Danger peptide receptor signaling in plants ensures basal immunity upon pathogen‐induced depletion of BAK1

Pathogens infect a host by suppressing defense responses induced upon recognition of microbe‐associated molecular patterns (MAMPs). Despite this suppression, MAMP receptors mediate basal resistance to limit host susceptibility, via a process that is poorly understood. The Arabidopsis leucine‐rich repeat (LRR) receptor kinase BAK1 associates and functions with different cell surface LRR receptors for a wide range of ligands, including MAMPs. We report that BAK1 depletion is linked to defense activation through the endogenous PROPEP peptides (Pep epitopes) and their LRR receptor kinases PEPR1/PEPR2, despite critical defects in MAMP signaling. In bak1‐knockout plants, PEPR elicitation results in extensive cell death and the prioritization of salicylate‐based defenses over jasmonate‐based defenses, in addition to elevated proligand and receptor accumulation. BAK1 disruption stimulates the release of PROPEP3, produced in response to Pep application and during pathogen challenge, and renders PEPRs necessary for basal resistance. These findings are biologically relevant, since specific BAK1 depletion coincides with PEPR‐dependent resistance to the fungal pathogen Colletotrichum higginsianum. Thus, the PEPR pathway ensures basal resistance when MAMP‐triggered defenses are compromised by BAK1 depletion.     

Immune responses induced by oligogalacturonides are differentially affected by AvrPto and loss of BAK1/BKK1 and PEPR1/PEPR2

Plants possess an innate immune system capable of restricting invasion by most potential pathogens. At the cell surface, the recognition of microbe‐associated molecular patterns (MAMPs) and/or damage‐associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs) represents the first event for the prompt mounting of an effective immune response. Pathogens have evolved effectors that block MAMP‐triggered immunity. The Pseudomonas syringae effector AvrPto abolishes immunity triggered by the peptide MAMPs flg22 and elf18, derived from the bacterial flagellin and elongation factor Tu, respectively, by inhibiting the kinase function of the corresponding receptors FLS2 and EFR, as well as their co‐receptors BAK1 and BKK1. Oligogalacturonides (OGs), a well‐known class of DAMPs, are oligomers of α‐1,4‐linked galacturonosyl residues, released on partial degradation of the plant cell wall homogalacturonan. We show here that AvrPto affects only a subset of the OG‐triggered immune responses and that, among these responses, only a subset is affected by the concomitant loss of BAK1 and BKK1. However, the antagonistic effect on auxin‐related responses is not affected by either AvrPto or the loss of BAK1/BKK1. These observations reveal an unprecedented complexity among the MAMP/DAMP response cascades. We also show that the signalling system mediated by Peps, another class of DAMPs, and their receptors PEPRs, contributes to OG‐activated immunity. We hypothesize that OGs are sensed through multiple and partially redundant perception/transduction complexes, some targeted by AvrPto, but not necessarily comprising BAK1 and BKK1.     

Negative control of BAK1 by protein phosphatase 2A during plant innate immunity

Recognition of pathogen‐associated molecular patterns (PAMPs) by surface‐localized pattern‐recognition receptors (PRRs) activates plant innate immunity, mainly through activation of numerous protein kinases. Appropriate induction of immune responses must be tightly regulated, as many of the kinases involved have an intrinsic high activity and are also regulated by other external and endogenous stimuli. Previous evidences suggest that PAMP‐triggered immunity (PTI) is under constant negative regulation by protein phosphatases but the underlying molecular mechanisms remain unknown. Here, we show that protein Ser/Thr phosphatase type 2A (PP2A) controls the activation of PRR complexes by modulating the phosphostatus of the co‐receptor and positive regulator BAK1. A potential PP2A holoenzyme composed of the subunits A1, C4, and B’η/ζ inhibits immune responses triggered by several PAMPs and anti‐bacterial immunity. PP2A constitutively associates with BAK1 in planta. Impairment in this PP2A‐based regulation leads to increased steady‐state BAK1 phosphorylation, which can poise enhanced immune responses. This work identifies PP2A as an important negative regulator of plant innate immunity that controls BAK1 activation in surface‐localized immune receptor complexes.     

The Arabidopsis CERK1‐associated kinase PBL27 connects chitin perception to MAPK activation

Perception of microbe‐associated molecular patterns by host cell surface pattern recognition receptors (PRRs) triggers the intracellular activation of mitogen‐activated protein kinase (MAPK) cascades. However, it is not known how PRRs transmit immune signals to MAPK cascades in plants. Here, we identify a complete phospho‐signaling transduction pathway from PRR‐mediated pathogen recognition to MAPK activation in plants. We found that the receptor‐like cytoplasmic kinase PBL27 connects the chitin receptor complex CERK1‐LYK5 and a MAPK cascade. PBL27 interacts with both CERK1 and the MAPK kinase kinase MAPKKK5 at the plasma membrane. Knockout mutants of MAPKKK5 compromise chitin‐induced MAPK activation and disease resistance to Alternaria brassicicola. PBL27 phosphorylates MAPKKK5 in vitro, which is enhanced by phosphorylation of PBL27 by CERK1. The chitin perception induces disassociation between PBL27 and MAPKKK5 in vivo. Furthermore, genetic evidence suggests that phosphorylation of MAPKKK5 by PBL27 is essential for chitin‐induced MAPK activation in plants. These data indicate that PBL27 is the MAPKKK kinase that provides the missing link between the cell surface chitin receptor and the intracellular MAPK cascade in plants.     

Nucleocytoplasmic trafficking is essential for BAK1‐ and BKK1‐mediated cell‐death control

BRI1‐ASSOCIATED KINASE 1 (BAK1) was initially identified as a co‐receptor of the brassinosteroid (BR) receptor BRI1. Genetic analyses also revealed that BAK1 and its closest homolog BAK1‐LIKE 1 (BKK1) regulate a BR‐independent cell‐death control pathway. The double null mutant bak1 bkk1 displays a salicylic acid‐ and light‐dependent cell‐death phenotype even without pathogen invasion. Molecular mechanisms of the spontaneous cell death mediated by BAK1 and BKK1 remain unknown. Here we report our identification of a suppressor of bak1 bkk1 (sbb1–1). Genetic analyses indicated that cell‐death symptoms in a weak double mutant, bak1–3 bkk1–1, were completely suppressed by the loss‐of‐function mutation in SBB1, which encodes a nucleoporin (NUP) 85‐like protein. Genetic analyses also demonstrated that individually knocking out three other nucleoporin genes from the SBB1‐located sub‐complex was also able to rescue the cell‐death phenotype of bak1–3 bkk1–1. In addition, a DEAD‐box RNA helicase, DRH1, was identified in the same protein complex as SBB1 via a proteomic approach. The drh1 mutation also rescues the cell‐death symptoms of bak1–3 bkk1–1. Further analyses indicated that export of poly(A)⁺ RNA was greatly blocked in the nup and drh1 mutants, resulting in accumulation of significant levels of mRNAs in the nuclei. Over‐expression of a bacterial NahG gene to inactivate salicylic acid also rescues the cell‐death phenotype of bak1–3 bkk1–1. Mutants suppressing cell‐death symptoms always showed greatly reduced salicylic acid contents. These results suggest that nucleocytoplasmic trafficking, especially of molecules directly or indirectly involved in endogenous salicylic acid accumulation, is critical in BAK1‐ and BKK1‐mediated cell‐death control.     

The Pseudomonas syringae effector HopF2 suppresses Arabidopsis immunity by targeting BAK1

Pseudomonas syringae delivers a plethora of effector proteins into host cells to sabotage immune responses and modulate physiology to favor infection. The P. syringae pv. tomato DC3000 effector HopF2 suppresses Arabidopsis innate immunity triggered by multiple microbe‐associated molecular patterns (MAMP) at the plasma membrane. We show here that HopF2 possesses distinct mechanisms for suppression of two branches of MAMP‐activated MAP kinase (MAPK) cascades. In addition to blocking MKK5 (MAPK kinase 5) activation in the MEKK1 (MAPK kinase kinase 1)/MEKKs–MKK4/5–MPK3/6 cascade, HopF2 targets additional component(s) upstream of MEKK1 in the MEKK1–MKK1/2–MPK4 cascade and the plasma membrane‐localized receptor‐like cytoplasmic kinase BIK1 and its homologs. We further show that HopF2 directly targets BAK1, a plasma membrane‐localized receptor‐like kinase that is involved in multiple MAMP signaling. The interaction between BAK1 and HopF2 and between two other P. syringae effectors, AvrPto and AvrPtoB, was confirmed in vivo and in vitro. Consistent with BAK1 as a physiological target of AvrPto, AvrPtoB and HopF2, the strong growth defects or lethality associated with ectopic expression of these effectors in wild‐type Arabidopsis transgenic plants were largely alleviated in bak1 mutant plants. Thus, our results provide genetic evidence to show that BAK1 is a physiological target of AvrPto, AvrPtoB and HopF2. Identification of BAK1 as an additional target of HopF2 virulence not only explains HopF2 suppression of multiple MAMP signaling at the plasma membrane, but also supports the notion that pathogen virulence effectors act through multiple targets in host cells.     

BAK1 is required for the attenuation of ethylene‐inducing xylanase (Eix)‐induced defense responses by the decoy receptor LeEix1

Elicitor recognition plays a key role in the reaction of plants to pathogens and the induction of plant defense responses. Furthermore, plant–microbe interactions involve numerous regulatory systems essential for plant defense against pathogens. Ethylene‐inducing xylanase (Eix) is a potent elicitor of plant defense responses in specific cultivars of tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum). The Eix receptors (LeEix1 and LeEix2) belong to a superclade of leucine‐rich repeat receptor‐like proteins (RLP) with a signal for receptor‐mediated endocytosis, which was shown to be essential for proper induction of defense responses. Both receptors are able to bind Eix, while only LeEix2 mediates defense responses. Here we demonstrate that LeEix1 heterodimerizes with LeEix2 upon application of the Eix elicitor. We show that LeEix1 attenuates Eix‐induced internalization and signaling of the LeEix2 receptor. Furthermore, we demonstrate, using yeast two‐hybrid and in planta bimolecular fluorescence complementation assays, that the brassinosteroid co‐receptor, BAK1, binds LeEix1 but not LeEix2. In BAK1‐silenced plants, LeEix1 was no longer able to attenuate plant responses to Eix, indicating that BAK1 is required for this attenuation. We suggest that LeEix1 functions as a decoy receptor for LeEix2, a function which requires BAK1.     

Ubiquitination of pattern recognition receptors in plant innate immunity

Lacking an adaptive immune system, plants largely rely on plasma membrane‐resident pattern recognition receptors (PRRs) to sense pathogen invasion. The activation of PRRs leads to the profound immune responses that coordinately contribute to the restriction of pathogen multiplication. Protein post‐translational modifications dynamically shape the intensity and duration of the signalling pathways. In this review, we discuss the specific regulation of PRR activation and signalling by protein ubiquitination, endocytosis and degradation, with a particular focus on the bacterial flagellin receptor FLS2 (flagellin sensing 2) in Arabidopsis.     

Receptor‐mediated recognition of mycobacterial pathogens

Mycobacteria are a genus of bacteria that range from the non‐pathogenic Mycobacterium smegmatis to Mycobacterium tuberculosis, the causative agent of tuberculosis in humans. Mycobacteria primarily infect host tissues through inhalation or ingestion. They are phagocytosed by host macrophages and dendritic cells. Here, conserved pathogen‐associated molecular patterns (PAMPs) on the surface of mycobacteria are recognized by phagocytic pattern recognition receptors (PRRs). Several families of PRRs have been shown to non‐opsonically recognize mycobacterial PAMPs, including membrane‐bound C‐type lectin receptors, membrane‐bound and cytosolic Toll‐like receptors and cytosolic NOD‐like receptors. Recently, a possible role for intracellular cytosolic PRRs in the recognition of mycobacterial pathogens has been proposed. Here, we discuss currentideas on receptor‐mediated recognition of mycobacterial pathogens by macrophages and dendritic cells.     

VDAC2 is required for truncated BID‐induced mitochondrial apoptosis by recruiting BAK to the mitochondria

Truncated BID (tBID), a proapoptotic BCL2 family protein, induces BAK/BAX‐dependent release of cytochrome c and other mitochondrial intermembrane proteins to the cytosol to induce apoptosis. The voltage‐dependent anion channels (VDACs) are the primary gates for solutes across the outer mitochondrial membrane (OMM); however, their role in apoptotic OMM permeabilization remains controversial. Here, we report that VDAC2^?/? (V2^?/?) mouse embryonic fibroblasts (MEFs) are virtually insensitive to tBID‐induced OMM permeabilization and apoptosis, whereas VDAC1^?/?, VDAC3^?/? and VDAC1^?/?/VDAC3^?/? MEFs respond normally to tBID. V2^?/? MEFs regain tBID sensitivity after VDAC2 expression. Furthermore, V2^?/? MEFs are deficient in mitochondrial BAK despite normal tBID–mitochondrial binding and BAX/BAK expression. tBID sensitivity of BAK^?/? MEFs is also reduced, although not to the same extent as V2^?/? MEFs, which might result from their strong overexpression of BAX. Indeed, addition of recombinant BAX also sensitized V2^?/? MEFs to tBID. Thus, VDAC2 acts as a crucial component in mitochondrial apoptosis by allowing the mitochondrial recruitment of BAK, thereby controlling tBID‐induced OMM permeabilization and cell death.     

Novel Rosaceae plant elicitor peptides as sustainable tools to control Xanthomonas arboricola pv. pruni in Prunus spp.

Fruit crops are regarded as important health promoters and constitute a major part of global agricultural production, and Rosaceae species are of high economic impact. Their culture is threatened by bacterial diseases, whose control is based on preventative treatments using compounds of limited efficacy and negative environmental impact. One of the most economically relevant examples is the pathogen Xanthomonas arboricola pv. pruni (Xap) affecting Prunus spp. The plant immune response against pathogens can be triggered and amplified by plant elicitor peptides (Peps), perceived by specific receptors (PEPRs). Although they have been described in various angiosperms, scarce information is available on Rosaceae species. Here, we identified the Pep precursor (PROPEP), Pep and PEPR orthologues of 10 Rosaceae species and confirmed the presence of the Pep/PEPR system in this family. We showed the perception and elicitor activity of Rosaceae Peps using the Prunus–Xap pathosystem as proof‐of‐concept. Treatment with nanomolar doses of Peps induced the corresponding PROPEP and a set of defence‐related genes in Prunus leaves, and enhanced resistance against Xap. Peps from the same species had the highest efficiencies. Rosaceae Peps could potentially be used to develop natural, targeted and environmentally friendly strategies to enhance the resistance of Prunus species against biotic attackers.     

The population genetic test Tajima's D identifies genes encoding pathogen‐associated molecular patterns and other virulence‐related genes in Ralstonia solanacearum

The detection of pathogen‐associated molecular patterns (PAMPs) by plant pattern recognition receptors (PRRs) is an essential part of plant immunity. Until recently, elf18, an epitope of elongation factor‐Tu (EF‐Tu), was the sole confirmed PAMP of Ralstonia solanacearum, the causal agent of bacterial wilt disease, limiting our understanding of R. solanacearum–plant interactions. Therefore, we set out to identify additional R. solanacearum PAMPs based on the hypothesis that genes encoding PAMPs are under selection to avoid recognition by plant PRRs. We calculated Tajima's D, a population genetic test statistic which identifies genes that do not evolve neutrally, for 3003 genes conserved in 37 R. solanacearum genomes. The screen flagged 49 non‐neutrally evolving genes, including not only EF‐Tu but also the gene for Cold Shock Protein C, which encodes the PAMP csp22. Importantly, an R. solanacearum allele of this PAMP was recently identified in a parallel independent study. Genes coding for efflux pumps, some with known roles in virulence, were also flagged by Tajima's D. We conclude that Tajima's D is a straightforward test to identify genes encoding PAMPs and other virulence‐related genes in plant pathogen genomes.     

The PTI-suppressing Avr2 effector from Fusarium oxysporum suppresses mono-ubiquitination and plasma membrane dissociation of BIK1

Plant pathogens use effector proteins to target host processes involved in pathogen perception, immune signalling, or defence outputs. Unlike foliar pathogens, it is poorly understood how root-invading pathogens suppress immunity. The Avr2 effector from the tomato root- and xylem-colonizing pathogen Fusarium oxysporum suppresses immune signalling induced by various pathogen-associated molecular patterns (PAMPs). It is unknown how Avr2 targets the immune system. Transgenic AVR2 Arabidopsis thaliana phenocopies mutants in which the pattern recognition receptor (PRR) co-receptor BRI1-ASSOCIATED RECEPTOR KINASE (BAK1) or its downstream signalling kinase BOTRYTIS-INDUCED KINASE 1 (BIK1) are knocked out. We therefore tested whether these kinases are Avr2 targets. Flg22-induced complex formation of the PRR FLAGELLIN SENSITIVE 2 and BAK1 occurred in the presence and absence of Avr2, indicating that Avr2 does not affect BAK1 function or PRR complex formation. Bimolecular fluorescence complementation assays showed that Avr2 and BIK1 co-localize in planta. Although Avr2 did not affect flg22-induced BIK1 phosphorylation, mono-ubiquitination was compromised. Furthermore, Avr2 affected BIK1 abundance and shifted its localization from nucleocytoplasmic to the cell periphery/plasma membrane. Together, these data imply that Avr2 may retain BIK1 at the plasma membrane, thereby suppressing its ability to activate immune signalling. Because mono-ubiquitination of BIK1 is required for its internalization, interference with this process by Avr2 could provide a mechanistic explanation for the compromised BIK1 mobility upon flg22 treatment. The identification of BIK1 as an effector target of a root-invading vascular pathogen identifies this kinase as a conserved signalling component for both root and shoot immunity.     

BAK7 displays unequal genetic redundancy with BAK1 in brassinosteroid signaling and early senescence in arabidopsis

BRI1-Associated kinase1 (BAK1), a five leucine-rich-repeat containing receptor-like serine/threonine kinase, has been shown to have dual functions: mediating brassinosteroid (BR) signaling and acting in the BR-independent plant defense response. Sequence analysis has revealed that BAK1 has two homologs, BAK7 and BAK8. Because BAK8 deviates from the canonical RD kinase motif, we focused on the functional analysis of BAK7. The expression pattern and tissues in which BAK7 appeared partially overlapped with those observed for BAK1. Expression levels of BAK7 increased in the bak1 mutant. Overexpression of BAK7 rescued the bri1 mutant phenotype, indicating that BAK7 can compensate for BAK1 in BR-mediated processes, especially in the absence of BAK1. However, root and hypocotyl elongation patterns of transgenic plants overexpressing BAK1 or BAK7 appeared to be different from the patterns observed in a BRI1 overexpressor. Furthermore, the sensitivity of transgenic plants overexpressing BAK7 to brassinazole, a biosynthetic inhibitor of brassinolide (BL), did not change compared to that of wild-type plants. In addition, we generated transgenic plants expressing BAK7 RNA interference constructs and found severe growth retardation and early senescence in these lines. Taken together, these results suggest that BAK7 is a component of the BR signaling pathway, with varying degrees of genetic redundancy with BAK1, and that it affects plant growth via BL-independent pathways in vivo.     

BAK1‐mediated phosphorylation of canonical G protein alpha during flagellin signaling in Arabidopsis

Heterotrimeric G proteins consisting of Gα, Gβ and Gγ are conserved signaling hubs in eukaryotes. Without analogs to canonical animal G protein‐coupled receptors, plant cells are thought to use RGS1 and a yet unknown mechanism to regulate the activity of Gα. Meanwhile, the exact role of canonical Gα in plant innate immunity remains controversial. Here, we report multiple immune deficiencies in the null allele of Arabidopsis Gα (GPA1) in response to bacterial flg22 elicitor, clarifying a positive regulatory role of GPA1 in flg22 signaling. We also detect overall increased phosphorylation of GPA1 but reduced phosphorylation at Thr¹⁹ upon flg22 elicitation. Interestingly, flg22 could not induce phosphorylation of GPA1^T19A and GPA1^T19D, suggesting that the dynamic Thr¹⁹ phosphorylation is required for GPA1 to respond to flg22. Moreover, flg22‐induced GPA1 phosphorylation is largely abolished in the absence of BAK1 in vivo, and BAK1 could phosphorylate GPA1 but not GPA1^T19A in vitro at the phosphorylation sites identified in vivo, suggesting BAK1 is likely the kinase for GPA1 phosphorylation in response to flg22. Furthermore, the T19A mutation could promote flg22‐induced association, rather than dissociation, between GPA1 and RGS1. Taken together, our findings shed new insights into the function and regulation of GPA1 in Arabidopsis defense signaling.     

An EFR-Cf-9 chimera confers enhanced resistance to bacterial pathogens by SOBIR1- and BAK1-dependent recognition of elf18

The transfer of well-studied native and chimeric pattern recognition receptors (PRRs) to susceptible plants is a proven strategy to improve host resistance. In most cases, the ectodomain determines PRR recognition specificity, while the endodomain determines the intensity of the immune response. Here we report the generation and characterization of the chimeric receptor EFR-Cf-9, which carries the ectodomain of the Arabidopsis thaliana EF-Tu receptor (EFR) and the endodomain of the tomato Cf-9 resistance protein. Both transient and stable expression of EFR-Cf-9 triggered a robust hypersensitive response (HR) upon elf18 treatment in tobacco. Co-immunoprecipitation and virus-induced gene silencing studies showed that EFR-Cf-9 constitutively interacts with SUPPRESSOR OF BIR1-1 (SOBIR1) co-receptor, and requires both SOBIR1 and kinase-active BRI1-ASSOCIATED KINASE1 (BAK1) for its function. Transgenic plants expressing EFR-Cf-9 were more resistant to the (hemi)biotrophic bacterial pathogens Pseudomonas amygdali pv. tabaci (Pta) 11528 and Pseudomonas syringae pv. tomato DC3000, and mounted an HR in response to high doses of Pta 11528 and P. carotovorum. Taken together, these data indicate that the EFR-Cf-9 chimera is a valuable tool for both investigating the molecular mechanisms responsible for the activation of defence responses by PRRs, and for potential biotechnological use to improve crop disease resistance.     

High levels of cyclic‐di‐GMP in plant‐associated Pseudomonas correlate with evasion of plant immunity

The plant innate immune system employs plasma membrane‐localized receptors that specifically perceive pathogen/microbe‐associated molecular patterns (PAMPs/MAMPs). This induces a defence response called pattern‐triggered immunity (PTI) to fend off pathogen attack. Commensal bacteria are also exposed to potential immune recognition and must employ strategies to evade and/or suppress PTI to successfully colonize the plant. During plant infection, the flagellum has an ambiguous role, acting as both a virulence factor and also as a potent immunogen as a result of the recognition of its main building block, flagellin, by the plant pattern recognition receptors (PRRs), including FLAGELLIN SENSING2 (FLS2). Therefore, strict control of flagella synthesis is especially important for plant‐associated bacteria. Here, we show that cyclic‐di‐GMP [bis‐(3′‐5′)‐cyclic di‐guanosine monophosphate], a central regulator of bacterial lifestyle, is involved in the evasion of PTI. Elevated cyclic‐di‐GMP levels in the pathogen Pseudomonas syringae pv. tomato (Pto) DC3000, the opportunist P. aeruginosa PAO1 and the commensal P. protegens Pf‐5 inhibit flagellin synthesis and help the bacteria to evade FLS2‐mediated signalling in Nicotiana benthamiana and Arabidopsis thaliana. Despite this, high cellular cyclic‐di‐GMP concentrations were shown to drastically reduce the virulence of Pto DC3000 during plant infection. We propose that this is a result of reduced flagellar motility and/or additional pleiotropic effects of cyclic‐di‐GMP signalling on bacterial behaviour.     

The multifaceted function of BAK1/SERK3: Plant immunity to pathogens and responses to insect herbivores

Almost a decade ago BRI1-associated kinase 1 (BAK1) was identified as a co-receptor of brassinosteroid (BR) insensitive 1 (BRI1), the receptor for BRs, which plays an essential role in transducing BR signaling to regulate plant development. BAK1 is also critical in resistance to various pathogens. BAK1 rapidly binds to certain receptors for pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) after the perception of pathogen elicitors and is required for the full elicitation of pathogen-induced defense responses, such as the activation of the mitogen-activated protein kinase 6 (MPK6) and production of reactive oxygen species. Thus, BAK1 functions in both BR signaling and PAMP-triggered immunity (PTI). Recently BAK1 was also found to play an important role in mediating defense responses against an insect herbivore (Manduca sexta) of Nicotiana attenuata. In this interaction, BAK1 positively modulates wound- or herbivore feeding-induced accumulation of jasmonic acid (JA) and JA-isoleucine (JA-Ile). This mini-review summarizes recent advances in our understanding of the functions of BAK1 in resistance to pathogens and herbivores.Key words: BAK1, defense, herbivore, immunity, insect, jasmonate, pathogen, wounding     

The tomato I gene for Fusarium wilt resistance encodes an atypical leucine‐rich repeat receptor‐like protein whose function is nevertheless dependent on SOBIR1 and SERK3/BAK1

We have identified the tomato I gene for resistance to the Fusarium wilt fungus Fusarium oxysporum f. sp. lycopersici (Fol) and show that it encodes a membrane‐anchored leucine‐rich repeat receptor‐like protein (LRR‐RLP). Unlike most other LRR‐RLP genes involved in plant defence, the I gene is not a member of a gene cluster and contains introns in its coding sequence. The I gene encodes a loopout domain larger than those in most other LRR‐RLPs, with a distinct composition rich in serine and threonine residues. The I protein also lacks a basic cytosolic domain. Instead, this domain is rich in aromatic residues that could form a second transmembrane domain. The I protein recognises the Fol Avr1 effector protein, but, unlike many other LRR‐RLPs, recognition specificity is determined in the C‐terminal half of the protein by polymorphic amino acid residues in the LRRs just preceding the loopout domain and in the loopout domain itself. Despite these differences, we show that I/Avr1‐dependent necrosis in Nicotiana benthamiana depends on the LRR receptor‐like kinases (RLKs) SERK3/BAK1 and SOBIR1. Sequence comparisons revealed that the I protein and other LRR‐RLPs involved in plant defence all carry residues in their last LRR and C‐terminal LRR capping domain that are conserved with SERK3/BAK1‐interacting residues in the same relative positions in the LRR‐RLKs BRI1 and PSKR1. Tyrosine mutations of two of these conserved residues, Q922 and T925, abolished I/Avr1‐dependent necrosis in N. benthamiana, consistent with similar mutations in BRI1 and PSKR1 preventing their interaction with SERK3/BAK1.