Download 2.8MB ppt

RNA silencing as an immune system in plants
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
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 posttranscriptional gene silencing (PTGS)
RNA silencing is a surveillance mechanism that controls
invading nucleic acids (transposons, viruses)
35S
TGS of promoter-homologous transgenes
asNiR
t
dsRNA
PTGS of mRNA-homologous (trans)genes
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
Viruses inhibit PTGS but not TGS
271 x 35S-GUS (PTGS NiR)
271 x 35S-GUS (PTGS NiR)
mock
TVCV
CMV
271 x 35S-GUS (TGS GUS)
mock
TEV
mock
CMV
TEV
TEV
35S-GUS
PTGS spreads systemically like viruses
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
PTGS-deficient mutants are hypersusceptible to CMV infection
C
25S
CMV
CMV
CMV
CMV
mock
CMV
mock
hen1-4
L1
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