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Common themes in plant pathogenesis: suppression of host defenses

Common themes in plant pathogenesis: suppression of host defenses. Successful pathogens overcome innate host defenses In the case of viruses, they suppress RNA silencing. A general model for RNA silencing, RNAi and PTGS. RISC = RNA-induced silencing complex siRNA = small interfering RNA.

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Common themes in plant pathogenesis: suppression of host defenses

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  1. Common themes in plant pathogenesis: suppression of host defenses Successful pathogens overcome innate host defenses In the case of viruses, they suppress RNA silencing

  2. A general model for RNA silencing, RNAi and PTGS RISC = RNA-induced silencing complex siRNA = small interfering RNA György Hutvágner and Phillip D Zamore RNAi: nature abhors a double-strand Current Opinion in Genetics & Development 2002, 12:225–232

  3. Vaucheret. 2006. Genes Dev. 20:759

  4. Vaucheret. 2006. Genes Dev. 20:759

  5. 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

  6. Concept of viral suppressor arose out of studies of a phenomenon called “synergy” Virus synergy occurs in doubly infected hosts The sum is greater than the parts

  7. Virus synergy between ZYMV and CMV in squash plants Healthy CMV-Fny ZYMV-AT Devastating in the field AT+Fny

  8. Potyvirus genome organization VPg P1 HC-Pro Poly(A) P3 CI VPg-NIa NIb CP NIa P1 HC-Pro Polyprotein VPgNIa 6K CI CP NIb VPg P3 HC-Pro P1 Protein responsible for synergism maps to HC-Pro in potyviruses

  9. The protein responsible for synergism suppresses post-transcriptional gene silencing Also achieved by expressing HC-Pro from a heterologous virus or if HC-Pro transgenic plants are crossed to silenced GUS or GFP plants Infect plant from D with potyvirus (PVY) and now the plant is able to express GFP again. Gene silencing of GFP is suppressed. Baulcombe, Carrington, Vance labs all reported this activity for HC-Pro in same month in 1998

  10. PTGS suppression is a widespread phenomenon Note: Correlation between proteins known to be involved in LD movement and/or efficient accumulation Voinnet et al. PNAS 1999

  11. Suppression of GFP silencing in N. benthamiana by different viruses or their proteins Use PVX as vector to express other viral proteins. Works because in this system PVX does not suppress silencing. ACMV is geminivirus (a ssDNA virus) Voinnet et al. 1999 PNAS

  12. More suppression of GFP silencing in N. benthamiana by different viruses or their proteins Voinnet et al. 1999 PNAS

  13. Silencing suppressors act on different stages of PTGS. Comparison of PVY-HC-Pro to CMV-2b. Brigneti et al. EMBO 1998

  14. Experiments to assess stage of silencing affected by HC-Pro • Using transgenic plants that are already silenced • (i.e. silencing is in the maintenance phase) • Using Agrobacterium infiltration to introduce • GUS and HC-Pro for transient expression A and B = Controls to demonstrate system works C = Infiltration of HC-Pro results in suppression of silencing in lines where silencing is established. Llave, Kasschau, and Carrington. 2000 PNAS

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

  16. Tombusvirus P19 binds to duplex siRNA Scholthof (2006) Nat. Rev. Microbiol.

  17. Waterhouse (2006) Science 313:54-55

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