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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
– 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)
Viral genome- compact
- various arrangement and strategies of expression
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
- independently in the majority of viruses – various mechanisms
Suppressors of RNA silencing
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
symptoms: depletion of metabolites, defence reactions, suppressor side effects, …
- chronic degenerative desease decreasing fitness
chlorotic lesions mozaic growth reduction
intervein chloroses necroses leaf curling
Viral infection
Spreading of viral infectionWithin 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!
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
TMV
Viral capsidsCapsomers – structural subunits (one or more capsid
proteins)
Basic shapes:
A. Helical – capsomers in helical arrangement
(e.g. Tobacco mosaic virus)
EM of helical capsids
B. Polyhedral – capsomers form usually triangles arranged to polyhedron (usually icosahedr – twenty sides)
- various number of proteins in a capsomer
12 pentagons20 hexagons
Viral capsids
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 virusese.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
Replication of RNA(+) virusesssRNA(+) = 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
AA
G
N’ N’ G
N CG
A
NN’ N’
N N N N
N’N’
GU
A
C
N
HUG
ACN
C
G
NH = A,C,T
yellow NTs + 3 short dsRNA regions necessary for cleavage(but also sufficient = possible induction of cleavage in trans)
cleavage site
AA
N CG
NN N N N
HUG
ACN
G
N
G
N’ N’ GA
N’ N’ N’N’
GU
A
C
N
C
introduced inducing RNA
cleaved RNA
minimal requirements of cleaved RNA: H = A,C,T
cleavage site