Plants and microbial pathogens co-evolve their mechanisms of detection and evasion, respectively. Plants detect the presence of conserved Pathogen Associated Molecular Patterns (PAMPs) via cell surface localised pattern-recognition receptors and activate PAMP-triggered immunity (PTI). Pathogens, however, have evolved mechanisms to suppress PTI by delivering ‘effector’ proteins into the host cell. Effectors attenuate plant immunity by interfering with the function of their host target proteins. In turn, plants evolved disease resistance (R) proteins that can recognize corresponding effectors (thereafter termed avirulent effector) and activate a strong defence system known as effector-triggered immunity (ETI), which includes rapid transcriptional reprogramming and programmed cell death in the infected cells.
We are interested in understanding the mechanisms of plant innate immunity and microbial pathogenesis. In particular, we focus on the following topics.
1. How do RRS1-RPS4 paired R proteins recognize pathogen effectors and activate immunity?
The Resistance to Ralstonia solanacearum 1 (RRS1) and Resistance to Pseudomonas syringae 4 (RPS4) function together to recognize at least 3 unrelated pathogen effectors, PopP2, an acetyl-transferase from Ralstonia solanacearum, AvrRps4, an anti-parallel coil-coiled protein from Pseudomonas syringae pv. pisi and an unknown effector from a fungal pathogen Colletotrichum higginsianum. RRS1 and RPS4 belong to the TIR (Toll-Interleukin 1 like receptor) class of Nucleotide-Binding Leucine-Rich Repeat (NB-LRR) R proteins and, in collaboration with Jonathan Jones’ (The Sainsbury Lab, UK), Bostjan Kobe’s (Univ. of Queensland, Australia) and Peter Dodds’s (CSIRO, Australia) groups, we previously identified the crucial residues for heterodimerization of TIR domains of RRS1 and RPS4 in planta(Williams et al., Science, 2014). AvrRps4 and PopP2 bind to the RRS1 C-terminal domain which contains a WRKY transcription factor domain. PopP2-mediated acetylation of lysine residues in RRS1-WRKY domain leads to dissociation of RRS1 from W-box (binding DNA sequences of WRKY transcription factors) and activates defence (Sarris et al., Cell, 2015).
In order to understand the detailed mechanisms by which RRS1-RPS4 receptor complex activates immunity, we use the suppressors of sensitive to low humidity 1 (slh1) mutants that shows a temperature-dependent auto-immunity due to a mutation in RRS1 WRKY DNA-binding domain (Noutoushi et al., Plant Journal 2005). In collaboration with Jonathan Jones’ group, we discovered that the vast majority of suppressor of slh1-mediated immunity (sushi) mutants carry mutations in RPS4. This indicates that RPS4 acts cooperatively with RRS1 to activate defence (Sohn et al., PLoS Genetics, 2014). Currently we are characterizing the loss of function mutations found in RRS1 coding sequence from the sushi screen to gain further information about RRS1-mediated auto-immune responses.
2. What is the genetic basis for the recognition of Pseudomonas syringaeeffector HopZ5?
Pseudomonas syringae pv. actinidiaeV-13, a vigorous pathogen of kiwifruit, secretes a type III effector HopZ5 that triggers an accession-specific immunity in the non-host plant Arabidopsis thaliana. HopZ5 is a member of the YopJ (a type III secretion-dependent effector from Yersinia) superfamily of putative acetyltransferases that target a large number of plant and animal proteins to enhance bacterial virulence.
In order to understand the genetic and biochemical bases of HopZ5-triggered immunity, we focus on the identification of corresponding immune receptor that recognizes HopZ5 in Arabidopsis thaliana.
3. How does Malus MR5 recognize Erwinia amylovora avirulence effector AvrRpt2?
Erwinia amylovora is an important pathogen of apple and pear plants. It is the causal agent of fire blight which causes serious crop loss worldwide. Recently, it was reported that a CC-NB-LRR class plant immune receptor, MR5, confers disease resistance to E. amylovora strains carrying a type III secretion-dependent avirulence effector avrRpt2 (Broggini et al., Plant Biotachnol J 2014). AvrRpt2 is also present in a well-characterized bacterial pathogen Pseudomonas syringae pathovartomato. Pseudomonas syringaeAvrRpt2, a cysteine protease, eliminates its host target protein, RIN4, and activates RPS2-dependent immunity in Arabidopsis thaliana.
Our aim is to better understand the molecular basis of AvrRpt2 recognition by MR5 and hope to contribute in developing new apple varieties with enhanced disease resistance to fire blight.