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
Capsid proteins
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. Viral infection
symptoms: depletion of metabolites, defence reactions, suppressor side effects, … - chronic degenerative desease decreasing fitness
chlorotic lesions
mozaic
growth reduction
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
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!

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
latent period (hours to days), life-long (also transmission to progeny)

11. Viral capsids
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. Viral capsids
Polyhedral – capsomers form usually triangles arranged to polyhedron (usually icosahedr – twenty sides)
- various number of proteins 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 ssRNA 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)
ssRNA viruses

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

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