StWRKY8 transcription factor regulates benzylisoquinoline alkaloid pathway in potato conferring resistance to late blight

The resistance to late blight is either qualitative or quantitative in nature. Quantitative resistance is durable, but challenging due to polygenic inheritance. In the present study, the diploid potato genotypes resistant and susceptible to late blight, were profiled for metabolites. Tissue specific metabolite analysis of benzylisoquinoline alkaloids (BIAs) in response to pathogen infection revealed increased accumulation of morphinone, codeine-6-glucuronide and morphine-3-glucuronides. These BIAs are antimicrobial compounds and possibly involved in cell wall reinforcement, especially through cross-linking cell wall pectins. Quantitative reverse transcription-PCR studies revealed higher expressions of TyDC, NCS, COR-2 and StWRKY8 transcription factor genes, in resistant genotypes than in susceptible genotype, following pathogen inoculation. A luciferase transient expression assay confirmed the binding of the StWRKY8 TF to promoters of downstream genes, elucidating a direct regulatory role on BIAs biosynthetic genes. Sequence analysis of StWRKY8 in potato genotypes revealed polymorphism in the WRKY DNA binding domain in the susceptible genotype, which is important for the regulatory function of this gene. A complementation assay of StWRKY8 in Arabidopsis wrky33 mutant background was associated with decreased fungal biomass. In conclusion, StWRKY8 regulates the biosynthesis of BIAs that are both antimicrobial and reinforce cell walls to contain the pathogen to initial infection.

The Cell Death Triggered by the Nuclear Localized RxLR Effector PITG_22798 from Phytophthora infestans Is Suppressed by the Effector AVR3b

Phytopathogenic oomycetes, such as Phytophthora infestans, potentially secrete many RxLR effector proteins into plant cells to modulate plant immune responses and promote colonization. However, the molecular mechanisms by which these RxLR effectors suppress plant immune responses are largely unknown. Here we describe an RxLR effector PITG_22798 (Gene accession: XM_002998349) that was upregulated during early infection of potato by P. infestans. By employment of agroinfiltration, we observed that PITG_22798 triggers cell death in Nicotiana benthamiana. Confocal microscopic examination showed that PITG_22798-GFP (Green Fluorescent Protein) located in the host nucleus when expressed transiently in N. benthamiana leaves. A nuclear localization signal (NLS) domain of PITG_22798 is important for nuclear localization and cell death-inducing activity. Sequence alignment and transient expression showed that PITG_22798 from diverse P. infestans isolates are conserved, and transient expression of PITG_22798 enhances P. infestans colonization of N. benthamiana leaves, which suggests that PITG_22798 contributes to P. infestans infection. PITG_22798-triggered cell death is dependent on SGT1-mediated signaling and is suppressed by the P. infestans avirulence effector 3b (AVR3b). The present research provides a clue for further investigation of how P. infestans effector PITG_22798 associates with and modulates host immunity.

The Loricrin-Like Protein (LLP) of Phytophthora infestans Is Required for Oospore Formation and Plant Infection

Loricrin-like protein (LLP) is characterized by a high content of glycine residues and is a major component of plant cell wall. Here, we identified a Phytophthora infestans ortholog of plant LLP, named PiLLP. In P. infestans, PiLLP is strongly expressed in asexual and sexual developmental stages, including in sporangia, zoospores and germinating cysts, and during oospore formation, as well as in the early stages of infection and during hydrogen peroxide stress. Compared with the wild type, the PiLLP-silenced transformants were defective in oospore formation, had slower colony expansion rates, produced less sporangia with lower germination and zoospore-release rates, and were more sensitive to hydrogen peroxide. Moreover, Nile red staining, and PiLLP-red fluorescent protein fusions indicated that PiLLP is involved in oogonia formation. The silenced transformants also had severely diminished virulence levels that could be partially restored with diphenyleneiodium treatments. The analysis of catalase activity showed a decrease of catalase activity in silenced transformants. Thus, PiLLP is important for sexual and asexual reproduction, and is required for oxidative stress tolerance and plant infection.

A survey of metabolic changes in potato leaves by NMR-based metabolic profiling in relation to resistance to late blight disease under field conditions

Non-targeted nuclear magnetic resonance (NMR)-based metabolic profiling was applied to potato leaves to survey metabolic changes associated with late blight resistance under field conditions. Potato plants were grown in an experimental field, and the compound leaves with no visible symptoms were collected from 20 cultivars/lines at two sampling time points: (i) the time of initial presentation of symptoms in susceptible cultivars and (ii) 12 days before this initiation. ¹ H NMR spectra of the foliar metabolites soluble in deuterium oxide- or methanol-d₄ -based buffers were measured and used for multivariate analysis. Principal component analysis for six cultivars at symptom initiation showed a class separation corresponding to their levels of late blight resistance. This separation was primarily explained by higher levels of malic acid, methanol, and rutin and a lower level of sucrose in the resistant cultivars than in the susceptible ones. Partial least squares regression revealed that the levels of these metabolites were strongly associated with the disease severity measured in this study under field conditions. These associations were observed only for the leaves harvested at the symptom initiation stage, but not for those collected 12 days beforehand. Subsequently, a simple, alternative enzymatic assay for l-malic acid was used to estimate late blight resistance, as a model for applying the potential metabolic marker obtained. This study demonstrated the potential of metabolomics for field-grown plants in combination with targeted methods for quantifying marker levels, moving towards marker-assisted screening of new cultivars with durable late blight resistance. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of a genetically modified potato on a non-target aphid are outweighed by cultivar differences

Insect-plant interactions may be unintentionally affected when introducing genetically modified (GM) crops into an agro-ecosystem. Our aim was to test the non-target effects of a late blight-resistant GM potato on Myzus persicae in greenhouse and climate room experiments and understand how position and number of R gene insertions can affect non-targets in GM events. We also aimed to compare results to baseline differences among three conventional potato varieties varying in resistance to late blight. Aphid development and survival were affected by some GM events in the first generation, though effects disappeared in the second generation. Effects were not dependent on the presence of a marker gene or the insertion of a second resistance gene. Positional effects of gene insertion influenced aphid performance on certain GM events. However, aphid fitness varied considerably more between conventional potato varieties than between Désirée and the GM events. Comparing different GM events to the non-transformed variety is relevant, since unintended effects of insertion can occur. Our protocols can be recommended for in planta risk assessments with aphids. Ecological perspective is gained by selecting several measured endpoints and by comparing the results with a baseline of conventional cultivars.

Knocking down expression of the auxin-amidohydrolase IAR3 alters defense responses in Solanaceae family plants

In plants, indole-3-acetic acid (IAA) amido hydrolases (AHs) participate in auxin homeostasis by releasing free IAA from IAA-amino acid conjugates. We investigated the role of IAR3, a member of the IAA amido hydrolase family, in the response of Solanaceous plants challenged by biotrophic and hemi-biotrophic pathogens. By means of genome inspection and phylogenic analysis we firstly identified IAA-AH sequences and putative IAR3 orthologs in Nicotiana benthamiana, tomato and potato. We evaluated the involvement of IAR3 genes in defense responses by using virus-induced gene silencing. We observed that N. benthamiana and tomato plants with knocked-down expression of IAR3 genes contained lower levels of free IAA and presented altered responses to pathogen attack, including enhanced basal defenses and higher tolerance to infection in susceptible plants. We showed that IAR3 genes are consistently up-regulated in N. benthamiana and tomato upon inoculation with Phytophthora infestans and Cladosporium fulvum respectively. However, IAR3 expression decreased significantly when hypersensitive response was triggered in transgenic tomato plants coexpressing the Cf-4 resistance gene and the avirulence factor Avr4. Altogether, our results indicate that changes in IAR3 expression lead to alteration in auxin homeostasis that ultimately affects plant defense responses.

The Solanum demissum R8 late blight resistance gene is an Sw-5 homologue that has been deployed worldwide in late blight resistant varieties

The potato late blight resistance gene R8 has been cloned. R8 is found in five late blight resistant varieties deployed in three different continents. R8 recognises Avr8 and is homologous to the NB-LRR protein Sw-5 from tomato. The broad spectrum late blight resistance gene R8 from Solanum demissum was cloned based on a previously published coarse map position on the lower arm of chromosome IX. Fine mapping in a recombinant population and bacterial artificial chromosome (BAC) library screening resulted in a BAC contig spanning 170 kb of the R8 haplotype. Sequencing revealed a cluster of at least ten R gene analogues (RGAs). The seven RGAs in the genetic window were subcloned for complementation analysis. Only one RGA provided late blight resistance and caused recognition of Avr8. From these results, it was concluded that the newly cloned resistance gene was indeed R8. R8 encodes a typical intracellular immune receptor with an N-terminal coiled coil, a central nucleotide binding site and 13 C-terminal leucine rich repeats. Phylogenetic analysis of a set of representative Solanaceae R proteins shows that R8 resides in a clearly distinct clade together with the Sw-5 tospovirus R protein from tomato. It was found that the R8 gene is present in late blight resistant potato varieties from Europe (Sarpo Mira), USA (Jacqueline Lee, Missaukee) and China (PB-06, S-60). Indeed, when tested under field conditions, R8 transgenic potato plants showed broad spectrum resistance to the current late blight population in the Netherlands, similar to Sarpo Mira.

Phytophthora infestans Argonaute 1 binds microRNA and small RNAs from effector genes and transposable elements

Phytophthora spp. encode large sets of effector proteins and distinct populations of small RNAs (sRNAs). Recent evidence has suggested that pathogen-derived sRNAs can modulate the expression of plant defense genes. Here, we studied the sRNA classes and functions associated with Phytophthora infestans Argonaute (Ago) proteins. sRNAs were co-immunoprecipitated with three PiAgo proteins and deep sequenced. Twenty- to twenty-two-nucleotide (nt) sRNAs were identified as the main interaction partners of PiAgo1 and high enrichment of 24-26-nt sRNAs was seen in the PiAgo4-bound sample. The frequencies and sizes of transposable element (TE)-derived sRNAs in the different PiAgo libraries suggested diversified roles of the PiAgo proteins in the control of different TE classes. We further provide evidence for the involvement of PiAgo1 in the P. infestans microRNA (miRNA) pathway. Protein-coding genes are probably regulated by the shared action of PiAgo1 and PiAgo5, as demonstrated by analysis of differential expression. An abundance of sRNAs from genes encoding host cell death-inducing Crinkler (CRN) effectors was bound to PiAgo1, implicating this protein in the regulation of the expanded CRN gene family. The data suggest that PiAgo1 plays an essential role in gene regulation and that at least two RNA silencing pathways regulate TEs in the plant-pathogenic oomycete P. infestans.

Trade-offs and evolution of thermal adaptation in the Irish potato famine pathogen Phytophthora infestans

Temperature is one of the most important environmental parameters with crucial impacts on nearly all biological processes. Due to anthropogenic activity, average air temperatures are expected to increase by a few degrees in coming decades, accompanied by an increased occurrence of extreme temperature events. Such global trends are likely to have various major impacts on human society through their influence on natural ecosystems, food production and biotic interactions, including diseases. In this study, we used a combination of statistical genetics, experimental evolution and common garden experiments to investigate the evolutionary potential for thermal adaptation in the potato late blight pathogen, Phytophthora infestans, and infer its likely response to changing temperatures. We found a trade-off associated with thermal adaptation to heterogeneous environments in P. infestans, with the degree of the trade-off peaking approximately at the pathogen's optimum growth temperature. A genetic trade-off in thermal adaptation was also evidenced by the negative association between a strain's growth rate and its thermal range for growth, and warm climates selecting for a low pathogen growth rate. We also found a mirror effect of phenotypic plasticity and genetic adaptation on growth rate. At below the optimum, phenotypic plasticity enhances pathogen's growth rate but nature selects for slower growing genotypes when temperature increases. At above the optimum, phenotypic plasticity reduces pathogen's growth rate but natural selection favours for faster growing genotypes when temperature increases further. We conclude from these findings that the growth rate of P. infestans will only be marginally affected by global warming.

Nuclear DNA content of the hybrid plant pathogen Phytophthora andina determined by flow cytometry

Phytophthora andina is a heterothallic plant pathogen of Andean solanaceous hosts and is an interspecific hybrid of P. infestans and an unknown Phytophthora species. The objective of this study was to estimate the nuclear DNA content of isolates in three clonal lineages of P. andina relative to P. infestans Twelve isolates of P. andina and six isolates of P. infestans were measured for nuclear DNA content by propidium iodide-stained flow cytometry. We found that the DNA content of P. andina was similar but slightly smaller, on average, than that of our sample of P. infestans isolates. This is consistent with P. andina being a homoploid hybrid rather than allopolyploid hybrid. Nuclear DNA content was more variable among a smaller sample of P. infestans isolates, including a putative triploid isolate from Mexico, but small differences in nuclear DNA content were also observed among P. andina isolates. Both species appear to be able to tolerate significant variation in genome size.

Interaction between the moss Physcomitrella patens and Phytophthora: a novel pathosystem for live-cell imaging of subcellular defence

Live-cell imaging of plant-pathogen interactions is often hampered by the tissue complexity and multicell layered nature of the host. Here, we established a novel pathosystem with the moss Physcomitrella patens as host for Phytophthora. The tip-growing protonema cells of this moss are ideal for visualizing interactions with the pathogen over time using high-resolution microscopy. We tested four Phytophthora species for their ability to infect P. patens and showed that P. sojae and P. palmivora were only rarely capable to infect P. patens. In contrast, P. infestans and P. capsici frequently and successfully penetrated moss protonemal cells, showed intracellular hyphal growth and formed sporangia. Next to these successful invasions, many penetration attempts failed. Here the pathogen was blocked by a barrier of cell wall material deposited in papilla-like structures, a defence response that is common in higher plants. Another common response is the upregulation of defence-related genes upon infection and also in moss we observed this upregulation in tissues infected with Phytophthora. For more advanced analyses of the novel pathosystem we developed a special set-up that allowed live-cell imaging of subcellular defence processes by high-resolution microscopy. With this set-up, we revealed that Phytophthora infection of moss induces repositioning of the nucleus, accumulation of cytoplasm and rearrangement of the actin cytoskeleton, but not of microtubules.

Loop-mediated isothermal amplification for detection of the tomato and potato late blight pathogen, Phytophthora infestans

AIMS

To design and validate a colorimetric loop-mediated isothermal amplification assay for rapid detection of Phytophthora infestans DNA.

METHODS AND RESULTS

Two sets of loop-mediated isothermal amplification (LAMP) primers were designed and evaluated for their sensitivity and specificity for P. infestans. ITSII primers targeted a portion of the internal transcribed spacer region of ribosomal DNA. These primers had a limit of detection of 2 pg P. infestans DNA and cross-reacted with the closely related species Phytophthora nicotianae. Rgn86_2 primers, designed to improve assay specificity, targeted a portion of a conserved hypothetical protein. These primers had a limit of detection of 200 pg P. infestans DNA and did not cross-react with P. nicotianae. The specificity of the Rgn86_2 assay was tested further using the closely related species P. andina, P. ipomoeae, P. mirabilis and P. phaseoli. Cross-reactions occurred with P. andina and P. mirabilis, but neither species occurs on tomato or potato. Both primer sets were able to detect P. infestans DNA extracted from tomato late blight leaf lesions.

CONCLUSIONS

Two colorimetric LAMP assays detected P. infestans DNA from pure cultures as well as infected leaf tissue. The ITSII primers had higher sensitivity, and the Rgn86_2 primers had higher specificity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of a LAMP assay for the detection of P. infestans, the causal organism of potato and tomato late blight. These assays have potential for immediate utility in plant disease research and diagnostic laboratories.

Mitochondrial DNA assessment of Phytophthora infestans isolates from potato and tomato in Ethiopia reveals unexpected diversity

Mitochondrial DNA (mtDNA) haplotypes were determined using restriction fragment length polymorphism (RFLP) for P. infestans sampled from 513 foliar lesions of late blight found on potato and tomato in different regions of Ethiopia. Among the four reported mitochondrial haplotypes of Phytophthora infestans, Ia, Ib and IIb were detected in 93 % of the samples analyzed but the vast majority of these were Ia. The remaining 7 % represented a previously unreported haplotype. DNA sequencing of this new haplotype also confirmed a single base nucleotide substitution that resulted in loss of EcoRI restriction site and gain of two additional MspI sites in cox1 and atp1 genes, respectively. There were 28 polymorphic sites among all nucleotide sequences including five reference isolates. Sites with alignment gaps were observed in P4 with one nucleotide deletion in 11 Ethiopian isolates. None of the reference sequence produced frame-shifts, with the exception of the 3-nucleotide deletion in the P4 region by Phytophthora andina, a feature that can be used to distinguish the new Ethiopian isolates from P. andina. While a distinguishing molecular data presented here clearly separated them from P. infestans, 7 % of the isolates that share this feature formed an important component of the late blight pathogen causing disease on Solanum tuberosum in Ethiopia. Thus, these Ethiopian isolates could represent a novel Phytophthora species reported for the first time here.

Molecular cloning, functional characterization and expression of potato (Solanum tuberosum) 1-deoxy-d-xylulose 5-phosphate synthase 1 (StDXS1) in response to Phytophthora infestans

1-Deoxy-D-xylulose 5-phosphate synthase (DXS) catalyzes the initial step of the plastidial 2C-methyl-D-erythritol-4-phosphate (DOXP-MEP) pathway involved in isoprenoid biosynthesis. In this study, we cloned the complete cDNA of potato DXS gene that was designated StDXS1. StDXS1 cDNA encodes for 719 amino acid residues, with MW of 77.8 kDa, and is present in one copy in the potato genome. Phylogenetic analysis and protein sequence alignments assigned StDXS1 to a group with DXS homologues from closely related species and exhibited homodomain identity with known DXS proteins from other plant species. Late blight symptoms occurred in parallel with a reduction in StDXS1 transcript levels, which may be associated with the levels of isoprenoids that contribute to plant protection against pathogens. Subcellular localization indicated that StDXS1 targets the chloroplasts where isoprenoids are synthesized. Arabidopsis expressing StDXS1 showed a higher accumulation of carotenoids and chlorophyll as compared to wild type controls. Lower levels of ABA and GA were detected in the transgenic DXS lines as compared to control plants, which reflected on higher germination rates of the transgenic DXS lines. No changes were detected in JA or SA contents. Selected downstream genes in the DOXP-MEP pathway, especially GGPPS genes, were up-regulated in the transgenic lines.

Genomic Characterization of a South American Phytophthora Hybrid Mandates Reassessment of the Geographic Origins of Phytophthora infestans

As the oomycete pathogen causing potato late blight disease, Phytophthora infestans triggered the famous 19th-century Irish potato famine and remains the leading cause of global commercial potato crop destruction. But the geographic origin of the genotype that caused this devastating initial outbreak remains disputed, as does the New World center of origin of the species itself. Both Mexico and South America have been proposed, generating considerable controversy. Here, we readdress the pathogen's origins using a genomic data set encompassing 71 globally sourced modern and historical samples of P. infestans and the hybrid species P. andina, a close relative known only from the Andean highlands. Previous studies have suggested that the nuclear DNA lineage behind the initial outbreaks in Europe in 1845 is now extinct. Analysis of P. andina's phased haplotypes recovered eight haploid genome sequences, four of which represent a previously unknown basal lineage of P. infestans closely related to the famine-era lineage. Our analyses further reveal that clonal lineages of both P. andina and historical P. infestans diverged earlier than modern Mexican lineages, casting doubt on recent claims of a Mexican center of origin. Finally, we use haplotype phasing to demonstrate that basal branches of the clade comprising Mexican samples are occupied by clonal isolates collected from wild Solanum hosts, suggesting that modern Mexican P. infestans diversified on Solanum tuberosum after a host jump from a wild species and that the origins of P. infestans are more complex than was previously thought.

An effector of the Irish potato famine pathogen antagonizes a host autophagy cargo receptor

Plants use autophagy to safeguard against infectious diseases. However, how plant pathogens interfere with autophagy-related processes is unknown. Here, we show that PexRD54, an effector from the Irish potato famine pathogen Phytophthora infestans, binds host autophagy protein ATG8CL to stimulate autophagosome formation. PexRD54 depletes the autophagy cargo receptor Joka2 out of ATG8CL complexes and interferes with Joka2's positive effect on pathogen defense. Thus, a plant pathogen effector has evolved to antagonize a host autophagy cargo receptor to counteract host defenses.

DOI:http://dx.doi.org/10.7554/eLife.10856.001

Plants and other living organisms can survive stress and starvation by digesting and recycling parts of their own cells. This process is known as autophagy and it involves engulfing cellular material inside spherical structures called autophagosomes, before delivering it to sites in the cell where digestive enzymes can break the material down. A form of autophagy, known as selective autophagy, can specifically degrade toxic substances such as disease-causing microbes. Selective autophagy works through proteins called autophagy cargo receptors that define which molecules are targeted for degradation. However, it was not clear whether autophagy protects plants from infections, or how much disease-causing microbes interfere with this process for their own benefit.

The microbe that causes late blight of potatoes (called Phytophthora infestans) is infamous for triggering widespread famines in Ireland in the 19^th century. This disease-causing microbe continues to pose a serious threat to food security today, and parasitizes plant tissues by releasing proteins called effectors that enter the plant’s cells to subvert the plant’s physiology and counteract its defenses.

Dagdas, Belhaj et al. now report that an effector from P. infestans, called PexRD54, can bind to autophagy-related protein from potato, called ATG8CL, and stimulate the formation of autophagosomes. Further experiments revealed that the PexRD54 effector could outcompete a plant autophagy cargo receptor that would otherwise bind to ATG8CL. This plant cargo receptor contributes to the plant’s defences, and by preventing it from interacting with ATG8CL, PexRD54 makes the plant more susceptible to infection by P. infestans.

These findings show that the PexRD54 effector has evolved to interact with an autophagy-related protein to counteract the plant’s defences. Dagdas, Belhaj et al. suggest that PexRD54 might do this by activating autophagy to selectively eliminate some of the molecules that the plant use to defend itself. Furthermore, P. infestans might also benefit from the nutrients that are released when cellular material is broken down via autophagy. Future work could test these two hypotheses and explore whether other effectors from disease-causing microbes work in a similar way.

DOI:http://dx.doi.org/10.7554/eLife.10856.002

Transcriptional dynamics of Phytophthora infestans during sequential stages of hemibiotrophic infection of tomato

Hemibiotrophic plant pathogens, such as the oomycete Phytophthora infestans, employ a biphasic infection strategy, initially behaving as biotrophs, where minimal symptoms are exhibited by the plant, and subsequently as necrotrophs, feeding on dead plant tissue. The regulation of this transition and the breadth of molecular mechanisms that modulate plant defences are not well understood, although effector proteins secreted by the pathogen are thought to play a key role. We examined the transcriptional dynamics of P. infestans in a compatible interaction with its host tomato (Solanum lycopersicum) at three infection stages: biotrophy; the transition from biotrophy to necrotrophy; and necrotrophy. The expression data suggest a tight temporal regulation of many pathways associated with the suppression of plant defence mechanisms and pathogenicity, including the induction of putative cytoplasmic and apoplastic effectors. Twelve of these were experimentally evaluated to determine their ability to suppress necrosis caused by the P. infestans necrosis-inducing protein PiNPP1.1 in Nicotiana benthamiana. Four effectors suppressed necrosis, suggesting that they might prolong the biotrophic phase. This study suggests that a complex regulation of effector expression modulates the outcome of the interaction.

Utilizing "Omic" Technologies to Identify and Prioritize Novel Sources of Resistance to the Oomycete Pathogen Phytophthora infestans in Potato Germplasm Collections

The greatest threat to potato production world-wide is late blight, caused by the oomycete pathogen Phytophthora infestans. A screen of 126 wild diploid Solanum accessions from the Commonwealth Potato Collection (CPC) with P. infestans isolates belonging to the genotype 13-A2 identified resistances in the species S. bulbocastanum, S. capsicibaccatum, S. microdontum, S. mochiquense, S. okadae, S. pinnatisectum, S. polyadenium, S. tarijense, and S. verrucosum. Effector-omics, allele mining, and diagnostic RenSeq (dRenSeq) were utilized to investigate the nature of resistances in S. okadae accessions. dRenSeq in resistant S. okadae accessions 7129, 7625, 3762, and a bulk of 20 resistant progeny confirmed the presence of full-length Rpi-vnt1.1 under stringent mapping conditions and corroborated allele mining results in the accessions 7129 and 7625 as well as Avr-vnt1 recognition in transient expression assays. In contrast, susceptible S. okadae accession 3761 and a bulk of 20 susceptible progeny lacked sequence homology in the 5' end compared to the functional Rpi-vnt1.1 gene. Further evaluation of S. okadae accessions with P. infestans isolates that have a broad spectrum of virulence demonstrated that, although S. okadae accessions 7129, 7625, and 7629 contain functional Rpi-vnt1.1, they also carry a novel resistance gene. We provide evidence that existing germplasm collections are important sources of novel resistances and that "omic" technologies such as dRenSeq-based genomics and effector-omics are efficacious tools to rapidly explore the diversity within these collections.

Transcription factor StWRKY1 regulates phenylpropanoid metabolites conferring late blight resistance in potato

Quantitative resistance is polygenically controlled and durable, but the underlying molecular and biochemical mechanisms are poorly understood. Secondary cell wall thickening is a critical process in quantitative resistance, regulated by transcriptional networks. This paper provides compelling evidence on the functionality of StWRKY1 transcription factor, in a compatible interaction of potato-Phytophthora infestans, to extend our knowledge on the regulation of the metabolic pathway genes leading to strengthening the secondary cell wall. A metabolomics approach was used to identify resistance-related metabolites belonging to the phenylpropanoid pathway and their biosynthetic genes regulated by StWRKY1. The StWRKY1 gene in resistant potato was silenced to decipher its role in the regulation of phenylpropanoid pathway genes to strengthen the secondary cell wall. Sequencing of the promoter region of StWRKY1 in susceptible genotypes revealed the absence of heat shock elements (HSEs). Simultaneous induction of both the heat shock protein (sHSP17.8) and StWRKY1 following pathogen invasion enables functioning of the latter to interact with the HSE present in the resistant StWRKY1 promoter region. EMSA and luciferase transient expression assays further revealed direct binding of StWRKY1 to promoters of hydroxycinnamic acid amide (HCAA) biosynthetic genes encoding 4-coumarate:CoA ligase and tyramine hydroxycinnamoyl transferase. Silencing of the StWRKY1 gene was associated with signs of reduced late blight resistance by significantly increasing the pathogen biomass and decreasing the abundance of HCAAs. This study provides convincing evidence on the role of StWRKY1 in the regulation of downstream genes to biosynthesize HCAAs, which are deposited to reinforce secondary cell walls.

BABA-primed defense responses to Phytophthora infestans in the next vegetative progeny of potato

The transcript of the PR1 gene accumulation as an informative marker of systemic acquired resistance (SAR) was analyzed in β-aminobutyric acid (BABA) primed potato in the short-lasting (3 days) and long-lasting (28 days) time periods after induction and in the vegetative descendants of primed plants derived from tubers and from in vitro seedlings. BABA pretreatment resulted either in minimal or no PR1 gene expression, but sequential treatment with BABA followed by virulent Phytophthora infestans provided data on the imprint of post-stress information and its duration until fertilization, in the form of an enhanced PR1 transcript accumulation and a transient increase of basal resistance to the late blight disease. The primed state for defense of the susceptible potato cultivar was transmitted to its vegetative progeny as a potentiated PR1 mRNA accumulation following challenge inoculation. However, variation was observed between vegetative accessions of the BABA-primed potato genotype in responsiveness to disease. In contrast to plants derived from tubers, potato propagated through in vitro seedlings largely lost inducible resistance traits, although itretained primed PR1 gene expression.

A novel workflow correlating RNA-seq data to Phythophthora infestans resistance levels in wild Solanum species and potato clones

Comparative transcriptomics between species can provide valuable understanding of plant-pathogen interactions. Here, we focus on wild Solanum species and potato clones with varying degree of resistance against Phytophthora infestans, which causes the devastating late blight disease in potato. The transcriptomes of three wild Solanum species native to Southern Sweden, Solanum dulcamara, Solanum nigrum, and Solanum physalifolium were compared to three potato clones, Desiree (cv.), SW93-1015 and Sarpo Mira. Desiree and S. physalifolium are susceptible to P. infestans whereas the other four have different degrees of resistance. By building transcript families based on de novo assembled RNA-seq across species and clones and correlating these to resistance phenotypes, we created a novel workflow to identify families with expanded or depleted number of transcripts in relation to the P. infestans resistance level. Analysis was facilitated by inferring functional annotations based on the family structure and semantic clustering. More transcript families were expanded in the resistant clones and species and the enriched functions of these were associated to expected gene ontology (GO) terms for resistance mechanisms such as hypersensitive response, host programmed cell death and endopeptidase activity. However, a number of unexpected functions and transcripts were also identified, for example transmembrane transport and protein acylation expanded in the susceptible group and a cluster of Zinc knuckle family proteins expanded in the resistant group. Over 400 expressed putative resistance (R-)genes were identified and resistant clones Sarpo Mira and SW93-1015 had ca 25% more expressed putative R-genes than susceptible cultivar Desiree. However, no differences in numbers of susceptibility (S-)gene homologs were seen between species and clones. In addition, we identified P. infestans transcripts including effectors in the early stages of P. infestans-Solanum interactions.

Immune activation mediated by the late blight resistance protein R1 requires nuclear localization of R1 and the effector AVR1

Resistance against oomycete pathogens is mainly governed by intracellular nucleotide-binding leucine-rich repeat (NLR) receptors that recognize matching avirulence (AVR) proteins from the pathogen, RXLR effectors that are delivered inside host cells. Detailed molecular understanding of how and where NLR proteins and RXLR effectors interact is essential to inform the deployment of durable resistance (R) genes. Fluorescent tags, nuclear localization signals (NLSs) and nuclear export signals (NESs) were exploited to determine the subcellular localization of the potato late blight protein R1 and the Phytophthora infestans RXLR effector AVR1, and to target these proteins to the nucleus or cytoplasm. Microscopic imaging revealed that both R1 and AVR1 occurred in the nucleus and cytoplasm, and were in close proximity. Transient expression of NLS- or NES-tagged R1 and AVR1 in Nicotiana benthamiana showed that activation of the R1-mediated hypersensitive response and resistance required localization of the R1/AVR1 pair in the nucleus. However, AVR1-mediated suppression of cell death in the absence of R1 was dependent on localization of AVR1 in the cytoplasm. Balanced nucleocytoplasmic partitioning of AVR1 seems to be a prerequisite. Our results show that R1-mediated immunity is activated inside the nucleus with AVR1 in close proximity and suggest that nucleocytoplasmic transport of R1 and AVR1 is tightly regulated.

Plant-mediated gene silencing restricts growth of the potato late blight pathogen Phytophthora infestans

Phytophthora infestans is an oomycete that causes severe damage to potato, and is well known for its ability to evolve rapidly in order to overcome resistant potato varieties. An RNA silencing strategy was evaluated here to clarify if small interfering RNA homologous to selected genes in P. infestans could be targeted from the plant host to reduce the magnitude of the infection. As a proof-of-concept, a hairpin RNA (hp-RNA) construct using the GFP marker gene was designed and introduced in potato. At 72 hpi, a 55-fold reduction of the signal intensity of a corresponding GFP expressing P. infestans strain on leaf samples of transgenic plants, compared with wild-type potato, was detected. This suggests that an RNA interference construct in the potato host could be processed and target a transcript of the pathogen. Three genes important in the infection process of P. infestans, PiGPB1, PiCESA2, and PiPEC, together with PiGAPDH taking part in basic cell maintenance were subsequently tested using an analogous transgenic strategy. Out of these gene candidates, the hp-PiGPB1 targeting the G protein β-subunit (PiGPB1) important for pathogenicity resulted in most restricted disease progress. Further, Illumina sequencing of inoculated transgenic potato leaves revealed sRNAs of 24/25 nt size homologous to the PiGPB1 gene in the transgenic plants indicating post-transcriptional silencing of the target gene. The work demonstrates that a host-induced gene-silencing approach is functional against P. infestans but is highly dependent on target gene for a successful outcome. This finding broadens the arsenal of control strategies to this important plant disease.

Methanol extract of mycelia from Phytophthora infestans-induced resistance in potato

Plants recognize certain microbial compounds as elicitors in their active defence mechanisms. It has been shown that a series of defence reactions are induced in potato plant cells after treatment with water-soluble hyphal wall components prepared from Phytophthora infestans. In this study, a methanol extract from mycelia of P. infestans (MEM), which contains lipophilic compounds, was used as another elicitor for the induction of the defence reactions in potato. MEM elicitor induced reactive oxygen species (ROS), especially O2(-) and H2O2 production, and nitric oxide (NO) generation in potato leaves and suspension-cultured cells. Hypersensitive cell death was detected in potato leaves within 6-8 h after MEM elicitor treatment. The accumulation of phytoalexins was detected by MEM elicitor treatment in potato tubers. In potato suspension-cultured cells, several defence-related genes were induced by MEM elicitors, namely Strboh, Sthsr203J, StPVS3, StPR1, and StNR5, which regulate various defence-related functions. Enhanced resistance against P. infestans was found in MEM-treated potato plants. These results suggested that MEM elicitor is recognized by host and enhances defence activities to produce substances inhibitory to pathogens.

Rerouting of plant late endocytic trafficking toward a pathogen interface

A number of plant pathogenic and symbiotic microbes produce specialized cellular structures that invade host cells where they remain enveloped by host-derived membranes. The mechanisms underlying the biogenesis and functions of host-microbe interfaces are poorly understood. Here, we show that plant late endocytic trafficking is diverted toward the extrahaustorial membrane (EHM); a host-pathogen interface that develops in plant cells invaded by Irish potato famine pathogen Phytophthora infestans. A late endosome and tonoplast marker protein Rab7 GTPase RabG3c, but not a tonoplast-localized sucrose transporter, is recruited to the EHM, suggesting specific rerouting of vacuole-targeted late endosomes to a host-pathogen interface. We revealed the dynamic nature of this process by showing that, upon activation, a cell surface immune receptor traffics toward the haustorial interface. Our work provides insight into the biogenesis of the EHM and reveals dynamic processes that recruit membrane compartments and immune receptors to this host-pathogen interface.