Review of innate and specific immunity in plants and animals

Amino acid mutation of succinate dehydrogenase complex induced resistance to benzovindiflupyr in Magnaporthe oryzae

Magnaporthe oryzae is a rice blast pathogen that seriously threatens rice yield. Benzovindiflupyr is a succinate dehydrogenase inhibitor (SDHI) fungicide that effectively controls many crop diseases. Benzovindiflupyr has a strong inhibitory effect on M. oryzae; however, control of rice blast by benzovindiflupyr and risk of resistance to benzovindiflupyr are not well studied in this pathogen. In this study, six benzovindiflupyr-resistant strains were obtained by domestication induced in the laboratory. The MoSdhBH245D mutation was the cause of M. oryzae resistance to benzovindiflupyr, which was verified through succinate dehydrogenase (SDH) activity assays, molecular docking, and site-specific mutations. Survival fitness analysis showed no significant difference between the benzovindiflupyr-resistant and parent strains. Positive cross-resistance to benzovindiflupyr and other SDHIs and negative cross-resistance to azoxystrobin were observed. Therefore, the risk of benzovindiflupyr resistance in M. oryzae might be medium to high. It should be combined with other classes of fungicides (tebuconazole and azoxystrobin) to slow the development of resistance.

A novel lectin receptor kinase gene, AtG-LecRK-I.2, enhances bacterial pathogen resistance through regulation of stomatal immunity in Arabidopsis

The S-locus lectin receptor kinases (G-LecRKs) have been suggested as receptors for microbe/damage-associated molecular patterns (MAMPs/DAMPs) and to be involved in the pathogen defense responses, but the functions of most G-LecRKs in biotic stress response have not been characterized. Here, we identified a member of this family, G-LecRK-I.2, that positively regulates flg22- and Pseudomonas syringae pv. tomato (Pst) DC3000-induced stomatal closure. G-LecRK-I.2 was rapidly phosphorylated under flg22 treatment and could interact with the FLS2/BAK1 complex. Two T-DNA insertion lines, glecrk-i.2-1 and glecrk-i.2-2, had lower levels of reactive oxygen species (ROS) and nitric oxide (NO) production in guard cells, as compared with the wild-type Col-0, under Pst DC3000 infection. Also, the immunity marker genes CBP60g and PR1 were induced at lower levels under Pst DC3000 hrcC- infection in glecrk-i.2-1 and glecrk-i.2-2. The GUS reporter system also revealed that G-LecRK-I.2 was expressed only in guard cells. We also found that G-LecRK-I.2 could interact H+-ATPase AHA1 to regulate H+-ATPase activity in the guard cells. Taken together, our results show that G-LecRK-I.2 plays an important role in regulating stomatal closure under flg22 and Pst DC3000 treatments and in ROS and NO signaling specifically in guard cells.

Differential Responses of Antioxidant Enzymes and Lignin Metabolism in Susceptible and Resistant Sweetpotato Cultivars during Root-Knot Nematode Infection

Root-knot nematodes (RKN) cause significant damage to sweetpotato plants and cause significant losses in yield and quality. Reactive oxygen species (ROS) play an important role in plant defenses, with levels of ROS-detoxifying antioxidant enzymes tightly regulated during pathogen infection. In this study, ROS metabolism was examined in three RKN-resistant and three RKN-susceptible sweetpotato cultivars. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were assessed, as was lignin-related metabolism. In RKN-infected roots, both resistant and susceptible cultivars increased SOD activity to produce higher levels of hydrogen peroxide (H₂O₂). However, H₂O₂ removal by CAT activity differed between cultivars, with susceptible cultivars having higher CAT activity and lower overall H₂O₂ levels. In addition, the expression of phenylpropanoid-related phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase genes, which encode enzymes involved in lignin metabolism, were higher in resistant cultivars, as were total phenolic and lignin contents. Enzyme activities and H₂O₂ levels were examined during the early (7 days) and late (28 days) phases of infection in representative susceptible and resistant cultivars, revealing contrasting changes in ROS levels and antioxidant responses in the different stages of infection. This study suggests that differences in antioxidant enzyme activities and ROS regulation in resistant and susceptible cultivars might explain reduced RKN infection in resistant cultivars, resulting in smaller RKN populations and overall higher resistance to infection and infestation by RKNs.

Beyond the List: Bioagent-Agnostic Signatures Could Enable a More Flexible and Resilient Biodefense Posture Than an Approach Based on Priority Agent Lists Alone

As of 2021, the biothreat policy and research communities organize their efforts around lists of priority agents, which elides consideration of novel pathogens and biotoxins. For example, the Select Agents and Toxins list is composed of agents that historic biological warfare programs had weaponized or that have previously caused great harm during natural outbreaks. Similarly, lists of priority agents promulgated by the World Health Organization and the National Institute of Allergy and Infectious Diseases are composed of previously known pathogens and biotoxins. To fill this gap, we argue that the research/scientific and biodefense/biosecurity communities should categorize agents based on how they impact their hosts to augment current list-based paradigms. Specifically, we propose integrating the results of multi-omics studies to identify bioagent-agnostic signatures (BASs) of disease-namely, patterns of biomarkers that accurately and reproducibly predict the impacts of infection or intoxication without prior knowledge of the causative agent. Here, we highlight three pathways that investigators might exploit as sources of signals to construct BASs and their applicability to this framework. The research community will need to forge robust interdisciplinary teams to surmount substantial experimental, technical, and data analytic challenges that stand in the way of our long-term vision. However, if successful, our functionality-based BAS model could present a means to more effectively surveil for and treat known and novel agents alike.

The Evolutionary Significance of Generalist Viruses with Special Emphasis on Plant Viruses and their Hosts

The host range of a virus is defined as the number of species a virus potentially infects. The specialist virus infects one or few related species while the generalist virus infects several different species, possibly in different families. Origin of generalist viruses from their specialist nature and the expansion of the host range of the generalist virus occur with the host shift event in which the virus encounters and adapts to a new host. Host shift events have resulted in the majority of the newly emerging viral diseases. This review discusses the advantages and disadvantages of generalist over specialist viruses and the unique features of plant viruses and their hosts that result in a higher incidence of generalist viruses in plants.

4-Hydroxy-3-methyl-2(1H)-quinolone, originally discovered from a Brassicaceae plant, produced by a soil bacterium of the genus Burkholderia sp.: determination of a preferred tautomer and antioxidant activity

4-Hydroxy-3-methyl-2(1H)-quinolone (1), a molecule known for a long time and recently discovered from a Brassicaceae plant Isatis tinctoria without providing sufficient evidence to support the structure, was isolated from a fermentation extract of Burkholderia sp. 3Y-MMP isolated from a soil by a Zn2+ enrichment culture. Detailed spectroscopic analyses by MS and NMR, combined with 13C chemical shift comparison with literature values of the related compounds and a synthetic preparation of 1, allowed its first full NMR characterization and identification of 2-quinolone but not 2-quinolinol (2) as the preferred tautomer for this heterocyclic system. While the metal-chelating activity was negligible, compound 1 at 10 μM, a concentration lower than that in liquid production cultures, quenched hydroxy radical-induced chemiluminescence emitted by luminol by 86%. Because some Burkholderia species are pathogenic to plants and animals, the above result suggests that 1 is a potential antioxidant to counteract reactive oxygen species-based immune response in the host organisms.

Sustainable Crop Protection, Global Climate Change, Food Security and Safety-Plant Immunity at the Crossroads

The development of novel strategies of plant disease management is crucial in view of the growing demand of sustainability in agri-food chains. The use of agrochemicals is not without risk for the consumer and environment in terms of their residues in food, feed, water bodies and harmful effects on nontarget organisms. However, because of the high global annual yield losses attributable to plant diseases and also due to global climate changes that have exacerbated some phytosanitary emergences, chemical input in agriculture is mandatory. In this complex scenario, the use of agrochemicals that boost the plant immune system represents a relatively novel approach in crop protection. These plant protection products are not antimicrobial or fungicidal agents, but include both natural and synthetic elicitors and plant activators that only target the host immune system, with no biocide mechanism of action. In general, these products present a number of strengths: they leave no residue and should not select resistant pathogen strains, they can be used to control virus diseases, and can increase the levels of bioactive phytochemicals in plant foods.

Perspectives on the utilization of resistance mechanisms from host and nonhost plants for durable protection of Brassica crops against Alternaria blight

BACKGROUND

Alternaria brassicae, the causal organism of Alternaria blight, is a necrotroph infecting crops of the Brassicaceae family at all growth stages. To circumvent this problem, several disease management strategies are being used in the field, and disease-resistant varieties have also been developed. However, no strategy has proven completely successful, owing to the high variability in virulence among A. brassicae isolates, which causes a diverse spectrum of symptoms. Nonhost resistance (NHR) is a robust and broad-spectrum defense mechanism available in plants, and the exploitation of gene pools from plant species that are nonhost to A. brassicae could serve as novel sources of resistance.

METHODOLOGY

We searched the literature using key words relevant to this study in various search engines, such as PubMed, Web of Science, and Google Scholar, as well as certain journal websites. The literature was retrieved, sorted, and mined to extract data pertinent to the present review.

RESULTS

In this review, we have comprehensively covered the recent progress made in developing Alternaria blight resistance in Brassica crops by exploiting host germplasm. We also enumerate the potential NHR sources available for A. brassicae and the NHR layers possibly operating against this pathogen. In addition, we propose different strategies for identifying NHR-related genes from nonhost plants and testing their relevance in imparting broad-spectrum resistance when transferred to host plants.

CONCLUSION

This review will help broaden the current knowledge base pertaining to the resistance sources available in host germplasm, the exploitation of NHR mechanisms, and their applications in protecting Brassica crops from Alternaria blight. The insights might also be applicable to a wider repertoire of plant pathogens.

Green Technology: Bacteria-Based Approach Could Lead to Unsuspected Microbe⁻Plant⁻Animal Interactions

The recent and massive revival of green strategies to control plant diseases, mainly as a consequence of the Integrated Pest Management (IPM) rules issued in 2009 by the European Community and the increased consumer awareness of organic products, poses new challenges for human health and food security that need to be addressed in the near future. One of the most important green technologies is biocontrol. This approach is based on living organisms and how these biocontrol agents (BCAs) directly or indirectly interact as a community to control plant pathogens and pest. Although most BCAs have been isolated from plant microbiomes, they share some genomic features, virulence factors, and trans-kingdom infection abilities with human pathogenic microorganisms, thus, their potential impact on human health should be addressed. This evidence, in combination with the outbreaks of human infections associated with consumption of raw fruits and vegetables, opens new questions regarding the role of plants in the human pathogen infection cycle. Moreover, whether BCAs could alter the endophytic bacterial community, thereby leading to the development of new potential human pathogens, is still unclear. In this review, all these issues are debated, highlighting that the research on BCAs and their formulation should include these possible long-lasting consequences of their massive spread in the environment.

Duckweed (Lemna minor) and Alfalfa (Medicago sativa) as Bacterial Infection Model Systems

Alternative animal host models of bacterial infection have been developed which reproduce some of the disease conditions observed in higher animals. Analogously, plants are useful for modeling bacterial pathogenesis, in some cases revealing broadly conserved infection mechanisms. Similar to animals, plants have been shown to possess innate immune systems that respond to invading viruses, bacteria, and fungi. Plant infection models often yield results faster, are more convenient, and less expensive than many animal infection models. Here, we describe the use of two different plant-based infection models for the discovery of virulence genes and factors involved in bacterial pathogenesis.

Deep sequencing analysis reveals a TMV mutant with a poly(A) tract reduces host defense responses in Nicotiana benthamiana

Tobacco mosaic virus (TMV) possesses an upstream pseudoknotted domain (UPD), which is important for replication. After substituting the UPD with an internal poly(A) tract (43 nt), a mutant TMV-43A was constructed. TMV-43A replicated slower than TMV and induced a non-lethal mosaic symptom in Nicotiana benthamiana. In this study, deep sequencing was performed to detect the differences of small RNA profiles between TMV- and TMV-43A-infected N. benthamiana. The results showed that TMV-43A produced lesser amount of virus-derived interfering RNAs (vsiRNAs) than that of TMV. However, the distributions of vsiRNAs generation hotspots between TMV and TMV-43A were similar. Expression of genes related to small RNA biogenesis in TMV-43A-infected N. benthamiana was significantly lower than that of TMV, which leads to generation of lesser vsiRNAs. The expressions of host defense response genes were up-regulated after TMV infection, as compared to TMV-43A-infected plants. Host defense response to TMV-43A infection was lower than that to TMV. The absence of UPD might contribute to the reduced host response to TMV-43A. Our study provides valuable information in the role of the UPD in eliciting host response genes after TMV infection in N. benthamiana. (187 words).

Catechol-Based Hydrogel for Chemical Information Processing

Catechols offer diverse properties and are used in biology to perform various functions that range from adhesion (e.g., mussel proteins) to neurotransmission (e.g., dopamine), and mimicking the capabilities of biological catechols have yielded important new materials (e.g., polydopamine). It is well known that catechols are also redox-active and we have observed that biomimetic catechol-modified chitosan films are redox-active and possess interesting molecular electronic properties. In particular, these films can accept, store and donate electrons, and thus offer redox-capacitor capabilities. We are enlisting these capabilities to bridge communication between biology and electronics. Specifically, we are investigating an interactive redox-probing approach to access redox-based chemical information and convert this information into an electrical modality that facilitates analysis by methods from signal processing. In this review, we describe the broad vision and then cite recent examples in which the catechol–chitosan redox-capacitor can assist in accessing and understanding chemical information. Further, this redox-capacitor can be coupled with synthetic biology to enhance the power of chemical information processing. Potentially, the progress with this biomimetic catechol–chitosan film may even help in understanding how biology uses the redox properties of catechols for redox signaling.

Transcriptome analysis of Meloidogyne incognita encumbered by Pasteuria penetrans endospores provides new insights into bacteria and nematode interaction

Root-knot nematodes (RKN,Meloidogynespp.) are one of the most harmful pests in agriculture.Pasteuria penetrans, an obligate hyperparasite of RKN, is an effective biological control agent. However, little is known about the molecular mechanisms of interaction betweenP. penetransand the second-stage juvenile (J2) ofMeloidogyne incognita. Here, we used transcriptome sequencing to characterise the differential expression profiles between control J2 ofM. incognitaand J2 encumbered byP. penetransendospores. A total of 445 genes were found to be differentially expressed, including 406 up-regulated and 39 down-regulated genes. Thirty-seven putative immune-related genes encoding collagens, cytochrome P450, lysozymes and other active proteins were identified. Genes involved in the ‘biosynthesis of unsaturated fatty acids’ pathway and several core sets of immune effectors were up-regulated, indicating conserved immune mechanisms among different nematodes. Down-regulation of cytochrome P450-related genes might suggest a specific defence response ofM. incognitaencumbered byP. penetransendospores.

Roles of the NLRP3 inflammasome in the pathogenesis of diabetic nephropathy

Diabetic nephropathy (DN) is a serious complication of diabetes mellitus, and persistent inflammation in circulatory and renal tissues is an important pathophysiological basis for DN. The essence of the microinflammatory state is the innate immune response, which is central to the occurrence and development of DN. Members of the inflammasome family, including both "receptors" and "regulators", are key to the inflammatory immune response. Nucleotide binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) and other inflammasome components are able to detect endogenous danger signals, resulting in activation of caspase-1 as well as interleukin (IL)-1β, IL-18 and other cytokines; these events stimulate the inflammatory cascade reaction, which is crucial for DN. Hyperglycaemia, hyperlipidaemia and hyperuricaemia can activate the NLRP3 inflammasome, which then mediates the occurrence and development of DN through the K⁺ channel model, the lysosomal damage model and the active oxygen cluster model. In this review, we survey the involvement of the NLRP3 inflammasome in various signalling pathways and highlight different aspects of their influence on DN. We also explore the important effects of the NLRP3 inflammasome on kidney function and structural changes that occur during DN development and progression. It is becoming more evident that NLRP3 inflammasome targeting has therapeutic potential for the treatment of DN.

Crosstalk of Signaling Mechanisms Involved in Host Defense and Symbiosis Against Microorganisms in Rice

Rice is one of the most important food crops, feeding about half population in the world. Rice pathogens cause enormous damage to rice production worldwide. In plant immunity research, considerable progress has recently been made in our understanding of the molecular mechanisms underlying microbe-associated molecular pattern (MAMP)-triggered immunity. Using genome sequencing and molecular techniques, a number of new MAMPs and their receptors have been identified in the past two decades. Notably, the mechanisms for chitin perception via the lysine motif (LysM) domain-containing receptor OsCERK1, as well as the mechanisms for bacterial MAMP (e.g. flg22, elf18) perception via the leucine-rich repeat (LRR) domain-containing receptors FLS2 and EFR, have been clarified in rice and Arabidopsis, respectively. In chitin signaling in rice, two direct substrates of OsCERK1, Rac/ROP GTPase guanine nucleotide exchange factor OsRacGEF1 and receptor-like cytoplasmic kinase OsRLCK185, have been identified as components of the OsCERK1 complex and are rapidly phosphorylated by OsCERK1 in response to chitin. Interestingly, OsCERK1 also participates in symbiosis with arbuscular mycorrhizal fungi (AMF) in rice and plays a role in the recognition of short-chitin molecules (CO4/5), which are symbiotic signatures included in AMF germinated spore exudates and induced by synthetic strigolactone. Thus, OsCERK1 contributes to both immunity and symbiotic responses. In this review, we describe recent studies on pathways involved in rice immunity and symbiotic signaling triggered by interactions with microorganisms. In addition, we describe recent advances in genetic engineering by using plant immune receptors and symbiotic microorganisms to enhance disease resistance of rice.

Fungal Innate Immunity Induced by Bacterial Microbe-Associated Molecular Patterns (MAMPs)

Plants and animals detect bacterial presence through Microbe-Associated Molecular Patterns (MAMPs) which induce an innate immune response. The field of fungal-bacterial interaction at the molecular level is still in its infancy and little is known about MAMPs and their detection by fungi. Exposing Fusarium graminearum to bacterial MAMPs led to increased fungal membrane hyperpolarization, a putative defense response, and a range of transcriptional responses. The fungus reacted with a different transcript profile to each of the three tested MAMPs, although a core set of genes related to energy generation, transport, amino acid production, secondary metabolism, and especially iron uptake were detected for all three. Half of the genes related to iron uptake were predicted MirA type transporters that potentially take up bacterial siderophores. These quick responses can be viewed as a preparation for further interactions with beneficial or pathogenic bacteria, and constitute a fungal innate immune response with similarities to those of plants and animals.

Psd1 binding affinity toward fungal membrane components as assessed by SPR: The role of glucosylceramide in fungal recognition and entry

Psd1 is a plant defensin that has antifungal activity against several pathogenic and nonpathogenic fungi. Previous analysis of Psd1 chemical shift perturbations by nuclear magnetic resonance (NMR) spectroscopy demonstrated that this defensin interacts with phospholipids and the sphingolipid glucosylceramide isolated from Fusarium solani (GlcCer(Fusarium solani)). In this study, these interactions were evaluated by real-time surface plasmon resonance (SPR) analysis. The data obtained demonstrated that Psd1 could bind more strongly to small unilamellar vesicles (SUV)-containing GlcCer(Fusarium solani) than to SUV that was composed of phosphatidylcholine (PC) alone or was enriched with GlcCer that had been isolated from soybeans. An increase in the SPR response after cholesterol or ergosterol incorporation in PC-SUV was detected; however, SUV composed of PC:Erg (7:3; molar:molar) became unstable in the presence of Psd1, suggesting membrane destabilization. We also observed a lack of Psd1 internalization in Candida albicans strains that were deficient in the glucosyl ceramide synthase gene. Together, these data indicate that GlcCer is essential for Psd1 anchoring in the fungal plasma membrane as well as internalization.

Phenylpropanoid Defences in Nicotiana tabacum Cells: Overlapping Metabolomes Indicate Common Aspects to Priming Responses Induced by Lipopolysaccharides, Chitosan and Flagellin-22

Plants have evolved both constitutive and inducible defence strategies to cope with different biotic stimuli and stresses. Exposure of a plant to a challenging stress can lead to a primed state that allows it to launch a more rapid and stronger defence. Here we applied a metabolomic approach to study and compare the responses induced in Nicotiana tabacum cells by microbe-associated molecular pattern (MAMP) molecules, namely lipopolysaccharides (LPS), chitosan (CHT) and flagellin-22 (FLG22). Early response metabolites, extracted with methanol, were analysed by UHPLC-MS/MS. Using multivariate statistical tools the metabolic profiles induced by these elicitors were analysed. In the metabolic fingerprint of these agents a total of 19 cinnamic acid derivatives conjugated to quinic acids (chlorogenic acids), shikimic acid, tyramine, polyamines or glucose were found as discriminant biomarkers. In addition, treatment with the phytohormones salicylic acid (SA), methyljasmonic acid (MJ) and abscisic acid (ABA) resulted in differentially-induced phenylpropanoid pathway metabolites. The results indicate that the phenylpropanoid pathway is activated by these elicitors while hydroxycinnamic acid derivatives are commonly associated with the metabolic response to the MAMPs, and that the activated responses are modulated by both SA and MJ, with ABA not playing a role.

Lubricin/Proteoglycan 4 binds to and regulates the activity of Toll-Like Receptors In Vitro

Proteoglycan 4 (PRG4/lubricin) is secreted by cells that reside in articular cartilage and line the synovial joint. Lubricin may play a role in modulating inflammatory responses through interaction with CD44. This led us to examine if lubricin could be playing a larger role in the modulation of inflammation/immunity through interaction with Toll-like receptors (TLRs). Human Embryonic Kidney (HEK) cells overexpressing TLRs 2, 4 or 5 and surface plasmon resonance were employed to determine if full length recombinant human lubricin was able to bind to and activate TLRs. Primary human synovial fibroblasts were also examined using flow cytometry and Luminex multiplex ELISA. A rat destabilization model of osteoarthritis (OA) was used to determine if lubricin injections were able to regulate pain and/or inflammation in vivo. Lubricin can bind to and regulate the activity of TLRs, leading to downstream changes in inflammatory signalling independent of HA. We confirmed these findings in vivo through intra-articular injections of lubricin in a rat OA model where the inhibition of systemic inflammatory signaling and reduction in pain were observed. Lubricin plays an important role in regulating the inflammatory environment under both homeostatic and tissue injury states.

NLRP3 inflammasome activation is essential for paraquat-induced acute lung injury

The innate immune response is important in paraquat-induced acute lung injury, but the exact pathways involved are not elucidated. The objectives of this study were to determine the specific role of the NLRP3 inflammasome in the process. Acute lung injury was induced by administering paraquat (PQ) intraperitoneally. NLRP3 inflammasome including NLRP3, ASC, and caspase-1 mRNA and protein expression in lung tissue and IL-1β and IL-18 levels in BALF were detected at 4, 8, 24, and 72 h after PQ administration in rats. Moreover, rats were pretreated with 10, 30, and 50 mg/kg NLRP3 inflammasome blocker glybenclamide, respectively, 1 h before PQ exposure. At 72 h after PQ administration, lung histopathology changes, NLRP3, ASC, and caspase-1 protein expression, as well as secretion of cytokines including IL-1β and IL-18 in BALF were investigated. The NLRP3 inflammasome including NLRP3, ASC, caspase-1 expression, and cytokines IL-1β and IL-18 levels in PQ poisoning rats were significantly higher than that in the control group. NLRP3 inflammasome blocker glybenclamide pretreatment attenuated lung edema, inhibited the NLRP3, ASC, and caspase-1 activation, and reduced IL-1β and IL-18 levels in BALF. In the in vitro experiments, IL-1β and IL-18 secreted from RAW264.7 mouse macrophages treated with paraquat were attenuated by glybenclamide. In conclusion, paraquat can induce IL-1β/IL-18 secretion via NLRP3-ASC-caspase-1 pathway, and the NLRP3 inflammasome is essential for paraquat-induced acute lung injury.

Chitosan-induced antiviral activity and innate immunity in plants

Immunity represents a trait common to all living organisms, and animals and plants share some similarities. Therefore, in susceptible host plants, complex defence machinery may be stimulated by elicitors. Among these, chitosan deserves particular attention because of its proved efficacy. This survey deals with the antiviral activity of chitosan, focusing on its perception by the plant cell and mechanism of action. Emphasis has been paid to benefits and limitations of this strategy in crop protection, as well as to the potential of chitosan as a promising agent in virus disease control.

Self/nonself perception in plants in innate immunity and defense

The ability to distinguish 'self' from 'nonself' is the most fundamental aspect of any immune system. The evolutionary solution in plants to the problems of perceiving and responding to pathogens involves surveillance of nonself, damaged-self and altered-self as danger signals. This is reflected in basal resistance or non-host resistance, which is the innate immune response that protects plants against the majority of pathogens. In the case of surveillance of nonself, plants utilize receptor-like proteins or -kinases (RLP/Ks) as pattern recognition receptors (PRRs), which can detect conserved pathogen/microbe-associated molecular pattern (P/MAMP) molecules. P/MAMP detection serves as an early warning system for the presence of a wide range of potential pathogens and the timely activation of plant defense mechanisms. However, adapted microbes express a suite of effector proteins that often interfere or act as suppressors of these defenses. In response, plants have evolved a second line of defense that includes intracellular nucleotide binding leucine-rich repeat (NB-LRR)-containing resistance proteins, which recognize isolate-specific pathogen effectors once the cell wall has been compromised. This host-immunity acts within the species level and is controlled by polymorphic host genes, where resistance protein-mediated activation of defense is based on an 'altered-self' recognition mechanism.

Polyphenols and the human brain: plant “secondary metabolite” ecologic roles and endogenous signaling functions drive benefits

Flavonoids and other polyphenols are ubiquitous plant chemicals that fulfill a range of ecologic roles for their home plant, including protection from a range of biotic and abiotic stressors and a pivotal role in the management of pathogenic and symbiotic soil bacteria and fungi. They form a natural part of the human diet, and evidence suggests that their consumption is associated with the beneficial modulation of a number of health-related variables, including those related to cardiovascular and brain function. Over recent years, the consensus as to the mechanisms responsible for these effects in humans has shifted away from polyphenols having direct antioxidant effects and toward their modulation of cellular signal transduction pathways. To date, little consideration has been given to the question of why, rather than how, these plant-derived chemicals might exert these effects. Therefore, this review summarizes the evidence suggesting that polyphenols beneficially affect human brain function and describes the current mechanistic hypotheses explaining these effects. It then goes on to describe the ecologic roles and potential endogenous signaling functions that these ubiquitous phytochemicals play within their home plant and discusses whether these functions drive their beneficial effects in humans via a process of “cross-kingdom” signaling predicated on the many conserved similarities in plant, microbial, and human cellular signal transduction pathways.

Ergosterol, an orphan fungal microbe-associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant-fungus interactions are mostly mediated by microbe-associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β-glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an 'orphan' fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant-fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems-based understanding of the dynamics associated with molecular mechanisms in plant-ergosterol interactions and associated host responses.

Common duckweed (Lemna minor) is a versatile high-throughput infection model for the Burkholderia cepacia complex and other pathogenic bacteria

Members of the Burkholderia cepacia complex (Bcc) have emerged in recent decades as problematic pulmonary pathogens of cystic fibrosis (CF) patients, with severe infections progressing to acute necrotizing pneumonia and sepsis. This study presents evidence that Lemna minor (Common duckweed) is useful as a plant model for the Bcc infectious process, and has potential as a model system for bacterial pathogenesis in general. To investigate the relationship between Bcc virulence in duckweed and Galleria mellonella (Greater wax moth) larvae, a previously established Bcc infection model, a duckweed survival assay was developed and used to determine LD₅₀ values. A strong correlation (R² = 0.81) was found between the strains’ virulence ranks in the two infection models, suggesting conserved pathways in these vastly different hosts. To broaden the application of the duckweed model, enteropathogenic Escherichia coli (EPEC) and five isogenic mutants with previously established LD₅₀ values in the larval model were tested against duckweed, and a strong correlation (R² = 0.93) was found between their raw LD₅₀ values. Potential virulence factors in B. cenocepacia K56-2 were identified using a high-throughput screen against single duckweed plants. In addition to the previously characterized antifungal compound (AFC) cluster genes, several uncharacterized genes were discovered including a novel lysR regulator, a histidine biosynthesis gene hisG, and a gene located near the gene encoding the recently characterized virulence factor SuhB_Bc. Finally, to demonstrate the utility of this model in therapeutic applications, duckweed was rescued from Bcc infection by treating with bacteriophage at 6-h intervals. It was observed that phage application became ineffective at a timepoint that coincided with a sharp increase in bacterial invasion of plant tissue. These results indicate that common duckweed can serve as an effective infection model for the investigation of bacterial virulence factors and therapeutic strategies to combat them.

Bacterial discrimination by dictyostelid amoebae reveals the complexity of ancient interspecies interactions

BACKGROUND

Amoebae and bacteria interact within predator-prey and host-pathogen relationships, but the general response of amoeba to bacteria is not well understood. The amoeba Dictyostelium discoideum feeds on, and is colonized by, diverse bacterial species, including Gram-positive [Gram(+)] and Gram-negative [Gram(-)] bacteria, two major groups of bacteria that differ in structure and macromolecular composition.

RESULTS

Transcriptional profiling of D. discoideum revealed sets of genes whose expression is enriched in amoebae interacting with different species of bacteria, including sets that appear specific to amoebae interacting with Gram(+) or with Gram(-) bacteria. In a genetic screen utilizing the growth of mutant amoebae on a variety of bacteria as a phenotypic readout, we identified amoebal genes that are only required for growth on Gram(+) bacteria, including one that encodes the cell-surface protein gp130, as well as several genes that are only required for growth on Gram(-) bacteria, including one that encodes a putative lysozyme, AlyL. These genes are required for parts of the transcriptional response of wild-type amoebae, and this allowed their classification into potential response pathways.

CONCLUSIONS

We have defined genes that are critical for amoebal survival during feeding on Gram(+), or Gram(-), bacteria that we propose form part of a regulatory network that allows D. discoideum to elicit specific cellular responses to different species of bacteria in order to optimize survival.

Nonhost resistance of tomato to the bean pathogen Pseudomonas syringae pv. syringae B728a is due to a defective E3 ubiquitin ligase domain in avrptobb728a

The bean pathogen Pseudomonas syringae pv. syringae B728a expresses homologs of the type III effectors AvrPto and AvrPtoB, either of which can trigger resistance in tomato cultivars expressing Pto and Prf genes. We found that strain B728a also elicits nonhost resistance in tomato cultivars VFNT Cherry and Moneymaker that lack Pto but express other members of the Pto family (e.g., SlFen and SlPtoC). Here, we show that the AvrPtoB homolog from B728a, termed AvrPtoBB728a (also known as HopAB1), is recognized by 'VFNT Cherry' and 'Moneymaker' when the effector is expressed in P. syringae pv. syringae 61, a strain lacking the avrPto or avrPtoB homolog. Using a gene-silencing approach, this recognition was shown to involve one or more Pto family members and Prf. AvrPtoBB728a interacted with SlFen, SlPtoC, and SlPtoD, in addition to Pto, in a yeast two-hybrid assay. In P. syringae pv. tomato DC3000, the C-terminal domain of AvrPtoB is an E3 ubiquitin ligase that ubiquitinates Fen, causing its degradation and leading to disease susceptibility. Although the C-terminal domain of AvrPtoBB728a shares 69% amino acid identity with that of AvrPtoB, we found that it has greatly reduced E3 ligase activity and is unable to ubiquitinate Fen in an in vitro ubiquitination assay. Thus, the nonhost resistance of 'VFNT Cherry' and 'Moneymaker' to B728a appears to be due to recognition of AvrPtoBB728 as a result of the effector's reduced E3 ligase activity, which prevents it from facilitating degradation of a Pto family member. We speculate that the primary plant host of B728a lacks a Fen-like protein and that, therefore, the E3 ligase of AvrPtoBB728 was unnecessary for pathogenicity and has diverged and become ineffective.

Plant cytokine or phytocytokine

Peptide hormones play an important role in plant growth and development. Some of them are secreted by stem cells and also regulate plant immunity through cell-cell communication and reprogramming the expression of immune related genes, such as CLAVATA3p (CLV3p) and phytosulfokine (PSK). These peptides play similar roles as cytokines in plant innate immunity. As explosive progress of plant omics, more and more such functional peptides will be discovered. I recommend that they should be named as plant cytokines or phytocytokines. This nomenclature will be convenient for study of plant secretory peptides and plant innate immunity.

Pathogenesis in tuberculosis: transcriptomic approaches to unraveling virulence mechanisms and finding new drug targets

Tuberculosis (TB) remains a major health problem worldwide. Attempts to control this disease have proved difficult owing to our poor understanding of the pathobiology of Mycobacterium tuberculosis and the emergence of strains that are resistant to multiple drugs currently available for treatment. Genome-wide expression profiling has provided new insight into the transcriptome signatures of the bacterium during infection, notably of macrophages and dendritic cells. These data indicate that M. tuberculosis expresses numerous genes to evade the host immune responses, to suit its intracellular life style, and to respond to various antibiotic drugs. Among the intracellularly induced genes, several have functions in lipid metabolism, cell wall synthesis, iron uptake, oxidative stress resistance, protein secretion, or inhibition of apoptosis. Herein we review these findings and discuss possible ways to exploit the data to understand the complex etiology of TB and to find new effective drug targets.

Glycerol-3-phosphate is a critical mobile inducer of systemic immunity in plants

Glycerol-3-phosphate (G3P) is an important metabolite that contributes to the growth and disease-related physiologies of prokaryotes, plants, animals and humans alike. Here we show that G3P serves as the inducer of an important form of broad-spectrum immunity in plants, termed systemic acquired resistance (SAR). SAR is induced upon primary infection and protects distal tissues from secondary infections. Genetic mutants defective in G3P biosynthesis cannot induce SAR but can be rescued when G3P is supplied exogenously. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues, and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues, which occurs through the symplast. These observations, along with the fact that dir1 plants accumulate reduced levels of G3P in their petiole exudates, suggest that the cooperative interaction of DIR1 and G3P orchestrates the induction of SAR in plants.

Herbal extracts and phytochemicals: plant secondary metabolites and the enhancement of human brain function

Humans consume a wide range of foods, drugs, and dietary supplements that are derived from plants and which modify the functioning of the central nervous sytem (CNS). The psychoactive properties of these substances are attributable to the presence of plant secondary metabolites, chemicals that are not required for the immediate survival of the plant but which are synthesized to increase the fitness of the plant to survive by allowing it to interact with its environment, including pathogens and herbivorous and symbiotic insects. In many cases, the effects of these phytochemicals on the human CNS might be linked either to their ecological roles in the life of the plant or to molecular and biochemical similarities in the biology of plants and higher animals. This review assesses the current evidence for the efficacy of a range of readily available plant-based extracts and chemicals that may improve brain function and which have attracted sufficient research in this regard to reach a conclusion as to their potential effectiveness as nootropics. Many of these candidate phytochemicals/extracts can be grouped by the chemical nature of their potentially active secondary metabolite constituents into alkaloids (caffeine, nicotine), terpenes (ginkgo, ginseng, valerian, Melissa officinalis, sage), and phenolic compounds (curcumin, resveratrol, epigallocatechin-3-gallate, Hypericum perforatum, soy isoflavones). They are discussed in terms of how an increased understanding of the relationship between their ecological roles and CNS effects might further the field of natural, phytochemical drug discovery.

Emerging role of antioxidants in the protection of uveitis complications

Current understanding of the role of oxidative stress in ocular inflammatory diseases indicates that antioxidant therapy may be important to optimize the treatment. Recently investigated antioxidant therapies for ocular inflammatory diseases include various vitamins, plant products and reactive oxygen species scavengers. Oxidative stress plays a causative role in both non-infectious and infectious uveitis complications, and novel strategies to diminish tissue damage and dysfunction with antioxidant therapy may ameliorate visual complications. Preclinical studies with experimental animals and cultured cells demonstrate significant anti-inflammatory effects of a number of promising antioxidant agents. Many of these antioxidants are under clinical trial for various inflammatory diseases other than uveitis such as cardiovascular, rheumatoid arthritis and cancer. Well planned interventional clinical studies in the field of ocular inflammation will be necessary to sufficiently investigate the potential medical benefits of antioxidant therapies for uveitis. This review summarizes the recent investigations of novel antioxidant agents for ocular inflammation, with selected studies focused on uveitis.

Devil inside: does plant programmed cell death involve the endomembrane system?

Eukaryotic cells have to constantly cope with environmental cues and integrate developmental signals. Cell survival or death is the only possible outcome. In the field of animal biology, tremendous efforts have been put into the understanding of mechanisms underlying cell fate decision. Distinct organelles have been proven to sense a broad range of stimuli and, if necessary, engage cell death signalling pathway(s). Over the years, forward and reverse genetic screens have uncovered numerous regulators of programmed cell death (PCD) in plants. However, to date, molecular networks are far from being deciphered and, apart from the autophagic compartment, no organelles have been assigned a clear role in the regulation of cellular suicide. The endomembrane system (ES) seems, nevertheless, to harbour a significant number of cell death mediators. In this review, the involvement of this system in the control of plant PCD is discussed in-depth, as well as compared and contrasted with what is known in animal and yeast systems.

Uromyces appendiculatus infection in BTH-treated bean plants: ultrastructural details of a lost fight

The mechanisms of BTH [benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester]-induced resistance against bean rust caused by Uromyces appendiculatus have been explored in Phaseolus vulgaris by light and transmission electron microscopy, following the infection progression in plants challenged 7 days after treatment. While BTH did not affect uredospore germination and fungal penetration in the substomatal cavity, a first impairment to the colonization appeared evident about 48-96 h after inoculation, with alterations of infection hypha structure and reduction in mycelium expansion. No differences were found in this phase regarding the formation and ultrastructure of haustoria in untreated and BTH-treated plants, except for the deposition of electron-opaque material in the extrahaustorial matrix of the latter. A second and decisive impairment in fungal progression was observed at 7-10 days after inoculation when host cell penetrated, or in close contact with the fungal hyphae, were impregnated by phenolic compounds. The same was observed in fungal walls, particularly around haustoria, thus hampering the biotrophic habitus of the fungus and further mycelium spreading. This, in turn, prevented the evasion of fungal reproductive structures, the uredinia, and the appearance of visible symptoms. No particular ultrastructural alterations were observed in most of the penetrated cells, even at late stages of infection, indicating that BTH treatment does not induce host cells to respond with a hypersensitive reaction (HR). A parallel time course of the expression of phenylalanine ammonia lyase (PAL) gene, the key enzyme for the synthesis of phenylpropanoidic phytoalexins and many other phenolics, has shown that PAL mRNA is strongly and persistently transcripted in BTH-treated plants since the 6th h after treatment, though no apparent ultrastructural alterations were detectable up to some days after pathogen challenging. This indicates that BTH, at the employed concentration of 0.3 mM, directly activates the plant's own defences, thus accounting for the observed full protection against bean rust.

Plant-eriophyoid mite interactions: cellular biochemistry and metabolic responses induced in mite-injured plants. Part I

This review is a comprehensive study of recent advances related to cytological, biochemical and physiological changes induced in plants in response to eriophyoid mite attack. It has been shown that responses of host plants to eriophyoids are variable. Most of the variability is due to individual eriophyoid mite-plant interactions. Usually, the direction and intensity of changes in eriophyoid-infested plant organs depend on mite genotype, density, or the feeding period, and are strongly differentiated relative to host plant species, cultivar, age and location. Although the mechanisms of changes elicited by eriophyoid mites within plants are not fully understood, in many cases the qualitative and quantitative biochemical status of mite-infested plants are known to affect the performance of consecutive herbivorous arthropods. In future, elucidation of the pathways from eriophyoid mite damage to plant gene activation will be necessary to clarify plant responses and to explain variation in plant tissue damage at the feeding and adjacent sites.

R proteins as fundamentals of plant innate immunity

Plants are attacked by a wide spectrum of pathogens, being the targets of viruses, bacteria, fungi, protozoa, nematodes and insects. Over the course of their evolution, plants have developed numerous defense mechanisms including the chemical and physical barriers that are constitutive elements of plant cell responses locally and/or systemically. However, the modern approach in plant sciences focuses on the evolution and role of plant protein receptors corresponding to specific pathogen effectors. The recognition of an invader's molecules could be in most cases a prerequisite sine qua non for plant survival. Although the predicted three-dimensional structure of plant resistance proteins (R) is based on research on their animal homologs, advanced technologies in molecular biology and bioinformatics tools enable the investigation or prediction of interaction mechanisms for specific receptors with pathogen effectors. Most of the identified R proteins belong to the NBS-LRR family. The presence of other domains (including the TIR domain) apart from NBS and LRR is fundamental for the classification of R proteins into subclasses. Recently discovered additional domains (e.g. WRKY) of R proteins allowed the examination of their localization in plant cells and the role they play in signal transduction during the plant resistance response to biotic stress factors. This review focuses on the current state of knowledge about the NBS-LRR family of plant R proteins: their structure, function and evolution, and the role they play in plant innate immunity.

Signaling by ROS drives inflammasome activation

Inflammasomes are innate immune signaling pathways that sense pathogens and injury to direct the proteolytic maturation of inflammatory cytokines such as IL-1beta and IL-18. Among inflammasomes, the NLRP3/NALP3 inflammasome is the most studied. However, little is known on the molecular mechanisms that mediate its assembly and activation. Recent findings suggest that ROS are produced by NLRP3/NALP3 activators and are essential secondary messengers signaling NLRP3/NALP3 inflammasome activation.

Thiamin confers enhanced tolerance to oxidative stress in Arabidopsis

Thiamin and thiamin pyrophosphate (TPP) are well known for their important roles in human nutrition and enzyme catalysis. In this work, we present new evidence for an additional role of these compounds in the protection of cells against oxidative damage. Arabidopsis (Arabidopsis thaliana) plants subjected to abiotic stress conditions, such as high light, cold, osmotic, salinity, and oxidative treatments, accumulated thiamin and TPP. Moreover, the accumulation of these compounds in plants subjected to oxidative stress was accompanied by enhanced expression of transcripts encoding thiamin biosynthetic enzymes. When supplemented with exogenous thiamin, wild-type plants displayed enhanced tolerance to oxidative stress induced by paraquat. Thiamin application was also found to protect the reactive oxygen species-sensitive ascorbate peroxidase1 mutant from oxidative stress. Thiamin-induced tolerance to oxidative stress was accompanied by decreased production of reactive oxygen species in plants, as evidenced from decreased protein carbonylation and hydrogen peroxide accumulation. Because thiamin could protect the salicylic acid induction-deficient1 mutant against oxidative stress, thiamin-induced oxidative protection is likely independent of salicylic acid signaling or accumulation. Taken together, our studies suggest that thiamin and TPP function as important stress-response molecules that alleviate oxidative stress during different abiotic stress conditions.

Chemical diversity and defence metabolism: how plants cope with pathogens and ozone pollution

Chemical defences represent a main trait of the plant innate immune system. Besides regulating the relationship between plants and their ecosystems, phytochemicals are involved both in resistance against pathogens and in tolerance towards abiotic stresses, such as atmospheric pollution. Plant defence metabolites arise from the main secondary metabolic routes, the phenylpropanoid, the isoprenoid and the alkaloid pathways. In plants, antibiotic compounds can be both preformed (phytoanticipins) and inducible (phytoalexins), the former including saponins, cyanogenic glycosides and glucosinolates. Chronic exposure to tropospheric ozone (O(3)) stimulates the carbon fluxes from the primary to the secondary metabolic pathways to a great extent, inducing a shift of the available resources in favour of the synthesis of secondary products. In some cases, the plant defence responses against pathogens and environmental pollutants may overlap, leading to the unspecific synthesis of similar molecules, such as phenylpropanoids. Exposure to ozone can also modify the pattern of biogenic volatile organic compounds (BVOC), emitted from plant in response to herbivore feeding, thus altering the tritrophic interaction among plant, phytophagy and their natural enemies. Finally, the synthesis of ethylene and polyamines can be regulated by ozone at level of S-adenosylmethionine (SAM), the biosynthetic precursor of both classes of hormones, which can, therefore, mutually inhibit their own biosynthesis with consequence on plant phenotype.

Plant cell death and cellular alterations induced by ozone: key studies in Mediterranean conditions

An account of histo-cytological and ultrastructural studies on ozone effect on crop and forest species in Italy is given, with emphasis on induced cell death and the underlying mechanisms. Cell death phenomena possibly due to ambient O(3) were recorded in crop and forest species. In contrast, visible O(3) effects on Mediterranean vegetation are often unclear. Microscopy is thus suggested as an effective tool to validate and evaluate O(3) injury to Mediterranean vegetation. A DAB-Evans blue staining was proposed to validate O(3) symptoms at the microscopic level and for a pre-visual diagnosis of O(3) injury. The method has been positively tested in some of the most important crop species, such as wheat, tomato, bean and onion and, with some restriction, in forest species, and it also allows one to gain some very useful insights into the mechanisms at the base of O(3) sensitivity or tolerance.

Nutritional traits of bean ( Phaseolus vulgaris ) seeds from plants chronically exposed to ozone pollution

The effect of chronic exposure to ozone pollution on nutritional traits of bean ( Phaseolus vulgaris L. cv. Borlotto Nano Lingua di Fuoco) seeds from plants grown in filtered and nonfiltered open-top chambers (OTCs) has been investigated. Results showed that, among seed macronutrients, ozone significantly raised total lipids, crude proteins, and dietary fiber and slightly decreased total free amino acid content, although with a significant reduction of asparagine, lysine, valine, methionine, and glycine, compensated by a conspicuous augmentation of ornithine and tryptophan. Phytosterol analysis showed a marked increase of beta-sitosterol, stigmasterol, and campesterol in seeds collected from nonfiltered OTCs. With regard to secondary metabolites, ozone exposure induced a slight increase of total polyphenol content, although causing a significant reduction of some flavonols (aglycone kaempferol and its 3-glucoside derivative) and hydroxycinnamates (caffeic, p-coumaric, and sinapic acids). Total anthocyanins decreased significantly, too. Nevertheless, ozone-exposed seeds showed higher antioxidant activity, with higher Trolox equivalent antioxidant capacity (TEAC) values than those measured in seeds collected from filtered air.

Chitosan as a MAMP, searching for a PRR

Chitosan, a deacetylated chitin derivative, behaves like a general elicitor, inducing a non-host resistance and priming a systemic acquired immunity. The defence responses elicited by chitosan include rising of cytosolic H(+) and Ca(2+), activation of MAP-kinases, callose apposition, oxidative burst, hypersensitive response (HR), synthesis of abscissic acid (ABA), jasmonate, phytoalexins and pathogenesis related (PR) proteins. Putative receptors for chitosan are a chitosan-binding protein, recently isolated, and possibly the chitin elicitor-binding protein (CEBiP). Nevertheless, it must be pointed out that biological activity of chitosan, besides the plant model, strictly depends on its physicochemical properties (deacetylation degree, molecular weight and viscosity), and that there is a threshold for chitosan concentration able to switch the induction of a cell death programme into necrotic cell death (cytotoxicity).