Plant Viruses

1. Plant Viruses Dr. Ernest Hiebert Plant Pathology, 1444 Fifield Hall 392 3631 216 ehi@ifas.ufl.edu

2. Introduction • Overview of plant viruses. • Life cycle of a typical RNA plant virus • Genomic organization and replication strategies of selected plant viruses • How are plant viruses controlled?

3. Relative sizes and comparative virion structures resolved by x-ray crystallography

5. Genus Tobamovirus ssRNA(+), rod-shaped virion morphology, no known vector Tobacco mosaic virus

8. Genus Tobamovirus ssRNA(+), rod-shaped virion morphology, no known vector Tobacco mosaic virus 6395 nts Cap at 5’ MT = capping, HEL = helicase, POL = polymerase,

9. Genomic organization and expression of type member Tobacco mosaic virus of the genus Tobamovirus • Rod-shaped virions 18 nm diameter and 300 nm long. • Virions very stable. • Coat protein MW = 17.5 kDa • No known vectors.

10. Genomic organization and expression of type member Tobacco mosaic virus of the genus Tobamovirus • Genome ssRNA, messenger sense, monopartite, 6395 nucleotides, contains at least four genes. • Capped 5’ end, t-RNA at 3’ end. • Viral RNA consists of a 5'-end untranslated region of 68 nts. • Open reading frame at the 5’end codes for a 126 kDa protein and a readthrough of the UAG terminator signal by plant suppressor Tyr-tRNA to produce 183 kDa protein. • Genes downstream expressed via subgenomic RNAs. Genome expression strategies involve subgenomic RNAs, and read-through translation

11. How do plant viruses initiate infection? Virus must enter via a wound in the plant cell wall and must be at the site during injury.

12. Cotranslational disassembly Replication

13. Schematic presentation of the“life cycle” of tobacco mosaic virus

14. Assembly

15. Virus spread from infection site • Replication within infected cell • Virions or viral genome move cell-to-cell via plasmodesmata • Facilitated by a virus-encoded movement protein. • Long distance movement via vascular tissue

16. Cell-to-cell movement via plasmodesmata, channels through plant cell walls which allow for passage of metabolites between adjacent cells

18. Illustration showing the spread of Tobacco mosaic virus in a tomato plant Agrios, G. N. Plant Pathology, 4e, Academic Press

19. Family Potyviridae, Genus Potyvirus ssRNA (+) flexous rod-shaped morphology, vectored by aphids Potato virus Y, necrotic strain infecting potato

20. Family Potyviridae, Genus Potyvirus ssRNA (+) flexous rod-shaped morphology, vectored by aphids ~9500 nts HC-PRO= helper factor (vector), protease, suppression of host defense AI amorphous inclusions NI nuclear inclusions Light micrograph of infected cell showing cylindrical inclusions (CI)

21. Potyvirus life cycle Encyclopedia of Virology, 2e, Academic Press

22. Viral translation strategies Polyprotein synthesis Picornaviruses- Entire (+) sense RNA genome is translated into a single large polyprotein. Processing is carried out by two virus encoded proteases 2A pro and 3C pro. Flaviviruses- Viral precursor proteins are processed by cellular proteases. The (+) sense RNA genome is translated into a polyprotein precursor processed by viral serine protease and by host signal peptidase. Potyvirus group of plant viruses- Potato virus Y and tobacco etch virus contain a (+) sense genome RNA of around 10,000 bases which has a single open reading frame. This polyprotein is processed by viral encoded proteases. Potyviruses Comoviruses

23. Family Bunyaviridae, genus Tospovirus, type member Tomato spotted wilt virus (TSWV)Negative and ambisense , single-stranded RNA Negative sense single stranded RNA virus

24. Family Bunyaviridae, genus Tospovirus, type member Tomato spotted wilt virus (TSWV)

25. Family Bunyaviridae, genus Tospovirus, type member tomato spotted wilt virus (TSWV)vectored by thrips, replicates both in the vector and in the plant host

26. Family Bunyaviridae, genus Tospovirus, type member Tomato spotted wilt virus (TSWV)

27. Family Bunyaviridae, genus Tospovirus, type member Tomato spotted wilt virus (TSWV)

28. Family Bunyaviridae, genus Tospovirus, type member Tomato spotted wilt virus (TSWV) The ORFs of the M and S RNAs are expressed by subgenomic mRNAs. Expression strategy includes tripartite genomic organization, cap snatching, negative/ambisense coding, subgenomic mRNAs, and proteolytic processing.

30. Circular ssDNA Family Geminiviridae Genus Begomovirus Tomato yellow leaf curlvirus

31. Circular ssDNA Family Geminiviridae Genus Begomovirus Tomato yellow leaf curl virus 2700 nts V2 movement V1 coat protein C1 replication C2 suppression of host C3 replication enhancement C4 pathogenicity IR origin of replication

32. Circular ssDNA Family Geminiviridae Genus Begomovirus

34. Conventional Approaches for the control of plant viruses • Breeding for resistance to plant viruses and virus vectors • Control of plant viruses by cross-protection • Control of Vectors • Insecticides • Exclusion of Plant Viruses • Quarantine and certification • Avoidance of plant viruses through cultural practices • Certification for plant viruses • Seed, root stocks, fruit tree propagative material • Elimination • Heat treatment, meristem tip culture, and tip micrografting

35. Molecular Approaches for the control of plant viruses Pathogen-derived resistance - gene silencing

36. Post-transcriptional gene silencing RNA silencing acts as a natural defense mechanism against invasive nucleic acids, including viruses, transposons and perhaps other highly repetitive genomic sequences. RNA silencing also plays a pivotal role in plant and animal development by providing an elegant system of gene control that can occur through RNA degradation, translational inhibition or chromatin modification.

38. http://www.sciencemag.org/cgi/reprint/309/5740/1518a.pdf

39. Symptoms due to PTGS

41. Commercial zucchini with engineered resistance to Zucchini yellow mosaic potyvirus

42. An article in “Discover” Magazine January 2000 describes how ultraviolet absorbing film over greenhouses can protect against insect-borne viruses. Yehezkel Antignus . 2000. Manipulation of wavelength-dependent behaviour of insects: an IPM tool to impede insects and restrict epidemics of insect-borne viruses. Virus Research 71 (2000) 213–220