Effects of targeted replacement of the tomatinase gene on the interaction of Septoria lycopersici with tomato plants

Identification and Genetic Dissection of Resistance to Red Crown Rot Disease in a Diverse Soybean Germplasm Population

Red crown rot (RCR) disease caused by Calonectria ilicicola negatively impacts soybean yield and quality. Unfortunately, the knowledge of the genetic architecture of RCR resistance in soybeans is limited. In this study, 299 diverse soybean accessions were used to explore their genetic diversity and resistance to RCR, and to mine for candidate genes via emergence rate (ER), survival rate (SR), and disease severity (DS) by a multi-locus random-SNP-effect mixed linear model of GWAS. All accessions had brown necrotic lesions on the primary root, with five genotypes identified as resistant. Nine single-nucleotide polymorphism (SNP) markers were detected to underlie RCR response (ER, SR, and DS). Two SNPs colocalized with at least two traits to form a haplotype block which possessed nine genes. Based on their annotation and the qRT-PCR, three genes, namely Glyma.08G074600, Glyma.08G074700, and Glyma.12G043600, are suggested to modulate soybean resistance to RCR. The findings from this study could serve as the foundation for breeding RCR-tolerant soybean varieties, and the candidate genes could be validated to deepen our understanding of soybean response to RCR.

Functional Analyses of a Small Secreted Cysteine-Rich Protein ThSCSP_14 in Tilletia horrida

Tilletia horrida is a biotrophic basidiomycete fungus that causes rice kernel smut, one of the most significant diseases in hybrid rice-growing areas worldwide. Little is known about the pathogenic mechanisms and functions of effectors in T. horrida. Here, we performed functional studies of the effectors in T. horrida and found that, of six putative effectors tested, only ThSCSP_14 caused the cell death phenotype in epidermal cells of Nicotiana benthamiana leaves. ThSCSP_14 was upregulated early on during the infection process, and the encoded protein was secreted. The predicted signal peptide (SP) of ThSCSP_14 was required for its ability to induce the necrosis phenotype. Furthermore, the ability of ThSCSP_14 to trigger cell death in N. benthamiana depended on suppressing the G2 allele of Skp1 (SGT1), required for Mla12 resistance (RAR1), heat-shock protein 90 (HSP90), and somatic embryogenesis receptor-like kinase (SERK3). It is important to note that ThSCSP_14 induced a plant defense response in N. benthamiana leaves. Hence, these results demonstrate that ThSCSP_14 is a possible effector that plays an essential role in T. horrida-host interactions.

Secreted Glycoside Hydrolase Proteins as Effectors and Invasion Patterns of Plant-Associated Fungi and Oomycetes

During host colonization, plant-associated microbes, including fungi and oomycetes, deliver a collection of glycoside hydrolases (GHs) to their cell surfaces and surrounding extracellular environments. The number and type of GHs secreted by each organism is typically associated with their lifestyle or mode of nutrient acquisition. Secreted GHs of plant-associated fungi and oomycetes serve a number of different functions, with many of them acting as virulence factors (effectors) to promote microbial host colonization. Specific functions involve, for example, nutrient acquisition, the detoxification of antimicrobial compounds, the manipulation of plant microbiota, and the suppression or prevention of plant immune responses. In contrast, secreted GHs of plant-associated fungi and oomycetes can also activate the plant immune system, either by acting as microbe-associated molecular patterns (MAMPs), or through the release of damage-associated molecular patterns (DAMPs) as a consequence of their enzymatic activity. In this review, we highlight the critical roles that secreted GHs from plant-associated fungi and oomycetes play in plant-microbe interactions, provide an overview of existing knowledge gaps and summarize future directions.

Bitter and sweet make tomato hard to (b)eat

The glycoalkaloid saponin α-tomatine is a tomato-specific secondary metabolite that accumulates to millimolar levels in vegetative tissues and has antimicrobial and antinutritional activity that kills microbial pathogens and deters herbivorous insects. We describe recent insights into the biosynthetic pathway of α-tomatine synthesis and its regulation. We discuss the mode of action of α-tomatine by physically interacting with sterols, thereby disrupting membranes, and how tomato protects itself from its toxic action. Tomato pathogenic microbes can enzymatically hydrolyze, and thereby inactivate, α-tomatine using either of three distinct types of glycosyl hydrolases. We also describe findings that extend well beyond the simple concept of plants producing toxins and pathogens inactivating them. There are reports that toxicity of α-tomatine is modulated by external pH, that α-tomatine can trigger programmed cell death in fungi, that cellular localization matters for the impact of α-tomatine on invading microbes, and that α-tomatine breakdown products generated by microbial hydrolytic enzymes can modulate plant immune responses. Finally, we address a number of outstanding questions that deserve attention in the future.

Transcriptome dynamics underlying elicitor-induced defense responses against Septoria leaf spot disease of tomato (Solanum lycopersicum L.)

Elicitor-induced defense response against potential plant pathogens has been widely reported in several crop plants; however, transcriptome dynamics underlying such defense response remains elusive. Our previous study identified and characterized a novel elicitor, κ-carrageenan, from Kappaphycus alvarezii, a marine red seaweed. Our preliminary studies have shown that the elicitor-treatment enhances the tolerance of a susceptible tomato cultivar to Septoria lycopersici (causative agent of leaf spot disease). To gain further insights into the genes regulated during elicitor treatment followed by pathogen infection, we have performed RNA-Seq experiments under different treatments, namely, control (untreated and uninfected), elicitor treatment, pathogen infection alone, and elicitor treatment followed by pathogen infection. To validate the results, forty-three genes belonging to five different classes, namely, ROS activating and detoxifying enzyme encoding genes, DEAD-box RNA helicase genes, autophagy-related genes, cysteine proteases, and pathogenesis-related genes, were chosen. Expression profiling of each gene was performed using qRT-PCR, and the data was correlated with the RNA-seq data. Altogether, the study has pinpointed a repertoire of genes that could be potential candidates for further functional characterization to provide insights into novel elicitor-induced fungal defense and develop transgenic lines resistant to foliar diseases.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00970-y.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

Disarming the Host: Detoxification of Plant Defense Compounds During Fungal Necrotrophy

While fungal biotrophs are dependent on successfully suppressing/subverting host defenses during their interaction with live cells, necrotrophs, due to their lifestyle are often confronted with a suite of toxic metabolites. These include an assortment of plant defense compounds (PDCs) which can demonstrate broad antifungal activity. These PDCs can be either constitutively present in plant tissue or induced in response to infection, but are nevertheless an important obstacle which needs to be overcome for successful pathogenesis. Fungal necrotrophs have developed a number of strategies to achieve this goal, from the direct detoxification of these compounds through enzymatic catalysis and modification, to the active transport of various PDCs to achieve toxin sequestration and efflux. Studies have shown across multiple pathogens that the efficient detoxification of host PDCs is both critical for successful infection and often a determinant factor in pathogen host range. Here, we provide a broad and comparative overview of the various mechanisms for PDC detoxification which have been identified in both fungal necrotrophs and fungal pathogens which depend on detoxification during a necrotrophic phase of infection. Furthermore, the effect that these mechanisms have on fungal host range, metabolism, and disease control will be discussed.

Antifungal Activity of Beauveria bassiana Endophyte against Botrytis cinerea in Two Solanaceae Crops

Botrytis cinerea causes substantial losses in tomato and chili pepper crops worldwide. Endophytes have shown the potential for the biological control of diseases. The colonization ability of native endophyte strains of Beauveria bassiana and their antifungal effect against B. cinerea were evaluated in Solanaceae crops. Root drenching with B. bassiana was applied, and endophytic colonization capacity in roots, stems, and leaves was determined. The antagonistic activity was evaluated using in vitro dual culture and also plants by drenching the endophyte on the root and by pathogen inoculation in the leaves. Ten native strains were endophytes of tomato, and eight were endophytes of chili pepper. All strains showed significant in vitro antagonism against B. cinerea (30–36%). A high antifungal effect was observed, and strains RGM547 and RGM644 showed the lowest percentage of the surface affected by the pathogen. Native strains of B. bassiana colonized tomato and chili pepper tissues and provided important levels of antagonism against B. cinerea.

The Host Stomatal Density Determines Resistance to Septoria gentianae in Japanese Gentian

Plant stomata represent the main battlefield for host plants and the pathogens that enter plant tissues via stomata. Septoria spp., a group of ascomycete fungi, use host plant stomata for invasion and cause serious damage to agricultural plants. There is no evidence, however, showing the involvement of stomata in defense systems against Septoria infection. In this study, we isolated Septoria gentianae 20-35 (Sg20-35) from Gentiana triflora showing gentian leaf blight disease symptoms in the field. Establishment of an infection system using gentian plants cultured in vitro enabled us to observe the Sg20-35 infection process and estimate its virulence in several gentian cultivars or lines. Sg20-35 also entered gentian tissues via stomata and showed increased virulence in G. triflora compared with G. scabra and their interspecific hybrid. Notably, the susceptibility of gentian cultivars to Sg20-35 was associated with their stomatal density on the adaxial but not abaxial leaf surface. Treatment of EPIDERMAL PATTERNING FACTOR-LIKE 9 (EPFL9/STOMAGEN) peptides, a small secreted peptide controlling stomatal density in Arabidopsis thaliana, increased stomatal density on the adaxial side of gentian leaves as well. Consequently, treated plants showed enhanced susceptibility to Sg20-35. These results indicate that stomatal density on the adaxial leaf surface is one of the major factors determining the susceptibility of gentian cultivars to S. gentianae and suggest that stomatal density control may represent an effective strategy to confer Septoria resistance.

Metabolomics of Solanum lycopersicum Infected with Phytophthora infestans Leads to Early Detection of Late Blight in Asymptomatic Plants

Tomato crops suffer attacks of various pathogens that cause large production losses. Late blight caused by Phytophthora infestans is a devastating disease in tomatoes because of its difficultly to control. Here, we applied metabolomics based on liquid chromatography⁻mass spectrometry (LC-MS) and metabolic profiling by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) in combination with multivariate data analysis in the early detection of late blight on asymptomatic tomato plants and to discriminate infection times of 4, 12, 24, 36, 48, 60, 72 and 96 h after inoculation (hpi). MALDI-MS and LC-MS profiles of metabolites combined with multivariate data analysis are able to detect early-late blight-infected tomato plants, and metabolomics based on LC-MS discriminates infection times in asymptomatic plants. We found the metabolite tomatidine as an important biomarker of infection, saponins as early infection metabolite markers and isocoumarin as early and late asymptomatic infection marker along the post infection time. MALDI-MS and LC-MS analysis can therefore be used as a rapid and effective method for the early detection of late blight-infected tomato plants, offering a suitable tool to guide the correct management and application of sanitary defense approaches. LC-MS analysis also appears to be a suitable tool for identifying major metabolites of asymptomatic late blight-infected tomato plants.

The Impact of Steroidal Glycoalkaloids on the Physiology of Phytophthora infestans, the Causative Agent of Potato Late Blight

Steroidal glycoalkaloids (SGAs) are plant secondary metabolites known to be toxic to animals and humans and that have putative roles in defense against pests. The proposed mechanisms of SGA toxicity are sterol-mediated disruption of membranes and inhibition of cholinesterase activity in neurons. It has been suggested that phytopathogenic microorganisms can overcome SGA toxicity by enzymatic deglycosylation of SGAs. Here, we have explored SGA-mediated toxicity toward the invasive oomycete Phytophthora infestans, the causative agent of the late blight disease in potato and tomato, as well as the potential for SGA deglycosylation by this species. Our growth studies indicate that solanidine, the nonglycosylated precursor of the potato SGAs α-chaconine and α-solanine, has a greater physiological impact than its glycosylated forms. All of these compounds were incorporated into the mycelium, but only solanidine could strongly inhibit the growth of P. infestans in liquid culture. Genes encoding several glycoside hydrolases with potential activity on SGAs were identified in the genome of P. infestans and were shown to be expressed. However, we found no indication that deglycosylation of SGAs takes place. We present additional evidence for apparent host-specific adaptation to potato SGAs and assess all results in terms of future pathogen management strategies.

Bactericidal Effect of Tomatidine-Tobramycin Combination against Methicillin-Resistant Staphylococcus aureus and Pseudomonas aeruginosa Is Enhanced by Interspecific Small-Molecule Interactions

This study investigated the antibacterial activity of the plant alkaloid tomatidine (TO) against Staphylococcus aureus grown in the presence of Pseudomonas aeruginosa. Since the P. aeruginosa exoproduct 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) is known to cause a respiratory deficiency in S. aureus and respiratory-deficient S. aureus are known to be hypersensitive to TO, we assessed kill kinetics of TO (8 μg/ml) against S. aureus in coculture with P. aeruginosa. Kill kinetics were also assessed using P. aeruginosa mutants deficient in the production of different exoproducts and quorum sensing-related compounds. After 24 h in coculture, TO increased the killing of S. aureus by 3.4 log₁₀ CFU/ml in comparison to that observed in a coculture without TO. The effect of TO was abolished when S. aureus was in coculture with the lasRrhlR, pqsA, pqsL, or lasA mutant of P. aeruginosa. The bactericidal effect of TO against S. aureus in coculture with the pqsL mutant was restored by supplemental HQNO. In an S. aureus monoculture, the combination of HQNO and TO was bacteriostatic, indicating that the pqsL mutant produced an additional factor required for the bactericidal effect. The bactericidal activity of TO was also observed against a tobramycin-resistant methicillin-resistant S. aureus (MRSA) in coculture with P. aeruginosa, and the addition of tobramycin significantly suppressed the growth of both microorganisms. TO shows a strong bactericidal effect against S. aureus when cocultured with P. aeruginosa. The combination of TO and tobramycin may represent a new treatment approach for cystic fibrosis patients frequently cocolonized by MRSA and P. aeruginosa.

Detoxification of α-tomatine by Cladosporium fulvum is required for full virulence on tomato

· α-Tomatine is an antifungal glycoalkaloid that provides basal defense to tomato (Solanum lycopersicum). However, tomato pathogens overcome this basal defense barrier by the secretion of tomatinases that degrade α-tomatine into the less fungitoxic compounds β-tomatine and tomatidine. Although pathogenic on tomato, it has been reported that the biotrophic fungus Cladosporium fulvum is unable to detoxify α-tomatine. · Here, we present a functional analysis of the glycosyl hydrolase (GH10), CfTom1, which is orthologous to fungal tomatinases. · We show that C. fulvum hydrolyzes α-tomatine into tomatidine in vitro and during the infection of tomato, which is fully attributed to the activity of CfTom1, as shown by the heterologous expression of this enzyme in tomato. Accordingly, ∆cftom1 mutants of C. fulvum are more sensitive to α-tomatine and are less virulent than the wild-type fungus on tomato. · Although α-tomatine is thought to be localized in the vacuole, we show that it is also present in the apoplast, where it is hydrolyzed by CfTom1 on infection. The accumulation of tomatidine during infection appears to be toxic to tomato cells and does not suppress defense responses, as suggested previously. Altogether, our results show that CfTom1 is responsible for the detoxification of α-tomatine by C. fulvum, and is required for full virulence of this fungus on tomato.

Saponin Synthesis and Function

Saponins are one of the most numerous and diverse groups of plant natural products. They serve a range of ecological roles including plant defence against disease and herbivores and possibly as allelopathic agents in competitive interactions between plants. Some saponins are also important pharmaceuticals, and the underexplored biodiversity of plant saponins is likely to prove to be a vital resource for future drug discovery. The biological activity of saponins is normally attributed to the amphipathic properties of these molecules, which consist of a hydrophobic triterpene or sterol backbone and a hydrophilic carbohydrate chain, although some saponins are known to have potent biological activities that are dependent on other aspects of their structure. This chapter will focus on the biological activity and the synthesis of some of the best-studied examples of plant saponins and on recent developments in the identification of the genes and enzymes responsible for saponin synthesis.

Plant defense compounds: systems approaches to metabolic analysis

Systems biology attempts to answer biological questions by integrating across diverse genomic data sets. With the increasing ability to conduct genomics experiments, this integrative approach is being rapidly applied across numerous biological research communities. One of these research communities investigates how plants utilize secondary metabolites or defense metabolites to defend against attack by pathogens and other biotic organisms. This use of systems biology to integrate across transcriptomics, metabolomics, and genomics is significantly enhancing the rate of discovery of genes, metabolites, and bioactivities for plant defense compounds as well as extending our knowledge of how these compounds are regulated. Plant defense compounds are also providing a unique proving platform to develop new approaches that enhance the ability to conduct systems biology with existing and previously unforseen genomics data sets. This review attempts to illustrate both how systems biology is helping the study of plant defense compounds and vice versa.

Tomatidine inhibits replication of Staphylococcus aureus small-colony variants in cystic fibrosis airway epithelial cells

Small-colony variants (SCVs) often are associated with chronic Staphylococcus aureus infections, such as those encountered by cystic fibrosis (CF) patients. We report here that tomatidine, the aglycon form of the plant secondary metabolite tomatine, has a potent growth inhibitory activity against SCVs (MIC of 0.12 μg/ml), whereas the growth of normal S. aureus strains was not significantly altered by tomatidine (MIC, >16 μg/ml). The specific action of tomatidine was bacteriostatic for SCVs and was clearly associated with their dysfunctional electron transport system, as the presence of the electron transport inhibitor 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) caused normal S. aureus strains to become susceptible to tomatidine. Inversely, the complementation of SCVs' respiratory deficiency conferred resistance to tomatidine. Tomatidine provoked a general reduction of macromolecular biosynthesis but more specifically affected the incorporation of radiolabeled leucine in proteins of HQNO-treated S. aureus at a concentration corresponding to the MIC against SCVs. Furthermore, tomatidine inhibited the intracellular replication of a clinical SCV in polarized CF-like epithelial cells. Our results suggest that tomatidine eventually will find some use in combination therapy with other traditional antibiotics to eliminate persistent forms of S. aureus.

Distinct colonization patterns and cDNA-AFLP transcriptome profiles in compatible and incompatible interactions between melon and different races of Fusarium oxysporum f. sp. melonis

BACKGROUND

Fusarium oxysporum f. sp. melonis Snyd. & Hans. (FOM) causes Fusarium wilt, the most important infectious disease of melon (Cucumis melo L.). The four known races of this pathogen can be distinguished only by infection on appropriate cultivars. No molecular tools are available that can discriminate among the races, and the molecular basis of compatibility and disease progression are poorly understood. Resistance to races 1 and 2 is controlled by a single dominant gene, whereas only partial polygenic resistance to race 1,2 has been described. We carried out a large-scale cDNA-AFLP analysis to identify host genes potentially related to resistance and susceptibility as well as fungal genes associated with the infection process. At the same time, a systematic reisolation procedure on infected stems allowed us to monitor fungal colonization in compatible and incompatible host-pathogen combinations.

RESULTS

Melon plants (cv. Charentais Fom-2), which are susceptible to race 1,2 and resistant to race 1, were artificially infected with a race 1 strain of FOM or one of two race 1,2 w strains. Host colonization of stems was assessed at 1, 2, 4, 8, 14, 16, 18 and 21 days post inoculation (dpi), and the fungus was reisolated from infected plants. Markedly different colonization patterns were observed in compatible and incompatible host-pathogen combinations. Five time points from the symptomless early stage (2 dpi) to obvious wilting symptoms (21 dpi) were considered for cDNA-AFLP analysis. After successful sequencing of 627 transcript-derived fragments (TDFs) differentially expressed in infected plants, homology searching retrieved 305 melon transcripts, 195 FOM transcripts expressed in planta and 127 orphan TDFs. RNA samples from FOM colonies of the three strains grown in vitro were also included in the analysis to facilitate the detection of in planta-specific transcripts and to identify TDFs differentially expressed among races/strains.

CONCLUSION

Our data suggest that resistance against FOM in melon involves only limited transcriptional changes, and that wilting symptoms could derive, at least partially, from an active plant response.We discuss the pathogen-derived transcripts expressed in planta during the infection process and potentially related to virulence functions, as well as transcripts that are differentially expressed between the two FOM races grown in vitro. These transcripts provide candidate sequences that can be further tested for their ability to distinguish between races.Sequence data from this article have been deposited in GenBank, Accession Numbers: HO867279-HO867981.

Plant antimicrobial agents and their effects on plant and human pathogens

To protect themselves, plants accumulate an armoury of antimicrobial secondary metabolites. Some metabolites represent constitutive chemical barriers to microbial attack (phytoanticipins) and others inducible antimicrobials (phytoalexins). They are extensively studied as promising plant and human disease-controlling agents. This review discusses the bioactivity of several phytoalexins and phytoanticipins defending plants against fungal and bacterial aggressors and those with antibacterial activities against pathogens affecting humans such as Pseudomonas aeruginosa and Staphylococcus aureus involved in respiratory infections of cystic fibrosis patients. The utility of plant products as “antibiotic potentiators” and “virulence attenuators” is also described as well as some biotechnological applications in phytoprotection.

Streptomyces scabies 87-22 possesses a functional tomatinase

The actinomycete Streptomyces scabies 87-22 is the causal agent of common scab, an economically important disease of potato and taproot crops. Sequencing of the S. scabies 87-22 genome revealed the presence of a gene with high homology to the gene encoding the alpha-tomatine-detoxifying enzyme tomatinase found in fungal tomato pathogens. The tomA gene from S. scabies 87-22 was cotranscribed with a putative family 1 glycosyl hydrolase gene, and purified TomA protein was active only on alpha-tomatine and not potato glycoalkaloids or xylans. Tomatinase-null mutants were more sensitive to alpha-tomatine than the wild-type strain in a disk diffusion assay. Interestingly, tomatine affected only aerial mycelium and not vegetative mycelium, suggesting that the target(s) of alpha-tomatine is not present during vegetative growth. Severities of disease for tomato seedlings affected by S. scabies 87-22 wild-type and DeltatomA1 strains were indistinguishable, suggesting that tomatinase is not important in pathogenicity on tomato plants. However, conservation of tomA on a pathogenicity island in S. acidiscabies and S. turgidiscabies suggests a role in plant-microbe interaction.

Accumulation of the hormone abscisic acid (ABA) at the infection site of the fungus Cercospora beticola supports the role of ABA as a repressor of plant defence in sugar beet

Inducible plant defence responses in sugar beet (Beta vulgaris L.) leaves are repressed during the early phase of infection by the fungus Cercospora beticola. In this report, we show that the concentration of the plant hormone abscisic acid (ABA) increases in sugar beet leaves during C. beticola infection. After an initial burst of ABA induced by inoculation of the fungus, elevated ABA concentrations were detected during the fungal penetration and colonization phases 3-9 days after inoculation. Fifteen days after inoculation, with visible onset of the necrotic phase of infection, the strongly elevated ABA concentrations in infected leaves were at levels similar to drought-stressed plants. A synthetic promoter composed of four copies of the ABA-responsive element (ABRE) A2 and the coupling element CE3 of the ABA-inducible barley gene HVA1 was strongly induced by ABA and C. beticola infection in transgenic sugar beet leaves. Analysis of the spatial pattern of promoter activity revealed that the ABA-inducible promoter was locally activated at the fungal infection sites. Furthermore, expression of the basic leucine zipper transcription factor AREB1 was induced by drought stress and fungal infection in the sugar beet. Application of ABA reduced the promoter activity of the phenylalanine ammonia lyase (BvPAL) gene, and this effect was observed with the -34 to +248 BvPAL promoter region. This region is equivalent to the core promoter, which is necessary for the suppression of BvPAL expression by C. beticola, as recently shown. These data indicate that ABA accumulation and activation of the ABA-dependent signalling cascade are the primary cause of suppression of BvPAL expression during infection of sugar beet leaves.

Tomatinase from Fusarium oxysporum f. sp. lycopersici is required for full virulence on tomato plants

Saponin detoxification enzymes from pathogenic fungi are involved in the infection process of their host plants. Fusarium oxysporum f. sp lycopersici, a tomato pathogen, produces the tomatinase enzyme Tom1, which degrades alpha-tomatine to less toxic derivates. To study the role of the tom1 gene in the virulence of F. oxysporum, we performed targeted disruption and overexpression of the gene. The infection process of tomato plants inoculated with transformants constitutively producing Tom1 resulted in an increase of symptom development. By contrast, tomato plants infected with the knockout mutants showed a delay in the disease process, indicating that Tom1, although not essential for pathogenicity, is required for the full virulence of F. oxysporum. Total tomatinase activity in the disrupted strains was reduced only 25%, leading to beta(2)-tomatine as the main hydrolysis product of the saponin in vitro. In silico analysis of the F. oxysporum genome revealed the existence of four additional putative tomatinase genes with identities to tomatinases from family 3 of glycosyl hydrolases. These might be responsible for the remaining tomatinase activity in the Deltatom1 mutants. Our results indicate that detoxification of alpha-tomatine in F. oxysporum is carried out by several tomatinase activities, suggesting the importance of these enzymes during the infection process.

A high throughput targeted gene disruption method for Alternaria brassicicola functional genomics using linear minimal element (LME) constructs

Alternaria brassicicola causes black spot disease of cultivated Brassicas and has been used consistently as a necrotrophic fungal pathogen for studies with Arabidopsis. In A. brassicicola, mutant generation has been the most rate-limiting step for the functional analysis of individual genes due to low efficiency of both transformation and targeted integration. To improve the targeted gene disruption efficiency as well as to expedite gene disruption construct production, we used a short linear construct with minimal elements, an antibiotic resistance selectable marker gene, and a 250- to 600-bp-long partial target gene. The linear minimal element (LME) constructs consistently produced stable transformants for diverse categories of genes. Typically, 100% of the transformants were targeted gene disruption mutants when using the LME constructs, compared with inconsistent transformation and usually less than 10% targeted gene disruption with circular plasmid disruption constructs. Each mutant displayed a unique molecular signature thought to originate from endogenous exonuclease activities in fungal cells. Our data suggests that a DNA double-stranded break repair mechanism (DSBR) functions to increase targeting efficiency. This method is advantageous for high throughput gene disruption, overexpression, and reporter gene introduction within target genes, especially for asexual filamentous fungi where genetic approaches are unfavorable.

The arms race continues: battle strategies between plants and fungal pathogens

Plants are under constant attack by a vast array of pathogens. To impede their attackers they use both broad-spectrum and pathogen-specific defence mechanisms. The arms race between plants and fungal pathogens is fascinatingly varied, and what might be elicited as a plant defence mechanism against a pathogen could promote or enhance the virulence of other pathogens. Fungi use countermeasures to detoxify plant antimicrobial compounds and to evade host resistance mechanisms. Certain fungal species also manipulate the host hormone balance to create an environment that is beneficial to their survival. Several lines of evidence indicate a co-evolutionary arms race in which both plants and fungi can respond to changes that occur in their opponents.

Identification of a tomatinase in the tomato-pathogenic actinomycete Clavibacter michiganensis subsp. michiganensis NCPPB382

The insertion site of a transposon mutant of Clavibacter michiganensis subsp. michiganensis NCPPB382 was cloned and found to be located in the gene tomA encoding a member of the glycosyl hydrolase family 10. The intact gene was obtained from a cosmid library of C. michiganensis subsp. michiganensis. The deduced protein TomA (543 amino acids, 58 kDa) contains a predicted signal peptide and two domains, the N-terminal catalytic domain and a C-terminal fibronectin III-like domain. The closest well-characterized relatives of TomA were tomatinases from fungi involved in the detoxification of the tomato saponin alpha-tomatine which acts as a growth inhibitor. Growth inhibition of C. michiganensis subsp. michiganensis by alpha-tomatine was stronger in the tomA mutants than in the wild type. Tomatinase activity assayed by deglycosylation of alpha-tomatine to tomatidine was demonstrated in concentrated culture supernatants of C. michiganensis subsp. michiganensis. No activity was found with the tomA mutants. However, neither the transposon mutant nor a second mutant constructed by gene disruption was affected in virulence on the tomato cv. Moneymaker.

RNA silencing as a tool for exploring gene function in ascomycete fungi

We have developed a pHANNIBAL-like silencing vector, pSilent-1, for ascomycete fungi, which carries a hygromycin resistance cassette and a transcriptional unit for hairpin RNA expression with a spacer of a cutinase gene intron from the rice blast fungus Magnaporthe oryzae. In M. oryzae, a silencing vector with the cutinase intron spacer (147 bp) showed a higher efficiency in silencing of the eGFP gene than did those with a spacer of a GUS gene fragment or a longer intron (850 bp) of a chitin binding protein gene. Application of pSilent-1 to two M. oryzae endogenous genes, MPG1 and polyketide synthase-like gene, resulted in various degrees of silencing of the genes in 70-90% of the resulting transformants. RNA silencing was also induced by a pSilent-1-based vector in Colletotrichum lagenarium at a slightly lower efficiency than in M. oryzae, indicating that this silencing vector should provide a useful reverse genetic tool in ascomycete fungi.

Tissue distribution of cholinesterases and anticholinesterases in native and transgenic tomato plants

Acetylcholinesterase, a major component of the central and peripheral nervous systems, is ubiquitous among multicellular animals, where its main function is to terminate synaptic transmission by hydrolyzing the neurotransmitter, acetylcholine. However, previous reports describe cholinesterase activities in several plant species and we present data for its presence in tomato plants. Ectopic expression of a recombinant form of the human enzyme and the expression pattern of the transgene and the accumulation of its product in transgenic tomato plants are described. Levels of acetylcholinesterase activity in different tissues are closely effected by and can be separated from alpha-tomatine, an anticholinesterase steroidal glycoalkaloid. The recombinant enzyme can also be separated from the endogenous cholinesterase activity by its subcellular localization and distinct biochemical properties. Our results provide evidence for the co-existence in tomato plants of both acetylcholinesterase activity and a steroidal glycoalkaloid with anticholinesterase activity and suggest spatial mutual exclusivity of these antagonistic activities. Potential functions, including roles in plant-pathogen interactions are discussed.

Fusarium oxysporum: exploring the molecular arsenal of a vascular wilt fungus

SUMMARY Taxonomy: Vascular wilt fungus; Ascomycete although sexual stage is yet to be found. The most closely related teleomorphic group, Gibberella, is classified within the Pyrenomycetes.

HOST RANGE

Very broad at the species level. More than 120 different formae speciales have been identified based on specificity to host species belonging to a wide range of plant families. Disease symptoms: Initial symptoms of vascular wilt include vein clearing and leaf epinasty, followed by stunting, yellowing of the lower leafs, progressive wilting of leaves and stem, defoliation and finally death of the plant. In cross-sections of the stem, a brown ring is evident in the area of the vascular bundles. Some formae speciales are not primarily vascular pathogens but cause foot- and rootrot or bulbrot. Economic importance: Causes severe losses on most vegetables and flowers, several field crops such as cotton and tobacco, plantation crops such as banana, plantain, coffee and sugarcane, and a few shade trees.

CONTROL

Use of resistant varieties is the only practical measure for controlling the disease in the field. Under greenhouse conditions, soil sterilization can be performed. Alternative control methods with potential for the future include soil solarization and biological control with antagonistic bacteria or fungi.

USEFUL WEBSITES

http://www.fgsc.net/fus.htm, http://www-genome.wi.mit.edu/annotation/fungi/fusarium/, http://www.cbs.knaw.nl/fusarium/database.html.

Resistance and susceptibility of plants to fungal pathogens

Plants are under continuous threat of infection by pathogens endowed with diverse strategies to colonize their host. Comprehensive biochemical and genetic approaches are now starting to reveal the complex signaling pathways that mediate plant disease resistance. Initiation of defense signaling often involves specific recognition of invading pathogens by the products of specialized host resistance (R) genes. Potential resistance signaling components have been identified by mutational analyses to be required for specific resistance in the model Arabidopsis and some crop species. Strikingly, many of the components share similarity to that of innate immune systems in animals. Evidence is also accumulating that plant pathogens have a number of ways to evade host defenses during the early stages of infection, similar to animal pathogens. These strategies are becoming much better understood in a number of plant-pathogen interactions. In this review, we focus on the current knowledge of host factors that control plant resistance and susceptibility to fungal pathogens. The knowledge accumulated in these studies will serve a fundamental basis for combating diseases in strategic molecular agriculture.

A saponin-detoxifying enzyme mediates suppression of plant defences

Plant disease resistance can be conferred by constitutive features such as structural barriers or preformed antimicrobial secondary metabolites. Additional defence mechanisms are activated in response to pathogen attack and include localized cell death (the hypersensitive response). Pathogens use different strategies to counter constitutive and induced plant defences, including degradation of preformed antimicrobial compounds and the production of molecules that suppress induced plant defences. Here we present evidence for a two-component process in which a fungal pathogen subverts the preformed antimicrobial compounds of its host and uses them to interfere with induced defence responses. Antimicrobial saponins are first hydrolysed by a fungal saponin-detoxifying enzyme. The degradation product of this hydrolysis then suppresses induced defence responses by interfering with fundamental signal transduction processes leading to disease resistance.

Constitutive plant toxins and their role in defense against herbivores and pathogens

Most recent investigations have focused on induced, rather than constitutive, plant defenses. Yet significant research has helped to illuminate some of the principal characteristics of constitutive defenses, including mechanisms of action and synergistic effects, as well as strategies used by herbivores and pathogens to circumvent them.

Pathogenicity genes of phytopathogenic fungi

Summary Recently many fungal genes have been identified that, when disrupted, result in strains with a reduction or total loss of disease symptoms. Such pathogenicity genes are the subject of this review. The large number of pathogenicity genes identified is due to the application of tagged mutagenesis techniques (random or targeted). Genes have been identified with roles in the formation of infection structures, cell wall degradation, overcoming or avoiding plant defences, responding to the host environment, production of toxins, and in signal cascades. Additionally, genes with no database matches and with 'novel' functions have also been found. Improved technologies for mutation analysis and for sequencing and analysing fungal genomes hold promise for identifying many more pathogenicity genes.

New (and used) approaches to the study of fungal pathogenicity

The fungi are the most economically important plant pathogens and continue to be the focus of extensive research with a wide variety of methodologies. Enhancements in microscopy techniques have increased our ability to visualize the intimate interaction of fungi and their host plants. Improving methods allow pharmacological inhibition and genetic dissection of the determinants of fungal pathogenicity in a gene-by-gene approach. Identification and analysis of genes differentially transcribed in ways pertinent to pathogenicity continues to be a frequent research approach. Genome-wide analysis is gaining favor in biological research and fungal plant pathogens are no exception. Several industrial research groups are exploring fungal plant pathogenesis based on genomic sequence data and genome-wide mutagenesis. In March 2001 the first publicly available complete genome of a filamentous fungus (Neurospora crassa) was released. N. crassa is of course a saprophyte and there is no complete sequence available for a plant pathogenic fungus in public databases. However, freely accessible entire genome sequences for both plant pathogenic fungi and their hosts are on the horizon. Sequence availability promises to revolutionize the rate at which data relevant to disease processes will be accrued. In this review we describe approaches currently applied to the study of plant pathogenic fungi and explore developments of potential future benefit with existing technologies not yet applied to this group of important organisms.