1. Virus-host interactions
Strategies viruses use to replicate their genomes in
susceptible host cells – “replication”
Strategies viruses use to move their genomes throughout
susceptible host plants – “cell-to-cell movement”
-Strategies viruses use to suppress host defenses

2. Plant viruses cause many different symptoms
Little Cherry
Plant viruses cause many different symptoms
Vein-banding
Flower Breaking
Necrosis
Tissue Deformation

3. Virions of plant viruses
Rigid rod
Icosahedral/
spherical
Flexuous rod

4. Plant virus
classes

5. Genes encoded by Tobacco mosaic virus

6. Viral Pathogenesis
Within the plant, viruses must complete three major steps in order
to infect a susceptible plant host.
Infection of
single cells
Cell-to-cell
movement
Long-distance
movement
Replication
Replication
Movement to
adjacent cells
Replication
Movement to
adjacent cells
cells throughout
the plant

7. Early steps in plant virus infection
Huang et al Curr. Opin. Virol.

8. Roles of host factors in (+) RNA virus replication
Huang et al Curr. Opin. Virol.

9. Assembly of viral replication complexes
Mine and Okuno Curr. Opin. Virol.

10. Virus movement
After the virus replicates it has to be able to move to new cells
and new tissues/organs in order to systemically infect
Infection of
single cells
Cell-to-cell
movement
Long-distance
movement
Replication
Replication
Movement to
adjacent cells
Replication
Movement to
adjacent cells
cells throughout
the plant

11. General view of virus cell-to-cell and long-distance movement
Hipper et al Front. Plant Sci.

12. Steps in cell-to-cell movement
Viruses need to leave sites of replication
Viruses need to locate the plasmodesmata
Viruses need to pass through plasmodesmata
Cell-to-cell
movement
Viral encoded “movement proteins (MPs)”
facilitate these steps. Most MPs are
Multifunctional.
MPs are required for movement
MPs bind to virus genomes
MPs interact with plant cytoskeleton
MPs localize to plasmodesmata
MPs gate plasmodesmata
Replication
Movement to
adjacent cells

13. Genes encoded by Tobacco mosaic virus

14. Replication and early steps in movement
Hyodo et al Front. Plant Sci.

15. Plasmodesmata
Lucas Virology. 344:

16. Structure of plasmodesmata and comparison to
viral particles

17. Model for trafficking through plasmodesmata
Lucas Virology.
344:

18. Model for TMV movement
Lucas Virology. 344:

19. Proposed accumulation and movement pathway for TMV
Liu and Nelson Front. Plant Sci.

20. Suppression of host defenses
Successful pathogens overcome innate host defense responses by targeting signaling, defense gene expression, or defense gene function.
Suppression of RNA silencing
Breaking through or breaking down physical or biochemical barriers
Suppression (or modulation) of basal defense
Suppression (or modulation) of (R gene–mediated HR or cell death)

21. The RNA silencing pathways – innate resistance to viruses
RNA silencing is logically an antiviral defense mechanism
Small RNAs corresponding to viruses can be found in infected tissues
Mutation of RNA silencing components can lead to increased virus infection
Viruses must protect themselves against this degradation pathway

22. Plant antiviral RNA silencing
Waterhouse (2006) Science 313:54-55

23. Anti-viral RNA silencing and its supporession by plant viruses
Incarbone and Dunoyer Trends Plant Sci.

24. Tombusvirus P19 is a silencing suppressor
Scholthof (2006) Nat. Rev. Microbiol.

25. Antiviral RNA silencing: defense and counter defense
Incarbone and Dunoyer Trends Plant Sci.

26. Perturbation of miRNA pathways to enhance resistance or viral pathogenesis
Incarbone and Dunoyer Trends Plant Sci.

27. Can virus infection be beneficial?
Roossinck PLoS Pathog.

28. New overlapping viral ORFs
Chung et al PNAS. 105:5897
Ling et al Virology 446:397

29. Summary
- Plant viruses encode proteins that direct the replication and movement
of their genomes
Viral replication occurs in association with host membranes and host factors
Viral movement is directed by movement proteins that serve many functions:
binding the viral genome
transporting the viral genome to plasmodesmata
gating plasmodesmata
trafficking through plasmodesmata
RNA silencing
an antiviral defense
plant viruses encode suppressors of RNA silencing
RNA silencing suppressors function by a variety of mechanisms