Plant Viruses Dr Ernest Hiebert Plant Pathology 1444

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

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?

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

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

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

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 k. Da • No known vectors.

Genomic organization and expression of type member Tobacco mosaic virus of the genus Tobamovirus Genome ss. RNA, 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 k. Da protein and a readthrough of the UAG terminator signal by plant suppressor Tyr-t. RNA to produce 183 k. Da protein. • Genes downstream expressed via subgenomic RNAs. Genome expression strategies involve subgenomic RNAs, and read-through translation

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.

Cotranslational disassembly Replication

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

Assembly

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

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

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

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

Family Potyviridae, Genus Potyvirus ss. RNA (+) 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)

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

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 2 A pro and 3 C 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

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

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

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

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

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

Circular ss. DNA Family Geminiviridae Genus Begomovirus Tomato yellow leaf curl virus

Circular ss. DNA Family Geminiviridae Genus Begomovirus Tomato yellow leaf curl virus 2700 nts V 2 movement V 1 coat protein C 1 replication C 2 suppression of host C 3 replication enhancement C 4 pathogenicity IR origin of replication

Circular ss. DNA Family Geminiviridae Genus Begomovirus

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

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

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.

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

Symptoms due to PTGS

Commercial zucchini with engineered resistance to Zucchini yellow mosaic potyvirus

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