Secretomic climax in plant–fungal interactions
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9th European Conference on Fungal Genetics, Edinburgh, April, 2008
The 9th European Conference on Fungal Genetics (ECFG9) was held in the beautiful city of Edinburgh (UK). Over five-hundred participants assembled in the venerable halls of the University of Edinburgh to learn about current progress in the field of fungal genetics research. An excellent scientific programme accompanied by a catalogue of social events made for a memorable meeting. Malt whisky tasting, Scottish dancing and the organizing committee dressed in kilts at the conference dinner are just some examples of what was experienced by the delegates during their stay in Edinburgh.
‘... small secreted proteins could act as putative effectors manipulating plant detection systems to establish compatible interactions’
Recent insights in plant–fungal interactions
The ECFG covers a wide range of fungal genetics and molecular research. However, one striking observation from this year's meeting was the considerable number of plant–fungal interaction presentations and posters given, underlining just how fast this particular field is advancing. During the morning plenary sessions, Nick Talbot (University of Exeter, UK) brought us more details about the biology of the model pathogen, Magnaporthe grisea, responsible for rice blast disease. He described through cytological approaches the mitotic control that takes place during the development of the appressorium, the specialized infection structure necessary for successful plant infection. The extensive data now available for this plant pathogen makes it an excellent model, which contributes greatly to our understanding of the plant infection process. Francis Martin (INRA Nancy, France) presented some of the typical features found in the coding space of the genome of the symbiotic fungus Laccaria bicolor. He reported the presence of hundreds of L. bicolor-specific genes coding for small secreted proteins, some being highly regulated in the symbiotic tissues compared with free-living mycelium. Using immunolocalization it has been demonstrated that one promising mycorrhiza-induced small secreted protein (MISSP7) accumulated in the Hartig net, the plant–fungal mixed tissue where nutrient exchange takes place between the two partners of the mutualistic association (Martin et al., 2008).
The inside men – getting deeper into fungal cells
Living cell biology has reached an amazing level of description for biological systems. Aside from the obvious scientific reasons, one not in touch with the latest developments in this fast-moving field of science may have felt like they were viewing a ‘Hollywood blockbuster’ with special effects when watching some of the presentations in this session. At every ECFG, cell biologists are able to take us deeper into fungal cells, allowing us to experience ‘live’ highly complex biological mechanisms. In this way, several talks from the plenary morning sessions made quite an impression on the ECFG9 audience. For example, the ‘ping-pong’ response of Green Fluorescent Protein (GFP)-labeled Mitogen Activated Protein (MAK2) kinase at the tips of anastomosis tubes during Neurospora crassa conidial germling fusion presented by Nick Read (University of Edinburgh, UK) was one excellent example of how close we can now get to a biological process. Gero Steinberg (University of Exeter, UK) introduced the laser-excitation microscopy device – that he stated as being the most sensitive – which provides a promising technique for deciphering the cytoskeleton dynamics in hyphal growth using the plant pathogen model Ustilago maydis. For the first time the number of dynein units delivered to the tips of developing fungal cells, to remain there or go backwards on microtubules, was quantified. Such counting was made possible by approximating the level of fluorescence of a single GFP unit signal in an ingenious counting system set on the nucleopore protein complex. These developments, together with new imaging technologies and the improved quality of fluorescent labeling, will strongly influence the field of fungal biology in the future.
Secretomics: towards the holy grail of plant–fungal interactions
In recent years, the search for secreted polypeptide effector molecules has emerged as one of the ‘hot’ topics in plant–microbe interactions. Just glancing at the meeting abstracts it could be seen that this trend has reached a new climax. After the publication of several fungal genomes belonging to species that interact with plants, comparative genomics is now offering more clues and leads about how a microbe is able to breach its host defense and establish a biotrophic interaction or gain access to its nutrient source. One of the key areas involved in controlling plant–fungal interactions is the extracellular interface between the two organisms. The complex interplay among signals, cell-wall-degrading and/or remodelling enzymes, and small secreted proteins that are released into this extracellular space and eventually delivered into the host cell, determines the ability, or inability, of the host to trigger its defense system against the invading organism (de Witt, 2007). The comprehension of general mechanisms controlling such events is yet to be reached owing to the high diversity of secreted proteins described in the proteomes of many fungal species interacting with plants. However, recent publications have identified impressive catalogs of small secreted proteins expressed during plant–fungus interactions (Catanzariti et al., 2006; Kämper et al., 2006; Martin et al., 2008). More candidates presented at the ECFG9 in symbiont or pathogen fungal species extended this list (see below). The number of specific genes described for a given species is quite striking and may underline the specificity of the interaction process between a pathogen and a symbiont, or between distinct pathogens and their plant-hosts after a long-lasting interaction history. To date, very few small fungal-secreted proteins have been explored in great detail; however, more studies are expected after in-depth examination of fungal genome contents (Mueller et al., 2008a,b). The New Phytologist Trust will support a symposium entitled ‘Effectors in plant–microbe interactions’ in September 2009 with the aim of developing a better understanding of effector proteins.
Many of the presentations given in the plant–fungal interaction parallel session stated the importance of modulating the plant defense response or recognition of the symbiotic partner in pathogenic as well as in mutualistic interactions. The experimental set ups included examination of whole secretomes (U. maydis, L. bicolor), immunolocalization, the yeast-based signal sequence trap (YSST), sequencing of tissue-specific cDNA libraries and infection studies with bacterial type III delivery vectors. The Yeast 2 Hybrid system, which suffered bad press as a result of its tedious screening process and high false-positive rates, appears to have experienced a renaissance, with many presentations and posters utilizing this approach to track proteins targeted by fungal effectors in planta.
Regine Kahmann (Max Planck Institute, Marburg, Germany) presented the latest advances concerning gene clusters from U. maydis that encode secreted proteins. Candidate genes expressed during plant infection influence pathogenicity of the fungus, and the selective deletion of specific clusters or genes impacts upon the distinct defense responses from the plant (Kämper et al., 2006). Kahmann described, in great detail, how these secreted effectors may shape the interaction with the host after secretion. The research she presented addressed the localization of secreted effector molecules, their interacting partners and how plant defense responses are impacted. Several genes encode proteins presumably translocated into the nucleus of the host plant cell, and a deletion strain of an interesting candidate fails to penetrate and elicits plant cell death, thus suggesting a possible involvement in the suppression of the host defense response. Despite these new insights into the biology of this model pathogen, Kahmann expressed caution because most of the data were obtained using heterologous systems rather than from experiments performed in the host plant, which is necessary to demonstrate unequivocally the presumed roles of these U. maydis-secreted proteins.
Several other presentations focused on the latest advances in our understanding of different plant–fungal interactions (with both pathogens and symbionts); these also described repertoires of secreted proteins. Richard O’Connell (Max Planck Institute, Köln, Germany) and Barbara Valent (Kansas State University, Manhattan, USA) reported sets of small secreted proteins in Colleotrichium higginsianum and M. grisea, respectively, which could contain putative effector genes. Similarly, Natalia Requena (University of Karlsruhe, Germany) presented prospective work that aims to identify secreted proteins from the endomycorrhizal fungus, Glomus intraradices, which could have roles in the control of the plant defense response and orientation to symbiosis development. She introduced the use of a yeast signal sequence trap method (Lee et al., 2006) to identify effectors in presymbiotic and symbiotic tissues of Medicago truncatula.
Ecogenomic perspectives with more basidiomycete genomes to come
Understanding the growth, health and physiology of plants in nature cannot be achieved without taking into account the diversity of microbial species that interact continuously in their changing ecosystems. An excellent example of this is the Populus community genome project that aims to sequence not only the genome of the model tree species Populus trichocarpa but also those of several microbes interacting with this tree (Martin et al., 2004, 2008; Tuskan et al., 2006). Following the close of the ECFG9, Basidio3, the third basidiomycete satellite meeting, was held. A statement was made by Igor Grigoriev (Joint Genome Institute (JGI), USA) about the basidiomycete genomes that would be sequenced in the coming years at the JGI. After this opening contribution, several presentations discussed future studies where these genomes would aid or underpin the research. The fungal genome projects presented included saprotrophs, such as the brown-rot Postia placenta, the dry-rot Serpula lacrymans and Pleurotus ostreatus; the gymnosperm root pathogen Heterobasidion annosum and the poplar leaf rust Melampsora larici-populina; and the ectomycorrhizal symbiont Paxillus involutus. All these species play major roles in diverse niches of terrestrial ecosystems and greatly impact upon diversity through direct interactions – beneficial or detrimental – or through mobilization and cycling of organic matter. Such progress in the genomics of basidiomycetes will provide an unparalleled opportunity to identify key points in interspecific and fungal–environment interactions and increase our understanding of how these species effect the functioning of the ecosystem (Martin & Slater, 2007).
The shape of things to come
Recent research in the field of plant–fungal interactions has drawn attention to the possibility that small secreted proteins could act as putative effectors manipulating plant detection systems to establish compatible interactions. Over the last few years fungal genomics has uncovered hundreds of candidate effectors of potential interest to the scientific community. These not only help in deciphering the mechanisms that underlie plant–fungal interactions but also in addressing crucial issues such as compatibility and recognition. As shown at the ECFG9, postgenomic studies are now underway in model plant–fungus interactions, and there is more to come from other models that are benefiting from recent genomic efforts. These studies, together with comparative genomics (possibly with more fungal genomes to come), provide a very promising outlook for the ECFG10.