Brett M. Tyler

The Top 10 oomycete pathogens in molecular plant pathology

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.

Mechanisms and evolution of virulence in oomycetes.

Many destructive diseases of plants and animals are caused by oomycetes, a group of eukaryotic pathogens important to agricultural, ornamental, and natural ecosystems. Understanding the mechanisms underlying oomycete virulence and the genomic processes by which those mechanisms rapidly evolve is essential to developing effective long-term control measures for oomycete diseases. Several common mechanisms underlying oomycete virulence, including protein toxins and cell-entering effectors, have emerged from comparing oomycetes with different genome characteristics, parasitic lifestyles, and host ranges. Oomycete genomes display a strongly bipartite organization in which conserved housekeeping genes are concentrated in syntenic gene-rich blocks, whereas virulence genes are dispersed into highly dynamic, repeat-rich regions. There is also evidence that key virulence genes have been acquired by horizontal transfer from other eukaryotic and prokaryotic species.

A Phytophthora sojae Glycoside Hydrolase 12 Protein Is a Major Virulence Factor during Soybean Infection and Is Recognized as a PAMP

Phytophthora sojae produces the xyloglucan-degrading enzyme XEG1 as a virulence factor; this apoplastic effector is also recognized via the plant’s PAMP recognition machinery. We identified a glycoside hydrolase family 12 (GH12) protein, XEG1, produced by the soybean pathogen Phytophthora sojae that exhibits xyloglucanase and β-glucanase activity. It acts as an important virulence factor during P. sojae infection but also acts as a pathogen-associated molecular pattern (PAMP) in soybean (Glycine max) and solanaceous species, where it can trigger defense responses including cell death. GH12 proteins occur widely across microbial taxa, and many of these GH12 proteins induce cell death in Nicotiana benthamiana. The PAMP activity of XEG1 is independent of its xyloglucanase activity. XEG1 can induce plant defense responses in a BAK1-dependent manner. The perception of XEG1 occurs independently of the perception of ethylene-inducing xylanase. XEG1 is strongly induced in P. sojae within 30 min of infection of soybean and then slowly declines. Both silencing and overexpression of XEG1 in P. sojae severely reduced virulence. Many P. sojae RXLR effectors could suppress defense responses induced by XEG1, including several that are expressed within 30 min of infection. Therefore, our data suggest that PsXEG1 contributes to P. sojae virulence, but soybean recognizes PsXEG1 to induce immune responses, which in turn can be suppressed by RXLR effectors. XEG1 thus represents an apoplastic effector that is recognized via the plant’s PAMP recognition machinery.

Efficient disruption and replacement of an effector gene in the oomycete Phytophthora sojae using CRISPR/Cas9

Summary P hytophthora sojae is an oomycete pathogen of soybean. As a result of its economic importance, P . sojae has become a model for the study of oomycete genetics, physiology and pathology. The lack of efficient techniques for targeted mutagenesis and gene replacement have long hampered genetic studies of pathogenicity in Phytophthora species. Here, we describe a CRISPR/Cas9 system enabling rapid and efficient genome editing in P . sojae. Using the RXLR effector gene Avr4/6 as a target, we observed that, in the absence of a homologous template, the repair of Cas9‐induced DNA double‐strand breaks (DSBs) in P . sojae was mediated by non‐homologous end‐joining (NHEJ), primarily resulting in short indels. Most mutants were homozygous, presumably as a result of gene conversion triggered by Cas9‐mediated cleavage of non‐mutant alleles. When donor DNA was present, homology‐directed repair (HDR) was observed, which resulted in the replacement of Avr4/6 with the NPT II gene. By testing the specific virulence of several NHEJ mutants and HDR‐mediated gene replacements in soybean, we have validated the contribution of Avr4/6 to recognition by soybean R gene loci, Rps4 and Rps6, but also uncovered additional contributions to resistance by these two loci. Our results establish a powerful tool for the study of functional genomics in Phytophthora, which provides new avenues for better control of this pathogen.

A paralogous decoy protects Phytophthora sojae apoplastic effector PsXEG1 from a host inhibitor

Host-pathogen point-counterpoint The arms race between pathogen and host is a well-known phenomenon. Ma et al. have now identified how an enzymatically inactive protein can abet a pathogens infectivity. The pathogenic oomycete Phytophthora sojae secretes xyloglucanase that damages soybean cell walls. Soybean, in turn, secretes a defense protein that binds to and inactivates the xyloglucanase. To counteract this plant defense, the oomycete deploys a product of its own gene duplication: an inactive enzyme that binds the plants defense protein. With the defense protein unproductively bound to the decoy, the oomycete can successfully invade the soybean cells. Science, this issue p. 710 A soybean pathogen releases an extracellular protein that can neutralize plant defenses. The extracellular space (apoplast) of plant tissue represents a critical battleground between plants and attacking microbes. Here we show that a pathogen-secreted apoplastic xyloglucan-specific endoglucanase, PsXEG1, is a focus of this struggle in the Phytophthora sojae–soybean interaction. We show that soybean produces an apoplastic glucanase inhibitor protein, GmGIP1, that binds to PsXEG1 to block its contribution to virulence. P. sojae, however, secretes a paralogous PsXEG1-like protein, PsXLP1, that has lost enzyme activity but binds to GmGIP1 more tightly than does PsXEG1, thus freeing PsXEG1 to support P. sojae infection. The gene pair encoding PsXEG1 and PsXLP1 is conserved in many Phytophthora species, and the P. parasitica orthologs PpXEG1 and PpXLP1 have similar functions. Thus, this apoplastic decoy strategy may be widely used in Phytophthora pathosystems.

Crystal structure of the effector AvrLm4-7 of Leptosphaeria maculans reveals insights into its translocation into plant cells and recognition by resistance proteins.

The avirulence gene AvrLm4-7 of Leptosphaeria maculans, the causal agent of stem canker in Brassica napus (oilseed rape), confers a dual specificity of recognition by two resistance genes (Rlm4 and Rlm7) and is strongly involved in fungal fitness. In order to elucidate the biological function of AvrLm4-7 and understand the specificity of recognition by Rlm4 and Rlm7, the AvrLm4-7 protein was produced in Pichia pastoris and its crystal structure was determined. It revealed the presence of four disulfide bridges, but no close structural analogs could be identified. A short stretch of amino acids in the C terminus of the protein, (R/N)(Y/F)(R/S)E(F/W), was well-conserved among AvrLm4-7 homologs. Loss of recognition of AvrLm4-7 by Rlm4 is caused by the mutation of a single glycine to an arginine residue located in a loop of the protein. Loss of recognition by Rlm7 is governed by more complex mutational patterns, including gene loss or drastic modifications of the protein structure. Three point mutations altered residues in the well-conserved C-terminal motif or close to the glycine involved in Rlm4-mediated recognition, resulting in the loss of Rlm7-mediated recognition. Transient expression in Nicotiana benthamiana (tobacco) and particle bombardment experiments on leaves from oilseed rape suggested that AvrLm4-7 interacts with its cognate R proteins inside the plant cell, and can be translocated into plant cells in the absence of the pathogen. Translocation of AvrLm4-7 into oilseed rape leaves is likely to require the (R/N)(Y/F)(R/S)E(F/W) motif as well as an RAWG motif located in a nearby loop that together form a positively charged region.

Phylogenetic and transcriptional analysis of an expanded bZIP transcription factor family in Phytophthora sojae

BackgroundBasic leucine zipper (bZIP) transcription factors are present exclusively in eukaryotes and constitute one of the largest and most diverse transcription factor families. The proteins are responsible for central developmental and physiological processes in plants, animals, and fungi, including the pathogenicity of fungal plant pathogens. However, there is limited understanding of bZIPs in oomycetes, which are fungus-like organisms in the kingdom Stramenopila. Oomycetes include many destructive plant pathogens, including the well-studied species Phytophthora sojae, which causes soybean stem and root rot.ResultsCandidate bZIPs encoded in the genomes of P. sojae and four other oomycetes, two diatoms, and two fungal species were predicted using bioinformatic methods. Comparative analysis revealed expanded numbers of bZIP candidates in oomycetes, especially the Phytophthora species, due to the expansion of several novel bZIP classes whose highly conserved asparagines in basic DNA-binding regions were substituted by other residues such as cysteine. The majority of these novel bZIP classes were mostly restricted to oomycetes. The large number of novel bZIPs appears to be the result of widespread gene duplications during oomycete evolution. The majority of P. sojae bZIP candidates, including both conventional and novel bZIP classes, were predicted to contain canonical protein secondary structures. Detection of gene transcripts using digital gene expression profiling and qRT-PCR suggested that most of the candidates were not pseudogenes. The major transcriptional shifts of bZIPs occurred during the zoosporangia/zoospore/cyst and host infection stages. Several infection-associated bZIP genes were identified that were positively regulated by H2O2 exposure.ConclusionsThe identification of large classes of bZIP proteins in oomycetes with novel bZIP motif variants, that are conserved and developmentally regulated and thus presumably functional, extends our knowledge of this important family of eukaryotic transcription factors. It also lays the foundation for detailed studies of the roles of these proteins in development and infection in P. sojae and other oomycetes.

Phytophthora megakarya and Phytophthora palmivora, Closely Related Causal Agents of Cacao Black Pod Rot, Underwent Increases in Genome Sizes and Gene Numbers by Different Mechanisms

Phytophthora megakarya (Pmeg) and Phytophthora palmivora (Ppal) are closely related species causing cacao black pod rot. Although Ppal is a cosmopolitan pathogen, cacao is the only known host of economic importance for Pmeg. Pmeg is more virulent on cacao than Ppal. We sequenced and compared the Pmeg and Ppal genomes and identified virulence-related putative gene models (PGeneM) that may be responsible for their differences in host specificities and virulence. Pmeg and Ppal have estimated genome sizes of 126.88 and 151.23 Mb and PGeneM numbers of 42,036 and 44,327, respectively. The evolutionary histories of Pmeg and Ppal appear quite different. Postspeciation, Ppal underwent whole-genome duplication whereas Pmeg has undergone selective increases in PGeneM numbers, likely through accelerated transposable element-driven duplications. Many PGeneMs in both species failed to match transcripts and may represent pseudogenes or cryptic genetic reservoirs. Pmeg appears to have amplified specific gene families, some of which are virulence-related. Analysis of mycelium, zoospore, and in planta transcriptome expression profiles using neural network self-organizing map analysis generated 24 multivariate and nonlinear self-organizing map classes. Many members of the RxLR, necrosis-inducing phytophthora protein, and pectinase genes families were specifically induced in planta. Pmeg displays a diverse virulence-related gene complement similar in size to and potentially of greater diversity than Ppal but it remains likely that the specific functions of the genes determine each species’ unique characteristics as pathogens.

Genetic resources for advanced biofuel production described with the Gene Ontology

Dramatic increases in research in the area of microbial biofuel production coupled with high-throughput data generation on bioenergy-related microbes has led to a deluge of information in the scientific literature and in databases. Consolidating this information and making it easily accessible requires a unified vocabulary. The Gene Ontology (GO) fulfills that requirement, as it is a well-developed structured vocabulary that describes the activities and locations of gene products in a consistent manner across all kingdoms of life. The Microbial ENergy processes Gene Ontology () project is extending the GO to include new terms to describe microbial processes of interest to bioenergy production. Our effort has added over 600 bioenergy related terms to the Gene Ontology. These terms will aid in the comprehensive annotation of gene products from diverse energy-related microbial genomes. An area of microbial energy research that has received a lot of attention is microbial production of advanced biofuels. These include alcohols such as butanol, isopropanol, isobutanol, and fuels derived from fatty acids, isoprenoids, and polyhydroxyalkanoates. These fuels are superior to first generation biofuels (ethanol and biodiesel esterified from vegetable oil or animal fat), can be generated from non-food feedstock sources, can be used as supplements or substitutes for gasoline, diesel and jet fuels, and can be stored and distributed using existing infrastructure. Here we review the roles of genes associated with synthesis of advanced biofuels, and at the same time introduce the use of the GO to describe the functions of these genes in a standardized way.

Efficient Genome Editing in the Oomycete Phytophthora sojae Using CRISPR/Cas9

Phytophthora is a filamentous fungus‐like microorganism, but belongs to the oomycetes, in the kingdom Stramenopila. Phytophthora species are notorious as plant destroyers, causing multibillion‐dollar damage to agriculture and natural ecosystems worldwide annually. For a long time, genome editing has been unattainable in oomycetes, because of their extremely low rate of homologous recombination. The recent implementation of the CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR‐associated) system in the soybean pathogen Phytophthora sojae, an experimental model for oomycetes, has opened up a powerful new research capability for the oomycete community. Here, we describe a detailed protocol for CRISPR/Cas9‐mediated genome editing in P. sojae, including single guide RNA (sgRNA) design and construction, efficient gene replacement, and mutant‐screening strategies. This protocol should be generally applicable for most culturable oomycetes. We also describe an optimized transformation method that is useful for other Phytophthora spp. including P. capsici and P. parasitica.