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Licciardello G, Scuderi G, Russo M, Bazzano M, Bar-Joseph M, Catara AF. Minor Variants of Orf1a, p33, and p23 Genes of VT Strain Citrus Tristeza Virus Isolates Show Symptomless Reactions on Sour Orange and Prevent Superinfection of Severe VT Isolates. Viruses 2023; 15:2037. [PMID: 37896814 PMCID: PMC10612028 DOI: 10.3390/v15102037] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2023] [Revised: 09/28/2023] [Accepted: 09/29/2023] [Indexed: 10/29/2023] Open
Abstract
The control of tristeza quick decline (QD) of citrus is based on the use of rootstocks that are tolerant or resistant to the Citrus tristeza virus (CTV), but some of them show bio-agronomic limits. The application of cross-protection (CP) has been insufficiently explored. The present study examined the possibility of QD control by cross-protection (CP) following reports showing the dependence of the CP strategy on the close genetic relationships between the protective and challenging CTV isolates. Taking advantage of deep sequencing technologies, we located six naturally infected trees harboring no-seedling yellow (no-SY) and no QD decline (mild) VT isolates and used these for challenge inoculation with three QD VT isolates. Symptom monitoring showed that all six Sicilian mild no-SY isolates, based on their genomic relatedness and mild symptoms reactions, provide effective protection against the three severe local VT isolates. The differences between the six mild and three severe isolates were confined to just a few nucleotide variations conserved in eight positions of three CTV genes (p23, p33, and Orf1a). These results confirm that the superinfection exclusion (SIE mechanism) depends on close genetic relatedness between the protective and challenging severe VT strain isolates. Ten years of investigation suggest that CP could turn into an efficient strategy to contain CTV QD infections of sweet orange trees on SO rootstock.
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Affiliation(s)
- Grazia Licciardello
- CREA—Council for Agricultural Research and Economics, Research Centre for Olive, Citrus and Tree Fruit, 95024 Acireale, Italy
| | - Giuseppe Scuderi
- Agrobiotech Soc. Coop. z.i. Blocco Palma I, Stradale Lancia 57, 95121 Catania, Italy; (G.S.); (M.R.); (M.B.)
| | - Marcella Russo
- Agrobiotech Soc. Coop. z.i. Blocco Palma I, Stradale Lancia 57, 95121 Catania, Italy; (G.S.); (M.R.); (M.B.)
| | - Marina Bazzano
- Agrobiotech Soc. Coop. z.i. Blocco Palma I, Stradale Lancia 57, 95121 Catania, Italy; (G.S.); (M.R.); (M.B.)
| | - Moshe Bar-Joseph
- The S. Tolkowsky Laboratory, Department of Plant Pathology, The Volcani Center, Agricultural Research Organization, Bet Dagan 50250, Israel;
| | - Antonino F. Catara
- Formerly, Department of Phytosanitary Science and Technologies, University of Catania, 95123 Catania, Italy;
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2
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Folimonova SY, Sun YD. Citrus Tristeza Virus: From Pathogen to Panacea. Annu Rev Virol 2022; 9:417-435. [DOI: 10.1146/annurev-virology-100520-114412] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Citrus tristeza virus (CTV) is the most destructive viral pathogen of citrus. During the past century, CTV induced grave epidemics in citrus-growing areas worldwide that have resulted in a loss of more than 100 million trees. At present, the virus continues to threaten citrus production in many different countries. Research on CTV is accompanied by distinctive challenges stemming from the large size of its RNA genome, the narrow host range limited to slow-growing Citrus species and relatives, and the complexity of CTV populations. Despite these hurdles, remarkable progress has been made in understanding the CTV-host interactions and in converting the virus into a tool for crop protection and improvement. This review focuses on recent advances that have shed light on the mechanisms underlying CTV infection. Understanding these mechanisms is pivotal for the development of means to control CTV diseases and, ultimately, turn this virus into an ally. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Affiliation(s)
- Svetlana Y. Folimonova
- Department of Plant Pathology, University of Florida, Gainesville, Florida, USA
- Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, Florida, USA
| | - Yong-Duo Sun
- Department of Plant Pathology, University of Florida, Gainesville, Florida, USA
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3
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Pasin F, Daròs JA, Tzanetakis IE. OUP accepted manuscript. FEMS Microbiol Rev 2022; 46:6534904. [PMID: 35195244 PMCID: PMC9249622 DOI: 10.1093/femsre/fuac011] [Citation(s) in RCA: 21] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2021] [Revised: 02/02/2022] [Accepted: 02/08/2022] [Indexed: 11/17/2022] Open
Abstract
Potyviridae, the largest family of known RNA viruses (realm Riboviria), belongs to the picorna-like supergroup and has important agricultural and ecological impacts. Potyvirid genomes are translated into polyproteins, which are in turn hydrolyzed to release mature products. Recent sequencing efforts revealed an unprecedented number of potyvirids with a rich variability in gene content and genomic layouts. Here, we review the heterogeneity of non-core modules that expand the structural and functional diversity of the potyvirid proteomes. We provide a family-wide classification of P1 proteinases into the functional Types A and B, and discuss pretty interesting sweet potato potyviral ORF (PISPO), putative zinc fingers, and alkylation B (AlkB)—non-core modules found within P1 cistrons. The atypical inosine triphosphate pyrophosphatase (ITPase/HAM1), as well as the pseudo tobacco mosaic virus-like coat protein (TMV-like CP) are discussed alongside homologs of unrelated virus taxa. Family-wide abundance of the multitasking helper component proteinase (HC-pro) is revised. Functional connections between non-core modules are highlighted to support host niche adaptation and immune evasion as main drivers of the Potyviridae evolutionary radiation. Potential biotechnological and synthetic biology applications of potyvirid leader proteinases and non-core modules are finally explored.
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Affiliation(s)
- Fabio Pasin
- Corresponding author: Instituto de Biología Molecular y Celular de Plantas (IBMCP), Consejo Superior de Investigaciones Científicas-Universitat Politècnica de València (CSIC-UPV), UPV Building 8E, Ingeniero Fausto Elio, 46011 Valencia, Spain. E-mail:
| | - José-Antonio Daròs
- Instituto de Biología Molecular y Celular de Plantas (IBMCP), Consejo Superior de Investigaciones Científicas-Universitat Politècnica de València (CSIC-UPV), 46011 Valencia, Spain
| | - Ioannis E Tzanetakis
- Department of Entomology and Plant Pathology, Division of Agriculture, University of Arkansas System, 72701 Fayetteville, AR, USA
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4
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Wen S, Wang G, Yang Z, Wang Y, Rao M, Lu Q, Hong N. Next-Generation Sequencing Combined With Conventional Sanger Sequencing Reveals High Molecular Diversity in Actinidia Virus 1 Populations From Kiwifruit Grown in China. Front Microbiol 2020; 11:602039. [PMID: 33391218 PMCID: PMC7774462 DOI: 10.3389/fmicb.2020.602039] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2020] [Accepted: 11/11/2020] [Indexed: 01/04/2023] Open
Abstract
Kiwifruit (Actinidia spp.) is native to China. Viral disease–like symptoms are common on kiwifruit plants. In this study, six libraries prepared from total RNA of leaf samples from 69 kiwifruit plants were subjected to next-generation sequencing (NGS). Actinidia virus 1 (AcV-1), a tentative species in the family Closteroviridae, was discovered in the six libraries. Two full-length and two near-full genome sequences of AcV-1 variants were determined by Sanger sequencing. The genome structure of these Chinese AcV-1 variants was identical to that of isolate K75 and consisted of 12 open reading frames (ORFs). Analyses of these sequences together with the NGS-derived contig sequences revealed high molecular diversity in AcV-1 populations, with the highest sequence variation occurring at ORF1a, ORF2, and ORF3, and the available variants clustered into three phylogenetic clades. For the first time, our study revealed different domain compositions in the viral ORF1a and molecular recombination events among AcV-1 variants. Specific reverse transcriptase–polymerase chain reaction assays disclosed the presence of AcV-1 in plants of four kiwifruit species and unknown Actinidia spp. in seven provinces and one city.
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Affiliation(s)
- Shaohua Wen
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China.,Key Laboratory of Horticultural Crop (Fruit Trees) Biology and Germplasm Creation of the Ministry of Agriculture, Wuhan, China
| | - Guoping Wang
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Zuokun Yang
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Yanxiang Wang
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Min Rao
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Qian Lu
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
| | - Ni Hong
- Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China.,Key Laboratory of Horticultural Crop (Fruit Trees) Biology and Germplasm Creation of the Ministry of Agriculture, Wuhan, China
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5
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Pasin F, Shan H, García B, Müller M, San León D, Ludman M, Fresno DH, Fátyol K, Munné-Bosch S, Rodrigo G, García JA. Abscisic Acid Connects Phytohormone Signaling with RNA Metabolic Pathways and Promotes an Antiviral Response that Is Evaded by a Self-Controlled RNA Virus. PLANT COMMUNICATIONS 2020; 1:100099. [PMID: 32984814 PMCID: PMC7518510 DOI: 10.1016/j.xplc.2020.100099] [Citation(s) in RCA: 32] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/20/2020] [Revised: 07/03/2020] [Accepted: 07/07/2020] [Indexed: 05/13/2023]
Abstract
A complex network of cellular receptors, RNA targeting pathways, and small-molecule signaling provides robust plant immunity and tolerance to viruses. To maximize their fitness, viruses must evolve control mechanisms to balance host immune evasion and plant-damaging effects. The genus Potyvirus comprises plant viruses characterized by RNA genomes that encode large polyproteins led by the P1 protease. A P1 autoinhibitory domain controls polyprotein processing, the release of a downstream functional RNA-silencing suppressor, and viral replication. Here, we show that P1Pro, a plum pox virus clone that lacks the P1 autoinhibitory domain, triggers complex reprogramming of the host transcriptome and high levels of abscisic acid (ABA) accumulation. A meta-analysis highlighted ABA connections with host pathways known to control RNA stability, turnover, maturation, and translation. Transcriptomic changes triggered by P1Pro infection or ABA showed similarities in host RNA abundance and diversity. Genetic and hormone treatment assays showed that ABA promotes plant resistance to potyviral infection. Finally, quantitative mathematical modeling of viral replication in the presence of defense pathways supported self-control of polyprotein processing kinetics as a viral mechanism that attenuates the magnitude of the host antiviral response. Overall, our findings indicate that ABA is an active player in plant antiviral immunity, which is nonetheless evaded by a self-controlled RNA virus.
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Affiliation(s)
- Fabio Pasin
- Centro Nacional de Biotecnología (CNB-CSIC), 28049 Madrid, Spain
- Agricultural Biotechnology Research Center, Academia Sinica, 11529 Taipei, Taiwan
| | - Hongying Shan
- Centro Nacional de Biotecnología (CNB-CSIC), 28049 Madrid, Spain
| | - Beatriz García
- Centro Nacional de Biotecnología (CNB-CSIC), 28049 Madrid, Spain
| | - Maren Müller
- Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Facultad de Biología, Universidad de Barcelona, 08028 Barcelona, Spain
| | - David San León
- Centro Nacional de Biotecnología (CNB-CSIC), 28049 Madrid, Spain
| | - Márta Ludman
- Agricultural Biotechnology Institute, National Agricultural Research and Innovation Centre, 2100 Gödöllő, Hungary
| | - David H. Fresno
- Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Facultad de Biología, Universidad de Barcelona, 08028 Barcelona, Spain
| | - Károly Fátyol
- Agricultural Biotechnology Institute, National Agricultural Research and Innovation Centre, 2100 Gödöllő, Hungary
| | - Sergi Munné-Bosch
- Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Facultad de Biología, Universidad de Barcelona, 08028 Barcelona, Spain
| | - Guillermo Rodrigo
- Institute for Integrative Systems Biology (I2SysBio), CSIC-University of Valencia, 46980 Paterna, Spain
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Sanfaçon H. Modulation of disease severity by plant positive-strand RNA viruses: The complex interplay of multifunctional viral proteins, subviral RNAs and virus-associated RNAs with plant signaling pathways and defense responses. Adv Virus Res 2020; 107:87-131. [PMID: 32711736 DOI: 10.1016/bs.aivir.2020.04.003] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Plant viruses induce a range of symptoms of varying intensity, ranging from severe systemic necrosis to mild or asymptomatic infection. Several evolutionary constraints drive virus virulence, including the dependence of viruses on host factors to complete their infection cycle, the requirement to counteract or evade plant antiviral defense responses and the mode of virus transmission. Viruses have developed an array of strategies to modulate disease severity. Accumulating evidence has highlighted not only the multifunctional role that viral proteins play in disrupting or highjacking plant factors, hormone signaling pathways and intracellular organelles, but also the interaction networks between viral proteins, subviral RNAs and/or other viral-associated RNAs that regulate disease severity. This review focusses on positive-strand RNA viruses, which constitute the majority of characterized plant viruses. Using well-characterized viruses with different genome types as examples, recent advances are discussed as well as knowledge gaps and opportunities for further research.
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Affiliation(s)
- Hélène Sanfaçon
- Summerland Research and Development Centre, Agriculture and Agri-Food Canada, Summerland, BC, Canada.
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7
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Dao TNM, Kang SH, Bak A, Folimonova SY. A Non-Conserved p33 Protein of Citrus Tristeza Virus Interacts with Multiple Viral Partners. MOLECULAR PLANT-MICROBE INTERACTIONS : MPMI 2020; 33:859-870. [PMID: 32141354 DOI: 10.1094/mpmi-11-19-0328-fi] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The RNA genome of citrus tristeza virus (CTV), one of the most damaging viral pathogens of citrus, contains 12 open reading frames resulting in production of at least 19 proteins. Previous studies on the intraviral interactome of CTV revealed self-interaction of the viral RNA-dependent RNA polymerase, the major coat protein (CP), p20, p23, and p33 proteins, while heterologous interactions between the CTV proteins have not been characterized. In this work, we examined interactions between the p33 protein, a nonconserved protein of CTV, which performs multiple functions in the virus infection cycle and is needed for virus ability to infect the extended host range, with other CTV proteins shown to mediate virus interactions with its plant hosts. Using yeast two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays, we demonstrated that p33 interacts with three viral proteins, i.e., CP, p20, and p23, in vivo and in planta. Coexpression of p33, which is an integral membrane protein, resulted in a shift in the localization of the p20 and p23 proteins toward the subcellular crude-membrane fraction. Upon CTV infection, the four proteins colocalized in the CTV replication factories. In addition, three of them, CP, p20, and p23, were found in the p33-formed membranous structures. Using bioinformatic analyses and mutagenesis, we found that the N-terminus of p33 is involved in the interactions with all three protein partners. A potential role of these interactions in virus ability to infect the extended host range is discussed.
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Affiliation(s)
- Thi Nguyet Minh Dao
- University of Florida, Plant Pathology Department, Gainesville, FL 32611, U.S.A
| | - Sung-Hwan Kang
- University of Florida, Plant Pathology Department, Gainesville, FL 32611, U.S.A
| | - Aurélie Bak
- University of Florida, Plant Pathology Department, Gainesville, FL 32611, U.S.A
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8
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Mann KS, Sanfaçon H. Expanding Repertoire of Plant Positive-Strand RNA Virus Proteases. Viruses 2019; 11:v11010066. [PMID: 30650571 PMCID: PMC6357015 DOI: 10.3390/v11010066] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2018] [Revised: 01/11/2019] [Accepted: 01/12/2019] [Indexed: 12/13/2022] Open
Abstract
Many plant viruses express their proteins through a polyprotein strategy, requiring the acquisition of protease domains to regulate the release of functional mature proteins and/or intermediate polyproteins. Positive-strand RNA viruses constitute the vast majority of plant viruses and they are diverse in their genomic organization and protein expression strategies. Until recently, proteases encoded by positive-strand RNA viruses were described as belonging to two categories: (1) chymotrypsin-like cysteine and serine proteases and (2) papain-like cysteine protease. However, the functional characterization of plant virus cysteine and serine proteases has highlighted their diversity in terms of biological activities, cleavage site specificities, regulatory mechanisms, and three-dimensional structures. The recent discovery of a plant picorna-like virus glutamic protease with possible structural similarities with fungal and bacterial glutamic proteases also revealed new unexpected sources of protease domains. We discuss the variety of plant positive-strand RNA virus protease domains. We also highlight possible evolution scenarios of these viral proteases, including evidence for the exchange of protease domains amongst unrelated viruses.
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Affiliation(s)
- Krin S Mann
- Summerland Research and Development Centre, Agriculture and Agri-Food Canada, Summerland, BC V0H 1Z0, Canada.
| | - Hélène Sanfaçon
- Summerland Research and Development Centre, Agriculture and Agri-Food Canada, Summerland, BC V0H 1Z0, Canada.
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9
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Ruiz-Ruiz S, Navarro B, Peña L, Navarro L, Moreno P, Di Serio F, Flores R. Citrus tristeza virus: Host RNA Silencing and Virus Counteraction. Methods Mol Biol 2019; 2015:195-207. [PMID: 31222705 DOI: 10.1007/978-1-4939-9558-5_14] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
To dissect the host RNA silencing response incited by citrus tristeza virus (CTV, genus Closterovirus), a (+) ssRNA of ~19300 nt, and the counter reaction deployed by the virus via its three RNA silencing suppressors (RSS), the small RNAs (sRNAs) of three virus-host combinations were deep sequenced. The subsequent analysis indicated that CTV sRNAs (1) constitute more than half of the total sRNAs in the susceptible Mexican lime and sweet orange, while only 3.5% in the restrictive sour orange; (2) are mostly of 21-22 nt, with those of (+) sense predominating slightly; and (3) derive from all the CTV genome, as evidenced by its entire recomposition from viral sRNA contigs but adopt an asymmetric pattern with a hotspot mapping at the 3'-terminal ~2500 nt. The citrus homologues of Arabidopsis Dicer-like (DCL) 4 and 2 most likely generate the 21 and 22 nt CTV sRNAs, respectively, by dicing the gRNA and the 3' co-terminal sgRNAs and, particularly, their double-stranded forms accumulating in infected cells. The plant sRNA profile, very similar and dominated by the 24 nt sRNAs in the three mock-inoculated controls, displayed a major reduction of the 24 nt sRNAs in Mexican lime and sweet orange, but not in sour orange. CTV infection also influences the levels of certain microRNAs.The high accumulation of CTV sRNAs in two of the citrus hosts examined suggests that it is not their synthesis, but their function, the target of the RSS encoded by CTV: p25 (intercellular), p23 (intracellular) and p20 (both). The two latter might block the loading of CTV sRNAs into the RNA silencing complex or interfere with it through alternative mechanisms. Of the three CTV RSS, p23 is the one that has been more thoroughly studied. It is a multifunctional RNA-binding protein with a putative Zn finger domain and basic motifs that (1) has no homologues in other closteroviruses, (2) accumulates in the nucleolus and plasmodesmata, (3) regulates the asymmetric balance of CTV (+) and (-) RNA strands, and (4) induces CTV syndromes and stimulates systemic infection in certain citrus species when expressed as a transgene ectopically or in phloem-associated cells.
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Affiliation(s)
- Susana Ruiz-Ruiz
- Instituto de Biología Molecular y Celular de Plantas, Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Valencia, Spain
| | - Beatriz Navarro
- Istituto per la Protezione Sostenibile delle Piante, Consiglio Nazionale delle Ricerche, Bari, Italy
| | - Leandro Peña
- Instituto de Biología Molecular y Celular de Plantas, Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Valencia, Spain.,Instituto Valenciano de Investigaciones Agrarias, Moncada, Spain
| | - Luis Navarro
- Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigaciones Agrarias (IVIA), Valencia, Spain
| | - Pedro Moreno
- Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigaciones Agrarias (IVIA), Valencia, Spain
| | - Francesco Di Serio
- Istituto per la Protezione Sostenibile delle Piante, Consiglio Nazionale delle Ricerche, Bari, Italy
| | - Ricardo Flores
- Instituto de Biología Molecular y Celular de Plantas, Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Valencia, Spain.
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Folimonova SY, Tilsner J. Hitchhikers, highway tolls and roadworks: the interactions of plant viruses with the phloem. CURRENT OPINION IN PLANT BIOLOGY 2018; 43:82-88. [PMID: 29476981 DOI: 10.1016/j.pbi.2018.02.001] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/30/2017] [Revised: 02/07/2018] [Accepted: 02/08/2018] [Indexed: 05/24/2023]
Abstract
The phloem is of central importance to plant viruses, providing the route by which they spread throughout their host. Compared with virus movement in non-vascular tissue, phloem entry, exit, and long-distance translocation usually involve additional viral factors and complex virus-host interactions, probably, because the phloem has evolved additional protection against these molecular 'hitchhikers'. Recent progress in understanding phloem trafficking of endogenous mRNAs along with observations of membranous viral replication 'factories' in sieve elements challenge existing conceptions of virus long-distance transport. At the same time, the central role of the phloem in plant defences against viruses and the sophisticated viral manipulation of this host tissue are beginning to emerge.
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Affiliation(s)
| | - Jens Tilsner
- Biomedical Sciences Research Complex, University of St Andrews, BMS Building, North Haugh, St Andrews, Fife KY16 9ST, United Kingdom; Cell and Molecular Sciences, The James Hutton Institute, Invergowrie, Dundee DD2 5DA, United Kingdom.
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