Petra Boevink

The focus of my research is the cell biology of plant-pathogen interactions, in particular between the notorious oomycete pathogen Phytophthora infestans, the causal agent of the devastating potato late blight, and its hosts. This pathogen manipulates plant defence responses on multiple levels, suggesting complex exchanges of signals between host and pathogen. I work closely with Paul Birch (of Dundee University Plant Sciences, based at the James Hutton Institute) and Steve Whisson and other members of the strong Phytophthora team in Dundee and we are part of the Dundee Effector Consortium which encourages collaboration and exchange between teams working on different pathogen/pest-plant interactions. 

P. infestans is a hemi-biotroph and the biotrophic phase of growth is essential for infection. It represents a point of vulnerability and is the stage at which the pathogen must exert the most control over the host defences. To do this the pathogen secretes a broad range of pathogenicity factors, referred to as effectors, many of which are protein. The P. infestans genome encodes hundreds of genes for small, secreted RXLR (+/-EER) effectors (their conserved motif similar to the one involved in protein translocation from the malaria parasite into host cells). The formation of haustoria, finger-like projections that breach the plant cell wall to make contact with the cell membrane, is crucial for the biotrophic phase of infection. We have shown that effectors are secreted at haustoria and that RXLR effectors are translocated into host cells.

We have characterised the activities of several RXLR effectors. These investigations reveal how the pathogen targets a broad range of host pathways that contribute to plant defence, often using multiple effectors to target different elements within the same pathway. 

One fo the first effectors we examined, PiAVR3a, an essential RXLR effector, has 14 candidate host protein interactions. The complexities of the interaction between PiAVR3a and the E3 ligase CMPG1 alone took several years to unravel. The nuclear-targeted effector Pi04314, one of the most highly expressed effectors and one that gives a large boost to P. infestans infection when co-expressed in leaves, binds to isoforms of the protein phosphatase type 1 catalytic domain (PP1c) and forms a host-pathogen holoenzyme, exploiting the phosphatase activity to benefit infection. Similarly the nucleolar-associated effector Pi04089 exploits the activity of a KH type of RNA binding protein (StKRBP1), Pi02860 targets a predicted potato E3 ligase NPH3/RPT2-like 1 (StNRL1) and Pi17316 uses MAP3K StVIK1. Thus StPP1c's the StKRBP1, StNRL1 and StVIK1 are all susceptibility (S) factors. This reveals the potential of S factors as novel targets for development of plant resistance.

PiAVR3a is detected by R3a within the plant cell cytosol and the RXLR-EER motif is required for the recognition. We showed the RXLR-EER motif is required for translocation of PiAVR3a into the plant cytosol using a GUS fusion. Subsequently, using effectors that we showed to be nuclear localised when expressed in plant cells, we managed to observe effector translocation in live cells with fluorescent protein fusions. Inhibitor studies revealed that apolastic effectors and other pathogenicity proteins are conventionally secreted while the translocated RXLR efectors are secreted by a different pathway, i.e. they are non-conventionally secreted. Current work is investigating how RXLR effectors are targeted to the non-conventional route and how that results in them being translocated into host cells. We have shown that uptake of a range of RXLR effectors by plant cells involves clathrin-mediated endocytosis, the next question is, how are they then released from endosomes to reach their host targets in the cytosol or other locations?

The imaging and cell biology facilites and capabilites at The James Hutton Institute are managed by Alison Roberts, Kath Wright and Sean Chapman in addition to their own research activities. High quality imaging work is not possible without great training, ongoing support and facility management.