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With the increasing availability of plant pathogen genomes, secreted proteins that aid infection (effectors) have emerged as key factors that help to govern plant-microbe interactions. The conserved CRN (CRinkling and Necrosis) effector family was first described in oomycetes by their capacity to induce host cell death. Despite recent advances towards the elucidation of CRN virulence functions, the relevance of CRN-induced cell death remains unclear. In planta over-expression of PcCRN83_152, a CRN effector from Phytophthora capsici, causes host cell death and boosts P. capsici virulence. We used these features to ask whether PcCRN83_152-induced cell death is linked to its virulence function. By randomly mutating this effector, we generated PcCRN83_152 variants with no cell death (NCD) phenotypes, which were subsequently tested for activity towards enhanced virulence. We showed that a subset of PcCRN83_152 NCD variants retained their ability to boost P. capsici virulence. Moreover, NCD variants were shown to have a suppressive effect on PcCRN83_152-mediated cell death. Our work shows that PcCRN83_152-induced cell death and virulence function can be separated. Moreover, if these findings hold true for other cell death-inducing CRN effectors, this work, in turn, will provide a framework for studies aimed at unveiling the virulence functions of these effectors.

Summary With the increasing availability of plant pathogen genomes, secreted proteins that aid infection (effectors) have emerged as key factors that help to govern plant–microbe interactions. The conserved CRN (CRinkling and Necrosis) effector family was first described in oomycetes by their capacity to induce host cell death. Despite recent advances towards the elucidation of CRN virulence functions, the relevance of CRN‐induced cell death remains unclear. In planta over‐expression of PcCRN83_152, a CRN effector from Phytophthora capsici, causes host cell death and boosts P. capsici virulence. We used these features to ask whether PcCRN83_152‐induced cell death is linked to its virulence function. By randomly mutating this effector, we generated PcCRN83_152 variants with no cell death (NCD) phenotypes, which were subsequently tested for activity towards enhanced virulence. We showed that a subset of PcCRN83_152 NCD variants retained their ability to boost P. capsici virulence. Moreover, NCD variants were shown to have a suppressive effect on PcCRN83_152‐mediated cell death. Our work shows that PcCRN83_152‐induced cell death and virulence function can be separated. Moreover, if these findings hold true for other cell death‐inducing CRN effectors, this work, in turn, will provide a framework for studies aimed at unveiling the virulence functions of these effectors.

SUMMARY Phytophthora capsici is a highly dynamic and destructive pathogen of vegetables. It attacks all cucurbits, pepper, tomato and eggplant, and, more recently, snap and lima beans. The disease incidence and severity have increased significantly in recent decades and the molecular resources to study this pathogen are growing and now include a reference genome. At the population level, the epidemiology varies according to the geographical location, with populations in South America dominated by clonal reproduction, and populations in the USA and South Africa composed of many unique genotypes in which sexual reproduction is common. Just as the impact of crop loss as a result of P. capsici has increased in recent decades, there has been a similar increase in the development of new tools and resources to study this devastating pathogen.Phytophthora capsici presents an attractive model for understanding broad-host-range oomycetes, the impact of sexual recombination in field populations and the basic mechanisms of Phytophthora virulence. Taxonomy: Kingdom Chromista; Phylum Oomycota; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; Genus Phytophthora; Species capsici. Disease symptoms: Symptoms vary considerably according to the host, plant part infected and environmental conditions. For example, in dry areas (e.g. southwestern USA and southern France), infection on tomato and bell or chilli pepper is generally

Long considered intractable organisms by fungal genetic research standards, the oomycetes have recently moved to the centre stage of research on plant-microbe interactions. Recent work on oomycete effector evolution, trafficking and function has led to major conceptual advances in the science of plant pathology. In this review, we provide a historical perspective on oomycete genetic research and summarize the state of the art in effector biology of plant pathogenic oomycetes by describing what we consider to be the 10 most important concepts about oomycete effectors. Fil: Schornack, Sebastian. The Sainsbury Laboratory; Reino Unido Fil: Huitema, Edgar. The Sainsbury Laboratory; Reino Unido Fil: Cano, Liliana M.. The Sainsbury Laboratory; Reino Unido Fil: Bozkurt, Tolga O.. The Sainsbury Laboratory; Reino Unido Fil: Oliva, Ricardo. The Sainsbury Laboratory; Reino Unido Fil: Van Damme, Mireille. The Sainsbury Laboratory; Reino Unido Fil: Schwizer, Simon. The Sainsbury Laboratory; Reino Unido Fil: Raffaele, Sylvain. The Sainsbury Laboratory; Reino Unido Fil: Chaparro Garcia, Angela. The Sainsbury Laboratory; Reino Unido Fil: Farrer, Rhys. The Sainsbury Laboratory; Reino Unido Fil: Segretin, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. The Sainsbury Laboratory; Reino Unido Fil: Bos, Jorunn. The Sainsbury Laboratory; Reino Unido Fil: Haas, Brian J.. Harvard University; Estados Unidos. Massachusetts Institute of Technology; Estados Unidos Fil: Zody, Michael C.. Massachusetts Institute of Technology; Estados Unidos. Harvard University; Estados Unidos Fil: Nusbaum, Chad. Massachusetts Institute of Technology; Estados Unidos. Harvard University; Estados Unidos Fil: Win, Joe. The Sainsbury Laboratory; Reino Unido Fil: Thines, Marco. The Sainsbury Laboratory; Reino Unido. Universidad de Hohenheim; Alemania Fil: Kamoun, Sophien. The Sainsbury Laboratory; Reino Unido

SUMMARY The molecular basis of non-host resistance, or species-specific resistance, remains one of the major unknowns in the study of plant-microbe interactions. In this paper, we describe the characterization of a non-host pathosystem involving the model plant Arabidopsis thaliana and the economically important and destructive oomycete pathogen Phytophthora infestans. Cytological investigations into the early stages of this interaction revealed the germination of P. infestans cysts on Arabidopsis leaves, direct penetration of epidermal cells, formation of infection vesicles and occasionally secondary hyphae, followed by a typical hypersensitive response. P. infestans biomass dynamics during infection of Arabidopsis was monitored using kinetic PCR, revealing an increase in biomass during the first 24 h after inoculation, followed by a decrease in the later stages. Transgenic reporter lines and RNA blot analyses were used to characterize the defence responses induced following P. infestans infection. Significant induction of PDF1.2 was observed at 48 h after inoculation, whereas elevated levels of PR gene expression were detected three days after inoculation. To further characterize this defence response, DNA microarray analyses were carried out to determine the expression profiles for c. 11 000 Arabidopsis cDNAs 16 h after infection. These analyses revealed a significant overlap between Arabidopsis non-host response and other defence-related treatments described in the literature. In particular, non-host response to P. infestans was clearly associated with activation of the jasmonate pathway. The described Arabidopsis-P. infestans pathosystem offers excellent prospects for improving our understanding of non-host resistance.

SUMMARY Transcripts encoding elicitin-like protein domains were identified from similarity searches of Phytophthora sojae expressed sequence tags and were characterized with regard to molecular structure and elicitor activity. The P. sojae elicitin family consists of at least nine genes with products similar to previously described elicitins (SOJA-2, SOJB, SOJ2, SOJ3, SOJ5, SOJ6 and SOJ7) or highly diverged from known sequences (SOJX and SOJY). The predicted structural features of seven (SOJA-2, SOJB, SOJ2, SOJ3, SOJ6, SOJX and SOJY) of the elicitin preproteins were compared. All of the predicted elicitins possess a leader signal sequence and a core elicitin domain. Five (SOJ2, SOJ3, SOJ6, SOJX and SOJY) of the characterized elicitins also contain a variable C-terminal region. In addition, SOJX and SOJY contain a C-terminal hydrophobic membrane-spanning domain. An analysis of expression patterns of the elicitin transcripts showed that SOJA-2, SOJB, SOJ2, SOJ3 and SOJ6 were expressed in axenically grown mycelia and during infection, but not in zoospores. In contrast, SOJX and SOJY were predominantly and specifically expressed in zoospores. Selected elicitin domains were also tested for the induction of the hypersensitive response (HR) in Nicotiana spp. All of the elicitin protein domains tested induced the HR, except for SOJX and SOJY. Overall, the results show that the P. sojae elicitin gene family is large and diverse, with varying patterns of expression and HR-inducing activity.

Phytophthora capsici is a highly dynamic and destructive pathogen of vegetables. It attacks all cucurbits, pepper, tomato and eggplant, and, more recently, snap and lima beans. The disease incidence and severity have increased significantly in recent decades and the molecular resources to study this pathogen are growing and now include a reference genome. At the population level, the epidemiology varies according to the geographical location, with populations in South America dominated by clonal reproduction, and populations in the USA and South Africa composed of many unique genotypes in which sexual reproduction is common. Just as the impact of crop loss as a result of P. capsici has increased in recent decades, there has been a similar increase in the development of new tools and resources to study this devastating pathogen. Phytophthora capsici presents an attractive model for understanding broad‐host‐range oomycetes, the impact of sexual recombination in field populations and the basic mechanisms of Phytophthora virulence. Taxonomy: Kingdom Chromista; Phylum Oomycota; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; Genus Phytophthora; Species capsici. Disease symptoms: Symptoms vary considerably according to the host, plant part infected and environmental conditions. For example, in dry areas (e.g. southwestern USA and southern France), infection on tomato and bell or chilli pepper is generally on the roots and crown, and the infected plants have a distinctive black/brown lesion visible at the soil line (Fig. 1). In areas in which rainfall is more common (e.g. eastern USA), all parts of the plant are infected, including the roots, crown, foliage and fruit (Fig. 1). Root infections cause damping off in seedlings, whereas, in older plants, it is common to see stunted growth, wilting and, eventually, death. For tomatoes, it is common to see significant adventitious root growth just above an infected tap root, and the stunted plants, although severely compromised, may not die. For many cucurbit fruit, the expanding lesions produce fresh sporangia over days (or even weeks depending on the size of the fruit) and the fruit often look as if they have been dipped in white powdered confectioner's sugar (Fig. 1). Generally, hyphae do not emerge from infected plants or fruit (common with Pythium infections) and all that is visible on the surface of an infected plant is sporangia. Importance: Phytophthora capsici presents an oomycete worst‐case scenario to growers as it has a broad host range, often produces long‐lived dormant sexual spores, has extensive genotypic diversity and has an explosive asexual disease cycle. It is becoming increasingly apparent that novel control strategies are needed to safeguard food production from P. capsici and other oomycetes. Considering that P. capsici is easy to grow, mate and manipulate in the laboratory and infects many plant species, this pathogen is a robust model for investigations, particularly those related to sexual reproduction, host range and virulence. Useful websites: Phytophthora capsici genome database: http://genome.jgi‐psf.org/Phyca11/Phyca11.home.html. Molecular tools to identify Phytophthora isolates: http://phytophthora‐id.org.

SUMMARY The molecular basis of non-host resistance, or species-specific resistance, remains one of the major unknowns in the study of plant-microbe interactions. In this paper, we describe the characterization of a non-host pathosystem involving the model plant Arabidopsis thaliana and the economically important and destructive oomycete pathogen Phytophthora infestans. Cytological investigations into the early stages of this interaction revealed the germination of P. infestans cysts on Arabidopsis leaves, direct penetration of epidermal cells, formation of infection vesicles and occasionally secondary hyphae, followed by a typical hypersensitive response. P. infestans biomass dynamics during infection of Arabidopsis was monitored using kinetic PCR, revealing an increase in biomass during the first 24 h after inoculation, followed by a decrease in the later stages. Transgenic reporter lines and RNA blot analyses were used to characterize the defence responses induced following P. infestans infection. Significant induction of PDF1.2 was observed at 48 h after inoculation, whereas elevated levels of PR gene expression were detected three days after inoculation. To further characterize this defence response, DNA microarray analyses were carried out to determine the expression profiles for c. 11 000 Arabidopsis cDNAs 16 h after infection. These analyses revealed a significant overlap between Arabidopsis non-host response and other defence-related treatments described in the literature. In particular, non-host response to P. infestans was clearly associated with activation of the jasmonate pathway. The described Arabidopsis-P. infestans pathosystem offers excellent prospects for improving our understanding of non-host resistance.

SUMMARY Transcripts encoding elicitin-like protein domains were identified from similarity searches of Phytophthora sojae expressed sequence tags and were characterized with regard to molecular structure and elicitor activity. The P. sojae elicitin family consists of at least nine genes with products similar to previously described elicitins (SOJA-2, SOJB, SOJ2, SOJ3, SOJ5, SOJ6 and SOJ7) or highly diverged from known sequences (SOJX and SOJY). The predicted structural features of seven (SOJA-2, SOJB, SOJ2, SOJ3, SOJ6, SOJX and SOJY) of the elicitin preproteins were compared. All of the predicted elicitins possess a leader signal sequence and a core elicitin domain. Five (SOJ2, SOJ3, SOJ6, SOJX and SOJY) of the characterized elicitins also contain a variable C-terminal region. In addition, SOJX and SOJY contain a C-terminal hydrophobic membrane-spanning domain. An analysis of expression patterns of the elicitin transcripts showed that SOJA-2, SOJB, SOJ2, SOJ3 and SOJ6 were expressed in axenically grown mycelia and during infection, but not in zoospores. In contrast, SOJX and SOJY were predominantly and specifically expressed in zoospores. Selected elicitin domains were also tested for the induction of the hypersensitive response (HR) in Nicotiana spp. All of the elicitin protein domains tested induced the HR, except for SOJX and SOJY. Overall, the results show that the P. sojae elicitin gene family is large and diverse, with varying patterns of expression and HR-inducing activity.