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 mozaic growth reduction intervein chloroses necroses 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 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 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 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

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) 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

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