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Phytobacterial Type III Effectors HopX1, HopAB1 and HopF2 Enhance Sense-Post-Transcriptional Gene Silencing Independently of Plant R Gene-Effector Recognition

August 2011 , Volume 24 , Number  8
Pages  907 - 917

Panagiotis F. Sarris,1,2 Shang Gao,3 Konstantinos Karademiris,2 Hailing Jin,3 Kriton Kalantidis,1,2 and Nickolas J. Panopoulos1,2

1Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology, Hellas, PO Box 1527, Heraklion 71110, Crete, Greece; 2Department of Biology, University of Crete, 71409 Heraklion, Greece; 3Department of Plant Pathology and Microbiology, Center for Plant Cell Biology and Institute for Integrative Genome Biology, University of California, Riverside 92521, U.S.A.

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Accepted 23 March 2011.

Plant- and animal-pathogenic bacteria deploy a variable arsenal of type III effector proteins (T3EP) to manipulate host defense. Specific biochemical functions and molecular or subcellular targets have been demonstrated or proposed for a growing number of T3EP but remain unknown for the majority of them. Here, we show that transient expression of genes coding certain bacterial T3EP (HopAB1, HopX1, and HopF2), which did not elicit hypersensitive response (HR) in transgenic green fluorescent protein (GFP) Nicotiana benthamiana 16C line, enhanced the sense post-transcriptional gene silencing (S-PTGS) triggered by agrodelivery of a GFP-expressing cassette and the silencing enhancement could be blocked by two well-known viral silencing suppressors. Further analysis using genetic truncations and site-directed mutations showed that the receptor recognition domains of HopAB1 and HopX1 are not involved in enhancing silencing. Our studies provide new evidence that phytobacterial pathogen T3EP manipulate the plant small interfering RNA pathways by enhancing silencing efficiency in the absence of effector-triggered immunity signaling and suggest that phytopathogenic bacterial effectors affect host RNA silencing in yet other ways than previously described.

© 2011 The American Phytopathological Society