PsHint1, associated with the G-protein α subunit PsGPA1, is required for the chemotaxis and pathogenicity of Phytophthora sojae

A Novel FYVE Domain-Containing Protein Kinase, PsZFPK1, Plays a Critical Role in Vegetative Growth, Sporangium Formation, Oospore Production, and Virulence in Phytophthora sojae

Proteins containing both FYVE and serine/threonine kinase catalytic (STKc) domains are exclusive to protists. However, the biological function of these proteins in oomycetes has rarely been reported. In the Phytophthora sojae genome database, we identified five proteins containing FYVE and STKc domains, which we named PsZFPK1, PsZFPK2, PsZFPK3, PsZFPK4, and PsZFPK5. In this study, we characterized the biological function of PsZFPK1 using a CRISPR/Cas9-mediated gene replacement system. Compared with the wild-type strain, P6497, the PsZFPK1-knockout mutants exhibited significantly reduced growth on a nutrient-rich V8 medium, while a more pronounced defect was observed on a nutrient-poor Plich medium. The PsZFPK1-knockout mutants also showed a significant increase in sporangium production. Furthermore, PsZFPK1 was found to be essential for oospore production and complete virulence but dispensable for the stress response in P. sojae. The N-terminal region, FYVE and STKc domains, and T602 phosphorylation site were found to be vital for the function of PsZFPK1. Conversely, these domains were not required for the localization of PsZFPK1 protein in the cytoplasm. Our results demonstrate that PsZFPK1 plays a critical role in vegetative growth, sporangium formation, oospore production, and virulence in P. sojae.

Phase-specific transcriptional patterns of the oomycete pathogen Phytophthora sojae unravel genes essential for asexual development and pathogenic processes

Oomycetes are filamentous microorganisms easily mistaken as fungi but vastly differ in physiology, biochemistry, and genetics. This commonly-held misconception lead to a reduced effectiveness by using conventional fungicides to control oomycetes, thus it demands the identification of novel functional genes as target for precisely design oomycetes-specific microbicide. The present study initially analyzed the available transcriptome data of the model oomycete pathogen, Phytophthora sojae, and constructed an expression matrix of 10,953 genes across the stages of asexual development and host infection. Hierarchical clustering, specificity, and diversity analyses revealed a more pronounced transcriptional plasticity during the stages of asexual development than that in host infection, which drew our attention by particularly focusing on transcripts in asexual development stage to eventually clustered them into 6 phase-specific expression modules. Three of which respectively possessing a serine/threonine phosphatase (PP2C) expressed during the mycelial and sporangium stages, a histidine kinase (HK) expressed during the zoospore and cyst stages, and a bZIP transcription factor (bZIP32) exclusive to the cyst germination stage were selected for down-stream functional validation. In this way, we demonstrated that PP2C, HK, and bZIP32 play significant roles in P. sojae asexual development and virulence. Thus, these findings provide a foundation for further gene functional annotation in oomycetes and crop disease management.

An Oomycete-Specific Leucine-Rich Repeat-Containing Protein Is Involved in Zoospore Flagellum Development in Phytophthora sojae

A leucine-rich repeat (LRR) is a widespread structural motif of 20 to 30 amino acids with characteristic repetitive sequences rich in leucine. LRR-containing proteins are critical for ligand recognition and binding, participating in plant development and defense. Like plants, oomycetes also harbor genes encoding LRR-containing proteins, but their functions remain largely unknown. We identified a zoospore-upregulated gene from Phytophthora sojae with LRRs and an extra structural maintenance of chromosomes-like domain. We generated knockout and complemented knockout strains of this LRR protein and found that its deletion resulted in a pronounced reduction in zoospore mobility and chemotaxis, cyst germination, and virulence. Interestingly, micro-examination of zoospores under a scanning electron microscope revealed irregularly shaped zoospores without flagella in these deletion mutants. In addition, the reintroduction of this LRR protein into the knockout mutant reversed all the deficiencies. Our data demonstrate a critical role for the Phytophthora LRR protein in modulating zoospore development, which impairs migration to the host soybean and affects the spread of Phytophthora pathogens.

Signal and regulatory mechanisms involved in spore development of Phytophthora and Peronophythora

Oomycetes cause hundreds of destructive plant diseases, threatening agricultural production and food security. These fungus-like eukaryotes show multiple sporulation pattern including the production of sporangium, zoospore, chlamydospore and oospore, which are critical for their survival, dispersal and infection on hosts. Recently, genomic and genetic technologies have greatly promoted the study of molecular mechanism of sporulation in the genus Phytophthora and Peronophythora. In this paper, we characterize the types of asexual and sexual spores and review latest progress of these two genera. We summarize the genes encoding G protein, mitogen-activated protein kinase (MAPK) cascade, transcription factors, RNA-binding protein, autophagy-related proteins and so on, which function in the processes of sporangium production and cleavage, zoospore behaviors and oospore formation. Meanwhile, various molecular, chemical and electrical stimuli in zoospore behaviors are also discussed. Finally, with the molecular mechanism of sporulation in Phytophthora and Peronophythora is gradually being revealed, we propose some thoughts for the further research and provide the alternative strategy for plant protection against phytopathogenic oomycetes.

Maize and Common Bean Seed Exudates Mediate Part of Nonhost Resistance to Phytophthora sojae Prior to Infection

Phytophthora sojae does not infect nonhost maize (Zea mays) but infects nonhost common bean (Phaseolus vulgaris) under inoculation. Soybean seed exudates participate in mediating host resistance to P. sojae before infection. This study aims to elucidate the role of seed exudates in mediating the nonhost resistance of maize and common bean to P. sojae before infection. The behaviors of P. sojae zoospores in response to the seed exudates were determined using an assay chamber and a concave slide. The proteomes of P. sojae zoospores in response to the seed exudates were analyzed with the tandem mass tag method. The key proteins were quantitatively verified by parallel reaction monitoring. Maize seed exudates exerted a repellent effect on zoospores of P. sojae. This result explains why zoospores sense repelling signaling molecules in maize seed exudates that weaken and strongly inhibit chemotaxis signals in the phosphatidylinositol signaling pathway and arachidonic acid metabolism pathway. Common bean seed exudates did not exhibit any attraction to the zoospores because the guanine nucleotide-binding protein signaling pathway, which is responsible for transmitting chemotactic signals, had no significant change. The proteins protecting the cell membrane structure were significantly downregulated, and the early apoptosis signal glutathione was enhanced in zoospores responding to common bean seed exudates, which resulted in dissolution of the cysts. Maize and common bean seed exudates mediate part of the nonhost resistance to P. sojae via different mechanisms before infection. The immunity of maize to P. sojae is caused by the repellent effect of maize seed exudates on zoospores. Common bean seed exudates participate in mediating nonhost resistance by dissolving the cysts.

The Mevalonate Pathway Is Important for Growth, Spore Production, and the Virulence of Phytophthora sojae

The mevalonate (MVA) pathway in eukaryotic organisms produces isoprenoids, sterols, ubiquinone, and dolichols. These molecules are vital for diverse cellular functions, ranging from signaling to membrane integrity, and from post-translational modification to energy homeostasis. However, information on the MVA pathway in Phytophthora species is limited. In this study, we identified the MVA pathway genes and reconstructed the complete pathway in Phytophthora sojae in silico. We characterized the function of the MVA pathway of P. sojae by treatment with enzyme inhibitor lovastatin, deletion of the geranylgeranyl diphosphate synthase gene (PsBTS1), and transcriptome profiling analysis. The MVA pathway is ubiquitously conserved in Phytophthora species. Under lovastatin treatment, mycelial growth, spore production, and virulence of P. sojae were inhibited but the zoospore encystment rate increased. Heterozygous mutants of PsBTS1 showed slow growth, abnormal colony characteristics, and mycelial morphology. Mutants showed decreased numbers of sporangia and oospores as well as reduced virulence. RNA sequencing analysis identified the essential genes in sporangia formation were influenced by the enzyme inhibitor lovastatin. Our findings elucidate the role of the MVA pathway in P. sojae and provide new insights into the molecular mechanisms underlying the development, reproduction, and virulence of P. sojae and possibly other oomycetes. Our results also provide potential chemical targets for management of plant Phytophthora diseases.

Phytophthora sojae leucine-rich repeat receptor-like kinases: diverse and essential roles in development and pathogenicity

Leucine-rich repeat receptor-like kinases (LRR-RLKs) are critical signal receptors in plant development and defense. Like plants, oomycete pathogen genomes also harbor LRR-RLKs, but their functions remain largely unknown. Here, we systematically characterize all the 24 LRR-RLK genes (PsRLKs) from Phytophthora sojae, which is a model of oomycete pathogens. Although none of them was required for vegetative growth, the specific PsRLKs are important for stress responses, zoospore production, zoospores chemotaxis, and pathogenicity. Interestingly, the Gα subunit PsGPA1 interacts with the five chemotaxis-related PsRLKs via their intracellular kinase domains, and expression of PsGPA1 gene is downregulated in the three mutants (ΔPsRLK17/22/24). Moreover, we generated the PsRLK-PsRLK interaction network of P. sojae and found that PsRLK21, together with PsRLK10 or PsRLK17, regulate virulence by direct association. Taken together, our results reveal the diverse roles of LRR-RLKs in modulating P. sojae development, interaction with soybean, and responses to diverse environmental factors.

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Systematically functional analysis of LRR-RLK family with 24 members in P. sojae

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Five chemotaxis-related PsRLKs directly interact with Gα protein PsGPA1

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PsRLKs form an interaction network in P. sojae

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The complex PsRLK21-PsRLK10/17 jointly regulates pathogenesis

Phytophthora zoospores: From perception of environmental signals to inoculum formation on the host-root surface

To explore moist soils and to target host plants, phytopathogenic Phytophthora species utilize the sensory and propulsion capabilities of the biflagellate unicellular zoospores they produce. Zoospore motion and interactions with the microenvironment are of primary importance for Phytophthora physiology. These are also of critical significance for plant pathology in early infection sequential events and their regulation: the directed zoospore migration toward the host, the local aggregation and adhesion at the host penetration site. In the soil, these early events preceding the root colonization are orchestrated by guidance factors, released from the soil particles in water films, or emitted within microbiota and by host plants. This signaling network is perceived by zoospores and results in coordinated behavior and preferential localization in the rhizosphere. Recent computational and structural studies suggest that rhizospheric ion and plant metabolite sensing is a key determinant in driving zoospore motion, orientation and aggregation. To reach their target, zoospores respond to various molecular, chemical and electrical stimuli. However, it is not yet clear how these signals are generated in local soil niches and which gene functions govern the sensing and subsequent responses of zoospores. Here we review studies on the soil, microbial and host-plant factors that drive zoospore motion, as well as the adaptations governing zoospore behavior. We propose several research directions that could be explored to characterize the role of zoospore microbial ecology in disease.

Phytopathogenic oomycetes: a review focusing on Phytophthora cinnamomi and biotechnological approaches

The Phytophthora genus is composed, mainly, of plant pathogens. This genus belongs to the Oomycete class, also known as "pseudo-fungi", within the Chromista Kingdom. Phytophthora spp. is highlighted due to the significant plant diseases that they cause, which represents some of the most economically and cultural losses, such as European chestnut ink disease, which is caused by P. cinnamomi. Currently, there have been four genome assemblies placed at the National Center for Biotechnology Information (NCBI), although the progress to understand and elucidate the pathogenic process of P. cinnamomi by its genome is progressing slowly. In this review paper, we aim to report and discuss the recent findings related to P. cinnamomi and its genomic information. Our research is based on paper databases that reported probable functions to P. cinnamomi proteins using sequence alignments, bioinformatics, and biotechnology approaches. Some of these proteins studied have functions that are proposed to be involved in the asexual sporulation and zoosporogenesis leading to the host colonization and consequently associated with pathogenicity. Some remarkable genes and proteins discussed here are related to oospore development, inhibition of sporangium formation and cleavage, inhibition of flagellar assembly, blockage of cyst germination and hyphal extension, and biofilm proteins. Lastly, we report some biotechnological approaches using biological control, studies with genome sequencing of P. cinnamomi resistant plants, and gene silencing through RNA interference (iRNA).

Organize, Don't Agonize: Strategic Success of Phytophthora Species

Plants are constantly challenged by various environmental stressors ranging from abiotic-sunlight, elevated temperatures, drought, and nutrient deficits, to biotic factors-microbial pathogens and insect pests. These not only affect the quality of harvest but also the yield, leading to substantial annual crop losses, worldwide. Although plants have a multi-layered immune system, phytopathogens such as species of the oomycete genus Phytophthora, can employ elaborate mechanisms to breach this defense. For the last two decades, researchers have focused on the co-evolution between Phytophthora and interacting hosts to decouple the mechanisms governing their molecular associations. This has provided a comprehensive understanding of the pathobiology of plants affected by oomycetes. Ultimately, this is important for the development of strategies to sustainably improve agricultural production. Therefore, this paper discusses the present-day state of knowledge of the strategic mode of operation employed by species of Phytophthora for successful infection. Specifically, we consider motility, attachment, and host cell wall degradation used by these pathogenic species to obtain nutrients from their host. Also discussed is an array of effector types from apoplastic (hydrolytic proteins, protease inhibitors, elicitins) to cytoplastic (RxLRs, named after Arginine-any amino acid-Leucine-Arginine consensus sequence and CRNs, for CRinkling and Necrosis), which upon liberation can subvert the immune response and promote diseases in plants.

The high-affinity phosphodiesterase PcPdeH is involved in the polarized growth and pathogenicity of Phytophthora capsici

The cAMP signaling pathway has been shown to be important in controlling morphological changes and pathogenicity in plant pathogens. In the present study, we identified PcPdeH, a gene encoding a high-affinity phosphodiesterase (PDE), which is a key regulator of the cAMP signaling pathway. To elucidate the function of PcPdeH, PcPdeH-knockout mutants were obtained using a type II CRISPR/Cas9 system in Phytophthora capsici. The knockout transformants of PcPdeH showed vegetative growth defects and abnormal cyst germination. Infection assays indicated that compared with the wild type, PcPdeH-knockout mutants showed significantly reduced virulence on pepper and tobacco leaves and exhibited increased (1.5-2-fold) cAMP levels relative to the wild-type and CK strains. Based on these phenotypic features, we propose that PcPdeH is crucial for vegetative growth, cyst germination and pathogenicity in P. capsici.

G protein α subunit suppresses sporangium formation through a serine/threonine protein kinase in Phytophthora sojae

Eukaryotic heterotrimeric guanine nucleotide-binding proteins consist of α, β, and γ subunits, which act as molecular switches to regulate a number of fundamental cellular processes. In the oomycete pathogen Phytophthora sojae, the sole G protein α subunit (Gα; encoded by PsGPA1) has been found to be involved in zoospore mobility and virulence, but how it functions remains unclear. In this study, we show that the Gα subunit PsGPA1 directly interacts with PsYPK1, a serine/threonine protein kinase that consists of an N-terminal region with unknown function and a C-terminal region with a conserved catalytic kinase domain. We generated knockout and knockout-complemented strains of PsYPK1 and found that deletion of PsYPK1 resulted in a pronounced reduction in the production of sporangia and oospores, in mycelial growth on nutrient poor medium, and in virulence. PsYPK1 exhibits a cytoplasmic-nuclear localization pattern that is essential for sporangium formation and virulence of P. sojae. Interestingly, PsGPA1 overexpression was found to prevent nuclear localization of PsYPK1 by exclusively binding to the N-terminal region of PsYPK1, therefore accounting for its negative role in sporangium formation. Our data demonstrate that PsGPA1 negatively regulates sporangium formation by repressing the nuclear localization of its downstream kinase PsYPK1.

Effects of Different G-Protein α-Subunits on Growth, Development and Secondary Metabolism of Monascus ruber M7

Strains of Monascus filamentous fungal species have been used to produce fermented foods in Asian countries, such as China, Japan, and The Korean Peninsula, for nearly 2,000 years. At present, their fermented products are widely used as food additives and nutraceutical supplements worldwide owing to their production of beneficial secondary metabolites. Heterotrimeric G-protein signaling pathways participate in regulating multiple biological processes in fungi. Previously, we identified three Monascus ruber M7 G-protein α subunits (Mga1–3) and demonstrated that Mga1 can regulate growth, reproduction and some secondary metabolites’ production. Here, we systematically analyzed and compared the roles of mga1–3 by combining single- and double-gene(s) knockouts and their transcriptomic data. First, mga2 and mga3 knock-out mutants and pairwise combinations of mga1–3 deletion strains were generated. Then the changes in growth, development and the main secondary metabolites, Monascus pigments and citrinin, in these mutants were systematically compared with M. ruber M7. Moreover, RNA-Seq analyses of these mutants were performed. All three Gα subunits worked together to regulate biological processes in M. ruber M7, with Mga1 playing a major role, while Mga2 and Mga3 playing supplemental roles. According to the existing literatures which we can find, gene knock-out mutants of the pairwise combination of mga1–3 and their transcriptome analysis are first reported in this study. The current results have clearly demonstrated the functional division of Mga1–3 in M. ruber M7, and could provide a deeper understanding of the effects of different Gα subunits on growth, development and secondary metabolism in other filamentous fungi.

Exchanges at the Plant-Oomycete Interface That Influence Disease

Molecular exchanges between plants and biotrophic, hemibiotrophic, and necrotrophic oomycetes affect disease progression.

Generating Gene Silenced Mutants in Phytophthora sojae

Phytophthora species are notorious pathogens of plants and cause enormous damage to agriculture. In order to understand Phytophthora pathogenesis, gene silencing and knockout methods are important for the investigation of gene functions. Although CRISPR/Cas9-based gene knockout procedures have been developed in Phytophthora sojae and Phytophthora capsici, it may not always be the best choice especially when knockout mutation leads to lethality. Therefore, gene silencing is a very useful tool for functional analysis of target genes in Phytophthora. This chapter introduces a gene silencing protocol for the soybean pathogen P. sojae. An expression cassette is incorporated into the P. sojae genome through PEG-mediated protoplast transformation, which leads to constitutive production of antisense RNA transcripts. These transcripts are able to target mRNAs through sequence complementarity and effectively reduce the expression of the target genes.

Phytophthora cinnamomi

Phytophthora cinnamomi is one of the most devastating plant pathogens in the world. It infects close to 5000 species of plants, including many of importance in agriculture, forestry and horticulture. The inadvertent introduction of P. cinnamomi into natural ecosystems, including a number of recognized Global Biodiversity Hotspots, has had disastrous consequences for the environment and the biodiversity of flora and fauna. The genus Phytophthora belongs to the Class Oomycetes, a group of fungus-like organisms that initiate plant disease through the production of motile zoospores. Disease control is difficult in agricultural and forestry situations and even more challenging in natural ecosystems as a result of the scale of the problem and the limited range of effective chemical inhibitors. The development of sustainable control measures for the future management of P. cinnamomi requires a comprehensive understanding of the cellular and molecular basis of pathogen development and pathogenicity. The application of next-generation sequencing technologies to generate genomic and transcriptomic data promises to underpin a new era in P. cinnamomi research and discovery. The aim of this review is to integrate bioinformatic analyses of P. cinnamomi sequence data with current knowledge of the cellular and molecular basis of P. cinnamomi growth, development and plant infection. The goal is to provide a framework for future research by highlighting potential pathogenicity genes, shedding light on their possible functions and identifying suitable targets for future control measures.

TAXONOMY

Phytophthora cinnamomi Rands; Kingdom Chromista; Phylum Oomycota or Pseudofungi; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; genus Phytophthora.

HOST RANGE

Infects about 5000 species of plants, including 4000 Australian native species. Host plants important for agriculture and forestry include avocado, chestnut, macadamia, oak, peach and pineapple.

DISEASE SYMPTOMS

A root pathogen which causes rotting of fine and fibrous roots, but which can also cause stem cankers. Root damage may inhibit water movement from roots to shoots, leading to dieback of young shoots. USEFUL WEBSITES: http://fungidb.org/fungidb/; http://genome.jgi.doe.gov/Phyci1/Phyci1.home.html; http://www.ncbi.nlm.nih.gov/assembly/GCA_001314365.1; http://www.ncbi.nlm.nih.gov/assembly/GCA_001314505.1.