Abstract
Activation of plant immunity relies on recognition of pathogen effectors by several classes of plant resistance proteins. To discover the underlying molecular mechanisms of effector recognition by the Arabidopsis thaliana RECOGNITION OF PERONOSPORA PARASITICA1 (RPP1) resistance protein, we adopted an Agrobacterium tumefaciens-mediated transient protein expression system in tobacco (Nicotiana tabacum), which allowed us to perform coimmunoprecipitation experiments and mutational analyses. Herein, we demonstrate that RPP1 associates with its cognate effector ARABIDOPSIS THALIANA RECOGNIZED1 (ATR1) in a recognition-specific manner and that this association is a prerequisite step in the induction of the hypersensitive cell death response of host tissue. The leucine-rich repeat (LRR) domain of RPP1 mediates the interaction with ATR1, while the Toll/Interleukin1 Receptor (TIR) domain facilitates the induction of the hypersensitive cell death response. Additionally, we demonstrate that mutations in the TIR and nucleotide binding site domains, which exhibit loss of function for the induction of the hypersensitive response, are still able to associate with the effector in planta. Thus, our data suggest molecular epistasis between signaling activity of the TIR domain and the recognition function of the LRR and allow us to propose a model for ATR1 recognition by RPP1.
Highlights
Plants have evolved a multilevel innate immune system to protect them against infection by a diverse range of pathogens, including viruses, bacteria, fungi, oomycetes, and nematodes
The ARABIDOPSIS THALIANA RECOGNIZED1 (ATR1) sequences obtained from different H. arabidopsidis strains contained many polymorphisms and were named after the strain they were isolated from; that is, ATR1-Emoy2 comes from H. arabidopsidis strain Emoy2
Our genetic and molecular analyses of the oomycete effector protein ATR1 and its cognate resistance protein RECOGNITION OF PERONOSPORA PARASITICA1 (RPP1) provide a mechanistic insight into the perception of an oomycete pathogen inside the plant cell
Summary
Plants have evolved a multilevel innate immune system to protect them against infection by a diverse range of pathogens, including viruses, bacteria, fungi, oomycetes, and nematodes. Upon association with PAMPs, the pattern-recognition receptors activate a downstream mitogen-activated protein kinase signaling cascade that culminates in transcriptional activation and generation of the innate immune responses (Chisholm et al, 2006; Jones and Dangl, 2006). This line of defense, called PAMPtriggered immunity, is commonly suppressed by a successful pathogen in order to establish infection. To interfere with PAMPtriggered immunity, pathogens from different kingdoms of life
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