SummaryArabidopsis resistance to the necrotrophic fungus Plectosphaerella cucumerina is complex and depends on the ethylene, jasmonic acid and salicylic acid signaling pathways. A quantitative trait loci (QTL) analysis of resistance to this fungus was performed using two populations of recombinant inbred lines. Three loci QRP1-QRP3 (for Quantitative Resistance to Plectosphaerella) were identified and mapped on chromosome 2 (QRP1 and QRP2) and 5 (QRP3). QRP1, the locus showing the strongest effect, was found to correspond to the ERECTA (ER) gene that encodes a receptor-like-kinase (RLK), which has been previously implicated in plant development, and resistance to the bacterium Ralstonia solanacearum. The leucine-rich repeat and the kinase domains of ERECTA were specifically required for resistance to P. cucumerina, as er mutant alleles impaired in any of these domains showed enhanced susceptibility to this fungus, but not to other virulent pathogens. The involvement of the ER-signaling pathway in resistance to P. cucumerina was supported by the fact that three mutants defective in this pathway, elk2, elk5 and elk4 (agb1-1), which encodes the b-subunit of Arabidopsis heterotrimeric G protein, were also impaired in their resistance to this fungus. The putative function of the Arabidopsis heterotrimeric G protein in resistance to P. cucumerina suggested by the enhanced susceptibility of agb1-1 was corroborated by the demonstration that a null allele (gpa1-4) of the G protein a-subunit showed enhanced resistance to this pathogen. Deposition of b-1,3-glucan callose at infection sites was specifically impaired in er-1 and agb1-1 mutants upon P. cucumerina inoculation. Taken together, these data suggest a putative function of ERECTA and heterotrimeric G protein in P. cucumerina perception.

In plant innate immunity, the surface-exposed leucine-rich repeat receptor kinases EFR and FLS2 mediate recognition of the bacterial pathogen-associated molecular patterns EF-Tu and flagellin, respectively. We identified the Arabidopsis stromal-derived factor-2 (SDF2) as being required for EFR function, and to a lesser extent FLS2 function. SDF2 resides in an endoplasmic reticulum (ER) protein complex with the Hsp40 ERdj3B and the Hsp70 BiP, which are components of the ER-quality control (ER-QC). Loss of SDF2 results in ER retention and degradation of EFR. The differential requirement for ER-QC components by EFR and FLS2 could be linked to N-glycosylation mediated by STT3a, a catalytic subunit of the oligosaccharyltransferase complex involved in co-translational N-glycosylation. Our results show that the plasma membrane EFR requires the ER complex SDF2-ERdj3B-BiP for its proper accumulation, and provide a demonstration of a physiological requirement for ER-QC in transmembrane receptor function in plants. They also provide an unexpected differential requirement for ER-QC and N-glycosylation components by two closely related receptors.

The Arabidopsis heterotrimeric G-protein controls defense responses to necrotrophic and vascular fungi. The agb1 mutant impaired in the Gβ subunit displays enhanced susceptibility to these pathogens. Gβ/AGB1 forms an obligate dimer with either one of the Arabidopsis Gγ subunits (γ1/AGG1 and γ2/AGG2). Accordingly, we now demonstrate that the agg1 agg2 double mutant is as susceptible as agb1 plants to the necrotrophic fungus Plectosphaerella cucumerina. To elucidate the molecular basis of heterotrimeric G-protein-mediated resistance, we performed a comparative transcriptomic analysis of agb1-1 mutant and wild-type plants upon inoculation with P. cucumerina. This analysis, together with metabolomic studies, demonstrated that G-protein-mediated resistance was independent of defensive pathways required for resistance to necrotrophic fungi, such as the salicylic acid, jasmonic acid, ethylene, abscisic acid, and tryptophan-derived metabolites signaling, as these pathways were not impaired in agb1 and agg1 agg2 mutants. Notably, many mis-regulated genes in agb1 plants were related with cell wall functions, which was also the case in agg1 agg2 mutant. Biochemical analyses and Fourier Transform InfraRed (FTIR) spectroscopy of cell walls from G-protein mutants revealed that the xylose content was lower in agb1 and agg1 agg2 mutants than in wild-type plants, and that mutant walls had similar FTIR spectratypes, which differed from that of wild-type plants. The data presented here suggest a canonical functionality of the Gβ and Gγ1/γ2 subunits in the control of Arabidopsis immune responses and the regulation of cell wall composition.

Yeast two-hybrid technology is used to build a high-quality protein interaction network centered on Arabidopsis G-protein coupled signaling. The interactions uncovered are without precedent in animals and fungi and help identify new cellular roles for G-protein signaling in plants.

Systemic acquired resistance (SAR) is an inducible immune response that depends on ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1).Here, we show that Arabidopsis (Arabidopsis thaliana) EDS1 is required for both SAR signal generation in primary infected leaves and SAR signal perception in systemic uninfected tissues. In contrast to SAR signal generation, local resistance remains intact in eds1 mutant plants in response to Pseudomonas syringae delivering the effector protein AvrRpm1. We utilized the SAR-specific phenotype of the eds1 mutant to identify new SAR regulatory proteins in plants conditionally expressing AvrRpm1. Comparative proteomic analysis of apoplastenriched extracts from AvrRpm1-expressing wild-type and eds1 mutant plants led to the identification of 12 APOPLASTIC, EDS1-DEPENDENT (AED) proteins. The genes encoding AED1, a predicted aspartyl protease, and another AED, LEGUME LECTIN-LIKE PROTEIN1 (LLP1), were induced locally and systemically during SAR signaling and locally by salicylic acid (SA) or its functional analog, benzo 1,2,3-thiadiazole-7-carbothioic acid S-methyl ester. Because conditional overaccumulation of AED1-hemagglutinin inhibited SAinduced resistance and SAR but not local resistance, the data suggest that AED1 is part of a homeostatic feedback mechanism regulating systemic immunity. In llp1 mutant plants, SAR was compromised, whereas the local resistance that is normally associated with EDS1 and SA as well as responses to exogenous SA appeared largely unaffected. Together, these data indicate that LLP1 promotes systemic rather than local immunity, possibly in parallel with SA. Our analysis reveals new positive and negative components of SAR and reinforces the notion that SAR represents a distinct phase of plant immunity beyond local resistance.

Screening of a genomic library from tomato plants (Lycopersicon esculentum) with a cDNA probe encoding a subtilisin-like protease (PR-P69) that is induced at the transcriptional level following pathogen attack (Tornero, P., Conejero, V., and Vera, P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 6332-6337) resulted in the isolation of a cluster of genomic clones that comprise a tandem of four different subtilisin-like protease genes (P69A, P69B, P69C, and P69D). Sequence analyses and comparison of the encoded proteins revealed that all are closely related (79 to 88% identity), suggesting that all are derived from a common ancestral gene. mRNA expression analysis as well as studies of transgenic plants transformed with promoter-␤-glucuronidase fusions for each of these genes revealed that the four genes exhibit differential transcriptional regulation and expression patterns. P69A and P69D are expressed constitutively, but with different expression profiles during development, whereas the P69B and P69C genes show expression following infection with Pseudomonas syringae and are also up-regulated by salicylic acid. We propose that these four P69-like proteases, as members of a complex gene family of plant subtilisin-like proteases, may be involved in a number of specific proteolytic events that occur in the plant during development and/or pathogenesis.Proteolysis is fundamental for the normal functioning of multicellular organisms and plays key roles in a variety of processes such as development, physiology, defense and stress responses, and adaptation to the changing environment. In plants, despite the importance of all these processes and involvement of different classes of proteinases (Refs. 1-5, and references therein), it still remains to be defined more precisely what components and molecular mechanisms are responsible for regulating specific aspects of protein degradation/processing. A major task for research will be to determine which pathway of proteolysis is responsible for the degradation of particular proteins.The serine proteases are one of the best characterized groups of proteolytic enzymes in higher organisms. They can be grouped in six clans, of which one of the largest is the subtilisin-like clan (EC 3.4.21.14) that includes over 200 different members. Despite this wealth of knowledge, very little is know about subtilisin-like proteases in plants. Recently, we and others have shown the existence of members of this clan in plants, including Arabidopsis (6), tomato (7, 8), melon (9), and Lilium plants (10). According to a recent classification (11), the subtilisin-like proteases from plants can be grouped within the Pyrolysin subfamily, which is highly related to the Kexin subfamily of proteases involved in the posttranslational processing of peptide hormones (12, 13). Comparative molecular, biochemical, and cellular studies indicate that the subgroup of plant subtilisin-like enzymes are characterized by the presence of a large polypeptide sequence insertion preceding the reactive Ser residue and/or lon...

SUMMARYFungal cell walls, which are essential for environmental adaptation and host colonization by the fungus, have been evolutionarily selected by plants and animals as a source of microbe-associated molecular patterns (MAMPs) that, upon recognition by host pattern recognition receptors (PRRs), trigger immune responses conferring disease resistance. Chito-oligosaccharides [b-1,4-N-acetylglucosamine oligomers, (GlcNAc) n ] are the only glycosidic structures from fungal walls that have been well-demonstrated to function as MAMPs in plants. Perception of (GlcNAc) 4-8 by Arabidopsis involves CERK1, LYK4 and LYK5, three of the eight members of the LysM PRR family. We found that a glucan-enriched wall fraction from the pathogenic fungus Plectosphaerella cucumerina which was devoid of GlcNAc activated immune responses in Arabidopsis wild-type plants but not in the cerk1 mutant. Using this differential response, we identified the non-branched 1,3-b-D-(Glc) hexasaccharide as a major fungal MAMP. Recognition of 1,3-b-D-(Glc) 6 was impaired in cerk1 but not in mutants defective in either each of the LysM PRR family members or in the PRR-co-receptor BAK1. Transcriptomic analyses of Arabidopsis plants treated with 1,3-b-D-(Glc) 6 further demonstrated that this fungal MAMP triggers the expression of immunity-associated genes. In silico docking analyses with molecular mechanics and solvation energy calculations corroborated that CERK1 can bind 1,3-b-D-(Glc) 6 at effective concentrations similar to those of (GlcNAc) 4 . These data support that plants, like animals, have selected as MAMPs the linear 1,3-b-D-glucans present in the walls of fungi and oomycetes. Our data also suggest that CERK1 functions as an immune co-receptor for linear 1,3-b-D-glucans in a similar way to its proposed function in the recognition of fungal chito-oligosaccharides and bacterial peptidoglycan MAMPs.

In plants, resistance to necrotrophic pathogens depends on the interplay between different hormone systems, such as those regulated by salicylic acid (SA), jasmonic acid (JA), ethylene, and abscisic acid. Repression of auxin signaling by the SA pathway was recently shown to contribute to antibacterial resistance. Here, we demonstrate that Arabidopsis auxin signaling mutants axr1, axr2, and axr6 that have defects in the auxin-stimulated SCF (Skp1-Cullin-F-box) ubiquitination pathway exhibit increased susceptibility to the necrotrophic fungi Plectosphaerella cucumerina and Botrytis cinerea. Also, stabilization of the auxin transcriptional repressor AXR3 that is normally targeted for removal by the SCF-ubiquitin/proteasome machinery occurs upon P. cucumerina infection. Pharmacological inhibition of auxin transport or proteasome function each compromise necrotroph resistance of wild-type plants to a similar extent as in non-treated auxin response mutants. These results suggest that auxin signaling is important for resistance to the necrotrophic fungi P. cucumerina and B. cinerea. SGT1b (one of two Arabidopsis SGT1 genes encoding HSP90/HSC70 co-chaperones) promotes the functions of SCF E3-ubiquitin ligase complexes in auxin and JA responses and resistance conditioned by certain Resistance (R) genes to biotrophic pathogens. We find that sgt1b mutants are as resistant to P. cucumerina as wild-type plants. Conversely, auxin/SCF signaling mutants are uncompromised in RPP4-triggered resistance to the obligate biotrophic oomycete, Hyaloperonospora parasitica. Thus, the predominant action of SGT1b in R gene-conditioned resistance to oomycetes appears to be at a site other than assisting SCF E3-ubiquitin ligases. However, genetic additivity of sgt1b axr1 double mutants in susceptibility to H. parasitica suggests that SCF-mediated ubiquitination contributes to limiting biotrophic pathogen colonization once plant-pathogen compatibility is established.