Transcriptome Analysis of Potato Infected with the Necrotrophic Pathogen Alternaria solani.

Potato early blight is caused by the necrotrophic fungus Alternaria solani and can result in yield losses of up to 50% if left uncontrolled. At present, the disease is controlled by chemical fungicides, yet rapid development of fungicide resistance renders current control strategies unsustainable. On top of that, a lack of understanding of potato defences and the quantitative nature of resistance mechanisms against early blight hinders the development of more sustainable control methods. Necrotrophic pathogens, compared to biotrophs, pose an extra challenge to the plant, since common defence strategies to biotic stresses such as the hypersensitive response and programmed cell death are often beneficial for necrotrophs. With the aim of unravelling plant responses to both the early infection stages (i.e., before necrosis), such as appressorium formation and penetration, as well as to later responses to the onset of necrosis, we present here a transcriptome analysis of potato interactions with A. solani from 1 h after inoculation when the conidia have just commenced germination, to 48 h post inoculation when multiple cell necrosis has begun. Potato transcripts with putative functions related to biotic stress tolerance and defence against pathogens were upregulated, including a putative Nudix hydrolase that may play a role in defence against oxidative stress. A. solani transcripts encoding putative pathogenicity factors, such as cell wall degrading enzymes and metabolic processes that may be important for infection. We therefore identified the differential expression of several potato and A. solani transcripts that present a group of valuable candidates for further studies into their roles in immunity or disease development.

Visualising the ionome in resistant and susceptible plant-pathogen interactions.

At the morphological and anatomical levels, the ionome, or the elemental composition of an organism, is an understudied area of plant biology. In particular, the ionomic responses of plant-pathogen interactions are scarcely described, and there are no studies on immune reactions. In this study we explored two X-ray fluorescence (XRF)-based ionome visualisation methods (benchtop- and synchrotron-based micro-XRF [µXRF]), as well as the quantitative inductively coupled plasma optical emission spectroscopy (ICP-OES) method, to investigate the changes that occur in the ionome of compatible and incompatible plant-pathogen interactions. We utilised the agronomically important and comprehensively studied interaction between potato (Solanum tuberosum) and the late blight oomycete pathogen Phytophthora infestans as an example. We used one late blight-susceptible potato cultivar and two resistant transgenic plant lines (only differing from the susceptible cultivar in one or three resistance genes) both in control and P. infestans-inoculated conditions. In the lesions from the compatible interaction, we observed rearrangements of several elements, including a decrease of the mobile macronutrient potassium (K) and an increase in iron (Fe) and manganese (Mn), compared with the tissue outside the lesion. Interestingly, we observed distinctly different distribution patterns of accumulation at the site of inoculation in the resistant lines for calcium (Ca), magnesium (Mg), Mn and silicon (Si) compared to the susceptible cultivar. The results reveal different ionomes in diseased plants compared to resistant plants. Our results demonstrate a technical advance and pave the way for deeper studies of the plant-pathogen ionome in the future.

Pathogen-Mediated Stomatal Opening: A Previously Overlooked Pathogenicity Strategy in the Oomycete Pathogen Phytophthora infestans.

Phytophthora infestans, the most damaging oomycete pathogen of potato, is specialized to grow sporangiophore through opened stomata for secondary inoculum production. However, it is still unclear which metabolic pathways in potato are manipulated by P. infestans in the guard cell-pathogen interactions to open the stomata. Here microscopic observations and cell biology were used to investigate antagonistic interactions between guard cells and the oomycete pathogen. We observed that the antagonistic interactions started at the very beginning of infection. Stomatal movement is an important part of the immune response of potato to P. infestans infection and this occurs through guard cell death and stomatal closure. We observed that P. infestans appeared to manipulate metabolic processes in guard cells, such as triacylglycerol (TAG) breakdown, starch degradation, H2O2 scavenging, and NO catabolism, which are involved in stomatal movement, to evade these stomatal defense responses. The signal transduction pathway of P. infestans-induced stomatal opening likely starts from H2O2 and NO scavenging, along with TAG breakdown while the subsequent starch degradation reinforces the opening process by strengthening guard cell turgor and opening the stomata to their maximum aperture. These results suggest that stomata are a barrier stopping P. infestans from completing its life cycle, but this host defense system can be bypassed through the manipulation of diverse metabolic pathways that may be induced by P. infestans effector proteins.

Altitudinal Heterogeneity of UV Adaptation in Phytophthorainfestans Is Associated with the Spatial Distribution of a DNA Repair Gene.

Climate change is considered a major threat to society and nature. UV irradiation is the most important environmental genotoxic agent. Thus, how elevated UV irradiation may influence human health and ecosystems has generated wide concern in the scientific community, as well as with policy makers and the public in general. In this study, we investigated patterns and mechanisms of UV adaptation in natural ecosystems by studying a gene-specific variation in the potato late blight pathogen, Phytophthora infestans. We compared the sequence characteristics of radiation sensitive 23 (RAD23), a gene involved in the nucleotide excision repair (NER) pathway and UV tolerance, in P. infestans isolates sampled from various altitudes. We found that lower genetic variation in the RAD23 gene was caused by natural selection. The hypothesis that UV irradiation drives this selection was supported by strong correlations between the genomic characteristics and altitudinal origin (historic UV irradiation) of the RAD23 sequences with UV tolerance of the P. infestans isolates. These results indicate that the RAD23 gene plays an important role in the adaptation of P. infestans to UV stress. We also found that different climatic factors could work synergistically to determine the evolutionary adaptation of species, making the influence of climate change on ecological functions and resilience more difficult to predict. Future attention should aim at understanding the collective impact generated by simultaneous change in several climate factors on species adaptation and ecological sustainability, using state of the art technologies such as experimental evolution, genome-wide scanning, and proteomics.

Horizontal Gene Transfer and Tandem Duplication Shape the Unique CAZyme Complement of the Mycoparasitic Oomycetes Pythium oligandrum and Pythium periplocum.

Crop protection strategies that are effective but that reduce our reliance on chemical pesticides are urgently needed to meet the UN sustainable development goals for global food security. Mycoparasitic oomycetes such as Pythium oligandrum and Pythium periplocum, have potential for the biological control of plant diseases that threaten crops and have attracted much attention due to their abilities to antagonize plant pathogens and modulate plant immunity. Studies of the molecular and genetic determinants of mycoparasitism in these species have been less well developed than those of their fungal counterparts. Carbohydrate-active enzymes (CAZymes) from P. oligandrum and P. periplocum are predicted to be important components of mycoparasitism, being involved in the degradation of the cell wall of their oomycete and fungal prey species. To explore the evolution of CAZymes of these species we performed an in silico identification and comparison of the full CAZyme complement (CAZyome) of the two mycoparasitic Pythium species (P. oligandrum and P. periplocum), with seven other Pythium species, and four Phytophthora species. Twenty CAZy gene families involved in the degradation of cellulose, hemicellulose, glucan, and chitin were expanded in, or unique to, mycoparasitic Pythium species and several of these genes were expressed during mycoparasitic interactions with either oomycete or fungal prey, as revealed by RNA sequencing and quantitative qRT-PCR. Genes from three of the cellulose and chitin degrading CAZy families (namely AA9, GH5_14, and GH19) were expanded via tandem duplication and predominantly located in gene sparse regions of the genome, suggesting these enzymes are putative pathogenicity factors able to undergo rapid evolution. In addition, five of the CAZy gene families were likely to have been obtained from other microbes by horizontal gene transfer events. The mycoparasitic species are able to utilize complex carbohydrates present in fungal cell walls, namely chitin and N-acetylglucosamine for growth, in contrast to their phytopathogenic counterparts. Nonetheless, a preference for the utilization of simple sugars for growth appears to be a common trait within the oomycete lineage.

What are the Top 10 Unanswered Questions in Molecular Plant-Microbe Interactions?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.The past few decades have seen major discoveries in the field of molecular plant-microbe interactions. As the result of technological and intellectual advances, we are now able to answer questions at a level of mechanistic detail that we could not have imagined possible 20 years ago. The MPMI Editorial Board felt it was time to take stock and reassess. What big questions remain unanswered? We knew that to identify the fundamental, overarching questions that drive our research, we needed to do this as a community. To reach a diverse audience of people with different backgrounds and perspectives, working in different areas of plant-microbe interactions, we queried the more than 1,400 participants at the 2019 International Congress on Molecular Plant-Microbe Interactions meeting in Glasgow. This group effort resulted in a list of ten, broad-reaching, fundamental questions that influence and inform our research. Here, we introduce these Top 10 unanswered questions, giving context and a brief description of the issues. Each of these questions will be the subject of a detailed review in the coming months. We hope that this process of reflecting on what is known and unknown and identifying the themes that underlie our research will provide a framework to use going forward, giving newcomers a sense of the mystery of the big questions and inspiring new avenues and novel insights.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Effect of RNA silencing suppression activity of chrysanthemum virus B p12 protein on small RNA species.

Chrysanthemum virus B encodes a multifunctional p12 protein that acts as a transcriptional activator in the nucleus and as a suppressor of RNA silencing in the cytoplasm. Here, we investigated the impact of p12 on accumulation of major classes of small RNAs (sRNAs). The results show dramatic changes in the sRNA profiles characterised by an overall reduction in sRNA accumulation, changes in the pattern of size distribution of canonical siRNAs and in the ratio between sense and antisense strands, lower abundance of siRNAs with a U residue at the 5'-terminus, and changes in the expression of certain miRNAs, most of which were downregulated.

Efficient RNA silencing suppression activity of Potato Mop-Top Virus 8K protein is driven by variability and positive selection.

Previously, we investigated the evolution of Potato mop-top virus (PMTV) ORFs. Results indicate that positive selection acts exclusively on an ORF encoding the 8K protein, a weak viral suppressor of RNA silencing (VSR). However, how the extraordinary variability contributes to 8K-mediated RNA silencing suppression remains unknown. Here, we characterized the RNA silencing suppression activity of the 8K protein from seven diverse isolates. We show that 8K encoded by isolate P1 exhibits stronger RNA silencing suppression activity than the 8K protein from six other isolates. Mutational analyses revealed that Ser-50 is critical for these differences. By comparing small RNA profiles we found a lower abundance of siRNAs with U residue at the 5'-terminus after expression of the P1 8K compared to expression of 8K from isolate P125, an isolate with weak VSR activity. These results provide new clues as to the role of positive selection in shaping activities of VSRs.

Infection mechanisms and putative effector repertoire of the mosquito pathogenic oomycete Pythium guiyangense uncovered by genomic analysis.

Pythium guiyangense, an oomycete from a genus of mostly plant pathogens, is an effective biological control agent that has wide potential to manage diverse mosquitoes. However, its mosquito-killing mechanisms are almost unknown. In this study, we observed that P. guiyangense could utilize cuticle penetration and ingestion of mycelia into the digestive system to infect mosquito larvae. To explore pathogenic mechanisms, a high-quality genome sequence with 239 contigs and an N50 contig length of 1,009 kb was generated. The genome assembly is approximately 110 Mb, which is almost twice the size of other sequenced Pythium genomes. Further genome analysis suggests that P. guiyangense may arise from a hybridization of two related but distinct parental species. Phylogenetic analysis demonstrated that P. guiyangense likely evolved from common ancestors shared with plant pathogens. Comparative genome analysis coupled with transcriptome sequencing data suggested that P. guiyangense may employ multiple virulence mechanisms to infect mosquitoes, including secreted proteases and kazal-type protease inhibitors. It also shares intracellular Crinkler (CRN) effectors used by plant pathogenic oomycetes to facilitate the colonization of plant hosts. Our experimental evidence demonstrates that CRN effectors of P. guiyangense can be toxic to insect cells. The infection mechanisms and putative virulence effectors of P. guiyangense uncovered by this study provide the basis to develop improved mosquito control strategies. These data also provide useful knowledge on host adaptation and evolution of the entomopathogenic lifestyle within the oomycete lineage. A deeper understanding of the biology of P. guiyangense effectors might also be useful for management of other important agricultural pests.

Tolerance and overcompensation to infection by Phytophthora infestans in the wild perennial climber Solanum dulcamara.

Studies of infection by Phytophthora infestans-the causal agent of potato late blight-in wild species can provide novel insights into plant defense responses, and indicate how wild plants might be influenced by recurrent epidemics in agricultural fields. In the present study, our aim was to investigate if different clones of Solanum dulcamara (a relative of potato) collected in the wild differ in resistance and tolerance to infection by a common European isolate of P. infestans. We performed infection experiments with six S. dulcamara genotypes (clones) both in the laboratory and in the field and measured the degree of infection and plant performance traits. In the laboratory, the six evaluated genotypes varied from resistant to susceptible, as measured by degree of infection 20 days post infection. Two of the four genotypes susceptible to infection showed a quadratic (concave downward) relationship between the degree of infection and shoot length, with maximum shoot length at intermediate values of infection. This result suggests overcompensation, that is, an increase in growth in infected individuals. The number of leaves decreased with increasing degree of infection, but at different rates in the four susceptible genotypes, indicating genetic variation for tolerance. In the field, the inoculated genotypes did not show any disease symptoms, but plant biomass at the end of the growing season was higher for inoculated plants than for controls, in-line with the overcompensation detected in the laboratory. We conclude that in S. dulcamara there are indications of genetic variation for both resistance and tolerance to P. infestans infection. Moreover, some genotypes displayed overcompensation. Learning about plant tolerance and overcompensation to infection by pathogens can help broaden our understanding of plant defense in natural populations and help develop more sustainable plant protection strategies for economically important crop diseases.

Draft Genome Sequence for the Tree Pathogen Phytophthora plurivora.

Species from the genus Phytophthora are well represented among organisms causing serious diseases on trees. Phytophthora plurivora has been implicated in long-term decline of woodland trees across Europe. Here we present a draft genome sequence of P. plurivora, originally isolated from diseased European beech (Fagus sylvatica) in Malmö, Sweden. When compared with other sequenced Phytophthora species, the P. plurivora genome assembly is relatively compact, spanning 41 Mb. This is organized in 1,919 contigs and 1,898 scaffolds, encompassing 11,741 predicted genes, and has a repeat content of approximately 15%. Comparison of allele frequencies revealed evidence for tetraploidy in the sequenced isolate. As in other sequenced Phytophthora species, P. plurivora possesses genes for pathogenicity-associated RXLR and Crinkle and Necrosis effectors, predominantly located in gene-sparse genomic regions. Comparison of the P. plurivora RXLR effectors with orthologs in other sequenced species in the same clade (Phytophthora multivora and Phytophthora capsici) revealed that the orthologs were likely to be under neutral or purifying selection.

Genome Sequence Resource for the Oomycete Taro Pathogen Phytophthora colocasiae.

Phytophthora colocasiae is a phytopathogenic oomycete that causes leaf blight and corm rot on taro (Colocasia esculenta), an important staple crop in the tropics. The impact of P. colocasiae is a serious concern for food security in Asian and Oceanic regions. Vietnamese strain 7290 of P. colocasiae was sequenced (Illumina) to assemble a draft genome of 56.6 Mb, comprised of 19,853 scaffolds and 19,984 predicted protein-coding genes. As in other Phytophthora species, P. colocasiae possesses numerous pathogenicity-related genes, such as the RxLR class of effectors. This draft genome sequence of P. colocasiae provides a resource to underpin the first steps in determining the molecular mechanisms of disease development in this pathosystem.

Draft genome of the oomycete pathogen Phytophthora cactorum strain LV007 isolated from European beech (Fagus sylvatica).

Phytophthora cactorum is a broad host range phytopathogenic oomycete. P. cactorum strain LV007 was isolated from a diseased European Beech (Fagus sylvatica) in Malmö, Sweden in 2016. The draft genome of P. cactorum strain LV007 is 67.81 Mb. It contains 15,567 contigs and 21,876 predicted protein-coding genes. As reported for other phytopathogenic Phytophthora species, cytoplasmic effector proteins including RxLR and CRN families were identified. The genome sequence has been deposited at DDBJ/ENA/GenBank under the accession NBIJ00000000. The version described in this paper is version NBIJ01000000.

Draft Genome Sequence of the Mycoparasitic Oomycete Pythium oligandrum Strain CBS 530.74.

The oomycete Pythium oligandrum is a mycoparasite and licenced biological control agent. Here, we report the draft genome sequence of P. oligandrum strain CBS 530.74, which is 36.80 Mb. It contains 341 scaffolds and 11,647 predicted protein-coding genes. As reported for plant-pathogenic Pythium species, RXLR-type effector sequences are absent.

Draft Genome Sequence of the Mycoparasitic Oomycete Pythium periplocum Strain CBS 532.74.

The oomycete Pythium periplocum is an aggressive mycoparasite of a number of plant pathogenic fungi and oomycetes and therefore has potential as a biological control agent. Here, we report the first draft genome sequence of P. periplocum, which comprises 35.89 Mb. It contains 1,043 scaffolds and 14,399 predicted protein-coding genes.

Comparative mitochondrial genome analysis of Pythium insidiosum and related oomycete species provides new insights into genetic variation and phylogenetic relationships.

Oomycetes are eukaryotic microorganisms, which are phylogenetically distinct from the true-fungi, which they resemble morphologically. While many oomycetes are pathogenic to plants, Pythium insidiosum is capable of infecting humans and animals. Mitochondrial (mt) genomes are valuable genetic resources for exploring the evolution of eukaryotes. During the course of 454-based nuclear genome sequencing, we identified a complete 54.9 kb mt genome sequence, containing 2 large inverted repeats, from P. insidiosum. It contains 65 different genes (including 2 ribosomal RNA genes, 25 transfer RNA genes and 38 genes encoding NADH dehydrogenases, cytochrome b, cytochrome c oxidases, ATP synthases, and ribosomal proteins). Thirty-nine of the 65 genes have two copies, giving a total of 104 genes. A set of 30 conserved protein-coding genes from the mt genomes of P. insidiosum, 11 other oomycetes, and 2 diatoms (outgroup) were used for phylogenetic analyses. The oomycetes can be classified into 2 phylogenetic groups, in relation to their taxonomic lineages: Saprolegnialean and Peronosporalean. P. insidiosum is more closely related to Pythium ultimum than other oomycetes. In conclusion, the complete mt genome of P. insidiosum was successfully sequenced, assembled, and annotated, providing a useful genetic resource for exploring the biology and evolution of P. insidiosum and other oomycetes.

Infection of the brown alga Ectocarpus siliculosus by the oomycete Eurychasma dicksonii induces oxidative stress and halogen metabolism.

Pathogens are increasingly being recognized as key evolutionary and ecological drivers in marine ecosystems. Defence mechanisms of seaweeds, however, have mostly been investigated by mimicking infection using elicitors. We have established an experimental pathosystem between the genome brown model seaweed Ectocarpus siliculosus and the oomycete Eurychasma dicksonii as a powerful new tool to investigate algal responses to infection. Using proteomics, we identified 21 algal proteins differentially accumulated in response to Eu. dicksonii infection. These include classical algal stress response proteins such as a manganese superoxide dismutase, heat shock proteins 70 and a vanadium bromoperoxidase. Transcriptional profiling by qPCR confirmed the induction of the latter during infection. The accumulation of hydrogen peroxide was observed at different infection stages via histochemical staining. Inhibitor studies confirmed that the main source of hydrogen peroxide is superoxide converted by superoxide dismutase. Our data give an unprecedented global overview of brown algal responses to pathogen infection, and highlight the importance of oxidative stress and halogen metabolism in these interactions. This suggests overlapping defence pathways with herbivores and abiotic stresses. We also identify previously unreported actors, in particular a Rad23 and a plastid-lipid-associated protein, providing novel insights into the infection and defence processes in brown algae.

RNAseq and Proteomics for Analysing Complex Oomycete Plant Interactions.

The oomycetes include some of the most devastating plant pathogens. In this review we discuss the latest results from oomycete and plant studies with emphasis on interaction studies. We focus on the outcomes of RNAseq and proteomics studies and some pitfalls of these approaches. Both pathogenic interactions and biological control are discussed. We underline the usefulness of studies at several levels of complexity from studies of one organism, up to two or more and within agricultural fields (managed settings) up to wild ecosystems. Finally we identify areas of future interest such as detailed interactome studies, dual RNAseq studies, peptide modification studies and population/meta omics with or without biological control agents.

Draft Genome Sequence of the Pathogenic Oomycete Pythium insidiosum Strain Pi-S, Isolated from a Patient with Pythiosis.

Pythium insidiosum is an oomycete that causes a life-threatening infectious disease called pythiosis in humans and animals living in tropical and subtropical countries. Here, we report the first draft genome sequence of P. insidiosum. The genome of P. insidiosum is 53.2 Mb and contains 14,962 open reading frames.

Auto-aggregation in zoospores of Phytophthora infestans: the cooperative roles of bioconvection and chemotaxis.

Phytophthora infestans is a highly destructive plant pathogen. It was the cause of the infamous Irish potato famine in the nineteenth century and remains to this day a significant global problem with associated costs estimated at $3 billion annually. Key to the success of this pathogen is the dispersal of free-swimming cells called zoospores. A poorly understood aspect of zoospore behaviour is auto-aggregation--the spontaneous formation of large-scale patterns in cell density. Current competing hypotheses suggest that these patterns are formed by one of two distinct mechanisms: chemotaxis and bioconvection. In this paper, we present mathematical and experimental results that together provide strong evidence that auto-aggregation can only result from a combination of these mechanisms, each having a distinct, time-separated role. A better understanding of the underlying infection mechanisms of P. infestans and potentially other Phytophthora species will in the longer term lead to advances in preventative treatment and thus potentially significant savings in socio-economic costs.