Understanding the Parastagonospora nodorum–wheat interaction; is it as simple as we think?

S Breen1, S Williams1,2, M Outram2, B Kobe1 and PS Solomon1

  1. Plant Sciences Division, Research School of Biology, The Australian National University
  2. School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience and Australian Infectious Diseases Research Centre, University of Queensland

As part of a larger project in the Solomon laboratory to elucidate the mechanisms of the necrotrophic effectors from Parastagonospora nodorum, a search for potential host protein binding partners for the P. nodorum Tox3 effector was conducted. From this work, the host target of Tox3, TaPR-1-1 was identified through a Yeast Two Hybrid approach. Importantly, the interaction of Tox3 and TaPR-1-1 was confirmed in planta using Co-IP. Recently, it was demonstrated that the P. nodorum effector, ToxA interacted with another TaPR-1 protein closely related to TaPR-1-1, TaPR-1-5 (Lu et al., 2014). Our analysis confirmed that Tox3 was also able to bind to TaPR-1-5 leading to the hypothesis that Tox3 targeted the basic group of PR-1 proteins. We then assessed if Tox3 could interact with other classes of TaPR-1 proteins (basic with C-terminal extention (CTE) and acidic). These analyses showed that Tox3 can interact with basic and acidic proteins but not with basic proteins with a CTE. Subsequent site-directed mutagenesis experiments have identified critical amino acids involved in this interaction for both Tox3 and the TaPR-1 proteins allowing us to produce an interaction between SnTox3 and basic proteins with a CTE. A recent study by Chen et al, 2014 showed that a peptide derived from the C-terminus of the Tomato PR-1b regulates immune signalling. This peptide is conserved in the TaPR1 proteins however it does not appear to be involved in the binding of SnTox3. These findings have raised the question as to the role of the PR-1 proteins during infection and the consequence of it binding to the Tox3 effector. Is the binding of Tox3 to these PR-1 proteins required for necrosis, or is it a mechanism to dampen host defence? Does the CAPE1 peptide play a role in increasing or decreasing pathogenicity of this organism?