Preface to
Biology of Plant-Microbe Interactions
Volume 5

Due to two major hurricanes in the Caribbean Region, the XII. International Congress on Molecular Plant-Microbe Interactions (MPMI), like the Mayan city of Uxmal, was built and rebuilt three times and finally held from Wednesday, December 14th to Sunday, December 19th, 2005, in the beautiful, peaceful white city of Mérida in Yucatán, México. There were around 160 oral presentations, in the format of plenary, concurrent, and mini-symposia sessions. Additionally, over 450 posters were presented and displayed during the entire congress. We had a total of 538 attendees from all over the World: 6 from Argentina, 8 from Australia, 9 from Belgium, 1 from the Czech Republic, 10 from Brazil, 30 from Canada, 2 from Colombia, 3 from Denmark, 1 from Finland, 36 from France, 27 from Germany, 1 from Israel, 6 from Italy, 33 from Japan, 10 from Korea, 65 from Mexico, 3 from New Zealand, 37 from The Netherlands, 1 from Poland, 1 from Peru, 1 from Russia, 10 from Spain, 1 from South Africa, 2 from Sweden, 7 from Switzerland, 41 from UK, 185 from USA, and 1 from Venezuela.

In Merida, the MPMI community celebrated a feast of great science, exploring the cutting edge aspects of plant-microbe interactions. Major themes of the XII IC-MPMI, illustrating the thriving vitality of this field, included: Recognition of Pathogens by Plants; Down-Stream Signalling Events in Plant-Microbe Recognition; Plant Cellular and Physiological Responses in Plant-Microbe Interactions; Plant Molecular Mechanisms of Resistance to Microbes; The Innate Immune Response in Plant-Microbe and Animal-Microbe Interactions; Post-Transcriptional Regulation and Gene Silencing; Cell-Cell Communication: Ecology and Evolution in the Interactions between Plant and Microbes; Biocontrol and Quorum Sensing; Virulence Effectors and Molecular Mimicry; Interactions between Plants and Virus; Interactions of Plants and Symbiotic Bacteria; Interactions between Plants and Mycorrhizal Fungi; Interactions between Pathogenic Fungi and Plants; Interactions between Plants and Nematodes, and Plants and Insects.

Jens Stougaard presented the opening lecture, addressing the stunning recent progress achieved in his group and in the field on plant-Rhizobium and plant mycorrhizal fungi interactions. In the last few years, the identity of plant receptors and key elements in the down-stream signalling pathways, involved the recognition and plant host responses to these symbionts, have been revealed. Symbiotic nitrogen-fixing cells are giant cells that are filled with thousands of nitrogen-fixing bacteroids. Eva Kondorosi showed that in the Sinorhizobium meliloti –Medicago truncatula symbiosis, endoreduplication cycles are essential for normal nodule development, which requires the cell cycle switch gene, ccs52A. The function of CCS52A is to transform actively dividing meristematic cells into non-dividing cells, causing an increase of the nuclear DNA content of cells form 2C up to 64C. Also, bacterial differentiation into bacteroids involves amplification of the S. meliloti genome. Two other highlights in this mutualistic interaction were presented by Jan Kijne who has studied how rhizobia adhere to plant cell surfaces, and Ann Hirsch who showed that several symbiotic genes are important for biofilm formation. Richard Jefferson from CAMBIA showed that not only Agrobacterium but other Rhizobiaceae are able to transfer genes to plants. This finding sets exciting new possibilities for plant transformation and biotechnology.

A forefront of research in plant-microbe interactions is the role of Ca2+ as a second messenger in signalling pathways. Carroll Vance presented that in the model legume Medicago truncatula, a novel family of six calmodulin-like proteins (CaMLs) was expressed specifically in root nodules within the symbiosome space. Striking evidence was presented that these ubiquitous Ca2+-binding gene products were recruited for symbiosis purposes and became essential for an effective symbiosis. In this respect, Jens Stougaard presented evidence that Calcium/Calmodulin-dependent kinases represent key elements in the down-stream signalling pathway (calcium spiking) response to Nod factors. Furthermore, a gain-of-function mutation in this gene had a spontaneous nodulation phenotype in M. trucatula, in the absence of Nod factor. An elegant analysis of Nod factor-induced Ca2+ signalling in legumes was presented by Allan Downie, using Confocal imaging of a Ca2+-sensitive molecule (cameleon) construct in roots and root hairs of M. truncatula. His results indicate that Ca2+-spiking has a nuclear location and that the frequency of spiking is different from the root cap to the non-elongating root hair zone, suggesting that the frequency of spiking in susceptible root hair cells might be critical.

Anemie Emons showed evidences that the actin cytoskeleton plays an important role in the interaction between legumes and Rhizobium, in particular that root-hair exocytosis could be affected after the addition of Nod factors. That actin cytoskeleton plays a key role was also elegantly addressed by Cardenas et al. Using fluorescently labeled cytochalasin, the polymerization sites in living root hair cells could be observed. After Nod factor addition there is an increased fluorescent signal indicating that more actin microfilaments plus end terminals are available. Gary Stacey and Luis Cardenas pointed out that reactive oxygen species (ROS) is one of the earliest responses to Nod factors.

Other highlights of the Congress were the emergent topics of innate immune response and the intriguing non-host resistance response of plants. In this regard, Sheng-Yang He, Sarah Grant showed a galore of molecular artillery displayed by pathogenic bacteria that suppress the plant innate immune response. Interestingly, the secretion of effector proteins, that are delivered into the plant cells, is not exclusive for bacteria. Diverse fungi such as Melampsora lini, presented by Peter Dodds, secrete small polypeptide effector molecules that may directly interact with host (R) resistance proteins. In Phytophthora infestans, Sophien Kamoun elegantly showed that there are more than 200 genes that likely encode secreted effectors (the secretome) in this fungus. Also, Regine Kahmann presented that in Ustilago maydis there is a set of novel genes, some of them involved in iron uptake, which are required for full pathogenesis. Furthermore, Pierre de Wit, showed that this seems to be the case for Avr2 which is a peptide secreted by Clodosporium fulbum into the apoplast of tomatoe leaves, and a virulence protein secreted by Magnaporthe which was presented by Jonathan Walton. Another highlight in the recognition of pathogens by plants was presented by Greg Martin who showed that plant pathogens use molecular mimicry of host proteins. In particular, the C-terminus of the bacterial effector AvrPtoB mimics a host E3 ligase to suppress Programmed Cell Death (PCD) associated with plant immunity. In this regard, the use of a U-box protein represents a novel mechanism of bacterial pathogenesis.

Adrienne Hardham elegantly illustrated with GFP-tagged cellular components that plant cells respond to fungal invasion attempts by a rapid, polarized reorganization of the actin cytoskeleton and also a quick cytoplasmic aggregation. Paul Schulze-Lefert discussed the existence of at least two separable mechanisms operating in non-host resistance, potentially explaining why in nature non-host resistance is more durable in comparison to race–specific resistance. A widespread class of biotrophic fungi, powdery mildews, have evolved specific means to sabotage the resistance response, by blocking a SNARE complex assembly at the plasma membrane. The SNARE protein-resistance mechanism, concomitant with the rapid actin cytoskeleton reorganization, at the cell periphery probably represents the first line of defence against invading pathogens in plant non-host resistance.

Other exciting highlights presented at the Congress dealt with the mechanisms orchestrated by plants and animals to activate defenses after perceiving pathogen-associated molecular patterns (PAMPs). In Arabidopsis, perception of flagellin increases resistance to the bacterium Pseudomonas syringae. Two research groups in conjunction, the groups of Jonathan Jones and Olivier Voinnet, showed that a flagellin-derived peptide induces a plant microRNA (miRNA) that negatively regulates messenger RNAs involved in the auxin response (the F-box auxin receptors: TIR1, AFB2, AFB3, but not AFB1). Repression of auxin signaling restricts P. syringae growth, implicating auxin in disease susceptibility and miRNA-mediated suppression of auxin signaling in resistance. Another striking result along the line of flagellin perception was presented by Silke Robatzek, from Thomas Boller’s group in Basel, who elegantly demonstrated the internalization of the flagellin receptor in response to ligand binding. Georg Felix (also from the Basel group) highlighted the parallels between innate immunity in animals and basal disease resistance in plants. Other major elicitor found in bacteria resides in EF-Tu. As in the case of flagellin, plants appear to recognize a specific highly conserved epitope in the N-terminal region of EF-Tu. A mutant in the receptor kinase EFR lacks the binding sites for EF-Tu as well as EF-Tu responses, indicating that EFR is a functional receptor.

An outstanding plenary session on the animal innate immune response was another highlight and incentive to plant and animal biologist to understand the parallels, evolution and universal conservation of this response.

There was also a strong contribution to the understanding of the plant-nematode and plant-mycorrhizal fungi interaction; Andrea Genre presented an interesting model derived from results obtained from GFP-protein fusions to several cell components which revealed the formation of a new apparatus required for the mycorrizal fungus interaction and colonization, this modeling provides evidence for an active role of the cytoskeleton. Beatriz Xoconostle also showed macromolecular trafficking from endomycorrhiza as evidenced by GFP translocation from the fungal symbiont to the plant, again indicating an active communication at the macromolecular level. Fréderic Debellé presented that a diffusible factor produced by the mycorrhizal fungus (Myc factor?) also stimulates lateral root formation. The Myc factor effect depends on DMI1, 2 but not DMI3. Debellé proposed that symbiosis-related lateral root formation could be used as a biological test to purify and characterize this putative Myc factor.

David Bird presented interesting evidence that plants respond to rhizobacterial Nod factors and root-knot nematodes factor (Nem Factor) via a shared response pathway, albeit the Nem factor is not required for parasitism but enhance it. Pierre Abad identified that plant actin cytoskeleton and in particular formin genes are involved in the feeding site (formation of giant cells) induced by root-knot nematodes.

A very special moment of the XII International Congress on MPMI was a moving ceremony led by Ann Hirsch, commemorating the late Georges Truchet for his outstanding contributions to understand Rhizobium-legume interactions, among these the identification of Nod factors.

The fact that this congress actually happened in spite of raging winds of defeat and became a scientific success was due to the strong will and commitment of many members of the MPMI Society and attendees that came to Mérida, on top of additional personal and economical costs. This is also a clear sign of scientific robustness and maturity of the MPMI Community which defeated extreme adverse conditions and the temptation to surrender. In fact, this congress was the integrative operation of valued heroic individual actions that contributed to the fulfillment of this major task.

Thanks to Sheng-Yang He and Ken Shirasu who enthusiastically organized a workshop directed to Graduate Students and Postdoctoral Scientists based on careers choices and opportunities, and to Jonathan Walton and Carroll Vance, who wrote grants for student scholarships. Special thanks to Roberto Ruíz, Beatriz Xoconostle and the staff from the printing facilities at CINVESTAV, who took as a personal task, far beyond duty, twice printing and pasting the book of abstracts.

Thanks to Amy Hope and Kim Flanegan and the rest of the staff at MPMI headquarters for logistic support, the authorities of UNAM, CINVESTAV, CONACYT and the Mexican Biochemical Society in Mexico who partially supported or loaned seeding funds for the Congress. To colleagues from CICY, the local research center in Merida. Thanks to the European Molecular Biology Organization (EMBO), the American Society for Microbiology (ASM), and CAMBIA who provided scholarships for speakers and students from developing countries. Thanks to our local committee, to the excellent secretarial support from Teresa Castillo, and from our students and young scientists who enthusiastically assisted the sessions. Many thanks to Nayeli Quinto ( who made the splendid design of the web page and the cover for the book of abstracts also in CD format. The professional and diligent effort from Ricardo Ciria, Alma Valle, Antonia Olivares and Mario Trejo from IBT-UNAM, is kindly acknowledged. Special thanks to the local authorities of Yucatán who provided logistic support and entertainment during the Congress.

Last but not least, a special mention to those colleagues, students and postdoctoral scientists, who came to Cancun in July, and finally could not make it to Mérida in December, for health conditions, adverse weather conditions, or personal and economical reasons, but nevertheless invested effort, time and money in the attempt. In some cases, registration money was never asked to be refunded, as a conscious altruistic contribution to sustain the congress. All these individual efforts and donations were extremely valuable because this congress had double expenses and was held with half of the resources originally planned and received for the Cancun event.

Federico Sánchez, Chair of the Congress, Coeditor
Carmen Quinto, Coeditor
Isabel López-Lara, Coeditor
Otto Geiger, Coeditor
Universidad Nacional Autónoma de México, Cuernavaca

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The International Society for Molecular Plant-Microbe Interactions