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Plant virus

A discussion on plant viruses

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Plant virus

  1. 1. Pawan Kumar M.Sc. Bioinformatics 1st semester PK 1
  2. 2. Learning Objectives  Introduction to Plant viruses  Infections  Virus life cycle  Transmission  Structure  Classification  Replication  Symptom  Identification  Control. PK 2
  3. 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 3
  4. 4. chronic degenerative desease decreasing fitness growth reductionchlorotic lesions mozaic intervein chloroses necroses leaf curling Viral infection PK 4
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  6. 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) PK 6
  7. 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. PK 7
  8. 8. Virus life cycle Invasion Genome uncoating, expression and replicationPK 8
  9. 9. Systemic transport through phloem PK 9
  10. 10. Spreading of viral infection Within a plant - plasmodesmata (movement proteiny) - vascular tissue (phloem) Movement proteins: - interaction with virion - interaction with plasmodesmata (increase of size exclussion limit) PK 10
  11. 11. Spreading of viral infection Between 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 11
  12. 12. 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 PK 12
  13. 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 PK 13
  14. 14. TMV Viral capsids Capsomers – structural subunits (one or more capsid proteins) Basic shapes: A. Helical – capsomers in helical arrangement (e.g. Tobacco mosaic virus) PK 14
  15. 15. capsomers form usually triangles arranged to polyhedron (usually icosahedra – twenty sides) - various number of proteins in a capsomer. 12 pentagons 20 hexagons Viral capsids PK 15
  16. 16. Classification of plant viruses - genom/replication  ssRNA, also dsRNA, ssDNA, dsDNA  ssRNA - coding ssRNA(+) - non-coding ssRNA(-) - replication via RT (also dsDNA viruses) PK 16
  17. 17. 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) PK 17
  18. 18. Replication cycle of ssDNA viruses (Geminiviridae) PK 18
  19. 19. RNA viruses PK 19
  20. 20. dsRNA viruses e.g. Phytoreoviridae - 12 dsRNA segments, - viral polymerase - transcription in cytoplasma (viroplasma) - minus strands synthetized after encapsidation PK 20
  21. 21. ssRNA viruses  Ss RNA(RT):Pseudoviridae again derived from retrotransposons.  Classical RNA viruses: enkapsidation of + or –RNA  RNA+ : most frequent (Tombusviridae, Bromoviridae, Potyviridae)  RNA- : Rhabdoviridae all propagate also in insect vectors RNA dep. RNA-polymerase in capsid. PK 21
  22. 22. Ss RNA (RT) viruses: PK 22 Reverse transcription is applied to first make ds DNA and then transcription and translation.
  23. 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. PK 23
  24. 24. Infection cycle of TMV. PK 24
  25. 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. PK 25
  26. 26. Some example PK 26
  27. 27. PK 27
  28. 28. Symptoms: PK 28 Systemic Symptoms:  General: Abnormal growth and developmental malformations (reduced growth, dwarfing, stunting of plants, etc.) Reduced life span of plants, death.  Foliage symptoms: Mosaics Yellows Ring spots.  Other: Symptoms on stems, fruit, roots.  Local symptoms: Necrotic lesions.
  29. 29. Detection and identification PK 29  Symptoms (possible causes by nutrient deficiencies, toxicities, insects, etc.  dsRNA isolation.  PCR, RT-PCR (for known viruses).  Virus genome sequencing.  Microarray.
  30. 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). PK 30
  31. 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.). PK 31
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