Plant Innate Immune Response Research Articles

The Effect of Hydrolysis and Protein Source on the Efficacy of Protein Hydrolysates as Plant Resistance Inducers against Powdery Mildew

The substitution of synthetic chemical pesticides has become a priority in agriculture and the induction of plant resistance by protein hydrolysates may offer a sustainable alternative. The protein source, method and degree of hydrolysis, as well as the amino acid and peptide composition may affect the efficacy of protein hydrolysates in protecting plants against pathogens. The aim of this work was to clarify the effect of enzymatic and acid hydrolysis on different plant protein sources (soybean, rapeseed and guar protein meal), in terms of efficacy against the powdery mildew of courgette plants (caused by Podosphaera xanthii). The enzymatic hydrolysates were obtained by incubating each protein suspension with a measured amount of Alcalase or Flavourzyme at 50°C for 24 h, in order to obtain an enzyme/substrate ratio of 1% and 50%. The chemical hydrolysates were obtained by acid hydrolysis using 6 N H2SO4 at 121°C for 15 min (6N A), or at 100°C for 8 h (6N B) respectively. Preventive foliar treatments with guar hydrolysates produced with both enzymatic (Alcalase 50% E/S ratio) and chemical (6N B) hydrolysis significantly reduced disease symptoms compared to the non-hydrolysed protein source. A positive correlation was found between efficacy and the degree of hydrolysis of guar acid hydrolysates, suggesting that the hydrolysis method may enhance the functional properties of the original protein source. In addition, positive correlations were found between the efficacy of guar hydrolysates and concentrations of specific peptides and amino acids. In conclusion, our results showed that the biocontrol effect of plant protein hydrolysates was related to the original protein source and two specific hydrolysis processes improved the functional properties of guar, producing peptide fragments and free amino acids that may be involved in the regulation of innate immune response in plants.

Autophagy induction in tobacco leaves infected by potato virus YO and its putative roles

Autophagy plays a critical role in the innate immune response of plants to pathogen infection. In the present study, we examined autophagy induced by potato virus Y ordinary strain (PVYO) infection in tobacco (Nicotiana benthamiana). Enzyme-linked immunosorbent assays revealed that the number of virus particles in the plant peaked at 2 weeks post-inoculation and then gradually decreased.Additionally, the amount of virus increased significantly in the 3rd and 4th leaves distal to the inoculated leaf and decreased slightly in the 5th leaf. Within 2 weeks of PVYO inoculation, the tobacco leaves showed typical symptoms of Potyvirus inoculation, including mottling, yellowing, a mosaic pattern, and necrotic tissue changes at the inoculated site. Based on an ultrastructural analysis of the PVYO-infected tobacco leaves, virus aggregates appeared as longitudinal and transverse arrays and pinwheels, which are typical of Potyvirus inoculation. Moreover, PVYO infection caused changes in the number, size, and shape of chloroplasts, whereas the number of plastogranules increased markedly. Furthermore, double-membrane autophagosome-like vacuoles, including electron-dense materials, laminated structures, and cellular organelles, were found. The induction of autophagy after the PVYO infection of tobacco leaves was further confirmed by the expression of lipidated microtubule-associated protein 1 light chain 3 (LC3)-II, an autophagy marker and p62, an autophagy adaptor protein. The LC3-II levels increased daily over the 4-week period. Although virus inoculation was performed systemically on the basal leaves of the plants, LC3-II was expressed throughout the leaves and the expression was higher in leaves distal to the inoculated leaf. Moreover, PVYO infection caused the activation of stress-activated protein kinases/c-Jun N-terminal kinases. Therefore, PVYO infection-induced autophagy was positively correlated with the virus content, suggesting that autophagy induction following PVYO infection is involved in the anti-pathogen response of the host.

The Innate Immune Signaling System as a Regulator of Disease Resistance and Induced Systemic Resistance Activity AgainstVerticillium dahliae

In the last decades, the plant innate immune responses against pathogens have been extensively studied, while biocontrol interactions between soilborne fungal pathogens and their hosts have received much less attention. Treatment of Arabidopsis thaliana with the nonpathogenic bacterium Paenibacillus alvei K165 was shown previously to protect against Verticillium dahliae by triggering induced systemic resistance (ISR). In the present study, we evaluated the involvement of the innate immune response in the K165-mediated protection of Arabidopsis against V. dahliae. Tests with Arabidopsis mutants impaired in several regulators of the early steps of the innate immune responses, including fls2, efr-1, bak1-4, mpk3, mpk6, wrky22, and wrky29 showed that FLS2 and WRKY22 have a central role in the K165-triggered ISR, while EFR1, MPK3, and MPK6 are possible susceptibility factors for V. dahliae and bak1 shows a tolerance phenomenon. The resistance induced by strain K165 is dependent on both salicylate and jasmonate-dependent defense pathways, as evidenced by an increased transient accumulation of PR1 and PDF1.2 transcripts in the aerial parts of infected plants treated with strain K165.

Transcriptomics of tomato plants infected with TYLCSV or expressing the central TYLCSV Rep protein domain uncover changes impacting pathogen response and senescence

To establish a successful infection viruses need to overcome plant innate immune responses and redirect host gene expression for their multiplication and diffusion. Tomato yellow leaf curl Sardinia virus (TYLCSV) is a geminivirus, which causes significant economic losses in tomato. The multifunctional replication associated geminivirus protein (Rep) has an important role during viral infection. In particular, the Rep central domain spanning from aa 120 to 180 is known to interact with viral and host factors. In this study, we used long serial analysis of gene expression to analyse the transcriptional profiles of transgenic tomato plants expressing the first 210 amino acids of TYLCSV Rep (Rep210) and TYLCSV-infected wild-type tomato plants (Wt-Ty). Also, we compared these profiles with those of transgenic Rep130 tomatoes. Comparison of Wt-Ty and Rep210 libraries with the wild-type one identified 118 and 203 differentially expressed genes (DEGs), respectively. Importantly, 55% of Wt-Ty DEGs were in common with Rep210, and no ones showed opposite expression. Conversely, a negligible overlap was found between Rep130 DEGs and Wt-Ty and Rep210 ones. TYLCSV- and Rep210-repressed genes, but not induced ones, overlapped with the leaf senescence process. Interestingly, TYLCSV upregulates expression of genes involved in the negative regulation of programmed cell death (PCD), several of which were also regulated by the abscisic acid. Rep210 upregulated genes related to defence response, immune system processes and negative regulation of PCD. Collectively, our results support a model in which the Rep central domain has a pivotal role in redirecting host plant gene expression.

Plant Innate Immune Response: Qualitative and Quantitative Resistance

ABSTRACTPlant diseases, caused by microbes, threaten world food, feed, and bioproduct security. Plant resistance has not been effectively deployed to improve resistance in plants for lack of understanding of biochemical mechanisms and genetic bedrock of resistance. With the advent of genome sequencing, the forward and reverse genetic approaches have enabled deciphering the riddle of resistance. Invading pathogens produce elicitors and effectors that are recognized by the host membrane-localized receptors, which in turn induce a cascade of downstream regulatory and resistance metabolite and protein biosynthetic genes (R) to produce resistance metabolites and proteins, which reduce pathogen advancement through their antimicrobial and cell wall enforcement properties. The resistance in plants to pathogen attack is expressed as reduced susceptibility, ranging from high susceptibility to hypersensitive response, the shades of gray. The hypersensitive response or cell death is considered as qualitative resistance, while the remainder of the reduced susceptibility is considered as quantitative resistance. The resistance is due to additive effects of several resistance metabolites and proteins, which are produced through a network of several hierarchies of plant R genes. Plants recognize the pathogen elicitors or receptors and then induce downstream genes to eventually produce resistance metabolites and proteins that suppress the pathogen advancement in plant. These resistance genes (R), against qualitative and quantitative resistance, can be identified in germplasm collections and replaced in commercial cultivars, if nonfunctional, based on genome editing to improve plant resistance.

Biotrophic Fungi Infection and Plant Defense Mechanism

Biotrophic pathogens derive nutrients from living cells by maintaining host viability. This host Maintenance sustain through highly specialized structural and biochemical relations. For valuable virulence activity biotrophic fungi have: highly developed infection structures; limited secretory activity, carbohydrate rich and protein-containing interfacial layers; long-term suppression of host defense; haustoria that used for nutrient absorption and metabolism. Plant defenses biotrophic fungal pathogen by penetration resistance and program cell death (PCD). Plant strengthens cell wall and membrane to halt spore germination and prevent the formation of the haustorium by Penetration resistance. The second resistance mechanism applied inside the penetrated epidermal cell that terminates nutrient supply to fungi for further development by induction of invaded program cell death. Plant innate immune responses occur through two basic interconnected forms: pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) to activate defense signal molecules. However, Biotrophic fungi have several mechanisms to defend their effectors from plant receptor molecules. Once the fungal effector passes plant defense mechanism the plant will not resist. Subsequently the plant reduces production of defense signaling molecule like salicylic acid. This review overviews recent knowledge of biotrophic fungi infection and plant defense strategies.

Production of functional small interfering RNAs by an amino-terminal deletion mutant of human Dicer

Although RNA interference (RNAi) functions as a potent antiviral innate-immune response in plants and invertebrates, mammalian somatic cells appear incapable of mounting an RNAi response and few, if any, small interfering RNAs (siRNAs) can be detected. To examine why siRNA production is inefficient, we have generated double-knockout human cells lacking both Dicer and protein kinase RNA-activated. Using these cells, which tolerate double-stranded RNA expression, we show that a mutant form of human Dicer lacking the amino-terminal helicase domain can process double-stranded RNAs to produce high levels of siRNAs that are readily detectable by Northern blot, are loaded into RNA-induced silencing complexes, and can effectively and specifically inhibit the expression of cognate mRNAs. Remarkably, overexpression of this mutant Dicer, but not wild-type Dicer, also resulted in a partial inhibition of Influenza A virus-but not poliovirus-replication in human cells.

Chitosan nanoparticles: A positive modulator of innate immune responses in plants.

The immunomodulatory role of the natural biopolymer, chitosan, has already been demonstrated in plants, whilst its nanoparticles have only been examined for biomedical applications. In our present study, we have investigated the possible ability and mechanism of chitosan nanoparticles (CNP) to induce and augment immune responses in plants. CNP-treatment of leaves produced significant improvement in the plant’s innate immune response through induction of defense enzyme activity, upregulation of defense related genes including that of several antioxidant enzymes as well as elevation of the levels of total phenolics. It is also possible that the extracellular localization of CNP may also play a role in the observed upregulation of defense response in plants. Nitric oxide (NO), an important signaling molecule in plant defense, was also observed to increase following CNP treatment. However, such CNP-mediated immuno-stimulation was significantly mitigated when NO production was inhibited, indicating a possible role of NO in such immune induction. Taken together, our results suggest that CNP may be used as a more effective phytosanitary or disease control agent compared to natural chitosan for sustainable organic cultivation.

Membrane-associated virus replication complexes locate to plant conducting tubes

It is generally accepted that in order to establish a systemic infection in a plant, viruses move from the initially infected cell to the vascular tissues by cell-to-cell movement through plasmodesmata (PD), and load into the vascular conducting tubes (i.e. phloem sieve elements and xylem vessel elements) for long-distance movement. The viral unit in these movements can be a virion or a yet-to-be-defined ribonucleic protein (RNP) complex. Using live-cell imaging, our laboratory has previously demonstrated that membrane-bound replication complexes move cell-to-cell during turnip mosaic virus (TuMV) infection. Our recent study shows that these membrane-bound replication complexes end up in the vascular conducting tubes, which is likely the case for potato virus X (PVX) also. The presence of TuMV-induced membrane complexes in xylem vessels suggests that viral components could also be found in other apoplastic regions of the plant, such as the intercellular space. This possibility may have implications regarding how we approach the study of plant innate immune responses against viruses.

Capping protein integrates multiple MAMP signalling pathways to modulate actin dynamics during plant innate immunity.

Plants and animals perceive diverse microbe-associated molecular patterns (MAMPs) via pattern recognition receptors and activate innate immune signalling. The actin cytoskeleton has been suggested as a target for innate immune signalling and a key transducer of cellular responses. However, the molecular mechanisms underlying actin remodelling and the precise functions of these rearrangements during innate immunity remain largely unknown. Here we demonstrate rapid actin remodelling in response to several distinct MAMP signalling pathways in plant epidermal cells. The regulation of actin dynamics is a convergence point for basal defence machinery, such as cell wall fortification and transcriptional reprogramming. Our quantitative analyses of actin dynamics and genetic studies reveal that MAMP-stimulated actin remodelling is due to the inhibition of capping protein (CP) by the signalling lipid, phosphatidic acid. In addition, CP promotes resistance against bacterial and fungal phytopathogens. These findings demonstrate that CP is a central target for the plant innate immune response.

Comparative conventional- and quantum dot-labeling strategies for LPS binding site detection in Arabidopsis thaliana mesophyll protoplasts.

Lipopolysaccharide (LPS) from Gram-negative bacteria is recognized as a microbe-associated molecular pattern (MAMP) and not only induces an innate immune response in plants, but also stimulates the development of characteristic defense responses. However, identification and characterization of a cell surface LPS-receptor/binding site, as described in mammals, remains elusive in plants. As an amphiphilic, macromolecular lipoglycan, intact LPS potentially contains three MAMP-active regions, represented by the O-polysaccharide chain, the core and the lipid A. Binding site studies with intact labeled LPS were conducted in Arabidopsis thaliana protoplasts and quantified using flow cytometry fluorescence changes. Quantum dots (Qdots), which allow non-covalent, hydrophobic labeling were used as a novel strategy in this study and compared to covalent, hydrophilic labeling with Alexa 488. Affinity for LPS-binding sites was clearly demonstrated by concentration-, temperature-, and time-dependent increases in protoplast fluorescence following treatment with the labeled LPS. Moreover, this induced fluorescence increase was convincingly reduced following pre-treatment with excess unlabeled LPS, thereby indicating reversibility of LPS binding. Inhibition of the binding process is also reported using endo- and exocytosis inhibitors. Here, we present evidence for the anticipated presence of LPS-specific binding sites in Arabidopsis protoplasts, and furthermore propose Qdots as a more sensitive LPS-labeling strategy in comparison to the conventional Alexa 488 hydrazide label for binding studies.

The role of plant small RNAs in NB-LRR regulation.

Small RNAs constitute a fundamental layer of gene regulation for diverse biological processes in plants, including development, metabolism and stress responses. With the advance of high-throughput sequencing technologies and the rapid accumulation of transcriptomic data, the scope of regulation afforded by small RNAs has expanded to encompass plant innate immune responses. Plants have evolved the capacity to control the infection through intracellular surveillance proteins of the nucleotide binding site-leucine-rich repeat (NB-LRR) family that recognize pathogen-encoded effectors and initiate effector-triggered immunity. Emerging evidence indicates that plants have evolved to use specific microRNAs that target conserved domains of NB-LRR-encoding genes and trigger the production of a phased array of 21-nucleotide secondary small interfering RNAs to amplify the silencing effect. Herein, this review describes recent advances in understanding the roles of small RNAs in NB-LRR regulation that provide new insights into small RNA-mediated arms race between plants and their pathogens and discuss the unresolved questions and the future prospects for research on this topic.

β-glucans and eicosapolyenoic acids as MAMPs in plant-oomycete interactions: past and present.

Branched β-1,3-glucans and the eicosapolyenoic acids (EP) are among the best characterized oomycete elicitors that trigger innate immune responses in plants. These elicitors were identified over three decades ago, and they were useful in the study of the sequence of physiological, biochemical and molecular events that induce resistance in plants. However, in spite of the cross-kingdom parallels where these molecules are well-characterized as immune system modulators in animals, their perception and modes of action in plants remains obscure. Oomycetes are among the most important plant pathogens, responsible for diseases that devastate crops, ornamentals, and tree species worldwide. With the recent interest and advances in our understanding of innate immunity in plants, and the redefining of many of the classical elicitors as microbe-associated molecular patterns (MAMPs), it seems timely and important to reexamine β-glucans and EP using contemporary approaches. In this review, we highlight early studies of β-glucans and EP, discuss their roles as evolutionarily conserved signals, and consider their action in relation to current models of MAMP-triggered immunity.

Conventional Alexa 488 Hydrazide in Quantum dot labelling of conjugates for lipopolysaccharide binding studies in <i>Arabidopsis thaliana</i> protoplast

Conventional Alexa 488 Hydrazide in Quantum dot labelling of conjugates for lipopolysaccharide binding studies in Arabidopsis thaliana protoplast Lipopolysaccharide (LPS) is a complex lipoglycan that is found in the outer membrane of Gram-negative bacteria and composed of three regions namely the fatty acid Lipid A, a core region of short oligosaccharide chains and an O-antigen region of polysaccharide. When LPS is used as a MAMP/PAMP, it not only induces an innate immune response in plants but also stimulates the development of defense responses such as the immediate release of ROS/ROI, PR gene expression and activation of the hypersensitive response (HR), resulting in stronger subsequent pathogen interactions. The identification and characterization of the elusive LPS receptor or receptor complex in plants is thus of importance, since understanding the mechanism of perception and specific signal transduction pathways will clarify, and lead to the advancement of, basal resistance in plants in order to decrease plant crop losses due to pathogen attack. In mammals, LPS binds to LBP (LPS binding protein) to form a LPS-LBP complex which is translocated to MD2 with the presence or absence of its co-receptor, a glycosylphosphatidylinositol (GPI)-linked protein, CD14. The interaction occurs on the host membrane and triggers an inflammatory defense response through the signaling cascade activated by the interaction with Toll-like receptor 4 (TLR4). A similar LPS-receptor interaction is however, unknown in plants. To address this, biological binding studies with regard to concentration, incubation time and temperature, affinity, specificity and saturation were conducted using LPS labeled with Alexa 488 hydrazide. Although such labeling does not affect the biological activity in mammalian studies, the same cannot necessarily be said for plant systems. Thus, quantum dots, which allow non-covalent hydrophobic labelling of LPS, were further employed in binding studies. The conjugation to LPS was confirmed by transmission electron microscopy and results illustrated higher fluorescence values as compared to Alexa-LPS fluorescence analysis. Furthermore, inhibition of the process is also reported using Wortmannin and Brefeldin A as suitable endocytosis inhibitors.

Melatonin as a signal molecule triggering defense responses against pathogen attack in Arabidopsis and tobacco.

Melatonin plays pleiotropic roles in both animals and plants. The possible role of melatonin in plant innate immune responses was recently discovered. As an initial study, we employed Arabidopsis to determine whether melatonin is involved in defense against the virulent bacterial pathogen Pseudomonas syringae DC3000. The application of a 10 μM concentration of melatonin on Arabidopsis and tobacco leaves induced various pathogenesis-related (PR) genes, as well as a series of defense genes activated by salicylic acid (SA) and ethylene (ET), two key factors involved in plant defense response, compared to mock-treated leaves. The induction of these defense-related genes in melatonin-treated Arabidopsis matched an increase in resistance against the bacterium by suppressing its multiplication about ten-fold relative to the mock-treated Arabidopsis. Like melatonin, N-acetylserotonin also plays a role in inducing a series of defense genes, although serotonin does not. Furthermore, melatonin-induced PR genes were almost completely or partially suppressed in the npr1, ein2, and mpk6 Arabidopsis mutants, indicative of SA and ET dependency in melatonin-induced plant defense signaling. This suggests that melatonin may be a novel defense signaling molecule in plant-pathogen interactions.

The hnRNP-Q protein LIF2 participates in the plant immune response.

Eukaryotes have evolved complex defense pathways to combat invading pathogens. Here, we investigated the role of the Arabidopsis thaliana heterogeneous nuclear ribonucleoprotein (hnRNP-Q) LIF2 in the plant innate immune response. We show that LIF2 loss-of-function in A. thaliana leads to changes in the basal expression of the salicylic acid (SA)- and jasmonic acid (JA)- dependent defense marker genes PR1 and PDF1.2, respectively. Whereas the expression of genes involved in SA and JA biosynthesis and signaling was also affected in the lif2-1 mutant, no change in SA and JA hormonal contents was detected. In addition, the composition of glucosinolates, a class of defense-related secondary metabolites, was altered in the lif2-1 mutant in the absence of pathogen challenge. The lif2-1 mutant exhibited reduced susceptibility to the hemi-biotrophic pathogen Pseudomonas syringae and the necrotrophic ascomycete Botrytis cinerea. Furthermore, the lif2-1 sid2-2 double mutant was less susceptible than the wild type to P. syringae infection, suggesting that the lif2 response to pathogens was independent of SA accumulation. Together, our data suggest that lif2-1 exhibits a basal primed defense state, resulting from complex deregulation of gene expression, which leads to increased resistance to pathogens with various infection strategies. Therefore, LIF2 may function as a suppressor of cell-autonomous immunity. Similar to its human homolog, NSAP1/SYNCRIP, a trans-acting factor involved in both cellular processes and the viral life cycle, LIF2 may regulate the conflicting aspects of development and defense programs, suggesting that a conserved evolutionary trade-off between growth and defense pathways exists in eukaryotes.

SEC14 phospholipid transfer protein is involved in lipid signaling-mediated plant immune responses in Nicotiana benthamiana.

We previously identified a gene related to the SEC14-gene phospholipid transfer protein superfamily that is induced in Nicotiana benthamiana (NbSEC14) in response to infection with Ralstonia solanacearum. We here report that NbSEC14 plays a role in plant immune responses via phospholipid-turnover. NbSEC14-silencing compromised expression of defense–related PR-4 and accumulation of jasmonic acid (JA) and its derivative JA-Ile. Transient expression of NbSEC14 induced PR-4 gene expression. Activities of diacylglycerol kinase, phospholipase C and D, and the synthesis of diacylglycerol and phosphatidic acid elicited by avirulent R. solanacearum were reduced in NbSEC14-silenced plants. Accumulation of signaling lipids and activation of diacylglycerol kinase and phospholipases were enhanced by transient expression of NbSEC14. These results suggest that the NbSEC14 protein plays a role at the interface between lipid signaling-metabolism and plant innate immune responses.

The perplexing role of autophagy in plant innate immune responses.

Autophagy is a major intracellular process for the degradation of cytosolic macromolecules and organelles in the lysosomes or vacuoles for the purposes of regulating cellular homeostasis and protein and organelle quality control. In complex metazoan organisms, autophagy is highly engaged during the immune responses through interfaces either directly with intracellular pathogens or indirectly with immune signalling molecules. Studies over the last decade or so have also revealed a number of important ways in which autophagy shapes plant innate immune responses. First, autophagy promotes defence-associated hypersensitive cell death induced by avirulent or related pathogens, but restricts unnecessary or disease-associated spread of cell death. This elaborate regulation of plant host cell death by autophagy is critical during plant immune responses to the types of plant pathogens that induce cell death, which include avirulent biotrophic pathogens and necrotrophic pathogens. Second, autophagy modulates defence responses regulated by salicylic acid and jasmonic acid, thereby influencing plant basal resistance to both biotrophic and necrotrophic pathogens. Third, there is an emerging role of autophagy in virus-induced RNA silencing, either as an antiviral collaborator for targeted degradation of viral RNA silencing suppressors or an accomplice of viral RNA silencing suppressors for targeted degradation of key components of plant cellular RNA silencing machinery. In this review, we summarize this important progress and discuss the potential significance of the perplexing role of autophagy in plant innate immunity.

Deciphering the Responses of Root Border-Like Cells of Arabidopsis and Flax to Pathogen-Derived Elicitors

Plant pathogens including fungi and bacteria cause many of the most serious crop diseases. The plant innate immune response is triggered upon recognition of microbe-associated molecular patterns (MAMPs) such as flagellin22 and peptidoglycan. To date, very little is known of MAMP-mediated responses in roots. Root border cells are cells that originate from root caps and are released individually into the rhizosphere. Root tips of Arabidopsis (Arabidopsis thaliana) and flax (Linum usitatissimum) release cells known as "border-like cells." Whereas root border cells of pea (Pisum sativum) are clearly involved in defense against fungal pathogens, the function of border-like cells remains to be established. In this study, we have investigated the responses of root border-like cells of Arabidopsis and flax to flagellin22 and peptidoglycan. We found that both MAMPs triggered a rapid oxidative burst in root border-like cells of both species. The production of reactive oxygen species was accompanied by modifications in the cell wall distribution of extensin epitopes. Extensins are hydroxyproline-rich glycoproteins that can be cross linked by hydrogen peroxide to enhance the mechanical strength of the cell wall. In addition, both MAMPs also caused deposition of callose, a well-known marker of MAMP-elicited defense. Furthermore, flagellin22 induced the overexpression of genes involved in the plant immune response in root border-like cells of Arabidopsis. Our findings demonstrate that root border-like cells of flax and Arabidopsis are able to perceive an elicitation and activate defense responses. We also show that cell wall extensin is involved in the innate immunity response of root border-like cells.

Structural Basis for flg22-Induced Activation of the Arabidopsis FLS2-BAK1 Immune Complex

Flagellin perception in Arabidopsis is through recognition of its highly conserved N-terminal epitope (flg22) by flagellin-sensitive 2 (FLS2). Flg22 binding induces FLS2 heteromerization with BRASSINOSTEROID INSENSITIVE 1-associated kinase 1 (BAK1) and their reciprocal activation followed by plant immunity. Here, we report the crystal structure of FLS2 and BAK1 ectodomains complexed with flg22 at 3.06 angstroms. A conserved and a nonconserved site from the inner surface of the FLS2 solenoid recognize the C- and N-terminal segment of flg22, respectively, without oligomerization or conformational changes in the FLS2 ectodomain. Besides directly interacting with FLS2, BAK1 acts as a co-receptor by recognizing the C terminus of the FLS2-bound flg22. Our data reveal the molecular mechanisms underlying FLS2-BAK1 complex recognition of flg22 and provide insight into the immune receptor complex activation.