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
Little Cherry
Plant viruses
cause many
different
symptoms
Vein-banding
Tissue Deformation
Necrosis
Flower Breaking
Virions of plant viruses
Rigid rod
Icosahedral/
spherical
Flexuous rod
Plant virus
classes
Genes encoded by Tobacco mosaic virus
Viral Pathogenesis
Within the plant, viruses must complete three major steps in order
to infect a susceptible plant host.
Infection of
single cells
Replication
Cell-to-cell
movement
Replication
Movement to
adjacent cells
Long-distance
movement
Replication
Movement to
adjacent cells
Movement to
cells throughout
the plant
Early steps in plant virus infection
Huang et al. 2012. Curr. Opin. Virol.
Roles of host factors in (+) RNA virus replication
Huang et al. 2012. Curr. Opin. Virol.
Assembly of viral replication complexes
Mine and Okuno. 2012. Curr. Opin. Virol.
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
Replication
Cell-to-cell
movement
Replication
Movement to
adjacent cells
Long-distance
movement
Replication
Movement to
adjacent cells
Movement to
cells throughout
the plant
General view of virus cell-to-cell and long-distance movement
Hipper et al. 2013. Front. Plant Sci.
Steps in cell-to-cell movement
Cell-to-cell
movement
1)
Viruses need to leave sites of replication
2)
Viruses need to locate the plasmodesmata
3)
Viruses need to pass through plasmodesmata
Viral encoded “movement proteins (MPs)”
facilitate these steps. Most MPs are
Multifunctional.
Replication
Movement to
adjacent cells
1)
MPs are required for movement
2)
MPs bind to virus genomes
3)
MPs interact with plant cytoskeleton
4)
MPs localize to plasmodesmata
5)
MPs gate plasmodesmata
Genes encoded by Tobacco mosaic virus
Replication and early steps in movement
Hyodo et al. 2014. Front. Plant Sci.
Plasmodesmata
Lucas. 2006. Virology. 344:169-184
Structure of plasmodesmata and comparison to
viral particles
Model for trafficking through plasmodesmata
Lucas. 2006. Virology.
344:169-184
Model for TMV movement
Lucas. 2006. Virology. 344:169-184
Proposed accumulation and movement pathway for TMV
Liu and Nelson. 2013. Front. Plant Sci.
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)
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
Plant antiviral RNA silencing
Waterhouse (2006) Science 313:54-55
Anti-viral RNA silencing and its supporession by plant viruses
Incarbone and Dunoyer. 2013. Trends Plant Sci.
Tombusvirus P19 is a silencing suppressor
Scholthof (2006) Nat. Rev. Microbiol.
Antiviral RNA silencing: defense and counter defense
Incarbone and Dunoyer. 2013. Trends Plant Sci.
Perturbation of miRNA pathways to enhance resistance or viral pathogenesis
Incarbone and Dunoyer. 2013. Trends Plant Sci.
Can virus infection be beneficial?
Roossinck. 2013. PLoS Pathog.
New overlapping viral ORFs
Chung et al. 2008. PNAS. 105:5897
Ling et al. 2013. Virology 446:397
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