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Sun D, Li S, Niu L, Reid MS, Zhang Y, Jiang CZ. PhOBF1, a petunia ocs element binding factor, plays an important role in antiviral RNA silencing. JOURNAL OF EXPERIMENTAL BOTANY 2017; 68:915-930. [PMID: 28053190 PMCID: PMC6055658 DOI: 10.1093/jxb/erw490] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/07/2016] [Accepted: 12/09/2016] [Indexed: 05/09/2023]
Abstract
Virus-induced gene silencing (VIGS) is a common reverse genetics strategy for characterizing the function of genes in plants. The detailed mechanism governing RNA silencing efficiency triggered by viruses is largely unclear. Here, we reveal that a petunia (Petunia hybrida) ocs element binding factor, PhOBF1, one of the basic leucine zipper (bZIP) transcription factors, was up-regulated by Tobacco rattle virus (TRV) infection. Simultaneous silencing of PhOBF1 and a reporter gene, phytoene desaturase (PDS) or chalcone synthase (CHS), by TRV-based VIGS led to a failure of the development of leaf photobleaching or the white-corollas phenotype. PhOBF1 silencing caused down-regulation of RNA silencing-related genes, including RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonautes (AGOs). After inoculation with the TRV-PhPDS, PhOBF1-RNAi lines exhibited a substantially impaired PDS silencing efficiency, whereas overexpression of PhOBF1 resulted in a recovery of the silencing phenotype (photobleaching) in systemic leaves. A compromised resistance to TRV and Tobacco mosaic virus was found in PhOBF1-RNAi lines, while PhOBF1-overexpressing lines displayed an enhanced resistance to their infections. Compared with wild-type plants, PhOBF1-silenced plants accumulated lower levels of free salicylic acid (SA), salicylic acid glucoside, and phenylalanine, contrarily to higher levels of those in plants overexpressing PhOBF1. Furthermore, transcripts of a number of genes associated with the shikimate and phenylpropanoid pathways were decreased or increased in PhOBF1-RNAi or PhOBF1-overexpressing lines, respectively. Taken together, the data suggest that PhOBF1 regulates TRV-induced RNA silencing efficiency through modulation of RDRs, DCLs, and AGOs mediated by the SA biosynthesis pathway.
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Affiliation(s)
- Daoyang Sun
- Department of Landscape Architecture and Arts, Northwest A&F University, Yangling, Shaanxi, China
- Department of Plant Sciences, University of California Davis, Davis, CA, USA
| | - Shaohua Li
- Department of Landscape Architecture and Arts, Northwest A&F University, Yangling, Shaanxi, China
| | - Lixin Niu
- Department of Landscape Architecture and Arts, Northwest A&F University, Yangling, Shaanxi, China
| | - Michael S Reid
- Department of Plant Sciences, University of California Davis, Davis, CA, USA
| | - Yanlong Zhang
- Department of Landscape Architecture and Arts, Northwest A&F University, Yangling, Shaanxi, China
| | - Cai-Zhong Jiang
- Department of Plant Sciences, University of California Davis, Davis, CA, USA
- Crops Pathology and Genetic Research Unit, United States Department of Agriculture, Agricultural Research Service, Davis, CA, USA
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López-Gresa MP, Lisón P, Yenush L, Conejero V, Rodrigo I, Bellés JM. Salicylic Acid Is Involved in the Basal Resistance of Tomato Plants to Citrus Exocortis Viroid and Tomato Spotted Wilt Virus. PLoS One 2016; 11:e0166938. [PMID: 27893781 PMCID: PMC5125658 DOI: 10.1371/journal.pone.0166938] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2016] [Accepted: 11/07/2016] [Indexed: 11/18/2022] Open
Abstract
Tomato plants expressing the NahG transgene, which prevents accumulation of endogenous salicylic acid (SA), were used to study the importance of the SA signalling pathway in basal defence against Citrus Exocortis Viroid (CEVd) or Tomato Spotted Wilt Virus (TSWV). The lack of SA accumulation in the CEVd- or TSWV-infected NahG tomato plants led to an early and dramatic disease phenotype, as compared to that observed in the corresponding parental Money Maker. Addition of acibenzolar-S-methyl, a benzothiadiazole (BTH), which activates the systemic acquired resistance pathway downstream of SA signalling, improves resistance of NahG tomato plants to CEVd and TSWV. CEVd and TSWV inoculation induced the accumulation of the hydroxycinnamic amides p-coumaroyltyramine, feruloyltyramine, caffeoylputrescine, and feruloylputrescine, and the defence related proteins PR1 and P23 in NahG plants earlier and with more intensity than in Money Maker plants, indicating that SA is not essential for the induction of these plant defence metabolites and proteins. In addition, NahG plants produced very high levels of ethylene upon CEVd or TSWV infection when compared with infected Money Maker plants, indicating that the absence of SA produced additional effects on other metabolic pathways. This is the first report to show that SA is an important component of basal resistance of tomato plants to both CEVd and TSWV, indicating that SA-dependent defence mechanisms play a key role in limiting the severity of symptoms in CEVd- and TSWV-infected NahG tomato plants.
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Affiliation(s)
- M. Pilar López-Gresa
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
| | - Purificación Lisón
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
| | - Lynne Yenush
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
| | - Vicente Conejero
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
| | - Ismael Rodrigo
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
| | - José María Bellés
- Instituto de Biología Molecular y Celular de Plantas, Universitat Politècnica de València (UPV)- Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain
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53
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Katsarou K, Mavrothalassiti E, Dermauw W, Van Leeuwen T, Kalantidis K. Combined Activity of DCL2 and DCL3 Is Crucial in the Defense against Potato Spindle Tuber Viroid. PLoS Pathog 2016; 12:e1005936. [PMID: 27732664 PMCID: PMC5061435 DOI: 10.1371/journal.ppat.1005936] [Citation(s) in RCA: 46] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2016] [Accepted: 09/14/2016] [Indexed: 12/22/2022] Open
Abstract
Viroids are self replicating non-coding RNAs capable of infecting a wide range of plant hosts. They do not encode any proteins, thus the mechanism by which they escape plant defenses remains unclear. RNAi silencing is a major defense mechanism against virus infections, with the four DCL proteins being principal components of the pathway. We have used Nicotiana benthamiana as a model to study Potato spindle tuber viroid infection. This viroid is a member of the Pospiviroidae family and replicates in the nucleus via an asymmetric rolling circle mechanism. We have created knock-down plants for all four DCL genes and their combinations. Previously, we showed that DCL4 has a positive effect on PSTVd infectivity since viroid levels drop when DCL4 is suppressed. Here, we show that PSTVd levels remain decreased throughout infection in DCL4 knockdown plants, and that simultaneous knockdown of DCL1, DCL2 or DCL3 together with DCL4 cannot reverse this effect. Through infection of plants suppressed for multiple DCLs we further show that a combined suppression of DCL2 and DCL3 has a major effect in succumbing plant antiviral defense. Based on our results, we further suggest that Pospoviroids may have evolved to be primarily processed by DCL4 as it seems to be a DCL protein with less detrimental effects on viroid infectivity. These findings pave the way to delineate the complexity of the relationship between viroids and plant RNA silencing response.
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Affiliation(s)
- Konstantina Katsarou
- Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology-Hellas, Heraklion, Greece
| | | | - Wannes Dermauw
- Laboratory of Agrozoology, Department of Crop Protection, Faculty of Bioscience Engineering, Ghent University, Belgium
| | - Thomas Van Leeuwen
- Laboratory of Agrozoology, Department of Crop Protection, Faculty of Bioscience Engineering, Ghent University, Belgium
- Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, The Netherlands
| | - Kriton Kalantidis
- Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology-Hellas, Heraklion, Greece
- Department of Biology, University of Crete, Heraklion, Greece
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54
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Coutinho ID, Baker JM, Ward JL, Beale MH, Creste S, Cavalheiro AJ. Metabolite Profiling of Sugarcane Genotypes and Identification of Flavonoid Glycosides and Phenolic Acids. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY 2016; 64:4198-206. [PMID: 27152527 DOI: 10.1021/acs.jafc.6b01210] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Sugarcane is an important agricultural crop in the economy of tropical regions, and Brazil has the largest cultivated acreage in the world. Sugarcane accumulates high levels of sucrose in its stalks. Other compounds produced by sugarcane are currently not of economic importance. To explore potential coproducts, we have studied the chemical diversity of sugarcane genotypes, via metabolite profiling of leaves by NMR and LC-DAD-MS. Metabolites were identified via in-house and public databases. From the analysis of 60 HPLC-fractionated extracts, LC-DAD-MS detected 144 metabolites, of which 56 were identified (MS-MS and (1)H NMR), including 19 phenolics and 25 flavones, with a predominance of isomeric flavone C-glycosides. Multivariate analysis of the profiles from genotypes utilized in Brazilian breeding programs revealed clustering according to sugar, phenolic acid, and flavone contents.
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Affiliation(s)
- Isabel D Coutinho
- Instituto de Quı́mica, Universidade Estadual Paulista "Julio de Mesquita Filho" (UNESP) , Rua Prof. Francisco Degni 55, CEP 14800-060 Araraquara, São Paulo, Brazil
| | - John M Baker
- Plant Biology and Crop Science Department, Rothamsted Research , Harpenden AL5 2JQ, United Kingdom
| | - Jane L Ward
- Plant Biology and Crop Science Department, Rothamsted Research , Harpenden AL5 2JQ, United Kingdom
| | - Michael H Beale
- Plant Biology and Crop Science Department, Rothamsted Research , Harpenden AL5 2JQ, United Kingdom
| | - Silvana Creste
- Instituto Agronômico - Centro de Cana , Rodovia Antonio Duarte Nogueira Km 321, CP 206, CEP 14032-800 Ribeirão Preto, Brazil
| | - Alberto J Cavalheiro
- Instituto de Quı́mica, Universidade Estadual Paulista "Julio de Mesquita Filho" (UNESP) , Rua Prof. Francisco Degni 55, CEP 14800-060 Araraquara, São Paulo, Brazil
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55
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Collum TD, Culver JN. The impact of phytohormones on virus infection and disease. Curr Opin Virol 2015; 17:25-31. [PMID: 26656395 DOI: 10.1016/j.coviro.2015.11.003] [Citation(s) in RCA: 54] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2015] [Revised: 10/26/2015] [Accepted: 11/09/2015] [Indexed: 11/26/2022]
Abstract
Phytohormones play a critical role in nearly every aspect of plant biology, including development and pathogen defense. During virus infection disruption of the plant's normal developmental physiology has often been associated with alterations in phytohormone accumulation and signaling. Only recently has evidence emerged describing mechanistically how viruses modulate phytohormone levels and the impact these modulations have on plant physiology and virus biology. From these studies there is an emerging theme of virus directed manipulation of plant hormone responses to disarm defense responses and reprogram the cellular environment to enhance replication and spread. In this review we examine the impact viruses have on plant hormone systems and the effects of this phytohormone manipulation on virus biology.
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Affiliation(s)
- Tamara D Collum
- Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742, USA
| | - James N Culver
- Institute for Bioscience and Biotechnology Research, University of Maryland, College Park, MD 20742, USA; Department of Plant Science and Landscape Architecture, University of Maryland, College Park, MD 20742, USA.
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56
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Zhang S, Liu Y, Yu B. New insights into pri-miRNA processing and accumulation in plants. WILEY INTERDISCIPLINARY REVIEWS-RNA 2015; 6:533-45. [PMID: 26119101 DOI: 10.1002/wrna.1292] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/06/2015] [Revised: 05/12/2015] [Accepted: 05/22/2015] [Indexed: 12/31/2022]
Abstract
MicroRNAs (miRNAs) regulate many biological processes such as development, metabolism, and others. They are processed from their primary transcripts called primary miRNA transcripts (pri-miRNAs) by the processor complex containing the RNAse III enzyme, DICER-LIKE1 (DCL1), in plants. Consequently, miRNA biogenesis is controlled through altering pri-miRNA accumulation and processing, which is crucial for plant development and adaptation to environmental changes. Plant pri-miRNAs are transcribed by DNA-dependent RNA polymerase II (Pol II) and their levels are determined through transcription and degradation, whereas pri-miRNA processing is affected by its structure, splicing, alternative splicing, loading to the processor and the processor activity, which involve in many accessory proteins. Here, we summarize recent progresses related to pri-miRNA transcription, stability, and processing in plants.
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Affiliation(s)
- Shuxin Zhang
- Center for Plant Science Innovation & School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE, USA.,State Key Laboratory of Crop Biology, College of Life Sciences, Shandong Agricultural University, Tai'an, China
| | - Yuhui Liu
- Biotechnology Research Institute, Chinese Academy of Agricultural Sciences & Key Laboratory of Agricultural Genomics, Ministry of Agriculture, Beijing, China
| | - Bin Yu
- Center for Plant Science Innovation & School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE, USA
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57
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Alazem M, Lin NS. Roles of plant hormones in the regulation of host-virus interactions. MOLECULAR PLANT PATHOLOGY 2015; 16:529-40. [PMID: 25220680 PMCID: PMC6638471 DOI: 10.1111/mpp.12204] [Citation(s) in RCA: 220] [Impact Index Per Article: 24.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Hormones are tuners of plant responses to biotic and abiotic stresses. They are involved in various complicated networks, through which they modulate responses to different stimuli. Four hormones primarily regulate plant defence to pathogens: salicylic acid (SA), jasmonic acid (JA), ethylene (Et) and abscisic acid (ABA). In susceptible plants, viral infections result in hormonal disruption, which manifests as the simultaneous induction of several antagonistic hormones. However, these antagonistic hormones may exhibit some sequential accumulation in resistant lines. Virus propagation is usually restricted by the activation of the small interfering RNA (siRNA) antiviral machinery and/or SA signalling pathway. Several studies have investigated these two systems, using different model viruses. However, the roles of hormones other than SA, especially those with antagonistic properties, such as ABA, have been neglected. Increasing evidence indicates that hormones control components of the small RNA system, which regulates many processes (including the siRNA antiviral machinery and the microRNA system) at the transcriptional or post-transcriptional level. Consequently, cross-talk between the antagonistic SA and ABA pathways modulates plant responses at multiple levels. In this review, we summarize recent findings on the different roles of hormones in the regulation of plant-virus interactions, which are helping us to elucidate the fine tuning of viral and plant systems by hormones.
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Affiliation(s)
- Mazen Alazem
- Institute of Plant and Microbial Biology, Academia Sinica, 128 Sec. 2, Academia Rd, Nankang, Taipei, 11529, Taiwan
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58
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Faoro F, Gozzo F. Is modulating virus virulence by induced systemic resistance realistic? PLANT SCIENCE : AN INTERNATIONAL JOURNAL OF EXPERIMENTAL PLANT BIOLOGY 2015; 234:1-13. [PMID: 25804804 DOI: 10.1016/j.plantsci.2015.01.011] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/08/2014] [Revised: 01/17/2015] [Accepted: 01/20/2015] [Indexed: 06/04/2023]
Abstract
Induction of plant resistance, either achieved by chemicals (systemic acquired resistance, SAR) or by rhizobacteria (induced systemic resistance, ISR) is a possible and/or complementary alternative to manage virus infections in crops. SAR mechanisms operating against viruses are diverse, depending on the pathosystem, and may inhibit virus replication as well as cell-to-cell and long-distance movement. Inhibition is often mediated by salicylic acid with the involvement of alternative oxidase and reactive oxygen species. However, salicylate may also stimulate a separate downstream pathway, leading to the induction of an additional mechanism, based on RNA-dependent RNA polymerase 1-mediated RNA silencing. Thus, SAR and RNA silencing would closely cooperate in the defence against virus infection. Despite tremendous recent progress in the knowledge of SAR mechanisms, only a few compounds, including benzothiadiazole and chitosan have been shown to reduce the severity of systemic virus disease in controlled environment and, more modestly, in open field. Finally, ISR induction, has proved to be a promising strategy to control virus disease, particularly by seed bacterization with a mixture of plant growth-promoting rhizobacteria. However, the use of any of these treatments should be integrated with cultivation practices that reduce vector pressure by the use of insecticides, or by Bt crops.
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Affiliation(s)
- Franco Faoro
- Department of Agricultural and Environmental Sciences, University of Milan, Via Celoria 2, 20133 Milano, Italy; CNR, Institute for Sustainable Plant Protection, Strada delle Cacce 73, 10135 Turin, Italy.
| | - Franco Gozzo
- Department of Food, Environmental and Nutritional Sciences, Section of Chemistry and Biomolecular Sciences, University of Milano, Via Celoria 2, 20133 Milano, Italy
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Viroids, the simplest RNA replicons: How they manipulate their hosts for being propagated and how their hosts react for containing the infection. Virus Res 2015; 209:136-45. [PMID: 25738582 DOI: 10.1016/j.virusres.2015.02.027] [Citation(s) in RCA: 68] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2014] [Revised: 02/23/2015] [Accepted: 02/23/2015] [Indexed: 12/31/2022]
Abstract
The discovery of viroids about 45 years ago heralded a revolution in Biology: small RNAs comprising around 350 nt were found to be able to replicate autonomously-and to incite diseases in certain plants-without encoding proteins, fundamental properties discriminating these infectious agents from viruses. The initial focus on the pathological effects usually accompanying infection by viroids soon shifted to their molecular features-they are circular molecules that fold upon themselves adopting compact secondary conformations-and then to how they manipulate their hosts to be propagated. Replication of viroids-in the nucleus or chloroplasts through a rolling-circle mechanism involving polymerization, cleavage and circularization of RNA strands-dealt three surprises: (i) certain RNA polymerases are redirected to accept RNA instead of their DNA templates, (ii) cleavage in chloroplastic viroids is not mediated by host enzymes but by hammerhead ribozymes, and (iii) circularization in nuclear viroids is catalyzed by a DNA ligase redirected to act upon RNA substrates. These enzymes (and ribozymes) are most probably assisted by host proteins, including transcription factors and RNA chaperones. Movement of viroids, first intracellularly and then to adjacent cells and distal plant parts, has turned out to be a tightly regulated process in which specific RNA structural motifs play a crucial role. More recently, the advent of RNA silencing has brought new views on how viroids may cause disease and on how their hosts react to contain the infection; additionally, viroid infection may be restricted by other mechanisms. Representing the lowest step on the biological size scale, viroids have also attracted considerable interest to get a tentative picture of the essential characteristics of the primitive replicons that populated the postulated RNA world.
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60
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Campos L, Lisón P, López-Gresa MP, Rodrigo I, Zacarés L, Conejero V, Bellés JM. Transgenic tomato plants overexpressing tyramine N-hydroxycinnamoyltransferase exhibit elevated hydroxycinnamic acid amide levels and enhanced resistance to Pseudomonas syringae. MOLECULAR PLANT-MICROBE INTERACTIONS : MPMI 2014; 27:1159-69. [PMID: 25014592 DOI: 10.1094/mpmi-04-14-0104-r] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Hydroxycinnamic acid amides (HCAA) are secondary metabolites involved in plant development and defense that have been widely reported throughout the plant kingdom. These phenolics show antioxidant, antiviral, antibacterial, and antifungal activities. Hydroxycinnamoyl-CoA:tyramine N-hydroxycinnamoyl transferase (THT) is the key enzyme in HCAA synthesis and is induced in response to pathogen infection, wounding, or elicitor treatments, preceding HCAA accumulation. We have engineered transgenic tomato plants overexpressing tomato THT. These plants displayed an enhanced THT gene expression in leaves as compared with wild type (WT) plants. Consequently, leaves of THT-overexpressing plants showed a higher constitutive accumulation of the amide coumaroyltyramine (CT). Similar results were found in flowers and fruits. Moreover, feruloyltyramine (FT) also accumulated in these tissues, being present at higher levels in transgenic plants. Accumulation of CT, FT and octopamine, and noradrenaline HCAA in response to Pseudomonas syringae pv. tomato infection was higher in transgenic plants than in the WT plants. Transgenic plants showed an enhanced resistance to the bacterial infection. In addition, this HCAA accumulation was accompanied by an increase in salicylic acid levels and pathogenesis-related gene induction. Taken together, these results suggest that HCAA may play an important role in the defense of tomato plants against P. syringae infection.
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61
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Minoia S, Carbonell A, Di Serio F, Gisel A, Carrington JC, Navarro B, Flores R. Specific argonautes selectively bind small RNAs derived from potato spindle tuber viroid and attenuate viroid accumulation in vivo. J Virol 2014; 88:11933-45. [PMID: 25100851 PMCID: PMC4178711 DOI: 10.1128/jvi.01404-14] [Citation(s) in RCA: 76] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2014] [Accepted: 07/31/2014] [Indexed: 12/21/2022] Open
Abstract
The identification of viroid-derived small RNAs (vd-sRNAs) of 21 to 24 nucleotides (nt) in plants infected by viroids (infectious non-protein-coding RNAs of just 250 to 400 nt) supports their targeting by Dicer-like enzymes, the first host RNA-silencing barrier. However, whether viroids, like RNA viruses, are also targeted by the RNA-induced silencing complex (RISC) remains controversial. At the RISC core is one Argonaute (AGO) protein that, guided by endogenous or viral sRNAs, targets complementary RNAs. To examine whether AGO proteins also load vd-sRNAs, leaves of Nicotiana benthamiana infected by potato spindle tuber viroid (PSTVd) were agroinfiltrated with plasmids expressing epitope-tagged versions of AGO1, AGO2, AGO3, AGO4, AGO5, AGO6, AGO7, AGO9, and AGO10 from Arabidopsis thaliana. Immunoprecipitation analyses of the agroinfiltrated halos revealed that all AGOs except AGO6, AGO7, and AGO10 associated with vd-sRNAs: AGO1, AGO2, and AGO3 preferentially with those of 21 and 22 nt, while AGO4, AGO5, and AGO9 additionally bound those of 24 nt. Deep-sequencing analyses showed that sorting of vd-sRNAs into AGO1, AGO2, AGO4, and AGO5 depended essentially on their 5'-terminal nucleotides, with the profiles of the corresponding AGO-loaded vd-sRNAs adopting specific hot spot distributions along the viroid genome. Furthermore, agroexpression of AGO1, AGO2, AGO4, and AGO5 on PSTVd-infected tissue attenuated the level of the genomic RNAs, suggesting that they, or their precursors, are RISC targeted. In contrast to RNA viruses, PSTVd infection of N. benthamiana did not affect miR168-mediated regulation of the endogenous AGO1, which loaded vd-sRNAs with specificity similar to that of its A. thaliana counterpart. Importance: To contain invaders, particularly RNA viruses, plants have evolved an RNA-silencing mechanism relying on the generation by Dicer-like (DCL) enzymes of virus-derived small RNAs of 21 to 24 nucleotides (nt) that load and guide Argonaute (AGO) proteins to target and repress viral RNA. Viroids, despite their minimal genomes (non-protein-coding RNAs of only 250 to 400 nt), infect and incite disease in plants. The accumulation in these plants of 21- to 24-nt viroid-derived small RNAs (vd-sRNAs) supports the notion that DCLs also target viroids but does not clarify whether vd-sRNAs activate one or more AGOs. Here, we show that in leaves of Nicotiana benthamiana infected by potato spindle tuber viroid, the endogenous AGO1 and distinct AGOs from Arabidopsis thaliana that were overexpressed were associated with vd-sRNAs displaying the same properties (5'-terminal nucleotide and size) previously established for endogenous and viral small RNAs. Overexpression of AGO1, AGO2, AGO4, and AGO5 attenuated viroid accumulation, supporting their role in antiviroid defense.
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Affiliation(s)
- Sofia Minoia
- Instituto de Biología Molecular y Celular de Plantas (IBMCP), Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Valencia, Spain
| | | | - Francesco Di Serio
- Istituto per la Protezione Sostenibile delle Piante, Consiglio Nazionale delle Ricerche, Bari, Italy
| | - Andreas Gisel
- Istituto di Tecnologie Biomediche, Consiglio Nazionale delle Ricerche, Bari, Italy
| | | | - Beatriz Navarro
- Istituto per la Protezione Sostenibile delle Piante, Consiglio Nazionale delle Ricerche, Bari, Italy
| | - Ricardo Flores
- Instituto de Biología Molecular y Celular de Plantas (IBMCP), Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Valencia, Spain
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