As part of their immune system, plants have pattern recognition receptors (PRRs) that can detect a broad range of microbeassociated molecular patterns (MAMPs). Here, we identified a PRR of Arabidopsis thaliana with specificity for the bacterial MAMP eMax from xanthomonads. Response to eMax seems to be restricted to the Brassicaceae family and also varied among different accessions of Arabidopsis. In crosses between sensitive accessions and the insensitive accession Shakhdara, eMax perception mapped to RECEPTOR-LIKE PROTEIN1 (RLP1). Functional complementation of rlp1 mutants required gene constructs that code for a longer version of RLP1 that we termed ReMAX (for receptor of eMax). ReMAX/RLP1 is a typical RLP with structural similarity to the tomato (Solanum lycopersicum) RLP Eix2, which detects fungal xylanase as a MAMP. Attempts to demonstrate receptor function by interfamily transfer of ReMAX to Nicotiana benthamiana were successful after using hybrid receptors with the C-terminal part of ReMAX replaced by that of Eix2. These results show that ReMAX determines specificity for eMax. They also demonstrate hybrid receptor technology as a promising tool to overcome problems that impede interfamily transfer of PRRs to enhance pathogen detection in crop plants.

The receptor kinase EFR of Arabidopsis thaliana detects the microbe-associated molecular pattern elf18, a peptide that represents the N terminus of bacterial elongation factor Tu. Here, we tested subdomains of EFR for their importance in receptor function. Receptor-like kinases (RLKs) 2 form the biggest family of surface receptors in higher plants. Based on genomic sequence information, more than 600 genes in Arabidopsis thaliana and more than 1,000 genes in rice are predicted to encode RLKs (1, 2). RLKs have a common molecular structure consisting of a C-terminal cytoplasmic Ser/Thr protein kinase domain connected by a single-pass transmembrane motif to different types of N-terminal ectodomains facing extracytoplasmic compartments. Members of the RLK family play fundamental roles for cellular response programs regulating cell growth, morphogenesis, fertilization, abscission, plant defense, and interaction with symbionts (3-7). Nevertheless, the attribute "receptor-like" holds for most of the RLKs that still remain orphan with respect to their biological functions and their potential regulatory ligands. Concerning functional aspects of ligand interaction and transmembrane activation, the receptor kinase BRI1, functioning as the receptor for the growth hormone brassinolide, has been most thoroughly studied (8 -10). Thereby, the ligand interaction site could be localized to a small subdomain within the ectodomain of BRI1 (11), and activation was found to involve a ligand-dependent complex formation with a second receptor kinase termed BAK1 for BRI1-associated receptor kinase 1 (9, 10). Transient expression of tagged versions of EFR and EFR lacking its cytoplasmic domain in leaves ofSeveral RLKs have been assigned roles in recognition of pathogen attack. Examples for RLKs that function as pattern recognition receptors for identified ligands include XA21 from rice (12), FLS2 (13), EFR (14), and CERK1 (15) detecting microbe-associated molecular patterns (MAMPs), and AtPEPR1 (16) detecting wound-related, endogenous danger signals. Upon activation with their respective ligands, FLS2, EFR, and AtPEPR1 seem to form heteromeric complexes with the co-receptor BAK1. Thus, intriguingly, BAK1 appears to function in the transmembrane signaling of plant RLKs with very different signal output programs (17)(18)(19)(20).The receptor kinase EFR from A. thaliana responds to bacterial elongation factor Tu (EF-Tu) with the induction of defense and increased resistance (14, 21). Mutant plants lacking this perception system show increased susceptibility to infection by Agrobacterium tumefaciens. The epitope that acts as MAMP was identified as the acetylated N terminus of EF-Tu. Synthetic peptides like elf18 and elf26 which represent the N terminus of EF-Tu with at least 18 amino acids are fully active as MAMPs and stimulate responses at subnanomolar concentrations. A radiolabeled derivative of elf26 was used to demonstrate specific, high affinity binding sites in tissues expressing EFR. In affinity cross-linking experiments this radioli...

Evidence from field experiments indicates differential roles of sulfur and nitrogen supply for plant resistance against pathogens. Dissection of these observations in defined pathosystems and controlled nutritional conditions indicates an activation of plant sulfur metabolism in several incompatible and compatible interactions. Contents of cysteine and glutathione as markers of primary sulfate assimilation and stress response show increases in ARABIDOPSIS THALIANA upon infection, coinciding with the synthesis of sulfur-containing defence compounds. Similar increases of thiols were observed with necrotrophic, biotrophic, and hemibiotrophic pathogens. Sulfate supply was found to be neutral or beneficial for tolerance against fungal but neutral for bacterial pathogens under IN VITRO conditions. According to various reports and own observations the effects of nitrogen supply appeared to be neutral or harmful, depending on the pathogen. The activation of sulfur metabolism was a consequence of activation of gene expression as revealed by macroarray analysis of an A. THALIANA/ALTERNARIA BRASSICICOLA pathosystem. This activation appeared to be largely independent from sufficient or optimal sulfate supply and from the established sulfate deficiency response. The data suggest that plant-pathogen interactions and sulfur metabolism are linked by jasmonic acid as signal.

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