1. RNA silencing as an immune system in plants

2. Introduction of a (transgenic) additional copy of an endogenous
gene can trigger coordinated silencing of the transgene and
of the endogenous gene (co-suppression)
Transgene-induced co-suppression mimics some cases of
spontaneous or mutagene-induced duplication of endogenous genes

3. RNA silencing is induced by double-stranded RNA (dsRNA)
dsRNA corresponding to promoter sequences induce transcriptional gene silencing (TGS)
dsRNA corresponding to transcribed sequences induce post- transcriptional gene silencing (PTGS)
RNA silencing is a surveillance mechanism that controls invading nucleic acids (transposons, viruses)

4. TGS of promoter-homologous transgenes
asNiR t
35S
dsRNA
TGS of promoter-homologous transgenes
PTGS of mRNA-homologous (trans)genes

5. PTGS but not TGS produces a graft-transmissible silencing signal
35S-NiR
271
(PTGS NiR)
35S-NiR / 271
35S-GUS
271 x 35S-GUS
(TGS GUS)
35S-GUS / 271 x 35S-GUS

6. Viruses inhibit PTGS but not TGS
271 x 35S-GUS (PTGS NiR)
271 x 35S-GUS (PTGS NiR)
mock TVCV CMV TEV
271 x 35S-GUS (TGS GUS)
35S-GUS
mock TEV
mock CMV TEV

7. PTGS spreads systemically like viruses

8. Plant mutants defective for PTGS are viable
Proteins controlling PTGS in Arabidopsis
- SGS1 : ?
- SGS2/SDE1 : RNA-dependent RNA polymerase
- SGS3 : unknown function (coiled-coil)
- SGS4/AGO1 : unknown function (PPD)
- SGS5/HEN1 : unknown function

9. PTGS-deficient mutants are hypersusceptible to CMV infection
mock
CMV L1
hen1-4
mock
CMV
CMV
25S

10. The issue of an infection by a virus depends on the race between
plant defense and virus RNA silencing suppressing activities :
Strong suppressor --> infection
Weak suppressor --> tolerance
No suppressor --> recovery --> immunity