern structurally related to PGNs, in a wild-type manner. Notably, lym1, lym3, and lym3 lym1 mutant genotypes exhibit supersusceptibility to infection with virulent Pseudomonas syringae pathovar tomato DC3000. Defects in basal immunity in lym3 lym1 double mutants resemble those observed in lym1 and lym3 single mutants, suggesting that both proteins are part of the same recognition system. We further show that deletion of CERK1, a LysM receptor kinase that had previously been implicated in chitin perception and immunity to fungal infection in Arabidopsis, phenocopies defects observed in lym1 and lym3 mutants, such as peptidoglycan insensitivity and enhanced susceptibility to bacterial infection. Altogether, our findings suggest that plants share with metazoans the ability to recognize bacterial PGNs. However, as Arabidopsis LysM domain proteins LYM1, LYM3, and CERK1 form a PGN recognition system that is unrelated to metazoan PGN receptors, we propose that lineage-specific PGN perception systems have arisen through convergent evolution.

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.

Mitogen-activated protein kinase (MAPK)-mediated responses are in part regulated by the repertoire of MAPK substrates, which is still poorly elucidated in plants. Here, the in vivo enzyme-substrate interaction of the Arabidopsis thaliana MAP kinase, MPK6, with an ethylene response factor (ERF104) is shown by fluorescence resonance energy transfer. The interaction was rapidly lost in response to flagellin-derived flg22 peptide. This complex disruption requires not only MPK6 activity, which also affects ERF104 stability via phosphorylation, but also ethylene signaling. The latter points to a novel role of ethylene in substrate release, presumably allowing the liberated ERF104 to access target genes. Microarray data show enrichment of GCC motifs in the promoters of ERF104 -up-regulated genes, many of which are stress related. ERF104 is a vital regulator of basal immunity, as altered expression in both erf104 and overexpressors led to more growth inhibition by flg22 and enhanced susceptibility to a non-adapted bacterial pathogen.itogen-activated protein kinase (MAPK) cascades transduce external signals into cellular responses in eukaryotes (1). In plants, MAPKs orthologous to the Arabidopsis MPK3, MPK4, and MPK6 are activated by various stimuli including flg22, a bacterial flagellin-derived peptide that acts as a pathogen-associated molecular pattern (PAMP) (2-5). These three MAPKs control defense positively (MPK3/MPK6) (3, 6) or negatively (MPK4) (7).Many phytohormones have been shown to affect defense responses; but most progress has been made in regard to salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) (8). The tobacco MPK6 ortholog is activated by SA (9) and the Arabidopsis mpk4 mutant has elevated SA levels and enhanced pathogen resistance (7). Genetic evidence linking ET to MAPK signaling is also suggested by the negative regulator of the ET response, Constitutive Triple Response 1 (CTR1), a Raf-like kinase that was recently shown to control MPK3/6 activation via MKK9 (MAPK kinase 9) (10). Both JA and the ET precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), activate MPK6 in Arabidopsis (11, 12) but not in tobacco (13). Although responses may differ between plant species, the activation of MPK6 by ET/ACC is highly debated (14). In another report, ACC did not activate MPK6, but ET biosynthesis was positively regulated by MPK6 through posttranslational stabilization of the rate-limiting ACC synthase (ACS) isoforms, ACS2 and ACS6 (14,15).In addition to the cytoplasmic ACSs, MAPKs also target nuclear proteins (10,16,17); this may occur either after MAPK nuclear translocation following activation (18,19) or as preformed nuclear protein complexes (20). The latter would imply movement of the upstream MKKs into the nucleus to modify the MAPKs or, alternatively, that the activated MAPKs enter the nucleus to displace the inactive MAPK from preformed complexes. Examples of nuclear targets include the MPK4 substrates, MKS1 and two MKS1-interactors of the WRKY transcription factor family, WRKY25 and WRKY33 ...