Differential Quantitative Requirements for NPR1 Between Basal Immunity and Systemic Acquired Resistance in Arabidopsis thaliana

Non-expressor of pathogenesis-related (PR) genes1 (NPR1) is a key transcription coactivator of plant basal immunity and systemic acquired resistance (SAR). Two mutant alleles, npr1-1 and npr1-3, have been extensively used for dissecting the role of NPR1 in various signaling pathways. However, it is unknown whether npr1-1 and npr1-3 are null mutants. Moreover, the NPR1 transcript levels are induced two- to threefold upon pathogen infection or salicylic acid (SA) treatment, but the biological relevance of the induction is unclear. Here, we used molecular and biochemical approaches including quantitative PCR, immunoblot analysis, site-directed mutagenesis, and CRISPR/Cas9-mediated gene editing to address these questions. We show that npr1-3 is a potential null mutant, whereas npr1-1 is not. We also demonstrated that a truncated npr1 protein longer than the hypothesized npr1-3 protein is not active in SA signaling. Furthermore, we revealed that TGACG-binding (TGA) factors are required for NPR1 induction, but the reverse TGA box in the 5'UTR of NPR1 is dispensable for the induction. Finally, we show that full induction of NPR1 is required for basal immunity, but not for SAR, whereas sufficient basal transcription is essential for full-scale establishment of SAR. Our results indicate that induced transcript accumulation may be differentially required for different functions of a specific gene. Moreover, as npr1-1 is not a null mutant, we recommend that future research should use npr1-3 and potential null T-DNA insertion mutants for dissecting NPR1's function in various physiopathological processes.

Expression of Putative Defense Responses in Cannabis Primed by Pseudomonas and/or Bacillus Strains and Infected by Botrytis cinerea

Cannabis (Cannabis sativa L.) offers many industrial, agricultural, and medicinal applications, but is commonly threatened by the gray mold disease caused by the fungus Botrytis cinerea. With few effective control measures currently available, the use of beneficial rhizobacteria represents a promising biocontrol avenue for cannabis. To counter disease development, plants rely on a complex network of inducible defense pathways, allowing them to respond locally and systemically to pathogens attacks. In this study, we present the first attempt to control gray mold in cannabis using beneficial rhizobacteria, and the first investigation of cannabis defense responses at the molecular level. Four promising Pseudomonas (LBUM223 and WCS417r) and Bacillus strains (LBUM279 and LBUM979) were applied as single or combined root treatments to cannabis seedlings, which were subsequently infected by B. cinerea. Symptoms were recorded and the expression of eight putative defense genes was monitored in leaves by reverse transcription quantitative polymerase chain reaction. The rhizobacteria did not significantly control gray mold and all infected leaves were necrotic after a week, regardless of the treatment. Similarly, no systemic activation of putative cannabis defense genes was reported, neither triggered by the pathogen nor by the rhizobacteria. However, this work identified five putative defense genes (ERF1, HEL, PAL, PR1, and PR2) that were strongly and sustainably induced locally at B. cinerea's infection sites, as well as two stably expressed reference genes (TIP41 and APT1) in cannabis. These markers will be useful in future researches exploring cannabis defense pathways.

Mycovirus-Induced Hypervirulence of Leptosphaeria biglobosa Enhances Systemic Acquired Resistance to Leptosphaeria maculans in Brassica napus

Phoma stem canker (blackleg) is one of the most important diseases of winter oilseed rape (Brassica napus) worldwide and is caused by a complex that comprises at least two species: Leptosphaeria maculans and L. biglobosa. Screening a panel of field Leptosphaeria isolates from B. napus for the presence of mycoviruses revealed the presence of a novel double-stranded RNA quadrivirus in L. biglobosa and no viruses in L. maculans. Following elimination of the mycovirus, virus-infected and virus-free isogenic lines of L. biglobosa were created. A direct comparison of the growth and virulence of these isogenic lines illustrated that virus infection caused hypervirulence and resulted in induced systemic resistance toward L. maculans in B. napus following lower leaf preinoculation with the virus-infected isolate. Analysis of the plant transcriptome suggests that the presence of the virus leads to subtle alterations in metabolism and plant defenses. For instance, transcripts involved in carbohydrate and amino acid metabolism are enriched in plants treated with the virus-infected isolate, while pathogenesis-related proteins, chitinases and WRKY transcription factors are differentially expressed. These results illustrate the potential for deliberate inoculation of plants with hypervirulent L. biglobosa to decrease the severity of Phoma stem canker later in the growing season.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Phenolic Compound Induction in Plant-Microbe and Plant-Insect Interactions: A Meta-Analysis

Plants rely on a variety of ways to protect themselves from being fed upon, including de novo production of specific compounds such as those termed as phenolics. Phenolics are often described as important in plant health and numerous studies have concluded they increase as a result of insect feeding, pathogen infection, or beneficial microorganism colonization. However, there are some studies reaching differing conclusions. Therefore, meta-analyses were conducted to observe whether common trends in phenolic induction in plants can be made when they become hosts to insects or microorganisms. Four hypotheses were tested. The first was that total phenolics increase as a generic response, and meta-analyses confirmed that this occurs when plants are infested with insects or colonized by bacterial or fungal microorganisms, but not for oomycetes. The second hypothesis was that phenolic induction is different when a beneficial microorganism colonizes a plant vs. when a plant is infected by a pathogen. Beneficial bacteria, pathogenic bacteria, and beneficial fungi produced increased phenolic levels in plant hosts, but fungal pathogens did not. The third hypothesis was that insect feeding method on plant hosts determines if phenolics are induced. Chewing induced phenolics but piercing-sucking and wood-boring did not. Lastly, we used meta-analyses to determine if annual or perennials rely on phenolic induction in different amounts, and even though annuals had significantly increased phenolic levels but perennials did not, it was observed that phenolic induction was not statistically different when plant type was considered. These results demonstrate that phenolic induction is a common response in plant hosts exposed to feeding or colonization, with specific exceptions such a pathogenic fungi and piercing-sucking insects.

The Multifaceted Roles of Plant Hormone Salicylic Acid in Endoplasmic Reticulum Stress and Unfolded Protein Response

Different abiotic and biotic stresses lead to the accumulation of unfolded and misfolded proteins in the endoplasmic reticulum (ER), resulting in ER stress. In response to ER stress, cells activate various cytoprotective responses, enhancing chaperon synthesis, protein folding capacity, and degradation of misfolded proteins. These responses of plants are called the unfolded protein response (UPR). ER stress signaling and UPR can be regulated by salicylic acid (SA), but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in ER stress and UPR is summarized in model plants and crops to gain a better understanding of SA-regulated processes at the physiological, biochemical, and molecular levels.

Acibenzolar-S-Methyl Restricts Infection of Nicotiana benthamiana by Plantago Asiatica Mosaic Virus at Two Distinct Stages

Plant activators, including acibenzolar-S-methyl (ASM), are chemical compounds that stimulate plant defense responses to pathogens. ASM treatment inhibits infection by a variety of plant viruses, however, the mechanisms of this broad-spectrum and strong effect remain poorly understood. We employed green fluorescent protein (GFP)-expressing viruses and Nicotiana benthamiana plants to identify the infection stages that are restricted by ASM. ASM suppressed infection by three viral species, plantago asiatica mosaic virus (PlAMV), potato virus X (PVX), and turnip mosaic virus (TuMV), in inoculated cells. Furthermore, ASM delayed the long-distance movement of PlAMV and PVX, and the cell-to-cell (short range) movement of TuMV. The ASM-mediated delay of long-distance movement of PlAMV was not due to the suppression of viral accumulation in the inoculated leaves, indicating that ASM restricts PlAMV infection in at least two independent steps. We used Arabidopsis thaliana mutants to show that the ASM-mediated restriction of PlAMV infection requires the NPR1 gene but was independent of the dicer-like genes essential for RNA silencing. Furthermore, experiments using protoplasts showed that ASM treatment inhibited PlAMV replication without cell death. Our approach, using GFP-expressing viruses, will be useful for the analysis of mechanisms underlying plant activator-mediated virus restriction.

Exhibition of Local but Not Systemic Induced Phenolic Defenses in Vitis vinifera L. Affected by Brown Wood Streaking, Grapevine Leaf Stripe, and Apoplexy (Esca Complex)

Balance between constitutive and induced responses provides plants flexibility to cope with biotic stresses. This study tested the hypothesis that invasion of grapevine wood by esca-associated fungi induces the production of defensive compounds as part of locally- and systemically-induced responses. In a vineyard, different symptomatic expressions of "Esca complex" in Vitis vinifera L. 'Malvasia' were evaluated in annual inspections. Then, levels of phenolics and fatty acids were determined in asymptomatic leaves of brown wood streaking (BWS) and grapevine leaf stripe (GLSD) vines, and in symptomatic leaves of GLSD and apoplectic vines; the results were compared with levels in healthy vines. In asymptomatic leaves of BWS and some GLSD vines, levels of phenolics decreased, independent of the total phenolic group. Such responses were usually associated with an increase in levels of linoleic, γ-linolenic and arachidonic acids, well-known signal transduction mediators. In symptomatic leaves, levels of phenolics increased, which is consistent with a locally-induced response; the onset of symptoms coincided with the highest increases e.g., 35% for quercetin-3-O-glucuronide. Thus, the long latency period between trunk invasion by fungi and visible foliar damage and the year-to-year fluctuation in symptomatic expressions observed with "Esca complex" might be partially attributed to a better utilization of constitutive defenses.

Dynamic ubiquitination determines transcriptional activity of the plant immune coactivator NPR1

Activation of systemic acquired resistance in plants is associated with transcriptome reprogramming induced by the unstable coactivator NPR1. Immune-induced ubiquitination and proteasomal degradation of NPR1 are thought to facilitate continuous delivery of active NPR1 to target promoters, thereby maximising gene expression. Because of this potentially costly sacrificial process, we investigated if ubiquitination of NPR1 plays transcriptional roles prior to its proteasomal turnover. Here we show ubiquitination of NPR1 is a progressive event in which initial modification by a Cullin-RING E3 ligase promotes its chromatin association and expression of target genes. Only when polyubiquitination of NPR1 is enhanced by the E4 ligase, UBE4, it is targeted for proteasomal degradation. Conversely, ubiquitin ligase activities are opposed by UBP6/7, two proteasome-associated deubiquitinases that enhance NPR1 longevity. Thus, immune-induced transcriptome reprogramming requires sequential actions of E3 and E4 ligases balanced by opposing deubiquitinases that fine-tune activity of NPR1 without strict requirement for its sacrificial turnover.

Pipecolic Acid Is Induced in Barley upon Infection and Triggers Immune Responses Associated with Elevated Nitric Oxide Accumulation

Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi)biotrophic pathogens. Here, we studied the potential role of Pip in bacteria-induced systemic immunity in barley. Exudates of barley leaves infected with the systemic immunity-inducing pathogen Pseudomonas syringae pv. japonica induced immune responses in A. thaliana. The same leaf exudates contained elevated Pip levels compared with those of mock-treated barley leaves. Exogenous application of Pip induced resistance in barley against the hemibiotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity-inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus Pyrenophora teres. Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, X. translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in Pip-pretreated compared with mock-pretreated plants. Thus, the data suggest that Pip induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.

Studies on the mechanism of agricultural chemicals focused on plant hormone signals

In recent years, it has become clear that the crosstalk of various plant hormones controls plant growth and disease resistance. Plant hormone signals may also be involved in the actions of a variety of pesticides and disease control techniques used for crop protection. From this point of view, we have focused on plant hormones to analyze the mode of action of pesticides that function in plants. Disease resistance inducers are pesticides that induce systemic acquired resistance (SAR) by activating the salicylic acid (SA)-mediated signaling pathway. However, when under unfavorable climate conditions, such as cold and cloudy weather, the resistance inducers are not sufficiently effective. Since the environmental stress response mediated by abscisic acid (ABA) may affect disease resistance, extensive studies of tobacco and tomato plants were performed, which clarified that SAR induction was suppressed by ABA. On the other hand, it was shown that transient high temperature treatment enhanced disease resistance via SA biosynthesis. These results suggest that changes in temperature due to climate change have an impact on disease resistance. The mode of action of a plant-growth regulator was analyzed by focusing on plant hormones. Isoprothiolane (IPT), an active ingredient of Fuji-one, is used as a plant-growth regulator and a fungicide. In Arabidopsis thaliana, we demonstrated that jasmonic acid and ethylene are required for the root elongation-promoting effect of IPT. As shown above, mode-of-action studies on pesticides in relation to plant hormones will lead to the development of new techniques for the better cultivation and protection of crops.

Using Game Theory to Understand Systemic Acquired Resistance as a Bet-Hedging Option for Increasing Fitness When Disease Is Uncertain

Systemic acquired resistance (SAR) is a mechanism through which plants may respond to initial challenge by a pathogen through activation of inducible defense responses, thereby increasing resistance to subsequent infection attempts. Fitness costs are assumed to be incurred by plants induced for SAR, and several studies have attempted to quantify these costs. We developed a mathematical model, motivated by game-theoretic concepts, to simulate competition between hypothetical plant populations with and without SAR to examine conditions under which the phenomenon of SAR may have evolved. Data were gathered from various studies on fitness costs of induced resistance on life history traits in different plant hosts and scaled as a proportion of the values in control cohorts in each study (i.e., healthy plants unprimed for SAR). With unprimed healthy control plants set to a fitness value of 1, primed healthy plants incurred a fitness cost of about 10.4% (0.896, n = 157), primed diseased plants incurred a fitness cost of about 15.5% (0.845, n = 54), and unprimed diseased plants incurred a fitness cost of about 28.9% (0.711, n = 69). Starting from a small proportion of the population (0.5%) and competing against a population with constitutive defenses alone in stochastic simulations, the SAR phenotype almost always dominated the population after 1000 generations when the probability of disease was greater than or equal to 0.5 regardless of the probability for priming errors.

The Novel Cerato-Platanin-Like Protein FocCP1 from Fusarium oxysporum Triggers an Immune Response in Plants

Panama disease, or Fusarium wilt, the most serious disease in banana cultivation, is caused by Fusarium oxysporum f. sp. cubense (FOC) and has led to great economic losses worldwide. One effective way to combat this disease is by enhancing host plant resistance. The cerato-platanin protein (CPP) family is a group of small secreted cysteine-rich proteins in filamentous fungi. CPPs as elicitors can trigger the immune system resulting in defense responses in plants. In this study, we characterized a novel cerato-platanin-like protein in the secretome of Fusarium oxysporum f. sp. cubense race 4 (FOC4), named FocCP1. In tobacco, the purified recombinant FocCP1 protein caused accumulation of reactive oxygen species (ROS), formation of necrotic reaction, deposition of callose, expression of defense-related genes, and accumulation of salicylic acid (SA) and jasmonic acid (JA) in tobacco. These results indicated that FocCP1 triggered a hypersensitive response (HR) and systemic acquired resistance (SAR) in tobacco. Furthermore, FocCP1 enhanced resistance tobacco mosaic virus (TMV) disease and Pseudomonas syringae pv. tabaci 6605 (Pst. 6605) infection in tobacco and improved banana seedling resistance to FOC4. All results provide the possibility of further research on immune mechanisms of plant and pathogen interactions, and lay a foundation for a new biological strategy of banana wilt control in the future.

The Mediator kinase module serves as a positive regulator of salicylic acid accumulation and systemic acquired resistance

In plants, the calmodulin-binding transcription activators (CAMTAs) are required for transcriptional regulation of abiotic and biotic stress responses. Among them, CAMTA3 in Arabidopsis has been intensively studied and shown to function redundantly with CAMTA1 and CAMTA2 to negatively regulate plant immunity. The camta1/2/3 triple mutant accordingly exhibits severe dwarfism due to autoimmunity. Here, through a suppressor screen using camta1/2/3 triple mutant, we found that a mutation in Cyclin-Dependent Kinase 8 (CDK8) partially suppresses the dwarfism and constitutive resistance phenotypes of camta1/2/3. CDK8 positively regulates steady-state salicylic acid (SA) levels and systemic required resistance (SAR). The expression of SA biosynthesis genes such as ICS1 and EDS5 is down-regulated in cdk8 mutants under uninfected conditions, suggesting that CDK8 contributes to the transcriptional regulation of these SA pathway genes. Knocking out another Mediator kinase module member MED12 yielded similar defects including decreased steady-state SA level and compromised SAR, suggesting that the whole Mediator kinase module contributes to the transcriptional regulation of SA levels and SAR.

Sympatry and interference of divergent Microbotryum pathogen species

The impact of infectious diseases in natural ecosystems is strongly influenced by the degree of pathogen specialization and by the local assemblies of potential host species. This study investigated anther-smut disease, caused by fungi in the genus Microbotryum, among natural populations of plants in the Caryophyllaceae. A broad geographic survey focused on sites of the disease on multiple host species in sympatry. Analysis of molecular identities for the pathogens revealed that sympatric disease was most often due to co-occurrence of distinct, host-specific anther-smut fungi, rather than localized cross-species disease transmission. Flowers from sympatric populations showed that the Microbotryum spores were frequently moved between host species. Experimental inoculations to simulate cross-species exposure to the pathogens in these plant communities showed that the anther-smut pathogen was less able to cause disease on its regular host when following exposure of the plants to incompatible pathogens from another host species. These results indicate that multi-host/multi-pathogen communities are common in this system and they involve a previously hidden mechanism of interference between Microbotryum fungi, which likely affects both pathogen and host distributions.

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 resistance inducer acibenzolar-S-methyl (ASM) and its microencapsulated formulations: their long-lasting control efficacy against cucumber diseases and mitigation of phytotoxicity

BACKGROUND

The development of fungicide resistance by pathogens is a major limiting factor for the control of plant diseases. To combat resistance development, the use of broad-spectrum but nonfungitoxic resistance inducers such as acibenzolar-S-methyl (ASM) is a promising approach because the orchestrated mechanisms underlying systemic acquired resistance induced by ASM are less likely to be overcome easily by pathogens. However, phytotoxicity is the main limiting factor of ASM.

RESULTS

ASM was highly active at inducing systemic resistance against powdery and downy mildews, the two major cucumber diseases. Based on the duration of the control, ASM effectiveness against these diseases was maintained well in plastic greenhouses and the number of sprays could be reduced. Control efficacy after seed treatment with ASM and the applications of microencapsulated ASM was also high against mildews in pots as well as in greenhouse experiments, with no problematic phytotoxicity.

CONCLUSION

The use of ASM is a potential integrated pest management-based tactic to control cucumber powdery and downy mildews because its long-lasting efficacy allows the application of typical fungicides to be reduced. The risk for resistance development in mildew pathogens will also be reduced. ASM seed treatment as well as soil amendment with microencapsulated ASM is effective in lowering the risk for the phytotoxicity of this compound. © 2018 Society of Chemical Industry.

AtNPR4 from Arabidopsis thaliana: expression, purification, crystallization and crystallographic analysis

Salicylic acid (SA) is an important phytohormone that is involved in the regulation of plant defence, growth and development. A large number of proteins have been shown to have the ability to interact with SA, and NPR4 has been demonstrated to be a receptor of SA that plays significant roles in the innate immune response of plants. In this study, Spodoptera frugiperda (Sf9) cells were used to express full-length AtNPR4 from Arabidopsis thaliana. To facilitate crystallization, T4 lysozyme (T4L) was added to the N-terminus of the AtNPR4 protein. The recombinant T4L-AtNPR4 protein was expressed, purified and crystallized using the sitting-drop and hanging-drop vapour-diffusion methods. The T4L-AtNPR4 crystals have symmetry consistent with space group C2, with unit-cell parameters a = 93.7, b = 85.8, c = 88.2 Å, β = 90° and one molecule per asymmetric unit. The best crystal diffracted to a resolution of 2.75 Å. Structure determination is in progress.

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) and Related Family: Mechanistic Insights in Plant Disease Resistance

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) and related NPR1-like proteins are a functionally similar, yet surprisingly diverse family of transcription co-factors. Initially, NPR1 in Arabidopsis was identified as a positive regulator of systemic acquired resistance (SAR), paralogs NPR3 and NPR4 were later shown to be negative SAR regulators. The mechanisms involved have been the subject of extensive research and debate over the years, during which time a lot has been uncovered. The known roles of this protein family have extended to include influences over a broad range of systems including circadian rhythm, endoplasmic reticulum (ER) resident proteins and the development of lateral organs. Recently, important advances have been made in understanding the regulatory relationship between members of the NPR1-like protein family, providing new insight regarding their interactions, both with each other and other defense-related proteins. Most importantly the influence of salicylic acid (SA) on these interactions has become clearer with NPR1, NPR3, and NPR4 being considered bone fide SA receptors. Additionally, post-translational modification of NPR1 has garnered attention during the past years, adding to the growing regulatory complexity of this protein. Furthermore, growing interest in NPR1 overexpressing crops has provided new insights regarding the role of NPR1 in both biotic and abiotic stresses in several plant species. Given the wealth of information, this review aims to highlight and consolidate the most relevant and influential research in the field to date. In so doing, we attempt to provide insight into the mechanisms and interactions which underly the roles of the NPR1-like proteins in plant disease responses.

Cloning and characterization of a Mimulus lewisii NPR1 gene involved in regulating plant resistance to Rhizoctonia solani

The monkey flower Mimulus lewisii is a new emerging model plant for the study in corolla tube formation, pigmentation patterns and pollinator selection, etc. However, the cultivation and management of this plant are difficult due to its susceptibility to a wide range of pathogens and the lack of rigid varieties with high levels of resistance to pathogens. In this regard, genetic engineering is a promising tool that may possibly allow us to enhance the M. lewisii disease resistance against pathogens. Here, we reported the isolation and characterization of non-expressor of pathogenesis related gene 1 (NPR1) gene from M. lewisii. The phylogenetic tree constructed based on the deduced sequence of MlNPR1 with homologs from other species revealed that MlNPR1 grouped together with other known NPR1 proteins of Scrophulariaceae family, and was nearest to Mimulus guttatus. Furthermore, expression analysis showed that MlNPR1 was upregulated after SA treatment and fungal infection. To understand the defensive role of this gene, we overexpressed MlNPR1 in M. lewisii. The transgenic lines showed slight phenotypic abnormalities, but constitutive expression of MlNPR1 activates defense signaling pathways by priming the expression of antifungal PR genes. Moreover, MlNPR1 transgenic lines showed enhanced resistance to Rhizoctonia solani there was delay in symptoms and reduced disease severity than non-transgenic plants. Altogether, the present study suggests that increasing the expression level of MlNPR1 may be a promising approach for development of monkey flower cultivars with enhanced resistance to diseases.

Corrigendum: Zucchini Yellow Mosaic Virus Infection Limits Establishment and Severity of Powdery Mildew in Wild Populations of Cucurbita pepo

[This corrects the article DOI: 10.3389/fpls.2018.00792.].

Innate Immunity Activation and RNAi Interplay in Citrus Exocortis Viroid-Tomato Pathosystem

Although viroids are the smallest and simplest plant pathogens known, the molecular mechanisms underlying their pathogenesis remain unclear. To unravel these mechanisms, a dual approach was implemented consisting of in silico identification of potential tomato silencing targets of pospiviroids, and the experimental validation of these targets through the sequencing of small RNAs and RNA ends extracted from tomatoes infected with a severe isolate of Citrus exocortis viroid (CEVd). The generated RNA ends were also used to monitor the differentially-expressed genes. These analyses showed that when CEVd symptoms are well established: (i) CEVd are degraded by at least three Dicer-like (DCL) proteins and possibly by RNA-induced silencing complex (RISC), (ii) five different mRNAs are partially degraded through post-transcriptional gene silencing (PTGS), including argonaute 2a, which is further degraded in phasiRNAs, (iii) Dicer-like 2b and 2d are both upregulated and degraded in phasiRNAs, and (iv) CEVd infection induced a significant shift in gene expression allowing to explain the usual symptoms of pospiviroids on tomato and to demonstrate the constant activation of host innate immunity and systemic acquired resistance (SAR) by these pathogenic RNAs. Finally, based on in silico analysis, potential immunity receptor candidates of viroid-derived RNAs are suggested.

l-lysine metabolism to N-hydroxypipecolic acid: an integral immune-activating pathway in plants

l-lysine catabolic routes in plants include the saccharopine pathway to α-aminoadipate and decarboxylation of lysine to cadaverine. The current review will cover a third l-lysine metabolic pathway having a major role in plant systemic acquired resistance (SAR) to pathogen infection that was recently discovered in Arabidopsis thaliana. In this pathway, the aminotransferase AGD2-like defense response protein (ALD1) α-transaminates l-lysine and generates cyclic dehydropipecolic (DP) intermediates that are subsequently reduced to pipecolic acid (Pip) by the reductase SAR-deficient 4 (SARD4). l-pipecolic acid, which occurs ubiquitously in the plant kingdom, is further N-hydroxylated to the systemic acquired resistance (SAR)-activating metabolite N-hydroxypipecolic acid (NHP) by flavin-dependent monooxygenase1 (FMO1). N-hydroxypipecolic acid induces the expression of a set of major plant immune genes to enhance defense readiness, amplifies resistance responses, acts synergistically with the defense hormone salicylic acid, promotes the hypersensitive cell death response and primes plants for effective immune mobilization in cases of future pathogen challenge. This pathogen-inducible NHP biosynthetic pathway is activated at the transcriptional level and involves feedback amplification. Apart from FMO1, some cytochrome P450 monooxygenases involved in secondary metabolism catalyze N-hydroxylation reactions in plants. In specific taxa, pipecolic acid might also serve as a precursor in the biosynthesis of specialized natural products, leading to C-hydroxylated and otherwise modified piperidine derivatives, including indolizidine alkaloids. Finally, we show that NHP is glycosylated in Arabidopsis to form a hexose-conjugate, and then discuss open questions in Pip/NHP-related research.

Zucchini Yellow Mosaic Virus Infection Limits Establishment and Severity of Powdery Mildew in Wild Populations of Cucurbita pepo

Few studies have examined the combined effect of multiple parasites on host fitness. Previous work in the Cucurbita pepo pathosystem indicates that infection with Zucchini yellow mosaic virus (ZYMV) reduces exposure to a second insect-vectored parasite (Erwinia tracheiphila). In this study, we performed two large-scale field experiments employing wild gourds (Cucurbita pepo ssp. texana), including plants with a highly introgressed transgene conferring resistance to ZYMV, to examine the interaction of ZYMV and powdery mildew, a common fungal disease. We found that ZYMV-infected plants are more resistant to powdery mildew (i.e., less likely to experience powdery mildew infection and when infected with powdery mildew, have reduced severity of powdery mildew symptoms). As a consequence, during widespread viral epidemics, proportionally more transgenic plants get powdery mildew than non-transgenic plants, potentially mitigating the benefits of the transgene. A greenhouse study using ZYMV-inoculated and non-inoculated controls (non-transgenic plants) revealed that ZYMV-infected plants were more resistant to powdery mildew than controls, suggesting that the transgene itself had no direct effect on the powdery mildew resistance in our field study. Additionally, we found evidence of elevated levels of salicylic acid, a phytohormone that mediates anti-pathogen defenses, in ZYMV-infected plants, suggesting that viral infection induces a plant immune response (systemic acquired resistance), thereby reducing plant susceptibility to powdery mildew infection.

A critical role for Arabidopsis MILDEW RESISTANCE LOCUS O2 in systemic acquired resistance

Members of the MILDEW RESISTANCE LOCUS O (MLO) gene family confer susceptibility to powdery mildews in different plant species, and their existence therefore seems to be disadvantageous for the plant. We recognized that expression of the Arabidopsis MLO2 gene is induced after inoculation with the bacterial pathogen Pseudomonas syringae, promoted by salicylic acid (SA) signaling, and systemically enhanced in the foliage of plants exhibiting systemic acquired resistance (SAR). Importantly, distinct mlo2 mutant lines were unable to systemically increase resistance to bacterial infection after inoculation with P. syringae, indicating that the function of MLO2 is necessary for biologically induced SAR in Arabidopsis. Our data also suggest that the close homolog MLO6 has a supportive but less critical role in SAR. In contrast to SAR, basal resistance to bacterial infection was not affected in mlo2. Remarkably, SAR-defective mlo2 mutants were still competent in systemically increasing the levels of the SAR-activating metabolites pipecolic acid (Pip) and SA after inoculation, and to enhance SAR-related gene expression in distal plant parts. Furthermore, although MLO2 was not required for SA- or Pip-inducible defense gene expression, it was essential for the proper induction of disease resistance by both SAR signals. We conclude that MLO2 acts as a critical downstream component in the execution of SAR to bacterial infection, being required for the translation of elevated defense responses into disease resistance. Moreover, our data suggest a function for MLO2 in the activation of plant defense priming during challenge by P. syringae.