Induced Endocytosis of the Receptor Kinase FLS2

Protein degrons and degradation: Exploring substrate recognition and pathway selection in plants

Proteome composition is dynamic and influenced by many internal and external cues, including developmental signals, light availability, or environmental stresses. Protein degradation, in synergy with protein biosynthesis, allows cells to respond to various stimuli and adapt by reshaping the proteome. Protein degradation mediates the final and irreversible disassembly of proteins, which is important for protein quality control and to eliminate misfolded or damaged proteins, as well as entire organelles. Consequently, it contributes to cell resilience by buffering against protein or organellar damage caused by stresses. Moreover, protein degradation plays important roles in cell signaling, as well as transcriptional and translational events. The intricate task of recognizing specific proteins for degradation is achieved by specialized systems that are tailored to the substrate's physicochemical properties and subcellular localization. These systems recognize diverse substrate cues collectively referred to as "degrons," which can assume a range of configurations. They are molecular surfaces recognized by E3 ligases of the ubiquitin-proteasome system but can also be considered as general features recognized by other degradation systems, including autophagy or even organellar proteases. Here we provide an overview of the newest developments in the field, delving into the intricate processes of protein recognition and elucidating the pathways through which they are recruited for degradation.

A G-type lectin receptor kinase negatively regulates Arabidopsis immunity against root-knot nematodes

Root-knot nematodes (Meloidogyne spp., RKN) are responsible for extensive crop losses worldwide. During infection, they penetrate plant roots, migrate between plant cells, and establish feeding sites, known as giant cells, near the root vasculature. Previously, we found that nematode perception and early responses in plants were similar to those of microbial pathogens and required the BRI1-ASSOCIATED KINASE1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE3 (BAK1/SERK3) coreceptor in Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum). Here, we implemented a reverse genetic screen for resistance or sensitivity to RKN using Arabidopsis T-DNA alleles of genes encoding transmembrane receptor-like kinases to identify additional receptors involved in this process. This screen identified a pair of allelic mutations with enhanced resistance to RKN in a gene we named ENHANCED RESISTANCE TO NEMATODES1 (ERN1). ERN1 encodes a G-type lectin receptor kinase (G-LecRK) with a single-pass transmembrane domain. Further characterization showed that ern1 mutants displayed stronger activation of MAP kinases, elevated levels of the defense marker MYB51, and enhanced H2O2 accumulation in roots upon RKN elicitor treatments. Elevated MYB51 expression and ROS bursts were also observed in leaves of ern1 mutants upon flg22 treatment. Complementation of ern1.1 with 35S- or native promoter-driven ERN1 rescued the RKN infection and enhanced defense phenotypes. Our results indicate that ERN1 is an important negative regulator of immunity.

Dynamic spatial reorganization of BSK1 complexes in the plasma membrane underpins signal-specific activation for growth and immunity

Growth and immunity are opposing processes that compete for cellular resources, and proper resource allocation is crucial for plant survival. BSK1 plays a key role in the regulation of both growth and immunity by associating with BRI1 and FLS2, respectively. However, it remains unclear how two antagonistic signals co-opt BSK1 to induce signal-specific activation. Here we show that the dynamic spatial reorganization of BSK1 within the plasma membrane underlies the mechanism of signal-specific activation for growth or immunity. Resting BSK1 localizes to membrane rafts as complexes. Unlike BSK1-associated FLS2 and BRI1, flg22 or exogenous brassinosteroid (BR) treatment did not decrease BSK1 levels at the plasma membrane (PM) but rather induced BSK1 multimerization and dissociation from FLS2/BSK1 or BRI1/BSK1, respectively. Moreover, flg22-activated BSK1 translocated from membrane rafts to non-membrane-raft regions, whereas BR-activated BSK1 remained in membrane rafts. When applied together with flg22, BR suppressed various flg22-induced BSK1 activities such as BSK1 dissociation from FLS2/BSK1, BSK1 interaction with MAPKKK5, and BSK translocation together with MAPKKK5. Taken together, this study provides a unique insight into how the precise control of BSK1 spatiotemporal organization regulates the signaling specificity to balance plant growth and immunity.

Emerging mechanisms to fine-tune receptor kinase signaling specificity

Organisms need to constantly inform their cellular machinery about the biochemical and physical status of their surroundings to adapt and thrive. While some external signals are also sensed intracellularly, a considerable share of external information is registered already at the plasma membrane (PM). Receptor kinases (RKs) are crucial for plant cells to integrate such cues from the environment, from microbes, or from other cells to coordinate their physiological response and their development. Early studies on RK signaling depicted the path from external signal to internal response in a linear fashion, but recent findings show that these cellular information highways are highly interconnected and pass signals through molecular intersections. In this review, we first discuss how individual RKs simultaneously contribute to the transduction and deconvolution of a multitude of signals by controlled assembly into diverse RK complexes, exemplified by FERONIA signaling versatility. We then elaborate on how cells can exert highly localized control over the assembly, interaction and composition of such complexes in order to attain essential cellular output specificity.

Plant species-specific recognition of long and short β-1,3-linked glucans is mediated by different receptor systems

Plants survey their environment for the presence of potentially harmful or beneficial microbes. During colonization, cell surface receptors perceive microbe-derived or modified-self ligands and initiate appropriate responses. The recognition of fungal chitin oligomers and the subsequent activation of plant immunity are well described. In contrast, the mechanisms underlying β-glucan recognition and signaling activation remain largely unexplored. Here, we systematically tested immune responses towards different β-glucan structures and show that responses vary between plant species. While leaves of the monocots Hordeum vulgare and Brachypodium distachyon can recognize longer (laminarin) and shorter (laminarihexaose) β-1,3-glucans with responses of varying intensity, duration and timing, leaves of the dicot Nicotiana benthamiana activate immunity in response to long β-1,3-glucans, whereas Arabidopsis thaliana and Capsella rubella perceive short β-1,3-glucans. Hydrolysis of the β-1,6 side-branches of laminarin demonstrated that not the glycosidic decoration but rather the degree of polymerization plays a pivotal role in the recognition of long-chain β-glucans. Moreover, in contrast to the recognition of short β-1,3-glucans in A. thaliana, perception of long β-1,3-glucans in N. benthamiana and rice is independent of CERK1, indicating that β-glucan recognition may be mediated by multiple β-glucan receptor systems.

Multiple intramolecular trafficking signals in RESISTANCE TO POWDERY MILDEW 8.2 are engaged in activation of cell death and defense

The extrahaustorial membrane (EHM) is a host-derived interfacial membrane encasing the haustorium of powdery mildew fungi. Arabidopsis thaliana RESISTANCE TO POWDERY MILDEW 8.2 (RPW8.2) is specifically targeted to the EHM via two EHM-targeting signals. Here, we demonstrate that proper coordination between the trafficking forces engaged via the EHM-targeting signals and the nuclear localization signals (NLSs), as well as the nuclear export signals (NESs), in RPW8.2 is critical for the activation of cell death and defense. We show that in the absence of pathogens, RPW8.2 is partitioned between the cytoplasm and the nucleus, and turned over via both the 26S proteasome- and the vacuole-dependent pathways. Enhanced cytoplasmic localization of RPW8.2 by tagging it with a NES led to lethal cell death. By contrast, enhanced nuclear localization of RPW8.2 by adding an NLS to it resulted in resistance to powdery mildew. Whereas expression of the NES-containing C-terminal domain of RPW8.2 in the cytoplasm is sufficient to trigger cell death, no such cell death-inducing activity is found with RPW8.2 variants that contain the two EHM-targeting signals along with the NES-containing C-terminal domain. In addition, we present evidence for the involvement of a leaf senescence pathway in RPW8.2-mediated cell death and defense. Taken together, our data suggest that RPW8.2 is subject to adjustment by distinct and perhaps coordinated mechanisms for its localization and function via interaction with the multiple intramolecular trafficking signals, which should provide further insights into RPW8.2-activated, EHM-focused resistance against powdery mildew.

CaLRR-RLK1, a novel RD receptor-like kinase from Capsicum annuum and transcriptionally activated by CaHDZ27, act as positive regulator in Ralstonia solanacearum resistance

BACKGROUND

Bacterial wilt caused by Ralstonia solanacearum is one of the most important diseases in pepper worldwide, however, the molecular mechanism underlying pepper resistance to bacterial wilt remains poorly understood.

RESULTS

Herein, a novel RD leucine-rich repeat receptor-like kinase, CaLRR-RLK1, was functionally characterized in immunity against R. solanacearum. CaLRR-RLK1 was targeted exclusively to plasma membrane and was up-regulated by R. solanacearum inoculation (RSI) as well as by the exogenous application of salicylic acid (SA), methyl jasmonate (MeJA) or ethephon (ETH). The silencing of CaLRR-RLK1 led to enhanced susceptibility of pepper plants to RSI, accompanied by down-regulation of immunity-related genes including CaACO1, CaHIR1, CaPR4 and CaPO2. In contrast, transient overexpression of CaLRR-RLK1 triggered hypersensitive response (HR)-like cell death and H2O2 accumulation in pepper leaves, manifested by darker trypan blue and DAB staining respectively. In addition, the ectopic overexpression of CaLRR-RLK1 in tobacco plants enhanced resistance R. solanacearum, accompanied with the immunity associated marker genes including NtPR2, NtPR2, NtHSR203 and NtHSR515. Furthermore, it was found that CaHDZ27, a positive regulator in pepper response to RSI in our previous study, transcriptionally activated CaLRR-RLK1 by direct targeting its promoter probably in a CAATTATTG dependent manner.

CONCLUSION

The study revealed that CaLRR-RLK1 confers pepper resistance to R. solanacearum as the direct targeting of CaHDZ27.

A Plant Immune Receptor Degraded by Selective Autophagy

Plants recycle non-activated immune receptors to maintain a functional immune system. The Arabidopsis immune receptor kinase FLAGELLIN-SENSING 2 (FLS2) recognizes bacterial flagellin. However, the molecular mechanisms by which non-activated FLS2 and other non-activated plant PRRs are recycled remain not well understood. Here, we provide evidence showing that Arabidopsis orosomucoid (ORM) proteins, which have been known to be negative regulators of sphingolipid biosynthesis, act as selective autophagy receptors to mediate the degradation of FLS2. Arabidopsis plants overexpressing ORM1 or ORM2 have undetectable or greatly diminished FLS2 accumulation, nearly lack FLS2 signaling, and are more susceptible to the bacterial pathogen Pseudomonas syringae. On the other hand, ORM1/2 RNAi plants and orm1 or orm2 mutants generated by the CRISPR/Cas9-mediated gene editing have increased FLS2 accumulation and enhanced FLS2 signaling, and are more resistant to P. syringae. ORM proteins interact with FLS2 and the autophagy-related protein ATG8. Interestingly, overexpression of ORM1 or ORM2 in autophagy-defective mutants showed FLS2 abundance that is comparable to that in wild-type plants. Moreover, FLS2 levels were not decreased in Arabidopsis plants overexpressing ORM1/2 derivatives that do not interact with ATG8. Taken together, these results suggest that selective autophagy functions in maintaining the homeostasis of a plant immune receptor and that beyond sphingolipid metabolic regulation ORM proteins can also act as selective autophagy receptors.

Biogenesis and Function of Multivesicular Bodies in Plant Immunity

Multivesicular bodies (MVBs) are specialized endosomes that contain intraluminal vesicles generated from invagination and budding of the limiting membrane. In the endocytic pathway, MVBs are late endosomes whose content can be degraded through fusion with lysosomes/vacuoles or released into the extracellular space after fusion with the plasma membrane (PM). The proteins retained on the limiting membrane of MVBs are translocated to the membrane of lysosomes/vacuoles or delivered back to the PM. It has been long suspected that MVBs might fuse with the PM to form paramural bodies in plant cells, possibly leading to release of building blocks for deposition of papillae and antimicrobial molecules against invading pathogens. Over the past decade or so, major progress has been made in establishing the critical roles of MVBs and associated membrane trafficking in pathogen recognition, defense signaling, and deployment of defense-related molecules during plant immune responses. Regulatory proteins and signaling pathways associated with induced biogenesis and trafficking of MVBs during plant immune responses have also been identified and characterized. Recent successful isolation of plant extracellular vesicles and proteomic profiling of their content have provided additional support for the roles of MVBs in plant-pathogen interactions. In this review, we summarize the important progress and discuss how MVBs, particularly through routing of cellular components to different destinations, contribute to the complex network of plant immune system.

Capping Protein Modulates Actin Remodeling in Response to Reactive Oxygen Species during Plant Innate Immunity

Plants perceive microbe-associated molecular patterns and damage-associated molecular patterns to activate innate immune signaling events, such as bursts of reactive oxygen species (ROS). The actin cytoskeleton remodels during the first 5 min of innate immune signaling in Arabidopsis (Arabidopsis thaliana) epidermal cells; however, the immune signals that impinge on actin cytoskeleton and its response regulators remain largely unknown. Here, we demonstrate that rapid actin remodeling upon elicitation with diverse microbe-associated molecular patterns and damage-associated molecular patterns represent a conserved plant immune response. Actin remodeling requires ROS generated by the defense-associated NADPH oxidase, RBOHD. Moreover, perception of flg22 by its cognate receptor complex triggers actin remodeling through the activation of RBOHD-dependent ROS production. Our genetic studies reveal that the ubiquitous heterodimeric capping protein transduces ROS signaling to the actin cytoskeleton during innate immunity. Additionally, we uncover a negative feedback loop between actin remodeling and flg22-induced ROS production.

New Insights on Leucine-Rich Repeats Receptor-Like Kinase Orthologous Relationships in Angiosperms

Leucine-Rich Repeats Receptor-Like Kinase (LRR-RLK) genes represent a large and complex gene family in plants, mainly involved in development and stress responses. These receptors are composed of an LRR-containing extracellular domain (ECD), a transmembrane domain (TM) and an intracellular kinase domain (KD). To provide new perspectives on functional analyses of these genes in model and non-model plant species, we performed a phylogenetic analysis on 8,360 LRR-RLK receptors in 31 angiosperm genomes (8 monocots and 23 dicots). We identified 101 orthologous groups (OGs) of genes being conserved among almost all monocot and dicot species analyzed. We observed that more than 10% of these OGs are absent in the Brassicaceae species studied. We show that the ECD structural features are not always conserved among orthologs, suggesting that functions may have diverged in some OG sets. Moreover, we looked at targets of positive selection footprints in 12 pairs of OGs and noticed that depending on the subgroups, positive selection occurred more frequently either in the ECDs or in the KDs.

Biochemical characterization of the tomato phosphatidylinositol-specific phospholipase C (PI-PLC) family and its role in plant immunity

Plants possess effective mechanisms to quickly respond to biotic and abiotic stresses. The rapid activation of phosphatidylinositol-specific phospholipase C (PLC) enzymes occurs early after the stimulation of plant immune-receptors. Genomes of different plant species encode multiple PLC homologs belonging to one class, PLCζ. Here we determined whether all tomato homologs encode active enzymes and whether they can generate signals that are distinct from one another. We searched the recently completed tomato (Solanum lycopersicum) genome sequence and identified a total of seven PLCs. Recombinant proteins were produced for all tomato PLCs, except for SlPLC7. The purified proteins showed typical PLC activity, as different PLC substrates were hydrolysed to produce diacylglycerol. We studied SlPLC2, SlPLC4 and SlPLC5 enzymes in more detail and observed distinct requirements for Ca(2+) ions and pH, for both their optimum activity and substrate preference. This indicates that each enzyme could be differentially and specifically regulated in vivo, leading to the generation of PLC homolog-specific signals in response to different stimuli. PLC overexpression and specific inhibition of PLC activity revealed that PLC is required for both specific effector- and more general "pattern"-triggered immunity. For the latter, we found that both the flagellin-triggered response and the internalization of the corresponding receptor, Flagellin Sensing 2 (FLS2) of Arabidopsis thaliana, are suppressed by inhibition of PLC activity. Altogether, our data support an important role for PLC enzymes in plant defence signalling downstream of immune receptors. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

A survey of FLS2 genes from multiple citrus species identifies candidates for enhancing disease resistance to Xanthomonas citri ssp. citri

Pathogen-associated molecular patterns (PAMPs)-triggered immunity (PTI) is an important component of plant innate immunity. In a previous study, we showed that the PAMP flg22 from Xanthomonas citri ssp. citri (Xflg22), the causal agent of citrus canker, induced PTI in citrus, which correlated with the observed levels of canker resistance. Here, we identified and sequenced two bacterial flagellin/flg22 receptors (FLS2-1 and FLS2-2) from 'Duncan' grapefruit (Citrus paradisi, CpFLS2-1 and CpFLS2-2) and 'Sun Chu Sha' mandarin (C. reticulata, CrFLS2-1 and CrFLS2-2). We were able to isolate only one FLS2 from 'Nagami' kumquat (Fortunella margarita, FmFLS2-1) and gene flanking sequences suggest a rearrangement event that resulted in the deletion of FLS2-2 from the genome. Phylogenetic analysis, gene structure and presence of critical amino acid domains all indicate we identified the true FLS2 genes in citrus. FLS2-2 was more transcriptionally responsive to Xflg22 than FLS2-1, with induced expression levels higher in canker-resistant citrus than in susceptible ones. Interestingly, 'Nagami' kumquat showed the highest FLS2-1 steady-state expression levels, although it was not induced by Xflg22. We selected FmFLS2-1, CrFLS2-2 and CpFLS2-2 to further evaluate their capacity to enhance bacterial resistance using Agrobacterium-mediated transient expression assays. Both FmFLS2-1 and CrFLS2-2, the two proteins from canker-resistant species, conferred stronger Xflg22 responses and reduced canker symptoms in leaves of the susceptible grapefruit genotype. These two citrus genes will be useful resources to enhance PTI and achieve resistance against canker and possibly other bacterial pathogens in susceptible citrus types.

Phytophthora infestans RXLR-WY Effector AVR3a Associates with Dynamin-Related Protein 2 Required for Endocytosis of the Plant Pattern Recognition Receptor FLS2

Pathogens utilize effectors to suppress basal plant defense known as PTI (Pathogen-associated molecular pattern-triggered immunity). However, our knowledge of PTI suppression by filamentous plant pathogens, i.e. fungi and oomycetes, remains fragmentary. Previous work revealed that the co-receptor BAK1/SERK3 contributes to basal immunity against the potato pathogen Phytophthora infestans. Moreover BAK1/SERK3 is required for the cell death induced by P. infestans elicitin INF1, a protein with characteristics of PAMPs. The P. infestans host-translocated RXLR-WY effector AVR3a is known to supress INF1-mediated cell death by binding the plant E3 ligase CMPG1. In contrast, AVR3aKI-Y147del, a deletion mutant of the C-terminal tyrosine of AVR3a, fails to bind CMPG1 and does not suppress INF1-mediated cell death. Here, we studied the extent to which AVR3a and its variants perturb additional BAK1/SERK3-dependent PTI responses in N. benthamiana using the elicitor/receptor pair flg22/FLS2 as a model. We found that all tested variants of AVR3a suppress defense responses triggered by flg22 and reduce internalization of activated FLS2. Moreover, we discovered that AVR3a associates with the Dynamin-Related Protein 2 (DRP2), a plant GTPase implicated in receptor-mediated endocytosis. Interestingly, silencing of DRP2 impaired ligand-induced FLS2 internalization but did not affect internalization of the growth receptor BRI1. Our results suggest that AVR3a associates with a key cellular trafficking and membrane-remodeling complex involved in immune receptor-mediated endocytosis. We conclude that AVR3a is a multifunctional effector that can suppress BAK1/SERK3-mediated immunity through at least two different pathways.

Recruitment of PLANT U-BOX13 and the PI4Kβ1/β2 phosphatidylinositol-4 kinases by the small GTPase RabA4B plays important roles during salicylic acid-mediated plant defense signaling in Arabidopsis

Protection against microbial pathogens involves the activation of cellular immune responses in eukaryotes, and this cellular immunity likely involves changes in subcellular membrane trafficking. In eukaryotes, members of the Rab GTPase family of small monomeric regulatory GTPases play prominent roles in the regulation of membrane trafficking. We previously showed that RabA4B is recruited to vesicles that emerge from trans-Golgi network (TGN) compartments and regulates polarized membrane trafficking in plant cells. As part of this regulation, RabA4B recruits the closely related phosphatidylinositol 4-kinase (PI4K) PI4Kβ1 and PI4Kβ2 lipid kinases. Here, we identify a second Arabidopsis thaliana RabA4B-interacting protein, PLANT U-BOX13 (PUB13), which has recently been identified to play important roles in salicylic acid (SA)-mediated defense signaling. We show that PUB13 interacts with RabA4B through N-terminal domains and with phosphatidylinositol 4-phosphate (PI-4P) through a C-terminal armadillo domain. Furthermore, we demonstrate that a functional fluorescent PUB13 fusion protein (YFP-PUB13) localizes to TGN and Golgi compartments and that PUB13, PI4Kβ1, and PI4Kβ2 are negative regulators of SA-mediated induction of pathogenesis-related gene expression. Taken together, these results highlight a role for RabA4B and PI-4P in SA-dependent defense responses.

A moving view: subcellular trafficking processes in pattern recognition receptor-triggered plant immunity

A significant challenge for plants is to induce localized defense responses at sites of pathogen attack. Therefore, host subcellular trafficking processes enable accumulation and exchange of defense compounds, which contributes to the plant on-site defenses in response to pathogen perception. This review summarizes our current understanding of the transport processes that facilitate immunity, the significance of which is highlighted by pathogens reprogramming membrane trafficking through host cell translocated effectors. Prominent immune-related cargos of plant trafficking pathways are the pattern recognition receptors (PRRs), which must be present at the plasma membrane to sense microbes in the apoplast. We focus on the dynamic localization of the FLS2 receptor and discuss the pathways that regulate receptor transport within the cell and their link to FLS2-mediated immunity. One emerging theme is that ligand-induced late endocytic trafficking is conserved across different PRR protein families as well as across different plant species.

Membrane trafficking and autophagy in pathogen-triggered cell death and immunity

Plants respond to pathogen attack with dynamic rearrangements of the endomembrane system and rapid redirection of membrane traffic to facilitate effective host defence. Mounting evidence indicates the involvement of endocytic, secretory, and vacuolar trafficking pathways in immune receptor activation, signal transduction, and execution of multiple defence responses including programmed cell death (PCD). Autophagy is a conserved intracellular trafficking and degradation process and has been implicated in basal immunity as well as in some forms of immune receptor-mediated vacuolar cell death. However, the regulatory interplay of autophagy and other membrane trafficking pathways in PCD and defence responses remains obscure. This review therefore highlights recent advances in the understanding of autophagic and membrane trafficking during plant immunity, and discusses emerging molecular links and functional interconnections.

ESCRT-I mediates FLS2 endosomal sorting and plant immunity

The plant immune receptor FLAGELLIN SENSING 2 (FLS2) is present at the plasma membrane and is internalized following activation of its ligand flagellin (flg22). We show that ENDOSOMAL SORTING COMPLEX REQUIRED FOR TRANSPORT (ESCRT)-I subunits play roles in FLS2 endocytosis in Arabidopsis. VPS37-1 co-localizes with FLS2 at endosomes and immunoprecipitates with the receptor upon flg22 elicitation. Vps37-1 mutants are reduced in flg22-induced FLS2 endosomes but not in endosomes labeled by Rab5 GTPases suggesting a defect in FLS2 trafficking rather than formation of endosomes. FLS2 localizes to the lumen of multivesicular bodies, but this is altered in vps37-1 mutants indicating compromised endosomal sorting of FLS2 by ESCRT-I loss-of-function. VPS37-1 and VPS28-2 are critical for immunity against bacterial infection through a role in stomatal closure. Our findings identify that VPS37-1, and likewise VPS28-2, regulate late FLS2 endosomal sorting and reveals that ESCRT-I is critical for flg22-activated stomatal defenses involved in plant immunity.

Plants deploy plasma membrane immune receptors to survey their environment for potential threats. One of these receptors, FLAGELIN SENSING 2 (FLS2) recognizes bacterial flagellin (flg22) and thereby triggers a multitude of defense responses, enhancing immunity against infectious pathogens. Regulation of the subcellular localization of FLS2 is therefore an important aspect in plant disease resistance. FLS2 is known to shuttle between the plasma membrane and endosomal compartments but enters the late endosomal trafficking pathway upon ligand-dependent activation. A key question is the regulation of activated FLS2 in late endosomal trafficking. Here, we show that FLS2 is internalized into the lumen of multivesicular bodies and discovered by genetic inhibition that this step is regulated by components of the ENDOSOMAL SORTING COMPLEXES REQUIRED FOR TRANSPORT-I (ESCRT-I). Furthermore, we reveal that these ESCRT-I components play crucial roles in plant immunity impacting the flg22-triggered closure of stomata, prominent entry points of pathogenic bacteria, which occurred downstream of the known flg22 responses. These findings highlight the roles of endosomal trafficking in regulating FLS2 subcellular localization and plant immunity.

Structure-function aspects of extracellular leucine-rich repeat-containing cell surface receptors in plants

Plants exploit several types of cell surface receptors for perception of extracellular signals, of which the extracellular leucine-rich repeat (eLRR)-containing receptors form the major class. Although the function of most plant eLRR receptors remains unclear, an increasing number of these receptors are shown to play roles in innate immunity and a wide variety of developmental processes. Recent efforts using domain swaps, gene shuffling analyses, site-directed mutagenesis, interaction studies, and crystallographic analyses resulted in the current knowledge on ligand binding and the mechanism of activation of plant eLRR receptors. This review provides an overview of eLRR receptor research, specifically summarizing the recent understanding of interactions among plant eLRR receptors, their co-receptors and corresponding ligands. The functions of distinct eLRR receptor domains, and their role in structure, ligand perception and multimeric complex formation are discussed. [Figure: see text] Bart P.H.J. Thomma (Corresponding author).

Mapping FLS2 function to structure: LRRs, kinase and its working bits

The plasma membrane-localised FLAGELLIN SENSING 2 (FLS2) receptor is an important component of plant immunity against potentially pathogenic bacteria, acting to recognise the conserved flg22 peptide of flagellin. FLS2 shares the common structure of transmembrane receptor kinases with a receptor-like ectodomain composed of leucine-rich repeats (LRR) and an active intracellular kinase domain. Upon ligand binding, FLS2 dimerises with the regulatory LRR-receptor kinase BRI1-associated kinase 1, which in turn triggers downstream signalling cascades. Although lacking crystal structure data, recent advances have been made in our understanding of flg22 recognition based on structural and functional analyses of FLS2. These studies have revealed critical regions/residues of FLS2 and post-translational modifications that regulate the abundance and activity of this receptor. In this review, we present the current knowledge on the structural mechanism of the FLS2-flg22 interaction and subsequent receptor-mediated signalling.

Salicylic acid interferes with clathrin-mediated endocytic protein trafficking

Removal of cargos from the cell surface via endocytosis is an efficient mechanism to regulate activities of plasma membrane (PM)-resident proteins, such as receptors or transporters. Salicylic acid (SA) is an important plant hormone that is traditionally associated with pathogen defense. Here, we describe an unanticipated effect of SA on subcellular endocytic cycling of proteins. Both exogenous treatments and endogenously enhanced SA levels repressed endocytosis of different PM proteins. The SA effect on endocytosis did not involve transcription or known components of the SA signaling pathway for transcriptional regulation. SA likely targets an endocytic mechanism that involves the coat protein clathrin, because SA interfered with the clathrin incidence at the PM and clathrin-deficient mutants were less sensitive to the impact of SA on the auxin distribution and root bending during the gravitropic response. By contrast, SA did not affect the ligand-induced endocytosis of the flagellin sensing2 (FLS2) receptor during pathogen responses. Our data suggest that the established SA impact on transcription in plant immunity and the nontranscriptional effect of SA on clathrin-mediated endocytosis are independent mechanisms by which SA regulates distinct aspects of plant physiology.

Relocalization of late blight resistance protein R3a to endosomal compartments is associated with effector recognition and required for the immune response

An important objective of plant-pathogen interactions research is to determine where resistance proteins detect pathogen effectors to mount an immune response. Many nucleotide binding-Leucine-rich repeat (NB-LRR) resistance proteins accumulate in the plant nucleus following effector recognition, where they initiate the hypersensitive response (HR). Here, we show that potato (Solanum tuberosum) resistance protein R3a relocates from the cytoplasm to endosomal compartments only when coexpressed with recognized Phytophthora infestans effector form AVR3a(KI) and not unrecognized form AVR3a(EM). Moreover, AVR3a(KI), but not AVR3a(EM), is also relocalized to endosomes in the presence of R3a. Both R3a and AVR3a(KI) colocalized in close physical proximity at endosomes in planta. Treatment with brefeldin A (BFA) or wortmannin, inhibitors of the endocytic cycle, attenuated both the relocalization of R3a to endosomes and the R3a-mediated HR. No such effect of these inhibitors was observed on HRs triggered by the gene-for-gene pairs Rx1/PVX-CP and Sto1/IpiO1. An R3a(D501V) autoactive MHD mutant, which triggered HR in the absence of AVR3a(KI), failed to localize to endosomes. Moreover, BFA and wortmannin did not alter cell death triggered by this mutant. We conclude that effector recognition and consequent HR signaling by NB-LRR resistance protein R3a require its relocalization to vesicles in the endocytic pathway.

Spatio-temporal cellular dynamics of the Arabidopsis flagellin receptor reveal activation status-dependent endosomal sorting

The activity of surface receptors is location specific, dependent upon the dynamic membrane trafficking network and receptor-mediated endocytosis (RME). Therefore, the spatio-temporal dynamics of RME are critical to receptor function. The plasma membrane receptor flagellin sensing2 (FLS2) confers immunity against bacterial infection through perception of flagellin (flg22). Following elicitation, FLS2 is internalized into vesicles. To resolve FLS2 trafficking, we exploited quantitative confocal imaging for colocalization studies and chemical interference. FLS2 localizes to bona fide endosomes via two distinct endocytic trafficking routes depending on its activation status. FLS2 receptors constitutively recycle in a Brefeldin A (BFA)-sensitive manner, while flg22-activated receptors traffic via ARA7/Rab F2b- and ARA6/Rab F1-positive endosomes insensitive to BFA. FLS2 endocytosis required a functional Rab5 GTPase pathway as revealed by dominant-negative ARA7/Rab F2b. Flg22-induced FLS2 endosomal numbers were increased by Concanamycin A treatment but reduced by Wortmannin, indicating that activated FLS2 receptors are targeted to late endosomes. RME inhibitors Tyrphostin A23 and Endosidin 1 altered but did not block induced FLS2 endocytosis. Additional inhibitor studies imply the involvement of the actin-myosin system in FLS2 internalization and trafficking. Altogether, we report a dynamic pattern of subcellular trafficking for FLS2 and reveal a defined framework for ligand-dependent endocytosis of this receptor.

The INs and OUTs of pattern recognition receptors at the cell surface

Pattern recognition receptors (PRRs) enable plants to sense non-self molecules displayed by microbes to mount proper defense responses or establish symbiosis. In recent years the importance of PRR subcellular trafficking to plant immunity has become apparent. PRRs traffic through the endoplasmatic reticulum (ER) and the Golgi apparatus to the plasma membrane, where they recognize their cognate ligands. At the plasma membrane, PRRs can be recycled or internalized via endocytic pathways. By using genetic and biochemical tools in combination with bioimaging, the trafficking pathways and their role in PRR perception of microbial molecules are now being revealed.

Artificial ubiquitylation is sufficient for sorting of a plasma membrane ATPase to the vacuolar lumen of Arabidopsis cells

Sorting of transmembrane proteins into the inner vesicles of multivesicular bodies for subsequent delivery to the vacuole/lysosome can be induced by attachment of a single ubiquitin or K63-linked ubiquitin chains to the cytosolic portion of the cargo in yeast and mammals. In plants, large efforts have been undertaken to elucidate the mechanisms of vacuolar trafficking of soluble proteins. Sorting of transmembrane proteins, by contrast, is still largely unexplored. As a proof of principle, that ubiquitin is involved in vacuolar sorting in plants we show that a translational fusion of a single ubiquitin to the Arabidopsis plasma membrane ATPase PMA-EGFP is sufficient to induce its endocytosis and sorting into the vacuolar lumen. Sorting of the artificial reporter is not dependent on ubiquitin chain formation, but involves ubiquitin's hydrophobic patch and can be inhibited by coexpression of a dominant-negative version of the ESCRT (endosomal sorting complex required for transport) related protein AtSKD1 (SUPPRESSOR OF K+ TRANSPORT GROWTH DEFECT1). Our results suggest that ubiquitin can in principle act as vacuolar sorting signal in plants.

Artificial ubiquitylation is sufficient for sorting of a plasma membrane ATPase to the vacuolar lumen of Arabidopsis cells

Sorting of transmembrane proteins into the inner vesicles of multivesicular bodies for subsequent delivery to the vacuole/lysosome can be induced by attachment of a single ubiquitin or K63-linked ubiquitin chains to the cytosolic portion of the cargo in yeast and mammals. In plants, large efforts have been undertaken to elucidate the mechanisms of vacuolar trafficking of soluble proteins. Sorting of transmembrane proteins, by contrast, is still largely unexplored. As a proof of principle, that ubiquitin is involved in vacuolar sorting in plants we show that a translational fusion of a single ubiquitin to the Arabidopsis plasma membrane ATPase PMA-EGFP is sufficient to induce its endocytosis and sorting into the vacuolar lumen. Sorting of the artificial reporter is not dependent on ubiquitin chain formation, but involves ubiquitin's hydrophobic patch and can be inhibited by coexpression of a dominant-negative version of the ESCRT (endosomal sorting complex required for transport) related protein AtSKD1 (SUPPRESSOR OF K+ TRANSPORT GROWTH DEFECT1). Our results suggest that ubiquitin can in principle act as vacuolar sorting signal in plants.

Receptor-like kinase complexes in plant innate immunity

Receptor-like kinases (RLKs) are surface localized, transmembrane receptors comprising a large family of well-studied kinases. RLKs signal through their transmembrane and juxtamembrane domains with the aid of various interacting partners and downstream components. The N-terminal extracellular domain defines ligand specificity, and RLK families are sub-classed according to this domain. The most studied of these subfamilies include those with (1) leucine-rich repeat (LRR) domains, (2) LysM domains (LYM), and (3) the Catharanthus roseus RLK1-like (CrRLK1L) domain. These proteins recognize distinct ligands of microbial origin or ligands derived from intracellular protein/carbohydrate signals. For example, the pattern-recognition receptor (PRR) AtFLS2 recognizes flg22 from flagellin, and the PRR AtEFR recognizes elf18 from elongation factor (EF-Tu). Upon binding of their cognate ligands, the aforementioned RLKs activate generic immune responses termed pattern-triggered immunity (PTI). RLKs can form complexes with other family members and engage a variety of intracellular signaling components and regulatory pathways upon stimulation. This review focuses on interesting new data about how these receptors form protein complexes to exert their function.

Structure and function of endosomes in plant cells

Endosomes are a heterogeneous collection of organelles that function in the sorting and delivery of internalized material from the cell surface and the transport of materials from the Golgi to the lysosome or vacuole. Plant endosomes have some unique features, with an organization distinct from that of yeast or animal cells. Two clearly defined endosomal compartments have been studied in plant cells, the trans-Golgi network (equivalent to the early endosome) and the multivesicular body (equivalent to the late endosome), with additional endosome types (recycling endosome, late prevacuolar compartment) also a possibility. A model has been proposed in which the trans-Golgi network matures into a multivesicular body, which then fuses with the vacuole to release its cargo. In addition to basic trafficking functions, endosomes in plant cells are known to function in maintenance of cell polarity by polar localization of hormone transporters and in signaling pathways after internalization of ligand-bound receptors. These signaling functions are exemplified by the BRI1 brassinosteroid hormone receptor and by receptors for pathogen elicitors that activate defense responses. After endocytosis of these receptors from the plasma membrane, endosomes act as a signaling platform, thus playing an essential role in plant growth, development and defense responses. Here we describe the key features of plant endosomes and their differences from those of other organisms and discuss the role of these organelles in cell polarity and signaling pathways.

Symbiotic rhizobia bacteria trigger a change in localization and dynamics of the Medicago truncatula receptor kinase LYK3

To form nitrogen-fixing symbioses, legume plants recognize a bacterial signal, Nod Factor (NF). The legume Medicago truncatula has two predicted NF receptors that direct separate downstream responses to its symbiont Sinorhizobium meliloti. NOD FACTOR PERCEPTION encodes a putative low-stringency receptor that is responsible for calcium spiking and transcriptional responses. LYSIN MOTIF RECEPTOR-LIKE KINASE3 (LYK3) encodes a putative high-stringency receptor that mediates bacterial infection. We localized green fluorescent protein (GFP)-tagged LYK3 in M. truncatula and found that it has a punctate distribution at the cell periphery consistent with a plasma membrane or membrane-tethered vesicle localization. In buffer-treated control roots, LYK3:GFP puncta are dynamic. After inoculation with compatible S. meliloti, LYK3:GFP puncta are relatively stable. We show that increased LYK3:GFP stability depends on bacterial NF and NF structure but that NF is not sufficient for the change in LYK3:GFP dynamics. In uninoculated root hairs, LYK3:GFP has little codistribution with mCherry-tagged FLOTILLIN4 (FLOT4), another punctate plasma membrane-associated protein required for infection. In inoculated root hairs, we observed an increase in FLOT4:mCherry and LYK3:GFP colocalization; both proteins localize to positionally stable puncta. We also demonstrate that the localization of tagged FLOT4 is altered in plants carrying a mutation that inactivates the kinase domain of LYK3. Our work indicates that LYK3 protein localization and dynamics are altered in response to symbiotic bacteria.

Endocytosis in plant-microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant-microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.

Plant pattern-recognition receptor FLS2 is directed for degradation by the bacterial ubiquitin ligase AvrPtoB

BACKGROUND

An important layer of active defense in plant immunity is the detection of pathogen-associated molecular patterns (PAMPs) mediated by cell-surface receptors. For the establishment of disease, pathogens depend on the ability to overcome PAMP perception and disable plant signaling pathways activated in response to PAMPs. Pattern recognition receptors (PRRs) are therefore prime targets for pathogen effectors. FLS2, its coreceptor BAK1, and EFR encode receptor-like kinases that play a role in immunity against bacterial pathogens.

RESULTS

Here, we report that virulence of Pseudomonas syringae pv tomato DC3000 (PtoDC3000) in Arabidopsis is enhanced through the action of its effector AvrPtoB, which promotes degradation of FLS2. We show that AvrPtoB, through its N terminus, associates with FLS2 and BAK1, of which interaction with FLS2 is enhanced by flg22 activation. In vitro, AvrPtoB is active as an E3 ligase to catalyze polyubiquitination of the kinase domain of FLS2, a process confirmed in planta. Full enhancement of PtoDC3000 virulence appears to require the E3 ligase activity of AvrPtoB.

CONCLUSIONS

AvrPtoB, initially identified through its activation of hypersensitive resistance in tomato cultivars expressing the Pto kinase, is composed of at least two functional domains: the N terminus is responsible for interaction with Pto, and the C terminus carries an E3 ligase activity. Based on our findings, we propose that both domains of AvrPtoB act together to support the virulence of PtoDC3000 in Arabidopsis through their ability to eliminate FLS2 from the cell periphery, and probably also other PAMP sensors that are constitutively expressed or induced after pathogen challenge.

PAMP-triggered immunity: Early events in the activation of FLAGELLIN SENSITIVE2

The Arabidopsis FLAGELLIN SENSITIVE2 (FLS2) protein is a leucine-rich repeat receptor-like kinase (LRR-RLK) that plays important roles in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). The binding of bacterial flagellin, one of the PAMPs, to the extracellular domain of FLS2 leads to activation of signaling cascades resulting in activation or repression of a specific set of genes involved in plant defense. The mechanisms at the cell membrane that lead to the activation of this signalling pathway are, however, not fully understood. Recently, we have shown that after ligand-treatment the mobility of FLS2 in the cell membrane is reduced and that the activation of FLS2 does not involve its constitutive or ligand-dependent homodimerization. Our data together with recently published reports suggest that FLS2 activation involves its association with other proteins, including BRI1-associated kinase 1 (BAK1), another LRR-RLK, and localization to less mobile areas, probably lipid rafts, in a ligand-dependent manner to initiate PTI.