Adnan Ahmad

1. Plant virusesPlant viruses

2. Plant viruses
• Nucleic acid in a protein capsid
– Protein capsid
• protection of NA
• transfer (infection)
• no membrane envelop• no membrane envelop
– Nucleic acid
• different types
• infectious itself only in some viruses
• encode just few genes (x bacteriophages up to 70)
• Most enzymatic activities necessary for virus replication
provided by the host cell

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

4. Proteins encoded by plant viruses
• Capsid proteins
• Polymerases of NA
• Movement proteins
- transport through plasmodesmata
Present in most plant
viruses
• Suppressors of silencing
• Other specialized proteins:
– e.g. proteases (cleavage of polyproteins), modulators of cell cycle, replication
protein, helicases….
Varying representation of these proteins in plant viruses

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

6. EM of helical capsids

7. B. Polyhedral – capsomers form usually triangles arranged to
polyhedron (usually icosahedron – twenty sides)
- various number of proteins in a capsomer
12 pentagons
20 hexagons
Viral capsids

8. Classification of plant viruses
- genome/mode of replication
• ssDNA
• dsDNA (RT)• dsDNA (RT)
• ssRNA ssRNA(+) = coding
ssRNA(-) = non-coding
• dsRNA

9. DNA viruses
replication dsDNA - RNA intermediate (by transcription) + reverse tr.
ssDNA - dsDNA intermediate (by host DNA polymerase)
transcription - RNA polymerase II (from dsDNA)
constitutive strong promoters for transgenosis

10. Caulimoviridae – „pararetroviruses“
- derived from LTR retrotransposones (order of ORF, replication, …)
- rarely integrated = „endogenous pararetroviruses“
- 35S transcript > full genomic (with overlap)
- cDNA synthesis, circularization

11. ssDNA viruses - Geminiviridae
- ability to activate cell cycle - Why?
- replication protein: nicking (ssDNA break, binding, religation)
- dsDNA intermediate with nucleosomes
Replication cycle

12. RNA viruses
= retrotransposons

13. dsRNA viruses
e.g. Phytoreoviridae - 12 dsRNA segments,
- transcription in cytoplasma (viroplasma)
- viral RNA dep. RNA-polymerase in virion
- minus strands synthetized after encapsidation - WHY?
ssRNA viruses
RT – Pseudoviridae = retrotransposons
Classical RNA viruses – enkapsidation of + (coding) or –RNA
RNA- : Rhabdo- a Bunyaviridae
- all propagate also in insect vectors (animal origin)
- viral polymerase in capsid – WHY?
RNA+ : most frequent (Tombusviridae, Bromoviridae,
Potyviridae)
ssRNA viruses

14. Replication of RNA(+) viruses
ssRNA(+) = mRNA and template for replication
– Release of RNA, synthesis (translation) of polymerase (and other proteins)
– RNA replication (minus and plus strand synthesis)
– Translation of viral proteins (polymerase, capsid, movement, …)
– new virions – spontaneous assembly of capsid proteins on NA

15. Spreading of viral infection
Between plants
– natural barriers of entrance: cuticle, cell wall
– HOW to overcome them?
- mechanical injury, direct contact (wind)
- vectors – sucking insects,- 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!

16. Non-persistant
• adsorbtion on styletes
(specific binding sites on acrostyle)
• only transient infectiousness:
immediate, persists minutes to hours
Circulative
Transmission via sucking insects
Circulative
• circulation of virus in insect body – salivary glands
• infectiousness:
latent period (hours to days), gradually decreasing many days
Propagative
• virus replication in transmittor insect
• infectiousness:
latent period (hours to days), life-long (also transmission to progeny)

17. Spreading of viral infection
Within a plant
- plasmodesmata (movement proteins)
- vascular tissue (phloem)
Movement proteins: - interaction with virion OR
- interaction with plasmodesmata- interaction with plasmodesmata
(increase of size exclussion limit)

18. - independently in the majority of
viruses – various mechanisms
(in any step of the RNAi
Suppressors
of RNA
silencing
(in any step of the RNAi
pathway)
- responsible for some symptoms
of viral infection through
repression of RNAi regulated
developmental pathways!
Csorba et al. 2015

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

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

21. VIROIDs
– circular ssRNA, no protein envelop (capsid)
– genom size insufficient to encode proteins
(359 b = 1/10 of smallest RNA viruses)
- first sequenced eukaryotic patogen Potato Spindle Tuber

22. VIROIDs
Replication with host DNA dep.(!) RNA Pol II
- probably rolling circle
- concatemers of some viroids autocatalytically
cleaved by hammer-head ribozymecleaved by hammer-head ribozyme
- symptoms of infection – likely result from induced RNAi
non-specifically affecting expression of plant genes
- common features (origin?) with HDV (hepatitis D virus)

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

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