Summary

This study builds on previous publications \cite{Ade2007}\cite{Zhang2010} that describe the activity of the Pseudomonas syringae Type 3 host-translocated effector AvrPphB during infection of Arabidopsis thaliana . AvrPphB is a protease that cleaves the Arabidopsis receptor like cytoplasmic kinase PBS1, which in turn is guarded by the nucleotide-binding leucine-rich repeat (NLR) intracellular receptor RPS5. Upon sensing kinase cleavage by the effector, RPS5 is activated and triggers the hypersensitive immune response. Previous studies by Roger Innes group \cite{Kim2016}, discussed by \cite{Giannakopoulou2016} have provided a proof of concept for the use of this system as a platform to develop disease resistance to diverse plant pathogens. RPS5 recognition spectrum can be broadened by introducing cleavage sites recognized by different effector proteases into PBS1. In this paper, the authors explore natural responses to AvrPphB in plants other than Arabidopsis, notably barley (see Figure below). They uncovered a recognition system of AvrPphB in barley that is mechanistically analogous to Arabidopsis RPS5/PBS1 and potentially exploitable. Two barley orthologs of PBS1, HvPBS1-1 and HvPBS1-2, are cleaved by AvrPphB. By employing genome-wide association studies (GWAS) they singled out PBR1, a functional homolog of RPS5, as the NLR that mediates AvrPphB recognition. They further characterized this PBR1 and identified a PBR1 homolog in wheat implying that the system could be functional in this important cereal crop. 
We found the paper to be well written and we particularly enjoyed the original genetic mapping strategies employed to identify the candidate NLR responsible for AvrPphB recognition. One important implication of the findings reported by Carter, Helm et al. is that PBS1-derived decoys can be developed in barley and other cereals such as wheat, to detect host-translocated protease effectors.