(Author for correspondence: tel +1 250 404 3318; email David.Joly@agr.gc.ca)
30th New Phytologist Symposium: Immunomodulation by Plant-associated Organisms, in Fallen Leaf Lake, California, USA, September 2012
Many organisms such as bacteria, fungi, oomycetes, nematodes and insects grow, feed and/or reproduce in close association with plant hosts. To establish such intimate interactions, symbionts (either mutualistic or parasitic) secrete effectors into host tissues, which are molecules that modulate plant cell structures and processes (Win et al., 2012a). This last decade, advances in genomics have revealed that symbionts possess dozens to hundreds of effectors. Currently, the field is moving rapidly from effector identification towards effector characterization, which provides a better understanding of how these effectors promote the establishment of a successful relationship with host plants. The 30th New Phytologist Symposium clearly illustrated this theme, as an international panel of c. 150 scientists was brought together to discuss current efforts to decipher effector functions within a wide range of biological systems. The remote location of the meeting in the Sierra Nevada mountains of California, USA, promoted lively discussions between participants during and after the sessions, but also via social networks (the whole conference was covered by a twitter feed, #30NPS tag, available on http://storify.com/KamounLab/30th-new-phytologist-symposium-immunomodulation-by).
The Symposium consisted of five sessions: pathogenomics, effector secretion and trafficking, induction and suppression of host immunity, structural biology of effectors and their targets, and emerging systems. In this meeting report, we focus on three main aspects or ‘take home messages’ discussed during the conference: (1) the identification of effector partners in plants; (2) recent advances facilitated by live-cell imaging and structural biology; and (3) emerging systems in the field.
‘Omics’ approaches to identify effectors and their plant interactors
Advances in next-generation sequencing technologies have allowed the rapid identification of effectors from diverse phytopathogens. Brian Staskawicz (University of California, Berkeley, CA, USA) kicked off the ‘Pathogenomics’ session and presented a genomics approach to track the cassava bacterial blight outbreaks caused by various Xanthomonas strains. Particularly, high-throughput genomic sequencing of these Xanthomonas field strains collected across three continents and over the last 70 yr has led to the identification of a set of conserved effector genes. In the future, these core effectors will serve as molecular probes to find resistance genes in plants, ultimately providing powerful tools to combat cassava bacterial blight (Bart et al., 2012). Richard Michelmore (University of California, Davis, CA, USA) also applied a similar comparative genomics approach to identify effectors from various downy mildew species. While genome analyses and comparative genomics approaches are facilitating the identification of effectors, the next challenge in the field is to functionally characterize these effectors.
Identifying effector targets in plants represents one first step towards effector characterization. The effector field has made significant headway as evidenced by the many effector interactors presented at this meeting. Large-scale protein–protein interaction screen using yeast two-hybrid, while labor-intensive, has led to the successful identification of various effector interactors in plants (Mukhtar et al., 2011). Additionally, Savithramma Dinesh-Kumar (University of California, Davis, CA, USA) presented a high-throughput method to identify effector interactors using protein microarrays (Popescu et al., 2007). However, while we now have various methodologies to identify such interactors in plants, Sophien Kamoun (The Sainsbury Laboratory, Norwich, UK) brought up a key point that not all effector-associated proteins should be considered as targets. He proposed to distinguish between plant proteins that are required for effector function vs those plant proteins that are modulated by effectors. In other words, plant proteins that activate effectors, serve as cofactors, or enable effectors to traffic inside host tissues should be considered as ‘effector helpers’. By contrast, plant proteins that are directly modulated by an effector to promote the ability of the symbiont to colonize and spread on its host would represent bona fide ‘effector targets’ (Win et al., 2012a).
Insights from live-cell imaging and structural biology
Within the last 3 yr, live-cell imaging technology made it possible to study the dynamic localization of effectors in infected plant cells, as well as the resulting structural and molecular rearrangement of plant compartments. Sophien Kamoun (The Sainsbury Laboratory, Norwich, UK) illustrated how this method can provide insights into plant focal secretion during immunity and how some effectors can interfere with this process. Notably, he presented the relocalization of the Phytophthora infestans effector Avrblb2 around haustoria during cell infection, demonstrating that effectors can be used as molecular probes to dissect plant immune responses (Bozkurt et al., 2011). As well, Barbara Valent (Kansas State University, Manhattan, KS, USA) and Nick Talbot (University of Exeter, Exeter, UK) used live-cell imaging to disentangle mechanisms of targeted secretion of rice blast effectors into plant tissues. By using specific inhibitors, they showed that apoplastic and cytoplasmic effectors follow distinct secretory pathways in Magnaporthe oryzae hyphae. Interestingly, using reciprocal swapping experiments, they revealed that promoters of rice blast effectors determine whether the effectors are localized to the cytoplasmic or the apoplastic compartments in plants.
Structural biology is another emerging method in the field of effector biology, and recently provided a stream of novel data on eukaryotic effectors. Notably, Mark Banfield (John Innes Center, Norwich, UK) presented an elegant comparison of recently-solved tridimensional structures of several RxLR effectors from oomycetes. He showed that these effectors possess WY domains that do not share obvious sequence homology but adopt a similar tridimensional fold. This core structural organization is currently viewed as a common backbone around which effectors diversify during evolution (Win et al., 2012b). Apart from oomycete RxLR effectors, the identification of effector genes in microbial genomes has been difficult, due to high sequence variability and the lack of conserved domains. Structure-based identification of shared and conserved folds between effectors would greatly aid their identification. For instance, many candidate effectors from the poplar rust fungus Melampsora larici-populina are organized in large families that present almost no sequence homology, apart from highly conserved patterns of cysteines (Sébastien Duplessis, INRA, Nancy, France). Such conserved cysteines could be involved in disulfide bridges serving as core backbones supporting effector diversification (Hacquard et al., 2012).
Effectors everywhere: emerging systems
Effectors were first identified in pathogenic bacteria as avirulence determinants, and later demonstrated to be virulence factors (Staskawicz, 2012). Since then, effectors have been identified in a variety of plant pathogenic microorganisms, including fungi, oomycetes or nematodes. Recently, effectors were also discovered in insect pests such as aphids (Bos et al., 2010) and even in mutualistic fungi (Kloppholz et al., 2011; Plett et al., 2011), where they appear to modulate the same ‘hubs’ within the plant immune system that are targeted by pathogens' effectors. Indeed, as described earlier, research in these emerging systems is also moving from effector identification to effector characterization. Francis Martin (INRA, Nancy, France) first showed how Mycorrhiza-induced Small Secreted Protein 7 (MiSSP7) from Laccaria bicolor interacts with poplar JAZ6, a nuclear-localized jasmonate receptor. He also demonstrated that MiSSP7 blocks JA signaling, thereby allowing the formation of the Hartig net, the site of nutrient exchange between ectomycorrhizal fungi and plant roots. Interestingly, MiSSP7 is highly conserved, unlike most effectors from plant pathogens that are often polymorphic. Natalia Requena (Karlsruhe Institute of Technology, Karlsruhe, Germany) then presented SP7, a repeat-containing protein from the arbuscular mycorrhizal fungus Glomus intraradices, that interacts with and reduces the expression of the transcription factor ERF19 (Ethylene-responsive Factor 19) in the plant nucleus. Another intriguing mutualistic fungus is Piriformospora indica, which presents an early biotrophic growth on its host, followed by a cell-death associated phase (Zuccaro et al., 2011). This biphasic lifestyle, at the crossroad of biotrophy and saprotrophy, prompted the deciphering of its host-adapted colonization strategies and the analysis of its candidate effectors (posters presented by Alga Zuccaro and Maryam Rafiqi). The study of such species that blur the frontiers of lifestyle categories of microbes (i.e. biotrophs, hemibiotrophs, necrotrophs or saprotrophs) will undoubtedly provide tremendous insights into the spatial and temporal deployment of effectors associated with a particular colonization strategy.
Newcomers among the effector-bearing plant-associated organisms also include Gram-positive intracellular bacteria, phytoplasmas. Phytoplasmas do not possess a type III secretion system and deliver effectors via a Sec-dependent mechanism of translocation. Saskia Hogenhout (John Innes Center, Norwich, UK) showed that three of the phytoplasma effectors induce a phenotypic change when expressed in Arabidopsis thaliana (MacLean et al., 2011). In particular, she described how SAP11 destabilizes class II TCP transcription factors, inducing the overproduction of stems (witches' brooms) and leaf crinkling (Sugio et al., 2011). This interaction cuts the production of JA induced by the insect vector. Another effector, SAP54, interacts with MADS-box transcription factors, thereby inducing the formation of leafy and indeterminate flowers (MacLean et al., 2011). By hacking transcription factors that regulate plant development, the phenotype imposed by these effectors to the plant extends to a third organism (i.e. the insect vector) to promote the transmission of phytoplasmas. Finally, the meeting ended with what are clearly the new kids in town: the candidate effectors that are being identified from parasitic plants. Ken Shirasu (RIKEN, Yokohama, Japan) described its large-scale genome and transcriptome analyses of Striga species (commonly known as witches weed) as well as the development of a model system using the hemiparasite Phtheirospermum japonicum. With the identification of effectors from mutualistic organisms, animals (insects and nematodes), and now from plants, effector biology is clearly moving from its ‘pathogenocentric’ view towards all plant-associated organisms, illustrating the many facets of this dynamically evolving research area.
The authors warmly acknowledge Holly Slater, Helen Pinfield-Wells and Jill Brooke from New Phytologist as well as Sophien Kamoun and Brian Staskawicz for the organization of the meeting. Furthermore, the authors would like to thank Sophien Kamoun and Francis Martin for helpful comments on the manuscript.