1. Plant Science Initiative and Department of Plant Pathology, University of Nebraska, Lincoln, Nebraska 68588-0660, USA
2. School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588-0118, USA
3. Department of Agronomy and Horticulture, University of Nebraska, Lincoln, Nebraska 68588-0915, USA
4. Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, USA
5. Molecular Cell Physiology, University of Bielefeld, 33501 Bielefeld, Germany
6. These authors contributed equally to this work.

Correspondence to: James R. Alfano¹ Correspondence and requests for materials should be addressed to J.R.A. (Email: jalfano2@unl.edu).

Abstract

The bacterial plant pathogen Pseudomonas syringae injects effector proteins into host cells through a type III protein secretion system to cause disease. The enzymatic activities of most of P. syringae effectors and their targets remain obscure. Here we show that the type III effector HopU1 is a mono-ADP-ribosyltransferase (ADP-RT). HopU1 suppresses plant innate immunity in a manner dependent on its ADP-RT active site. The HopU1 substrates in Arabidopsis thaliana extracts were RNA-binding proteins that possess RNA-recognition motifs (RRMs). A. thaliana knockout lines defective in the glycine-rich RNA-binding protein GRP7 (also known as AtGRP7), a HopU1 substrate, were more susceptible than wild-type plants to P. syringae. The ADP-ribosylation of GRP7 by HopU1 required two arginines within the RRM, indicating that this modification may interfere with GRP7's ability to bind RNA. Our results suggest a pathogenic strategy where the ADP-ribosylation of RNA-binding proteins quells host immunity by affecting RNA metabolism and the plant defence transcriptome.

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