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