To overcome host innate immunity, Phytophthora infestans secretes numerous proteins to establish disease on potato (effectors). Some of these effectors are secreted into intercellular spaces while others are translocated inside host cells.

P. infestans research at SCRI aims to:

• determine which effector proteins are essential for pathogenicity
• identify peptide motifs responsible for translocation inside host cells
• understand the mechanisms of effector translocation
• determine effector localisation and function inside host cells
• identify triggers (avirulence proteins) of host resistance through genetics, genomics and high-throughput plant assays.

We have shown that a dual peptide motif RxLR-EER is required for effector protein translocation inside host cells (Armstrong et al. 2005; Rehmany et al. 2005; Birch et al. 2006; Bos et al. 2006; Whisson et al. 2007). Cell biology was used to demonstrate that the RXLR-EER effector, AVR3a, is secreted from haustoria and translocated from the extrahaustorial matrix to the inside of plant cells. More than 400 genes encode potential secreted RxLR-EER effectors and all those tested have been shown to be up-regulated during infection (Whisson et al. 2007). The availability of the P. infestans genome sequence has recently allowed us to identify the AVR2 gene triggering resistance in R2-containing plants.

RNAi is being used by Anna Avrova and Steve Whisson to investigate RXLR-EER effector function during infection and so far the expression of more than 20 has been silenced, resulting in a range of altered infection phenotypes.

Miles Armstrong, Eleanor Gilroy, Petra Boevink and Paul Birch are investigating the host targets of RXLR-EER proteins that are presumably manipulated during infection. A number of host proteins have been shown to interact with AVR3a, including the ubiquitin E3 ligase CMPG1, and experiments are under way to visualise RXLR-EER protein-target interactions in vivo and to investigate the biochemical events following such interactions.

Ingo Hein and Eleanor Gilroy are using cloned RXLR-EER-encoding P. infestans genes to screen wild potato germplasm (in the Commonwealth Potato Collection) for new sources of late blight resistance. They are using potato microarrays to investigate differential gene expression in compatible (susceptible) and incompatible (resistant) interactions with P. infestans.

Anna Avrova, Paul Birch, Petra Boevink and Steve Whisson are using a range of transcriptional profiling techniques and RNAi to investigate genes that are up-regulated at various developmental stages in the P. infestans infection cycle. In particular, a focus is on identifying genes responsible for early stages of the interaction with the host, particularly during appressorium development.

Image: Phytophthoras infestans transformants expressing AVR3a fused to monomeric red fluorescent protein are seen to secrete AVR3a from biotrophic structures called haustoria.

References

Whisson, S.C., Boevink, P.C., Moleleki, L., Avrova, A.O., Morales, J., Gilroy, E.M., Armstrong, M.R., Grouffaud, S., van West, P., Chapman, S., Hein, I., Toth, I.K., Pritchard, L., Birch, P.R.J. 2007. A translocation signal for delivery of oomycete effector proteins inside host plant cells. Nature 450, 115-118.

Bos, J.I., Kanneganti, T., Young, C., Cakir, C., Armstrong, M.R., Birch, P.R.J., Kamoun, S. 2006. The C-terminal half of Phytophthora infestans RXLR effector AVR3a is sufficient for triggering R3a-mediated hypersensitivity and suppressing INF1-induced cell death. Plant Journal 48, 165-176.

Birch, P.R.J., Rehmany, A.P., Pritchard, L., Kamoun, S., Beynon, J.L. 2006. Trafficking Arms: Oomycete Effectors Enter Host Plant Cells. Trends in Microbiology 14, 8-11.

Rehmany, A.P., Gordon, A., Rose, L.E., Allen, R.L., Armstrong, M.R., Whisson, S.C., Kamoun, S., Birch, P.R.J., Beynon, J.L. 2005. Differential recognition of highly divergent downy mildew avirulence gene alleles by RPP1 genes from two Arabidopsis lines. The Plant Cell 17, 1839-1850.

Armstrong, M.R., Whisson, S.C., Pritchard, L., Bos, J.I.B., Venter, E., Avrova, A.O., Rehmany, A.P., Böhme, U., Brooks, K., Cherevach, I., Hamlin, N., White, B., Fraser, A., Lord, A., Quail, M., Churcher, C., Hall, N., Berriman, M., Huang, S., Kamoun, S., Beynon, J.L., Birch, P.R.J. 2005. An Ancestral Oomycete Locus Contains Late Blight Avirulence Gene Avr3a, Encoding a Protein that is Recognised in the Host Cytoplasm. Proceedings of the National Academy of Sciences USA 102, 7766-7771.