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 post- transcriptional gene silencing (PTGS) RNA silencing is a surveillance mechanism that controls invading nucleic acids (transposons, viruses)

TGS of promoter-homologous transgenes asNiR t 35S dsRNA TGS of promoter-homologous transgenes 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 TEV 271 x 35S-GUS (TGS GUS) 35S-GUS mock TEV mock CMV TEV

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 mock CMV L1 hen1-4 mock CMV CMV 25S

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