Plant and animal cells have an assortment of receptors employed for pathogen surveillance. The Nucleotide-binding domain and Leucine-rich Repeat-containing (NLR) family of proteins are important receptors involved in cell autonomous surveillance. In animals, upon perception of virulence molecules or conserved molecular patterns from pathogens several NLRs initiate immune signalling by the induced proximity of signalling domains. In order to do this, these NLRs form oligomeric wheel-shaped complexes, called inflammasomes, that bring the N-terminal signalling domains of the component NLRs into close proximity and initiates downstream signalling. In plants, much less is known about signal initiation and transduction during NLR-mediated immunity. Plant NLRs recognise virulence factors that are often race specific, called effectors. I used the model NLR pair RRS1/RPS4 to investigate effector recognition by NLRs.
RRS1 and RPS4 have the archetypal TIR-NB-LRR architecture of a large subset of plant NLRs, but also contain non-canonical domains. Of particular note, RRS1 contains a C-terminal WRKY DNA-binding domain. RRS1 and RPS4 act in concert to recognise at least three effectors: AvrRps4 secreted by Pseudomonas syringae; PopP2, an acetyltransferase secreted by Ralstonia sotanacearum; and an unidentified effector from Cottetotrichum
higginsianum.
In this thesis, I present evidence that supports a new conceptual framework for effector recognition. In this model, the virulence target of PopP2 and AvrRps4 has fused to an NLR (in this case the WRKY domain in RRS1) and acts as a decoy, baiting the effector to attack the NLR and trigger immunity, instead of enhancing susceptibility by interfering with the function of its intended target. This mechanism of effector perception may be widespread in plants and I discuss other examples of NLRs with integrated atypical domains.
NLRs exert intramolecular inhibition of immune signalling in the absence of effectors in order to prevent autoimmunity. The domains of RRS1 C-terminal to the WRKY exert negative regulation of activation. Upon establishing the method of effector recognition by RRS1 and RPS4, I leverage this knowledge of the mechanism of RRS1 autoinhibition to engineer recognition of viral proteases.
In the final chapter of this thesis engineer a mammalian inflammasomeforming NLR system, fused to the signalling domain of RPS4, into plants to 1) introduce into plants an intracellular receptor that recognises conserved pathogen associated molecular patterns and 2) test if inducing the proximity of plant NLR signalling domains is sufficient for activation of NLR-mediated immunity
