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

Plant viruses cause many different symptoms

Necrosis

Vein-banding

Little Cherry

Flower BreakingTissue Deformation

Rigid rod Icosahedral/spherical

Flexuous rod

Virions of plant viruses

Plant virus classes

Genes encoded by Tobacco mosaic virus

Viral Pathogenesis

Within the plant, viruses must complete three major steps in orderto infect a susceptible plant host.

Infection ofsingle cells

Cell-to-cellmovement

Long-distancemovement

Replication ReplicationMovement to adjacent cells

ReplicationMovement to adjacent cellsMovement 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 cellsand new tissues/organs in order to systemically infect

Infection ofsingle cells

Cell-to-cellmovement

Long-distancemovement

Replication ReplicationMovement to adjacent cells

ReplicationMovement to adjacent cellsMovement to cells throughout the plant

General view of virus cell-to-cell and long-distance movement

Hipper et al. 2013. Front. Plant Sci.

Cell-to-cellmovement

ReplicationMovement to adjacent cells

Steps in 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 areMultifunctional.

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 toviral particles

Model for trafficking through plasmodesmata

Lucas. 2006. Virology. 344:169-184

Lucas. 2006. Virology. 344:169-184

Model for TMV movement

Liu and Nelson. 2013. Front. Plant Sci.

Proposed accumulation and movement pathway for TMV

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 throughthrough 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

Waterhouse (2006) Science 313:54-55

Plant antiviral RNA silencing

Incarbone and Dunoyer. 2013. Trends Plant Sci.

Anti-viral RNA silencing and its supporession by plant viruses

Scholthof (2006) Nat. Rev. Microbiol.

Tombusvirus P19 is a silencing suppressor

Incarbone and Dunoyer. 2013. Trends Plant Sci.

Antiviral RNA silencing: defense and counter defense

Incarbone and Dunoyer. 2013. Trends Plant Sci.

Perturbation of miRNA pathways to enhance resistance or viral pathogenesis

Can virus infection be beneficial?

Roossinck. 2013. PLoS Pathog.

Ling et al. 2013. Virology 446:397

Chung et al. 2008. PNAS. 105:5897

New overlapping viral ORFs

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 genometransporting the viral genome to plasmodesmatagating plasmodesmatatrafficking through plasmodesmata

- RNA silencingan antiviral defenseplant viruses encode suppressors of RNA silencing

RNA silencing suppressors function by a variety of mechanisms