Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

Many oomycete and fungal plant pathogens are obligate biotrophs, which extract nutrients only from living plant tissue and cannot grow apart from their hosts. Although these pathogens cause substantial crop losses, little is known about the molecular basis or evolution of obligate biotrophy. Here, we report the genome sequence of the oomycete Hyaloperonospora arabidopsidis (Hpa), an obligate biotroph and natural pathogen of Arabidopsis thaliana. In comparison with genomes of related, hemibiotrophic Phytophthora species, the Hpa genome exhibits dramatic reductions in genes encoding (i) RXLR effectors and other secreted pathogenicity proteins, (ii) enzymes for assimilation of inorganic nitrogen and sulfur, and (iii) proteins associated with zoospore formation and motility. These attributes comprise a genomic signature of evolution toward obligate biotrophy.

Plants and animals employ innate immune systems to cope with microbial infection. Pattern-triggered immunity relies on the recognition of microbe-derived patterns by pattern recognition receptors (PRRs). Necrosis and ethylene-inducing peptide 1-like proteins (NLPs) constitute plant immunogenic patterns that are unique, as these proteins are produced by multiple prokaryotic (bacterial) and eukaryotic (fungal, oomycete) species. Here we show that the leucine-rich repeat receptor protein (LRR-RP) RLP23 binds in vivo to a conserved 20-amino-acid fragment found in most NLPs (nlp20), thereby mediating immune activation in Arabidopsis thaliana. RLP23 forms a constitutive, ligand-independent complex with the LRR receptor kinase (LRR-RK) SOBIR1 (Suppressor of Brassinosteroid insensitive 1 (BRI1)-associated kinase (BAK1)-interacting receptor kinase 1), and recruits a second LRR-RK, BAK1, into a tripartite complex upon ligand binding. Stable, ectopic expression of RLP23 in potato (Solanum tuberosum) confers nlp20 pattern recognition and enhanced immunity to destructive oomycete and fungal plant pathogens, such as Phytophthora infestans and Sclerotinia sclerotiorum. PRRs that recognize widespread microbial patterns might be particularly suited for engineering immunity in crop plants.

Accessions of a plant species can show considerable genetic differences that are analyzed effectively by using recombinant inbred line (RIL) populations. Here we describe the results of genomewide expression variation analysis in an RIL population of Arabidopsis thaliana. For many genes, variation in expression could be explained by expression quantitative trait loci (eQTLs). The nature and consequences of this variation are discussed based on additional genetic parameters, such as heritability and transgression and by examining the genomic position of eQTLs versus gene position, polymorphism frequency, and gene ontology. Furthermore, we developed an approach for genetic regulatory network construction by combining eQTL mapping and regulator candidate gene selection. The power of our method was shown in a case study of genes associated with flowering time, a well studied regulatory network in Arabidopsis. Results that revealed clusters of coregulated genes and their most likely regulators were in agreement with published data, and unknown relationships could be predicted.natural variation

SUMMARYArabidopsis downy mildew resistant 6 (dmr6) mutants have lost their susceptibility to the downy mildew Hyaloperonospora arabidopsidis. Here we show that dmr6 is also resistant to the bacterium Pseudomonas syringae and the oomycete Phytophthora capsici. Resistance is accompanied by enhanced defense gene expression and elevated salicylic acid levels. The suppressive effect of the DMR6 oxygenase was confirmed in transgenic Arabidopsis lines overexpressing DMR6 that show enhanced susceptibility to H. arabidopsidis, P. capsici, and P. syringae. Phylogenetic analysis of the superfamily of 2-oxoglutarate Fe(II)-dependent oxygenases revealed a subgroup of DMR6-LIKE OXYGENASEs (DLOs). Within Arabidopsis, DMR6 is most closely related to DLO1 and DLO2. Overexpression of DLO1 and DLO2 in the dmr6 mutant restored the susceptibility to downy mildew indicating that DLOs negatively affect defense, similar to DMR6. DLO1, but not DLO2, is co-expressed with DMR6, showing strong activation during pathogen attack and following salicylic acid treatment. DMR6 and DLO1 differ in their spatial expression pattern in downy mildew-infected Arabidopsis leaves; DMR6 is mostly expressed in cells that are in contact with hyphae and haustoria of H. arabidopsidis, while DLO1 is expressed mainly in the vascular tissues near infection sites. Strikingly, the dmr6-3_dlo1 double mutant, that is completely resistant to H. arabidopsidis, showed a strong growth reduction that was associated with high levels of salicylic acid. We conclude that DMR6 and DLO1 redundantly suppress plant immunity, but also have distinct activities based on their differential localization of expression.