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.

Our results provide evidence for a new regulatory mechanism for innate immune receptors with BIR2 acting as a negative regulator of PAMP-triggered immunity by limiting BAK1-receptor complex formation in the absence of ligands.

Glutaredoxins are small heat-stable oxidoreductases that transfer electrons from glutathione (GSH) to oxidized cysteine residues, thereby contributing to protein integrity and regulation. In Arabidopsis thaliana, floral glutaredoxins ROXY1 and ROXY2 and pathogen-induced ROXY19/GRX480 interact with bZIP transcription factors of the TGACG (TGA) motif-binding family. ROXY1, ROXY2, and TGA factors PERIANTHIA, TGA9, and TGA10 play essential roles in floral development. In contrast, ectopically expressed ROXY19/GRX480 negatively regulates expression of jasmonic acid (JA)/ethylene (ET)-induced defense genes through an unknown mechanism that requires clade II transcription factors TGA2, TGA5, and/or TGA6. Here, we report that at least 17 of the 21 land plant-specific glutaredoxins encoded in the Arabidopsis genome interact with TGA2 in a yeast-two-hybrid system. To investigate their capacity to interfere with the expression of JA/ET-induced genes, we developed a transient expression system. Activation of the ORA59 (OCTADECANOID-RESPONSIVE ARABIDOPSIS AP2/ERF-domain protein 59) promoter by transcription factor EIN3 (ETHYLENE INSENSITVE 3) was suppressed by co-expressed ROXY19/GRX480. Suppression depended on the L**LL motif in the C-terminus of ROXY19/GRX480. This putative protein interaction domain was recently described as being essential for the TGA/ROXY interaction. Ten of the 17 tested ROXY proteins suppressed ORA59 promoter activity, which correlated with the presence of the C-terminal ALWL motif, which is essential for ROXY1 function in flower development. ROXY19/GRX480-mediated repression depended on the GSH binding site, suggesting that redox modification of either TGA factors or as yet unknown target proteins is important for the suppression of ORA59 promoter activity.

Short title: BIR3 negatively regulates and stabilizes BAK1One-sentence summary: The receptor kinase BIR3 negatively regulates cell surface receptor complexes and thereby prevents unwanted activation of immune and hormone responses.The authors responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantcell.org) are: Birgit Kemmerling (birgit.kemmerling@zmbp.uni-tuebingen.de) and Xiaofeng Wang (wangxff99@nwsuaf.edu.cn).ABSTRACT BAK1 is a co-receptor and positive regulator of multiple ligand-binding leucine-richrepeat receptor kinases (LRR-RKs) and is involved in brassinosteroid (BR)-dependent growth and development, innate immunity and cell death control. The BAK1-interacting LRR-RKs BIR2 and BIR3 were previously identified by proteomics analyses of in vivo BAK1 complexes. Here we show that BAK1-related pathways such as innate immunity and cell death control are affected by BIR3 in Arabidopsis Plant Cell Advance Publication. Published on August 25 2017, doi:10.1105/tpc.17.00376 ©2017 American Society of Plant Biologists. All Rights Reserved 2 thaliana. BIR3 also has a strong negative impact on BR signaling. BIR3 directly interacts with the BR receptor BRI1 and other ligand-binding receptors and negatively regulates BR signaling by competitive inhibition of BRI1. BIR3 is released from BAK1 and BRI1 after ligand exposure and directly affects the formation of BAK1 complexes with BRI1 or FLAGELLIN SENSING2. Double mutants of bak1 and bir3 show spontaneous cell death and constitutive activation of defense responses. BAK1 and its closest homolog BKK1 interact with and are stabilized by BIR3, suggesting that bak1 bir3 double mutants mimic the spontaneous cell death phenotype observed in bak1 bkk1 mutants via destabilization of BIR3 target proteins. Our results provide evidence for a negative regulatory mechanism for BAK1 receptor complexes in which BIR3 interacts with BAK1 and inhibits ligand-binding receptors to prevent BAK1 receptor complex formation.

SUMMARYIn eukaryotic cells, transduction of external stimuli into the nucleus to induce transcription and export of mRNAs for translation in the cytoplasm is mediated by nuclear pore complexes (NPCs) composed of nucleoporin proteins (Nups). We previously reported that Arabidopsis MOS3, encoding the homolog of vertebrate Nup96, is required for plant immunity and constitutive resistance mediated by the de-regulated Toll interleukin 1 receptor/nucleotide-binding/leucine-rich repeat (TNL)-type R gene snc1. In vertebrates, Nup96 is a component of the conserved Nup107-160 nuclear pore sub-complex, and implicated in immunity-related mRNA export. Here, we used a reverse genetics approach to examine the requirement for additional subunits of the predicted Arabidopsis Nup107-160 complex in plant immunity. We show that, among eight putative complex members, beside MOS3, only plants with defects in Nup160 or Seh1 are impaired in basal resistance. Constitutive resistance in the snc1 mutant and immunity mediated by TNL-type R genes also depend on functional Nup160 and have a partial requirement for Seh1. Conversely, resistance conferred by coiled coil-type immune receptors operates largely independently of both genes, demonstrating specific contributions to plant defense signaling. Our functional analysis further revealed that defects in nup160 and seh1 result in nuclear accumulation of poly(A) mRNA, and, in the case of nup160, considerable depletion of EDS1, a key positive regulator of basal and TNL-triggered resistance. These findings suggest that Nup160 is required for nuclear mRNA export and full expression of EDS1-conditioned resistance pathways in Arabidopsis.