Plant viruses
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
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)
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
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!
Example: suppressor P19 (tombusvirus)
– dual function
- homodimers P19 bind siRNA
- induce expression of miR168 – block of AGO1 translation
Burgyán, Havelda 2011
Viral infection
symptoms: depletion of metabolites, defence
reactions, suppressor side effects, …
- chronic degenerative desease decreasing fitness
chlorotic lesions
intervein chloroses
mozaic
necroses
growth reduction
leaf curling
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)
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!
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)
Viral capsids
Capsomers – structural subunits (one or more capsid
proteins)
Basic shapes:
A. Helical – capsomers in helical arrangement
(e.g. Tobacco mosaic virus)
TMV
EM of helical capsids
Viral capsids
B. Polyhedral – capsomers form usually triangles arranged to
polyhedron (usually icosahedr – twenty sides)
- various number of proteins in a capsomer
12 pentagons
20 hexagons
Classification of plant viruses
- genom/replication
• ssRNA, also dsRNA, ssDNA, dsDNA
• ssRNA
- coding
ssRNA(+)
- non-coding
ssRNA(-)
- replication via RT (also dsDNA viruses)
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)
Caulimoviridae
- derived from LTR retroTE (order of ORF, replication, tRNA primer)
- rarely integrated = „endogenous pararetroviruses“ – integrase?
- 35S transcript > full genomic
- circularization
Replication cycle of ssDNA viruses
(Geminiviridae)
– ability to activate cell cycle! Why?
RNA viruses
dsRNA viruses
e.g. Phytoreoviridae - 12 dsRNA segments,
- viral polymerase
- transcription in cytoplasma (viroplasma)
- minus strands synthetized after encapsidation
ssRNA viruses
- 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)
Replication of RNA(+) viruses
ssRNA(+) = mRNA and replication template
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
VIROIDs
– circular ssRNA, no protein envelop (capsid)
– genom size insufficient to encode proteins
(359 b = 1/10 of smallest RNA viruses)
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)
Hammerhead ribozyme
yellow NTs + 3 short dsRNA
regions necessary for cleavage
(but also sufficient = possible
induction of cleavage in trans)
N N
C G
A A U
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
cleavage site
H = A,C,T
cleavage site
minimal requirements
of cleaved RNA:
N N
C G
A A U
A
H
cleaved RNA
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
H = A,C,T