The ADP-ribosyltransferase HopU1 of P. syringae is required for full virulence in Arabidopsis thaliana and is known to suppress immune responses triggered by pattern recognition receptor (PRR)-mediated recognition of pathogen-associated molecular patterns (PAMPs). This suppression had been shown to occur through modification of plant glycine-rich RNA-binding protein 7 (GRP7), inhibiting its RNA-binding activity. Previous work had suggested that GRP7 is involved in inducing early and late immune responses (reactive oxygen species (ROS) production and callose deposition, respectively) and that A. thaliana lacking GRP7 is more susceptible to P. syringae infection than wild-type plants. However, which immunity-related RNAs and which aspect of the response HopU1 interferes with had remained unclear.
Because GRP7 is an RNA-binding protein, the authors reasoned that it might enhance the expression of immunity-related RNAs and that this step might be targeted by HopU1. Consistent with this hypothesis, GRP7 showed a highly specific in vivo association with the transcripts for FLS2 and EFR, two PRRs known to be involved in early and late plant immune responses. Moreover, mass spectrometry analysis revealed that GRP7 interacts with components of the translation machinery, indicating that it might promote translation of the two PRRs. Although HopU1 did not interfere with the association between GRP7 and the translation machinery, it did inhibit the binding of GRP7 to FLS2 and EFR mRNAs, and this inhibition was caused by mono-ADP-ribosylation of the GRP7 RNA-binding domain.
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