In the high valley of the Arc river in Haute Maurienne, France, glacier rivers and threatening seracs make loud roaring sounds, the snow is slowly retreating from the alpine meadows and hundreds of plants are rushing to generate their seeds in these highlands where winter lasts for 8 months of the year. In walking through this colorful cornocupia of flowering plants, enchanting forests of dwarf willows and rock lichens competing for light and nutrients – a peaceful struggle for life – it is hard to believe that a war is taking place in the entangled vegetal crowd. Necrotic spots, blisters and yellow pustules are the visible testimony of the invasion of plant leaves and stems by deadly bacterial and fungal parasites. Pathogenic microbes interact with their host cells to create unique niches for replication and dissemination. The invasion of plant tissues by these pathogenic microbes relies on a battery of secreted molecules, including exotoxins, phytohormone-related metabolites and hydrolytic enzymes (e.g. cellulases, pectinases and proteases) acting on the host tissues. Mutualist symbionts also use secreted metabolites to modify the host expression (Felten et al., 2009). Studies developed over the last five years have demonstrated that secreted proteins also act as powerful effectors. These secreted protein effectors are emerging as the prime weapons and decoys of plant parasites and also as targets for host recognition and immunity (McCann & Guttman, 2009; Panstruga & Dodds, 2009). Effectors are defined as molecules that manipulate host-cell structure and function, thereby facilitating infection (virulence factors or toxins) and/or triggering defense responses (avirulence factors or elicitors) (Kamoun, 2007). This dual (and conflicting) activity of effectors has been broadly reported in many plant–microbial pathosystems. The research topic is currently investigated by employing a variety of approaches (biochemical, physiological and developmental), together with cellular biology, bioinformatics, functional genomics and proteomics, rapidly moving towards genome-wide and evolutionary analyses (Kamoun, 2007; Schornack et al., 2008; McCann & Guttman, 2009; Panstruga & Dodds, 2009; Kebdani et al., 2010; Khang et al., 2010).
However, despite tremendous progress in recent years many questions still remain unanswered. Very little is known about the genes that determine whether effectors can or cannot suppress the basal defence (Niks & Marcel, 2009). To understand how molecular information is exchanged among species, and how this information is related to the functional character of the plant and microbial interface, the 22nd New Phytologist Symposium ‘Effectors in plant–microbe interactions’ brought together scientists working on plant–microbe interactions across a range of organisms (viruses, bacteria, fungi and nematodes) (Lebrun & Kamoun, 2010). In addition to topical review papers (Ciuffetti et al., pp. 911–919; Genin, pp. 920–928; Terauchi and Yoshida, pp. 929–939), this issue of New Phytologist contains a feature that presents some of the most exciting results discussed at the symposium. The papers herein discuss the evolution (Chen et al., pp. 941–956; Genin, pp. 920–928; Khrunyk et al., pp. 957–968; Nguyen et al., pp. 969–982), secretion (Szczesny et al., 2010b; pp. 983–1002), trafficking (Wang et al., pp. 1003–1017) and the target (Macho et al., pp. 1018–1033; Manning et al., pp. 1034–1047; Römer et al., pp. 1048–1057; Szczesny et al., 2010a, pp. 1058–1074) of effectors.
It appears that bacteria and eukaryotic microbes share a common small protein-coded ‘language’ to interact with plants that is distinct from general nutrient exchange. Effectors of bacteria and filamentous micro-organisms (fungi and Oomycetes) act on a wide range of plant cellular processes (Kamoun, 2007; Genin, 2010; Manning et al., 2010; Römer et al., 2010). However, to alter the host cell, effectors must reach a host target. Apoplastic effectors reside in the plant extracellular space, but most effectors investigated enter the host cell. The ability of microbial proteins to gain access to the host-cell cytoplasm, and subsequently to organelles and the nucleus, is therefore a crucial step in pathogenesis and probably in mutualistic symbiosis. There are several cellular mechanisms by which this protein secretion and subsequent host-cell entry can take place. Bacterial proteins can be auto-transported, they can pass through the general secretory pathway, or most importantly from the standpoint of virulence, they can be secreted by one of several specialized mechanisms found in pathogenic bacteria (McCann & Guttman, 2009; Genin, 2010). Many Gram-negative bacterial pathogens encode type III secretion systems (T3S), syringe-like macromolecular complexes, to directly deliver a cocktail of effector proteins into the host cell (Cornelis & Van Gijsegem, 2000; McCann & Guttman, 2009; Genin, 2010). Type II secretion (T2S) systems could also promote disease and contribute to the translocation of effector proteins that are delivered into the plant cell by the T3S system (Szczesny et al., 2010b).
As plant-invading fungi have a common phylogenetic origin and a long history of co-evolution with plants, they probably share ancestral functions involved in interactions with host plants. Studies on genes/functions involved in pathogenicity and symbiosis have highlighted a particular class of effectors corresponding to secreted small proteins (SSP). Genome-wide surveys and transcript profilings have shown that several SSPs belong to protein families in Magnaporthe grisea (a fungal rice leaf pathogen) (Terauchi and Yoshida, 2010), Ustilago maydis (corn smut) (Khrunyk et al., 2010), Leptospheria maculans (a fungal rapeseed leaf/stem pathogen), Melampsora lini (flax rust), Laccaria bicolor (a mycorrhizal symbiont) (Martin et al., 2008; Martin & Selosse, 2008) and Oomycete Phythophtora spp. (Kamoun, 2007; Kebdani et al., 2010). Genome-wide transcriptomics experiments showed that M. grisea, U. maydis and L. bicolor genes, specifically expressed during infection, mainly encode SPPs. Together, these recent studies strongly suggested that SSPs play a role in the interactions between plant-invading fungi and their host. The main hypothesis is that symbiotic and pathogenic fungi mobilize a rich assortment of effector SSPs that interact with, or manipulate, host plants during infection or symbiosis. This research topic is currently investigated by a wide range of approaches combining bioinformatics, functional genomics (transcriptomics, reverse genetics, biochemical/biological assays) and proteomics. As hundreds of species of fungi undergo genome sequencing and annotation, we are moving rapidly toward genome-wide analyses of fungal effectors (Kamoun, 2007; Terauchi and Yoshida, 2010). In contrast to bacterial effectors, eukaryotic effector proteins are internalized into the plant cell in the absence of the fungus or oomycete, suggesting that they do not require a microbe-encoded transport mechanism. Several effectors of eukaryotic microbes, including biotrophic and hemibiotrophic fungi and oomycetes, appear to fuse to the host membranes via binding of an N-terminal motif, RXLR (arginine, any amino acid, leucine, arginine), to phosphoinositol-3-phosphate (PI3P) (Kale et al., 2010).
One of the most pressing questions in the study of effectors is to characterize their biochemical activities to understand how they alter plant processes and increase the reproductive success of the pathogen. Recent findings illustrate a diversity of effector structures and activities (Kamoun, 2007; Genin). Many of them act as suppressors of specific plant defense through enzyme activities, such as proteases and/or acetyltransferases (Macho et al.; Szczesny et al., 2010a). Some effectors, such as the soybean cyst nematode (Heterodera glycines) CLE effector proteins, can also alter host gene expression and development in mimicking morphogenetic plant peptides (Wang et al.). The high number and diversity of SSPs lead to the proposal that they interfere with many aspects of the plant metabolism to allow the efficiency of the pathogenic or mutualistic process to be optimized.
Recent data suggest that host plants also use this protein-based language to control invading mutualistic microbes. It has been suggested that correct symbiosome development in Medicago truncatula requires the orderly secretion of nodule-specific cysteine-rich (NCR) peptides that are targeted to the bacteria and enter the bacterial membrane and cytosol through the coordinated up-regulation of a nodule-specific pathway (Van de Velde et al., 2010; Wang et al., 2010). As several thousands of novel plant SSPs have been found in Arabidopsis and Poplar by RNA-Seq and protein profiling (X. Yang, G. Hurst, G. Tuskan, pers. comm.), I hypothesize that plants hosting eukaryotic microbes are not a silent partner in the pathogenic or mutualistic symbiotic equation. Rather, they too – through the use of SSPs – might manipulate the growth and development of invading bacterial and fungi within their tissues for their own protection or benefit.
The scientific rewards from comprehensive research programs on microbial effectors include a greater fundamental understanding of the interactions between organisms at the community level and benefit for sustainable agriculture. A deeper understanding of the complex array of factors affecting host–pathogen interactions and co-evolution could indeed ensure efficient targeting of parasite-control methods in forest and agricultural ecosystems.