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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
t
asNiR
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
35S-NiR / 271
271 (PTGS NiR)
35S-GUS / 271 x 35S-GUS
35S-GUS
271 x 35S-GUS (TGS 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 CMV TEV
mock 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
C
L1
hen1-4
mock CMV CMV
mock CMV 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 --gt infection Weak suppressor --gt
tolerance No suppressor --gt recovery --gt immunity