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Compiled from reports by Dawn Arnold, (UWE, Bristol), Rebecca Lyons (Rothamsted), Tim Simpson (Imperial College), Chris Ridout (John Innes) and Eleanor Gilroy, Hui Liu, Juan Morales, Sonia Humphris (SCRI)
The Journey there
This year’s biannual MPMI meeting was held in Sorrento, Italy. Dawn Arnold and Rob Jackson decided to use the conference as a good excuse to drive to Sorrento. They left the UK on the Sunday to arrive in Sorrento on the Friday, the day before the conference started. The driving was initially quite reasonable, but became more ‘interesting’ the further south they went, which correlated very well with the increase in temperature. By the time they had picked up a couple of lab members from Naples airport, the ‘interesting’ driving had taken on a whole new meaning! This was particularly notable at a section where at least 10 lanes emerging from motorway toll booths was reduced down to 2 lanes. It was mayhem and forward progression took ages with every driver trying to get a centimetre advantage over every other one. They got through almost unscathed, apart from the scraped wing mirror which had literally been squeezed onto the central crash barrier. Approaching Sorrento however, the wisdom of holding the 13th MPMI conference in the shadow of Vesuvius was questioned, especially after the 12th MPMI had been disrupted not once but twice by Caribbean hurricanes. But Sorrento is a beautiful place and needless to say Vesuvius remained asleep. The Congress was attended by 1200 delegates from 59 countries. The scientific program was intense: sixty platform presentations, 937 posters as well as 5 special interest workshops were held over the course of five days.
PAMPs and defence responses
The IS-MPMI award lecture entitled “PAMPering elicitors: How flags and elfs learned to fly”, was presented by Thomas Boller who gave an entertaining account of his lab’s extensive contribution to the current understanding of pathogen-associated molecular patterns (PAMPs), and his initial struggle to get his important discoveries suitably recognised. The lecture set the scene for numerous platform and poster presentations discussing PAMPs and their role in basal defence. Delphine Chinchilla and John Rathjen both showed that the transmembrane receptor kinase 1, BAK1, is a key player in PAMP-triggered immunity. BAK1 interacts in vivo with the FLS2 pattern-recognition receptor in a flg22 ligand-dependent manner. Moreover, it appears that BAK1 plays a role in triggering responses not only to flg22 but to a range of PAMPs. Fred Ausubel presented Pseudomonas virulence factors able to block PAMP signalling, for example coronatine, which mimics JA and suppresses the flg22-mediated transcriptional activation of cytochrome P450 monooxygenases.
Peter Dodds described the recent work on Flax and Flax rust, the classic gene for gene system originally described by Flor. AVRM, AVRL, directly interact with the resistance protein, shown by the yeast 2 hybrid method. Availability of a crystal structure of AVR and R protein shows that AVR sits in the curve of the LRR, and that there are many areas of contact. But interestingly the LRR region alone was not sufficient for the interaction, and the P-loop (involved with ATP-binding) is also important. Paul Schulze-Lefert described experiments on Mla function. 5% of Mla resides in the nucleus where it associates with WRKY2 transcription factor. A P loop Mla mutant (defective in ATP binding) still localised to the nucleus, but did not associate with WRKY2. WRKY2 silencing leads to disease resistance, whereas WRKY2 over-expression completely over-rides Mla-mediated resistance. Cyril Zipfel described a screen for Arabidopsis mutants affected in their sensitivity to elf 18, a peptide surrogate that triggers PAMP-mediated immunity. He found an elfin mutant very susceptible to bacterial disease, with a defect in a novel secreted protein with conserved domains. The approach taken seems to provide a fertile resource for identifying PAMP-triggered signalling components. Jeff Dangl presented an excellent example of the guard hypothesis involving the host factor RIN4 in Arabidopsis as an intermediate between R-Avr interaction. RIN4 recognises three type III Pseudomonas effectors and activates the R genes RPM1 and RPS2 via the NDR1 protein. Three amino acids were found to be required for direct interaction between the three effectors and RIN4. Surprisingly, a rpm1 rpm2 mutant Arabidopsis plant inoculated with Pseudomonas that contained AvrB mounted a weak resistance response. This was mediated by another gene, TAO1, suggesting that the wrong R gene was targeted by RIN4. R gene mediated resistance therefore exhibits quantitative and epistatic effects.
There were many motivating talks focusing on the perception of signals from the edge of cells and how this can result in intracellular signalling in the host. Dan Klessig discussed his recent work on SA-binding proteins, one of which is methyl salicylate esterase and concluded that MeSA could be the long sought systemic signal for systemic acquired resistance. In other sessions focusing on host mechanisms for controlling interactions Dong amusingly deliberated on her interesting microarray findings, including that NPR1 appears to target protein-folding and secretion pathways since genes such as Sec6, clatherin and cyclophilin were differentially expressed. SNI1, a negative regulator of PR genes, appears to specifically de-repress NPR1- dependent genes, and contains armadillo repeats also found in B-catenin, which is a scaffold protein that shuttles between cytoplasm and nucleus. Robatzek expressed the importance of receptor ubiquitination and considered that AvrPtoB, which has E3-ligase activity, triggered ubiqituitin mediated receptor endocytosis.
Brian Staskawicz began by describing that T3SS effectors are delivered unfolded and may require proteins such as cyclophilins. Ulla Bonas’s work was also very interesting because it highlighted a plant promoter element (upa box) bound by the AvrBs3 effector which is highly conserved in Xanthamonas species suggesting direct manipulation of host expression by bacteria. Jim Alfano gave an excellent talk highlighting the role of another varied effector, HopU1, which interferes with ribosylation of plant RNA-binding proteins resulting in post-translation suppression of defences.
Another area of interest was quorum sensing (QS). QS is a mechanism for linking bacterial cell population density to virulence factor synthesis via small diffusible signal molecules, like Nacylhomoserine lactones (AHLs). These signal molecules control diverse physiological processes including bioluminescence, motility, production of extracellular polysaccharides, exoenzymes, antibiotic biosynthesis, plasmid conjugal transfer, biofilm development and virulence. Lindow explained that QS enhances epiphytic fitness, extracellular polysaccharide production, oxidative stress tolerance and virulence of Pseudomonas syringae in later stages of lesion formation.
However it does not suppress swarming motility as he found that a QS mutant strain increased the rates of invasion on bean leaves compared to wild-type after topical applications to bean plants. By studying Rhizobium etli, Michiels’ group demonstrated that mutants in the R. etli cin system, an AHL-based QS system, are no longer able to move over a semisolid surface. A similar discovery was reported by Kim et al. by studying Burkholderia glumae, the causative agent of rice grain rot. The N-actanoyl homoserine lactone (C8-HSL)-deficient mutant of B. glumae is aflagellate and has lost the ability to swim and swarm. Both swimming or swarming activities and flagellation of a QS mutant strain can be restored by providing C8-HSL exogenously suggesting that the flagella biosynthesis is under the control of QS. In the biocontrol strain Pseudomonas fluorescens CH40, Lapouge and colleagues reported that the plant protection ability of P. fluorescens depends, in part, on the production of extracellular enzymes and secondary metabolites, whose expression is controlled by QS. Interestingly, QS also involves several small RNAs (e.g. RsmXYZ) and Gac/Rsm signal transduction pathway at critical checkpoints of translation initiation and mRNA stability in P. fluorescens.
From the meeting it was very clear that Oomycetes have been consolidated as very exciting models for plant pathology research. The RxLR class of Oomycete effectors was a hot topic throughout the congress. These effectors have been discovered in the genomes of Hyaloperonospora parasitica, Phytophthora sojae, P. ramorum, P. capcisi and P. infestans and a similar motif (RxLx[E/Q]) has been described in the malaria parasite, Plasmodium falciparum. Evidence suggests that RxLR class effectors are detected by corresponding plant defence R genes inside the host cytoplasm. Jim Beynon analyzed the H. parasitica genome sequence and found 7740 putative secreted proteins. From those, 380 expressed RxLRs have been identified. 19 are conserved, 59 have between 1 and 10 amino acid changes and 22 of them have high levels of polymorphisms (more than 10 amino acid changes). As an example for H. parasitica host coevolution, Jim described the plant R protein RPP13 as having extreme variation in the LRR region and high variation in the corresponding avirulence RxLR protein ATR13, and pointed out that variation in the ATR13 protein is driven by RPP13.
Paul Birchs’ group showed strong evidence that the RxLR motif is involved in the mechanism of RxLR effector protein translocation inside the host cytoplasm. However this motif is not necessary for the protein function inside the host cell. The same function has been reported for the RxLx[E/Q] motif from the malaria parasite, which point towards a new conserved protein translocation mechanism in some eukaryotic pathogens. It is apparent that the C’ terminal part of the RxLR effector is responsible for its function. Brett Tyler discussed emerging evidence about the function of some RxLR effectors like an increase in P. sojae virulence to soybean when overexpressing Avr1b in soybean hairy root cells and suppression of programmed cell death in soybean triggered by BAX or Avr4/6. Sophien Kamoun’s talk provided a clever summary of several reviews he wrote recently on oomycete genomics.
Pierre de Wit explained the role of the Cladosporium fulvum effector Avr4 in chitin-binding, which enables it to protect hyphae of several chitin containing fungi. AVR2, which inhibits cysteine protease RCR3, is also an effector that facilitates virulence in absence of Cf-2. Based on the recent genome sequencing of Ustilago maydis, Regine Kahmann presented updates into proteins required for biotrophy. Deletion of the gene cluster 19A diminished virulence, but allowed normal development of U. maydis until spore production. By analysing separate gene knockouts, she found that 3 genes of the cluster work together to induce tumour formation and anthocyanin production, and suppress necrosis. However, in contrast, knockout of the 19A gene cluster in the related smut fungus, Sporisorium reilianum only slightly reduced its pathogenicity.
On pathogen virulence, Richard Oliver gave a fascinating account of the evolution of a virulence gene cluster in Pyrenophora tritici repentis. He proposed that the cluster, which is associated with transposases, had come from Septoria nodorum possibly via hyphal anastomosis. Francis Martin, described whole genome analysis in Laccaria bicolour, a fungal symbiont of trees. The fungus had apparently undergone massive loss of genes encoding cell wall degrading enzymes for adaptation to symbiosis, but maintained non-plant degrading enzymes such as peptidases and chitinases associated with decay of organic matter (including insect cuticles). It was as though the fungus had evolved two genomes; one for survival in the leaf litter, the other for tree root tips. Christina Cuomo, talked about sequencing of Puccinia graminis, a pathogen with a genome size estimated to be 86 MB. They now have 81. 5 MB sequenced in 4557 contigs, 47. 2 % of which is repetitive DNA. Avirulence gene AVRT6 has been localised to an 18kb region, corresponding to a genetic distance of 24 cM (less that 1kB/cM!). They are performing Solexa sequencing on UG99, an aggressive strain first identified in Uganda in 1999. They aim to identify SNP’s to track the spread of this strain.
Another hot topic, RNA silencing, was introduced by James Carrington. He used examples of viral silencing suppressors to examine miRNA and siRNA silencing pathways. Mutant plants with different DICER genes knocked out revealed functional redundancy between DICERs in Arabidopsis, and he proposed the existence of a selective pressure for the evolution of new DICERs. Exciting new data demonstrating the numerous roles of RNA silencing in plant biology was presented by Olivier Voinnett and members of his research group. They showed that in addition to antiviral resistance and modulation of developmental pathways, miRNAs are also involved in resistance to bacteria via the activation of PAMP-triggered pathways. As a counter-defence, Pseudomonas virulence factors were found to suppress the production of such miRNAs. In another presentation, Arabidopsis plants were inoculated with a hairpin construct derived from a virus normally confined to the phloem to induce RNA silencing. Surprisingly, siRNAs moved away from the phloem companion cells, suggesting that cells are immunised ahead of infection. It was also shown that the miRNA and antiviral RNA silencing pathways converged at AGO1, explaining why plants transformed with the cucumber mosaic virus 2b silencing suppressor gene (which interacts directly with AGO1) are developmentally deformed. Plants may have evolved a counter defence against viral silencing suppressors: Voinnet suggested that R genes may interact with the RISC-AGO complex and / or guide viral silencing suppressors away from the RNA silencing machinery. Virus induced gene silencing (VIGS) was heavily exploited in the many pathosystems presented. Also on the virology side of the congress, Dinesh-Kumar presented a new host protein facilitating the N resistance protein-TMV interaction. Using elegant in vivo split YFP fusion analyses, he showed that TMV p50 protein not only interacts with the tobacco host protein NIP1, but is required for NIP1 to move from the chloroplast into the cytoplasm and interacts with the N protein to activate defence signalling in the nucleus.
Andrew Love showed that Arabidopsis plants transformed with the CaMV silencing suppressor and pathogenicity determinant P6 contained increased levels of NPR1 protein, and furthermore, that NPR1 was confined to the nucleus in such plants, allowing the pathogen to dupe the plant into thinking it had already increased SA and thereby stopping activation of a SA-activated defence response. Martin Fuchs provided evidence that the perceived risks of commercialisation of virus resistant plants containing pathogen derived transgenes, such as functional complementation and recombination were unfounded following studies of transgenic squash and grape, which were commercially released 15 years ago in the USA.
For the connoisseur, lectures could also be found relating to, among other subjects, parasitism by Root-Knot nematodes (RKNs). The lecture by Valerie Williamson focussed upon Meloidogyne hapla and its potential for assuming the role of a model organism. RKN’s are attracted to root tips, attacking just behind the root cap. Her group isolated gland specific cDNA’s, identifying lots of novel genes, some of which were similar to bacterial genes and so might have arisen by horizontal transmission. They found a candidate Avirulence gene (GcI) by cDNA AFLP, and showed that it belonged to a gene family but with no close homologues. Since there is no transformation or genetics for RKN, they used RNAI of CgI gene to show gain of virulence.
Other talks on the quirkier side of plant pathology included one by Conseulo de Molales on Cuscuta pentagona, an obligate parasite of plants. With a series of fascinating videos, she tested volatiles that elicit directional growth of the parasite towards the host. Cuscuta can apparently make a choice between wheat and the preferred host tomatoes, emission of hexenyl acetate resulting in a negative choice for wheat. In addition, Jonathan Gressel talked about use of a transgenic Fusarium to control Orobanche, another parasitic plant (Broomrapes). The transgenic fungus expressing a necrosis inducing peptide (Nep1) gave over 90% control of the parasite. However, this biocontrol method will not be developed commercially because of the problem of releasing a genetically modified plant pathogen into the environment. Nevertheless, it tells a fascinating story about the development of a potential solution to an important problem endemic in many parts of the world.
In the evenings those delegates masochistic enough to fend off the allure of Sorrento’s restaurants could engross themselves in specialised workshops of their choice. Of these one workshop focusing upon Gene Ontology and annotation proved particularly interesting. Organised by Candace Collmer and Trudy Torto-Alalibo, the aim was the introduction and promotion of a relatively recent contribution to the ‘Gene Ontology’ (GO), known as ‘Plant- Associated Microbe Gene Ontology’ (PAMGO). The GO has the aim of introducing universal expressions (known as ontologies) that will describe gene products by their 1) biological processes, 2) cellular components and 3) molecular functions. By doing so it is an attempt to unify descriptions across databases resulting in easier comparisons and data mining. The system (accessed via the tool ‘AmiGO’) provides, amongst other functions, aid to genome sequence annotators and those performing microarray analysis to study attributes of gene products. PAMGO further extends the GO to the study of microbe/host interactions. Examples of information contained to date relate to several species of bacteria (e.g. Pseudomonas syringae pv. tomato) fungi (e.g. Magnaporthe grisea), nematode (e.g. Meloidogyne hapla) and oomycetes (e.g. Phytophthora ramorum). It is a universally accessible tool that is continually evolving under the direction of the users. As such it may prove to be a very useful tool from now on.
However, this conference was not all work and no play. Each day the delegates were treated to fine food and wine. Halfway through the conference a tour of the famous town of Pompeii was organised. This town once buried during an eruption by the great Vesuvius in A. D. 79 now stands silently crumbling – a relic to the once great civilisation of the Rome. Guided tours were provided and for a brief period that afternoon the town was swamped by a sea of yellow hatted scientists all wilting under a blazing sun. Curiously, the majority of these eminent intellectuals were insistent on visiting the ‘Lupanare’, a place of ill repute adorned with rather eye-opening frescos.
As the final day came to a close a ceremony was held where awards were given for the best posters. Professor Matteo Lorito and his colleagues were also thanked for organising what had turned out to be a really magnificent conference. Finally, to cap it all off the day ended with a very pleasant dinner featuring fillet of grouper and Neapolitan pastries, all to the serenade of traditional Neapolitan folk songs. So with friendships made and old renewed we look forward to Quebec in 2009!
And finally the thoughts of the BSPP President, Richard Cooper
My impressions were of heat, high humidity (the hotels air handling system did not cope), and maybe too many PAMPs and Type III effectors. There are real advances in these areas but it was saturation coverage. In fairness, the meeting was extremely well organised throughout and mostly well balanced. With concurrent sessions there was never a shortage of things to do or hear. This is the strength and weakness of MPMI meetings; how to be in two or three talks at the same time? Surprisingly though, I met those colleagues that I needed to meet; the long lunches in a spectacular high location were a blessing and a chance to grab that colleague for an hour or so. Progress in the field of interactions appears to be logarithmic. It is truly exciting, but for anyone like me with many interests, it is now impossible to keep up on all fronts. The wine at lunchtime didn’t help.
We set up and manned the BSPP stand which was in a great position and received much interest. I have to add that it surpassed the MPMI stand, which was primarily selling trinkets!