Activation of Plant Innate Immunity by Extracellular High Mobility Group Box 3 and Its Inhibition by Salicylic Acid

Intricacies of plants' innate immune responses and their dynamic relationship with fungi: A review

The role of the plant innate immune system in the defense and symbiosis processes becomes integral in a complex network of interactions between plants and fungi. An understanding of the molecular characterization of the plant innate immune system is crucial because it constitutes plants' self-defense shield against harmful fungi, while creating mutualistic relationships with beneficial fungi. Due to the plant-induced awareness and their complexity of interaction with fungi, sufficient assessment of the participation of the plant innate immune system in ecological balance, agriculture, and maintenance of an infinite ecosystem is mandatory. Given the current global challenge, such as the surge of plant-infectious diseases, and pursuit of sustainable forms of agriculture; it is imperative to understand the molecular language of communication between plants and fungi. That knowledge can be practically used in diverse areas, e.g., in agriculture, new tactics may be sought after to try new methods that boost crop receptiveness against fungal pathogens and reduce the dependence on chemical management. Also, it could boost sustainable agricultural practices via enhancing mycorrhizal interactions that promote nutrient absorption and optimum cropping with limited exposure of environmental contamination. Moreover, this review offers insights that go beyond agriculture and can be manipulated to boost plant conservation, environmental restoration, and quality understanding of host-pathogen interactions. Consequently, this specific review paper has offered a comprehensive view of the complex plant innate immune-based responses with fungi and the mechanisms in which they interact.

Genome-Wide Identification and Expression Analysis of the High-Mobility Group B (HMGB) Gene Family in Plant Response to Abiotic Stress in Tomato

High-mobility group B (HMGB) proteins are a class of non-histone proteins associated with eukaryotic chromatin and are known to regulate a variety of biological processes in plants. However, the functions of HMGB genes in tomato (Solanum lycopersicum) remain largely unexplored. Here, we identified 11 members of the HMGB family in tomato using BLAST. We employed genome-wide identification, gene structure analysis, domain conservation analysis, cis-acting element analysis, collinearity analysis, and qRT-PCR-based expression analysis to study these 11 genes. These genes were categorized into four groups based on their unique protein domain structures. Despite their structural diversity, all members contain the HMG-box domain, a characteristic feature of the HMG superfamily. Syntenic analysis suggested that tomato SlHMGBs have close evolutionary relationships with their homologs in other dicots. The promoter regions of SlHMGBs are enriched with numerous cis-elements related to plant growth and development, phytohormone responsiveness, and stress responsiveness. Furthermore, SlHMGB members exhibited distinct tissue-specific expression profiles, suggesting their potential roles in regulating various aspects of plant growth and development. Most SlHMGB genes respond to a variety of abiotic stresses, including salt, drought, heat, and cold. For instance, SlHMGB2 and SlHMGB4 showed positive responses to salt, drought, and cold stresses. SlHMGB1, SlHMGB3, and SlHMGB8 were involved in responses to two types of stress: SlHMGB1 responded to drought and heat, while SlHMGB3 and SlHMGB8 responded to salt and heat. SlHMGB6 and SlHMGB11 were solely regulated by drought and heat stress, respectively. Under various treatment conditions, the number of up-regulated genes significantly outnumbered the down-regulated genes, implying that the SlHMGB family may play a crucial role in mitigating abiotic stress in tomato. These findings lay a foundation for further dissecting the precise roles of SlHMGB genes.

The nematode effector calreticulin competes with the high mobility group protein OsHMGB1 for binding to the rice calmodulin-like protein OsCML31 to enhance rice susceptibility to Meloidogyne graminicola

The root-knot nematode Meloidogyne graminicola secretes effectors into rice tissues to modulate host immunity. Here, we characterised MgCRT1, a calreticulin protein of M. graminicola, and identified its target in the plant. In situ hybridisation showed MgCRT1 mRNA accumulating in the subventral oesophageal gland in J2 nematodes. Immunolocalization indicated MgCRT1 localises in the giant cells during parasitism. Host-induced gene silencing of MgCRT1 reduced the infection ability of M. graminicola, while over-expressing MgCRT1 enhanced rice susceptibility to M. graminicola. A yeast two-hybrid approach identified the calmodulin-like protein OsCML31 as an interactor of MgCRT1. OsCML31 interacts with the high mobility group protein OsHMGB1 which is a conserved DNA binding protein. Knockout of OsCML31 or overexpression of OsHMGB1 in rice results in enhanced susceptibility to M. graminicola. In contrast, overexpression of OsCML31 or knockout of OsHMGB1 in rice decreases susceptibility to M. graminicola. The GST-pulldown and luciferase complementation imaging assay showed that MgCRT1 decreases the interaction of OsCML31 and OsHMGB1 in a competitive manner. In conclusion, when M. graminicola infects rice and secretes MgCRT1 into rice, MgCRT1 interacts with OsCML31 and decreases the association of OsCML31 with OsHMGB1, resulting in the release of OsHMGB1 to enhance rice susceptibility.

Pattern recognition receptors as potential therapeutic targets for developing immunological engineered plants

There is an urgent need to combat pathogen infestations in crop plants to ensure food security worldwide. To counter this, plants have developed innate immunity mediated by Pattern Recognition Receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and damage- associated molecular patterns (DAMPs). PRRs activate Pattern-Triggered Immunity (PTI), a defence mechanism involving intricate cell-surface and intracellular receptors. The diverse ligand-binding ectodomains of PRRs, including leucine-rich repeats (LRRs) and lectin domains, facilitate the recognition of MAMPs and DAMPs. Pathogen resistance is mediated by a variety of PTI responses, including membrane depolarization, ROS production, and the induction of defence genes. An integral part of intracellular immunity is the Nucleotide-binding Oligomerization Domain, Leucine-rich Repeat proteins (NLRs) which recognize and respond to effectors in a potent manner. Enhanced understanding of PRRs, their ligands, and downstream signalling pathways has contributed to the identification of potential targets for genetically modified plants. By transferring PRRs across plant species, it is possible to create broad-spectrum resistance, potentially offering innovative solutions for plant protection and global food security. The purpose of this chapter is to provide an update on PRRs involved in disease resistance, clarify the mechanisms by which PRRs recognize ligands to form active receptor complexes and present various applications of PRRs and PTI in disease resistance management for plants.

Recognition of nonself is necessary to activate Drosophila's immune response against an insect parasite

BACKGROUND

Innate immune responses can be activated by pathogen-associated molecular patterns (PAMPs), danger signals released by damaged tissues, or the absence of self-molecules that inhibit immunity. As PAMPs are typically conserved across broad groups of pathogens but absent from the host, it is unclear whether they allow hosts to recognize parasites that are phylogenetically similar to themselves, such as parasitoid wasps infecting insects.

RESULTS

Parasitoids must penetrate the cuticle of Drosophila larvae to inject their eggs. In line with previous results, we found that the danger signal of wounding triggers the differentiation of specialized immune cells called lamellocytes. However, using oil droplets to mimic infection by a parasitoid wasp egg, we found that this does not activate the melanization response. This aspect of the immune response also requires exposure to parasite molecules. The unidentified factor enhances the transcriptional response in hemocytes and induces a specific response in the fat body.

CONCLUSIONS

We conclude that a combination of danger signals and the recognition of nonself molecules is required to activate Drosophila's immune response against parasitic insects.

The clade III subfamily of OsSWEETs directly suppresses rice immunity by interacting with OsHMGB1 and OsHsp20L

The clade III subfamily of OsSWEETs includes transmembrane proteins necessary for susceptibility to bacterial blight (BB). These genes are targeted by the specific transcription activator-like effector (TALE) of Xanthomonas oryzae pv. oryzae and mediate sucrose efflux for bacterial proliferation. However, the mechanism through which OsSWEETs regulate rice immunity has not been fully elucidated. Here, we demonstrated that the cytosolic carboxyl terminus of OsSWEET11a/Xa13 is required for complementing susceptibility to PXO99 in IRBB13 (xa13/xa13). Interestingly, the C-terminus of ZmXa13, the maize homologue of OsSWEET11a/Xa13, could perfectly substitute for the C-terminus of OsSWEET11a/Xa13. Furthermore, OsSWEET11a/Xa13 interacted with the high-mobility group B1 (OsHMGB1) protein and the small heat shock-like protein OsHsp20L through the same regions in the C-terminus. Consistent with the physical interactions, knockdown or knockout of either OsHMGB1 or OsHsp20L caused an enhanced PXO99-resistant phenotype similar to that of OsSWEET11a/OsXa13. Surprisingly, the plants in which OsHMGB1 or OsHsp20L was repressed developed increased resistance to PXO86, PXO61 and YN24, which carry TALEs targeting OsSWEET14/Xa41 or OsSWEET11a/Xa13. Additionally, OsHsp20L can interact with all six members of clade III OsSWEETs, whereas OsHMGB1 can interact with five other members in addition to OsSWEET12. Overall, we revealed that OsHMGB1 and OsHsp20L mediate conserved BB susceptibility by interacting with clade III OsSWEETs, which are candidates for breeding broad-spectrum disease-resistant rice.

Macrophage-derived exosomal HMGB3 regulates silica-induced pulmonary inflammation by promoting M1 macrophage polarization and recruitment

BACKGROUND

Chronic inflammation and fibrosis are characteristics of silicosis, and the inflammatory mediators involved in silicosis have not been fully elucidated. Recently, macrophage-derived exosomes have been reported to be inflammatory modulators, but their role in silicosis has not been explored. The purpose of the present study was to investigate the role of macrophage-derived exosomal high mobility group box 3 (HMGB3) in silica-induced pulmonary inflammation.

METHODS

The induction of the inflammatory response and the recruitment of monocytes/macrophages were evaluated by immunofluorescence, flow cytometry and transwell assays. The expression of inflammatory cytokines was examined by RT-PCR and ELISA, and the signalling pathways involved were examined by western blot analysis.

RESULTS

HMGB3 expression was increased in exosomes derived from silica-exposed macrophages. Exosomal HMGB3 significantly upregulated the expression of inflammatory cytokines, activated the STAT3/MAPK (ERK1/2 and p38)/NF-κB pathways in monocytes/macrophages, and promoted the migration of these cells by CCR2.

CONCLUSIONS

Exosomal HMGB3 is a proinflammatory modulator of silica-induced inflammation that promotes the inflammatory response and recruitment of monocytes/macrophages by regulating the activation of the STAT3/MAPK/NF-κB/CCR2 pathways.

Metabolic Responses of the Microalga Neochloris oleoabundans to Extracellular Self- and Nonself-DNA

Stressed organisms identify intracellular molecules released from damaged cells due to trauma or pathogen infection as components of the innate immune response. These molecules called DAMPs (Damage-Associated Molecular Patterns) are extracellular ATP, sugars, and extracellular DNA, among others. Animals and plants can recognize their own DNA applied externally (self-exDNA) as a DAMP with a high degree of specificity. However, little is known about the microalgae responses to damage when exposed to DAMPs and specifically to self-exDNAs. Here we compared the response of the oilseed microalgae Neochloris oleoabundans to self-exDNA, with the stress responses elicited by nonself-exDNA, methyl jasmonate (MeJA) and sodium bicarbonate (NaHCO3). We analyzed the peroxidase enzyme activity related to the production of reactive oxygen species (ROS), as well as the production of polyphenols, lipids, triacylglycerols, and phytohormones. After 5 min of addition, self-exDNA induced peroxidase enzyme activity higher than the other elicitors. Polyphenols and lipids were increased by self-exDNA at 48 and 24 h, respectively. Triacylglycerols were increased with all elicitors from addition and up to 48 h, except with nonself-exDNA. Regarding phytohormones, self-exDNA and MeJA increased gibberellic acid, isopentenyladenine, and benzylaminopurine at 24 h. Results show that Neochloris oleoabundans have self-exDNA specific responses.

Secondary metabolites related to the resistance of Psidium spp. against the nematode Meloidogyneenterolobii

The guava tree (Psidium guajava) is a tropical species native to South America and is recognized as the 11th most economically important fruit tree in Brazil. However, the presence of the nematode Meloidogyne enterolobii and the fungus Fusarium solani in the roots of guava plants leads to the development of root galls, causing significant damage. In contrast, the species P. guineense and P. cattleianum have been identified as resistant and immune to the nematode, respectively. In this study, the researchers aimed to compare the metabolomic profiles of infected and uninfected roots of P. guajava, P. cattleianum, and P. guineense using mass spectrometry coupled with liquid chromatography (LC-MS). The goal was to identify secondary metabolites that could potentially be utilized as biochemical resources for nematode control. The findings of the study demonstrated that the plant metabolism of all three species undergoes alterations in response to the phytopathogen inoculation. By employing molecular networks, the researchers identified that the secondary metabolites affected by the infection, whether produced or suppressed, are primarily of a polar chemical nature. Further analysis of the database confirmed the polar nature of the regulated substances after infection, specifically hydrolysable tannins and lignans in P. guineense and P. cattleianum. Interestingly, a group of non-polar substances belonging to the terpene class was also identified in the resistant and immune species. This suggests that these terpenes may act as inhibitors of M. enterolobii, working as repellents or as molecules that can reduce oxidative stress during the infection process, thus enhancing the guava resistance to the nematode. Overall, this study provides valuable insights into the metabolic alterations occurring in different Psidium spp. in response to M. enterolobii infection. The identification of specific secondary metabolites, particularly terpenes, opens up new possibilities for developing effective strategies to control the nematode and enhance guava resistance.

Conservation and divergence of flg22, pep1 and nlp20 in activation of immune response and inhibition of root development

Many pattern-recognition receptors (PRRs) and their corresponding ligands have been identified. However, it is largely unknown how similar and different these ligands are in inducing plant innate immunity and affecting plant development. In this study, we examined three well characterized ligands in Arabidopsis thaliana, namely flagellin 22 (flg22), plant elicitor peptide 1 (pep1) and a conserved 20-amino-acid fragment found in most necrosis and ethylene-inducing peptide 1-like proteins (nlp20). Our quantitative analyses detected the differences in amplitude in the early immune responses of these ligands, with nlp20-induced responses typically being slower than those mediated by flg22 and pep1. RNA sequencing showed the shared differentially expressed genes (DEGs) was mostly enriched in defense response, whereas nlp20-regulated genes represent only a fraction of those genes differentially regulated by flg22 and pep1. The three elicitors all inhibited primary root growth, especially pep1, which inhibited both auxin transport and signaling pathway. In addition, pep1 significantly inhibited the cell division and genes involved in cell cycle. Compared with flg22 and nlp20, pep1 induced much stronger expression of its receptor in roots, suggesting a potential positive feedback regulation in the activation of immune response. Despite PRRs and their co-receptor BAK1 were necessary for both PAMP induced immune response and root growth inhibition, bik1 mutant only showed impaired defense response but relatively normal root growth inhibition, suggesting BIK1 acts differently in these two biological processes.

Ambivalent response in pathogen defense: A double-edged sword?

Plants possess effective immune systems that defend against most microbial attackers. Recent plant immunity research has focused on the classic binary defense model involving the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our current understanding comes from studies that relied on information derived from a limited number of pathosystems, newer studies concerning the incredibly diverse interactions between plants and microbes are providing additional insights into other novel mechanisms. Here, we review the roles of both classical and more recently identified components of defense signaling pathways and stress hormones in regulating the ambivalence effect during responses to diverse pathogens. Because of their different lifestyles, effective defense against biotrophic pathogens normally leads to increased susceptibility to necrotrophs, and vice versa. Given these opposing forces, the plant potentially faces a trade-off when it mounts resistance to a specific pathogen, a phenomenon referred to here as the ambivalence effect. We also highlight a novel mechanism by which translational control of the proteins involved in the ambivalence effect can be used to engineer durable and broad-spectrum disease resistance, regardless of the lifestyle of the invading pathogen.

Dorsal switch protein 1 as a damage signal in insect gut immunity to activate dual oxidase via an eicosanoid, PGE2

Various microbiota including beneficial symbionts reside in the insect gut. Infections of pathogens cause dysregulation of the microflora and threaten insect survival. Reactive oxygen species (ROS) have been used in the gut immune responses, in which its production is tightly regulated by controlling dual oxidase (Duox) activity via Ca2+ signal to protect beneficial microflora and gut epithelium due to its high cytotoxicity. However, it was not clear how the insects discriminate the pathogens from the various microbes in the gut lumen to trigger ROS production. An entomopathogenic nematode (Steinernema feltiae) infection elevated ROS level in the gut lumen of a lepidopteran insect, Spodoptera exigua. Dorsal switch protein 1 (DSP1) localized in the nucleus in the midgut epithelium was released into plasma upon the nematode infection and activated phospholipase A2 (PLA2). The activated PLA2 led to an increase of PGE2 level in the midgut epithelium, in which rising Ca2+ signal up-regulated ROS production. Inhibiting DSP1 release by its specific RNA interference (RNAi) or specific inhibitor, 3-ethoxy-4-methoxyphenol, treatment failed to increase the intracellular Ca2+ level and subsequently prevented ROS production upon the nematode infection. A specific PLA2 inhibitor treatment also prevented the up-regulation of Ca2+ and subsequent ROS production upon the nematode infection. However, the addition of PGE2 to the inhibitor treatment rescued the gut immunity. DSP1 release was not observed at infection with non-pathogenic pathogens but detected in plasma with pathogenic infections that would lead to damage to the gut epithelium. These results indicate that DSP1 acts as a damage-associated molecular pattern in gut immunity through DSP1/PLA2/Ca2+/Duox.

Identification of resistance gene analogs of the NBS-LRR family through transcriptome probing and in silico prediction of the expressome of Dalbergia sissoo under dieback disease stress

Dalbergia sissoo is an important timber tree, and dieback disease poses a dire threat to it toward extinction. The genomic record of D. sissoo is not available yet on any database; that is why it is challenging to probe the genetic elements involved in stress resistance. Hence, we attempted to unlock the genetics involved in dieback resistance through probing the NBS-LRR family, linked with mostly disease resistance in plants. We analyzed the transcriptome of D. sissoo under dieback challenge through DOP-rtPCR analysis using degenerate primers from conserved regions of NBS domain-encoded gene sequences. The differentially expressed gene sequences were sequenced and in silico characterized for predicting the expressome that contributes resistance to D. sissoo against dieback. The molecular and bioinformatic analyses predicted the presence of motifs including ATP/GTP-binding site motif A (P-loop NTPase domain), GLPL domain, casein kinase II phosphorylation site, and N-myristoylation site that are the attributes of proteins encoded by disease resistance genes. The physicochemical characteristics of identified resistance gene analogs, subcellular localization, predicted protein fingerprints, in silico functional annotation, and predicted protein structure proved their role in disease and stress resistance.

Activation of plant immunity by exposure to dinitrogen pentoxide gas generated from air using plasma technology

Reactive nitrogen species (RNS) play an important role in plant immunity as signaling factors. We previously developed a plasma technology to partially convert air molecules into dinitrogen pentoxide (N₂O₅), an RNS whose physiological action is poorly understood. To reveal the function of N₂O₅ gas in plant immunity, Arabidopsis thaliana was exposed to plasma-generated N₂O₅ gas once (20 s) per day for 3 days, and inoculated with Botrytis cinerea, Pseudomonas syringae pv. tomato DC3000 (Pst), or cucumber mosaic virus strain yellow (CMV(Y)) at 24 h after the final N₂O₅ gas exposure. Lesion size with B. cinerea infection was significantly (P < 0.05) reduced by exposure to N₂O₅ gas. Propagation of CMV(Y) was suppressed in plants exposed to N₂O₅ gas compared with plants exposed to the air control. However, proliferation of Pst in the N₂O₅-gas-exposed plants was almost the same as in the air control plants. These results suggested that N₂O₅ gas exposure could control plant disease depending on the type of pathogen. Furthermore, changes in gene expression at 24 h after the final N₂O₅ gas exposure were analyzed by RNA-Seq. Based on the gene ontology analysis, jasmonic acid and ethylene signaling pathways were activated by exposure of Arabidopsis plants to N₂O₅ gas. A time course experiment with qRT-PCR revealed that the mRNA expression of the transcription factor genes, WRKY25, WRKY26, WRKY33, and genes for tryptophan metabolic enzymes, CYP71A12, CYP71A13, PEN2, and PAD3, was transiently induced by exposure to N₂O₅ gas once for 20 s peaking at 1–3 h post-exposure. However, the expression of PDF1.2 was enhanced beginning from 6 h after exposure and its high expression was maintained until 24–48 h later. Thus, enhanced tryptophan metabolism leading to the synthesis of antimicrobial substances such as camalexin and antimicrobial peptides might have contributed to the N₂O₅-gas-induced disease resistance.

Plant immunity by damage-associated molecular patterns (DAMPs)

Recognition by plant receptors of microbe-associated molecular patterns (MAMPs) and pathogenicity effectors activates immunity. However, before evolving the capacity of perceiving and responding to MAMPs and pathogenicity factors, plants, like animals, must have faced the necessity to protect and repair the mechanical wounds used by pathogens as an easy passage into their tissue. Consequently, plants evolved the capacity to react to damage-associated molecular patterns (DAMPs) with responses capable of functioning also in the absence of pathogens. DAMPs include not only primarily cell wall (CW) fragments but also extracellular peptides, nucleotides and amino acids that activate both local and long-distance systemic responses and, in some cases, prime the subsequent responses to MAMPs. It is conceivable that DAMPs and MAMPs act in synergy to activate a stronger plant immunity and that MAMPs exploit the mechanisms and transduction pathways traced by DAMPs. The interest for the biology and mechanism of action of DAMPs, either in the plant or animal kingdom, is expected to substantially increase in the next future. This review focuses on the most recent advances in DAMPs biology, particularly in the field of CW-derived DAMPs.

HMGB1-Like Dorsal Switch Protein 1 Triggers a Damage Signal in Mosquito Gut to Activate Dual Oxidase via Eicosanoids

Several mosquitoes transmit human pathogens by blood feeding, with the gut being the main entrance for the pathogens. Thus, the gut epithelium defends the pathogens by eliciting potent immune responses. However, it was unclear how the mosquito gut discriminates pathogens among various microflora in the lumen. This study proposed a hypothesis that a damage signal might be specifically induced by pathogens in the gut. The Asian tiger mosquito, Aedes albopictus, encodes dorsal switch protein 1 (Aa-DSP1) as a putative damage-associated molecular pattern (DAMP). Aa-DSP1 was localized in the nucleus of the midgut epithelium in naïve larvae. Upon infection by a pathogenic bacterium, Serratia marcescens, Aa-DSP1 was released to hemocoel and activated phospholipase A2 (PLA2). The activated PLA2 increased the level of prostaglandin E2 (PGE2) in the gut and subsequently increased Ca2+ signal to produce reactive oxygen species (ROS) via dual oxidase (Duox). Inhibition of Aa-DSP1 via RNA interference or specific inhibitor treatment failed to increase PGE2/Ca2+ signal upon the bacterial infection. Thus, the inhibitors specifically targeting eicosanoid biosynthesis significantly prevented the upregulation of ROS production in the gut and enhanced mosquito mortality after the bacterial infection. However, such inhibitory effects were rescued by adding PGE2. These suggest that Aa-DSP1 plays an important role in immune response of the mosquito gut as a DAMP during pathogen infection by triggering a signaling pathway, DSP1/PLA2/Ca2+/Duox.

Fungal oxysterol-binding protein-related proteins promote pathogen virulence and activate plant immunity

Oxysterol-binding protein-related proteins (ORPs) are a conserved class of lipid transfer proteins that are closely involved in multiple cellular processes in eukaryotes, but their roles in plant-pathogen interactions are mostly unknown. We show that transient expression of ORPs of Magnaporthe oryzae (MoORPs) in Nicotiana benthamina plants triggered oxidative bursts and cell death; treatment of tobacco Bright Yellow-2 suspension cells with recombinant MoORPs elicited the production of reactive oxygen species. Despite ORPs being normally described as intracellular proteins, we detected MoORPs in fungal culture filtrates and intercellular fluids from barley plants infected with the fungus. More importantly, infiltration of Arabidopsis plants with recombinant Arabidopsis or fungal ORPs activated oxidative bursts, callose deposition, and PR1 gene expression, and enhanced plant disease resistance, implying that ORPs may function as endogenous and exogenous danger signals triggering plant innate immunity. Extracellular application of fungal ORPs exerted an opposite impact on salicylic acid and jasmonic acid/ethylene signaling pathways. Brassinosteroid Insensitive 1-associated Kinase 1 was dispensable for the ORP-activated defense. Besides, simultaneous knockout of MoORP1 and MoORP3 abolished fungal colony radial growth and conidiation, whereas double knockout of MoORP1 and MoORP2 compromised fungal virulence on barley and rice plants. These observations collectively highlight the multifaceted role of MoORPs in the modulation of plant innate immunity and promotion of fungal development and virulence in M. oryzae.

Salicylic Acid in Root Growth and Development

In plants, salicylic acid (SA) is a hormone that mediates a plant’s defense against pathogens. SA also takes an active role in a plant’s response to various abiotic stresses, including chilling, drought, salinity, and heavy metals. In addition, in recent years, numerous studies have confirmed the important role of SA in plant morphogenesis. In this review, we summarize data on changes in root morphology following SA treatments under both normal and stress conditions. Finally, we provide evidence for the role of SA in maintaining the balance between stress responses and morphogenesis in plant development, and also for the presence of SA crosstalk with other plant hormones during this process.

An Anecdote on Prospective Protein Targets for Developing Novel Plant Growth Regulators

Phytohormones are the main regulatory molecules of core signalling networks associated with plant life cycle regulation. Manipulation of hormone signalling cascade enables the control over physiological traits of plant, which has major applications in field of agriculture and food sustainability. Hence, stable analogues of these hormones are long sought after and many of them are currently known, but the quest for more effective, stable and economically viable analogues is still going on. This search has been further strengthened by the identification of the components of signalling cascade such as receptors, downstream cascade members and transcription factors. Furthermore, many proteins of phytohormone cascades are available in crystallized forms. Such crystallized structures can provide the basis for identification of novel interacting compounds using in silico approach. Plenty of computational tools and bioinformatics software are now available that can aid in this process. Here, the metadata of all the major phytohormone signalling cascades are presented along with discussion on major protein-ligand interactions and protein components that may act as a potential target for manipulation of phytohormone signalling cascade. Furthermore, structural aspects of phytohormones and their known analogues are also discussed that can provide the basis for the synthesis of novel analogues.

Damage-Associated Molecular Patterns (DAMPs) in Plant Innate Immunity: Applying the Danger Model and Evolutionary Perspectives

Danger signals trigger immune responses upon perception by a complex surveillance system. Such signals can originate from the infectious nonself or the damaged self, the latter termed damage-associated molecular patterns (DAMPs). Here, we apply Matzinger's danger model to plant innate immunity to discuss the adaptive advantages of DAMPs and their integration into preexisting signaling pathways. Constitutive DAMPs (cDAMPs), e.g., extracellular ATP, histones, and self-DNA, fulfill primary, conserved functions and adopt a signaling role only when cellular damage causes their fragmentation or localization to aberrant compartments. By contrast, immunomodulatory peptides (also known as phytocytokines) exclusively function as signals and, upon damage, are activated as inducible DAMPs (iDAMPs). Dynamic coevolutionary processes between the signals and their emerging receptors and shared co-receptors have likely linked danger recognition to preexisting, conserved downstream pathways.

Signaling Pathways and Downstream Effectors of Host Innate Immunity in Plants

Phytopathogens, such as biotrophs, hemibiotrophs and necrotrophs, pose serious stress on the development of their host plants, compromising their yields. Plants are in constant interaction with such phytopathogens and hence are vulnerable to their attack. In order to counter these attacks, plants need to develop immunity against them. Consequently, plants have developed strategies of recognizing and countering pathogenesis through pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). Pathogen perception and surveillance is mediated through receptor proteins that trigger signal transduction, initiated in the cytoplasm or at the plasma membrane (PM) surfaces. Plant hosts possess microbe-associated molecular patterns (P/MAMPs), which trigger a complex set of mechanisms through the pattern recognition receptors (PRRs) and resistance (R) genes. These interactions lead to the stimulation of cytoplasmic kinases by many phosphorylating proteins that may also be transcription factors. Furthermore, phytohormones, such as salicylic acid, jasmonic acid and ethylene, are also effective in triggering defense responses. Closure of stomata, limiting the transfer of nutrients through apoplast and symplastic movements, production of antimicrobial compounds, programmed cell death (PCD) are some of the primary defense-related mechanisms. The current article highlights the molecular processes involved in plant innate immunity (PII) and discusses the most recent and plausible scientific interventions that could be useful in augmenting PII.

The novel peptide NbPPI1 identified from Nicotiana benthamiana triggers immune responses and enhances resistance against Phytophthora pathogens

In plants, recognition of small secreted peptides, such as damage/danger-associated molecular patterns (DAMPs), regulates diverse processes, including stress and immune responses. Here, we identified an SGPS (Ser-Gly-Pro-Ser) motif-containing peptide, Nicotiana tabacum NtPROPPI, and its two homologs in Nicotiana benthamiana, NbPROPPI1 and NbPROPPI2. Phytophthora parasitica infection and salicylic acid (SA) treatment induced NbPROPPI1/2 expression. Moreover, SignalP predicted that the 89-amino acid NtPROPPI includes a 24-amino acid N-terminal signal peptide and NbPROPPI1/2-GFP fusion proteins were mainly localized to the periplasm. Transient expression of NbPROPPI1/2 inhibited P. parasitica colonization, and NbPROPPI1/2 knockdown rendered plants more susceptible to P. parasitica. An eight-amino-acid segment in the NbPROPPI1 C-terminus was essential for its immune function and a synthetic 20-residue peptide, NbPPI1, derived from the C-terminus of NbPROPPI1 provoked significant immune responses in N. benthamiana. These responses led to enhanced accumulation of reactive oxygen species, activation of mitogen-activated protein kinases, and up-regulation of the defense genes Flg22-induced receptor-like kinase (FRK) and WRKY DNA-binding protein 33 (WRKY33). The NbPPI1-induced defense responses require Brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1). These results suggest that NbPPI1 functions as a DAMP in N. benthamiana; this novel DAMP provides a potentially useful target for improving plant resistance to Pytophthora pathogens.

Overexpression of OsHMGB707, a High Mobility Group Protein, Enhances Rice Drought Tolerance by Promoting Stress-Related Gene Expression

Drought stress adversely affects crop growth and productivity worldwide. In response, plants have evolved several strategies in which numerous genes are induced to counter stress. High mobility group (HMG) proteins are the second most abundant family of chromosomal proteins. They play a crucial role in gene transcriptional regulation by modulating the chromatin/DNA structure. In this study, we isolated a novel HMG gene, OsHMGB707, one of the candidate genes localized in the quantitative trait loci (QTL) interval of rice drought tolerance, and examined its function on rice stress tolerance. The expression of OsHMGB707 was up-regulated by dehydration and high salt treatment. Its overexpression significantly enhanced drought tolerance in transgenic rice plants, whereas its knockdown through RNA interference (RNAi) did not affect the drought tolerance of the transgenic rice plants. Notably, OsHMGB707-GFP is localized in the cell nucleus, and OsHMGB707 is protein-bound to the synthetic four-way junction DNA. Several genes were up-regulated in OsHMGB707-overexpression (OE) rice lines compared to the wild-type rice varieties. Some of the genes encode stress-related proteins (e.g., DREB transcription factors, heat shock protein 20, and heat shock protein DnaJ). In summary, OsHMGB707 encodes a stress-responsive high mobility group protein and regulates rice drought tolerance by promoting the expression of stress-related genes.

Circular RNA 0001313 Knockdown Suppresses Non-Small Cell Lung Cancer Cell Proliferation and Invasion via the microRNA-452/HMGB3/ERK/MAPK Axis

Background

Non-small cell lung cancer (NSCLC) seriously endangers human health. Circular RNAs (circRNAs) regulate diverse types of cancers, including NSCLC. This study investigated the possible mechanism of circ0001313 in NSCLC.

Materials and Methods

Circ0001313 expression in NSCLC tissues was measured, and its correlation with clinicopathological features was analyzed. The binding relationships among circ0001313, microRNA (miR)-452 and HMGB3 were tested. The gain and loss of functions were performed to examine NSCLC cell malignant behaviors. After HMGB3 overexpression, ERK/MAPK pathway-related protein levels were detected. Subsequently, the rescue experiment was further performed using an ERK/MAPK pathway inhibitor PD98059.

Results

Abnormally elevated circ0001313 and decreased miR-452 in NSCLC cells were observed. Circ0001313 silencing or miR-452 overexpression significantly reduced NSCLC cell proliferation and invasion. Circ0001313 competitively bound to miR-452 to upregulate HMGB3, thus promoting NSCLC cell growth. HMGB3 overexpression activated the ERK/MAPK pathway to contribute to NSCLC development.

Conclusion

We highlighted that silencing of circ0001313 blunted the ERK/MAPK pathway via the miR-452/HMGB3 axis, thereby inhibiting NSCLC cell proliferation and invasion.

Breaking Bad News: Dynamic Molecular Mechanisms of Wound Response in Plants

Recognition and repair of damaged tissue are an integral part of life. The failure of cells and tissues to appropriately respond to damage can lead to severe dysfunction and disease. Therefore, it is essential that we understand the molecular pathways of wound recognition and response. In this review, we aim to provide a broad overview of the molecular mechanisms underlying the fate of damaged cells and damage recognition in plants. Damaged cells release the so-called damage associated molecular patterns to warn the surrounding tissue. Local signaling through calcium (Ca2+), reactive oxygen species (ROS), and hormones, such as jasmonic acid, activates defense gene expression and local reinforcement of cell walls to seal off the wound and prevent evaporation and pathogen colonization. Depending on the severity of damage, Ca2+, ROS, and electrical signals can also spread throughout the plant to elicit a systemic defense response. Special emphasis is placed on the spatiotemporal dimension in order to obtain a mechanistic understanding of wound signaling in plants.

Non-steroidal Anti-inflammatory Drugs Target TWISTED DWARF1-Regulated Actin Dynamics and Auxin Transport-Mediated Plant Development

The widely used non-steroidal anti-inflammatory drugs (NSAIDs) are derivatives of the phytohormone salicylic acid (SA). SA is well known to regulate plant immunity and development, whereas there have been few reports focusing on the effects of NSAIDs in plants. Our studies here reveal that NSAIDs exhibit largely overlapping physiological activities to SA in the model plant Arabidopsis. NSAID treatments lead to shorter and agravitropic primary roots and inhibited lateral root organogenesis. Notably, in addition to the SA-like action, which in roots involves binding to the protein phosphatase 2A (PP2A), NSAIDs also exhibit PP2A-independent effects. Cell biological and biochemical analyses reveal that many NSAIDs bind directly to and inhibit the chaperone activity of TWISTED DWARF1, thereby regulating actin cytoskeleton dynamics and subsequent endosomal trafficking. Our findings uncover an unexpected bioactivity of human pharmaceuticals in plants and provide insights into the molecular mechanism underlying the cellular action of this class of anti-inflammatory compounds.

Deciphering the Binding of Salicylic Acid to Arabidopsis thaliana Chloroplastic GAPDH-A1

Salicylic acid (SA) has an essential role in the responses of plants to pathogens. SA initiates defence signalling via binding to proteins. NPR1 is a transcriptional co-activator and a key target of SA binding. Many other proteins have recently been shown to bind SA. Amongst these proteins are important enzymes of primary metabolism. This fact could stand behind SA’s ability to control energy fluxes in stressed plants. Nevertheless, only sparse information exists on the role and mechanisms of such binding. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was previously demonstrated to bind SA both in human and plants. Here, we detail that the A1 isomer of chloroplastic glyceraldehyde 3-phosphate dehydrogenase (GAPA1) from Arabidopsis thaliana binds SA with a K_D of 16.7 nM, as shown in surface plasmon resonance experiments. Besides, we show that SA inhibits its GAPDH activity in vitro. To gain some insight into the underlying molecular interactions and binding mechanism, we combined in silico molecular docking experiments and molecular dynamics simulations on the free protein and protein–ligand complex. The molecular docking analysis yielded to the identification of two putative binding pockets for SA. A simulation in water of the complex between SA and the protein allowed us to determine that only one pocket—a surface cavity around Asn35—would efficiently bind SA in the presence of solvent. In silico mutagenesis and simulations of the ligand/protein complexes pointed to the importance of Asn35 and Arg81 in the binding of SA to GAPA1. The importance of this is further supported through experimental biochemical assays. Indeed, mutating GAPA1 Asn35 into Gly or Arg81 into Leu strongly diminished the ability of the enzyme to bind SA. The very same cavity is responsible for the NADP⁺ binding to GAPA1. More precisely, modelling suggests that SA binds to the very site where the pyrimidine group of the cofactor fits. NADH inhibited in a dose-response manner the binding of SA to GAPA1, validating our data.

Overexpression of CsHMGB Alleviates Phytotoxicity and Propamocarb Residues in Cucumber

Cucumber (Cucumis sativus L.) is one of the most economically important fruits of the Cucurbitaceae family, therefore consideration of potential pesticide residues in the fruit in the context of cucumber breeding and production programs is important. Propamocarb (a pesticide commonly used to prevent downy mildew) is widely used in cucumber cultivation, but the molecular mechanism underlying the degradation and metabolism of propamocarb in cucumber is not well understood. We screened a candidate CsHMGB gene (CsaV3-5G28190) for response to propamocarb exposure using transcriptome data. The coding region of CsHMGB was 624 bp in length and encoded the conserved HMB-box region. CsHMGB expression differed significantly between the "D0351" genotype, which accumulated low levels of propamocarb, and the "D9320" genotype, which accumulated high levels of propamocarb. CsHMGB expression was positively correlated with propamocarb levels in the cucumber peel. CsHMGB expression was upregulated in the fruit peels of the "D0351" genotype following exposure to propamocarb stress for 3-120 h, but no difference was observed in expression between propamocarb treatment and control for the "D9320" genotype. For the "D0351" genotype, CsHMGB expression was higher in the fruit peels and leaves than that in female flowers; expression was moderate in the stems and fruit pulps, and weak in male flowers and roots. The CsHMGB protein was targeted to the nucleus in Arabidopsis protoplasts and in the epidermis of Nicotiana benthamiana leaves. We measured MDA, O₂ ^-, and H₂O₂ levels in cucumber plants and found that they were likely to accumulate reactive oxygen species (ROS) in response to propamocarb stress. Analysis of antioxidant enzyme activity (SOD, POD, CAT, APX, GPX, GST, and GR) and the ascorbate-glutathione (AsA-GSH) system showed that the resistance of the plants was reduced and the levels of propamocarb residue was increased in CsHMGB-silenced plants in response to propamocarb stress. Conversely, overexpression of CsHMGB promoted glutathione-dependent detoxification by AsA-GSH system and improved the antioxidant potential, reduced the accumulation of ROS. Ultimately, the metabolism of propamocarb in cucumber was increased via increase in the wax levels and the stomatal conductance.

In Vitro Analytical Approaches to Study Plant Ligand-Receptor Interactions

State-of-the-art in vitro methods characterize receptor-ligand interactions, highlighting experiment strategies, advantages and limitations.

Perception of Damaged Self in Plants

Plants use specific receptor proteins on the cell surface to detect host-derived danger signals released in response to attacks by pathogens or herbivores and activate immune responses against them.

Exogenous Treatment with Glutamate Induces Immune Responses in Arabidopsis

Plant resistance inducers (PRIs) are compounds that protect plants from diseases by activating immunity responses. Exogenous treatment with glutamate (Glu), an important amino acid for all living organisms, induces resistance against fungal pathogens in rice and tomato. To understand the molecular mechanisms of Glu-induced immunity, we used the Arabidopsis model system. We found that exogenous treatment with Glu induces resistance against pathogens in Arabidopsis. Consistent with this, transcriptome analyses of Arabidopsis seedlings showed that Glu significantly induces the expression of wound-, defense-, and stress-related genes. Interestingly, Glu activates the expression of genes induced by pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns at much later time points than the flg22 peptide, which is a bacterial-derived PAMP. The Glu receptor-like (GLR) proteins GLR3.3 and GLR3.6 are involved in the early expression of Glu-inducible genes; however, the sustained expression of these genes does not require the GLR proteins. Glu-inducible gene expression is also not affected by mutations in genes that encode PAMP receptors (EFR, FLS2, and CERK1), regulators of pattern-triggered immunity (BAK1, BKK1, BIK1, and PBL1), or a salicylic acid biosynthesis enzyme (SID2). The treatment of roots with Glu activates the expression of PAMP-, salicylic acid-, and jasmonic acid-inducible genes in leaves. Moreover, the treatment of roots with Glu primes chitin-induced responses in leaves, possibly through transcriptional activation of LYSIN-MOTIF RECEPTOR-LIKE KINASE 5 (LYK5), which encodes a chitin receptor. Because Glu treatment does not cause discernible growth retardation, Glu can be used as an effective PRI.

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1) releases latent defense signals in stems with reduced lignin content

There is considerable interest in engineering plant cell wall components, particularly lignin, to improve forage quality and biomass properties for processing to fuels and bioproducts. However, modifying lignin content and/or composition in transgenic plants through down-regulation of lignin biosynthetic enzymes can induce expression of defense response genes in the absence of biotic or abiotic stress. Arabidopsis thaliana lines with altered lignin through down-regulation of hydroxycinnamoyl CoA:shikimate/quinate hydroxycinnamoyl transferase (HCT) or loss of function of cinnamoyl CoA reductase 1 (CCR1) express a suite of pathogenesis-related (PR) protein genes. The plants also exhibit extensive cell wall remodeling associated with induction of multiple cell wall-degrading enzymes, a process which renders the corresponding biomass a substrate for growth of the cellulolytic thermophile Caldicellulosiruptor bescii lacking a functional pectinase gene cluster. The cell wall remodeling also results in the release of size- and charge-heterogeneous pectic oligosaccharide elicitors of PR gene expression. Genetic analysis shows that both in planta PR gene expression and release of elicitors are the result of ectopic expression in xylem of the gene ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1), which is normally expressed during anther and silique dehiscence. These data highlight the importance of pectin in cell wall integrity and the value of lignin modification as a tool to interrogate the informational content of plant cell walls.

Salicylic Acid Targets Protein Phosphatase 2A to Attenuate Growth in Plants

Plants, like other multicellular organisms, survive through a delicate balance between growth and defense against pathogens. Salicylic acid (SA) is a major defense signal in plants, and the perception mechanism as well as downstream signaling activating the immune response are known. Here, we identify a parallel SA signaling that mediates growth attenuation. SA directly binds to A subunits of protein phosphatase 2A (PP2A), inhibiting activity of this complex. Among PP2A targets, the PIN2 auxin transporter is hyperphosphorylated in response to SA, leading to changed activity of this important growth regulator. Accordingly, auxin transport and auxin-mediated root development, including growth, gravitropic response, and lateral root organogenesis, are inhibited. This study reveals how SA, besides activating immunity, concomitantly attenuates growth through crosstalk with the auxin distribution network. Further analysis of this dual role of SA and characterization of additional SA-regulated PP2A targets will provide further insights into mechanisms maintaining a balance between growth and defense.

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SA modulates root development independently of NPR1-mediated canonical signaling

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SA attenuates growth through crosstalk with the auxin transport network

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SA upregulates the phosphorylation status of PIN auxin efflux carriers through PP2A

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SA directly targets A subunits of PP2A, inhibiting the activity of the complex

Disrupting the Homeostasis of High Mobility Group Protein Promotes the Systemic Movement of Bamboo mosaic virus

Viruses hijack various organelles and machineries for their replication and movement. Ever more lines of evidence indicate that specific nuclear factors are involved in systemic trafficking of several viruses. However, how such factors regulate viral systemic movement remains unclear. Here, we identify a novel role for Nicotiana benthamiana high mobility group nucleoprotein (NbHMG1/2a) in virus movement. Although infection of N. benthamiana with Bamboo mosaic virus (BaMV) decreased NbHMG1/2a expression levels, nuclear-localized NbHMG1/2a protein was shuttled out of the nucleus into cytoplasm upon BaMV infection. NbHMG1/2a knockdown or even overexpression did not affect BaMV accumulation in inoculated leaves, but it did enhance systemic movement of the virus. Interestingly, the positive regulator Rap-GTPase activation protein 1 was highly upregulated upon infection with BaMV, whereas the negative regulator thioredoxin h protein was greatly reduced, no matter if NbHMG1a/2a was silenced or overexpressed. Our findings indicate that NbHMG1/2a may have a role in plant defense responses. Once its homeostasis is disrupted, expression of relevant host factors may be perturbed that, in turn, facilitates BaMV systemic movement.

Phosphoproteomics Profiling of Cotton (Gossypium hirsutum L.) Roots in Response to Verticillium dahliae Inoculation

Verticillium wilt is a plant vascular disease causing severe yield and quality losses in many crops and is caused by the soil-borne plant pathogenic fungus Verticillium dahliae. To investigate the molecular mechanisms of the cotton-V. dahliae interaction, a time-course phosphoproteomic analysis of roots of susceptible and resistant cotton lines in response to V. dahliae was performed. In total, 1716 unique phosphoproteins were identified in the susceptible (S) and resistant (R) cotton lines. Of these, 359 phosphoproteins were significantly different in R1 (1 day after V. dahliae inoculation) vs R0 (mock) group and 287 phosphoproteins in R2 (3 days after V. dahliae inoculation) vs R0 group. Moreover, 263 proteins of V. dahliae-regulated phosphoproteins were significantly changed in S1 (1 day after V. dahliae inoculation) vs S0 (mock) group and 197 proteins in S2 (3 days after V. dahliae inoculation) vs S0 group. Thirty phosphoproteins were significantly changed and common to the resistant and susceptible cotton lines following inoculation with V. dahliae. Specifically, 92 phosphoproteins were shared in both in R1 vs R0 and R2 vs R0 but not in susceptible cotton lines. There were 38 common phosphoproteins shared in both S1 vs S0 and S2 vs S0 but not in resistant cotton lines. GO terms and Kyoto Encyclopedia of Genes and Genomes pathway analyses displayed an abundance of known and novel phosphoproteins related to plant-pathogen interactions, signal transduction, and metabolic processes, which were correlated with resistance against fungal infection. These data provide new perspectives and inspiration for understanding molecular defense mechanisms of cotton roots against V. dahliae infection.

Salicylic Acid Binding Proteins (SABPs): The Hidden Forefront of Salicylic Acid Signalling

Salicylic acid (SA) is a phytohormone that plays important roles in many aspects of plant life, notably in plant defenses against pathogens. Key mechanisms of SA signal transduction pathways have now been uncovered. Even though details are still missing, we understand how SA production is regulated and which molecular machinery is implicated in the control of downstream transcriptional responses. The NPR1 pathway has been described to play the main role in SA transduction. However, the mode of SA perception is unclear. NPR1 protein has been shown to bind SA. Nevertheless, NPR1 action requires upstream regulatory events (such as a change in cell redox status). Besides, a number of SA-induced responses are independent from NPR1. This shows that there is more than one way for plants to perceive SA. Indeed, multiple SA-binding proteins of contrasting structures and functions have now been identified. Yet, all of these proteins can be considered as candidate SA receptors and might have a role in multinodal (decentralized) SA input. This phenomenon is unprecedented for other plant hormones and is a point of discussion of this review.

In Silico Phylogenetic and Structural Analyses of Plant Endogenous Danger Signaling Molecules upon Stress

The plant innate immune system has two major branches, the pathogen-triggered immunity and the effector-triggered immunity (ETI). The effectors are molecules released by plant attackers to evade host immunity. In addition to the foreign intruders, plants possess endogenous instigators produced in response to general cellular injury termed as damage-associated molecular patterns (DAMPs). In plants, DAMPs or alarmins are released by damaged, stressed, or dying cells following abiotic stress such as radiation, oxidative and drought stresses. In turn, a cascade of downstream signaling events is initiated leading to the upregulation of defense or response-related genes. In the present study, we have investigated more thoroughly the conservation status of the molecular mechanisms implicated in the danger signaling primarily in plants. Towards this direction, we have performed in silico phylogenetic and structural analyses of the associated biomolecules in taxonomically diverse plant species. On the basis of our results, the defense mechanisms appear to be largely conserved within the plant kingdom. Of note, the sequence and/or function of several components of these mechanisms was found to be conserved in animals, as well. At the same time, the molecules involved in plant defense were found to form a dense protein-protein interaction (PPi) network, suggesting a crosstalk between the various defense mechanisms to a variety of stresses, like oxidative stress.

Cytoskeletal Exposure in the Regulation of Immunity and Initiation of Tissue Repair

This article reviews and discusses emerging evidence suggesting an evolutionarily-conserved connection between injury-associated exposure of cytoskeletal proteins and the induction of tolerance to infection, repair of tissue damage and restoration of homeostasis. While differences exist between vertebrates and invertebrates with respect to the receptor(s), cell types, and effector mechanisms involved, the response to exposed cytoskeletal proteins appears to be protective and to rely on a conserved signaling cassette involving Src family kinases, the nonreceptor tyrosine kinase Syk, and tyrosine phosphatases. A case is made for research programs that integrate different model organisms in order to increase the understanding of this putative response to tissue damage.

Damage-Associated Molecular Pattern-Triggered Immunity in Plants

As a universal process in multicellular organisms, including animals and plants, cells usually emit danger signals when suffering from attacks of microbes and herbivores, or physical damage. These signals, termed as damage-associated molecular patterns (DAMPs), mainly include cell wall or extracellular protein fragments, peptides, nucleotides, and amino acids. Once exposed on cell surfaces, DAMPs are detected by plasma membrane-localized receptors of surrounding cells to regulate immune responses against the invading organisms and promote damage repair. DAMPs may also act as long-distance mobile signals to mediate systemic wounding responses. Generation, release, and perception of DAMPs, and signaling events downstream of DAMP perception are all rigorously modulated by plants. These processes integrate together to determine intricate mechanisms of DAMP-triggered immunity in plants. In this review, we present an extensive overview on our current understanding of DAMPs in plant immune system.

Nucleic Acid Sensing in Mammals and Plants: Facts and Caveats

The accumulation of nucleic acids in aberrant compartments is a signal of danger: fragments of cytosolic or extracellular self-DNA indicate cellular dysfunctions or disruption, whereas cytosolic fragments of nonself-DNA or RNA indicate infections. Therefore, nucleic acids trigger immunity in mammals and plants. In mammals, endosomal Toll-like receptors (TLRs) sense single-stranded (ss) or double-stranded (ds) RNA or CpG-rich DNA, whereas various cytosolic receptors sense dsDNA. Although a self/nonself discrimination could favor targeted immune responses, no sequence-specific sensing of nucleic acids has been reported for mammals. Specific immune responses to extracellular self-DNA versus DNA from related species were recently reported for plants, but the underlying mechanism remains unknown. The subcellular localization of mammalian receptors can favor self/nonself discrimination based on the localization of DNA fragments. However, autoantibodies and diverse damage-associated molecular patterns (DAMPs) shuttle DNA through membranes, and most of the mammalian receptors share downstream signaling elements such as stimulator of interferon genes (STING) and the master transcription regulators, nuclear factor (NF)-κB, and interferon regulatory factor 3 (IRF3). The resulting type I interferon (IFN) response stimulates innate immunity against multiple threats-from infection to physical injury or endogenous DNA damage-all of which lead to the accumulation of eDNA or cytoplasmatic dsDNA. Therefore, no or only low selective pressures might have favored a strict self/nonself discrimination in nucleic acid sensing. We conclude that the discrimination between self- and nonself-DNA is likely to be less strict-and less important-than assumed originally.

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

Pattern Recognition Receptors—Versatile Genetic Tools for Engineering Broad-Spectrum Disease Resistance in Crops

Infestations of crop plants with pathogens pose a major threat to global food supply. Exploiting plant defense mechanisms to produce disease-resistant crop varieties is an important strategy to control plant diseases in modern plant breeding and can greatly reduce the application of agrochemicals. The discovery of different types of immune receptors and a detailed understanding of their activation and regulation mechanisms in the last decades has paved the way for the deployment of these central plant immune components for genetic plant disease management. This review will focus on a particular class of immune sensors, termed pattern recognition receptors (PRRs), that activate a defense program termed pattern-triggered immunity (PTI) and outline their potential to provide broad-spectrum and potentially durable disease resistance in various crop species—simply by providing plants with enhanced capacities to detect invaders and to rapidly launch their natural defense program.

High-mobility group box 1 protein orchestrates responses to tissue damage via inflammation, innate and adaptive immunity, and tissue repair

A single protein, HMGB1, directs the triggering of inflammation, innate and adaptive immune responses, and tissue healing after damage. HMGB1 is the best characterized damage-associated molecular pattern (DAMP), proteins that are normally inside the cell but are released after cell death, and allow the immune system to distinguish between antigens that are dangerous or not. Notably, cells undergoing severe stress actively secrete HMGB1 via a dedicated secretion pathway: HMGB1 is relocated from the nucleus to the cytoplasm and then to secretory lysosomes or directly to the extracellular space. Extracellular HMGB1 (either released or secreted) triggers inflammation and adaptive immunological responses by switching among multiple oxidation states, which direct the mutually exclusive choices of different binding partners and receptors. Immune cells are first recruited to the damaged tissue and then activated; thereafter, HMGB1 supports tissue repair and healing, by coordinating the switch of macrophages to a tissue-healing phenotype, activation and proliferation of stem cells, and neoangiogenesis. Inevitably, HMGB1 also orchestrates the support of stressed but illegitimate tissues: tumors. Concomitantly, HMGB1 enhances the immunogenicity of mutated proteins in the tumor (neoantigens), promoting anti-tumor responses and immunological memory. Tweaking the activities of HMGB1 in inflammation, immune responses and tissue repair could bring large rewards in the therapy of multiple medical conditions, including cancer.

Pattern recognition receptors and signaling in plant-microbe interactions

Plants solely rely on innate immunity of each individual cell to deal with a diversity of microbes in the environment. Extracellular recognition of microbe- and host damage-associated molecular patterns leads to the first layer of inducible defenses, termed pattern-triggered immunity (PTI). In plants, pattern recognition receptors (PRRs) described to date are all membrane-associated receptor-like kinases or receptor-like proteins, reflecting the prevalence of apoplastic colonization of plant-infecting microbes. An increasing inventory of elicitor-active patterns and PRRs indicates that a large number of them are limited to a certain range of plant groups/species, pointing to dynamic and convergent evolution of pattern recognition specificities. In addition to common molecular principles of PRR signaling, recent studies have revealed substantial diversification between PRRs in their functions and regulatory mechanisms. This serves to confer robustness and plasticity to the whole PTI system in natural infections, wherein different PRRs are simultaneously engaged and faced with microbial assaults. We review the functional significance and molecular basis of PRR-mediated pathogen recognition and disease resistance, and also an emerging role for PRRs in homeostatic association with beneficial or commensal microbes.

Sensing Danger: Key to Activating Plant Immunity

In both plants and animals, defense against pathogens relies on a complex surveillance system for signs of danger. Danger signals may originate from the infectious agent or from the host itself. Immunogenic plant host factors can be roughly divided into two categories: molecules which are passively released upon cell damage ('classical' damage-associated molecular patterns, DAMPs), and peptides which are processed and/or secreted upon infection to modulate the immune response (phytocytokines). We highlight the ongoing challenge to understand how plants sense various danger signals and integrate this information to produce an appropriate immune response to diverse challenges.