BSPP News Autumn 1998 - Online EditionThe Newsletter of the British Society for Plant Pathology
Number 33, Autumn 1998
Review of ICPP98
The 7th International Congress of Plant Pathology in Edinburgh, from 9th to 14th August, was a truly impressive event, with around 2000 lectures and posters covering almost all aspects of plant pathology. The following are short reviews of particular aspects of the Congress, for the benefit of those who weren't able to attend, and for those who were torn between two or even three parallel sessions. I thank the authors for writing these articles at very short notice, in just the week after the conference.
Jon Green, University of Birmingham, U.K.
The last time I was in Edinburgh for a conference we were almost blown off Arthur's seat so it was a pleasant change to have a week with decent weather! The new conference centre for the meeting was excellent, in particular the large lecture theatre - although a bigger screen would have been helpful.
The highlights of the meeting for our group were the Wednesday sessions on `Cell Biology of Plant-Pathogen Interactions' and `Molecular Biology of Fungal Pathogenicity'. In the first session, Michelle Heath (Toronto, Canada) gave an excellent overview describing the different stages of plant-pathogen interactions lead-ing to the hypersensitive response and programmed cell death. This was followed by a series of talks which showed the power of combining molecular approaches with cell biology and biochemistry.
In the pathogenicity session it was interesting to compare studies on the biotrophic fungi with more tractable systems. Nick Talbot (Exeter, U.K.) reviewed the large amount of information accumulating on pathogenicity of Magnaporthe grisea, focussing on his own work with the hydrophobin MPG1. John Manners (Brisbane, Australia) described a novel pathogenicity gene of Colletotrichum gloeosporioides, disruption of which caused a hypersensitive response in the host.
There was also progress reported in work on biotrophic fungi. Sarah Gurr (Oxford, U.K.) described studies on triggers and signals involved in spore germination inr Erysiphe graminis. Christine Struck (Konstanz, Germany) described her work on genes expres-sed in haustoria of the rust fungus Uromyces fabae. Several transporters have been identified after complement-ation of yeast mutants.
In spite of sickness, a broken bone and getting lost inumerable times, the Birmingham group had a good week culminating in a haggis feast on the last night.
Integrated approaches to resistance gene function
Mohammed Benghezal, Australian National University, Canberra
Developments in the molecular genetics of resistance were reviewed in sessions organised by Pierre de Wit (Wageningen, The Netherlands) and Richard Michelmore (Davis, U.S.A.). Despite the isolation of an increasing number of resistance genes (R genes), their mode of action still remains unclear. In parallel, several pathogen avirulence genes (AVR genes) have also been cloned, confirming Flor's gene-for-gene relationship. In other words, the specific interaction between R and AVR genes leads to the activation of plant defences and resistance.
A simple model would be a direct binding of the AVR gene's product to the R gene's product, in a ligand-receptor manner. But only one example of direct R-AVR interaction of this kind has been reported so far. Thus the dogma of a resistance gene product being the direct target for the elicitor molecule does not yet have strong support. Finally the activation of plant defences upon triggering by the elicitor still remains unclear, partly because of its complexity, but mainly because of the enormous lack of biochemical data.
To gain insight in the mechanism of action of R genes, a biochemical characterisation of the key players at plant-pathogen's interface is now necessary: subcellular localisation of R and AVR gene products, together with purification of the encoded proteins for structural analysis and biochemical investigation. The task is challenging because of the weak expression of the R genes in plant and the difficulty of expressing these genes in heterologous expression systems. Hence an integrated approach, based on biochemistry, cell biology, genetics and molecular biology is urgently needed. This will lead not only to a better understanding of the plant resistance but also to design new and more subtle strategies to engineer broad spectrum resistant crops of commercial importance.
Diversity of plants and their defences
Robert Grant-Downton, University of Oxford, U.K.
A prominent yet enigmatic theme of ICPP 98 was diversity, initiated by the lecture `Plant Disease and Biodiversity' given by the Conference President, Professor David Ingram (Edinburgh, U.K.). The erosion of biodiversity touches plant pathology in many ways, most significantly through the loss of wild relatives of crop plants. It is depressing that progress in breeding strategies and genetic manipulation coincides with the imperilment of valuable and irreplaceable genetic resources.
Perhaps there is a parallel between 19th century explorers who grappled with organismic diversity and today's molecular biologists. The jungle of signalling pathways in plant-pathogen interactions is slowly being explored. Salicylic acid mediated disease resistance is becoming more familiar terrain; there is strong evidence for SA-binding proteins and a MAP-kinase relay which elegantly links boosted SA levels to defence gene expression.
Nevertheless, previously unrecognised pathways involved in defence are emerging such as the mysterious SA-independent and microscopic-wound-induced pathways leading to enhanced resistance. Is a diversity of signalling pathways to be expected, given that a number of host-pathogen pairs are under study? Studies of resistance at the cellular level hinted this may be the case, as subtle differences in the hypersensitive response have been observed in different systems. This underlines a potential dilemma in the field of molecular plant pathology: is it more valuable to invest in a few model systems to elucidate great molecular detail or to gain more practical information by studying, with less resolution, the many economically important host-pathogen interactions?
A balance would seem most sensible, based on R gene transgenics. In some cases, R genes function well in the genetic and biochemical environment of a distantly related species, yet in other cases the introduced R gene causes developmental aberrations. Only when a greater diversity of R genes and their evolutionary origins have been explored will transgenic crops with broad-spectrum resistance become commercial possibilities.
Understanding and engineering resistance
Penny Brading, John Innes Centre, Norwich, U.K.
A session on "Genetically-engineered Resistance: Modified Natural Resistance", led by Kim Hammond-Kosack (Monsanto, Belgium,) raised the question of whether genetically-engineered resistance genes can provide effective and durable future solutions to plant disease in agriculture. Two different strategies for triggering resistance in transgenic plants were described in tomato. Kim demonstrated how a transposon-tagged resistance (R) gene (Cf-9) in the same transgenic plant as its corresponding avirulence gene (Avr9) can lead to induction of defence responses if R gene function is occasionally restored by transposon excision.
Using the same resistance/avirulence gene interaction, Martin Stuiver (Zeneca MOGEN, The Netherlands) then described another technique to activate genetically-engineered resistance, in which the transformed avirulence gene was under control of a pathogen-inducible promoter and only expressed at the site of attempted infection. In both cases, moderate protection against several fungal and viral biotrophs was achieved.
Evidence was presented that the signaling events triggered by the Cf-9 gene in response to the fungal AVR9 peptide can also effectively stop a viral pathogen. This finding indicates that the mechanisms by which plants resist different classes of pathogen are overlapping.
Again, using Cf-9 as an example, it was suggested that R gene pyramiding may improve resistance durability because some of the R gene homologues at the Cf-9 resistance locus are functional and resistance to Cladosporium fulvum in Cf-9-containing tomato cultivars is unusually durable.
Caius Rommens (Monsanto, U.S.A.) outlined several strategies for engineering broad spectrum disease resistance. Use of sequence homology to clone R genes from resistant sources and transfer them to crops of interest is one approach. Another strategy involves over-expression of pathogen-inducible genes which encode antimicrobial products. Over-expression of defence-related signaling molecules is a third strategy. To identify novel signaling proteins, database technologies are being used to identify genes that are induced or suppressed during resistance responses.
An example of over-expression of a resistance signaling protein was described by Xin Li (North Carolina, U.S.A.). Overexpression of NPR1, a key defence pathway signaling protein in Arabidopsis, resulted in enhanced resistance to Pseudomonas syringae and Peronospora parasitica.
A paper by W. Broekaert (Leuven, Belgium) then described studies with Arabidopsis mutants which indicated that different pathogens activate one of two independent resistance pathways. Resistance mechanisms against some pathogens require salicylic acid (SA) and induce specific pathogenesis related genes. However an SA-independent pathway involving jasmonic acid (JA) and ethylene which induces a distinct set of jasmonic acid-dependent antimicrobial protein genes has now been shown to be activated by several other pathogens. Understanding the interplay of these different pathways may lead to new strategies for engineering resistance.
Evolution of resistance genes
Jeremy Burdon, CSIRO, Canberra, Australia
For me, one particular highlight of ICPP98 was Richard Michelmore's keynote address entitled, `Genomic organization of resistance genes and exploiting germplasm resources' that was part of Symposium 3.4, `Conventional and Enhanced Breeding Strategies for Resistance to Pathogens'.
One of the intriguing conundrums of hostpathogen coevolutionary interactions has been the apparently overriding and constant advantage pathogens have in any "arms race" by virtue of their high reproductive potential and the generation of new virulence by deletions and other loss of function mutations. In contrast, because the appearance of new resistance specificities has been seen as requiring a gain of function, its occurrence has been predicted to occur far more infrequently.
In the past few years a degree of balance has been restored to this arms race through the outcome of elegant work on the fine structure of resistance genes which has indicated that new specificities may be generated by intragenic recombination. Michelmore's presentation took a hard look at this idea and suggested that the rate of unequal crossingover at resistance loci is too low to explain the rapid generation of new specificities that appears to be needed to explain the potential hundreds of resistance genes to be found in all plant species. Genome rearrangements may be responsible for generating some new specificities. However, he argued that the primary evolutionary force is divergent evolution on orthologues with a birth and death process responsible for changes in copy number.
The implications of these and other ideas concerning the generation of new specificities are profound. In the longterm, a clear understanding of the mechanisms and frequency whereby new resistances are generated will have significant implications for our understanding of the evolution of geneforgene systems, the dynamics of natural associations, and even the perennial argument about whether fitness costs associated with "unnecessary" resistance and virulence exist.
New technologies and applications for DNA diagnostics
Emily Taylor, National Institute for Agricultural Botany, Cambridge, U.K.
An evening session entitled `Pathogen detection by real time fluorescence PCR' concentrated on the use of Perkin Elmer's TaqManTM sequence detection system.
Juliane Henderson (Brisbane, Australia) presented a case study of setting up a TaqMan assay for Clavibacter xyli subsp. xyli, a bacterial pathogen of sugar cane. The assay could detect one bacterial cell, and this sensitivity was useful in highlighting a contamination problem with sample collection that was not detected with the conventional PCR-ELISA test. This indicates the need for careful experimental design for PCR to include controls to monitor contamination as well as procedures to minimise false positives.
Peter Bonants (Wageningen, The Netherlands) described the various types of probes which could be used for real time fluorescence PCR. This included use of an isothermal NASBA assay which amplified RNA from the potato virus, PLRV, and was then combined with fluorescent detection by a `molecular beacon'. This is a sequence-specific oligonucleotide which undergoes a fluorogenic conformational change upon hybridisation to its target which results in an increase of fluorescence. He also mentioned the use of fluorescent hybridisation probes, SunriseTM primers and future technologies which include DNA chips and oligo arrays.
An alternative to the Perkin Elmer TaqMan system for real time fluorescent detection is the Lightcycler from Idaho Technologies. The Lightcycler is capable of performing a PCR assay in fifteen minutes with real time detection.
Felix Jaffe (NIAB, U.K.) presented a poster on the PCR detection and quantitation of Erwinia carotovora subsp. atroseptica using this equipment. The real time monitoring enables one to observe the kinetics of the PCR reaction and use this data for quantitation purposes.
I am sure that this new generation of PCR and probe detection technology will have great impact as molecular tools for advancing diagnostics, if the initial capital costs do not prove to be prohibitive.
Population structure of Phytophthora species
Lee Baines, University of Wales, Bangor
André Drenth (Brisbane, Australia) described the recent history of the homothallic Phytophthora sojae in Australia. P. sojae was first introduced into Australia in 1979 as a single race, but rapidly diversified, overcoming resistance genes in local cultivars. Using low-copy RFLP markers, Drenth was able to show that levels of genotypic diversity were low, and that the population of P. sojae was dominated by a single clonal lineage.
Drenth also referred to Phytophthora cinnamomi, which in contrast to P. sojae, is heterothallic. P. cinnamomi has been present in Australia for over a century. Drenth, again with low-copy RFLP markers has shown that the genotypic diversity in Australia is low.
Greg Forbes (CIP, Ecuador) presented a review of the current literature on the regional differences in populations of Phytophthora infestans. In North America and Europe, sexual recombination is found in several regions, whereas in many developing nations the populations are dominated by one or two clonal lineages. Host specificity was also highlighted; in North America and Europe some genotypes are found on both tomato and potato, but genotypes in developing countries seem to be highly host specific.
Bill Fry (Cornell University, U.S.A.) was able to shed further light on populations of P. infestans New genotypes of P. infestans have been introduced in Europe and are replacing the previously dominant clonal line (US-1). New strains have also reached South America, Southeast Asia and North America. South American populations are still mainly clonal, while Japan and Korea are dominated by a few newly introduced genotypes. New genotypes of P. infestans appear to be absent in Australia and South Africa. The new strains in the USA were introduced in the 1980s and seem to be much more aggressive than the previous US-1 clone. Fry suggests that this higher level of fitness may be due to the gradual accumulation of detrimental mutations in the previously dominant asexual lineages.
"The best conference of my life"
Bruce Fitt, IACR Rothamsted, U.K.
A splendid view of Edinburgh castle from the bus greeted me as I arrived for ICPP98 on 7th August. The impressive architecture and festive atmosphere made Edinburgh an excellent venue for the congress. The rain soon stopped to provide a fine week for the delegates ("the best week of the summer" according to the bus driver as I left the city on 14th August). I made my way to Pollock Halls, set against a spectacular backdrop of Arthur's seat, where the facilities were excellent and the ample Scottish breakfasts provided the energy needed for tackling the day's scientific programmes.
On the Saturday, I attended the pre-congress workshop on blackleg (stem canker) of oilseed rape. We had some lively discussions on topics ranging from the molecular biology of the pathogen to the epidemiology of the disease in different countries (including reasons why it does not usually occur in Scotland!) Participants had to test their practical skills by trying out a new Canadian diagnostic kit for the pathogen.
After the Royal opening ceremony on Monday, I concentrated on the epidemiology programme at the congress. Highlights included discussions on the role and measurement of leaf wetness in pathogen infection, the effects of leaf diseases on host physiology in relation to retention of green leaf area, the use of models of different types in plant disease epidemiology, including studies on pathogen dispersal/growth above/below ground, and the development of decision support systems which are appropriate for growers in developed or developing countries. I also attended the IT session, where we discovered what lessons can be learnt from the social behaviour of ants. It was difficult to find time to get round all the posters most relevant to my interests, with their wealth of new information.
Outside of the formal sessions, this congress provided me with many opportunities for discussions with colleagues from many different parts of the international community of plant pathologists. I am very grateful to those colleagues who worked so hard to put together an excellent programme and to the BSPP for providing a bursary to contribute to my expenses.
A view from North Africa
Sonia Hamza, Tunisian National Agronomy Institute, Tunis
I am a lecturer in plant pathology at the Institut National Agronomique de Tunisie, where my research interest is in Septoria tritici, a major disease of wheat. I therefore had several objectives in attending ICPP98. First, I wished to update my knowledge of plant pathology. Second, I wanted to meet other Septoria researchers. Third, I was able to present two posters which described my own research.
Many lectures were given on molecular mechanisms of plant-pathogen interactions. Some of them are very interesting and it was useful that the organisers of ICPP98 assembled the abstracts of the papers in a book. The most interesting part of the conference for me was an evening meeting on `Resistance of wheat to Septoria tritici', where I had the opportunity of meeting several researchers who work in the same field.
I thank the organisers for the generous grant which enabled me to attend the conference, and also for the surprising opportunity of having lunch with Princess Anne.
Particular thanks to Sonia Hamza, who found a little time to write this review despite getting married a week after the Congress! Congratulations and best wishes, Sonia, from your pathologist colleagues.
A view from an organiser
Mike Cooke, University College, Dublin, Ireland
I approached ICPP 98 with a degree of apprehension. As a member of the Finance Committee I was aware that a few hiccups were afoot in relation to various matters, particularly the electronic processing system for Abstracts. Within our committee specifically, we had encountered many problems relating to fund raising and sponsorship. But under the expert chairmanship of Professor D. Gareth Jones we had managed to overcome most of these before the start of the Congress. My particular task of raising money for the Bursary Fund for overseas delegates had proven to be particularly rewarding.
However, my fears were unfounded and on arrival at the EICC I was particularly impressed at the lack of long queues for delegate registration. My experiences at past Congresses had conditioned me to a one hour wait for processing at Edinburgh. I was through the system within 10 minutes which I thought was most impressive. The efficiency continued. As far as I am aware, no major problems occurred that were at least obvious to delegates in the organisation of this massive two thousand plus delegate event.
One would have to be impressed by the efficient EICC staff who seemed to be continuously in contact with a central control office by mobile phone. The staff were courteous to delegates and extremely helpful. I suppose one would expect good service for the fees paid, which were considerable for most people.
I shall always remember the Marquee for the poster sessions. A circus-like atmosphere might be putting it a little strongly, but there were moments, like the boards refusing to accept the velcro strip, when comedy came into operation. Towards the end of the meeting the high winds certainly made their presence known under the canvass, but the structure survived and the poster sessions were very successful.
For my own part I was very happy with the international exposure afforded to the three posters myself and my research students presented at the meeting. All 300 reprints on offer were taken which proves something (I'm not sure exactly what!) My only criticism of the poster sessions would be the lack of opportunity for the authors to present their work orally to those who might be interested. It is increasingly the case that in poster sessions in international meetings time is made available for oral presentations at the poster site. I expect the logistics of allowing this at ICPP98, where there were nearly 1500 posters, would be daunting.
I was most interested in the time devoted to sustainable systems at the meeting. I cannot remember an international congress with such a large element on this topic, reflecting, of course, changing attitudes towards this area of agriculture. Martin Wolfe chaired an excellent all-day session on sustainable systems which resulted in much discussion. I also very much enjoyed the public forum on `Global Food Security' organised by Clive James with contributions from the distinguished Norman Borlaug and others.
There were many other symposia which provided stimulating science and one became spoilt for choice. The whole Congress was a mind-blowing experience academically and socially, and it was nice to descend into the exhibition for some relative relaxation when the going got rough. There were some excellent book bargains to be had, particularly the European Handbook of Plant Diseases at £10.00. Some new books were launched at the Congress, notably `The Epidemiology of Plant Diseases', edited by D. Gareth Jones (and priced by Kluwer at £170.00 - the book was later stolen!).
All in all, a most enjoyable experience, culminating socially in the Farewell Ceilidh and dinner - a most spectacular transformation of the exhibition hall was achieved. The chairman of the local Arrangements Committee, Bill Rennie, certainly looked spectacular in his kilt! We now all look forward to ICPP 2003 at Christchurch, New Zealand. The organisers will certainly have a hard act to follow!