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Lionel Navarro

Small RNA-directed Control of the Host Immune Response and its Targeting by Bacterial Effectors

Our current research interests are to understand the role and regulation of small RNAs in plant innate immunity and to unravel the mechanisms by which bacterial effectors interfere with RNA silencing in plants and humans. We are mostly using the phytopathogenic bacterium Pseudomonas syringae pv. tomato strain DC3000 and the human pathogenic bacterium Shigella flexneri as model systems.

Current projects


Mechanisms of bacterial-mediated suppression of the microRNA pathway. The innate immune response is the first line of defence against pathogens, which plays a critical role in antimicrobial defence. This response is initiated by the perception of pathogen-derived signatures referred to as Pathogen or Microbe associated molecular patterns (PAMPs or MAMPs). Both plants and animals sense PAMPs and in turn differentially regulate a large set of immune responsive genes, among which microRNA (miRNAs) were recently identified. These small regulatory RNAs control gene expression mostly at the post-transcriptional level by inducing degradation and/or translational silencing of target mRNAs.We recently showed that growth of a type III secretion-defective mutant of Pseudomonas syringae as well as non-pathogenic bacteria was significantly enhanced in Arabidopsis miRNA-deficient mutants, implicating the miRNA pathway as major component of antibacterial defense. Accordingly, we have identified P. syringae type III secreted proteins that suppress multiple steps of the miRNA pathway. Our goal is now to identify the modes of action of these bacterial silencing suppressors and to determine whether analogous mechanisms of RNA silencing suppression are used by human pathogenic bacteria.

Role of the RNA-directed DNA methylation pathway in the regulation of the Arabidopsis immune response and the evolution of immune-response genes. In higher eukaryotes, the vast majority of the genome appears to be transcribed, leading to an extraordinary diversity of non- coding RNAs (ncRNAs). Whereas the functional significance of these ncRNAs is mostly unknown, increasing evidence suggests a role for these molecules in guiding chromatin modifications. In plants, a large proportion of ncRNAs is processed by the RNA silencing machinery to produce short interfering (si)RNAs. Some of them guide sequence specific DNA methylation through a phenomenon referred to as RNA-directed DNA methylation (RdDM). The RdDM pathway contributes to the transcriptional silencing (TGS) of transposons, repeats as well as some genes that carry repeats in their vicinity. We have recently shown that DNA demethylation is part of a plant immune response and, accordingly, have identified functionally relevant defense genes that are directly controlled by this epigenetic regulatory mechanism. Our goal is now to determine whether bacterial effectors could have evolved strategies to interfere directly with the Arabidopsis TGS machinery to enable disease. We are also investigating the contribution of the RdDM pathway in transgenerational immune priming and its eventual implication in the evolution of immune-response genes.

Selected publications

Zervudacki J, Yu A, Amesefe D, Wang J, Drouaud J, Navarro L and Deleris A. Transcriptional control and exploitation of an immune-responsive family of retrotransposons. Embo Journal (2018) e98482.

Thiébeauld O, Charvin M, Singla Rastogi M, Yang F, Pontier D, Pouzet C, Bapaume L, Li G, Deslandes L, Lagrange T, Alfano J.R, and Navarro L. A bacterial GW-effector targets Arabidopsis AGO1 to promote pathogenicity and induces Effector-triggered immunity by disrupting AGO1 homeostasis. BIORXIV (2017) 215590.

Deleris A, Halter T, Navarro L. DNA Methylation and Demethylation in Plant Immunity. Annu Rev Phytopathol (2016) 54: 579-603.

Boccara M, Sarazin A, Thiébeauld O, Jay F, Voinnet O, Navarro L, Colot V. The Arabidopsis miR472-RDR6 silencing pathway modulates PAMP- and Effector-triggered immunity through the post-transcriptional control of disease resistance genes. PLOS Pathogens (2014) 10: e1003883.

Yu A, Lepère G, Jay F, Wang J, Bapaume L, Wang Y, Abraham AL, Penterman J, Fischer RL, Voinnet O, Navarro L. Dynamics and biological relevance of DNA demethylation in Arabidopsis antibacterial defense. PNAS (2013) 110: 2389-2394.

Navarro L., Jay F., Nomura K., He S.Y., Voinnet O. Suppression of the microRNA pathway by bacterial effector proteins. Science (2008) 321: 964-967.




Arabidopsis system reporting silencing activity at the level of xylem (...)
Arabidopsis system reporting silencing activity at the level of xylem parenchyma cells
Arabidopsis system reporting silencing activity at the level of (...)
Arabidopsis system reporting silencing activity at the level of hydathodes
Arabidopsis reporter system that recapitulates the variegated phenotype (...)
Arabidopsis reporter system that recapitulates the variegated phenotype observed in natural epialleles
Human HeLa cells infected with Shigella flexneri (in red: Actin; in blue: (...)
Human HeLa cells infected with Shigella flexneri (in red: Actin; in blue: Shigella cells)
Propagation of a GFP-tagged Pto DC3000 in leaf secondary veins of an active (...)
Propagation of a GFP-tagged Pto DC3000 in leaf secondary veins of an active DNA demethylase mutant
Arabidopsis system reporting the transcriptional activation of a (...)
Arabidopsis system reporting the transcriptional activation of a retrotransposon in response to bacterial challenges
Nucleocytosolic accumulation of a bacterial silencing suppressor in a leaf (...)
Nucleocytosolic accumulation of a bacterial silencing suppressor in a leaf of Nicotiana benthamiana