The tomato powdery mildew fungus Oidium neolycopersici

Fighting Tomato Fungal Diseases with a Biocontrol Product Based on Amoeba Lysate

New solutions to reduce the use of chemical pesticides to combat plant diseases and to meet societal and political demands are needed to achieve sustainable agriculture. Tomato production, both in greenhouses and in open fields, is affected by numerous pathogens. The aim of this study is to assess the possibility of controlling both late blight and powdery mildew in tomatoes with a single biocontrol product currently under registration. The biocontrol product AXP12, based on the lysate of Willaertia magna C2c Maky, has already proved its efficacy against downy mildew of grapevine and potato late blight. Its ability to elicit tomato defenses and its efficacy in the greenhouse and in the field were tested. This study establishes that AXP12 stimulates the tomato genes involved in plant defense pathways and has the capacity to combat in greenhouse and field both late blight (Phytophtora infestans) and powdery mildew (Oidium neolycopersici and Leveillula taurica) of tomato.

Needles in fungal haystacks: Discovery of a putative a-factor pheromone and a unique mating strategy in the Leotiomycetes

The Leotiomycetes is a hugely diverse group of fungi, accommodating a wide variety of important plant and animal pathogens, ericoid mycorrhizal fungi, as well as producers of antibiotics. Despite their importance, the genetics of these fungi remain relatively understudied, particularly as they don't include model taxa. For example, sexual reproduction and the genetic mechanisms that underly this process are poorly understood in the Leotiomycetes. We exploited publicly available genomic and transcriptomic resources to identify genes of the mating-type locus and pheromone response pathway in an effort to characterize the mating strategies and behaviors of 124 Leotiomycete species. Our analyses identified a putative a-factor mating pheromone in these species. This significant finding represents the first identification of this gene in Pezizomycotina species outside of the Sordariomycetes. A unique mating strategy was also discovered in Lachnellula species that appear to have lost the need for the primary MAT1-1-1 protein. Ancestral state reconstruction enabled the identification of numerous transitions between homothallism and heterothallism in the Leotiomycetes and suggests a heterothallic ancestor for this group. This comprehensive catalog of mating-related genes from such a large group of fungi provides a rich resource from which in-depth, functional studies can be conducted in these economically and ecologically important species.

Overexpression of the C4 protein of tomato yellow leaf curl Sardinia virus increases tomato resistance to powdery mildew

Powdery mildew (PM) is one of the most important diseases of greenhouse and field-grown tomatoes. Viruses can intervene beneficially on plant performance in coping with biotic and abiotic stresses. Tomato yellow leaf curl Sardinia virus (TYLCSV) has been reported recently to induce tolerance against drought stress in tomato, and its C4 protein acts as the main causal factor of tolerance. However, its role in response to biotic stresses is still unknown. In this study, transgenic tomato plants carrying the TYLCSV C4 protein were exposed to biotic stress following the inoculation with Oidium neolycopersici, the causal agent of tomato PM. Phytopathological, anatomic, molecular, and physiological parameters were evaluated in this plant pathosystem. Heterologous TYLCSV C4 expression increased the tolerance of transgenic tomato plants to PM, not only reducing symptom occurrence, but also counteracting conidia adhesion and secondary hyphae elongation. Pathogenesis-related gene expression and salicylic acid production were found to be higher in tomato transgenic plants able to cope with PM compared to infected wild-type tomato plants. Our study contributes to unraveling the mechanism leading to PM tolerance in TYLCSV C4-expressing tomato plants. In a larger context, the findings of TYLCSV C4 as a novel PM defense inducer could have important implications in deepening the mechanisms regulating the management of this kind of protein to both biotic and abiotic stresses.

Contribution of a WRKY Transcription Factor, ShWRKY81, to Powdery Mildew Resistance in Wild Tomato

Tomato powdery mildew, caused by Oidium neolycopersici, is a destructive fungal disease that damages almost all of the aerial parts of tomato, causing devastating losses in tomato production worldwide. WRKY transcription factors are key regulators of plant immunity, but the roles of ShWRKYs in wild tomato Solanum habrochaites LA1777 against O. neolycopersici still remain to be uncovered. Here, we show that ShWRKY81 is an important WRKY transcription factor from wild tomato Solanum habrochaites LA1777, contributing to plant resistance against O. neolycopersici. ShWRKY81 was isolated and identified to positively modulate tomato resistance against On-Lz. The transient overexpression of the ShWRKY81-GFP (green fluorescent protein) fusion protein in Nicotiana benthamiana cells revealed that ShWRKY81 was localized in the nucleus. ShWRKY81 responded differentially to abiotic and biotic stimuli, with ShWRKY81 mRNA accumulation in LA1777 seedlings upon On-Lz infection. The virus-induced gene silencing of ShWRKY81 led to host susceptibility to On-Lz in LA1777, and a loss of H₂O₂ formation and hypersensitive response (HR) induction. Furthermore, the transcripts of ShWRKY81 were induced by salicylic acid (SA), and ShWRKY81-silenced LA1777 seedlings displayed decreased levels of the defense hormone SA and SA-dependent PRs gene expression upon On-Lz infection. Together, these results demonstrate that ShWRKY81 acts as a positive player in tomato powdery mildew resistance.

Combating powdery mildew: Advances in molecular interactions between Blumeria graminis f. sp. tritici and wheat

Wheat powdery mildew caused by a biotrophic fungus Blumeria graminis f. sp. tritici (Bgt), is a widespread airborne disease which continues to threaten global wheat production. One of the most chemical-free and cost-effective approaches for the management of wheat powdery mildew is the exploitation of resistant cultivars. Accumulating evidence has reported that more than 100 powdery mildew resistance genes or alleles mapping to 63 different loci (Pm1-Pm68) have been identified from common wheat and its wild relatives, and only a few of them have been cloned so far. However, continuous emergence of new pathogen races with novel degrees of virulence renders wheat resistance genes ineffective. An essential breeding strategy for achieving more durable resistance is the pyramiding of resistance genes into a single genotype. The genetics of host-pathogen interactions integrated with temperature conditions and the interaction between resistance genes and their corresponding pathogen a virulence genes or other resistance genes within the wheat genome determine the expression of resistance genes. Considerable progress has been made in revealing Bgt pathogenesis mechanisms, identification of resistance genes and breeding of wheat powdery mildew resistant cultivars. A detailed understanding of the molecular interactions between wheat and Bgt will facilitate the development of novel and effective approaches for controlling powdery mildew. This review gives a succinct overview of the molecular basis of interactions between wheat and Bgt, and wheat defense mechanisms against Bgt infection. It will also unleash the unsung roles of epigenetic processes, autophagy and silicon in wheat resistance to Bgt.

Germplasm Screening Using DNA Markers and Genome-Wide Association Study for the Identification of Powdery Mildew Resistance Loci in Tomato

Powdery mildew (PM), caused by Oidium spp. in tomato, is a global concern that leads to diminished yield. We aimed to evaluate previously reported DNA markers linked to powdery mildew resistance (PMR) and identify novel quantitative trait loci (QTLs) for PMR through a genome-wide association study in tomato. Sequencing analysis of the internal transcribed spacer (ITS) of a PM strain (PNU_PM) isolated from Miryang, Gyeongnam, led to its identification as Oidium neolycopersici. Thereafter, a PM bioassay was conducted for a total of 295 tomato accessions, among which 24 accessions (4 S. lycopersicum accessions and 20 accessions of seven wild species) showed high levels of resistance to PNU_PM. Subsequently, we genotyped 11 markers previously linked to PMR in 56 accessions. PMR-specific banding patterns were detected in 15/22 PMR accessions, while no such bands were observed in the powdery mildew-susceptible accessions. The genome-wide association study was performed using TASSEL and GAPIT, based on the phenotypic data of 290 accessions and 11,912 single nucleotide polymorphisms (SNPs) obtained from the Axiom® Tomato SNP Chip Array. Nine significant SNPs in chromosomes 1, 4, 6, 8, and 12, were selected and five novel QTL regions distinct from previously known PMR-QTL regions were identified. Of these QTL regions, three putative candidate genes for PMR were selected from chromosomes 4 and 8, including two nucleotide binding site-leucine rich repeat class genes and a receptor-like kinase gene, all of which have been identified previously as causative genes for PMR in several crop species. The SNPs discovered in these genes provide useful information for understanding the molecular basis of PMR and developing DNA markers for marker-assisted selection of PMR in tomato.

Actin-bundling protein fimbrin regulates pathogenicity via organizing F-actin dynamics during appressorium development in Colletotrichum gloeosporioides

Anthracnose caused by Colletotrichum gloeosporioides leads to serious economic loss to rubber tree yield and other tropical crops. The appressorium, a specialized dome-shaped infection structure, plays a crucial role in the pathogenesis of C. gloeosporioides. However, the mechanism of how actin cytoskeleton dynamics regulate appressorium formation and penetration remains poorly defined in C. gloeosporioides. In this study, an actin cross-linking protein fimbrin homologue (CgFim1) was identified in C. gloeosporioides, and the knockout of CgFim1 led to impairment in vegetative growth, conidiation, and pathogenicity. We then investigated the roles of CgFim1 in the dynamic organization of the actin cytoskeleton. We observed that actin patches and cables localized at the apical and subapical regions of the hyphal tip, and showed a disc-to-ring dynamic around the pore during appressorium development. CgFim1 showed a similar distribution pattern to the actin cytoskeleton. Moreover, knockout of CgFim1 affected the polarity of the actin cytoskeleton in the hyphal tip and disrupted the actin dynamics and ring structure formation in the appressorium, which prevented polar growth and appressorium development. The CgFim1 mutant also interfered with the septin structure formation. This caused defects in pore wall overlay formation, pore contraction, and the extension of the penetration peg. These results reveal the mechanism by which CgFim1 regulates the growth and pathogenicity of C. gloeosporioides by organizing the actin cytoskeleton.

Functional Characterization of Tomato ShROP7 in Regulating Resistance against Oidium neolycopersici

ROPs (Rho-like GTPases from plants) are a unique family of small GTP-binding proteins in plants and play vital roles in numerous cellular processes, including growth and development, abiotic stress signaling, and plant defense. In the case of the latter, the role of ROPs as response regulators to obligate parasitism remains largely enigmatic. Herein, we isolated and identified ShROP7 and show that it plays a critical role in plant immune response to pathogen infection. Real-time quantitative PCR analysis revealed that the expression of ShROP7 was significantly increased during incompatible interactions. To establish its requirement for resistance, we demonstrate that virus-induced gene silencing (VIGS) of ShROP7 resulted in increased susceptibility of tomato to Oidium neolycopersici (On) Lanzhou strain (On-Lz). Downstream resistance signaling through H₂O₂ and the induction of the hypersensitive response (HR) in ShROP7-silenced plants were significantly reduced after inoculating with On-Lz. Taken together, with the identification of ShROP7-interacting candidates, including ShSOBIR1, we demonstrate that ShROP7 plays a positive regulatory role in tomato powdery mildew resistance.

MLO Proteins from Tomato (Solanum lycopersicum L.) and Related Species in the Broad Phylogenetic Context

MLO proteins are a family of transmembrane proteins in land plants that play an important role in plant immunity and host–pathogen interactions, as well as a wide range of development processes. Understanding the evolutionary history of MLO proteins is important for understanding plant physiology and health. In the present work, we conducted a phylogenetic analysis on a large set of MLO protein sequences from publicly available databases, specifically emphasising MLOs from the tomato plant and related species. As a result, 4886 protein sequences were identified and used to construct a phylogenetic tree. In comparison to previous findings, we identified nine phylogenetic clades, revealed the internal structure of clades I and II as additional clades and showed the presence of monocotyledon species in all MLO clades. We identified a set of 19 protein motifs that allowed for the identification of particular clades. Sixteen SlMLO proteins from tomato were located in the phylogenetic tree and identified in relation to homologous sequences from other Solanaceae species. The obtained results could be useful for further work on the use of MLO proteins in the study of mildew resistance in Solanaceae and other plant families.

Infection Strategies and Pathogenicity of Biotrophic Plant Fungal Pathogens

Biotrophic plant pathogenic fungi are widely distributed and are among the most damaging pathogenic organisms of agriculturally important crops responsible for significant losses in quality and yield. However, the pathogenesis of obligate parasitic pathogenic microorganisms is still under investigation because they cannot reproduce and complete their life cycle on an artificial medium. The successful lifestyle of biotrophic fungal pathogens depends on their ability to secrete effector proteins to manipulate or evade plant defense response. By integrating genomics, transcriptomics, and effectoromics, insights into how the adaptation of biotrophic plant fungal pathogens adapt to their host populations can be gained. Efficient tools to decipher the precise molecular mechanisms of rust–plant interactions, and standardized routines in genomics and functional pipelines have been established and will pave the way for comparative studies. Deciphering fungal pathogenesis not only allows us to better understand how fungal pathogens infect host plants but also provides valuable information for plant diseases control, including new strategies to prevent, delay, or inhibit fungal development. Our review provides a comprehensive overview of the efforts that have been made to decipher the effector proteins of biotrophic fungal pathogens and demonstrates how rapidly research in the field of obligate biotrophy has progressed.

The Entomopathogenic Fungi Metarhizium brunneum and Beauveria bassiana Promote Systemic Immunity and Confer Resistance to a Broad Range of Pests and Pathogens in Tomato

Biocontrol agents can control pathogens by reenforcing systemic plant resistance through systemic acquired resistance (SAR) or induced systemic resistance (ISR). Trichoderma spp. can activate the plant immune system through ISR, priming molecular mechanisms of defense against pathogens. Entomopathogenic fungi (EPF) can infect a wide range of arthropod pests and play an important role in reducing pests' population. Here, we investigated the mechanisms by which EPF control plant diseases. We tested two well studied EPF, Metarhizium brunneum isolate Mb7 and Beauveria bassiana as the commercial product Velifer, for their ability to induce systemic immunity and disease resistance against several fungal and bacterial phytopathogens, and their ability to promote plant growth. We compared the activity of these EPF to an established biocontrol agent, Trichoderma harzianum T39, a known inducer of systemic plant immunity and broad disease resistance. The three fungal agents were effective against several fungal and bacterial plant pathogens and arthropod pests. Our results indicate that EPF induce systemic plant immunity and disease resistance by activating the plant host defense machinery, as evidenced by increases in reactive oxygen species production and defense gene expression, and that EPF promote plant growth. EPF should be considered as control means for Tuta absoluta. We demonstrate that, with some exceptions, biocontrol in tomato can be equally potent by the tested EPF and T. harzianum T39, against both insect pests and plant pathogens. Taken together, our findings suggest that EPF may find use in broad-spectrum pest and disease management and as plant growth promoting agents.

Joint Communication and Sensing: A Proof of Concept and Datasets for Greenhouse Monitoring Using LoRaWAN

In recent years, greenhouse-based precision agriculture (PA) has been strengthened by utilization of Internet of Things applications and low-power wide area network communication. The advancements in multidisciplinary technologies such as artificial intelligence (AI) have created opportunities to assist farmers further in detecting disease and poor nutrition of plants. Neural networks and other AI techniques need an initial set of measurement campaigns along with extensive datasets as a training set to baseline and evolve different applications. This paper presents LoRaWAN-based greenhouse monitoring datasets over a period of nine months. The dataset has both the network and sensing information from multiple sensor nodes for tomato crops in two different greenhouse environments. The goal is to provide the research community with a dataset to evaluate performance of LoRaWAN inside a greenhouse and develop more efficient PA monitoring techniques. In this paper, we carried out an exploratory data analysis to infer crop growth by analyzing just the LoRaWAN signals and without inclusion of any extra hardware. This work uses a multilayer perceptron artificial neural network to predict the weekly plant growth, trained using RSSI value from sensor data and manual measurement of plant height from the greenhouse. We developed this proof of concept of joint communication and sensing by using generated dataset from the “Proefcentrum Hoogstraten” greenhouse in Belgium. Results for the proposed method yield a root mean square error of 10% in detecting the average plant height inside a greenhouse. In future, we can use this concept of landscape sensing for different supplementary use-cases and to develop optimized methods.

Identification and Characterization of WRKY41, a Gene Conferring Resistance to Powdery Mildew in Wild Tomato (Solanum habrochaites) LA1777

WRKYs, a large family of transcription factors, are involved in plant response to biotic and abiotic stresses, but the role of them in tomato resistance to Oidium neolycopersici is still unclear. In this study, we evaluate the role of WRKYs in powdery mildew-resistant wild tomato (Solanum habrochaites) LA1777 defense against O. neolycopersici strain lz (On-lz) using a combination of omics, classical plant pathology- and cell biology-based approaches. A total of 27 WRKYs, belonging to group I, II, and III, were identified as differentially expressed genes in LA1777 against On-lz. It was found that expression of ShWRKY41 was increased after Pseudomonas syringae pv. tomato (Pst) DC3000, On-lz and Botrytiscinerea B05 inoculation or ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) treatment. GUS staining of ShWRKY41 promoter indicated that the expression of ShWRKY41 could be induced by SA and ethylene. Furthermore, ShWRKY41 gene silencing reduced the resistance to On-lz infection by decreasing the generation of H₂O₂ and HR in LA1777 seedlings. Overall, our research suggests that ShWRKY41 plays a positive role in defense activation and host resistance to O. neolycopersici in wild tomato (S. habrochaites) LA1777.

Dual protection: A tydeoid mite effectively controls both a problem pest and a key pathogen in tomato

BACKGROUND

The tomato russet mite (TRM), Aculops lycopersici, and powdery mildew (PM), Oidium neolycopersici, are two major problems in tomato cultivation for which no effective biocontrol solutions exist to date. In a greenhouse compartment, we investigated the potential of preventatively establishing the iolinid omnivorous mite Pronematus ubiquitus on potted tomato plants to control both pest and pathogen simultaneously.

RESULTS

Using Typha pollen, P. ubiquitus established well on tomato plants, with numbers reaching up to 250 motiles per tomato leaflet. The built-up population was capable of controlling subsequent infestations with both TRM and PM. This represents the first report of an arthropod protecting a crop against pests as well as disease.

CONCLUSION

The implementation of P. ubiquitus in tomato crops could be a real game-changer as it eliminates the need for repeated pesticide use or sulphur applications. The finding that arthropods can effectively control diseases opens up new opportunities for biological crop protection. © 2021 Society of Chemical Industry.

Precision Genome Engineering for the Breeding of Tomatoes: Recent Progress and Future Perspectives

Currently, poor biodiversity has raised challenges in the breeding and cultivation of tomatoes, which originated from the Andean region of Central America, under global climate change. Meanwhile, the wild relatives of cultivated tomatoes possess a rich source of genetic diversity but have not been extensively used for the genetic improvement of cultivated tomatoes due to the possible linkage drag of unwanted traits from their genetic backgrounds. With the advent of new plant breeding techniques (NPBTs), especially CRISPR/Cas-based genome engineering tools, the high-precision molecular breeding of tomato has become possible. Further, accelerated introgression or de novo domestication of novel and elite traits from/to the wild tomato relatives to/from the cultivated tomatoes, respectively, has emerged and has been enhanced with high-precision tools. In this review, we summarize recent progress in tomato precision genome editing and its applications for breeding, with a special focus on CRISPR/Cas-based approaches. Future insights and precision tomato breeding scenarios in the CRISPR/Cas era are also discussed.

Advances in application of genome editing in tomato and recent development of genome editing technology

Genome editing, a revolutionary technology in molecular biology and represented by the CRISPR/Cas9 system, has become widely used in plants for characterizing gene function and crop improvement. Tomato, serving as an excellent model plant for fruit biology research and making a substantial nutritional contribution to the human diet, is one of the most important applied plants for genome editing. Using CRISPR/Cas9-mediated targeted mutagenesis, the re-evaluation of tomato genes essential for fruit ripening highlights that several aspects of fruit ripening should be reconsidered. Genome editing has also been applied in tomato breeding for improving fruit yield and quality, increasing stress resistance, accelerating the domestication of wild tomato, and recently customizing tomato cultivars for urban agriculture. In addition, genome editing is continuously innovating, and several new genome editing systems such as the recent prime editing, a breakthrough in precise genome editing, have recently been applied in plants. In this review, these advances in application of genome editing in tomato and recent development of genome editing technology are summarized, and their leaving important enlightenment to plant research and precision plant breeding is also discussed.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

ShNPSN11, a vesicle-transport-related gene, confers disease resistance in tomato to Oidium neolycopersici

Tomato powdery mildew, caused by Oidium neolycopersici, is a fungal disease that results in severe yield loss in infected plants. Herein, we describe the function of a class of proteins, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which play a role in vesicle transport during defense signaling. To date, there have been no reports describing the function of tomato SNAREs during resistance signaling to powdery mildew. Using a combination of classical plant pathology-, genetics-, and cell biology-based approaches, we evaluate the role of ShNPSN11 in resistance to the powdery mildew pathogen O. neolycopersici. Quantitative RT-PCR analysis of tomato SNAREs revealed that ShNPSN11 mRNA accumulation in disease-resistant varieties was significantly increased following pathogen, compared with susceptible varieties, suggesting a role during induced defense signaling. Using in planta subcellular localization, we demonstrate that ShNPSN11 was primarily localized at the plasma membrane, consistent with the localization of SNARE proteins and their role in defense signaling and trafficking. Silencing of ShNPSN11 resulted in increased susceptibility to O. neolycopersici, with pathogen-induced levels of H2O2 and cell death elicitation in ShNPSN11-silenced lines showing a marked reduction. Transient expression of ShNPSN11 did not result in the induction of a hypersensitive cell death response or suppress cell death induced by BAX. Taken together, these data demonstrate that ShNPSNl11 plays an important role in defense activation and host resistance to O. neolycopersici in tomato LA1777.

A gain of function mutation in SlNRC4a enhances basal immunity resulting in broad-spectrum disease resistance

Plants rely on innate immunity to perceive and ward off microbes and pests, and are able to overcome the majority of invading microorganisms. Even so, specialized pathogens overcome plant defenses, posing a persistent threat to crop and food security worldwide, raising the need for agricultural products with broad, efficient resistance. Here we report a specific mutation in a tomato (S. lycopersicum) helper nucleotide-binding domain leucine-rich repeat H-NLR, SlNRC4a, which results in gain of function constitutive basal defense activation, in absence of PRR activation. Knockout of the entire NRC4 clade in tomato was reported to compromise Rpi-blb2 mediated immunity. The SlNRC4a mutant reported here possesses enhanced immunity and disease resistance to a broad-spectrum of pathogenic fungi, bacteria and pests, while lacking auto-activated HR or negative effects on plant growth and crop yield, providing promising prospects for agricultural adaptation in the war against plant pathogens that decrease productivity.

Lorena Pizarro, Meirav Leibman-Markus et al. explore the genetic mechanisms for plant innate immunity. They functionally characterize a gain of function mutation in SlNRC4a in tomato. They characterize the structure of the mutant protein and functionally demonstrate that it confers broad-spectrum resistance without triggering a hypersensitive response or negatively impacting plant growth and crop yield.

CRISPR/Cas9-targeted mutagenesis of the tomato susceptibility gene PMR4 for resistance against powdery mildew

BACKGROUND

The development of CRISPR/Cas9 technology has facilitated targeted mutagenesis in an efficient and precise way. Previously, RNAi silencing of the susceptibility (S) gene PowderyMildewResistance 4 (PMR4) in tomato has been shown to enhance resistance against the powdery mildew pathogen Oidium neolycopersici (On).

RESULTS

To study whether full knock-out of the tomato PMR4 gene would result in a higher level of resistance than in the RNAi-silenced transgenic plants we generated tomato PMR4 CRISPR mutants. We used a CRISPR/Cas9 construct containing four single-guide RNAs (sgRNAs) targeting the tomato PMR4 gene to increase the possibility of large deletions in the mutants. After PCR-based selection and sequencing of transformants, we identified five different mutation events, including deletions from 4 to 900-bp, a 1-bp insertion and a 892-bp inversion. These mutants all showed reduced susceptibility to On based on visual scoring of disease symptoms and quantification of relative fungal biomass. Histological observations revealed a significantly higher occurrence of hypersensitive response-like cell death at sites of fungal infection in the pmr4 mutants compared to wild-type plants. Both haustorial formation and hyphal growth were diminished but not completely inhibited in the mutants.

CONCLUSION

CRISPR/Cas-9 targeted mutagenesis of the tomato PMR4 gene resulted in mutants with reduced but not complete loss of susceptibility to the PM pathogen On. Our study demonstrates the efficiency and versatility of the CRISPR/Cas9 system as a powerful tool to study and characterize S-genes by generating different types of mutations.

The tomato Arp2/3 complex is required for resistance to the powdery mildew fungus Oidium neolycopersici

The actin-related protein 2/3 complex (Arp2/3 complex), a key regulator of actin cytoskeletal dynamics, has been linked to multiple cellular processes, including those associated with response to stress. Herein, the Solanum habrochaites ARPC3 gene, encoding a subunit protein of the Arp2/3 complex, was identified and characterized. ShARPC3 encodes a 174-amino acid protein possessing a conserved P21-Arc domain. Silencing of ShARPC3 resulted in enhanced susceptibility to the powdery mildew pathogen Oidium neolycopersici (On-Lz), demonstrating a role for ShARPC3 in defence signalling. Interestingly, a loss of ShARPC3 coincided with enhanced susceptibility to On-Lz, a process that we hypothesize is the result of a block in the activity of SA-mediated defence signalling. Conversely, overexpression of ShARPC3 in Arabidopsis thaliana, followed by inoculation with On-Lz, showed enhanced resistance, including the rapid induction of hypersensitive cell death and the generation of reactive oxygen. Heterologous expression of ShARPC3 in the arc18 mutant of Saccharomyces cerevisiae (i.e., ∆arc18) resulted in complementation of stress-induced phenotypes, including high-temperature tolerance. Taken together, these data support a role for ShARPC3 in tomato through positive regulation of plant immunity in response to O. neolycopersici pathogenesis.

Cloning of the Cytochrome b Gene From the Tomato Powdery Mildew Fungus Leveillula taurica Reveals High Levels of Allelic Variation and Heteroplasmy for the G143A Mutation

Leveillula taurica is a major pathogen of tomato and several other crops that can cause substantial yield losses in favorable conditions for the fungus. Quinone outside inhibitor fungicides (Q_oIs) are routinely used for the control of the pathogen in tomato fields across California, but their recurrent use could lead to the emergence of resistance against these compounds. Here, we partially cloned the cytochrome b gene from L. taurica (Lt cytb) and searched within populations of the fungus collected from tomato fields across California for mutations that confer resistance to Q_oIs. A total of 21 single nucleotide polymorphisms (SNPs) were identified within a 704 bp fragment of the Lt cytb gene analyzed, of which five were non-synonymous substitutions. Among the most frequent SNPs encountered within field populations of the pathogen was the G143A substitution that confers high levels of resistance against Q_oIs in several fungi. The other four amino acid substitutions were novel mutations, whose effect on Q_oI resistance is currently unknown. Sequencing of the Lt cytb gene from individual single-cell conidia of the fungus further revealed that most SNPs, including the one leading to the G143A substitution, were present in a heteroplasmic state, indicating the co-existence of multiple mitotypes in single cells. Analysis of the field samples showed that the G143A substitution is predominantly heteroplasmic also within field populations of L. taurica in California, suggesting that Q_oI resistance in this fungus is likely to be quantitative rather than qualitative.

CRISPR/Cas9: A Novel Weapon in the Arsenal to Combat Plant Diseases

Plant pathogens like virus, bacteria, and fungi incur a huge loss of global productivity. Targeting the dominant R gene resulted in the evolution of resistance in pathogens, which shifted plant pathologists' attention toward host susceptibility factors (or S genes). Herein, the application of sequence-specific nucleases (SSNs) for targeted genome editing are gaining more importance, which utilize the use of meganucleases (MN), zinc finger nucleases (ZFNs), transcription activator-like effector-based nucleases (TALEN) with the latest one namely clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). The first generation of genome editing technologies, due to their cumbersome nature, is becoming obsolete. Owing to its simple and inexpensive nature the use of CRISPR/Cas9 system has revolutionized targeted genome editing technology. CRISPR/Cas9 system has been exploited for developing resistance against virus, bacteria, and fungi. For resistance to DNA viruses (mainly single-stranded DNA viruses), different parts of the viral genome have been targeted transiently and by the development of transgenic plants. For RNA viruses, mainly the host susceptibility factors and very recently the viral RNA genome itself have been targeted. Fungal and bacterial resistance has been achieved mainly by targeting the host susceptibility genes through the development of transgenics. In spite of these successes CRISPR/Cas9 system suffers from off-targeting. This and other problems associated with this system are being tackled by the continuous discovery/evolution of new variants. Finally, the regulatory standpoint regarding CRISPR/Cas9 will determine the fate of using this versatile tool in developing pathogen resistance in crop plants.

Development of DNA markers for Slmlo1.1, a new mutant allele of the powdery mildew resistance gene SlMlo1 in tomato (Solanum lycopersicum)

Reductions in growth and quality due to powdery mildew (PM) disease cause significant economic losses in tomato production. Oidium neolycopersici was identified as the fungal species responsible for tomato PM disease in South Korea in the present study, based on morphological and internal transcribed spacer DNA sequence analyses of PM samples collected from two remote regions (Muju and Miryang). The genes involved in resistance to this pathogen in the tomato accession ‘KNU-12’ (Solanum lycopersicum var. cerasiforme) were evaluated, and the inheritance of PM resistance in ‘KNU-12’ was found to be conferred via simple Mendelian inheritance of a mutant allele of the PM susceptibility locus Ol-2 (SlMlo1). Full-length cDNA analysis of this newly identified mutant allele (Slmlo1.1) showed that a 1-bp deletion in its coding region led to a frameshift mutation possibly resulting in SlMlo1 loss-of-function. An alternatively spliced transcript of Slmlo1.1 was observed in the cDNA sequences of ‘KNU-12’, but its direct influence on PM resistance is unclear. A derived cleaved amplified polymorphic sequence (dCAPS) and a high-resolution melting (HRM) marker were developed based on the 1-bp deletion in Slmlo1.1, and could be used for efficient marker-assisted selection (MAS) using ‘KNU-12’ as the source for durable and broad-spectrum resistance to PM.

Comparative genome analyses reveal sequence features reflecting distinct modes of host-adaptation between dicot and monocot powdery mildew

BACKGROUND

Powdery mildew (PM) is one of the most important and widespread plant diseases caused by biotrophic fungi. Notably, while monocot (grass) PM fungi exhibit high-level of host-specialization, many dicot PM fungi display a broad host range. To understand such distinct modes of host-adaptation, we sequenced the genomes of four dicot PM biotypes belonging to Golovinomyces cichoracearum or Oidium neolycopersici.

RESULTS

We compared genomes of the four dicot PM together with those of Blumeria graminis f.sp. hordei (both DH14 and RACE1 isolates), B. graminis f.sp. tritici, and Erysiphe necator infectious on barley, wheat and grapevine, respectively. We found that despite having a similar gene number (6620-6961), the PM genomes vary from 120 to 222 Mb in size. This high-level of genome size variation is indicative of highly differential transposon activities in the PM genomes. While the total number of genes in any given PM genome is only about half of that in the genomes of closely related ascomycete fungi, most (~ 93%) of the ascomycete core genes (ACGs) can be found in the PM genomes. Yet, 186 ACGs were found absent in at least two of the eight PM genomes, of which 35 are missing in some dicot PM biotypes, but present in the three monocot PM genomes, indicating remarkable, independent and perhaps ongoing gene loss in different PM lineages. Consistent with this, we found that only 4192 (3819 singleton) genes are shared by all the eight PM genomes, the remaining genes are lineage- or biotype-specific. Strikingly, whereas the three monocot PM genomes possess up to 661 genes encoding candidate secreted effector proteins (CSEPs) with families containing up to 38 members, all the five dicot PM fungi have only 116-175 genes encoding CSEPs with limited gene amplification.

CONCLUSIONS

Compared to monocot (grass) PM fungi, dicot PM fungi have a much smaller effectorome. This is consistent with their contrasting modes of host-adaption: while the monocot PM fungi show a high-level of host specialization, which may reflect an advanced host-pathogen arms race, the dicot PM fungi tend to practice polyphagy, which might have lessened selective pressure for escalating an with a particular host.

Morphological differences between aerial and submerged sporidia of bio-fongicide Pseudozyma flocculosa CBS 16788

Pseudozyma flocculosa is a fungus very useful and highly efficient as a biocontrol agent against powdery mildew. The reproduction of this fungus occurs exclusively by asexual production of conidia or sporidia that are the most suitable form for agricultural use and seems to be the most resistant to storage conditions. Despite the advantages offered by P. flocculosa in biological control, the use of this fungus use remains largely limited compared to that of chemical fungicides, at least partly due to the difficulty to obtain sporidia resistant to adverse environmental stresses in submerged culture conditions. Under solid-state and submerged-state cultivation, P. flocculosa strain CBS 16788 produced different types of sporidia. The submerged sporidia (SS) appeared relatively uniform in size, which was 15,4 ± 1,6 μm μm long, and 2,8 ± 0.8 μm wide. The aerial sporidia (AS) varied in shape and size, with a mean length of 8,2 ± 3 μm and width of 2,3 ± 0.6 μm. Under scanning and transmission electron microscopy, the cell wall of submerged sporidia was thinner than that of aerial spores, and the surface was smooth in contrast to the aerial sporidia that had a tendency to have verrucous, brittle surface characteristics. The thickness of the aerial sporidia wall is due to the presence of an outer layer rich in melanin. The sporidia germination was compared on YMPD (yeast extract, malt extract, soy peptone, dextrose and agar) coated coverslips. The aerial sporidia did not show germ tubes until 5 h of incubation, while the submerged sporidia showed many germ tubes after the same time. The resistance against the adverse environmental conditions in relation to the type of sporidia of P. flocculosa is discussed.

Pathogen recognition in compatible plant-microbe interactions

Microbial infections in plant leaves remain a major challenge in agriculture. Hence an understanding of disease mechanisms at the molecular level is of paramount importance for identifying possible intervention points for their control. Whole-transcriptome changes during early disease stages in susceptible plant species are less well-documented than those of resistant ones. This study focuses on the differential transcriptional changes at 24 hours post inoculation (hpi) in tomato leaflets affected by three pathogens: (1) Phytophthora infestans, (2) Botrytis cinerea, and (3) Oidium neolycopersici. Grey mould (B. cinerea) was the disease that had progressed the most by 24 hpi, both in terms of visible symptoms as well as differential gene expression. By means of RNA-seq, we identified 50 differentially expressed tomato genes specifically induced by B. cinerea infection and 18 specifically induced by P. infestans infection at 24 hpi. Additionally, a set of 63 genes were differentially expressed during all three diseases when compared by a Bayesian approach to their respective mock infections. And Gene expression patterns were found to also depend on the inoculation technique. These findings suggest a specific and distinct transcriptional response in plant leaf tissue in reaction to B. cinerea and P. infestans invasion at 24 hpi, indicating that plants may recognize the attacking pathogen.

Bioassimilable sulphur provides effective control of Oidium neolycopersici in tomato, enhancing the plant immune system

BACKGROUND

Developments of alternatives to the use of chemical pesticides to control pests are focused on the induction of natural plant defences. The study of new compounds based on liquid bioassimilable sulphur and its effect as an inductor of the immune system of plants would provide an alternative option to farmers to enhance plant resistance against pathogen attacks such as powdery mildew. In order to elucidate the efficacy of this compound in tomato against powdery mildew, we tested several treatments: curative foliar, preventive foliar, preventive in soil drench and combining preventive in soil drench and curative foliar.

RESULTS

In all cases, treated plants showed lower infection development, better physiological parameters and a higher level of chlorophyll. We also observed better performance in parameters involved in plant resistance such as antioxidant response, callose deposition and hormonal levels.

CONCLUSION

The results indicate that preventive and curative treatments can be highly effective for the prevention and control of powdery mildew in tomato plants. Foliar treatments are able to stop the pathogen development when they are applied as curative. Soil drench treatments induce immune response mechanisms of plants, increasing significantly callose deposition and promoting plant development. © 2016 Society of Chemical Industry.

Suppression of Powdery Mildews by UV-B: Application Frequency and Timing, Dose, Reflectance, and Automation

Disease-suppressive effects of nighttime applications of ultraviolet-B (UV-B) were investigated at two irradiance levels (1.6 or 0.8 W/m²) in strawberry and rosemary plants inoculated with Podosphaera aphanis or Golovinomyces biocellatus, respectively. Plants were exposed to each irradiance level every third night for either 9 or 18 min, every night for either 3 or 6 min, or three times every night for either 1 or 2 min. Thus, over time, all plants received the same cumulative dose of UV-B, and severity of powdery mildew was reduced by 90 to 99% compared with untreated controls in both crops. Use of polished aluminum lamp reflectors and UV-B reflective surfaces on greenhouse benches significantly increased treatment efficacy. An automated apparatus consisting of an adjustable boom with directed airflow was used to move UV-B lamps over greenhouse benches at 25 or 50 cm/min. Directed airflow moved leaves on the subtending plants to better expose upper and lower surfaces to UV-B but directed airflow actually decreased the efficacy of UV-B treatments, possibly by dispersing conidia from lesions before they were exposed to a lethal dose of UV-B. Results indicate broad applicability of nighttime applications of UV-B to suppress powdery mildews, and that cumulative UV-B dose is an overriding factor determining efficacy. Finally, enhanced suppression on shaded or obscured tissues is more likely to be affected by reflective bench surfaces than through attempts to physically manipulate the foliage.

Induced resistance in Solanum lycopersicum by algal elicitor extracted from Sargassum fusiforme

Tomato (Solanum lycopersicum) production relies heavily on the use of chemical pesticides, which is undesired by health- and environment-concerned consumers. Environment-friendly methods of controlling tomato diseases include agroecological practices, organic fungicides, and biological control. Plants' resistance against pathogens is induced by applying agents called elicitors to the plants and would lead to disease prevention or reduced severity. We investigated the ability of a novel elicitor extracted from the brown sea algae (Sargassum fusiforme) to elicit induced resistance in tomato. The studied elicitor induced hypersensitive cell death and O2 (-) production in tomato tissues. It significantly reduced severities of late blight, grey mold, and powdery mildew of tomato. Taken together, our novel elicitor has not shown any direct antifungal activity against the studied pathogens, concluding that it is an elicitor of induced resistance.

Spatiotemporal patterns of induced resistance and susceptibility linking diverse plant parasites

Induced defenses mediate interactions between parasites sharing the same host plant, but the outcomes of these interactions are challenging to predict because of spatiotemporal variation in plant responses and differences in defense pathways elicited by herbivores or pathogens. Dissecting these mediating factors necessitates an approach that encompasses a diversity of parasitic feeding styles and tracks interactions over space and time. We tested indirect plant-mediated relationships across three tomato (Solanum lycopersicum) consumers: (1) the fungal pathogen-powdery mildew, Oidium neolycopersici; (2) a sap-feeding insect-silverleaf whitefly, Bemisia tabaci; and (3) a chewing insect-the leaf miner, Tuta absoluta. Further, we evaluated insect/pathogen responses on local vs. systemic leaves and over short (1 day) vs. long (4 days) time scales. Overall, we documented: (1) a bi-directional negative effect between O. neolycopersici and B. tabaci; (2) an asymmetrical negative effect of B. tabaci on T. absoluta; and (3) an asymmetrical positive effect of T. absoluta on O. neolycopersici. Spatiotemporal patterns varied depending on the species pair (e.g., whitefly effects on leaf miner performance were highly localized to the induced leaf, whereas effects on pathogen growth were both local and systemic). These results highlight the context-dependent effects of induced defenses on a diverse community of tomato parasites. Notably, the outcomes correspond to those predicted by phytohormonal theory based on feeding guild differences with key implications for the recent European invasion by T. absoluta.

Integration of Elicitors and Less-Susceptible Hybrids for the Control of Powdery Mildew in Organic Tomato Crops

Powdery mildew is a serious economic problem in Mediterranean tomato production. The disease is currently controlled by fungicides (especially sulfur) in both conventional and organic production. Four factorial greenhouse experiments were conducted in successive cropping seasons (autumn 2005, autumn 2006, spring 2006, and spring 2007) to assess the main and combined effects of (i) a less-susceptible hybrid ('Elpida'), (ii) chitin soil amendment, or (iii) Milsana or chitosan elicitor foliar treatments on the severity of powdery mildew and crop performance. Analysis of variance revealed significant effects of cropping season on disease severity and crop yield. All control practices reduced disease severity; reductions of approximately 40, 30, 15, and 15% were achieved by Milsana foliar treatment, the use of a less-susceptible hybrid, chitosan foliar treatment, and chitin soil amendment, respectively. In the season with the highest disease pressure, higher yields were obtained with the less-susceptible hybrid Elpida whereas, in the three seasons with no or low disease pressure, the use of the more susceptible hybrid 'Bison' produced higher yields. Milsana and sulfur treatments did not significantly affect yield (yields were slightly lower compared with untreated control plants) and, therefore, their use cannot be recommended based on the results obtained.

Inbreeding increases susceptibility to powdery mildew (Oidium neolycopersici) infestation in horsenettle (Solanum carolinense L)

Inbreeding is common in flowering plants, but relatively few studies have examined its effects on interactions between plants and other organisms, such as herbivores and pathogens. In a recent paper, we documented effects of inbreeding depression on plant volatile signaling phenotypes, including elevated constitutive volatile emissions (and consequently greater herbivore recruitment to inbred plants) but reduced emission of key herbivore-induced volatiles that attract predatory and parasitic insects to damaged plants. While the effects of inbreeding on plant-insect interactions have been explored in only a few systems, even less is known about its effects on plant-pathogen interactions. Here we report the effects of inbreeding on horsenettle susceptibility to powdery mildew (Oidium neolycopersici), including more rapid onset of infection in inbred plants, particularly when plants were not previously damaged. These data suggest that inbreeding may increase plant susceptibility to pathogen infection and, therefore, may potentially facilitate pathogen establishment in natural populations.

An avirulent tomato powdery mildew isolate induces localized acquired resistance to a virulent isolate in a spatiotemporal manner

Hypersensitive response (HR) of plant cells to the attack of pathogens induces resistance to subsequent attacks by a broad spectrum of pathogens, leading to acquired resistance. In this study, we characterized the localized acquired resistance (LAR) in the epidermal cells of tomato. First, we report the discovery of a new isolate of tomato powdery mildew occurring in Japan, KTP-02, which has a different virulence spectrum compared with the previously-characterized isolate, KTP-01. Using these two isolates, we investigated LAR phenomenon in the epidermal cells of tomato plants carrying the Ol-4 resistance gene. Ol-4 encodes a nucleotide-binding site leucine-rich repeat protein that triggers HR in the epidermal cells in response to KTP-01 but not KTP-02. We mounted a single conidium of KTP-01 on a single tomato epidermal cell and then monitored the progress of HR in that cell by live microscopy. Once HR occurred in that cell, we mounted a single conidium of KTP-02 on cells adjacent to or at one-cell distance from the first challenged cells, in different time points. With a digital microscope, we consecutively tracked the progress of HR (i.e., induction of LAR) in those cells. Results showed that, in tomato plants carrying the Ol-4 gene, HR to KTP-01 results in induction of HR in the adjacent epidermal cells challenged with KTP-02. Our results show that LAR can be triggered only in adjacent cell layer and lasts 24 to 48 h after HR occurred in the first cell. We did not observe the reverse phenomenon, induced susceptibility to KTP-01 by KTP-02. Altogether, we report an advanced technique for investigating LAR phenomena, and provide data on spatiotemporal characteristics of LAR in tomato epidermal cells.

Transgenic tomatoes showing higher glutathione peroxydase antioxidant activity are more resistant to an abiotic stress but more susceptible to biotic stresses

The function of selenium independent glutathione peroxidase (GPx) in response to biotic and abiotic stresses was investigated in transgenic tomato plants overexpressing an exogenous GPx and exhibiting a 50% increase in total GPx activity. GPx-overexpressing and control plants were challenged either by a mechanical stress or by infection with the biotrophic parasite Oidium neolycopersici or the necrotrophic parasite Botrytis cinerea. In mechanically stressed plants, internode growth was significantly less modified in GPx-overexpressing plants compared to controls. This stress resistant phenotype was not accompanied with any change in the global antioxidant response of the plants other than their increased GPx activity. Following infection by O. neolycopersici or by B. cinerea, lesion extension was increased in GPx-overexpressing plants compared with controls. These results showed that GPx overexpression provoked opposite effects in situations of biotic and abiotic challenges, suggesting a key role for this scavenger enzyme in controlling biotic and abiotic stress responses.

Polymorphic change of appressoria by the tomato powdery mildew Oidium neolycopersici on host tomato leaves reflects multiple unsuccessful penetration attempts

The appressorial shapes of the powdery mildews are an important clue to the taxonomy of the powdery mildew fungi, but the conidia of the tomato powdery mildew Oidium neolycopersici KTP-01 develop non-lobed, nipple-shaped, and moderately lobed or multilobed appressoria on the same leaves. To remove this ambiguity, we performed consecutive observations of sequential appressorial development of KTP-01 conidia with a high-fidelity digital microscope. Highly germinative conidia of KTP-01, collected from conidial pseudochains formed on the tomato leaves, were inoculated into host tomato and nonhost barley leaves or an artificial hydrophobic membrane (Parafilm). Events from germination initiation to appressorium formation were synchronous in all conidia on all materials used for inoculation, but post-appressorial behaviors varied among the materials. Appressoria on the membrane-stuck glass slide formed several projections at different portions of the appressoria to repeat unsuccessful penetration attempts. Similar unsuccessful penetration behavior by KTP-01 conidia was observed in the inoculations into leaves of barley plants, wild tomato species Lycopersicon peruvianum LA2172 (carrying the Ol-4 gene for powdery mildew resistance), and a susceptible host tomato (Lycopersicon esculentum) that had been inoculated with the barley powdery mildew (Blumeria graminis f. sp. hordei, race 1) conidia. On the barley leaves, all penetrations of KTP-01 were impeded by the papillae formed beneath the sites of the appressorial projections. On both the wild tomato and the race 1-inoculated cultivated tomato plants, KTP-01 conidia were prevented from forming functional haustoria by hypersensitive epidermal cell death; this hypersensitive reaction involved the Ol-4 gene in the wild tomato plants or the 'induced resistance' acquired by the nonpathogenic conidia previously inoculated into the cultivated tomato plants. All these KTP-01 conidia produced several projections on the appressoria during the repeated unsuccessful penetration attempts and eventually exhibited multilobed appressoria. On the host tomato leaves inoculated singly with KTP-01 conidia, fewer than 20% of the conidia located appressoria on the central part of target epidermal cells and succeeded in forming functional haustoria at the first penetration attempt without forming an appressorial projection. These conidia exhibited non-lobed appressoria. The remaining conidia, however, whose appressoria were located on/near the border of the target epidermal cells, were more likely to fail to penetrate at the first penetration, and then to develop additional projections for subsequent penetrations. Most conidia succeeded in forming functional haustoria at the second to fourth penetration attempts, but a few conidia failed to produce haustoria at all attempted penetrations. Eventually, the conidia that succeeded at the second penetration possessed a single appressorial projection (exhibiting the nipple-shaped appressoria), whereas the remaining conidia exhibited moderately lobed appressoria with two to four appressorial projections and multilobed appressoria, with more projections. Thus, the present study revealed that the basic shape of appressoria of KTP-01 was the non-lobed type, and that polymorphic changes of the appressoria occurred as a result of successive production of projections during repeated unsuccessful penetration attempts.

Collection of highly germinative pseudochain conidia of Oidium neolycopersici from conidiophores by electrostatic attraction

A population of simultaneously germinating conidia is an ideal inoculum of the powdery mildew pathogen, Oidium neolycopersici. In conditions of no or low wind velocity, O. neolycopersici successively stacks mature conidia on conidiophores in a chain formation (pseudochain), without releasing the precedent mature conidia. These pseudochain conidia represent a perfect inoculum, in which all conidia used for inoculation germinate simultaneously. However, we found that conidia must be collected before they fall to the leaf surface, because the germination rate was lower among conidia deposited on the leaf surface. We used an electrostatic spore collector to collect the pseudochain conidia, and their high germination rate was not affected by this treatment. The spore collector consisted of an electrified insulator probe, which created an electrostatic field around its pointed tip, and attracted conidia within its electric field. The attractive force created by the probe tip was directly proportional to voltage, and was inversely proportional to the distance between the tip and a target colony on a leaf. Pseudochain conidia were successfully collected by bringing the electrified probe tip close to target colonies on leaves. In this way, conidia were collected from colonies at 3-d intervals. This effectively collected all conidia from conidiophores before they dropped to the leaf surface. A high germination rate was observed among conidia attracted to the probe tip (95.5+/-0.6%). Conidia were easily suspended in water with added surfactant, and retained their germination ability. These conidia were infective and produced conidia in pseudochains on conidiophores after inoculation. The electrostatic spore collection method can be used to collect conidia as they form on conidiophores, thus obtaining an inoculum population in which all of the conidia germinate simultaneously.

Oidium neolycopersici: intraspecific variability inferred from amplified fragment length polymorphism analysis and relationship with closely related powdery mildew fungi infecting various plant species

Previous works indicated a considerable variation in the pathogenicity, virulence, and host range of Oidium neolycopersici isolates causing tomato powdery mildew epidemics in many parts of the world. In this study, rDNA internal transcribed spacer (ITS) sequences, and amplified fragment length polymorphism (AFLP) patterns were analyzed in 17 O. neolycopersici samples collected in Europe, North America, and Japan, including those which overcame some of the tomato major resistance genes. The ITS sequences were identical in all 10 samples tested and were also identical to ITS sequences of eight previously studied O. neolycopersici specimens. The AFLP analysis revealed a high genetic diversity in O. neolycopersici and indicated that all 17 samples represented different genotypes. This might suggest the existence of either a yet unrevealed sexual reproduction or other genetic mechanisms that maintain a high genetic variability in O. neolycopersici. No clear correlation was found between the virulence and the AFLP patterns of the O. neolycopersici isolates studied. The relationship between O. neolycopersici and powdery mildew anamorphs infecting Aquilegia vulgaris, Chelidonium majus, Passiflora caerulea, and Sedum alboroseum was also investigated. These anamorphs are morphologically indistinguishable from and phylogenetically closely related to O. neolycopersici. The cross-inoculation tests and the analyses of ITS sequences and AFLP patterns jointly indicated that the powdery mildew anamorphs collected from the above mentioned plant species all represent distinct, but closely related species according to the phylogenetic species recognition. All these species were pathogenic only to their original host plant species, except O. neolycopersici which infected S. alboroseum, tobacco, petunia, and Arabidopsis thaliana, in addition to tomato, in cross-inoculation tests. This is the first genome-wide study that investigates the relationships among powdery mildews that are closely related based on ITS sequences and morphology. The results indicate that morphologically indistinguishable powdery mildews that differed in only one to five single nucleotide positions in their ITS region are to be considered as different taxa with distinct host ranges.

Conditions for development of powdery mildew of tomato caused by Oidium neolycopersici

Oidium neolycopersici causes severe powdery mildew on all aerial parts of tomato, excluding the fruit. The objective of the present work was to examine factors that influence the development of O. neolycopersici on tomato and to identify potential methods for managing tomato powdery mildew. Under controlled conditions, the highest rates of conidial germination were observed at 25 degrees C, 99% relative humidity (RH) and minimal light, and the lowest on leaves adjacent to fruits. Optimal conditions for appressoria formation were 25 degrees C, RH ranging from 33 to 99%, and 1,750 lux light intensity. More conidia were formed at 20 degrees C, 70 to 85% RH, and 5,150 lux light intensity than at 16 and 26 degrees C, 99% RH, and 480 to 1,750 lux, respectively. Conidia survived and remained capable of germination for over four months when initially incubated at lower temperatures and higher RH, as compared with their fast decline under more extreme summer shade conditions. In growth chamber experiments, disease did not develop at 28 degrees C. Within the range of 70 to 99% RH, disease was less severe under the higher RH than the drier conditions. Disease was also less severe at lower light intensities. Data collected in three commercial-like greenhouse experiments involving various climate regimes were used to draw correlations regarding the effects of temperature and RH on the development of epidemics. Severity of powdery mildew was positively correlated with the duration of the range 15 to 25 degrees C, 1 to 4 weeks before disease evaluation (BDE), RH levels of 60 to 90% at 2 to 4 weeks BDE, and RH of 50 to 60% during the week BDE. Conversely, disease was negatively correlated with the duration of temperatures in the low and high ranges (5 to 15 degrees C and 35 to 40 degrees C) at 1 to 4 weeks BDE, with the duration of RH levels of 40% and below at 1 to 4 weeks BDE, and with 50 to 60% RH during the third week BDE. High (90 to 100%) RH was also negatively correlated with disease severity. These results suggest that the combination of high temperatures and low RH may help reduce O. neolycopersici powdery mildew severity in greenhouse tomatoes.

Natural genetic resources of Arabidopsis thaliana reveal a high prevalence and unexpected phenotypic plasticity of RPW8-mediated powdery mildew resistance

Here, an approach based on natural genetic variation was adopted to analyse powdery mildew resistance in Arabidopsis thaliana. Accessions resistant to multiple powdery mildew species were crossed with the susceptible Col-0 ecotype and inheritance of resistance was analysed. Histochemical staining was used to visualize archetypal plant defence responses such as callose deposition, hydrogen peroxide accumulation and host cell death in a subset of these ecotypes. In six accessions, resistance was likely of polygenic origin while 10 accessions exhibited evidence for a single recessively or semi-dominantly inherited resistance locus. Resistance in the latter accessions was mainly manifested at the terminal stage of the fungal life cycle by a failure of abundant conidiophore production. The resistance locus of several of these ecotypes was mapped to a genomic region containing the previously analysed atypical RPW8 powdery mildew resistance genes. Gene silencing revealed that members of the RPW8 locus were responsible for resistance to Golovinomyces orontii in seven accessions. These results suggest that broad-spectrum powdery mildew resistance in A. thaliana is predominantly of polygenic origin or based on RPW8 function. The findings shed new light on the natural variation of inheritance, phenotypic expression and pathogen range of RPW8-conditioned powdery mildew resistance.

Naturally occurring broad-spectrum powdery mildew resistance in a Central American tomato accession is caused by loss of mlo function

The resistant cherry tomato (Solanum lycopersicum var. cerasiforme) line LC-95, derived from an accession collected in Ecuador, harbors a natural allele (ol-2) that confers broad-spectrum and recessively inherited resistance to powdery mildew (Oidium neolycopersici). As both the genetic and phytopathological characteristics of ol-2-mediated resistance are reminiscent of powdery mildew immunity conferred by loss-of-function mlo alleles in barley and Arabidopsis, we initiated a candidate-gene approach to clone Ol-2. A tomato Mlo gene (SlMlo1) with high sequence-relatedness to barley Mlo and Arabidopsis AtMLO2 mapped to the chromosomal region harboring the Ol-2 locus. Complementation experiments using transgenic tomato lines as well as virus-induced gene silencing assays suggested that loss of SlMlo1 function is responsible for powdery mildew resistance conferred by ol-2. In progeny of a cross between a resistant line bearing ol-2 and the susceptible tomato cultivar Moneymaker, a 19-bp deletion disrupting the SlMlo1 coding region cosegregated with resistance. This polymorphism results in a frameshift and, thus, a truncated nonfunctional SlMlo1 protein. Our findings reveal the second example of a natural mlo mutant that possibly arose post-domestication, suggesting that natural mlo alleles might be evolutionarily short-lived due to fitness costs related to loss of mlo function.

The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi

The powdery mildew diseases, caused by fungal species of the Erysiphales, have an important economic impact on a variety of plant species and have driven basic and applied research efforts in the field of phytopathology for many years. Although the first taxonomic reports on the Erysiphales date back to the 1850's, advances into the molecular biology of these fungal species have been hampered by their obligate biotrophic nature and difficulties associated with their cultivation and genetic manipulation in the laboratory. The discovery in the 1990's of a few species of powdery mildew fungi that cause disease on Arabidopsis has opened a new chapter in this research field. The great advantages of working with a model plant species have translated into remarkable progress in our understanding of these complex pathogens and their interaction with the plant host. Herein we summarize advances in the study of Arabidopsis-powdery mildew interactions and discuss their implications for the general field of plant pathology. We provide an overview of the life cycle of the pathogens on Arabidopsis and describe the structural and functional changes that occur during infection in the host and fungus in compatible and incompatible interactions, with special emphasis on defense signaling, resistance pathways, and compatibility factors. Finally, we discuss the future of powdery mildew research in anticipation of the sequencing of multiple powdery mildew genomes. The cumulative body of knowledge on powdery mildews of Arabidopsis provides a valuable tool for the study and understanding of disease associated with many other obligate biotrophic pathogen species.

GFP technology for the study of biocontrol agents in tritrophic interactions: A case study with Pseudozyma flocculosa

GFP technology was applied to the biocontrol agent (BCA) Pseudozyma flocculosa to study its development and interactions at the tritrophic level plant-powdery mildew-BCA. Transformation experiments with GFP led to the production of a strongly fluorescent strain, Act-4, that displayed biocontrol traits typical of P. flocculosa WT. Following inundative applications, growth of P. flocculosa Act-4 was closely and almost exclusively associated with the colonies of the pathogen regardless of the powdery mildew species or the host plant tested. Development of P. flocculosa Act-4 on control leaves alone was extremely limited 24 h after its application and was typical of the epiphytic growth characterizing this type of yeast-like fungus. Based on the strong correlation between the colonization pattern of the different powdery mildew species tested and the presence of P. flocculosa Act-4, as determined by its fluorescence, it seems that growth of the BCA is dependant on the presence of powdery mildews. These results demonstrate that the GFP technology can be used to study plant-pathogen-BCA interactions and fulfill a wide array of purposes ranging from fundamental observations of the biocontrol behavior of a BCA to very applied ones serving some of the requirements for the registration of BCA's such as defining their environmental fate.

Tomato defense to the powdery mildew fungus: differences in expression of genes in susceptible, monogenic- and polygenic resistance responses are mainly in timing

Oidium neolycopersici is a causal agent of tomato powdery mildew. In this paper, gene expression profiles were investigated of susceptible, monogenic- and polygenic resistant tomato genotypes in response to O. neolycopersici infection by using cDNA-AFLP. Around 30,000 TDFs (Transcript Derived Fragments), representing approximately 22% of the transcriptome based on in silico estimation, were identified and 887 TDFs were differentially expressed (DE-TDFs) upon inoculation with O. neolycopersici spores. Forty-two percent of the identified DE-TDFs were detected in both the compatible and incompatible interactions, a subset of these were studied for their temporal patterns. All of these common induced DE-TDFs displayed an expression peak at 7 days post incoluation in monogenic resistant response but sustained up-regulation in the susceptible and the polygenic resistant response. While more than half of these common DE-TDFs showed earlier timing in incompatible interactions compared to compatible interaction. Only 2% of the identified DE-TDFs were specific to either the monogenic or the polygenic resistant response. By annotation of the 230 sequenced DE-TDFs we found that 34% of the corresponding transcripts were known to be involved in plant defense, whereas the other transcripts played general roles in signal transduction (11%), regulation (24%), protein synthesis and degradation (11%), energy metabolism (12%) including photosynthesis, photorespiration and respiration.

Formation of Conidial Pseudochains by Tomato Powdery Mildew Oidium neolycopersici

The formation of conidial pseudochains by the tomato powdery mildew Oidium neolycopersici on tomato leaves was monitored using a high-fidelity digital microscope. Individual living conidiophores that formed mature conidial cells at their apex were selected for observation. The conidial cells were produced during repeated division and elongation by the generative cells of the conidiophores. Under weak wind conditions (0.1 m/s), these conidial cells did not separate from each other to produce a chain of conidial cells (pseudochain). The pseudochains dropped from the conidiophores once four conidial cells were connected. The conidiophores resumed conidium production, followed by another cycle of pseudochain formation. The formation of pseudochains by tomato powdery mildew was not influenced by the ambient relative humidity. On the other hand, the conidial cells produced were easily wind dispersed without forming pseudochains when conidiophores were exposed to stronger winds (1.0 m/s). The present study successfully demonstrated that the pathogen required wind to disperse progeny conidia from the conidiophores and produced conidial pseudochains when the wind was below a critical level, independent of high relative humidity as reported previously.