Pseudomonas syringae pv. tomato DC3000 Type III Secretion Effector Polymutants Reveal an Interplay between HopAD1 and AvrPtoB

•An effectorless Pseudomonas syringae mutant was constructed to study effector interplay•The effector HopAD1 is sufficient to elicit cell death in Nicotiana benthamiana•An AvrPtoB mutant (AvrPtoBM3) suppresses HopAD1-triggered host cell death•AvrPtoBM3 interacts with MKK2, which is required for HopAD1-triggered cell death

SummaryThe bacterial pathogen Pseudomonas syringae pv. tomato DC3000 suppresses the two-tiered plant innate immune system by injecting a complex repertoire of type III secretion effector (T3E) proteins. Beyond redundancy and interplay, individual T3Es may interact with multiple immunity-associated proteins, rendering their analysis challenging. We constructed a Pst DC3000 polymutant lacking all 36 T3Es and restored individual T3Es or their mutants to explore the interplay among T3Es. The weakly expressed T3E HopAD1 was sufficient to elicit immunity-associated cell death in Nicotiana benthamiana. HopAD1-induced cell death was suppressed partially by native AvrPtoB and completely by AvrPtoBM3, which has mutations disrupting its E3 ubiquitin ligase domain and two known domains for interacting with immunity-associated kinases. AvrPtoBM3 also gained the ability to interact with the immunity-kinase MKK2, which is required for HopAD1-dependent cell death. Thus, AvrPtoB has alternative, competing mechanisms for suppressing effector-triggered plant immunity. This approach allows the deconvolution of individual T3E activities.

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