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Discovery, Development and Delivery in Plant Pathology: 7th Conference of the European Foundation for Plant Pathology & British Society for Plant Pathology Presidential Meeting
5-10 September 2004, Aberdeen.
Plant Pathology research at UWE covers both fundamental biology (eg proteomic analysis of downy mildew of peas and metabolic profiling of bacterial diseases of potatoes) as well as more applied aspects (such as development of biosensors and ‘electronic noses’ for early detection of infection in the field). Thus a meeting that drew together discovery, development and delivery was particularly welcome. Some aspects were of greater relevance to our specific interests, and so we focus in this report on biotrophic infections, pathogen detection and potato pathology.
Presentations in the ‘discovery’ session started with the Hyaloperonospora parasitica/Arabidopsis downy mildew pathosystem. Jim Beynon showed how analysis of pathogen genes can provide fundamental information on mechanisms of pathogenicity and host defence. He focussed on ‘pathogenicity effector molecules’, which include proteins likely to suppress defence mechanisms, alter host metabolism and modulate gene transcription. Three approaches were described: map-based cloning to identify avirulence genes; suppressive subtractive hybridisation to detect genes encoding proteins likely to be exported from pathogen cells and thus be candidate pathogenicity effectors; genomics, where recently released Phytophthora sequences provide exciting opportunities for comparative genomics of oomycetes. These libraries will also help those of us using a proteomics approach to identify proteins that are differentially regulated during infection. An example of the value of functional genomics was given by Sophien Kamoun for Phytophthora infestans. Data mining of sequence databases identified 18 extracellular protease inhibitor genes that appear to be deployed by the pathogen to suppress host defence responses. This provided an intriguing insight into the complexity of even just this single aspect of host-pathogen interaction, with a cascade of inhibitors matching a cascade of host proteases.
Proteomics has been applied by the Aberdeen Oomycete Group to identify proteins produced by appressoria and other pre-invasion structures formed by P. infestans. Proteins key to successful infection include those involved in amino acid and cellulose biosynthesis. The specific aim of a proteomic analysis of downy mildew infected peas, described in our first presentation, was to identify biomarkers that could be used in biosensor devices for pre-symptomatic diagnosis of infection in the field. This demonstrated a role for proteomics beyond the gleaning of fundamental information about host-pathogen interactions, bridging the gap between discovery and delivery.
cDNA-AFLP isolation of gene fragments from compatible interactions in leaf rust of wheat, caused by Puccinia triticina, was shown by Matt Dickinson to have enabled a variety of techniques to identify host and pathogen genes expressed at specific times during the infection process. The largest group of fungal genes had a metabolic function, whilst the plant genes typically had stress functions.
Of the fungal genes, about 30% had no significant homology in databases. A similar situation was reported by David Collinge for barley powdery mildew, where a differential display mRNA approach had identified some 36 genes up-regulated in infected epidermal cells. Whilst 5 and 9 transcripts were identified as plant and fungal respectively, a further 22 were unidentified. This is a common problem in research at the gene and protein levels, as these ‘unknowns’ may include the key effectors of compatibility and resistance. The focus for wheat rust is now on further functional analysis and localisation of expression in planta, whilst proteomics will be applied to screen for key proteins in the barley powdery mildew interaction.
The ‘development’ session was launched by Gerry Saddler’s overview of potato brown rot caused by Ralstonia solanacearum. He used two very different case studies to illustrate approaches to control. In Kenya, where brown rot is well established and now present in 71% of fields causing a 50% yield loss, biocontrol using avirulent mutants will be incorporated into integrated pest management schemes. Brown rot has been present in the UK since 1995, but out of about 10,000 samples of tubers tested, only 5 ware stocks have proved positive for the infection. Eradication of the indigenous alternate host (Solanum dulcamara) from the River Tay has eliminated what appears to have been one single introduction of the pathogen. Thus in the UK, exclusion and eradication are the main control strategies. This requires development of rigorous and reliable methods for detection, for example using an array of molecular diagnostics as reviewed by Rick Mumford. An alternative approach, using electronic sensors to detect finger-print volatile organic compounds for brown rot and ring rot within bulk quantities of imported potato tubers, was explored in our second contribution.
Serious concerns about fungicide resistance, not least to the relatively new strobilurins, means that reliance on fungicides for disease control is not sustainable. Although this message from Simon Oxley was not new, the stark reality is that often there is not much else in place to help control pathogenic fungi. The need for a shift away from fungicides and towards greater emphasis on host resistance was emphasised again by Peter Gladders in the ‘delivery’ session. With cultivars of crops generally selected for agronomic (yield) reasons instead of resistance to pathogens (explored by Didier Andrivon for potatoes and P. infestans in France), it was refreshing to learn of the coordinated, pre-emptive breeding programmes for resistance to cereal rust in Australia. Robert Park described how these have been used to incorporate resistance to rusts not yet present there, as well as for targeting endemic diseases by anticipating changes in the pathogen population. He estimated that this approach had saved the Australian grain industry some AUS$100 million. By comparison, it cost AUS$40 million in chemicals to control wheat stripe rust in 2003 following its appearance a year before, whilst US$184 million over 8-10 years is needed to develop just one new fungicide (Karl Kuck, Bayer CropScience).
In many parts of the world, safe and effective fungicides are beyond the reach of those farmers who may live on little more that one US$ a day. The problems of ‘delivery’ (ie converting R&D into practical messages) in resource-poor developing countries were addressed by Rebecca Nelson.
Central issues were the choice of communication strategy, for example through setting up ‘farmer field schools’ and the use of mass media, as well as awareness of the social and cultural context within which any new pest management approaches should be implemented. The latter was illustrated by Solveig Danielson in relation to seed quality as a prerequisite for successful crop production of potato in Uganda and rice in Bangladesh. Eric Boa introduced ‘ethnopathology’, the study of local farmers’ knowledge about the diseases affecting their crops. This has very practical applications for those providing advice on plant health in developing countries. For example, aphids on peach, peach leaf curl and smut on maize are all known by the same name in Bolivia, but clearly need very different control strategies.
Aberdeen had greeted us with the sea and sky matching its grey granite buildings, perhaps reflecting the serious nature of the challenge facing plant pathologists as their work contributes to food production. But the sun then shone from a clear blue sky for the rest of the conference, providing an appropriate back-drop for a meeting that broadened horizons and illuminated the many facets of our discipline. We thank the BSPP for providing travel grants, and the organisers of both the scientific and social programmes for their excellent planning and delivery. Finally, a special thanks to Steve Woodward and Janice Brodie for the post-meeting foray to a pine forest and whisky distillery. . . some of us never tire in our pursuit of fungi and their products!
Richard Amey & Peter Spencer-Phillips University of the West of England, Bristol
