Figure 1.

The diverse activities of AvrPtoB. AvrPtoB is delivered into the plant cell by the type III secretion system (T3SS) of Pseudomonas syringae. In tomato, full‐length AvrPtoB is recognized by Pto, whereas the N‐terminal region of AvrPtoB is recognized by Fen. Both Pto and Fen kinases signal effector‐triggered immunity (ETI) through the nucleotide‐binding leucine‐rich repeat (NB‐LRR) protein, Prf (not shown). Full‐length AvrPtoB ubiquitinates Fen, leading to its degradation and the suppression of ETI. One explanation for the ability of Pto to avoid ubiquitination and degradation by AvrPtoB is that it may inhibit ubiquitination by phosphorylating AvrPtoB near the E3 ubiquitin ligase domain. In the absence of AvrPtoB, CERK1/Bti9 and FLS2 trigger pathogen (or microbe)‐associated molecular pattern‐triggered immunity (PTI) in response to PAMPs (flagellin in the case of FLS2 and an unknown PAMP for CERK1/Bti9). AvrPtoB suppresses CERK1/Bti9‐ and FLS2‐mediated PTI, reportedly by promoting their degradation in Arabidopsis. In addition, the N‐terminal region of AvrPtoB is sufficient for interacting with the co‐receptor BAK1 and disrupting the PAMP‐induced FLS2–BAK1 complex. The N‐terminal region of AvrPtoB is also able to suppress mitogen‐activated protein (MAP) kinase activation, transcription of PAMP‐induced genes and microRNA (miRNA) production, most probably as a result of targeting pattern recognition receptors (PRRs). AvrPtoB expression is associated with increased production of the phytohormones abscisic acid (ABA) and ethylene. Both hormones antagonize the salicylic acid (SA)‐mediated immune response and ethylene is known to be important for symptom formation. HR, hypersensitive response; PM, plasma membrane.