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

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 Cell-to-cell movement Long-distance movement Replication Replication Movement to adjacent cells Replication Movement to adjacent cells 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 Cell-to-cell movement Long-distance movement Replication Replication Movement to adjacent cells Replication Movement to adjacent cells 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 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

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