1 Plant Viruses

Pawan KumarM.Sc. Bioinformatics

1st semester

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2 Learning Objectives

Introduction to Plant viruses Infections Virus life cycle Transmission Structure Classification Replication Symptom Identification Control.

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3 Plant viruses Non-cellular form of life. Nucleic acid in protein capsid (no membrane envelop). Virus is a nucleo-protein having RNA or DNA as a

genetic material. RNA or DNA may be ss or ds, ssRNA may be +ve or –ve

sense. The fundamental characteristic is their absolute

dependence on a living host organism for their reproduction.

Other necessary processes (enzymatic activities) by host cell.

Encode just few genes (x bacteriophages up to 70)

Most of plant virus infect a limited number of different plant species and a few have a wide host range.PK

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chronic degenerative desease decreasing fitness

growth reductionchlorotic lesions mozaic

intervein chloroses necroses leaf curling

Viral infection

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6 Plant response to inoculation with a virus

Plant is immune: virus does not replicate in protoplasts nor in cells of the intact plant

Infection is limited to initially infected cells: replication, but no cell-to-cell movement (could be due to an ineffectual virus movement protein)

Infection is limited to initial leaf: replication, cell-to-cell movement, but no long distance movement (inability to enter phloem or due to a host resistance gene-mediated response, i.e. hypersensitive response when the plant resistance gene product recognizes some viral product, local lesions formation)

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7 MODE OF ENTRY

Mechanically TMV, BCMV, PVX, PYDV. Vectors PLRV,TLCV, WTV. Pollen Grain necrotic ring spot, ring spot of soybean. Seeds or vegetative:Propagation BCMV, CMV, Pea stripe virus. Pinocyte CPMV, Pea enation mosaic Fungal parasite TNV, Barley yellow mosaic virus,

Tobacco stunt virus. Ectodesmata TMV. Epidermal hair TMV.

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8 Virus life cycle

Invasion

Genome uncoating, expression and replication

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Systemic transport through phloem

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10 Spreading of viral infectionWithin a plant

- plasmodesmata (movement proteiny)- vascular tissue (phloem) Movement proteins: - interaction with virion

- interaction with plasmodesmata (increase of size exclussion limit)

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11 Spreading of viral infectionBetween plants –

natural barriers of entrance: cuticle, cell wall

- mechanical injury, direct contact (wind)- vectors – sucking insects, other insects, nematods, fungi - grafting, root coalescence, - parasitic plants (Cuscuta)- vegetative propagation - some viruses also via seeds and polen!

Protection – elimination of infected plants and vector insects!PK

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Non-persistant

• adsorbtion on styletes(specific binding sites on acrostyle)

• infectiousness: immediate, persists only minutes to hours

Circulative

• circulation of virus in insect body – salivary glands• infectiousness:latent period (hours to days), gradually decreasing many days Propagative

• virus replication in transmittor• infectiousness:latent period (hours to days), life-long (also transmission to progeny)

Transmission via sucking insects

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13 Proteins encoded by plant viruses

Polymerases of NA (helicases) Movement proteins

- transport through plasmodesmata Capsid proteins Proteases

- cleavage of polyproteins.silencing

Different representation of these proteins in different viruses

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TMV

Viral capsidsCapsomers – structural subunits (one or more capsid

proteins)

Basic shapes:

A. Helical – capsomers in helical arrangement

(e.g. Tobacco mosaic virus)

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capsomers form usually triangles arranged to polyhedron (usually icosahedra – twenty sides)

- various number of proteins in a capsomer.

12 pentagons20 hexagons

Viral capsids

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16 Classification of plant viruses- genom/replication

ssRNA, also dsRNA, ssDNA, dsDNA

ssRNA - coding ssRNA(+)- non-coding ssRNA(-)- replication via RT (also dsDNA viruses)

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DNA Viruses- transcription by RNA polymerase II from dsDNA

dsDNA viruses – replication through RNA intermediate (reverse transcription)

ssDNA viruses – replication through dsDNA intermediate (by host DNA polymerase)

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Replication cycle of ssDNA viruses (Geminiviridae)

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RNA viruses

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20dsRNA viruses

e.g. Phytoreoviridae - 12 dsRNA segments, - viral polymerase- transcription in cytoplasma (viroplasma) - minus strands synthetized after encapsidation

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21 ssRNA viruses

Ss RNA(RT):Pseudoviridaeagain derived from retrotransposons.

Classical RNA viruses: enkapsidation of + or –RNA

RNA+ : most frequent (Tombusviridae, Bromoviridae, Potyviridae)

RNA- : Rhabdoviridaeall propagate also in insect vectorsRNA dep. RNA-polymerase in capsid.

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22 Ss RNA (RT) viruses:

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Reverse transcription is applied to first make ds DNA and then transcription and translation.

23 Ss RNA+ viruses: e.g. tobacco mosaic virus (TMV)

Release of RNA Translation of polymerase RNA replication Translation of viral proteins (polymerase, capsid, ….) new virions spontaneously pack with coating protein.

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24 Infection cycle of TMV.

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25 Ss RNA- viruses: -ve ss RNA require RDRP(RNA dependent RNA

polymerase). Which make +ve ss RNA from its original stand. These newly formed +ve ss RNA will make –ve ss

RNA and also translate to form protein capsid.

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26 Some example

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28 Symptoms:

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Systemic Symptoms: General:

Abnormal growth and developmental malformations (reduced growth, dwarfing, stunting of plants, etc.)

Reduced life span of plants, death. Foliage symptoms:

MosaicsYellows Ring spots.

Other: Symptoms on stems, fruit, roots.

Local symptoms: Necrotic lesions.

29 Detection and identification

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Symptoms (possible causes by nutrient deficiencies, toxicities, insects, etc.

dsRNA isolation. PCR, RT-PCR (for known viruses). Virus genome sequencing. Microarray.

30 Control of plant viruses

Keep out of an area through quarantine, inspection, and certification programs that would prevent or limit local or international virus movement and warranty the use of virus-free seed, tubers, budwood .

Eradication of diseased plants Controlling the insect vectors (oil sprays, repellents,

insecticides, predators…) Breeding plants for resistance Transgenic resistance (natural resistance genes, viral

sequences, genes from other sources).

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31 Reference: Prescott Harley Klein's Microbiology - Willey, Sherwood &

Woolvertion (7th ed.). Molecular Biology of the Gene - James D. Watson, Tania A.

Baker, Stephen P. Bell (5th ed.). Wikipedia source. YouTube source. Characteristic of Plant viral pathogen by Svetlana Folimonova

(Ass. Prof. -Dept. of Plant Pathology, University of Florida). Introduction to Plant Viruses, the Invisible Foe

(www.apsnet.org). Viron | Biology Field Guide | Biowars (www.biowars.com). Plant Virus replication by P.N. Sharma Department of Plant

Pathology, CSK HPKV, Palampur (H.P.).

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