Regulation of gene expression through microRNAs (miRNAs) and antiviral defense through small interfering RNAs (siRNAs) are aspects of RNA silencing1, a process originally discovered as an unintended consequence of plant transformation by disarmed Agrobacterium tumefaciens strains2. Although RNA silencing protects cells against foreign genetic elements3, its defensive role against virulent, tumor-inducing bacteria has remained unexplored. Here, we show that siRNAs corresponding to transferred-DNA oncogenes initially accumulate in virulent A. tumefaciens–infected tissues and that RNA interference–deficient plants are hypersusceptible to the pathogen. Successful infection relies on a potent antisilencing state established in tumors whereby siRNA synthesis is specifically inhibited. This inhibition has only modest side effects on the miRNA pathway, shown here to be essential for disease development. The similarities and specificities of the A. tumefaciens RNA silencing interaction are discussed and contrasted with the situation encountered with plant viruses.
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We thank P. Zambryski and M. Matzke for critical reading of the manuscript, S. Gelvin for providing strains and the root inoculation protocol, members of the Voinnet lab for discussions and R. Wagner's team for plant care. This work was supported by an Action thématique et incitative sur programme Jeune Chercheur award from the Centre National de la Recherche Scientifique and a postdoctoral fellowship to P.D. from the Federation of European Biochemical Societies. O.V. is recipient of a Young Investigator Award from the European Molecular Biology Organization.
The authors declare no competing financial interests.
A. thaliana plants expressing the P19 and P1-HcPro viral silencing suppressors show decreased susceptibility to A. tumefaciens. (PDF 456 kb)
Absence of secondary siRNAs in stems and tumors from GF-RNAi plants with the rdr6 mutation. (PDF 331 kb)
Optimal virus accumulation in stems of DAN2 plants carrying the rdr6 mutation, compared with tumors developing on similar DAN2 tissues. (PDF 185 kb)
Specific transcriptional repression of miR393 in tumors. (PDF 239 kb)
A. tumefaciens susceptibility assays in roots of the dcl2-1, dcl3-1 and rdr2-1 mutants. (PDF 807 kb)
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Dunoyer, P., Himber, C. & Voinnet, O. Induction, suppression and requirement of RNA silencing pathways in virulent Agrobacterium tumefaciens infections. Nat Genet 38, 258–263 (2006). https://doi.org/10.1038/ng1722
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