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In plants, host response to pathogenic  microbes  is driven both  by microbial  perception  and detection of modified-self. The Xanthomonas campestris  effector protein AvrAC/XopAC uridylylates the Arabidopsis  BIK1  kinase  to  dampen  basal  resistance and thereby promotes  bacterial  virulence. Here we show that PBL2, a paralog of BIK1, is similarly uridylylated by AvrAC.

However, in contrast to BIK1, PBL2 uridylylation is specifically required for host recognition of AvrAC to trigger immunity, but not AvrAC virulence. PBL2 thus acts as a decoy and enables AvrAC detection. AvrAC recognition also requires the RKS1 pseudokinase of the ZRK family and the NOD-like receptor ZAR1, which is known to recognize the Pseudomonas syringae effector HopZ1a. ZAR1 forms a stable  complex  with  RKS1, which  specifically  recruits PBL2 when the latter is uridylylated by AvrAC, triggering  ZAR1-mediated  immunity.  The results illustrate  how  decoy  substrates  and pseudokinases can specify and expand the capacity of the plant immune system.

Fig.: Immune reactions within the plant cell.

1. Plant cells detect the presence of bacterial compounds and activate immunity.
2. Bacteria inject proteins into the plant cells to neutralize this immunity.
3. In resistant plants, these neutralizers are recognized by a decoy. A stronger immunity is activated through a key NLR able of indirectly perceiving the presence of several bacterial neutralizers.