Plant VirusesPlant VirusesDr. Ernest Hiebert

Plant Pathology, 1444 Fifield Hall392 3631 216

ehi@ifas.ufl.edu

Introduction 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 TobamovirusssRNA(+), rod-shaped virion morphology, no known vector Tobacco mosaic virus

http://www.rci.rutgers.edu/~bhillman/comparative_virology/

http://www.rci.rutgers.edu/~bhillman/comparative_virology/

Genus TobamovirusssRNA(+), rod-shaped virion morphology, no known vector Tobacco mosaic virus

MT = capping, HEL = helicase, POL = polymerase,

6395 ntsCap at 5’

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.

Genomic organization and expression of type member Tobacco mosaic virus of the genus TobamovirusGenome 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

How do plant viruses initiate How do plant viruses initiate infection?infection?

Virus must enter via a wound in

the plant cell wall and mustbe at the site during injury.

Replication

Cotranslational disassembly

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

Assembly

Virus spread from Virus spread from infection siteinfection 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

Agrios, G. N. Plant Pathology, 4e, Academic Press

Illustration showing the spread of Tobacco mosaic virus in a tomato plant

Family Potyviridae, Genus PotyvirusssRNA (+) flexous rod-shaped morphology, vectored by aphidsPotato virus Y, necrotic strain infecting potato

Family Potyviridae, Genus PotyvirusssRNA (+) flexous rod-shaped morphology, vectored by aphids

HC-PRO= helper factor (vector), protease, suppression of host defenseAI amorphous inclusions

NI nuclear inclusions

Light micrograph of infected cell

showing cylindrical inclusions (CI)

~9500 nts

Potyvirus life cycle

Encyclopedia of Virology, 2e, Academic Press

Viral translation strategies

Polyprotein synthesisPicornaviruses- 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.

http://www.rci.rutgers.edu/~bhillman/comparative_virology/

PotyvirusesComoviruses

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

Negative sense single strandedRNA 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)

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.

Circular ssDNA Family GeminiviridaeGenus BegomovirusTomato yellow leaf curl virus

Circular ssDNA Family GeminiviridaeGenus BegomovirusTomato yellow leaf curl virus

V2 movement

V1 coat protein

C1 replication

C2 suppression of host

C3 replication enhancement

C4 pathogenicity IR origin of replication

2700 nts

Circular ssDNA Family GeminiviridaeGenus 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 virusesPathogen-derived resistance - 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.

Post-transcriptional gene silencingPost-transcriptional gene silencing

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

Symptoms due to PTGS

Commercial zucchini with engineered resistance toZucchini yellow mosaic potyvirus

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

An article in “Discover” Magazine January 2000 describes howultraviolet absorbing film over greenhouses can protect against insect-borne viruses.