Arabidopsis late blight: infection of a nonhost plant by Albugo laibachii enables full colonization by Phytophthora infestans

Fusarium-produced vitamin B6 promotes the evasion of soybean resistance by Phytophthora sojae

Plants can be infected by multiple pathogens concurrently in natural systems. However, pathogen-pathogen interactions have rarely been studied. In addition to the oomycete Phytophthora sojae, fungi such as Fusarium spp. also cause soybean root rot. In a 3-year field investigation, we discovered that P. sojae and Fusarium spp. frequently coexisted in diseased soybean roots. Out of 336 P. sojae-soybean-Fusarium combinations, more than 80% aggravated disease. Different Fusarium species all enhanced P. sojae infection when co-inoculated on soybean. Treatment with Fusarium secreted non-proteinaceous metabolites had an effect equal to the direct pathogen co-inoculation. By screening a Fusarium graminearum mutant library, we identified Fusarium promoting factor of Phytophthora sojae infection 1 (Fpp1), encoding a zinc alcohol dehydrogenase. Fpp1 is functionally conserved in Fusarium and contributes to metabolite-mediated infection promotion, in which vitamin B6 (VB6) produced by Fusarium is key. Transcriptional and functional analyses revealed that Fpp1 regulates two VB6 metabolism genes, and VB6 suppresses expression of soybean disease resistance-related genes. These results reveal that co-infection with Fusarium promotes loss of P. sojae resistance in soybean, information that will inform the sustainable use of disease-resistant crop varieties and provide new strategies to control soybean root rot.

Resistance strategies for defense against Albugo candida causing white rust disease

Albugo candida, the causal organism of white rust, is an oomycete obligate pathogen infecting crops of Brassicaceae family occurred on aerial part, including vegetable and oilseed crops at all growth stages. The disease expression is characterized by local infection appearing on the abaxial region developing white or creamy yellow blister (sori) on leaves and systemic infections cause hypertrophy and hyperplasia leading to stag-head of reproductive organ. To overcome this problem, several disease management strategies like fungicide treatments were used in the field and disease-resistant varieties have also been developed using conventional and molecular breeding. Due to high variability among A. candida isolates, there is no single approach available to understand the diverse spectrum of disease symptoms. In absence of resistance sources against pathogen, repetitive cultivation of genetically-similar varieties locally tends to attract oomycete pathogen causing heavy yield losses. In the present review, a deep insight into the underlying role of the non-host resistance (NHR) defence mechanism available in plants, and the strategies to exploit available gene pools from plant species that are non-host to A. candida could serve as novel sources of resistance. This work summaries the current knowledge pertaining to the resistance sources available in non-host germ plasm, the understanding of defence mechanisms and the advance strategies covers molecular, biochemical and nature-based solutions in protecting Brassica crops from white rust disease.

Characterization of host-effector transcription dynamics during pathogen infection in engineered late blight resistant potato

Phytophthora infestans, the etiologic agent of late blight, is a threat to potato production in areas with high humidity during the growing season. The oomycete pathogen is hemi-biotrophic, it establishes infection on living plant cells and then spreads, kills, and feeds off the necrotized plant tissue material. The interaction between host and pathogen is complex with dynamic pathogen RXLR effectors and potato NB-LRR resistance proteins actively competing for dominance and survival. Late blight protection was brought to several cultivars of potato through insertion of the wild potato (Solanum venturii) NB-LRR resistance gene Rpi-vnt1.1. We have established that the late blight protection trait, mediated by Rpi-vnt1.1, is effective despite low expression of RNA. The RNA expression dynamics of Rpi-vnt1.1 and the cognate pathogen RXLR effector, Avr-vnt1, were evaluated following spray inoculation with up to five different contemporary late blight isolates from North America and South America. Following inoculations, RXLR effector transcript profiles provided insight into interaction compatibility in relation to markers of the late blight hemi-biotrophic lifecycle.

The Arabidopsis WRR4A and WRR4B paralogous NLR proteins both confer recognition of multiple Albugo candida effectors

The oomycete Albugo candida causes white blister rust, an important disease of Brassica crops. Distinct races of A. candida are defined by their capacity to infect different host plant species. Each A. candida race encodes secreted proteins with a CX₂ CX₅ G ('CCG') motif that are polymorphic and show presence/absence variation, and are therefore candidate effectors. The White Rust Resistance 4 (WRR4) locus in Arabidopsis thaliana accession Col-0 contains three genes that encode intracellular nucleotide-binding domain leucine-rich repeat immune receptors. The Col-0 alleles of WRR4A and WRR4B confer resistance to multiple A. candida races, although both WRR4A and WRR4B can be overcome by the Col-0-virulent race 4 isolate AcEx1. Comparison of CCG candidate effectors in avirulent and virulent races, and transient co-expression of CCG effectors from four A. candida races in Nicotiana sp. or A. thaliana, revealed CCG effectors that trigger WRR4A- or WRR4B-dependent hypersensitive responses. We found eight WRR4A-recognised CCGs and four WRR4B-recognised CCGs, the first recognised proteins from A. candida for which the cognate immune receptors in A. thaliana are known. This multiple recognition capacity potentially explains the broad-spectrum resistance to several A. candida races conferred by WRR4 paralogues. We further show that of five tested CCGs, three confer enhanced disease susceptibility when expressed in planta, consistent with A. candida CCG proteins being effectors.

Exploiting breakdown in nonhost effector-target interactions to boost host disease resistance

Plants are resistant to most microbial species due to nonhost resistance (NHR), providing broad-spectrum and durable immunity. However, the molecular components contributing to NHR are poorly characterised. We address the question of whether failure of pathogen effectors to manipulate nonhost plants plays a critical role in NHR. RxLR (Arg-any amino acid-Leu-Arg) effectors from two oomycete pathogens, Phytophthora infestans and Hyaloperonospora arabidopsidis, enhanced pathogen infection when expressed in host plants (Nicotiana benthamiana and Arabidopsis, respectively) but the same effectors performed poorly in distantly related nonhost pathosystems. Putative target proteins in the host plant potato were identified for 64 P. infestans RxLR effectors using yeast 2-hybrid (Y2H) screens. Candidate orthologues of these target proteins in the distantly related non-host plant Arabidopsis were identified and screened using matrix Y2H for interaction with RxLR effectors from both P. infestans and H. arabidopsidis. Few P. infestans effector-target protein interactions were conserved from potato to candidate Arabidopsis target orthologues (cAtOrths). However, there was an enrichment of H. arabidopsidis RxLR effectors interacting with cAtOrths. We expressed the cAtOrth AtPUB33, which unlike its potato orthologue did not interact with P. infestans effector PiSFI3, in potato and Nicotiana benthamiana. Expression of AtPUB33 significantly reduced P. infestans colonization in both host plants. Our results provide evidence that failure of pathogen effectors to interact with and/or correctly manipulate target proteins in distantly related non-host plants contributes to NHR. Moreover, exploiting this breakdown in effector-nonhost target interaction, transferring effector target orthologues from non-host to host plants is a strategy to reduce disease.

Receptor-mediated nonhost resistance in plants

Nonhost resistance (NHR) is a plant immune response that prevents many microorganisms in the plant's environment from pathogenicity against the plant. Since successful pathogens have adapted to overcome the immune systems of their host, the durable nature of NHR has potential in the management of plant disease. At present, there is genetic and molecular evidence that the underlying molecular mechanisms of NHR are similar to the plant immune responses that occur in host plants following infection by adapted pathogens. We consider that the molecular basis of NHR is multilayered, conferred by physicochemical barriers and defense responses that are induced following molecular recognition events. Moreover, the relative contribution of each component may depend on evolutionary distances between host and nonhost plants of given pathogen species. This mini-review has focused on the current knowledge of plant NHR, especially the recognition of non-adapted pathogens by nonhost plants at the cellular level. Recent gains in understanding the roles of plasma membrane-localized pattern-recognition receptors (PRRs) and the cytoplasmic nucleotide-binding leucine-rich repeat receptors (NLRs) associated with these processes, as well as the genes involved, are summarized. Finally, we provide a theoretical perspective on the durability of receptor-mediated NHR and its practical potential as an innovative strategy for crop protection against pathogens.

Evolutionary trade-offs at the Arabidopsis WRR4A resistance locus underpin alternate Albugo candida race recognition specificities

The oomycete Albugo candida causes white rust of Brassicaceae, including vegetable and oilseed crops, and wild relatives such as Arabidopsis thaliana. Novel White Rust Resistance (WRR) genes from Arabidopsis enable new insights into plant/parasite co-evolution. WRR4A from Arabidopsis accession Columbia (Col-0) provides resistance to many but not all white rust races, and encodes a nucleotide-binding, leucine-rich repeat immune receptor. Col-0 WRR4A resistance is broken by AcEx1, an isolate of A. candida. We identified an allele of WRR4A in Arabidopsis accession Øystese-0 (Oy-0) and other accessions that confers full resistance to AcEx1. WRR4AOy-0 carries a C-terminal extension required for recognition of AcEx1, but reduces recognition of several effectors recognized by the WRR4ACol-0 allele. WRR4AOy-0 confers full resistance to AcEx1 when expressed in the oilseed crop Camelina sativa.

Shaping the leaf microbiota: plant-microbe-microbe interactions

The aerial portion of a plant, namely the leaf, is inhabited by pathogenic and non-pathogenic microbes. The leaf's physical and chemical properties, combined with fluctuating and often challenging environmental factors, create surfaces that require a high degree of adaptation for microbial colonization. As a consequence, specific interactive processes have evolved to establish a plant leaf niche. Little is known about the impact of the host immune system on phyllosphere colonization by non-pathogenic microbes. These organisms can trigger plant basal defenses and benefit the host by priming for enhanced resistance to pathogens. In most disease resistance responses, microbial signals are recognized by extra- or intracellular receptors. The interactions tend to be species specific and it is unclear how they shape leaf microbial communities. In natural habitats, microbe-microbe interactions are also important for shaping leaf communities. To protect resources, plant colonizers have developed direct antagonistic or host manipulation strategies to fight competitors. Phyllosphere-colonizing microbes respond to abiotic and biotic fluctuations and are therefore an important resource for adaptive and protective traits. Understanding the complex regulatory host-microbe-microbe networks is needed to transfer current knowledge to biotechnological applications such as plant-protective probiotics.

Host-specialized transcriptome of plant-associated organisms

Living organisms respond to their immediate environment by modulating their genetic programme to perform adapted functions. Eukaryotic organisms that associate with plants (fungi, oomycetes, insects, …) alter their transcriptome in a host-specific manner. Recent comparative transcriptomic studies revealed that host-specialized transcriptomes consist of a limited set of genes. Such a set typically encodes proteins that modulate host structures and functions (predicted effectors and other secreted proteins), control nutrient assimilation (proteases, transporters), and maintain cellular homeostasis (oxidoreductases, detoxification enzymes). We conclude by discussing open mechanistic and evolutionary questions and integrated approaches to move beyond descriptive studies.

Rapid evolution in plant-microbe interactions - a molecular genomics perspective

Rapid (co-)evolution at multiple timescales is a hallmark of plant-microbe interactions. The mechanistic basis for the rapid evolution largely rests on the features of the genomes of the interacting partners involved. Here, we review recent insights into genomic characteristics and mechanisms that enable rapid evolution of both plants and phytopathogens. These comprise fresh insights in allelic series of matching pairs of resistance and avirulence genes, the generation of novel pathogen effectors, the recently recognised small RNA warfare, and genomic aspects of secondary metabolite biosynthesis. In addition, we discuss the putative contributions of permissive host environments, transcriptional plasticity and the role of ploidy on the interactions. We conclude that the means underlying the rapid evolution of plant-microbe interactions are multifaceted and depend on the particular nature of each interaction.

Two Is Better Than One: Studying Ustilago bromivora-Brachypodium Compatibility by Using a Hybrid Pathogen

Pathogenic fungi can have devastating effects on agriculture and health. One potential challenge in dealing with pathogens is the possibility of a host jump (i.e., when a pathogen infects a new host species). This can lead to the emergence of new diseases or complicate the management of existing threats. We studied host specificity by using a hybrid fungus formed by mating two closely related fungi: Ustilago bromivora, which normally infects Brachypodium spp., and U. hordei, which normally infects barley. Although U. hordei was unable to infect Brachypodium spp., the hybrid could. These hybrids also displayed the same mating-type bias that had been observed in U. bromivora and provide evidence of a dominant spore-killer-like system on the sex chromosome of U. bromivora. By analyzing the genomic composition of 109 hybrid strains, backcrossed with U. hordei over four generations, we identified three regions associated with infection on Brachypodium spp. and 75 potential virulence candidates. The most strongly associated region was located on chromosome 8, where seven genes encoding predicted secreted proteins were identified. The fact that we identified several regions relevant for pathogenicity on Brachypodium spp. but that none were essential suggests that host specificity, in the case of U. bromivora, is a multifactorial trait which can be achieved through different subsets of virulence factors.

An evolutionary framework for host shifts - jumping ships for survival

Host jumping is a process by which pathogens settle in new host groups. It is a cornerstone in the evolution of pathogens, as it leads to pathogen diversification. It is unsurprising that host jumping is observed in facultative pathogens, as they can reproduce even if they kill their hosts. However, host jumps were thought to be rare in obligate biotrophic pathogens, but molecular phylogenetics has revealed that the opposite is true. Here, I review some concepts and recent findings and present several hypotheses on the matter. In short, pathogens evolve and diversify via host jumps, followed by radiation, specialisation and speciation. Host jumps are facilitated by, for example, effector innovations, stress, compatible pathogens and physiological similarities. Host jumping, subsequent establishment, and speciation takes place rapidly - within centuries and millennia rather than over millions of years. If pathogens are unable to evolve into neutral or mutualistic interactions with their hosts, they will eventually be removed from the host population, despite balancing trade-offs. Thus, generally, plant pathogens only survive in the course of evolution if they jump hosts. This is also reflected by the diversity patterns observed in many genera of plant pathogens, where it leads to a mosaic pattern of host groups over time, in which the original host group becomes increasingly obscure.

Phytophthora infestans Sporangia Produced in Artificial Media and Plant Lesions Have Subtly Divergent Transcription Profiles but Equivalent Infection Potential and Aggressiveness

Sporangia of the potato late blight agent Phytophthora infestans are often used in studies of pathogen biology and plant responses to infection. Investigations of spore biology can be challenging in oomycetes because their sporangia are physiologically active and change in response to environmental factors and aging. Whether sporangia from artificial media and plant lesions are functionally equivalent has been a topic of debate. To address these issues, we compared the transcriptomes and infection ability of sporangia from rye-sucrose media, potato and tomato leaflets, and potato tubers. Small differences were observed between the mRNA profiles of sporangia from all sources, including variation in genes encoding metabolic enzymes, cell-wall-degrading enzymes, and ABC transporters. Small differences in sporangia age also resulted in variation in the transcriptome. Taking care to use sporangia of similar maturity, we observed that those sourced from media or plant lesions had similar rates of zoospore release and cyst germination. There were also no differences in infection rates or aggressiveness on leaflets, based on single-spore inoculation assays. Such results are discordant with those of a recent publication in this journal. Nevertheless, we conclude that sporangia from plant and media cultures are functionally similar and emphasize the importance of using "best practices" in experiments with sporangia to obtain reliable results.

Oomycetes Used in Arabidopsis Research

Arabidopsis plants in their natural environment are susceptible to infection by oomycete pathogens, in particular to downy mildew and white rust diseases. These naturally occurring infectious agents have imposed evolutionary pressures on Arabidopsis populations and are therefore highly relevant for the study of host-pathogen co-evolution. In addition, the study of oomycete diseases, including infections caused by several Phytophthora species, has led to many scientific discoveries on Arabidopsis immunity and disease. Herein, we describe the major oomycete species used for experiments on Arabidopsis, and how these pathosystems have been used to provide significant insights into mechanistic and evolutionary aspects of plant-oomycete interactions. We also highlight understudied aspects of plant-oomycete interactions, as well as translational approaches, that can be productively addressed using the reference pathosystems described in this article.

Revisiting the Concept of Host Range of Plant Pathogens

Strategies to manage plant disease-from use of resistant varieties to crop rotation, elimination of reservoirs, landscape planning, surveillance, quarantine, risk modeling, and anticipation of disease emergences-all rely on knowledge of pathogen host range. However, awareness of the multitude of factors that influence the outcome of plant-microorganism interactions, the spatial and temporal dynamics of these factors, and the diversity of any given pathogen makes it increasingly challenging to define simple, all-purpose rules to circumscribe the host range of a pathogen. For bacteria, fungi, oomycetes, and viruses, we illustrate that host range is often an overlapping continuum-more so than the separation of discrete pathotypes-and that host jumps are common. By setting the mechanisms of plant-pathogen interactions into the scales of contemporary land use and Earth history, we propose a framework to assess the frontiers of host range for practical applications and research on pathogen evolution.

Plant nonhost resistance: paradigms and new environments

Nonhost resistance (NHR) protects plants from a large and diverse array of potential phytopathogens. Each phytopathogen can parasitise some plant species, but most plant species are nonhosts that are innately immune due to a series of physical, chemical and inducible defenses these nonadapted pathogens cannot overcome. New evidence supports the NHR paradigm that posits the inability of potential pathogens to colonise nonhost plants is frequently due to molecular incompatibility between pathogen virulence factors and plant cellular targets. While NHR is durable, it is not insurmountable. Environmental changes can facilitate pathogen host jumps or alternatively result in new encounters between previously isolated plant species and pathogens. Climate change is predicted to substantially alter the current distribution of plants and their pathogens which could result in parasitism of new plant species.

Intercellular cooperation in a fungal plant pathogen facilitates host colonization

Cooperation is associated with major transitions in evolution such as the emergence of multicellularity. It is central to the evolution of many complex traits in nature, including growth and virulence in pathogenic bacteria. Whether cells of multicellular parasites function cooperatively during infection remains, however, largely unknown. Here, we show that hyphal cells of the fungal pathogen Sclerotinia sclerotiorum reprogram toward division of labor to facilitate the colonization of host plants. Using global transcriptome sequencing, we reveal that gene expression patterns diverge markedly in cells at the center and apex of hyphae during Arabidopsis thaliana colonization compared with in vitro growth. We reconstructed a genome-scale metabolic model for S. sclerotiorum and used flux balance analysis to demonstrate metabolic heterogeneity supporting division of labor between hyphal cells. Accordingly, continuity between the central and apical compartments of invasive hyphae was required for optimal growth in planta Using a multicell model of fungal hyphae, we show that this cooperative functioning enhances fungal growth predominantly during host colonization. Our work identifies cooperation in fungal hyphae as a mechanism emerging at the multicellular level to support host colonization and virulence.

Genes involved in nonhost disease resistance as a key to engineer durable resistance in crops

Most potential pathogens fail to establish virulence for a plethora of plants found in nature. This intrinsic property to resist pathogen virulence displayed by organisms without triggering canonical resistance (R) genes has been termed nonhost resistance (NHR). While host resistance involves recognition of pathogen elicitors such as avirulence factors by bona fide R proteins, mechanism of NHR seems less obvious, often involving more than one gene. We can generally describe NHR in two steps: 1) pre-invasive resistance, either passive or active, which can restrict the pathogen from entering the host, and 2) post-invasive resistance, an active defense response that often results in hypersensitive response like programmed cell death and reactive oxygen species accumulation. While PAMP-triggered-immunity (PTI) is generally effective against nonhost pathogens, effector-triggered-immunity (ETI) can be effective against both host and nonhost pathogens. Prolonged interactions between adapted pathogens and their resistant host plants results in co-evolution, which can lead to new pathogen strains that can be virulent and cause disease on supposedly resistant host. In this context, engineering durable resistance by manipulating genes involved in NHR is an attractive approach for sustainable agriculture. Several genes involved in NHR have been characterized for their role in plant defense. In this review, we report genes involved in NHR identified to date and highlight a few examples where genes involved in NHR have been used to confer resistance in crop plants against economically important diseases.

Albugo candida race diversity, ploidy and host-associated microbes revealed using DNA sequence capture on diseased plants in the field

Physiological races of the oomycete Albugo candida are biotrophic pathogens of diverse plant species, primarily the Brassicaceae, and cause infections that suppress host immunity to other pathogens. However, A. candida race diversity and the consequences of host immunosuppression are poorly understood in the field. We report a method that enables sequencing of DNA of plant pathogens and plant-associated microbes directly from field samples (Pathogen Enrichment Sequencing: PenSeq). We apply this method to explore race diversity in A. candida and to detect A. candida-associated microbes in the field (91 A. candida-infected plants). We show with unprecedented resolution that each host plant species supports colonization by one of 17 distinct phylogenetic lineages, each with an unique repertoire of effector candidate alleles. These data reveal the crucial role of sexual and asexual reproduction, polyploidy and host domestication in A. candida specialization on distinct plant species. Our bait design also enabled phylogenetic assignment of DNA sequences from bacteria and fungi from plants in the field. This paper shows that targeted sequencing has a great potential for the study of pathogen populations while they are colonizing their hosts. This method could be applied to other microbes, especially to those that cannot be cultured.

Plant reference genes for development and stress response studies

Many reference genes are used by different laboratories for gene expression analyses to indicate the relative amount of input RNA/DNA in the experiment. These reference genes are supposed to show least variation among the treatments and with the control sets in a given experiment. However, expression of reference genes varies significantly from one set of experiment to the other. Thus, selection of reference genes depends on the experimental conditions. Sometimes the average expression of two or three reference genes is taken as standard. This review consolidated the details of about 120 genes attempted for normalization during comparative expression analysis in 16 different plants. Plant species included in this review are Arabidopsis thaliana, cotton (Gossypium hirsutum), tobacco (Nicotiana benthamiana and N. tabacum), soybean (Glycine max), rice (Oryza sativa), blueberry (Vaccinium corymbosum), tomato (Solanum lycopersicum), wheat (Triticum aestivum), potato (Solanum tuberosum), sugar cane (Saccharum sp.), carrot (Daucus carota), coffee (Coffea arabica), cucumber (Cucumis sativus), kiwi (Actinidia deliciosa) and grape (Vitis vinifera). The list includes model and cultivated crop plants from both monocot and dicot classes. We have categorized plant-wise the reference genes that have been used for expression analyses in any or all of the four different conditions such as biotic stress, abiotic stress, developmental stages and various organs and tissues, reported till date. This review serves as a guide during the reference gene hunt for gene expression analysis studies.

The powdery mildew-resistant Arabidopsis mlo2 mlo6 mlo12 triple mutant displays altered infection phenotypes with diverse types of phytopathogens

Arabidopsis thaliana mlo2 mlo6 mlo12 triple mutant plants exhibit complete immunity against infection by otherwise virulent obligate biotrophic powdery mildew fungi such as Golovinomyces orontii. While this phenotype is well documented, the interaction profile of the triple mutant with other microbes is underexplored and incomplete. Here, we thoroughly assessed and quantified the infection phenotypes of two independent powdery mildew-resistant triple mutant lines with a range of microbes. These microorganisms belong to three kingdoms of life, engage in diverse trophic lifestyles, and deploy different infection strategies. We found that interactions with microbes that do not directly enter leaf epidermal cells were seemingly unaltered or showed even enhanced microbial growth or symptom formation in the mlo2 mlo6 mlo12 triple mutants, as shown for Pseudomonas syringae and Fusarium oxysporum. By contrast, the mlo2 mlo6 mlo12 triple mutants exhibited reduced host cell entry rates by Colletotrichum higginsianum, a fungal pathogen showing direct penetration of leaf epidermal cells comparable to G. orontii. Together with previous findings, the results of this study strengthen the notion that mutations in genes MLO2, MLO6 and MLO12 not only restrict powdery mildew colonization, but also affect interactions with a number of other phytopathogens.

Interplay Between Innate Immunity and the Plant Microbiota

The innate immune system of plants recognizes microbial pathogens and terminates their growth. However, recent findings suggest that at least one layer of this system is also engaged in cooperative plant-microbe interactions and influences host colonization by beneficial microbial communities. This immune layer involves sensing of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) that initiate quantitative immune responses to control host-microbial load, whereas diversification of MAMPs and PRRs emerges as a mechanism that locally sculpts microbial assemblages in plant populations. This suggests a more complex microbial management role of the innate immune system for controlled accommodation of beneficial microbes and in pathogen elimination. The finding that similar molecular strategies are deployed by symbionts and pathogens to dampen immune responses is consistent with this hypothesis but implies different selective pressures on the immune system due to contrasting outcomes on plant fitness. The reciprocal interplay between microbiota and the immune system likely plays a critical role in shaping beneficial plant-microbiota combinations and maintaining microbial homeostasis.

Tomato root microbiota and Phytophthora parasitica-associated disease

BACKGROUND

Interactions between pathogenic oomycetes and microbiota residing on the surface of the host plant root are unknown, despite being critical to inoculum constitution. The nature of these interactions was explored for the polyphagous and telluric species Phytophthora parasitica.

RESULTS

Composition of the rhizospheric microbiota of Solanum lycopersicum was characterized using deep re-sequencing of 16S rRNA gene to analyze tomato roots either free of or partly covered with P. parasitica biofilm. Colonization of the host root surface by the oomycete was associated with a shift in microbial community involving a Bacteroidetes/Proteobacteria transition and Flavobacteriaceae as the most abundant family. Identification of members of the P. parasitica-associated microbiota interfering with biology and oomycete infection was carried out by screening for bacteria able to (i) grow on a P. parasitica extract-based medium (ii), exhibit in vitro probiotic or antibiotic activity towards the oomycete (iii), have an impact on the oomycete infection cycle in a tripartite interaction S. lycopersicum-P. parasitica-bacteria. One Pseudomonas phylotype was found to exacerbate disease symptoms in tomato plants. The lack of significant gene expression response of P. parasitica effectors to Pseudomonas suggested that the increase in plant susceptibility was not associated with an increase in virulence. Our results reveal that Pseudomonas spp. establishes commensal interactions with the oomycete. Bacteria preferentially colonize the surface of the biofilm rather than the roots, so that they can infect plant cells without any apparent infection of P. parasitica.

CONCLUSIONS

The presence of the pathogenic oomycete P. parasitica in the tomato rhizosphere leads to a shift in the rhizospheric microbiota composition. It contributes to the habitat extension of Pseudomonas species mediated through a physical association between the oomycete and the bacteria.

Albugo-imposed changes to tryptophan-derived antimicrobial metabolite biosynthesis may contribute to suppression of non-host resistance to Phytophthora infestans in Arabidopsis thaliana

Background

Plants are exposed to diverse pathogens and pests, yet most plants are resistant to most plant pathogens. Non-host resistance describes the ability of all members of a plant species to successfully prevent colonization by any given member of a pathogen species. White blister rust caused by Albugo species can overcome non-host resistance and enable secondary infection and reproduction of usually non-virulent pathogens, including the potato late blight pathogen Phytophthora infestans on Arabidopsis thaliana. However, the molecular basis of host defense suppression in this complex plant–microbe interaction is unclear. Here, we investigate specific defense mechanisms in Arabidopsis that are suppressed by Albugo infection.

Results

Gene expression profiling revealed that two species of Albugo upregulate genes associated with tryptophan-derived antimicrobial metabolites in Arabidopsis. Albugo laibachii-infected tissue has altered levels of these metabolites, with lower indol-3-yl methylglucosinolate and higher camalexin accumulation than uninfected tissue. We investigated the contribution of these Albugo-imposed phenotypes to suppression of non-host resistance to P. infestans. Absence of tryptophan-derived antimicrobial compounds enables P. infestans colonization of Arabidopsis, although to a lesser extent than Albugo-infected tissue. A. laibachii also suppresses a subset of genes regulated by salicylic acid; however, salicylic acid plays only a minor role in non-host resistance to P. infestans.

Conclusions

Albugo sp. alter tryptophan-derived metabolites and suppress elements of the responses to salicylic acid in Arabidopsis. Albugo sp. imposed alterations in tryptophan-derived metabolites may play a role in Arabidopsis non-host resistance to P. infestans. Understanding the basis of non-host resistance to pathogens such as P. infestans could assist in development of strategies to elevate food security.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0360-z) contains supplementary material, which is available to authorized users.

Arabidopsis late blight: infection of a nonhost plant by Albugo laibachii enables full colonization by Phytophthora infestans

The oomycete pathogen Phytophthora infestans causes potato late blight, and as a potato and tomato specialist pathogen, is seemingly poorly adapted to infect plants outside the Solanaceae. Here, we report the unexpected finding that P. infestans can infect Arabidopsis thaliana when another oomycete pathogen, Albugo laibachii, has colonized the host plant. The behaviour and speed of P. infestans infection in Arabidopsis pre-infected with A. laibachii resemble P. infestans infection of susceptible potato plants. Transcriptional profiling of P. infestans genes during infection revealed a significant overlap in the sets of secreted-protein genes that are induced in P. infestans upon colonization of potato and susceptible Arabidopsis, suggesting major similarities in P. infestans gene expression dynamics on the two plant species. Furthermore, we found haustoria of A. laibachii and P. infestans within the same Arabidopsis cells. This Arabidopsis-A. laibachii-P. infestans tripartite interaction opens up various possibilities to dissect the molecular mechanisms of P. infestans infection and the processes occurring in co-infected Arabidopsis cells.