1) Molecular functions of type 3 effectors
R. solanacearum possesses an abundant repertoire of Type 3 effectors. Since these effectors are injected by the bacterium into plant cells, the elucidation of Type 3 effector functions require the identification and characterization of their plant targets in order to understand their mode of action in the host cell. In the recent years, we have characterized a 7 member family of effectors called GALA which have LRR and F-box domains and which probably mimic some plant SCF component which possess E3 ubiquitin ligase activity. These GALA effectors are collectively required for full virulence on tomato and Arabidopsis, and the GALA7 effector is essential for virulence on the legume Medicago truncatula. These phenotypes on different host plants show that GALA effectors are important effectors that probably manipulate the plant ubiquitination system to promote disease.
Our group also characterized several effector proteins that specify host range of R. solanacearum GMI1000 towards several plants, such as, for example, the AvrA and PopP1 avirulence proteins which are recognized by the tobacco immune system and trigger a defensive hypersensitive response.
project members: Stephane Genin, Nemo Peeters, Fabienne Vailleau, Anne-Claire Cazalé, Patrick Barberis, David Lohou, Philippe Remigi
2) Regulation of virulence determinants
Mechanisms of bacterial adaptation to plants are also investigated through gene regulation approaches. In order to improve our understanding of the pathogenicity process in the early phases of plant infection, we have characterised the program triggered by HrpG, a transcriptional activator of the Type 3 Secretion System genes. In addition to Type 3-dependent genes, HrpG controls the expression of a large number of other virulence determinants likely involved in adaptation to life in the host. This approach is currently extended to other essential virulence regulators to provide an integrated view of the pathogenicity process.
Another way of investigation concerns the regulation of the translocation process of Type 3 effectors into plant cells. We recently identified a Type 3 chaperone controlling secretion of effectors and required for pathogenicity only on certain plants, suggesting that alteration of the secretion process can be detrimental on specific hosts.
project members: Stephane Genin, Fabienne Vailleau, Anne-Claire Cazalé, Patrick Barberis, David Lohou, Suvendra Rey
Collaborations: Prof. M. Valls (University of Barcelona)
Current funding: ANR 'Jeunes Chercheurs' (2011-2014) Project : PATHO-SWITCH. Coordinator: F. Vailleau. 210K€
3) Evolutionnary adaptation of the pathogen towards its host plants
Based on the still expanding host range described in the literature, it is known that R. solanacearum has a great adaptive potential that allows it to infect multiple hosts from distant botanical families. This offers a unique opportunity to study the molecular mechanisms governing this trait. Consequently, we recently initiated a project on the experimental evolution of R. solanacearum by serial passage experiments on a variety of plants followed by a complete genome re-sequencing of individuals with evolved beneficial adaptive traits. This approach should unravel functions important for the bacterial fitness in planta, and these functions could vary from one host to another.
Project members: Stephane Genin, Alice Guidot, Patrick Barberis, Wei Jiang
Collaborations: J. Gouzy (LIPM), Prof. C. Thébaut & Prof. J.B. Ferdy (EDB, Toulouse).
Current funding: * ANR 'GMGE' (2008-2011) Experimental evolution of the plant pathogen Ralstonia solanacearum: molecular bases of adaptation to host plants. Coordinator: S. Genin. 400K€
*Functional analysis of a candidate gene involved in the adaptation of Ralstonia solanacearum to host plants. INRA, SPE Department (2011-2012) Coordinator: A. Guidot 40K€