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Plant viruses

Plant viruses. Plant viruses. Nucleic acid in protein capsid (no membrane envelop) P rotein capsid – protection and transfer of NA Nucleic acid – infectious (in some viruses together with polymerases) Encode just few genes (x bacteriophages up to 70)

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Plant viruses

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  1. Plant viruses

  2. Plant viruses • Nucleic acid in protein capsid (no membrane envelop) • Protein capsid – protection and transfer of NA • Nucleic acid – infectious (in some viruses together with polymerases) • Encode just few genes (x bacteriophages up to 70) • Other necessary processes (enzymatic activities) by host cell

  3. Viral genome - compact - various arrangement and strategies of expression – formation of polyproteins – segmented genome (alt. more virions - e.g. Tobacco rattle virus) – alt. read-through stop codon (translational readthrough) – alternative frameshift during translation – overlapping reading frames: alt. translation starts (transcription from both strands) – IRES (cap independent initiation of translation)

  4. Proteins encoded by plant viruses • Polymerases of NA (helicases) • Movement proteins - transport through plasmodesmata • Capsidproteins • Proteases - cleavage of polyproteins • Suppressors of silencing Different representation of these proteins in different viruses

  5. Suppressors of RNA silencing • independently in the majority of viruses – various mechanisms Burgyán, Havelda 2011 • participate in symptoms of infection through repression of RNAi regulated developmental steps!

  6. Example: suppressor P19 (tombusvirus) • – dual function • homodimers P19 bind siRNA • induce expression of miR168 – block of AGO1 translation Burgyán, Havelda 2011

  7. Viralinfection symptoms: depletion of metabolites, defence reactions, suppressor side effects, …- chronic degenerative desease decreasing fitness growth reduction chlorotic lesions mozaic intervein chloroses necroses leaf curling

  8. 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)

  9. Spreading of viral infection Betweenplants– naturalbarriersofentrance: cuticle, cell wall - mechanicalinjury, directcontact (wind) - vectors – suckinginsects, otherinsects, nematods, fungi - grafting, rootcoalescence, - parasiticplants (Cuscuta) - vegetative propagation - somevirusesalsoviaseedsand polen! Protection – eliminationofinfectedplantsandvectorinsects!

  10. Transmission via sucking insects • 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)

  11. Viralcapsids Capsomers – structural subunits (one or more capsid proteins) Basic shapes: A. Helical– capsomers in helical arrangement (e.g. Tobacco mosaic virus) TMV

  12. EM of helical capsids

  13. Viralcapsids • Polyhedral– capsomersformusuallytrianglesarranged to polyhedron (usuallyicosahedr – twentysides) • - variousnumberofproteins in a capsomer 12 pentagons 20 hexagons

  14. Classification of plant viruses- genom/replication • ssRNA, also dsRNA, ssDNA, dsDNA • ssRNA - coding ssRNA(+) - non-coding ssRNA(-) - replication via RT (also dsDNA viruses)

  15. 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)

  16. Caulimoviridae • derived from LTR retroTE(order of ORF, replication, tRNA primer) • - rarely integrated = „endogenous pararetroviruses“ – integrase? • 35S transcript > full genomic • circularization

  17. Replication cycle of ssDNA viruses (Geminiviridae) – ability to activate cell cycle! Why?

  18. RNA viruses

  19. dsRNA viruses e.g. Phytoreoviridae - 12 dsRNA segments, - viral polymerase - transcription in cytoplasma (viroplasma) - minus strands synthetized after encapsidation • RT – Pseudoviridae – again derived from retrotransposons Classical RNA viruses – enkapsidation of + or –RNA • RNA- : Rhabdo- a Bunyaviridae • all propagate also in insect vectors • RNA dep. RNA-polymerase in capsid – why? • RNA+ : most frequent (Tombusviridae, Bromoviridae, Potyviridae) ssRNAviruses

  20. Replication of RNA(+) viruses ssRNA(+) = mRNA and replication template

  21. Replication cycle of 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 through „polymeration“ of capsid proteins on NA

  22. VIROIDs • circular ssRNA, no protein envelop (capsid) • genom size insufficient to encode proteins (359 b = 1/10 of smallest RNA viruses)

  23. VIROIDs - symptoms of infection – likely results from induced RNAi non-specifically affecting expression of plant genes - common features (origine?) with HDV (hepatitis D virus) Replication with host DNA dep.(!) RNA Pol II - probably rolling circle • concatemers of some viroids autocatalytically cleaved by hammer-head ribozyme e.g. Potato Spindle Tuber (the first sequenced eucaryotic patogen)

  24. H = A,C,T Hammerhead ribozyme yellow NTs + 3 short dsRNA regions necessary for cleavage (but also sufficient = possible induction of cleavage in trans) cleavage site N N C G A U A A H G N N N N N G N C N’ N’ N’ N’ C N’ C N’ G A U G G N A

  25. H = A,C,T cleavage site minimal requirements of cleaved RNA: N N C G A U A cleaved RNA A H G N N N N N G N C N’ N’ N’ N’ C N’ C N’ G A U introduced inducing RNA G G N A

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