BSPP News Spring 2001 - Online EditionThe Newsletter of the British Society for Plant Pathology
Number 38, Spring 2001
Conference and Travel Reports
2000: 7th International Symposium on the Microbiology of Aerial Plant
University of California, Berkeley, USA : 3 - 8 August, 2000
Steve Lindow and his team were host to around 100 participants at the University of California, Berkeley Campus in a meeting as diverse as the phyllosphere itself. This is a meeting held every five years focusing on the ecology of leaf surface microorganisms and how this impacts upon other disciplines, especially plant pathology and microbiology.
We were eased in gently by Thomas Whitham (Northern Arizona University) by his clear and bold examples of plants as heterogeneous habitats. For example, trees with juvenile and adult leaf forms restrict aphids to a small portion of the tree where they suffer increased competition and predation. But think twice before controlling gall aphids: their removal from an area will halve biodiversity of fauna and microflora because of interdependency.
Gwyn Beattie (Iowa State University) reviewed the effect of leaf surface waxes on bacterial colonisation. Anything which affects epicuticular waxes will affect bacterial populations, e.g. breeding for drought tolerance, global warming (both of which increase wax production and make it harder for bacteria to colonise), and air pollution (waxes absorb and adsorb pollutants which can affect bacterial growth, e.g. chlorinated hydrocarbons are potential C sources which can boost bacterial growth).
Lukas Schreiber (University of Wuerzburg, Germany) talked on a subject close to my own area of research, leaf surface wetting and cuticular permeability caused by bacterial epiphytes. Bacteria can wet waxy cuticles and decrease the surface tension of water via their outer walls and the production of biosurfactants. This assists bacteria to establish on leaf surfaces. Following establishment, bacteria will directly penetrate cuticles to gain access to nutrients: they do not have to rely on passive entry to the mesophyll via natural openings such as stomata and hydathodes.
Linda Kinkel (University of Minnesota) showed us how to model the effects of microbial aggregation on population dynamics, whilst her friendly protagonist, Chris Upper (University of Wisconsin) gave us actual field data on how the population dynamics of Pseudomonas syringae on bean are affected by immigration. Linda to Chris: "Your data are not what I would have predicted". Chris to Linda: "That's why field data are better than model data, Linda". OK, both have a part to play.
Biofilms are clusters of different species of bacteria and fungi on plant surfaces, and aggregates are clusters of a single bacterial species. These and the cross-talk of cells within them were the topics of a number of speakers. Studies of phenotypes and genotypes of bacteria within and between clusters and solitary cells are beginning to be made, to see if solitary cells behave differently to those growing in clusters. We'll see a lot more of this topic in the future. Studies on bacterial autoinduction (quorum sensing or self-regulation of population size) and the role of biofilms in plant surface ecology are blossoming. The horizontal gene pool provides a fitness advantage by gene exchange via plasmids - we now need to know what ecologically significant genes are important in this process. Ultimately: can biofilms be manipulated for disease control and biocontrol?
The meeting was dominated by studies of model systems which used bacterial, as opposed to fungal, epiphytes and pathogens, no doubt due to the relative ease with which bacteria can be studied compared to fungi. However, Richard Belanger (University of Laval, Quebec) gave an excellent account of interactions between leaf surface fungi with special reference to biocontrol, concluding that competition as a mode of action for biocontrol is best for unspecialised and specialised necrotrophic pathogens, whereas antibiosis is best for biotrophs and some specialised necrotrophs. Botrytis must be controlled in its saprophytic phase because it is so aggressive as a pathogen. He was sceptical about parasitism as a mode of action for biocontrol, although Odile Carisse (Agriculture & Agri Food, Quebec) may not agree, since the mycoparasite Microsphaeropsis sp. which she is investigating for biocontrol of Botrytis squamosa on onion appears very effective at reducing sclerotia-borne and air-borne infection, although it does this by attacking B.squamosa in its saprophytic phase as Belanger suggests is most effective. Richard Belanger, by the way, had some great Powerpoint cartoons of leaf surface colonisers which would be very suitable for a wider interpretation of phyllosphere interactions to the public.
Still on the biocontrol theme, Virginia Stockwell (USDA-ARS, Oregon) showed that it is not the indigenous microbes which prevented establishment and success of the potential biocontrol agents Pantoea agglomerans and Pseudomonas fluorescens against the fireblight pathogen Erwinia amylovora, but plain old unreliable weather. What became clear to me in hearing about biocontrol was that every system is different - generalisations are impossible to make.
Susan Hirano (University of Wisconsin) asked, "Just how important are pathogenicity genes from the hrp cluster and the gacS/gacA global regulatory genes, in field fitness of P. syringae pv syringae?". A nice well-defined piece of research with clear answers - the hrp type III secretion system and gacS contribute a lot to field fitness, gacA does not. With more on the global regulation theme, Staffan Kjelleberg (University of New South Wales, Australia) works with a fascinating marine system, particularly the colonists of the red alga Delisea pulchra. This alga produces furanones which strongly inhibit colonisation by marine bacteria, by interfering with their quorum sensing mechanisms. Furanones are potential natural antifoulants and are also under investigation as therapeutic agents against opportunistic Pseudomonas aeruginosa infection in cystic fibrosis patients.
The final day was mostly given over to a range of shorter papers. Peter Spencer-Phillips (University of West of England, Bristol) told us about the source of "malodours" in wheat grain and the development of ingenious electronic sensors to detect them. Steve Lindow described a GFP reporter system for detecting the heterogeneity of iron on leaves, and Antonius Suwanto (Bogor Agricultural University, Indonesia) described the epiphytic fitness of a pathogenicity mutant of Xanthomonas campestris pv glycines and its potential for biocontrol of this pathogen on soybean. Steve Lindow's students gave various presentations on the theme of his lab - describing and quantifying microbial dynamics and behaviour on leaf surfaces. This is important for manipulation of the phyllosphere microorganisms for disease control with reduced pesticides, biocontrol and novel control methods.
Two sessions were held where we discussed, as small groups, the contribution which research into the phyllosphere can make to advancing our knowledge in five areas: plant pathology, plant biology, general microbiology, food safety and the general public.
The social programme was laid back and Californian - a trip to Muir Woods to view the Redwoods, cruising the Bay and a barbeque in the hills above Berkeley gazing down on a chilly, foggy San Francisco. I was in "Berserkeley" 25 years ago with long hair and kaftan (oh yes) and I still find it a wonderful place to be - a mix of nationalities, artistes, intellectuals and wierdos, and that beautiful campus and setting.
It is not possible to do justice to everyone's talk here, or indeed the lively poster session. This was a packed, diverse and exciting meeting: you will be able to read more about it if you get hold of a copy of the Conference Proceedings when it comes out via APS Press in 2001. The next Phyllosphere meeting will be hosted by Mark Bailey at Oxford, U.K. in 2005 and since this is home territory for BSPP members, I can really recommend the meeting for the wide perspective it gives to us plant pathologists. Finally, I am very grateful to the BSPP for the award of a Travel Grant to enable me to attend and benefit from this meeting.
Scottish Agricultural College, Edinburgh