Patternrecognition receptors in eukaryotes initiate defence responses on detection of microbe-associated molecular patterns shared by many microbe species. The Leu-rich repeat receptor-like kinases FLS2 and EFR recognize the bacterial epitopes flg22 and elf18, derived from flagellin and elongation factor-Tu, respectively. We describe Arabidopsis 'priority in sweet life' (psl) mutants that show de-repressed anthocyanin accumulation in the presence of elf18. EFR accumulation and signalling, but not of FLS2, are impaired in psl1, psl2, and stt3a plants. PSL1 and PSL2, respectively, encode calreticulin3 (CRT3) and UDP-glucose:glycoprotein glycosyltransferase that act in concert with STT3A-containing oligosaccharyltransferase complex in an N-glycosylation pathway in the endoplasmic reticulum. However, EFR-signalling function is impaired in weak psl1 alleles despite its normal accumulation, thereby uncoupling EFR abundance control from quality control. Furthermore, salicylic acid-induced, but EFR-independent defence is weakened in psl2 and stt3a plants, indicating the existence of another client protein than EFR for this immune response. Our findings suggest a critical and selective function of N-glycosylation for different layers of plant immunity, likely through quality control of membrane-localized regulators.

Plants actively perceive and respond to perturbations in their cell walls which arise during growth, biotic and abiotic stresses. However, few components involved in plant cell wall integrity sensing have been described to date. Using a reverse-genetic approach, we identified the Arabidopsis thaliana leucine-rich repeat receptor kinase MIK2 as an important regulator of cell wall damage responses triggered upon cellulose biosynthesis inhibition. Indeed, loss-of-function mik2 alleles are strongly affected in immune marker gene expression, jasmonic acid production and lignin deposition. MIK2 has both overlapping and distinct functions with THE1, a malectin-like receptor kinase previously proposed as cell wall integrity sensor. In addition, mik2 mutant plants exhibit enhanced leftward root skewing when grown on vertical plates. Notably, natural variation in MIK2 (also named LRR-KISS) has been correlated recently to mild salt stress tolerance, which we could confirm using our insertional alleles. Strikingly, both the increased root skewing and salt stress sensitivity phenotypes observed in the mik2 mutant are dependent on THE1. Finally, we found that MIK2 is required for resistance to the fungal root pathogen Fusarium oxysporum. Together, our data identify MIK2 as a novel component in cell wall integrity sensing and suggest that MIK2 is a nexus linking cell wall integrity sensing to growth and environmental cues.

Recognition of molecular patterns characteristic of microbes or altered-self leads to immune activation in multicellular eukaryotes. In Arabidopsis thaliana, the leucine-rich-repeat receptor kinases FLAGELLIN-SENSING2 (FLS2) and EF-TU RECEPTOR (EFR) recognize bacterial flagellin and elongation factor EF-Tu (and their elicitor-active epitopes flg22 and elf18), respectively. Likewise, PEP1 RECEP-TOR1 (PEPR1) and PEPR2 recognize the elicitor-active Pep epitopes conserved in Arabidopsis ELICITOR PEPTIDE PRECURSORs (PROPEPs). Here we reveal that loss of ETHYLENE-INSENSITIVE2 (EIN2), a master signaling regulator of the phytohormone ethylene (ET), lowers sensitivity to both elf18 and flg22 in different defense-related outputs. Remarkably, in contrast to a large decrease in FLS2 expression, EFR expression and receptor accumulation remain unaffected in ein2 plants. Genome-wide transcriptome profiling has uncovered an inventory of EIN2-dependent and EFR-regulated genes. This dataset highlights important aspects of how ET modulates EFR-triggered immunity: the potentiation of salicylate-based immunity and the repression of a jasmonate-related branch. EFR requires ET signaling components for PROPEP2 activation but not for PROPEP3 activation, pointing to both ET-dependent and -independent engagement of the PEPR pathway during EFR-triggered immunity. Moreover, PEPR activation compensates the ein2 defects for a subset of EFR-regulated genes. Accordingly, ein2 pepr1 pepr2 plants exhibit additive defects in EFR-triggered antibacterial immunity, compared with ein2 or pepr1 pepr2 plants. Our findings suggest that the PEPR pathway not only mediates ET signaling but also compensates for its absence in enhancing plant immunity.microbe-associated molecular pattern | danger-associated molecular pattern | disease resistance | plant hormone | defense-related genes

Recognition of microbe-associated molecular patterns (MAMPs), conserved structures typical of a microbial class, triggers immune responses in eukaryotes. This is accompanied by a diverse set of physiological responses that are thought to enhance defense activity in plants. However, the extent and mechanisms by which MAMPinduced events contribute to host immunity are poorly understood. Here we reveal Arabidopsis priority in sweet life4 (psl4) and psl5 mutants that are insensitive to the bacterial elongation factor (EF)-Tu epitope elf18 but responsive to flagellin epitope flg22. PSL4 and PSL5, respectively, identify ␤-and ␣-subunits of endoplasmic reticulumresident glucosidase II, which is essential for stable accumulation and quality control of the elf18 receptor EFR but not the flg22 receptor FLS2. We notice that EFR signaling is partially and differentially impaired without a significant decrease of the receptor steady-state levels in 2 weakly dysfunctional gII␣ alleles, designated psl5-1 and rsw3. Remarkably, rsw3 plants exhibit marked supersusceptibility against a virulent bacterial phytopathogen despite nearly intact coactivation of MAPKs, reactive oxygen species, ethylene biosynthesis, and callose deposition in response to elf18, demonstrating that these signaling outputs alone are insufficient to mount effective immunity. However, rsw3 plants fail to maintain high transcript levels of defense-promoting WRKY, PR1, and PR2 genes at late time points (4 to 24 h) after elf18 elicitation. This points to an unexpected separation between initial and sustained activation of EFR-mediated signaling in the absence of proper glucosidase II-mediated endoplasmic reticulum quality control. Our findings strongly suggest the importance of sustained MAMP receptor signaling as a key step in the establishment of robust immunity.EFR ͉ ER quality control ͉ LRR RLK ͉ plant immunity

Summary Plant pathogens employ effector proteins to manipulate their hosts. Fusarium oxysporum f. sp. lycopersici (Fol), the causal agent of tomato wilt disease, produces effector protein Avr2. Besides being a virulence factor, Avr2 triggers immunity in I‐2 carrying tomato (Solanum lycopersicum). Fol strains that evade I‐2 recognition carry point mutations in Avr2 (e.g. Avr2R45H), but retain full virulence.Here we investigate the virulence function of Avr2 and determine its crystal structure. Transgenic tomato and Arabidopsis expressing either wild‐type ΔspAvr2 (deleted signal‐peptide) or the ΔspAvr2 R45H variant become hypersusceptible to fungal, and even bacterial infections, suggesting that Avr2 targets a conserved defense mechanism. Indeed, Avr2 transgenic plants are attenuated in immunity‐related readouts, including flg22‐induced growth inhibition, ROS production and callose deposition.The crystal structure of Avr2 reveals that the protein shares intriguing structural similarity to ToxA from the wheat pathogen Pyrenophora tritici‐repentis and to TRAF proteins. The I‐2 resistance‐breaking Avr2V41M, Avr2R45H and Avr2R46P variants cluster on a surface‐presented loop. Structure‐guided mutagenesis enabled uncoupling of virulence from I‐2‐mediated recognition.We conclude that I‐2‐mediated recognition is not based on monitoring Avr2 virulence activity, which includes suppression of immune responses via an evolutionarily conserved effector target, but by recognition of a distinct epitope.

Environmental adaptation of organisms relies on fast perception and response to external signals, which lead to developmental changes. Plant cell growth is strongly dependent on cell wall remodeling. However, little is known about cell wall‐related sensing of biotic stimuli and the downstream mechanisms that coordinate growth and defense responses. We generated genetically encoded pH sensors to determine absolute pH changes across the plasma membrane in response to biotic stress. A rapid apoplastic acidification by phosphorylation‐based proton pump activation in response to the fungus Fusarium oxysporum immediately reduced cellulose synthesis and cell growth and, furthermore, had a direct influence on the pathogenicity of the fungus. In addition, pH seems to influence cellulose structure. All these effects were dependent on the COMPANION OF CELLULOSE SYNTHASE proteins that are thus at the nexus of plant growth and defense. Hence, our discoveries show a remarkable connection between plant biomass production, immunity, and pH control, and advance our ability to investigate the plant growth‐defense balance.

Plants interact with a huge variety of soil microbes, ranging from pathogenic to mutualistic. The Fusarium oxysporum (Fo) species complex consists of ubiquitous soil inhabiting fungi that can infect and cause disease in over 120 different plant species including tomato, banana, cotton, and Arabidopsis. However, in many cases Fo colonization remains symptomless or even has beneficial effects on plant growth and/or stress tolerance. Also in pathogenic interactions a lengthy asymptomatic phase usually precedes disease development. All this indicates a sophisticated and fine-tuned interaction between Fo and its host. The molecular mechanisms underlying this balance are poorly understood. Plant hormone signaling networks emerge as key regulators of plant-microbe interactions in general. In this review we summarize the effects of the major phytohormones on the interaction between Fo and its diverse hosts. Generally, Salicylic Acid (SA) signaling reduces plant susceptibility, whereas Jasmonic Acid (JA), Ethylene (ET), Abscisic Acid (ABA), and auxin have complex effects, and are potentially hijacked by Fo for host manipulation. Finally, we discuss how plant hormones and Fo effectors balance the interaction from beneficial to pathogenic and vice versa.

Pathogens use effector proteins to manipulate their hosts. During infection of tomato, the fungus Fusarium oxysporum secretes the effectors Avr2 and Six5. Whereas Avr2 suffices to trigger I-2-mediated cell death in heterologous systems, both effectors are required for I-2-mediated disease resistance in tomato. How Six5 participates in triggering resistance is unknown. Using bimolecular fluorescence complementation assays we found that Avr2 and Six5 interact at plasmodesmata. Single-cell transformation revealed that a 2xRFP marker protein and Avr2-GFP only move to neighboring cells in the presence of Six5. Six5 alone does not alter plasmodesmatal transduction as 2xRFP was only translocated in the presence of both effectors. In SIX5-expressing transgenic plants, the distribution of virally expressed Avr2-GFP, and subsequent onset of I-2-mediated cell death, differed from that in wild-type tomato. Taken together, our data show that in the presence of Six5, Avr2 moves from cell to cell, which in susceptible plants contributes to virulence, but in I-2 containing plants induces resistance.