Bacterial effectors target BAK1-associated receptor complexes

Abstract

The long-standing association between hosts and microbes has generated some of most intricate relationships. The studies on molecular mechanisms of host-microbe interaction have been revealing many fascinating stories. Here we zoom in on a specific topic on the interplay between bacterial effectors and plant innate immune signaling. In particular, we will summarize our recent discovery that bacterial effector proteins, AvrPto and AvrPtoB, target plant immune signaling receptor complexes to interfere with host immune responses and development.

Figures and Tables

Figure 1 Bacterial effectors suppress PAMP immunity by targeting receptor signaling complexes. (A) In the absence of infection, PAMP receptor FLS2 remains at the resting stage and there is no apparent receptor complex formation. (B) Once challenged by microbes, FLS2 detects the invading microbes by binding to the ligand-bacterial flagellin, which allows FLS2 conformational change and heterodimerization with BAK1 to form a tight receptor complex. The functional PAMP receptor complex initiates downstream signaling, including activation of MAPK cascade, transcription factors and immune genes, thereby leading to plant resistance to microbes. (C) To be pathogenic, some successful pathogens deployed virulence weapons to tear down host immune system. For instance, type III effectors AvrPto and AvrPtoB, secreted from Pst DC3000 and delivered into host cells, target BAK1-associated PAMP receptor complexes and disrupt the complex formation, thereby impeding the immune signaling and leading to disease.

Figure 2 Multiple functions of AvrPto and AvrPtoB as virulence and avirulence factors. BAK1 is a signaling partner of BR receptor BRI 1, multiple PAMP receptors PRRs, and unknown receptor for cell death. By targeting BAK1, AvrPto and AvrPtoB efficiently suppress multiple signaling in plant immunity, development and cell death control, one stone killing two birds. Apparently, AvrPto also targets BAK1-independent PAMP signaling. In the presence of Pto and Prf, AvrPto triggers potent effector-mediated immunity in tomato.