Plant Immunity Directly or Indirectly Restricts the Injection of Type III Effectors by the Pseudomonas syringae Type III Secretion System

Plants perceive microorganisms by recognizing microbial molecules known as pathogen-associated molecular patterns (PAMPs) inducing PAMP-triggered immunity (PTI) or by recognizing pathogen effectors inducing effector-triggered immunity (ETI). The hypersensitive response (HR), a programmed cell death response associated with ETI, is known to be inhibited by PTI. Here, we show that PTI-induced HR inhibition is due to direct or indirect restriction of the type III protein secretion system's (T3SS) ability to inject type III effectors (T3Es). We found that the Pseudomonas syringae T3SS was restricted in its ability to inject a T3E-adenylate cyclase (CyaA) injection reporter into PTI-induced tobacco (Nicotiana tabacum) cells. We confirmed this restriction with a direct injection assay that monitored the in planta processing of the AvrRpt2 T3E. Virulent P. syringae strains were able to overcome a PAMP pretreatment in tobacco or Arabidopsis (Arabidopsis thaliana) and continue to inject a T3E-CyaA reporter into host cells. In contrast, ETI-inducing P. syringae strains were unable to overcome PTI-induced injection restriction. A P. syringae pv tomato DC3000 mutant lacking about one-third of its T3E inventory was less capable of injecting into PTI-induced Arabidopsis plant cells, grew poorly in planta, and did not cause disease symptoms. PTI-induced transgenic Arabidopsis expressing the T3E HopAO1 or HopF2 allowed higher amounts of the T3E-CyaA reporter to be injected into plant cells compared to wild-type plants. Our results show that PTI-induced HR inhibition is due to direct or indirect restriction of T3E injection and that T3Es can relieve this restriction by suppressing PTI.