BSPP News Summer 2002 - Online EditionThe Newsletter of the British Society for Plant Pathology
Number 42, Summer 2002
The 13th Crucifer Genetics Workshop was held in Davis, California. About 150 participants from 19 countries attended. As a PhD student working on stem canker/blackleg (caused by Leptosphaeria maculans), it was interesting for me to attend the Blackleg of Crucifers Workshop, held in conjunction with the Crucifier Genetics Workshop.
The Blackleg Workshop was organised by G. Seguin-Swartz (Agriculture & Agro-Food Canada) and attracted 35 participants from 11 countries. The first session was about isolate population structure. Shengyi Liu (Oil Crops Research Institute, CAAS, China) reported the identification of non-aggressive L. maculans (B-group) in China and discussed the risk of aggressive L. maculans (A-group) spreading into China. Soledade Pedras (University of Saskatchewan, Canada) reported on new phytotoxins produced by L. maculans isolates from western Canada. These isolates are virulent to traditionally stem canker resistant species and are becoming a new threat to the oilseed rape production in Canada.
The second session of Blackleg Workshop focused on disease resistance. Michel Renard (INRA, France) reported that some genetic factors involved in the resistance of B. juncea to L. maculans were transferred into oilseed rape lines through interspecific crosses. Nevertheless, the resistance introgressed into a very susceptible genetic backgroud had a weak potential durability. He discussed increasing the durability of resistance through selecting the genetic background. Hendrik Winter (INRA, France) presented work on the transfer of stem canker resistance genes from related species into Brassica napus. Plants from intergeneric hybrids between B. napus and Sinapis arvensis or Coincya monensis showed high cotyledon or adult plant resistance to L. maculans. However mixoploidies tended to decrease in subsequent backcross generations.
The last session of the Blackleg Workshop was on epidemiology and disease management. I presented my work on comparative epidemiology of A-group and B-group L. maculans, which provided more evidence that the two groups are two different species. Jean-Noel Aubertot (INRA, France) showed that cultural practices affected stem canker development in France. An early sowing date and low nitrogen availability during the vegetative stage decreased crown canker development. This work will contribute to the improvement of integrated disease management.
In the poster session, I was particularly interested in the poster 'Aggressive isolates of L. maculans found in Mexico' presented by Onesimo Moreno-Rico (Universidad Autonoma de Aguascalientes, Mexico). As I am working on the epidemiology of L. maculans, I was interested in the reason why the aggressive L. maculans (A-group) has spread into Mexico. Dr Onesimo Moreno-Rico believes that the fungus may have been introduced into Mexico via the import of contaminated seeds. In Mexico, blackleg has caused up to 70% yield losses and become a limiting factor in cauliflower production. This information has broadened my knowledge of L. maculans distribution in the world, and is useful for us, since we are considering a new project on preventing A-group L. maculans from spreading into China, where about 10 M ha of oilseed rape is grown. Up to now, only the non-aggressive L. maculans (B-group) has been identified in China. A-group L. maculans is more damaging than B-group, and there is a need to develop strategies to prevent the A-group spreading into China.
As I have worked previously on oilseed rape breeding, I was also interested in the Brassica breeding session of the Crucifer Genetics Workshop. Jinling Meng (Huazhong Agricultural University, China) spoke on 'An approach to create new types of Brassica napus'. He presented a strategy of using the A' genome in B. campestris (A'A') and the C' genome in B. carinata (BBC'C') to replace the A and C genomes in Brassica napus (AACC). Hybrids between the new type of B. napus (A'A'B'B') and common varieties of B. napus (AACC) have strong heterosis. He discussed some results of field trials and the implication of the new type of B. napus.
The Crucifer Workshop ended with a talk by Elizabeth Jeffery (University of Illinois, USA) on 'Nutraceuticals in Brassica'. She reported that crucifers, more that other vegetables produce substances which protect against a number of cancers. Chemoprotection by crucifers is mainly attributable to isothiocyanate hydrolysis products of the glucosinolate secondary metabolites that are unique to cruciferous vegetables. This is a piece of good news for people who like crucifer vegetables. It is nice to know the nutrients and function of our food.
The two workshops gave me the opportunity to meet scientists from international groups working on genetics and molecular biology of L. maculans, to update my knowledge of crucifer breeding and to make useful international contacts with other research groups. I am very grateful to the BSPP for the award of a Travel Grant to enable me to attend and benefit from the workshops.
University of Hertfordshire
The third International Bacterial Wilt Symposium (IBWS) took place at an isolated golfing complex near to the town of White River, an hour's flying time from Johannesburg and close to the Kruger National Park which provided a wonderful diversion from an intense and fascinating 5-day meeting. Over 100 enthusiastic delegates from 30 countries reflected the global importance of Ralstonia solanacearum, the causal agent of bacterial wilt, which affects more than 400 plant species including economically important crops such as potato, tomato, eggplant. Capsicum, tobacco, banana, ginger and groundnut.
The conference was opened by Prof. Luis Sequeira, a pioneer of bacterial wilt research, who reiterated the importance of the pathogen on livelihood and food availability worldwide, and highlighted renewed concern over the organism as a possible agent for bioterrrorism. In the keynote address, Dr. Bharati Patel from the Rockefeller Foundation outlined the current state of plant pathology in Africa and opportunities for collaboration on an international scale. Prof. Chris Hayward presented historical and forward-looking views on international collaboration and information exchange, drawing on his experiences since the first international conference on bacterial wilt in North Carolina in 1976. John Elphinstone, then reviewed the current global status of bacterial wilt disease, based on recent literature and expert opinions surveyed amongst the conference delegates.
The programme included 4 full days of high quality presentations and
posters over 7 sessions with additional evening discussion. Abstracts can
be viewed on the internet at http://ibws.nexenservices.com/. A particular
highlight of the conference was the presentation by Dr Christian
Boucher on outputs and prospects from the complete genomic sequence of
R. solanacearum, which he had published in 'Nature' the previous
week. Over 200 candidate pathogenicity genes have been identified
on the chromosome and megaplasmid.
A stimulating presentation by Dick Van Elsas (Plant Research International, the Nertherlands) on the fate, activity and spread of R. solanacearum in European habitats introduced the concept of a viable but nonculturable (VBNC) form of the bacterium potentially able to survive in soil and water at low temperature. Several other presentations and an evening discussion session raised several questions regarding the existence and significance of a VBNC form.
A key presentation on diversity and diagnosis was given by Mark Fegan (University of Queensland, Australia), who proposed a new classification system for the R. solanacearum species complex. The hierarchical scheme has three levels: phylotype, sequevar and clonal line. Four phylotypes are differentiated by ITS multiplex PCR which neatly separates the Asian, American, African and Indonesian origins of strains. Sequevars are separated by a greater than 1% variation in endophytic gene sequences (e.g. glucanase, hrpB genes), and clonal lines consist of strains with identical genetic fingerprints under analyses such as repPCR, PFGE and AFLP.
A final session dedicated to R. solanacearum on banana and plantain, reflected the continued global importance of moko, bugtok and blood diseases. Presentations covered increased spread of these diseases in countries such as Brazil, Indonesia, and the Philippines, and described improved diagnostic assays involving multiplex PCR.
In addition to the full scientific schedule, a day trip into the Kruger National Park provided an unforgetable insight into the wide diversity of animal, bird and plant life and an opportunity to gaze on views unchanged for thousands of years. With a 5:00 am start and temperatures exceeding 40 ºC during the day, the discussion in the bar on that particular evening was rather subdued.
British delegates were well represented at the conference. Julian Smith from NRI presented a paper on the fate in soil of an avirulent R. solanacearum mutant as a putative biocontrol agent, as well as a poster on the incidence of bacterial wilt in potato in Bolivia. A poster from SASA by John Wood et al. was presented by Gerry Saddler on the eradication of R. solanacearum from a Scottish waterway. Sean Simpkins from CSL presented a poster on the use of microarray technology to study differential gene expression in R. solanacearum in relation to temperature and host range. John Elphinstone, also from CSL, presented a poster on new diagnostic technologies ranging from on-site lateral-flow test kits to quantitative "Taqman" PCR assays in addition to the paper on current global bacterial wilt status.
The local organisers headed by Jody Terblanche (Tobacco and Cotton Research Institute, Rustenburg, South Africa) and the programme committee headed by Caitilyn Allen (University of Wisconsin) and Philippe Prior (INRA) created an excellent conference in a perfect environment. A book of the symposium papers entitled "Bacterial wilt: The disease and the Ralstonia solanacearum species complex" (C. Allen, P. Prior and C.A. Hayward eds.) will be published by APS Press and the British delegates agreed to form a committee, headed by Gerry Saddler, to organise the 4th IBWS in the UK in 4 or 5 years time.
Central Science Laboratory.
This year's Keystone Symposium 'Specificity and Crosstalk in Plant Signal Transduction' was held at the Granlibakken Resort in Tahoe City, California, USA. The scenic surroundings and the environment of world-class scientists made the conference particularly enjoyable and memorable.
'Specificity and crosstalk' featured in all the talks, and it became clear how intriguingly complicated signal transduction processes are in plants. Recurring themes which featured in many signalling pathways were the role of reactive oxygen species (ROS) and specificity via protein-protein interactions.
The keynote address of the Symposium was by Beverly Errede, on MAP kinase signalling cascades in yeast. She highlighted the varying degress of complexity that occur in MAPK pathways, and how particular input signals lead to highly specific outputs, regulated by scaffold proteins. As MAPK pathways in plants are now being revealed, this was important information, as the importance to identify these proteins in plants is now inevitable.
Hormones and signalling: Talks revealed (not surprisingly) that significant cross-talk occurs between various plant hormone signal transduction cascades. An interesting story was presented by Peter McCourt on the eternal search for an ABA receptor. Various ABA analogues have been used to identify ABA-insensitive mutants. This has still not yet led to the discovery of the ABA receptor, but has nevertheless identified novel targets of ABA signalling, such as the AP2 family of transcription factors ("….we did not get hold of the dancer, but we still enjoyed the dance!"). Mike Blatt and Fedora Sutton have both used a Xenopus oocyte system which involved microinjecting them with plant cDNAs, and looking for changes in ion conductance across the membrane, since ABA induces K+ and Cl- currents. However, neither have yet found an ABA receptor using this approach.
ABA signalling was featured in many of the talks, highlighting that novel signal molecules are still being identified, and also that some current work reveals some historical aspects of ABA research. Julian Schroeder's group has placed ABI1 and ABI2 in distinct positions in the ABA signalling cascade in stomatal guard cells, whereby ABA leads to ROS production via NADPH oxidase; ABI1 intercepts this upstream pathway, whereas ABI2 intercepts the pathway downstream of ROS production. Steve Neill's group described some exciting new discoveries on the role of nitric oxide as another novel signal in the ABA signalling cascade in guard cells (poster presentation). Novel ABA mutants are being identified, which intriguingly, appear to be in genes regulating RNA processing. This implies that ABA plays a central role in regulating the transcriptional machinery in a cell; interestingly, one of the first genes identified as being ABA-inducible is also related to transcription.
Calcium signalling comprised an entire session. The main message here was that guard cells have a calcium-reactive memory, and are programmed in a specific way in response to calcium oscillations. Moreover, redundancy is the key feature in guard cell signalling, because when one pathway is lost, another compensates for it.
Two-component signal circuitry featured in several presentations as
an important mechanism of signal perception, not just in plants and yeast,
but also cyanobacteria. Ildoo Hwang from Jen Sheen's group highlighted
lucidly its role in cytokinin signalling, using a protoplast transient
expression system as a model. Klaus Harter elucidated the role of a response
regulator protein ARR4 (which, interestingly, is also regulated by cytokinins)
in phytochrome B signalling, and hypothesised that prokaryotic histidine
kinases have probably evolved to become plant phytochromes. Kazuo Shinozaki's
group's current work is involved in histidine kinases as osmosensors: AtHK1
was identified as a negative regulator of osmotic stress in Arabidopsis.
Norio Murata talked about the histidine kinases in cyanobacteria – using
various mutants, they identified distinct sub-sets of target genes downstream
of the cold, salt and osmotic stress-sensing histidine kinases, again reflecting
considerable cross-talk (and specificity in output) between various stress
Genomics and post-genomics: An unmissable feature of many talks was the use of post-genomics approaches to address specific problems. Several groups are using microarray technology to identify genes involved in specific signalling pathways, e.g. phytochrome, low temperature and drought signalling. Of particular importance was Paul Haynes' talk (Torrey Mesa Research Institute). He presented some of their state-of-the-art faciltities at TMRI, which they have used to sequence and annotate the rice proteome. Their success using 2D gels and spot identification was particularly encouraging for the many frustrated 2D victims in the audience! An important revelation was that 2D gels can also be used successfully to identify protein binding partners in multicomplexes. Array work presented by Joel Kreps (TMRI) revealed considerable overlap in plant responses to cold, drought and salt stress; nevertheless it was interesting to note that there is some conservation between rice and Arabidopsis stress-regulated genes, yet quite distinct genes exist in both species.
Signalling in plant defence: Signalling pathways in plant defence is no exception to the rule of complexity. Jeff Dangl presented recent work identifying a protein (RIN4) which interacts with the resistance factor RPM1 and also with the bacterial avirulence factors AvrRpm1 and AvrB (the latter two appear to phosphorylate RIN4). New vocabulary in the plant disease resistance dictionary include the terms "resistosome" and "susceptisome", describing those proteins involved in either resistance or susceptibility, respectively. Critical determinants of plant disease resistance specificity appear to lie in specific protein-protein interactions. Thus, studying just the gene regulation is not sufficient; a detailed analysis of the protein and its binding partners are key to decipher any signalling pathway. Jonathan Jones presented some of his team's work on Cf signalling. Mutants that suppress Cf gene function have been identified; one such mutant, Rcr, suppressed Cf2-dependent but not Cf9-dependent resistance. Rcr3 encodes a cysteine protease, which in itself is interesting, considering the role of cysteine proteases (caspases, which have not yet been identified in plants) in the hypersensitive response. Another interesting aspect of Jonathan's talk was the drawing of analogies between Cf9-Avr9 interaction and flagellin perception. Flagellin is a bacterial elicitor which binds to specific proteins in the plasma membrane and activates a host of defence responses in Arabidopsis cells. Interestingly, the orthologues of Cf9-induced genes appear to also be induced by flagellin, indicating further complexities in the classical gene-for-gene hypothesis.
Signalling during plant defence was also featured in Jen Sheen's talk on MAPK pathways in Arabidopsis. Using an elegant transient expression system, protein interactions between various MAPK pathway components was shown. The elicitor flagellin activates the MAPKs AtMPK3 and 6 via AtMEKK1 via binding to the flagellin receptor; however AtMPK3 and 6 are also activated by hydrogen peroxide and auxin, but in this case, the interacting MAPKKK partner is ANP1. Specific target genes are activated by the specific stimuli. Thus several layers of complexity occur in every aspect of plant cell signalling, but the overall message might be that plants have a tremendous capacity to perceive and regulate responses to multiple stimuli, leading to a favourable outcome.
The Symposium was an excellent opportunity to interact with key players in plant cell signalling. It was certainly thought-provoking, and pleasing to learn that the way forward in this field was to make use of new post-genomic facilities to unravel some of the overlapping complexities of a wide spectrum of signalling pathways.
University of the West of England