BSPP News Spring 2001 - Online Edition

The Newsletter of the British Society for Plant Pathology
Number 38, Spring 2001 

Conference and Travel Reports

The 10th International Conference on Plant Pathogenic Bacteria
Charlottetown, Prince Edward Island, Canada:  July 2000

PEI, or The Island, is a bit off the beaten track for most foreigners. From London the 10 hour trip (or for IT 21 hours!) involved two-three flights via a major hub like Halifax, Toronto or Montreal. And that was if you were lucky; some delegates were delayed by up to six hours because overbooking by Air Canada resulted in transfer to later flights. (Also they lost RC's luggage which arrived 24 hours later sodden by Halifax airport rain). Situated at 46o latitude (equivalent to Lyon, France), PEI is famous in Canada for Anne of Green Gables, its seafood (particularly lobster and mussels), potatoes (supplier of French fries to Ronald McDonalds) and the warmest seawater north of the Carolinas, USA. The Island is generally composed of low, rolling hills with beautifully defined fields and pretty farmhouses. And it was warmer and drier than the UK!

Charlottetown is the capital city of PEI, Canada's smallest, and one of the oldest, provinces. The town is characterised by shingle roofed, clapboarded houses and was the scene of the first conference on colonial union which led to the Confederation of Canada in 1867. The conference hotel, the Delta, is situated on the city waterfront overlooking the bay where the West and Hillsborough Rivers meet and flow into the Gulf of St Lawrence. Close to the hotel was a well-visited restaurant, Jakes, which turned out to be the mainstay for eating seafood chowder with some of the local ales.

One hundred and thirty five delegates from 30 countries (if you count Scotland as separate now!) attended this meeting. The meeting schedule comprised seven symposia, many with concurrent oral paper sessions, over four days. After opening remarks from the conference chairman, Dr Solke De Boer, the keynote address was given by Dr Ann Vidaver, Chief Scientist for the US Dept. of Agriculture Programme. She was invited to communicate her thoughts on the challenges and prospects with plant associated bacteria in the next millennium. Interestingly, she first highlighted the challenges for 2000 that were posed by her in a talk in 1981: Biocontrol and synergy, control/suppression of disease, genetic engineering, and useful pathogens. Dr Vidaver then went on to spell out the current challenges and prospects of plant pathologists, along the way highlighting potential problems that face the agriculture industry. For instance, what changes in the natural microbial flora would accompany the introduction of transgenic plants and what is the role of endophytes? Because of the current costs and regulatory climate, fewer synthetic chemicals are becoming available for preventative/curative disease problems. Now is when we need more chemicals, particularly wide host range chemicals, for stimulating resistance in plants. How can we best use our knowledge of signal transduction in disease management? She identified a need to be able to cultivate "non-cultivable" microbes, possibly using plant tissues/organs as growth media. Better detection and identification tools are needed for use in the field. Our knowledge of host and pathogen genomes should be put to good use for creating new products, for plant vaccination and for better understanding of the targets of susceptibility. And new technology should be put to use: such as satellites for monitoring the spread of infectious diseases and virtual reality for "telemedicine" in plants.  The new needs and areas of investigation for the next millennium are chemical and spatial analysis; community and co-operative analysis; the early events of disease/resistance; isolation of new, and fastidious, pathogens, disease management using predictive models.

Disease eradication was inevitably a common theme amongst several speakers. Jaap Janse highlighted one of the few reports of disease eradication in the world. Citrus canker was eradicated from Thursday Island, Australia only to be re-introduced on the stump of a West Indian lime. Dr Janse stressed the importance of checking material for latent infections before trading as well as inspection by importing countries and education of growers and inspectors. Mike Daniels gave an excellent overview, covering the progression from molecular biology to disease control and highlighted three topical areas: study of plants (R genes, signalling genes, and induced genes), study of pathogens (pathogenicity genes) and exploitation. He pointed out the remarkable advances in the study of plant pathogens; in 80 years we have gone from studying the roles of extracellular polysaccharides and toxins to the first sequencing of an entire genome of a plant pathogen (Xylella fastidiosa). Brazilian scientists carried out this impressive, latter feat. Efforts are now being made to sequence more plant pathogen genomes which apparently include: Agrobacterium tumefaciens, Ralstonia solanacearum, Pseudomonas syringae, Erwinia spp and Xanthomonascampestris pv campestris.

The knowledge gained from genome sequencing can be used for bioinformatic analysis, expression studies, protein-protein interactions and mutagenesis. TJ Burr talked about the future development of biological and chemical controls. Streptomycin and oxytetracycline are commonly used antibiotics, but because of a short lifespan of activity and the increasing spread of resistance among pathogens, there is a need for more research into alternatives. Copper is also out of favour because of plasmid-mediated resistance that spreads through apple orchards. Currently, only a few commercial companies are carrying out research into alternatives such as bactericides and oxolinic acid (the latter for fireblight control). The new kid on the block, "Messenger" TM, (syn Erwinia amylovora harpin) was highlighted as an "all singing, all dancing" elicitor of systemic acquired resistance and would confer resistance to a wide range of pathogens. This was hammered home (with more pictures than data, let it be said) by one of the co-developers Dr Steve Beer. Resistance was afforded against aphids, bacteria and fungi and could be used on a wide range of plants, such as roses, cotton food crops and tobacco (see Nature Biotech, June 2000, "Protein biopesticide"). If this proves to be effective, teachers of pathology note - a great example of a commercial application resulting from in depth study of  plant-pathogen interactions.

There were several papers and posters on Xanthomonas blight of cassava by RC's and Kerstin Wydra's group (Gottingen); the Bath contribution concerned attempts (such as by AFLP analysis) to unravel the complex nature of polygenic resistance of this major, yet understudied tropical crop.

There were many heated debates among the taxonomists about nomenclature of organisms. The "simple taxonomist", Louis Gardan, stated that co-operation is needed in the scientific establishment to augment a better standard of classification. John Young felt that phylogenetic classification, which is based on ancestral relationships, is not an adequate method. He went on to compare the species and genus concepts. The species concept states that a bacterial species is defined as a population whose strains share >70 % DNA-DNA hybridisation. Dr Young then asked why Pantoea and Erwinia should be considered as separate species when they are over 96 % similar? This topic was discussed further in the round table meeting on Erwinia (see later). An excellent talk by Giuseppe Firrao (Uni. of Udine, Italy) examined the status of the phytoplasmas and was gratified that they were now regarded along with plant pathogenic bacteria. Classification of the phytoplasmas is based on the symptom class, the plant host range and the vector specificity.

In the Genetics of Pathogenicity session RJ presented a paper describing the isolation and characterisation of virulence genes in Pseudomonas syringae pathovars. David Coplin explained the complex system, comprising four regulatory genes (hrpL, hrpS, hrpXY), for expression of hrp genes in Pantoea stewartii.  A. Penaloza-Vazquez showed that hrp genes are not required for the production of coronatine, thus demonstrating individual regulatory systems for virulence genes. Dawn Arnold (UWE) highlighted the location of similar DNA sequences flanking avirulence genes and showed amplification of novel avirulence genes from a range of P. syringae pathovars.

The second day broached "HR and virulence genes". Steve Beer (Cornell) began the session by detailing the application of knowledge gained from the HR elicitor harpin (mentioned above). Gail Preston followed with a fascinating study of a type III secretion cluster found in the saprophytic plant growth-promoting organism Pseudomonas fluorescens. P. fluorescens elicited a HR when an avirulence gene was expressed in the organism thus demonstrating functionality of the type III genes. This posed an intriguing question as to why a saprophytic organism should carry a cluster of genes normally associated with pathogenic bacteria. The following talk, by Frank White, described some excellent experiments for elucidating that avirulence genes in Xanthomonas sp. are virulence determinants as well. He showed that nuclear localisation signals in the protein were required for virulence and avirulence. AvrXa7 was localised to the nucleus of plant cells and bound only double-stranded DNA; does this protein therefore emulate a eukaryotic transcription factor? On the Tuesday evening were four round table discussions, for the four main groups of plant pathogenic bacteria. The Xanthomonas/Pseudomonas session was rather timid. One of the possible problems, which reflects the current interest in these pathogens, was that there were about 50 people attending and there was a reticence to engage in passionate debating. One of the main questions put to the group was whether there should be a continuation of the "Pseudomonas syringae and related pathogens" meeting in the coming years. Although the consensus was "yes", this issue remains unresolved.

The round table on Erwinia was chaired by IT and included many of the leading Erwinia biologists. The session dealt with  topics from "how to find funds to sequence these pathogens?" to "do we accept Pectobacterium as the new genus name for Erwinia carotovora and others?". On the first issue, all agreed that an international effort was required if we were to succeed in sequencing these pathogens. Indeed, we are currently awaiting the outcome of a USDA application on the sequencing of E. chrysanthemi, E. amylovora and E. stewartii, in which a number of non-US labs  have been asked to participate in bioinformatic aspects of the work. On the second issue, with the taxonomists still debating the justifications for renaming members of the Erwinia genus, it was decided that the genus name Pectobacterium (yet another attempt to introduce the name - the first being in 1945) would not be adopted at present. It is unfortunate, therefore, that some international DNA databases are already cataloguing "Erwinia" sequences under this new name!

In the "Detection and Diagnosis" session A. Matthysse provided an interesting insight to chromosomal genes of Agrobacterium tumefaciens that were required for both attachment of the bacteria to plant cells and virulence. Jan van der Wolf provided a nice overview of the current and upcoming technologies used for detection of plant pathogenic bacteria. The best course of action was to use multiplex detection methods and that PCR alone was not robust enough for current standards. Jeff Jones described a comprehensive piece of work classifying X. campestris pv. vesicatoria into different subgroups using a wide range of molecular and immunological techniques. The quantity of work that he and his co-workers have covered was considerable and a lesson in how to research the taxonomic status of an organism. Martin Romantshuk took a big leap forward in his presentation, showing video excerpts of bacterial colonisation in the plant leaf visualised by gfp expression. He showed that hrpA was expressed as soon as one hour after infection of the leaf and that bacteria on the leaf surface congregate in the stomata. Matthias Ullrich examined the importance of temperature in expression of different genes and he found that the optimal temperature for synthesis of the toxin coronatine and for levansucrase was 18 oC. However, transcription was temperature independent suggesting that the temperature control mechanism lies at another stage of the biosynthetic pathway. On the Wednesday afternoon, the delegates were whisked away on tours around the Island. RJ took "Tour B" which included a local mussel packing factory and a visit to the 9 mile long Confederation Bridge, which spans the Northumberland Strait and links the Island to the mainland. RC and IT went on "Tour A" to the old community of Orwell and Macphail (writer and physician) homestead which now has an ecological forestry project to return some of the native species to deforested areas (a welcome break from potato fields!). Mussel tasting and compulsory testing of the remarkably balmy south coastal waters prepared us for the final convergence with the other tours on a holiday park, the Rainbow Valley, where drinks and lobster (some managed to obtain two) dinner were served. Local families provided music, singing and dancing entertainment, but all too soon we were all taken back to Charlottetown – a relatively sober excursion!

On the final day, Mark Wilson described how a hrpG mutant of X. c. pv. vesicatoria pre-inoculated on tomato plants appeared to be able to protect the plants from pathogenic bacteria. Even though no type III pilus is produced by this mutant, the plant seems to recognise the bacterium as a potential pathogen. Ian Toth described two novel approaches to find genes involved in the interaction of Erwinia carotovora subspecies atroseptica with potato. Using cDNA-AFLP analyses and genome mapping using BAC libraries, Ian and co-workers have identified a number of genes hitherto unknown in this pathogen, including the entire hrp/dsp cluster. Klaus Rudolph brought the conference to a close by providing an overview of the last 35 years of phytobacteriology research. He paid special emphasis to the pathogenicity of Pseudomonas syringae describing the likely roles of toxins and extracellular polysaccharides in disease.

The conference was very organised, the presentations were diverse and of high quality and the size and venue ensured many new contacts and discussions. The conference wouldn't have been complete, however, without the loss of more bags on the journey home (this time IT's). The next meeting is due to take place in 2004, possibly in York or in Heidelberg or Frankfurt, Germany. England versus Germany again…at least it won't come down to penalties!
Thanks to BSPP for providing travel grants to attend the meeting.

Robert Jackson, University of theWest of England
Ian Toth, SCRI, Dundee
Richard Cooper, University of Bath