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The awr Gene Family Encodes a Novel Class of Ralstonia solanacearum Type III Effectors Displaying Virulence and Avirulence Activities

July 2012 , Volume 25 , Number  7
Pages  941 - 953

Montserrat Solé,1 Crina Popa,1 Oriane Mith,1 Kee Hoon Sohn,2 Jonathan D. G. Jones,2 Laurent Deslandes,3 and Marc Valls1

1Dept. Genètica, Universitat de Barcelona, Av. Diagonal 643 Annex, 08028 Barcelona, and Centre de Recerca Agrigenòmica, Edifici CRAG, Campus UAB, 08193 Bellaterra, Catalonia, Spain; 2The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, U.K.; 3Laboratoire des Interactions Plantes Micro-organismes, UMR CNRS-INRA 2594/441, 31320 Castanet Tolosan, France

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Accepted 4 March 2012.

We present here the characterization of a new gene family, awr, found in all sequenced Ralstonia solanacearum strains and in other bacterial pathogens. We demonstrate that the five paralogues in strain GMI1000 encode type III-secreted effectors and that deletion of all awr genes severely impairs its capacity to multiply in natural host plants. Complementation studies show that the AWR (alanine-tryptophan-arginine tryad) effectors display some functional redundancy, although AWR2 is the major contributor to virulence. In contrast, the strain devoid of all awr genes (Δawr1-5) exhibits enhanced pathogenicity on Arabidopsis plants. A gain-of-function approach expressing AWR in Pseudomonas syringae pv. tomato DC3000 proves that this is likely due to effector recognition, because AWR5 and AWR4 restrict growth of this bacterium in Arabidopsis. Transient overexpression of AWR in nonhost tobacco species caused macroscopic cell death to varying extents, which, in the case of AWR5, shows characteristics of a typical hypersensitive response. Our work demonstrates that AWR, which show no similarity to any protein with known function, can specify either virulence or avirulence in the interaction of R. solanacearum with its plant hosts.

This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.