Salicylic acid is a systemic signal and an inducer of pathogenesis-related proteins in virus-infected tobacco

Immunity priming uncouples the growth-defense trade-off in tomato

Plants have developed an array of mechanisms to protect themselves against pathogen invasion. The deployment of defense mechanisms is imperative for plant survival, but can come at the expense of plant growth, leading to the 'growth-defense trade-off' phenomenon. Following pathogen exposure, plants can develop resistance to further attack. This is known as induced resistance, or priming. Here, we investigated the growth-defense trade-off, examining how defense priming via systemic acquired resistance (SAR), or induced systemic resistance (ISR), affects tomato development and growth. We found that defense priming can promote, rather than inhibit, plant development, and that defense priming and growth trade-offs can be uncoupled. Cytokinin response was activated during induced resistance, and found to be required for the observed growth and disease resistance resulting from ISR activation. ISR was found to have a stronger effect than SAR on plant development. Our results suggest that growth promotion and induced resistance can be co-dependent, and that, in certain cases, defense priming can drive developmental processes and promote plant yield.

Tobamovirus infection aggravates gray mold disease caused by Botrytis cinerea by manipulating the salicylic acid pathway in tomato

Botrytis cinerea is the causative agent of gray mold disease, and infects more than 1400 plant species, including important crop plants. In tomato, B. cinerea causes severe damage in greenhouses and post-harvest storage and transport. Plant viruses of the Tobamovirus genus cause significant damage to various crop species. In recent years, the tobamovirus tomato brown rugose fruit virus (ToBRFV) has significantly affected the global tomato industry. Most studies of plant-microbe interactions focus on the interaction between the plant host and a single pathogen, however, in agricultural or natural environments, plants are routinely exposed to multiple pathogens. Here, we examined how preceding tobamovirus infection affects the response of tomato to subsequent infection by B. cinerea. We found that infection with the tobamoviruses tomato mosaic virus (ToMV) or ToBRFV resulted in increased susceptibility to B. cinerea. Analysis of the immune response of tobamovirus-infected plants revealed hyper-accumulation of endogenous salicylic acid (SA), upregulation of SA-responsive transcripts, and activation of SA-mediated immunity. Deficiency in SA biosynthesis decreased tobamovirus-mediated susceptibility to B. cinerea, while exogenous application of SA enhanced B. cinerea symptoms. These results suggest that tobamovirus-mediated accumulation of SA increases the plants' susceptibility to B. cinerea, and provide evidence for a new risk caused by tobamovirus infection in agriculture.

Salicylic Acid and Mobile Regulators of Systemic Immunity in Plants: Transport and Metabolism

Systemic acquired resistance (SAR) occurs when primary infected leaves produce several SAR-inducing chemical or mobile signals that are transported to uninfected distal parts via apoplastic or symplastic compartments and activate systemic immunity. The transport route of many chemicals associated with SAR is unknown. Recently, it was demonstrated that pathogen-infected cells preferentially transport salicylic acid (SA) through the apoplasts to uninfected areas. The pH gradient and deprotonation of SA may lead to apoplastic accumulation of SA before it accumulates in the cytosol following pathogen infection. Additionally, SA mobility over a long distance is essential for SAR, and transpiration controls the partitioning of SA into apoplasts and cuticles. On the other hand, glycerol-3-phosphate (G3P) and azelaic acid (AzA) travel via the plasmodesmata (PD) channel in the symplastic route. In this review, we discuss the role of SA as a mobile signal and the regulation of SA transport in SAR.

Ethylene-responsive factor ERF114 mediates fungal pathogen effector PevD1-induced disease resistance in Arabidopsis thaliana

APETALA2/ethylene-responsive factor (AP2/ERF) family transcription factors are well-documented in plant responses to a wide range of biotic and abiotic stresses, but their roles in mediating elicitor-induced disease resistance remains largely unexplored. PevD1 is a Verticillium dahliae secretory effector that can induce disease resistance in cotton and tobacco plants. In our previous work, Nicotiana benthamiana ERF114 (NbERF114) was identified in a screen of genes differentially expressed in response to PevD1 infiltration. Here, we found that the ortholog of NbERF114 in Arabidopsis thaliana (ERF114) also strongly responded to PevD1 treatment and transcripts were induced by Pseudomonas syringae pv. tomato (Pst) DC3000 infection. Loss of ERF114 function caused impaired disease resistance, while overexpressing ERF114 (OE-ERF114) enhanced resistance to Pst DC3000. Moreover, ERF114 mediated PevD1-induced disease resistance. RNA-sequencing analysis revealed that the transcript level of phenylalanine ammonia-lyase1 (PAL1) and its downstream genes were significantly suppressed in erf114 mutants compared with A. thaliana Col-0. Reverse transcription-quantitative PCR (RT-qPCR) analysis further confirmed that the PAL1 mRNA level was significantly elevated in overexpressing OE-ERF114 plants but reduced in erf114 mutants compared with Col-0. Chromatin immunoprecipitation-qPCR (ChIP-qPCR) and electrophoretic mobility shift assay verified that ERF114 directly bound to the promoter of PAL1. The gene expression profiles of ERF114 and PAL1 in oestradiol-inducible transgenic plants confirmed ERF114 could activate PAL1 transcriptional expression. Further investigation revealed that ERF114 positively modulated PevD1-induced lignin and salicylic acid accumulation, probably by activating PAL1 transcription.

Role of Promising Secondary Metabolites to Confer Resistance Against Environmental Stresses in Crop Plants: Current Scenario and Future Perspectives

Plants often face incompatible growing environments like drought, salinity, cold, frost, and elevated temperatures that affect plant growth and development leading to low yield and, in worse circumstances, plant death. The arsenal of versatile compounds for plant consumption and structure is called metabolites, which allows them to develop strategies to stop enemies, fight pathogens, replace their competitors and go beyond environmental restraints. These elements are formed under particular abiotic stresses like flooding, heat, drought, cold, etc., and biotic stress such as a pathogenic attack, thus associated with survival strategy of plants. Stress responses of plants are vigorous and include multifaceted crosstalk between different levels of regulation, including regulation of metabolism and expression of genes for morphological and physiological adaptation. To date, many of these compounds and their biosynthetic pathways have been found in the plant kingdom. Metabolites like amino acids, phenolics, hormones, polyamines, compatible solutes, antioxidants, pathogen related proteins (PR proteins), etc. are crucial for growth, stress tolerance, and plant defense. This review focuses on promising metabolites involved in stress tolerance under severe conditions and events signaling the mediation of stress-induced metabolic changes are presented.

GhENODL6 Isoforms from the Phytocyanin Gene Family Regulated Verticillium Wilt Resistance in Cotton

Verticillium wilt (VW), a fungal disease caused by Verticillium dahliae, currently devastates cotton fiber yield and quality seriously, yet few resistance germplasm resources have been discovered in Gossypium hirsutum. The cotton variety Nongda601 with suitable VW resistance and high yield was developed in our lab, which supplied elite resources for discovering resistant genes. Early nodulin-like protein (ENODL) is mainly related to nodule formation, and its role in regulating defense response has been seldom studied. Here, 41 conserved ENODLs in G. hirsutum were identified and characterized, which could divide into four subgroups. We found that GhENODL6 was upregulated under V. dahliae stress and hormonal signal and displayed higher transcript levels in resistant cottons than the susceptible. The GhENODL6 was proved to positively regulate VW resistance via overexpression and gene silencing experiments. Overexpression of GhENODL6 significantly enhanced the expressions of salicylic acid (SA) hormone-related transcription factors and pathogenicity-related (PR) protein genes, as well as hydrogen peroxide (H₂O₂) and SA contents, resulting in improved VW resistance in transgenic Arabidopsis. Correspondingly, in the GhENODL6 silenced cotton, the expression levels of both phenylalanine ammonia lyase (PAL) and 4-coumarate-CoA ligase (4CL) genes significantly decreased, leading to the reduced SA content mediating by the phenylalanine ammonia lyase pathway. Taken together, GhENODL6 played a crucial role in VW resistance by inducing SA signaling pathway and regulating the production of reactive oxygen species (ROS). These findings broaden our understanding of the biological roles of GhENODL and the molecular mechanisms underlying cotton disease resistance.

CabZIP23 Integrates in CabZIP63-CaWRKY40 Cascade and Turns CabZIP63 on Mounting Pepper Immunity against Ralstonia solanacearum via Physical Interaction

CabZIP63 and CaWRKY40 were previously found to be shared in the pepper defense response to high temperature stress (HTS) and to Ralstonia solanacearum inoculation (RSI), forming a transcriptional cascade. However, how they activate the two distinct defense responses is not fully understood. Herein, using a revised genetic approach, we functionally characterized CabZIP23 in the CabZIP63-CaWRKY40 cascade and its context specific pepper immunity activation against RSI by interaction with CabZIP63. CabZIP23 was originally found by immunoprecipitation-mass spectrometry to be an interacting protein of CabZIP63-GFP; it was upregulated by RSI and acted positively in pepper immunity against RSI by virus induced gene silencing in pepper plants, and transient overexpression in Nicotiana benthamiana plants. By chromatin immunoprecipitation (ChIP)-qPCR and electrophoresis mobility shift assay (EMSA), CabZIP23 was found to be directly regulated by CaWRKY40, and CabZIP63 was directly regulated by CabZIP23, forming a positive feedback loop. CabZIP23-CabZIP63 interaction was confirmed by co-immunoprecipitation (CoIP) and bimolecular fluorescent complimentary (BiFC) assays, which promoted CabZIP63 binding immunity related target genes, including CaPR1, CaNPR1 and CaWRKY40, thereby enhancing pepper immunity against RSI, but not affecting the expression of thermotolerance related CaHSP24. All these data appear to show that CabZIP23 integrates in the CabZIP63-CaWRKY40 cascade and the context specifically turns it on mounting pepper immunity against RSI.

Induced Systemic Resistance for Improving Plant Immunity by Beneficial Microbes

Plant beneficial microorganisms improve the health and growth of the associated plants. Application of beneficial microbes triggers an enhanced resistance state, also termed as induced systemic resistance (ISR), in the host, against a broad range of pathogens. Upon the activation of ISR, plants employ long-distance systemic signaling to provide protection for distal tissue, inducing rapid and strong immune responses against pathogens invasions. The transmission of ISR signaling was commonly regarded to be a jasmonic acid- and ethylene-dependent, but salicylic acid-independent, transmission. However, in the last decade, the involvement of both salicylic acid and jasmonic acid/ethylene signaling pathways and the regulatory roles of small RNA in ISR has been updated. In this review, the plant early recognition, responsive reactions, and the related signaling transduction during the process of the plant-beneficial microbe interaction was discussed, with reflection on the crucial regulatory role of small RNAs in the beneficial microbe-mediated ISR.

Spatial-Temporal Response of Reactive Oxygen Species and Salicylic Acid Suggest Their Interaction in Pumpkin Rootstock-Induced Chilling Tolerance in Watermelon Plants

Grafting with pumpkin rootstock could improve chilling tolerance in watermelon, and salicylic acid (SA) as a signal molecule is involved in regulating plant tolerance to chilling and other abiotic stresses. To clarify the mechanism in pumpkin rootstock-induced systemic acquired acclimation in grafted watermelon under chilling stress, we used self-grafted (Cl/Cl) and pumpkin rootstock-grafted (Cl/Cm) watermelon seedlings to study the changes in lipid peroxidation, photosystem II (PSII) activity and antioxidant metabolism, the spatio-temporal response of SA biosynthesis and H2O2 accumulation to chilling, and the role of H2O2 signal in SA-induced chilling tolerance in grafted watermelon. The results showed that pumpkin rootstock grafting promoted SA biosynthesis in the watermelon scions. Chilling induced hydrolysis of conjugated SA into free SA in the roots and accumulation of free SA in the leaves in Cl/Cm plants. Further, pumpkin rootstock grafting induced early response of antioxidant enzyme system in the roots and increased activities of ascorbate peroxidase and glutathione reductase in the leaves, thus maintaining cellular redox homeostasis. Exogenous SA improved while the inhibition of SA biosynthesis reduced chilling tolerance in Cl/Cl seedlings. The application of diphenyleneiodonium (DPI, inhibitor of NADPH oxidase) and dimethylthiourea (DMTU, H2O2 scavenger) decreased, while exogenous H2O2 improved the PSII activity in Cl/Cl plants under chilling stress. Additionally, the decrease of the net photosynthetic rate in DMTU- and DPI-pretreated Cl/Cl plants under chilling conditions could be alleviated by subsequent application of H2O2 but not SA. In conclusion, pumpkin rootstock grafting induces SA biosynthesis and redistribution in the leaves and roots and participates in the regulation of antioxidant metabolism probably through interaction with the H2O2 signal, thus improving chilling tolerance in watermelon.

Biosynthesis and Roles of Salicylic Acid in Balancing Stress Response and Growth in Plants

Salicylic acid (SA) is an important plant hormone with a critical role in plant defense against pathogen infection. Despite extensive research over the past 30 year or so, SA biosynthesis and its complex roles in plant defense are still not fully understood. Even though earlier biochemical studies suggested that plants synthesize SA from cinnamate produced by phenylalanine ammonia lyase (PAL), genetic analysis has indicated that in Arabidopsis, the bulk of SA is synthesized from isochorismate (IC) produced by IC synthase (ICS). Recent studies have further established the enzymes responsible for the conversion of IC to SA in Arabidopsis. However, it remains unclear whether other plants also rely on the ICS pathway for SA biosynthesis. SA induces defense genes against biotrophic pathogens, but represses genes involved in growth for balancing defense and growth to a great extent through crosstalk with the growth-promoting plant hormone auxin. Important progress has been made recently in understanding how SA attenuates plant growth by regulating the biosynthesis, transport, and signaling of auxin. In this review, we summarize recent progress in the biosynthesis and the broad roles of SA in regulating plant growth during defense responses. Further understanding of SA production and its regulation of both defense and growth will be critical for developing better knowledge to improve the disease resistance and fitness of crops.

The Possibility of Using Paulownia elongata S. Y. Hu × Paulownia fortunei Hybrid for Phytoextraction of Toxic Elements from Post-Industrial Wastes with Biochar

The potential of the Paulownia hybrid for the uptake and transport of 67 elements along with the physiological response of plants cultivated in highly contaminated post-industrial wastes (flotation tailings—FT, and mining sludge—MS) was investigated. Biochar (BR) was added to substrates to limit metal mobility and facilitate plant survival. Paulownia could effectively uptake and translocate B, Ca, K, P, Rb, Re and Ta. Despite severe growth retardation, chlorophyll biosynthesis was not depleted, while an increased carotenoid content was noted for plants cultivated in waste materials. In Paulownia leaves and roots hydroxybenzoic acids (C6-C1) were dominant phenolics, and hydroxycinnamic acids/phenylpropanoids (C6-C3) and flavonoids (C6-C3-C6) were also detected. Plant cultivation in wastes resulted in quantitative changes in the phenolic fraction, and a significant drop or total inhibition of particular phenolics. Cultivation in waste materials resulted in increased biosynthesis of malic and succinic acids in the roots of FT-cultivated plants, and malic and acetic acids in the case of MS/BR substrate. The obtained results indicate that the addition of biochar can support the adaptation of Paulownia seedlings growing on MS, however, in order to limit unfavorable changes in the plant, an optimal addition of waste is necessary.

Heat-killed endophytic bacterium induces robust plant defense responses against important pathogens

Stress caused by pathogens strongly damages plants. Developing products to control plant disease is an important challenge in sustainable agriculture. In this study, a heat-killed endophytic bacterium (HKEB), Bacillus aryabhattai, is used to induce plant defense against fungal and bacterial pathogens, and the main defense pathways used by the HKEB to activate plant defense are revealed. The HKEB induced high protection against different pathogens through the salicylic and jasmonic acid pathways. We report the presence of gentisic acid in the HKEB for the first time. These results show that HKEBs may be a useful tool for the management of plant diseases.

The glycosyltransferase UGT76B1 modulates N-hydroxy-pipecolic acid homeostasis and plant immunity

The tradeoff between growth and defense is a critical aspect of plant immunity. Therefore, the plant immune response needs to be tightly regulated. Salicylic acid (SA) is an important plant hormone regulating defense against biotrophic pathogens. Recently, N-hydroxy-pipecolic acid (NHP) was identified as another regulator for plant innate immunity and systemic acquired resistance (SAR). Although the biosynthetic pathway leading to NHP formation is already been identified, how NHP is further metabolized is unclear. Here, we present UGT76B1 as a uridine diphosphate-dependent glycosyltransferase (UGT) that modifies NHP by catalyzing the formation of 1-O-glucosyl-pipecolic acid in Arabidopsis thaliana. Analysis of T-DNA and clustered regularly interspaced short palindromic repeats (CRISPR) knock-out mutant lines of UGT76B1 by targeted and nontargeted ultra-high performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS) underlined NHP and SA as endogenous substrates of this enzyme in response to Pseudomonas infection and UV treatment. ugt76b1 mutant plants have a dwarf phenotype and constitutive defense response which can be suppressed by loss of function of the NHP biosynthetic enzyme FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1). This suggests that elevated accumulation of NHP contributes to the enhanced disease resistance in ugt76b1. Externally applied NHP can move to distal tissue in ugt76b1 mutant plants. Although glycosylation is not required for the long-distance movement of NHP during SAR, it is crucial to balance growth and defense.

Signals in systemic acquired resistance of plants against microbial pathogens

After a local infection by the microbial pathogens, plants will produce strong resistance in distal tissues to cope with the subsequent biotic attacks. This type of the resistance in the whole plant is termed as systemic acquired resistance (SAR). The priming of SAR can confer the robust defense responses and the broad-spectrum disease resistances in plants. In general, SAR is activated by the signal substances generated at the local sites of infection, and these small signaling molecules can be rapidly transported to the systemic tissues through the phloem. In the last two decades, numerous endogenous metabolites were proved to be the potential elicitors of SAR, including methyl salicylate (MeSA), azelaic acid (AzA), glycerol-3-phosphate (G3P), free radicals (NO and ROS), pipecolic acid (Pip), N-hydroxy-pipecolic acid (NHP), dehydroabietinal (DA), monoterpenes (α-pinene and β-pinene) and NAD(P). In the meantime, the proteins associated with the transport of these signaling molecules were also identified, such as DIR1 (DEFECTIVE IN INDUCED RESISTANCE 1) and AZI1 (AZELAIC ACID INDUCED 1). This review summarizes the recent findings related to synthesis, transport and interaction of the different signal substances in SAR.

Metabolic Mechanism of Plant Defense against Rice Blast Induced by Probenazole

The probenazole fungicide is used for controlling rice blast (Magnaporthe grisea) primarily by inducing disease resistance of the plant. To investigate the mechanism of induced plant defense, rice seedlings were treated with probenazole at 15 days post emergence, and non-treated plants were used for the control. The plants were infected with M. grisea 5 days after chemical treatment and incubated in a greenhouse. After 7 days, rice seedlings were sampled. The metabolome of rice seedlings was chemically extracted and analyzed using gas chromatography and mass spectrum (GC-MS). The GC-MS data were processed using analysis of variance (ANOVA), principal component analysis (PCA) and metabolic pathway elucidation. Results showed that probenazole application significantly affected the metabolic profile of rice seedlings, and the effect was proportionally leveraged with the increase of probenazole concentration. Probenazole resulted in a change of 54 metabolites. Salicylic acid, γ-aminobutyrate, shikimate and several other primary metabolites related to plant resistance were significantly up-regulated and some metabolites such as phenylalanine, valine and proline were down-regulated in probenazole-treated seedlings. These results revealed a metabolic pathway of rice seedlings induced by probenazole treatment regarding the resistance to M. grisea infection.

Cloning, Expression, and Purification of a Pathogenesis-Related Protein from Oenanthe javanica and Its Biological Properties

Pathogenesis-related (PR) proteins are inducible and accumulated in plants upon pathogen challenge for survival. Interest in these proteins has arisen in many fields of research, including areas of protein defense mechanisms and plant-derived allergens. In this study, we cloned a PR protein gene (OJPR) from Oenanthe javanica, which consisted of 465 bp with an approximate molecular mass of 16 kDa. The DNA and deduced amino acid sequences of OJPR were 87% similar to Pimpinella brachycarpa PR-1 together with a glycine-rich loop which is a signature motif of PR-10. In microarray analysis, OJPR-transfected Raw264.7 (OJPR⁺) upregulated high mobility group box 1 and protein kinase Cα, and downregulated chemokine ligand 3 and interleukin 1β which are all related to toll-like receptor 4 (TLR4) and inflammation. TAK-242 and PD98059 inhibited the activation by OJPR, suggesting that OJPR transduce TLR4-mediated signaling. Interestingly, OJPR increased anti-viral repertoires, including interferon (IFN)α, IFNγ, OAS1, and Mx1 in CD4⁺ primary T cells. Taken together, we concluded that OJPR may play a role in modulating host defense responses via TLR signal transduction and provide new insights into the therapeutic and diagnostic advantages as a potential bioactive protein.

Roles of ethylene, jasmonic acid, and salicylic acid and their interactions in frankincense resin production in Boswellia sacra Flueck. trees

The roles of ethylene, jasmonic acid, and salicylic acid and their interactions in frankincense resin production in Boswellia sacra trees growing in the drylands of Oman were studied. On March 18 (Experiment 1) and September 17 (Experiment 2), 2018, 32-year-old B. sacra trees with multiple trunks were selected at the Agricultural Experiment Station, Sultan Qaboos University, Oman. Various lanolin pastes containing Ethrel, an ethylene-releasing compound; methyl jasmonate; sodium salicylate; and combinations of these compounds were applied to debarked wounds 15 mm in diameter on the trunks. After a certain period, the frankincense resin secreted from each wound was harvested and weighed. The anatomical characteristics of the resin ducts were also studied in the bark tissue near the upper end of each wound. The combination of Ethrel and methyl jasmonate greatly enhanced frankincense resin production within 7 days in both seasons. The application of methyl jasmonate alone, sodium salicylate alone or a combination of both did not affect resin production. These findings suggest a high possibility of artificial enhancement of frankincense resin production by the combined application of Ethrel and methyl jasmonate to B. sacra trees.

Cognitive and Memory Functions in Plant Immunity

From the time of Thucydides in the 5th century BC, it has been known that specific recognition of pathogens and memory formation are critical components of immune functions. In contrast to the immune system of jawed vertebrates, such as humans and mice, plants lack a circulatory system with mobile immune cells and a repertoire of clonally distributed antigen receptors with almost unlimited specificities. However, without these systems and mechanisms, plants can live and survive in the same hostile environment faced by other organisms. In fact, they achieve specific pathogen recognition and elimination, with limited self-reactivity, and generate immunological memory, sometimes with transgenerational characteristics. Thus, the plant immune system satisfies minimal conditions for constituting an immune system, namely, the recognition of signals in the milieu, integration of that information, subsequent efficient reaction based on the integrated information, and memorization of the experience. In the previous report, this set of elements was proposed as an example of minimal cognitive functions. In this essay, I will first review current understanding of plant immunity and then discuss the unique features of cognitive activities, including recognition of signals from external as well as internal environments, autoimmunity, and memory formation. In doing so, I hope to reach a deeper understanding of the significance of immunity omnipresent in the realm of living organisms.

Salicylic Acid-Producing Endophytic Bacteria Increase Nicotine Accumulation and Resistance against Wildfire Disease in Tobacco Plants

Endophytic bacteria (EB) are both a novel source of bioactive compounds that confer phytopathogen resistance and inducers of secondary metabolites in host plants. Twenty-seven EB isolated from various parts of Metasequoia glyptostroboides, Ginkgo biloba, Taxus brevifolia, Pinus densiflora, Salix babylonica, and S. chaenomeloides could produce salicylic acid (SA). The highest producers were isolates EB-44 and EB-47, identified as Pseudomonas tremae and Curtobacterium herbarum, respectively. Nicotiana benthamiana grown from EB-44-soaked seeds exhibited a 2.3-fold higher endogenous SA concentration and increased resistance against P. syringae pv. tabaci, the causative agent of tobacco wildfire disease, than plants grown from water-soaked seeds. N benthamiana and N. tabacum grown from EB-44-treated seeds developed 33% and 54% disease lesions, respectively, when infected with P. syringae pv. tabaci, and showed increased height and weight, in addition to 4.6 and 1.4-fold increases in nicotine accumulation, respectively. The results suggest that SA-producing EB-44 can successfully colonize Nicotiana spp., leading to increased endogenous SA production and resistance to tobacco wildfire disease. The newly isolated EB can offer an efficient and eco-friendly solution for controlling wildfire disease and nicotine accumulation in Nicotiana, with additional application for other important crops to increase both productivity and the generation of bioactive compounds.

Insight Into Late Wilting Disease of Cucumber Demonstrates the Complexity of the Phenomenon in Fluctuating Environments

Some diseases are caused by coinfection of several pathogens in the same plant. However, studies on the complexity of these coinfection events under different environmental conditions are scarce. Our ongoing research involves late wilting disease of cucumber caused by coinfection of Cucumber green mottle mosaic virus (CGMMV) and Pythium spp. We specifically investigated the role of various temperatures (18, 25, 32°C) on the coinfection by CGMMV and two predominant Pythium species occurring in cucumber greenhouses under Middle Eastern climatic conditions. During the summer months, Pythium aphanidermatum was most common, whereas P. spinosum predominated during the winter-spring period. P. aphanidermatum preferred higher temperatures while P. spinosum preferred low temperatures and caused very low levels of disease at 32°C when the 6-day-old seedlings were infected with P. spinosum alone. Nevertheless, after applying a later coinfection with CGMMV on the 14-day-old plants, a synergistic effect was detected for both Pythium species at optimal and suboptimal temperatures, with P. spinosum causing high mortality incidence even at 32°C. The symptoms caused by CGMMV infection appeared earlier as the temperature increased. However, within each temperature, no significant influence of the combined infection was detected. Our results demonstrate the complexity of coinfection in changing environmental conditions and indicate its involvement in disease development and severity as compared with infection by each of the pathogens alone.

MdWRKY46-Enhanced Apple Resistance to Botryosphaeria dothidea by Activating the Expression of MdPBS3.1 in the Salicylic Acid Signaling Pathway

Salicylic acid (SA) is closely related to disease resistance of plants. WRKY transcription factors have been linked to the growth and development of plants, especially under stress conditions. However, the regulatory mechanism of WRKY proteins involved in SA production and disease resistance in apple is not clear. In this study, MdPBS3.1 responded to Botryosphaeria dothidea and enhanced resistance to B. dothidea. Electrophoretic mobility shift assays, yeast one-hybrid assays, and chromatin immunoprecipitation and quantitative PCR demonstrated that MdWRKY46 can directly bind to a W-box motif in the promoter of MdPBS3.1. Glucuronidase transactivation and luciferase analysis further showed that MdWRKY46 can activate the expression of MdPBS3.1. Finally, B. dothidea inoculation in transgenic apple calli and fruits revealed that MdWRKY46 improved resistance to B. dothidea by the transcriptional activation of MdPBS3.1. Viral vector-based transformation assays indicated that MdWRKY46 elevates SA content and transcription of SA-related genes, including MdPR1, MdPR5, and MdNPR1 in an MdPBS3.1-dependent way. These findings provide new insights into how MdWRKY46 regulates plant resistance to B. dothidea through the SA signaling pathway.

Maize phenylalanine ammonia-lyases contribute to resistance to Sugarcane mosaic virus infection, most likely through positive regulation of salicylic acid accumulation

Sugarcane mosaic virus (SCMV) is a pathogen of worldwide importance that causes dwarf mosaic disease on maize (Zea mays). Until now, few maize genes/proteins have been shown to be involved in resistance to SCMV. In this study, we characterized the role of maize phenylalanine ammonia-lyases (ZmPALs) in accumulation of the defence signal salicylic acid (SA) and in resistance to virus infection. SCMV infection significantly increased SA accumulation and expression of SA-responsive pathogenesis-related protein genes (PRs). Interestingly, exogenous SA treatment decreased SCMV accumulation and enhanced resistance. Both reverse transcription-coupled quantitative PCR and RNA-Seq data confirmed that expression levels of at least four ZmPAL genes were significantly up-regulated upon SCMV infection. Knockdown of ZmPAL expression led to enhanced SCMV infection symptom severity and virus multiplication, and simultaneously resulted in decreased SA accumulation and PR gene expression. Intriguingly, application of exogenous SA to SCMV-infected ZmPAL-silenced maize plants decreased SCMV accumulation, showing that ZmPALs are required for SA-mediated resistance to SCMV infection. In addition, lignin measurements and metabolomic analysis showed that ZmPALs are also involved in SCMV-induced lignin accumulation and synthesis of other secondary metabolites via the phenylpropanoid pathway. In summary, our results indicate that ZmPALs are required for SA accumulation in maize and are involved in resistance to virus infection by limiting virus accumulation and moderating symptom severity.

The cotton GhWIN2 gene activates the cuticle biosynthesis pathway and influences the salicylic and jasmonic acid biosynthesis pathways

BACKGROUND

Metabolic pathways are interconnected and yet relatively independent. Genes involved in metabolic modules are required for the modules to run. Study of the relationships between genes and metabolic modules improves the understanding of metabolic pathways in plants. The WIN transcription factor activates the cuticle biosynthesis pathway and promotes cuticle biosynthesis. The relationship between the WIN transcription factor and other metabolic pathways is unknown. Our aim was to determine the relationships between the main genes involved in cuticle biosynthesis and those involved in other metabolic pathways. We did this by cloning a cotton WIN gene, GhWIN2, and studying its influence on other pathways.

RESULTS

As with other WIN genes, GhWIN2 regulated expression of cuticle biosynthesis-related genes, and promoted cuticle formation. Silencing of GhWIN2 resulted in enhanced resistance to Verticillium dahliae, caused by increased content of salicylic acid (SA). Moreover, silencing of GhWIN2 suppressed expression of jasmonic acid (JA) biosynthesis-related genes and content. GhWIN2 positively regulated the fatty acid biosynthesis pathway upstream of the JA biosynthesis pathway. Silencing of GhWIN2 reduced the content of stearic acid, a JA biosynthesis precursor.

CONCLUSIONS

GhWIN2 not only regulated the cuticle biosynthesis pathway, but also positively influenced JA biosynthesis and negatively influenced SA biosynthesis.

Ethylene Response Factor ERF11 Activates BT4 Transcription to Regulate Immunity to Pseudomonas syringae

Pseudomonas syringae, a major hemibiotrophic bacterial pathogen, causes many devastating plant diseases. However, the transcriptional regulation of plant defense responses to P. syringae remains largely unknown. Here, we found that gain-of-function of BTB AND TAZ DOMAIN PROTEIN 4 (BT4) enhanced the resistance of Arabidopsis (Arabidopsis thaliana) to Pst DC3000 (Pseudomonas syringae pv. tomato DC3000). Disruption of BT4 also weakened the salicylic acid (SA)-induced defense response to Pst DC3000 in bt4 mutants. Further investigation indicated that, under Pst infection, transcription of BT4 is modulated by components of both the SA and ethylene (ET) signaling pathways. Intriguingly, the specific binding elements of ETHYLENE RESPONSE FACTOR (ERF) proteins, including dehydration responsive/C-repeat elements and the GCC box, were found in the putative promoter of BT4 Based on publicly available microarray data and transcriptional confirmation, we determined that ERF11 is inducible by salicylic acid and Pst DC3000 and is modulated by the SA and ET signaling pathways. Consistent with the function of BT4, loss-of-function of ERF11 weakened Arabidopsis resistance to Pst DC3000 and the SA-induced defense response. Biochemical and molecular assays revealed that ERF11 binds specifically to the GCC box of the BT4 promoter to activate its transcription. Genetic studies further revealed that the BT4-regulated Arabidopsis defense response to Pst DC3000 functions directly downstream of ERF11. Our findings indicate that transcriptional activation of BT4 by ERF11 is a key step in SA/ET-regulated plant resistance against Pst DC3000, enhancing our understanding of plant defense responses to hemibiotrophic bacterial pathogens.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

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.

MiRNA160 is associated with local defense and systemic acquired resistance against Phytophthora infestans infection in potato

To combat pathogen infection, plants employ local defenses in infected sites and elicit systemic acquired resistance (SAR) in distant tissues. MicroRNAs have been shown to play a significant role in local defense, but their association with SAR is unknown. In addition, no such studies of the interaction between potato and Phytophthora infestans have been reported. We investigated the role of miR160 in local and SAR responses to P. infestans infection in potato. Expression analysis revealed induced levels of miR160 in both local and systemic leaves of infected wild-type plants. miR160 overexpression and knockdown plants exhibited increased susceptibility to infection, suggesting that miR160 levels equivalent to those of wild-type plants may be necessary for mounting local defense responses. Additionally, miR160 knockdown lines failed to elicit SAR, and grafting assays indicated that miR160 is required in both local and systemic leaves to trigger SAR. Consistently, SAR-associated signals and genes were dysregulated in miR160 knockdown lines. Furthermore, analysis of the expression of defense and auxin pathway genes and direct regulation of StGH3.6, a mediator of salicylic acid-auxin cross-talk, by the miR160 target StARF10 revealed the involvement of miR160 in antagonistic cross-talk between salicylic acid-mediated defense and auxin-mediated growth pathways. Overall, our study demonstrates that miR160 plays a crucial role in local defense and SAR responses during the interaction between potato and P. infestans.

SA inhibits complex III activity to generate reactive oxygen species and thereby induces GA overproduction in Ganoderma lucidum

Ganoderma lucidum has high commercial value because it produces many active compounds, such as ganoderic acids (GAs). Salicylic acid (SA) was previously reported to induce the biosynthesis of GA in G. lucidum. In this study, we found that SA induces GA biosynthesis by increasing ROS production, and further research found that NADPH oxidase-silenced strains exhibited a partial reduction in the response to SA, resulting in the induction of increased ROS production. Furthermore, the localization of ROS shows that mitochondria are sources of ROS production in response to SA treatment. An additional analysis focused on the relationship between SA-induced ROS production and mitochondrial functions, and the results showed that inhibitors of mitochondrial complexes I and II exert approximately 40–50% superimposed inhibitory effects on the respiration rate and H₂O₂ content when co-administered with SA. However, no obvious superimposed inhibition effects were observed in the sample co-treated with mitochondrial complex III inhibitor and SA, implying that the inhibitor of mitochondrial complex III and SA might act on the same site in mitochondria. Additional experiments revealed that complex III activity was decreased 51%, 62% and 75% after treatment with 100, 200, and 400 µM SA, respectively. Our results highlight the finding that SA inhibits mitochondrial complex III activity to increase ROS generation. In addition, inhibition of mitochondrial complex III caused ROS accumulation, which plays an essential role in SA-mediated GA biosynthesis in G. lucidum. This conclusion was also demonstrated in complex III-silenced strains. To the best of our knowledge, this study provides the first demonstration that SA inhibits complex III activity to increase the ROS levels and thereby regulate secondary metabolite biosynthesis.

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Mitochondria as a source of salicylic acid (SA) induced reactive oxygen species (ROS) production in Ganoderma lucidum.

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SA induces the accumulation of ganoderic acids in Ganoderma lucidum by mitochondria ROS overproduction.

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SA inhibits mitochondrial complex III activity to increase ROS and thereby induces ganoderic acids biosynthesis.

Isochorismate-based salicylic acid biosynthesis confers basal resistance to Fusarium graminearum in barley

Salicylic acid (SA) plays an important role in signal transduction and disease resistance. In Arabidopsis, SA can be made by either of two biosynthetic branches, one involving isochorismate synthase (ICS) and the other involving phenylalanine ammonia-lyase (PAL). However, the biosynthetic pathway and the importance of SA remain largely unknown in Triticeae. Here, we cloned one ICS and seven PAL genes from barley, and studied their functions by their overexpression and suppression in that plant. Suppression of the ICS gene significantly delayed plant growth, whereas PAL genes, both overexpressed and suppressed, had no significant effect on plant growth. Similarly, suppression of ICS compromised plant resistance to Fusarium graminearum, whereas similar suppression of PAL genes had no significant effect. We then focused on transgenic plants with ICS. In a leaf-based test with F. graminearum, transgenic plants with an up-regulated ICS were comparable with wild-type control plants. By contrast, transgenic plants with a suppressed ICS lost the ability to accumulate SA during pathogen infection and were also more susceptible to Fusarium than the wild-type controls. This suggests that ICS plays a unique role in SA biosynthesis in barley, which, in turn, confers a basal resistance to F. graminearum by modulating the accumulation of H2 O2 , O2- and reactive oxygen-associated enzymatic activities. Although SA mediates systemic acquired resistance (SAR) in dicots, there was no comparable SAR response to F. graminearum in barley. This study expands our knowledge about SA biosynthesis in barley and proves that SA confers basal resistance to fungal pathogens.

Ten Prominent Host Proteases in Plant-Pathogen Interactions

Proteases are enzymes integral to the plant immune system. Multiple aspects of defence are regulated by proteases, including the hypersensitive response, pathogen recognition, priming and peptide hormone release. These processes are regulated by unrelated proteases residing at different subcellular locations. In this review, we discuss 10 prominent plant proteases contributing to the plant immune system, highlighting the diversity of roles they perform in plant defence.

Potyviral Gene-Silencing Suppressor HCPro Interacts with Salicylic Acid (SA)-Binding Protein 3 to Weaken SA-Mediated Defense Responses

The viral infection process is a battle between host defense response and pathogen antagonizing action. Several studies have established a tight link between the viral RNA silencing suppressor (RSS) and the repression of salicylic acid (SA)-mediated defense responses, nonetheless host factors directly linking an RSS and the SA pathway remains unidentified. From yeast two-hybrid analysis, we identified an interaction between the potyviral RSS helper-component proteinase (HCPro) and SA-binding protein SABP3. Co-localization and bimolecular fluorescence complementation analyses validated the direct in vivo interaction between Turnip mosaic virus (TuMV) HCPro and the Arabidopsis homologue of SABP3, AtCA1. Additionally, transient expression of TuMV HCPro demonstrated its ability to act as a negative regulator of AtCA1. When the plants of the AtCA1 knockout mutant line were inoculated with TuMV, our results indicated that AtCA1 is essential to restrict viral spreading and accumulation, induce SA accumulation, and trigger the SA pathway. Unexpectedly, the AtCA1 overexpression line also displayed a similar phenotype, suggesting that the constitutive expression of AtCA1 antagonizes the SA pathway. Taken together, our results depict AtCA1 as an essential regulator of SA defense responses. Moreover, the interaction of potyviral HCPro with this regulator compromises the SA pathway to weaken host defense responses and facilitate viral infection.

Arsenic forms and their combinations induce differences in phenolic accumulation in Ulmus laevis Pall

Total phenolics and the profile of phenolic acids and flavonoids were investigated in the roots and leaves of Ulmus laevis cultured on the medium with inorganic and organic arsenic - As(III), As(V) and DMA(V) at 0.06mM and their equimolar combinations. Further, the accumulation of salicylic acid (free and glucoside-bound) and lipid oxidation were assayed following a three-month long experiment. As treatment caused elevated production of phenolics, which was higher in photosynthetic tissue than in roots for all As forms and their combinations, and their overall content was correlated with the accumulation of organic As in roots and As(III) in leaves. The accumulation of organic As strongly induced shikimate-derived protocatechiuc acid in roots. Contrary to this, shikimate-derived phenolics (protocatechuic, gallic acids and 4-HBA) were suppressed in leaves, while the accumulation of C6C3 acids (caffeic, p-coumaric and chlorogenic) was stimulated by As(V) application. Surprisingly, these acids were not detected in the leaves of As(III)-treated plants, and mutually applied As(III) and DMA(V) reduced their content. DMA(V) negatively influenced the level of salicylic acid and its storage mechanism and this effect correlated with elevated MDA content in leaves. Quercetin accumulation was observed in both organs (mainly leaves) of DMA(V)-treated plants thereby proving its function in defensive response of Ulmus laevis to organic forms of As.

Perception of Salicylic Acid in Physcomitrella patens

Salicylic acid (SA) is a key signaling molecule in plant immunity. Two types of SA receptors, NPR1 and NPR3/NPR4, were reported to be involved in the perception of SA in Arabidopsis. SA is also synthesized in the non-vascular moss Physcomitrella patens following pathogen infection. Sequence analysis revealed that there is only one NPR1/NPR3/NPR4-like protein in P. patens. This agrees with the phylogenetic study that showed the divergence of NPR1 and NPR3/NPR4 from the same ancestor during the evolution of higher plants. Intriguingly, expression of the P. patens NPR1/NPR3/NPR4-like gene in Arabidopsis does not complement the constitutive defense phenotype of the npr3 npr4 double mutant, but can partially rescue the mutant phenotypes of npr1-1, suggesting that it functions as an NPR1-like positive regulator of SA-mediated immunity and P. patens does not have an SA receptor functioning similarly as NPR3/NPR4. Future characterization of the P. patens NPR1-like protein and analysis of its functions through knockout and biochemical approaches will help us better understand how SA is perceived and what its functions are in P. patens.

Biotic factors that induce the tomato Ve1 R-gene

In tomato, Verticillium resistance is determined by the Ve gene locus encoding two leucine-rich repeat-receptor-like proteins (Ve1, Ve2). The resistance function usually is attributed to Ve1 alone, with two known alleles: Ve1, encoding a resistance protein, and ve1, with a premature stop codon encoding a truncated product. We have examined further Ve-gene expression in resistant and susceptible near-isolines of Verticillium-infected Craigella tomatoes, using both quantitative RT-PCR and an alternative RFLP assay. Ve1 is induced differentially in resistant and susceptible plants, while Ve2 is constitutively expressed throughout disease development. Contrary to their putative role in Verticillium resistance, these profiles were observed even with compatible Verticillium interactions, some bacterial pathogens, and transgenic tomato plants expressing the fungal Ave1 effector. This suggests broader roles in disease and/or stress. To determine the contribution of plant hormones, abscisic acid, methyl jasmonate, naphthaleneacetic acid or salicylic acid were infused independently via the tomato root and effects on Ve1 induction were confirmed using biosynthesis mutants. While all the hormones modulated Ve1-gene induction, abscisic acid and salicylic acid were not required while jasmonic acid appears to play a more direct role.

Infestation of Broad Bean (Vicia faba) by the Green Stink Bug (Nezara viridula) Decreases Shoot Abscisic Acid Contents under Well-Watered and Drought Conditions

The response of broad bean (Vicia faba) plants to water stress alone and in combination with green stink bug (Nezara viridula) infestation was investigated through measurement of: (1) leaf gas exchange; (2) plant hormone titres of abscisic acid (ABA) and its metabolites, and of salicylic acid (SA); and (3) hydrogen peroxide (H2O2) content. Furthermore, we evaluated the effects of experimentally water-stressed broad-bean plants on N. viridula performance in terms of adult host-plant preference, and nymph growth and survival. Water stress significantly reduced both photosynthesis (A) and stomatal conductance (gs ), while infestation by the green stink bug had no effects on photosynthesis but significantly altered partitioning of ABA between roots and shoots. Leaf ABA was decreased and root ABA increased as a result of herbivore attack, under both well-watered and water-deprived conditions. Water stress significantly impacted on SA content in leaves, but not on H2O2. However, infestation of N. viridula greatly increased both SA and H2O2 contents in leaves and roots, which suggests that endogenous SA and H2O2 have roles in plant responses to herbivore infestation. No significant differences were seen for green stink bug choice between well-watered and water-stressed plants. However, for green stink bug nymphs, plant water stress promoted significantly lower weight increases and significantly higher mortality, which indicates that highly water-stressed host plants are less suitable for N. viridula infestation. In conclusion two important findings emerged: (i) association of water stress with herbivore infestation largely changes plant response in terms of phytohormone contents; but (ii) water stress does not affect the preference of the infesting insects, although their performance was impaired.

Diverse mechanisms of resistance to Pseudomonas syringae in a thousand natural accessions of Arabidopsis thaliana

Plants are continuously threatened by pathogen attack and, as such, they have evolved mechanisms to evade, escape and defend themselves against pathogens. However, it is not known what types of defense mechanisms a plant would already possess to defend against a potential pathogen that has not co-evolved with the plant. We addressed this important question in a comprehensive manner by studying the responses of 1041 accessions of Arabidopsis thaliana to the foliar pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. We characterized the interaction using a variety of established methods, including different inoculation techniques, bacterial mutant strains, and assays for the hypersensitive response, salicylic acid (SA) accumulation and reactive oxygen species production . Fourteen accessions showed resistance to infection by Pst DC3000. Of these, two accessions had a surface-based mechanism of resistance, six showed a hypersensitive-like response while three had elevated SA levels. Interestingly, A. thaliana was discovered to have a recognition system for the effector AvrPto, and HopAM1 was found to modulate Pst DC3000 resistance in two accessions. Our comprehensive study has significant implications for the understanding of natural disease resistance mechanisms at the species level and for the ecology and evolution of plant-pathogen interactions.

Salicylic Acid-Dependent Plant Stress Signaling via Mitochondrial Succinate Dehydrogenase

Mitochondria are known for their role in ATP production and generation of reactive oxygen species, but little is known about the mechanism of their early involvement in plant stress signaling. The role of mitochondrial succinate dehydrogenase (SDH) in salicylic acid (SA) signaling was analyzed using two mutants: disrupted in stress response1 (dsr1), which is a point mutation in SDH1 identified in a loss of SA signaling screen, and a knockdown mutant (sdhaf2) for SDH assembly factor 2 that is required for FAD insertion into SDH1. Both mutants showed strongly decreased SA-inducible stress promoter responses and low SDH maximum capacity compared to wild type, while dsr1 also showed low succinate affinity, low catalytic efficiency, and increased resistance to SDH competitive inhibitors. The SA-induced promoter responses could be partially rescued in sdhaf2, but not in dsr1, by supplementing the plant growth media with succinate. Kinetic characterization showed that low concentrations of either SA or ubiquinone binding site inhibitors increased SDH activity and induced mitochondrial H₂O₂ production. Both dsr1 and sdhaf2 showed lower rates of SA-dependent H₂O₂ production in vitro in line with their low SA-dependent stress signaling responses in vivo. This provides quantitative and kinetic evidence that SA acts at or near the ubiquinone binding site of SDH to stimulate activity and contributes to plant stress signaling by increased rates of mitochondrial H₂O₂ production, leading to part of the SA-dependent transcriptional response in plant cells.

Modulation of host plant immunity by Tobamovirus proteins

BACKGROUND

To establish successful infection, plant viruses produce profound alterations of host physiology, disturbing unrelated endogenous processes and contributing to the development of disease. In tobamoviruses, emerging evidence suggests that viral-encoded proteins display a great variety of functions beyond the canonical roles required for virus structure and replication. Among these, their modulation of host immunity appears to be relevant in infection progression.

SCOPE

In this review, some recently described effects on host plant physiology of Tobacco mosaic virus (TMV)-encoded proteins, namely replicase, movement protein (MP) and coat protein (CP), are summarized. The discussion is focused on the effects of each viral component on the modulation of host defense responses, through mechanisms involving hormonal imbalance, innate immunity modulation and antiviral RNA silencing. These effects are described taking into consideration the differential spatial distribution and temporality of viral proteins during the dynamic process of replication and spread of the virus.

CONCLUSION

In discussion of these mechanisms, it is shown that both individual and combined effects of viral-encoded proteins contribute to the development of the pathogenesis process, with the host plant's ability to control infection to some extent potentially advantageous to the invading virus.

The Wheat Mediator Subunit TaMED25 Interacts with the Transcription Factor TaEIL1 to Negatively Regulate Disease Resistance against Powdery Mildew

Powdery mildew, caused by the biotrophic fungal pathogen Blumeria graminis f. sp. tritici, is a major limitation for the production of bread wheat (Triticum aestivum). However, to date, the transcriptional regulation of bread wheat defense against powdery mildew remains largely unknown. Here, we report the function and molecular mechanism of the bread wheat Mediator subunit 25 (TaMED25) in regulating the bread wheat immune response signaling pathway. Three homoalleles of TaMED25 from bread wheat were identified and mapped to chromosomes 5A, 5B, and 5D, respectively. We show that knockdown of TaMED25 by barley stripe mosaic virus-induced gene silencing reduced bread wheat susceptibility to the powdery mildew fungus during the compatible plant-pathogen interaction. Moreover, our results indicate that MED25 may play a conserved role in regulating bread wheat and barley (Hordeum vulgare) susceptibility to powdery mildew. Similarly, bread wheat ETHYLENE INSENSITIVE3-LIKE1 (TaEIL1), an ortholog of Arabidopsis (Arabidopsis thaliana) ETHYLENE INSENSITIVE3, negatively regulates bread wheat resistance against powdery mildew. Using various approaches, we demonstrate that the conserved activator-interacting domain of TaMED25 interacts physically with the separate amino- and carboxyl-terminal regions of TaEIL1, contributing to the transcriptional activation activity of TaEIL1. Furthermore, we show that TaMED25 and TaEIL1 synergistically activate ETHYLENE RESPONSE FACTOR1 (TaERF1) transcription to modulate bread wheat basal disease resistance to B. graminis f. sp. tritici by repressing the expression of pathogenesis-related genes and deterring the accumulation of reactive oxygen species. Collectively, we identify the TaMED25-TaEIL1-TaERF1 signaling module as a negative regulator of bread wheat resistance to powdery mildew.

Spatial and temporal regulation of biosynthesis of the plant immune signal salicylic acid

The plant hormone salicylic acid (SA) is essential for local defense and systemic acquired resistance (SAR). When plants, such as Arabidopsis, are challenged by different pathogens, an increase in SA biosynthesis generally occurs through transcriptional induction of the key synthetic enzyme isochorismate synthase 1 (ICS1). However, the regulatory mechanism for this induction is poorly understood. Using a yeast one-hybrid screen, we identified two transcription factors (TFs), NTM1-like 9 (NTL9) and CCA1 hiking expedition (CHE), as activators of ICS1 during specific immune responses. NTL9 is essential for inducing ICS1 and two other SA synthesis-related genes, phytoalexin-deficient 4 (PAD4) and enhanced disease susceptibility 1 (EDS1), in guard cells that form stomata. Stomata can quickly close upon challenge to block pathogen entry. This stomatal immunity requires ICS1 and the SA signaling pathway. In the ntl9 mutant, this response is defective and can be rescued by exogenous application of SA, indicating that NTL9-mediated SA synthesis is essential for stomatal immunity. CHE, the second identified TF, is a central circadian clock oscillator and is required not only for the daily oscillation in SA levels but also for the pathogen-induced SA synthesis in systemic tissues during SAR. CHE may also regulate ICS1 through the known transcription activators calmodulin binding protein 60g (CBP60g) and systemic acquired resistance deficient 1 (SARD1) because induction of these TF genes is compromised in the che-2 mutant. Our study shows that SA biosynthesis is regulated by multiple TFs in a spatial and temporal manner and therefore fills a gap in the signal transduction pathway between pathogen recognition and SA production.

Selectable marker-free co-expression of Nicotiana rustica CN and Nicotiana tabacum HAK1 genes improves resistance to tobacco mosaic virus in tobacco

The viral disease caused by tobacco mosaic virus (TMV) is the most prevalent viral disease in many tobacco production areas. A breeding strategy based on resistance genes is an effective method for improving TMV resistance in tobacco. Also, the physiological status of plants is also critical to disease resistance improvement. Potassium ion is one of the most abundant inorganic nutrients in plant cells, and mediates plant responses to abiotic and biotic stresses. Improving K+ content in soil by fertilising can enhance diseases resistance of crops. However, the K+ absorption in plants depends mostly on K+ transporters located in cytoplasmic membrane. Therefore, the encoding genes for K+ transporters are putative candidates to target for improving tobacco mosaic virus resistance. In this work, the synergistic effect of a N-like resistance gene CN and a tobacco putative potassium transporter gene HAK1 was studied. The results showed that TMV-resistance in CN-HAK1-containing tobaccos was significantly enhanced though a of strengthening leaf thickness and reduction in the size of necrotic spots compared with only CN-containing plants, indicating the improvement of potassium nutrition in plant cells could increase the tobacco resistance to TMV by reducing the spread of the virus. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis for TMV-CP expression in the inoculated leaf of the transgenic and wild-type plants also supported the conclusion. Further, the results of defence-related determination including antioxidative enzymes (AOEs) activity, salicylic acid (SA) content and the expression of resistance-related genes demonstrated CN with HAK1 synergistically enhanced TMV-resistance in transgenic tobaccos. Additionally, the HAK1- overexpression significantly improved the photosynthesis and K+-enriching ability in trans-CN-HAK1 tobaccos, compared with other counterparts. Finally, this work provides a method for screening new varieties of marker-free and safe transgenic antiviral tobacco.

Salicylic acid biosynthesis is enhanced and contributes to increased biotrophic pathogen resistance in Arabidopsis hybrids

Heterosis, the phenotypic superiority of a hybrid over its parents, has been demonstrated for many traits in Arabidopsis thaliana, but its effect on defence remains largely unexplored. Here, we show that hybrids between some A. thaliana accessions show increased resistance to the biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Comparisons of transcriptomes between these hybrids and their parents after inoculation reveal that several key salicylic acid (SA) biosynthesis genes are significantly upregulated in hybrids. Moreover, SA levels are higher in hybrids than in either parent. Increased resistance to Pst DC3000 is significantly compromised in hybrids of pad4 mutants in which the SA biosynthesis pathway is blocked. Finally, increased histone H3 acetylation of key SA biosynthesis genes correlates with their upregulation in infected hybrids. Our data demonstrate that enhanced activation of SA biosynthesis in A. thaliana hybrids may contribute to their increased resistance to a biotrophic bacterial pathogen.

Salicylic acid biosynthesis is enhanced and contributes to increased biotrophic pathogen resistance in Arabidopsis hybrids

Heterosis, the phenotypic superiority of a hybrid over its parents, has been demonstrated for many traits in Arabidopsis thaliana, but its effect on defence remains largely unexplored. Here, we show that hybrids between some A. thaliana accessions show increased resistance to the biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Comparisons of transcriptomes between these hybrids and their parents after inoculation reveal that several key salicylic acid (SA) biosynthesis genes are significantly upregulated in hybrids. Moreover, SA levels are higher in hybrids than in either parent. Increased resistance to Pst DC3000 is significantly compromised in hybrids of pad4 mutants in which the SA biosynthesis pathway is blocked. Finally, increased histone H3 acetylation of key SA biosynthesis genes correlates with their upregulation in infected hybrids. Our data demonstrate that enhanced activation of SA biosynthesis in A. thaliana hybrids may contribute to their increased resistance to a biotrophic bacterial pathogen.

The molecular basis for heterosis, the phenomenon whereby hybrid plants show phenotypic superiority to their parents, remains poorly understood. Here, Yang et al. show that salicylic acid biosynthesis is enhanced in hybrids of Arabidopsis thaliana and correlates with heterosis for pathogen defence.

Unmasking host and microbial strategies in the Agrobacterium-plant defense tango

Coevolutionary forces drive adaptation of both plant-associated microbes and their hosts. Eloquently captured in the Red Queen Hypothesis, the complexity of each plant-pathogen relationship reflects escalating adversarial strategies, but also external biotic and abiotic pressures on both partners. Innate immune responses are triggered by highly conserved pathogen-associated molecular patterns, or PAMPs, that are harbingers of microbial presence. Upon cell surface receptor-mediated recognition of these pathogen-derived molecules, host plants mount a variety of physiological responses to limit pathogen survival and/or invasion. Successful pathogens often rely on secretion systems to translocate host-modulating effectors that subvert plant defenses, thereby increasing virulence. Host plants, in turn, have evolved to recognize these effectors, activating what has typically been characterized as a pathogen-specific form of immunity. Recent data support the notion that PAMP-triggered and effector-triggered defenses are complementary facets of a convergent, albeit differentially regulated, set of immune responses. This review highlights the key players in the plant's recognition and signal transduction pathways, with a focus on the aspects that may limit Agrobacterium tumefaciens infection and the ways it might overcome those defenses. Recent advances in the field include a growing appreciation for the contributions of cytoskeletal dynamics and membrane trafficking to the regulation of these exquisitely tuned defenses. Pathogen counter-defenses frequently manipulate the interwoven hormonal pathways that mediate host responses. Emerging systems-level analyses include host physiological factors such as circadian cycling. The existing literature indicates that varying or even conflicting results from different labs may well be attributable to environmental factors including time of day of infection, temperature, and/or developmental stage of the host plant.

Signal regulators of systemic acquired resistance

Salicylic acid (SA) is an important phytohormone that plays a vital role in a number of physiological responses, including plant defense. The last two decades have witnessed a number of breakthroughs related to biosynthesis, transport, perception and signaling mediated by SA. These findings demonstrate that SA plays a crictical role in both local and systemic defense responses. Systemic acquired resistance (SAR) is one such SA-dependent response. SAR is a long distance signaling mechanism that provides broad spectrum and long-lasting resistance to secondary infections throughout the plant. This unique feature makes SAR a highly desirable trait in crop production. This review summarizes the recent advances in the role of SA in SAR and discusses its relationship to other SAR inducers.

R-gene-mediated resistance to plant viruses

Most of the reported dominant disease-resistance genes in plants, R genes, encode NB-LRR immune receptors. Plant genomes carry many NB-LRR type R genes that recognize specific pathogens and induce resistance against them. Thus, this immune system in plants is thought to perform similar functions as the adaptive immune system in animals. In this review, we provide an overview of the resistance mechanisms, evolution, and agricultural applications of R genes against plant viruses. We also introduce recent advances in research into the regulatory mechanisms of R gene expression, focusing on regulation by microRNAs and introns. One of the most intriguing phenomena that occur following R gene-mediated recognition of viruses is programmed cell death around the initial infection site, although its significance in the survival strategies of plants remains to be elucidated. We discuss the possible benefits for plants of inducing such programmed cell death based on our empirical observations and some hypotheses from an ecological point of view.