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. Pathogens manipulate 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. We have fruitful collaborations with Phytophthora researchers in New Zealand, South Africa and China.

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. A key group of these are the RXLR (+/-EER) effectors (their conserved motif is 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.

Characterising the activities of several RXLR effectors revealed 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. We are also investigating how effectors are delivered to host cells; what is the secretion pathway, the entry route and how effectors reach their final destinations.

One of the first effectors we examined, PiAVR3a, 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 forms a host-pathogen holoenzyme with protein phosphatase type 1 catalytic domain (PP1c), 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.

Initially we showed that RXLR-EER motif is required for translocation of PiAVR3a into the plant cytosol using GUS fusions. Subsequently, using effectors that we found were 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 were conventionally secreted while the RXLR efectors tested were secreted by a different pathway, i.e. non-conventionally secreted. We are investigating how RXLR effectors are targeted to this 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 questions we have are whether specific signals are involved in this endocytosis and how are effectors 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, Michael Porter 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.