Prof Donal O'Sullivan
Prof Donal O'Sullivan
Host - Dr Laurence Bindschedler
Ultrastructural, transcriptomic and genetic analysis of the Claviceps purpurea – wheat interaction
Claviceps purpurea, the ergot fungus, is an unusual pathogen in that it has a very broad natural host range (readily infecting >600 species from the grass family) but a highly sophisticated but physically very delimited infection strategy, with spore germination tubes following the route of the pollen tube through receptive stigma, with each infection event being limited to a single floret position. The macroscopic symptoms – exudation of conidiospores (honeydew) and the formation of a (highly toxic) fruiting body – the sclerotium – in place of a grain can be observed from 7-10 dpi, but all the critical early events that determine whether the pathogen successfully infects or not take place enveloped within the protective surroundings of the lemma, palea and glumes. Before our BBSRC-funded project began, the early host responses in the interaction had never been documented.
With this in mind, we set out to gather some baseline data on the first 7 days of the interaction. 3D-confocal imaging showed hyphal growth in the first 72hpi is highly directional and mainly limited to intercellular growth, that branching growth is triggered at around 72hpi. A timecourse RNA-Seq of dissected ovary tissue subjected to one of three treatments – mock inoculation, inoculation with Claviceps or pollination revealed that although for up to 3dpi, the transcriptome changes associated with infection are very subtle indeed, two major categories of gene function (hormone signalling and defence-related genes) are perturbed, and that specific transcriptional signatures are associated with the entry of Claviceps hyphae successively into the stigma/style, transmitting tissue and base of the ovary.
In a separate stream of work, we conducted QTL mapping of partial suppression of sclerotial development in a mapping population developed from a cross between two contrasting UK winter wheat varieties. The principal QTL effects were colocated with the rht1 and rht2 loci – homoeologous mutant DELLA proteins defective in transduction of gibberellin signalling, which gave another, independent line of evidence supporting the picture that subversion of normal phytohormone signalling processes is essential to the Claviceps purpurea infection strategy.
Current, ongoing work includes further QTL mapping of a clearer resistance phenotype found in a durum wheat accession, transgenic perturbation of GA levels in an ovary-enriched manner and a speculative attempt to target several Claviceps purpurea genes essential for pathogenesis with host-mediated RNAi.
The main research interests that I am pursuing at Reading are:
1. “Systems genetics of wheat yield” whereby we will be conducting very detailed phenotyping of a large multi-parent elite wheat mapping population in the field over the entire course of a crop cycle which will permit exquisitely detailed genetic dissection of yield into yield components. I am currently recruiting a postdoc and 1-2 PhD students to this project, so it will be a major activity.
2. Somewhat allied to this is our work on association genetics in wheat, whereby we have a thoroughly characterised and high density genotyped panel of 500 elite and progenitor UK varieties on which we are conducting extensive disease phenotyping across several EU countries and conducting GWA scans for markers associated with resistance of both the qualitative and quantitative kind.
3. Our work on ergot has highlighted the key undocumented roles of phytohormones in the ovary pre-anthesis, and I will be taking this subject forward, most likely from the perspective of how genetic variation in hormone signalling pathways can modulate yield rather than modulate ergot development.
4. Having developed a genetic toolkit for Vicia faba (faba bean/field bean), we are pursuing the mapping (and sometimes cloning) of genes underlying traits of interest in this important nitrogen-fixing grain legume.