Of genes and genomes, needles and haystacks: Blumeria graminis and functionality

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.

Identification of the interacting proteins of Bambusa pervariabilis × Dendrocalamopsis grandis in response to the transcription factor ApCtf1β in Arthrinium phaeospermum

Arthrinium phaeospermum is the main pathogen that causes Bambusa pervariabilis × Dendrocalamopsis grandis blight. It secretes the cutinase transcription factor ApCtf1β, which has been shown to play an important role in B. pervariabilis × D. grandis virulence. However, knowledge about the interaction target genes of ApCtf1β in B. pervariabilis × D. grandis remains limited. A cDNA library for the yeast two-hybrid system was constructed from B. pervariabilis × D. grandis shoots after 168 h treatment with A. phaeospermum. The library was identified as 1.20 × 10⁷ cfu, with an average insert >1,000 bp in size and a 100% positive rate, providing a database for the subsequent molecular study of the interaction between A. phaeospermum and B. pervariabilis × D. grandis. The yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), and glutathione-S-transferase (GST) pull-down assays were used to screen for and identify two ApCtf1β interacting target proteins, BDUbc and BDSKL1, providing a reliable theoretical basis to study the molecular mechanism underlying B. pervariabilis × D. grandis resistance in response to A. phaeospermum, which would, in turn, establish a platform to develop new strategies for the sustainable and effective control of the blight diseases of forest trees.

The Cell-Wall β-d-Glucan in Leaves of Oat (Avena sativa L.) Affected by Fungal Pathogen Blumeria graminis f. sp. avenae

In addition to the structural and storage functions of the (1,3; 1,4)-β-d-glucans (β-d-glucan), the possible protective role of this polymer under biotic stresses is still debated. The aim of this study was to contribute to this hypothesis by analyzing the β-d-glucans content, expression of related cellulose synthase-like (Csl) Cs1F6, CslF9, CslF3 genes, content of chlorophylls, and β-1,3-glucanase content in oat (Avena sativa L.) leaves infected with the commonly occurring oat fungal pathogen, Blumeria graminis f. sp. avenae (B. graminis). Its presence influenced all measured parameters. The content of β-d-glucans in infected leaves decreased in all used varieties, compared to the non-infected plants, but not significantly. Oats reacted differently, with Aragon and Vaclav responding with overexpression, and Bay Yan 2, Ivory, and Racoon responding with the underexpression of these genes. Pathogens changed the relative ratios regarding the expression of CslF6, CslF9, and CslF3 genes from neutral to negative correlations. However, changes in the expression of these genes did not statistically significantly affect the content of β-d-glucans. A very slight indication of positive correlation, but statistically insignificant, was observed between the contents of β-d-glucans and chlorophylls. Some isoforms of β-1,3-glucanases accumulated to a several-times higher level in the infected leaves of all varieties. New isoforms of β-1,3-glucanases were also detected in infected leaves after fungal infection.

ATP-Binding Cassette (ABC) Transporters in Fusarium Specific Mycoparasite Sphaerodes mycoparasitica during Biotrophic Mycoparasitism

Recent transcriptomic profiling has revealed importance membrane transporters such as ATP-binding cassette (ABC) transporters in fungal necrotrophic mycoparasites. In this study, RNA-Seq allowed rapid detection of ABC transcripts involved in biotrophic mycoparasitism of Sphaerodes mycoparasitica against the phytopathogenic and mycotoxigenic Fusarium graminearum host, the causal agent of Fusarium head blight (FHB). Transcriptomic analyses of highly expressed S. mycoparasitica genes, and their phylogenetic relationships with other eukaryotic fungi, portrayed the ABC transporters’ evolutionary paths towards biotrophic mycoparasitism. Prior to the in silico phylogenetic analyses, transmission electron microscopy (TEM) was used to confirm the formation of appressorium/haustorium infection structures in S. mycoparasitica during early (1.5 d and 3.5 d) stages of mycoparasitism. Transcripts encoding biotrophy-associated secreted proteins did uncover the enrolment of ABC transporter genes in this specific biocontrol mode of action, while tandem ABC and BUB2 (non-ABC) transcripts seemed to be proper for appressorium development. The next-generation HiSeq transcriptomic profiling of the mycoparasitic hypha samples, revealed 81 transcripts annotated to ABC transporters consisting of a variety of ABC-B (14%), ABC-C (22%), and ABC-G (23%), and to ABC-A, ABC-F, aliphatic sulfonates importer (TC 3.A.1.17.2), BtuF, ribose importer (TC 3.A.1.2.1), and unknown families. The most abundant transcripts belonged to the multidrug resistance exporter (TC 3.A.1.201) subfamily of the ABC-B family, the conjugate transporter (TC 3.A.1.208) subfamily of the ABC-C family, and the pleiotropic drug resistance (PDR) (TC 3.A.1.205) subfamily of the ABC-G family. These findings highlight the significance of ABC transporter genes that control cellular detoxification against toxic substances (e.g., chemical pesticides and mycotoxins) in sustaining a virulence of S. mycoparasitica for effective biotrophic mycoparasitism on the F. graminearum host. The findings of this study provide clues to better understand the biotrophic mycoparasitism of S. mycoparasitica interacting with the Fusarium host, which implies that the ABC transporter group of key proteins is involved in the mycoparasite’s virulence and multidrug resistance to toxic substances including cellular detoxification.

Thinopyrum intermedium TiAP1 interacts with a chitin deacetylase from Blumeria graminis f. sp. tritici and increases the resistance to Bgt in wheat

The biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt) is a crucial factor causing reduction in global wheat production. Wild wheat relatives, for example Thinopyrum intermedium, is one of the wild-used parents in wheat disease-resistant breeding. From T. intermedium line, we identified the aspartic protease gene, TiAP1, which is involved in resistance against Bgt. TiAP1 is a secreted protein that accumulates in large amounts at the infection sites of Bgt and extends to the intercellular space. Yeast two-hybrid, luciferase complementation imaging and bimolecular florescent complimentary analysis showed that TiAP1 interacted with the chitin deacetylase (BgtCDA1) of Bgt. The yeast expression, purification and in vitro test confirmed the chitin deacetylase activity of BgtCDA1. The bombardment and VIGS-mediated host-induced gene silencing showed that BgtCDA1 promotes the invasion of Bgt. Transcriptome analysis showed the cell wall xylan metabolism, lignin biosynthesis-related and defence genes involved in the signal transduction were up-regulated in the transgenic TiAP1 wheat induced by Bgt. The TiAP1 in wheat may inactivate the deacetylation function of BgtCDA1, cause chitin oligomers expose to wheat chitin receptor, then trigger the wheat immune response to inhibit the growth and penetration of Bgt, and thereby enhance the resistance of wheat to pathogens.

The Multifaceted Roles of Fungal Cutinases during Infection

Cuticles cover the aerial epidermis cells of terrestrial plants and thus represent the first line of defence against invading pathogens, which must overcome this hydrophobic barrier to colonise the inner cells of the host plant. The cuticle is largely built from the cutin polymer, which consists of C₁₆ and C₁₈ fatty acids attached to a glycerol backbone that are further modified with terminal and mid-chain hydroxyl, epoxy, and carboxy groups, all cross-linked by ester bonds. To breach the cuticle barrier, pathogenic fungal species employ cutinases—extracellular secreted enzymes with the capacity to hydrolyse the ester linkages between cutin monomers. Herein, we explore the multifaceted roles that fungal cutinases play during the major four stages of infection: (i) spore landing and adhesion to the host plant cuticle; (ii) spore germination on the host plant cuticle; (iii) spore germ tube elongation and the formation of penetrating structures; and (iv) penetration of the host plant cuticle and inner tissue colonisation. Using previous evidence from the literature and a comprehensive molecular phylogenetic tree of cutinases, we discuss the notion whether the lifestyle of a given fungal species can predict the activity nature of its cutinases.

War and Peas: Molecular Bases of Resistance to Powdery Mildew in Pea (Pisum sativum L.) and Other Legumes

Grain legumes, or pulses, have many beneficial properties that make them potentially attractive to agriculture. However, the large-scale cultivation of legumes faces a number of difficulties, in particular the vulnerability of the currently available cultivars to various diseases that significantly impair yields and seed quality. One of the most dangerous legume pathogens is powdery mildew (a common name for parasitic fungi of the order Erisyphales). This review examines the methods of controlling powdery mildew that are used in modern practice, including fungicides and biological agents. Special attention is paid to the plant genetic mechanisms of resistance, which are the most durable, universal and environmentally friendly. The most studied legume plant in this regard is the garden pea (Pisum sativum L.), which possesses naturally occurring resistance conferred by mutations in the gene MLO1 (Er1), for which we list here all the known resistant alleles, including er1-12 discovered by the authors of this review. Recent achievements in the genetics of resistance to powdery mildew in other legumes and prospects for the introduction of this resistance into other agriculturally important legume species are also discussed.

A Hyperspectral Library of Foliar Diseases of Wheat

This work established a hyperspectral library of important foliar diseases of wheat induced by different fungal pathogens, representing a time series from infection to symptom appearance for the purpose of detecting spectral changes. The data were generated under controlled conditions at the leaf scale. The transition from healthy to diseased leaf tissue was assessed, and spectral shifts were identified and used in combination with histological investigations to define developmental stages in pathogenesis for each disease. The spectral signatures of each plant disease that indicate a specific developmental stage during pathogenesis, defined as turning points, were combined into a spectral library. Machine learning analysis methods were applied and compared to test the potential of this library to detect and quantify foliar diseases in hyperspectral images. All evaluated classifiers had high accuracy (≤99%) for the detection and identification of both biotrophic and necrotrophic fungi. The potential of applying spectral analysis methods in combination with a spectral library for the detection and identification of plant diseases is demonstrated. Further evaluation and development of these algorithms should contribute to a robust detection and identification system for plant diseases at different developmental stages and the promotion and development of site-specific management techniques for plant diseases under field conditions.

The Plant Cuticle: An Ancient Guardian Barrier Set Against Long-Standing Rivals

The aerial surfaces of plants are covered by a protective barrier formed by the cutin polyester and waxes, collectively referred to as the cuticle. Plant cuticles prevent the loss of water, regulate transpiration, and facilitate the transport of gases and solutes. As the cuticle covers the outermost epidermal cell layer, it also acts as the first line of defense against environmental cues and biotic stresses triggered by a large array of pathogens and pests, such as fungi, bacteria, and insects. Numerous studies highlight the cuticle interface as the site of complex molecular interactions between plants and pathogens. Here, we outline the multidimensional roles of cuticle-derived components, namely, epicuticular waxes and cutin monomers, during plant interactions with pathogenic fungi. We describe how certain wax components affect various pre-penetration and infection processes of fungi with different lifestyles, and then shift our focus to the roles played by the cutin monomers that are released from the cuticle owing to the activity of fungal cutinases during the early stages of infection. We discuss how cutin monomers can activate fungal cutinases and initiate the formation of infection organs, the significant impacts of cuticle defects on the nature of plant-fungal interactions, along with the possible mechanisms raised thus far in the debate on how host plants perceive cutin monomers and/or cuticle defects to elicit defense responses.

Host-specificity factors in plant pathogenic fungi

Fortunately, no fungus can cause disease on all plant species, and although some plant-pathogenic fungi have quite a broad host range, most are highly limited in the range of plant species or even cultivars that they cause disease in. The mechanisms of host specificity have been extensively studied in many plant-pathogenic fungi, especially in fungal pathogens causing disease on economically important crops. Specifically, genes involved in host specificity have been identified during the last few decades. In this overview, we describe and discuss these host-specificity genes. These genes encode avirulence (Avr) proteins, proteinaceous host-specific toxins or secondary metabolites. We discuss the genomic context of these genes, their expression, polymorphism, horizontal transfer and involvement in pathogenesis.

The Regulatory Network of CMPG1-V in Wheat-Blumeria graminis f. sp. tritici Interaction Revealed by Temporal Profiling Using RNA-Seq

Wheat powdery mildew (Pm), caused by Blumeria graminis f. sp. tritici (Bgt), is a prevalent fungal disease. The diploid wheat relative Haynaldia villosa (H. villosa) showed broad-spectrum resistance (BSR) to Pm. A previous study reported an E3 ligase gene, CMPG1-V from H. villosa, showing BSR to Pm. To elucidate the regulatory network mediated by CMPG1-V, in this study, gene expression profiling of CMPG1-V transgenic plant (CMPG1-V_OE) and its receptor Yangmai 158 was analyzed and compared after Bgt inoculation at four infection stages. GO and KEGG analysis revealed obvious reprogramming of SA and ABA signaling, starch/sucrose metabolism, and photosynthesis in CMPG1-V_OE, compared with those in Yangmai 158. Transcripts of SA synthesis genes SARD1 and UGT, signaling factors TGA and PRs, and SnRKs in ABA signaling were specifically upregulated in CMPG1-V_OE rather than Yangmai 158. Transcripts of LHCII in photosynthesis, GLUC and TPP in starch/sucrose metabolism were also induced distinctly in CMPG1-V_OE. WGCNA analysis showed crucial regulatory candidates of CMPG1-V, involving serine/threonine-protein kinase in phosphorylation, glucosyltransferase in flavonoid biosynthesis, defense factor WRKYs, and peroxidase in oxidative stress. Our results facilitate the deciphering of the resistant regulatory network of CMPG1-V and the identification of key candidates which might be employed in breeding programs.

Transcriptome Analysis Identifies Candidate Genes and Functional Pathways Controlling the Response of Two Contrasting Barley Varieties to Powdery Mildew Infection

Powdery mildew caused by Blumeria graminis f. sp. hordei (Bgh) is one of the most serious diseases in barley. The numerous barley varieties across China provide valuable genetic resources to screen the resistant germplasm and to discover the primary genes of resistance to powdery mildew. In this study, Chinese barley variety Feng 7 was identified as a highly resistant genotype which limited Bgh colonization by cell apoptosis using leaf staining assay, while another variety Hua 30 showed high susceptibility. The performance of high resistance to Bgh in F₁ plants from the two varieties suggested dominant gene(s) controlled the resistance to powdery mildew in Feng 7. To understand the host transcriptional response to Bgh infection, these two barley varieties Feng 7 and Hua 30 were inoculated with Bgh, and their transcriptional profiling using RNA sequencing (RNA-seq) at four time points (12 h post-inoculation (hpi), 24 hpi, 48 hpi, and 72 hpi) were compared. 4318 differentially expressed genes (DEGs), including 2244 upregulated and 2074 downregulated genes, were detected in Feng 7, compared with Hua 30 at 12 hpi. 4907 DEGs (2488 upregulated and 2419 downregulated) were detected at 24 hpi. 4758 DEGs (2295 upregulated and 2463 downregulated) were detected at 48 hpi. 3817 DEGs (2036 upregulated and 1781 downregulated) were detected at 72 hpi. The results showed the number of DEGs between two varieties peaked at 24 hpi (for the upregulated) or 48 hpi (for the downregulated), which is matched with the processing of Bgh infection. In addition, the number of upregulated DEGs involved in the functional pathways of plant defense (mitogen-activated protein kinase (MAPK) pathway and plant hormone signal transduction) is elevated remarkably at 24 hpi. Six candidate genes (PR13, glutaredoxin, alcohol dehydrogenase, and cytochrome P450) were identified in Feng 7. All of them present continuous expression at higher levels upon Bgh infection, compared with the performance in Hua 30, which revealed the potential contribution to Feng 7 mediate resistance to Bgh. In conclusion, the candidate genes and relevant pathways provided key information towards understanding the defense of barley to Bgh attack and the molecular mechanisms of different genetic resistance to powdery mildew.

Analysis of cell wall synthesis and metabolism during early germination of Blumeria graminis f. sp. hordei conidial cells induced in vitro

As an obligate biotroph, Blumeria graminis f. sp. hordei (Bgh) cannot be grown in an axenic culture, and instead must be cultivated on its host species, Hordeum vulgare (barley). In this study an in vitro system utilizing n-hexacosanal, a constituent of the barley cuticle and known inducer of Bgh germination, was used to cultivate Bgh and differentiate conidia up to the appressorial germ tube stage for analysis. Transcriptomic and proteomic profiling of the appressorial germ tube stage revealed that there was a significant shift towards energy and protein production during the pre-penetrative phase of development, with an up-regulation of enzymes associated with cellular respiration and protein synthesis, modification and transport. Glycosidic linkage analysis of the cell wall polysaccharides demonstrated that during appressorial development an increase in 1,3- and 1,4-linked glucosyl residues and xylosyl residues was detected along with a significant decrease in galactosyl residues. The use of this in vitro cultivation method demonstrates that it is possible to analyse the pre-penetrative processes of Bgh development in the absence of a plant host.

UV-C irradiation compromises conidial germination, formation of appressoria, and induces transcription of three putative photolyase genes in the barley powdery mildew fungus, Blumeria graminis f. sp. hordei

UV-C irradiation is known to compromise germination of Blumeria graminis conidia and to reduce powdery mildew infestation. However, only scarce information is available on the effects of UV-C irradiation on B. graminis appressorium formation. Applying a Formvar® resin-based in vitro system allowed for analyzing B. graminis germination and appressorium formation in absence of plant defense. UV-C irradiation more strongly affected the differentiation of appressoria than conidial germination. In vivo and in vitro, a single dose of 100 J m^-2 UV-C was sufficient to reduce germination to less than 20 % and decrease appressorium formation to values below 5 %. UV-C irradiation negatively affected pustule size and conidiation. White light-mediated photoreactivation was most effective immediately after UV-C irradiation, indicating that a prolonged phase of darkness after UV-C treatment increases the efficacy of B. graminis control. UV-C irradiation increased transcript levels of three putative B. graminis photolyase genes, while mere white light or blue light irradiation did not contribute to the transcriptional up-regulation. Thus, UV-C irradiation effectively controls B. graminis infestation and proliferation by restricting prepenetration processes. Nevertheless, photoreactivation plays an important role in UV-C-based powdery mildew control in crops and hence has to be considered for planning specific irradiation schedules.

Histological and cytological studies of plant infection by Erysiphe euonymi-japonici

Powdery mildew caused by Erysiphe euonymi-japonici (Eej) is an increasingly serious fungal disease on Euonymus japonicus that is an important ornamental plant. However, little is currently known about infection and pathogenesis of Eej on E. japonicus. Here, we report plant infection by Eej at the histological and cytological levels. Eej caused severe disease symptoms with white and snow-like colonies on leaf surfaces of E. japonicus. Microscopic observations were conducted continuously to define infection process of Eej on E. japonicus. Eej conidia germinated to produce appressorial germ tubes on leaf surfaces and formed irregular haustoria in plant epidermal cells at 6 h post-inoculation (hpi) and 12 hpi, respectively. After uptaking nutrients from host cells by haustoria, Eej formed numerous hyphae and extensive colonization on leaf surfaces at 96 hpi and finally produced abundant conidiophores and new conidia on leaf surfaces at 168 hpi. In addition, there was consistently a single nucleus in different Eej infection structures and haustorial development could be divided into three major stages, including formation of penetration peg, formation of haustorial neck and initial haustorium, and maturation of haustorium. These results provide useful information for further determination of Eej pathogenesis and finally controlling the disease.

Combined Effect of CO2 and Temperature on Wheat Powdery Mildew Development

The effect of simulated climate changes by applying different temperatures and CO2 levels was investigated in the Blumeria graminis f. sp. tritici/wheat pathosystem. Healthy and inoculated plants were exposed in single phytotrons to six CO2+temperature combinations: (1) 450 ppm CO2/18-22°C (ambient CO2 and low temperature), (2) 850 ppm CO2/18-22°C (elevated CO2 and low temperature), (3) 450 ppm CO2/22-26°C (ambient CO2 and medium temperature), (4) 850 ppm CO2/22-26°C (elevated CO2 and medium temperature), (5) 450 ppm CO2/26-30°C (ambient CO2 and high temperature), and (6) 850 ppm CO2/26-30°C (elevated CO2 and high temperature). Powdery mildew disease index, fungal DNA quantity, plant death incidence, plant expression of pathogenesis-related (PR) genes, plant growth parameters, carbohydrate and chlorophyll content were evaluated. Both CO2 and temperature, and their interaction significantly influenced powdery mildew development. The most advantageous conditions for the progress of powdery mildew on wheat were low temperature and ambient CO2. High temperatures inhibited pathogen growth independent of CO2 conditions, and no typical powdery mildew symptoms were observed. Elevated CO2 did not stimulate powdery mildew development, but was detrimental for plant vitality. Similar abundance of three PR transcripts was found, and the level of their expression was different between six phytotron conditions. Real time PCR quantification of Bgt was in line with the disease index results, but this technique succeeded to detect the pathogen also in asymptomatic plants. Overall, future global warming scenarios may limit the development of powdery mildew on wheat in Mediterranean area, unless the pathogen will adapt to higher temperatures.

Transient Overexpression of HvSERK2 Improves Barley Resistance to Powdery Mildew

Somatic embryogenesis receptor-like kinases (SERKs) play an essential role in plant response to pathogen infection. Here we identified three SERK genes (HvSERK1/2/3) from barley, and aimed to determine their implication in defense responses to barley powdery mildew (Bgh). Although HvSERK1/2/3 share the characteristic domains of the SERK family, only HvSERK2 was significantly induced in barley leaves during Bgh infection. The expression of HvSERK2 was rapidly induced by hydrogen peroxide (H₂O₂) treatment, but not by treatment with salicylic acid (SA), methyl jasmonate (MeJA), ethephon (ETH), or abscisic acid (ABA). Bioinformatics analysis of the cloned HvSERK2 promoter revealed that it contains several elements responsible for defense responses against pathogens. Promoter functional analysis showed that the HvSERK2 promoter was induced by Bgh and H₂O₂. Subcellular localization analysis of HvSERK2 indicated that it is mainly located on the plasma membrane. Transient overexpression of HvSERK2 in epidermal cells of the susceptible barley cultivar Hua 30 reduced the Bgh haustorium index from 58.6% to 43.2%. This study suggests that the HvSERK2 gene plays a positive role in the improvement of barley resistance to powdery mildew, and provides new insight into the function of SERK genes in the biotic stress response of plants.

A Verticillium dahliae Extracellular Cutinase Modulates Plant Immune Responses

Cutinases have been implicated as important enzymes during the process of fungal infection of aerial plant organs. The function of cutinases in the disease cycle of fungal pathogens that invade plants through the roots has been less studied. Here, functional analysis of 13 cutinase (carbohydrate esterase family 5 domain-containing) genes (VdCUTs) in the highly virulent vascular wilt pathogen Verticillium dahliae Vd991 was performed. Significant sequence divergence in cutinase family members was observed in the genome of V. dahliae Vd991. Functional analyses demonstrated that only VdCUT11, as purified protein, induced cell death and triggered defense responses in Nicotiana benthamiana, cotton, and tomato plants. Virus-induced gene silencing showed that VdCUT11 induces plant defense responses in Nicotiana benthamania in a BAK1 and SOBIR-dependent manner. Furthermore, coinfiltration assays revealed that the carbohydrate-binding module family 1 protein (VdCBM1) suppressed VdCUT11-induced cell death and other defense responses in N. benthamiana. Targeted deletion of VdCUT11 in V. dahliae significantly compromised virulence on cotton plants. The cutinase VdCUT11 is an important secreted enzyme and virulence factor that elicits plant defense responses in the absence of VdCBM1.

Population structure of the ash dieback pathogen, Hymenoscyphus fraxineus, in relation to its mode of arrival in the UK

The ash dieback fungus, Hymenoscyphus fraxineus, a destructive, alien pathogen of common ash (Fraxinus excelsior), has spread across Europe over the past 25 years and was first observed in the UK in 2012. To investigate the relationship of the pathogen's population structure to its mode of arrival, isolates were obtained from locations in England and Wales, either where established natural populations of ash had been infected by wind-dispersed ascospores or where the fungus had been introduced on imported planting stock. Population structure was determined by tests for vegetative compatibility (VC), mating type and single-nucleotide polymorphisms (SNPs). VC heterogeneity was high at all locations, with 96% of isolate pairings being incompatible. Frequencies of the MAT1-1-1 and MAT1-2-1 idiomorphs were approximately equal, consistent with H. fraxineus being an obligate outbreeder. Most SNP variation occurred within study location and there was little genetic differentiation between the two types of location in the UK, or between pathogen populations in the UK and continental Europe. There was modest differentiation between UK subpopulations, consistent with genetic variation between source populations in continental Europe. However, there was no evidence of strong founder effects, indicating that numerous individuals of H. fraxineus initiated infection at each location, regardless of the route of pathogen transmission. The ssRNA virus HfMV1 was present at moderate to high frequencies in all UK subpopulations. The results imply that management of an introduced plant pathogen requires action against its spread at the continental level involving coordinated efforts by European countries.

Ozone triggers different defence mechanisms against powdery mildew (Blumeria graminis DC. Speer f. sp. tritici) in susceptible and resistant wheat genotypes

Ozone has been proposed as a convenient elicitor against pathogens since it is known to generate different reactive oxygen species (ROS) and induce nonspecific defence by altering gene expression. The mode of action and its interaction with other defence pathways are yet to be elucidated. Besides its negative effects on plants, ozone can be used for triggering defence against environmental stresses, including pathogens, when used at appropriate concentrations. Powdery mildew, caused by the obligate biotrophic fungus Blumera graminis f.sp. tritici (Bgt), is an important plant disease that reduces crop yield and quality. We hypothesised that ozone treatment may elicit defence against Bgt by inducing ROS signalling or other routes such as the salicylic acid (SA) or jasmonic acid (JA) pathways. We conducted experiments with Bgt-susceptible (cv. Pamukova) and resistant (cv. Tahirova) wheat (Triticum aestivum L,) cultivars and treated them with different ozone concentrations before Bgt inoculation. Stress response and defence-related features such as antioxidative enzyme activity; lipid peroxidation; H2O2 and Ca+2 levels; PR1, LOX, PAL and RBOH gene expression; and disease severity were assayed. Clear discrepancies between the responses of susceptible and resistant cultivars were found, suggesting that different defence routes were activated. Here, we showed that ozone treatment was effective for diminishing Bgt invasion in the susceptible cultivar in the short term, which was probably related to defence induced via the SA pathway. Moreover, the resistant cultivar Tahirova exhibited a different mode of action against the pathogen that was triggered by ozone treatment, plausibly related to the JA pathway.

Very-long-chain aldehydes induce appressorium formation in ascospores of the wheat powdery mildew fungus Blumeria graminis

Asexually produced conidia of the wheat powdery mildew fungus Blumeria graminis f. sp. tritici (Bgt) are known to perceive cuticular very-long-chain aldehydes as signal substances strongly stimulating germination and differentiation of infection structures in a concentration- and chain-length-dependent manner. Conidial germination and appressorium formation are widely prevented by the presence of free water on the host surface. However, sexually produced ascospores can differentiate immersed in water. Applying a Formvar^®-based in vitro-system showed that ascospore appressorium formation was strongly induced by the presence of wheat leaf cuticular wax. Similar to conidia, ascospore appressorium formation is triggered by the presence of very-long-chain aldehydes in a chain-length-dependent manner with n-octacosanal as the most inducing aldehyde. Surface hydrophobicity positively affected ascospore germination but not appressorium formation. Ascospores required significantly more time to complete the differentiation of appressoria and exhibited a more distinct dependence on the availability of free water than their conidial counterparts. Unlike conidia, ascospores showed a more variable germination and differentiation pattern even with a single germ tube differentiating an appressorium. Despite these differences our results demonstrate that a host surface recognition principle based on cuticular very-long-chain aldehydes is a common feature of B. graminis f. sp. tritici ascospores and conidia.

Reduction of Growth and Reproduction of the Biotrophic Fungus Blumeria graminis in the Presence of a Necrotrophic Pathogen

Crops are attacked by many potential pathogens with differing life-history traits, which raises the question of whether or not the outcome of infection by one pathogen may be modulated by a change in the host environment brought on by infection by another pathogen. We investigated the host-mediated interaction between the biotroph Blumeria graminis f.sp. tritici (Bgt), the powdery mildew pathogen of wheat, and the necrotroph Zymoseptoria tritici, which has a long latent, endophytic phase following which it switches to a necrotrophic phase, resulting in the disease symptoms of Septoria tritici blotch. Both diseases are potentially severe in humid temperate climates and are controlled by fungicides and by growing wheat varieties with partial resistance. The compatible interaction between Z. tritici and the host reduced the number, size, and reproductive capacity of mildew colonies that a normally virulent Bgt isolate would produce but did not significantly alter the early development of Bgt on the leaf. The effect on virulent Bgt was elicited only by viable spores of Z. tritici. Notably, this effect was seen before the necrotic foliar symptoms induced by Z. tritici were visible, which implies there is a physiological interaction during the latent, endophytic period of Z. tritici, which either takes place directly between this fungus and Bgt or is mediated by the wheat leaf. Information on how different pathogens interact in host plants may allow plant breeders and others to improve the design of screening trials and selection of germplasm.

Genome-wide identification, classification and expression analysis in fungal-plant interactions of cutinase gene family and functional analysis of a putative ClCUT7 in Curvularia lunata

Cutinase is described as playing various roles in fungal-plant pathogen interactions, such as eliciting host-derived signals, fungal spore attachment and carbon acquisition during saprophytic growth. However, the characteristics of the cutinase genes, their expression in compatible interactions and their roles in pathogenesis have not been reported in Curvularia lunata, an important leaf spot pathogen of maize in China. Therefore, a cutinase gene family analysis could have profound significance. In this study, we identified 13 cutinase genes (ClCUT1 to ClCUT13) in the C. lunata genome. Multiple sequence alignment showed that most fungal cutinase proteins had one highly conserved GYSQG motif and a similar DxVCxG[ST]-[LIVMF](3)-x(3)H motif. Gene structure analyses of the cutinases revealed a complex intron-exon pattern with differences in the position and number of introns and exons. Based on phylogenetic relationship analysis, C. lunata cutinases and 78 known cutinase proteins from other fungi were classified into four groups with subgroups, but the C. lunata cutinases clustered in only three of the four groups. Motif analyses showed that each group of cutinases from C. lunata had a common motif. Real-time PCR indicated that transcript levels of the cutinase genes in a compatible interaction between pathogen and host had varied expression patterns. Interestingly, the transcript levels of ClCUT7 gradually increased during early pathogenesis with the most significant up-regulation at 3 h post-inoculation. When ClCUT7 was deleted, pathogenicity of the mutant decreased on unwounded maize (Zea mays) leaves. On wounded maize leaves, however, the mutant caused symptoms similar to the wild-type strain. Moreover, the ClCUT7 mutant had an approximately 10 % reduction in growth rate when cutin was the sole carbon source. In conclusion, we identified and characterized the cutinase family genes of C. lunata, analyzed their expression patterns in a compatible host-pathogen interaction, and explored the role of ClCUT7 in pathogenicity. This work will increase our understanding of cutinase genes in other fungal-plant pathogens.

Are ineffective defence reactions potential target for induced resistance during the compatible wheat-powdery mildew interaction?

Powdery mildew caused by Blumeria graminis f.sp. tritici, an obligate aerial biotrophic fungus, would be one of the most damaging wheat (Triticum aestivum) diseases without the extensive use of conventional fungicides. In our study, the expression levels of some basal defence-related genes were investigated during a compatible interaction in order to evaluate wheat reactions to infection, along with the different stages of the infectious process in planta. As fungal conidia initiated their germination and developed appressorial germ tube (AGT), early defence reactions involved the expression of a lipoxygenase (LOX)- and an oxalate oxidase (OXO)-encoding genes, followed by activations of corresponding LOX (EC 1.13.11.12) and OXO (EC 1.2.3.4) activities, respectively. When penetration of AGT took place, up-regulation of chitinases (CHI) and PR1-encoding genes expression occurred along with an increase of CHI (EC 3.2.1.14) activity. Meanwhile, expression of a phenylalanine ammonia-lyase-encoding gene also took place. Up-regulation of a phospholipase C- and lipid transfer proteins-encoding genes expression occurred during the latest stages of infection. Neither the phi glutathione S-transferase (GST)-encoding gene expression nor the GST (EC 2.5.1.13) activity was modified upon wheat infection by powdery mildew. Whether these defence reactions during such a compatible interaction are markers of immunity or susceptibility, and whether they have the ability to contribute to protection upon modulation of their timing and their intensity by resistance inducers are discussed.

New Insights into the Life Cycle of the Wheat Powdery Mildew: Direct Observation of Ascosporic Infection in Blumeria graminis f. sp. tritici

Although Blumeria graminis is an intensively studied pathogen, an important part of its life cycle (namely, the way ascospores initiate primary infections on cereal leaves) has not yet been explored in detail. This study reports, for the first time, the direct observation of this process in B. graminis f. sp. tritici using light and confocal laser-scanning microscopy. All the germinated ascospores produced a single germ tube type both in vitro and on host plant surfaces; therefore, the ascosporic and conidial germination patterns are markedly different in this fungus, in contrast to other powdery mildews. Germinated ascospores penetrated the epidermal cells of wheat leaves and produced haustoria as known in the case of conidial infections. This work confirmed earlier studies reporting that B. graminis chasmothecia collected from the field do not contain mature ascospores, only asci filled with protoplasm; ascospore development is induced by moist conditions and is a fast process compared with other powdery mildews. Although ascosporic infections are frequent in B. graminis f. sp. tritici in the field, as shown by this study and other works as well, a recent analysis of the genomes of four isolates revealed the signs of clonal or near-clonal reproduction. Therefore, chasmothecia and ascospores are probably more important as oversummering structures than genetic recombination factors in the life cycle of this pathogen.

Formae speciales of cereal powdery mildew: close or distant relatives?

Powdery mildew is an important disease of cereals, affecting both grain yield and end-use quality. The causal agent of powdery mildew on cereals, Blumeria graminis, has been classified into eight formae speciales (ff.spp.), infecting crops and wild grasses. Advances in research on host specificity and resistance, and on pathogen phylogeny and origins, have brought aspects of the subspecific classification system of B. graminis into ff.spp. into question, because it is based on adaptation to certain hosts rather than strict host specialization. Cereals therefore cannot be considered as typical non-hosts to non-adapted ff.spp. We introduce the term 'non-adapted resistance' of cereals to inappropriate ff.spp. of B. graminis, which involves both pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is no clear distinction between the mechanisms of resistance to adapted and non-adapted ff.spp. Molecular evolutionary data suggest that the taxonomic grouping of B. graminis into different ff.spp. is not consistent with the phylogeny of the fungus. Imprecise estimates of mutation rates and the lack of genetic variation in introduced populations may explain the uncertainty with regard to divergence times, in the Miocene or Holocene epochs, of ff.spp. of B. graminis which infect cereal crop species. We propose that most evidence favours divergence in the Holocene, during the course of early agriculture. We also propose that the forma specialis concept should be retained for B. graminis pathogenic on cultivated cereals to include clades of the fungus which are strongly specialized to these hosts, i.e. ff.spp. hordei, secalis and tritici, as well as avenae from cultivated A. sativa, and that the forma specialis concept should no longer be applied to B. graminis from most wild grasses.

The wheat powdery mildew genome shows the unique evolution of an obligate biotroph

Wheat powdery mildew, Blumeria graminis forma specialis tritici, is a devastating fungal pathogen with a poorly understood evolutionary history. Here we report the draft genome sequence of wheat powdery mildew, the resequencing of three additional isolates from different geographic regions and comparative analyses with the barley powdery mildew genome. Our comparative genomic analyses identified 602 candidate effector genes, with many showing evidence of positive selection. We characterize patterns of genetic diversity and suggest that mildew genomes are mosaics of ancient haplogroups that existed before wheat domestication. The patterns of diversity in modern isolates suggest that there was no pronounced loss of genetic diversity upon formation of the new host bread wheat 10,000 years ago. We conclude that the ready adaptation of B. graminis f.sp. tritici to the new host species was based on a diverse haplotype pool that provided great genetic potential for pathogen variation.

Microwounding is a pivotal factor for the induction of actin-dependent penetration resistance against fungal attack

Induced penetration resistance is triggered by failed penetration attempts of nonpathogenic fungi. The resistance mechanism is an important nonhost reaction in plants that can block the invasion of filamentous pathogens such as fungi and oomycetes. However, it remains unclear whether the mechanical stimuli accompanying fungal penetration play a role in induced penetration resistance, whereas the perforation of the cell wall may provide significant stimuli to plant cells. Here, we used microneedles or biolistic bombardment to mimic fungal penetration pegs and a micromanipulation transfer technique of the bio-probe, a germling of Blumeria graminis hordei, to the wounded cells to demonstrate that microwounds derived from fungal penetration attempts may trigger induced penetration resistance in plant cells. When preinoculated with the nonpathogenic fungi Erysiphe pisi and Colletotrichum orbiculare, which were unable to penetrate a barley cell, the penetration of a bio-probe that was transferred by micromanipulation onto the same cell was completely blocked. Fungal penetration was essential to the triggering of induced penetration resistance because a penetration-peg-defective mutant of C. orbiculare completely lacked the ability to trigger resistance. The artificial microwounds significantly, but not completely, blocked the penetration of the bio-probe. Treatment with the actin polymerization inhibitor cytochalasin A or expression of the actin depolymerizing protein HvPro1 caused complete ablation of the induced penetration resistance triggered by either failed fungal penetration or artificial microwounds. These results strongly suggest that microwounding may trigger actin-dependent induced penetration resistance. Manipulation of induced penetration resistance may be a promising target to improve basic disease resistance in plants.

Transgenic Pm3 multilines of wheat show increased powdery mildew resistance in the field

Resistance (R) genes protect plants very effectively from disease, but many of them are rapidly overcome when present in widely grown cultivars. To overcome this lack of durability, strategies that increase host resistance diversity have been proposed. Among them is the use of multilines composed of near-isogenic lines (NILs) containing different disease resistance genes. In contrast to classical R-gene introgression by recurrent backcrossing, a transgenic approach allows the development of lines with identical genetic background, differing only in a single R gene. We have used alleles of the resistance locus Pm3 in wheat, conferring race-specific resistance to wheat powdery mildew (Blumeria graminis f. sp. tritici), to develop transgenic wheat lines overexpressing Pm3a, Pm3c, Pm3d, Pm3f or Pm3g. In field experiments, all tested transgenic lines were significantly more resistant than their respective nontransformed sister lines. The resistance level of the transgenic Pm3 lines was determined mainly by the frequency of virulence to the particular Pm3 allele in the powdery mildew population, Pm3 expression levels and most likely also allele-specific properties. We created six two-way multilines by mixing seeds of the parental line Bobwhite and transgenic Pm3a, Pm3b and Pm3d lines. The Pm3 multilines were more resistant than their components when tested in the field. This demonstrates that the difference in a single R gene is sufficient to cause host-diversity effects and that multilines of transgenic Pm3 wheat lines represent a promising strategy for an effective and sustainable use of Pm3 alleles.

Barley mildew and its elicitor chitosan promote closed stomata by stimulating guard-cell S-type anion channels

Stomatal closure is known to be associated with early defence responses of plant cells triggered by microbe-associated molecular patterns (MAMPs). However, the molecular mechanisms underlying these guard-cell responses have not yet been elucidated. We therefore studied pathogen-induced changes in ion channel activity in Hordeum vulgare guard cells. Barley mildew (Blumeria graminis) hyphae growing on leaves inhibited light-induced stomatal opening, starting at 9 h after inoculation, when appressoria had developed. Alternatively, stomatal closure was induced by nano-infusion of chitosan via open stomata into the sub-stomatal cavity. Experiments using intracellular double-barreled micro-electrodes revealed that mildew stimulated S-type (slow) anion channels in guard cells. These channels enable the efflux of anions from guard cells and also promote K(+) extrusion by altering the plasma membrane potential. Stimulation of S-type anion channels was also provoked by nano-infusion of chitosan. These data suggest that MAMPs of mildew hyphae penetrating the cuticle provoke activation of S-type anion channels in guard cells. In response, guard cells extrude K(+) salts, resulting in stomatal closure. Plasma membrane anion channels probably represent general targets of MAMP signaling in plants, as these elicitors depolarize the plasma membrane of various cell types.

HIGS: host-induced gene silencing in the obligate biotrophic fungal pathogen Blumeria graminis

Powdery mildew fungi are obligate biotrophic pathogens that only grow on living hosts and cause damage in thousands of plant species. Despite their agronomical importance, little direct functional evidence for genes of pathogenicity and virulence is currently available because mutagenesis and transformation protocols are lacking. Here, we show that the accumulation in barley (Hordeum vulgare) and wheat (Triticum aestivum) of double-stranded or antisense RNA targeting fungal transcripts affects the development of the powdery mildew fungus Blumeria graminis. Proof of concept for host-induced gene silencing was obtained by silencing the effector gene Avra10, which resulted in reduced fungal development in the absence, but not in the presence, of the matching resistance gene Mla10. The fungus could be rescued from the silencing of Avra10 by the transient expression of a synthetic gene that was resistant to RNA interference (RNAi) due to silent point mutations. The results suggest traffic of RNA molecules from host plants into B. graminis and may lead to an RNAi-based crop protection strategy against fungal pathogens.

Analysis of a Blumeria graminis-secreted lipase reveals the importance of host epicuticular wax components for fungal adhesion and development

The biotrophic powdery mildew fungus Blumeria graminis releases extracellular materials to the surface of fungal infection structures that facilitate anchoring them to hydrophobic plant surfaces prior to infection; however, the chemistry of fungal adhesives and the mechanism of adhesion remain largely unclear. Expressed sequence tag analysis led to identification of a secreted lipase, Lip1, from B. graminis. Expression of LIP1 is dramatically upregulated during the early stages of fungal development. Lip1, secreted to the surface of fungal cell walls, possesses lipolytic activity against a broad range of glycerides and releases alkanes and primary fatty alcohols from the epicuticular wax of wheat leaves. Of the epicuticular wax components released by Lip1 activity, long-chain alkanes are the most efficient cues for triggering appressorium formation. Pretreatment of wheat leaves with Lip1, thereby removing leaf surface wax, severely compromises components of fungal pathogenicity, including conidial adhesion, appressorium formation, and secondary hypha growth. Our data suggest that Lip1 activity releases cues from the host surface to promote pathogen development and infection.

Fungal effector proteins: past, present and future

The pioneering research of Harold Flor on flax and the flax rust fungus culminated in his gene-for-gene hypothesis. It took nearly 50 years before the first fungal avirulence (Avr) gene in support of his hypothesis was cloned. Initially, fungal Avr genes were identified by reverse genetics and map-based cloning from model organisms, but, currently, the availability of many sequenced fungal genomes allows their cloning from additional fungi by a combination of comparative and functional genomics. It is believed that most Avr genes encode effectors that facilitate virulence by suppressing pathogen-associated molecular pattern-triggered immunity and induce effector-triggered immunity in plants containing cognate resistance proteins. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either on the plasma membrane or inside the plant cell. Indirect recognition of an effector (also known as the guard model) implies that the virulence target of an effector in the host (the guardee) is guarded by the resistance protein (the guard) that senses manipulation of the guardee, leading to activation of effector-triggered immunity. In this article, we review the literature on fungal effectors and some pathogen-associated molecular patterns, including those of some fungi for which no gene-for-gene relationship has been established.

A proteomic analysis of powdery mildew (Blumeria graminis f.sp. hordei) conidiospores

Conidiospores are the asexual propagation units of many plant-pathogenic fungi. In this article, we report an annotated proteome map of ungerminated conidiospores of the ascomycete barley powdery mildew pathogen, Blumeria graminis f.sp. hordei. Using a combination of two-dimensional polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry, we have identified the proteins in 180 spots, which probably represent at least 123 distinct fungal gene products. Most of the identified proteins have a predicted function in carbohydrate, lipid or protein metabolism, indicating that the spore is equipped for the catabolism of storage compounds as well as for protein biosynthesis and folding on germination.

Fungal effector proteins

It is accepted that most fungal avirulence genes encode virulence factors that are called effectors. Most fungal effectors are secreted, cysteine-rich proteins, and a role in virulence has been shown for a few of them, including Avr2 and Avr4 of Cladosporium fulvum, which inhibit plant cysteine proteases and protect chitin in fungal cell walls against plant chitinases, respectively. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either inside the plant cell or on plasma membranes. Several secreted effectors function inside the host cell, but the uptake mechanism is not yet known. Variation observed among fungal effectors shows two types of selection that appear to relate to whether they interact directly or indirectly with their cognate resistance proteins. Direct interactions seem to favor point mutations in effector genes, leading to amino acid substitutions, whereas indirect interactions seem to favor jettison of effector genes.

Host perception and signal transduction studies in wild-type Blumeria graminis f. sp. hordei and a quinoxyfen-resistant mutant implicate quinoxyfen in the inhibition of serine esterase activity

BACKGROUND

Quinoxyfen is a potent and effective fungicide, hitherto considered to control powdery mildew disease by perturbing signal transduction during early germling differentiation. The aim of this paper is to understand the mode of action of quinoxyfen by comparing the perception of host-derived signals and signal relay in a wild-type Blumeria graminis f. sp. hordei EM Marchal (Bgh) (WT/IM82) and a quinoxyfen-resistant field isolate (QR/2B11).

RESULTS

QR/2B11 germinates more promiscuously on host-like and artificial surfaces than the quinoxyfen-sensitive WT/IM82. The pivotal role of host cuticle deprivation in the formation of hooked appressorial germ tubes (hAGTs) in WT/IM82 and a dramatic drop in germling differentiation in the presence of the mildewicide are demonstrated. QR/2B11 strain shows a dependence on host cuticle-like features for hAGT formation but no significant difference between germling differentiation in the presence or absence of quinoxyfen. PKC-inhibitor Ro 318220 induces morphological changes similar to those seen in quinoxyfen-treated germlings. PKC1 transcript accumulation is equivalently upregulated by quinoxyfen in QR/2B11 and WT/IM82 strains, but Bgh cutinase CUT1 transcript is 8 times more abundant in QR/2B11 conidia than in WT/IM82 conidia. Quinoxyfen inhibits serine esterase activity in WT/IM82, but not in QR/2B11.

CONCLUSION

Collectively, these data suggest that quinoxyfen interferes with the perception of host-derived signals required for full germling differentiation, and that QR/2B11 bypasses the need for such signals. Moreover, quinoxyfen appears to target serine esterase activity, with a downstream perturbation in signal transduction; this represents the first demonstrable biochemical difference between the quinoxyfen-resistant and -sensitive isolates.

Pathogen-derived nitric oxide influences formation of the appressorium infection structure in the phytopathogenic fungus Blumeria graminis

Nitric oxide (NO) is an important signal in plant resistance to pathogens. Here we report that NO is also generated by Blumeria graminis f.sp. hordei as a pathogenesis determinant on barley. Infection by B. graminis f.sp. hordei is dependent on appressorium formation in order to penetrate the host. Using fluorescent dye diaminofluorescein-2 diacetate (DAF-2DA) and confocal laser scanning microscopy, transient NO generation was detected within the B. graminis f.sp. hordei appressorium during its maturation. To confirm that NO was indeed being measured, DAF-2DA fluorescence was suppressed using a NO scavenger and a mammalian NO synthase inhibitor. Both chemicals affected the number of appressorial lobes produced by the fungus. These data indicate that NO plays a key role in formation of B. graminis f.sp. hordei appressoria.

Cutinase and hydrophobin interplay: A herald for pathogenesis?

Surface-penetrating phytopathogenic fungi frequently form appressoria. These are specialised infection structures pivotal to fungal ingress into the host. Recently, we demonstrated that one member of a family of cutinases in Magnaporthe grisea is involved in surface sensing, mediating appressorium differentiation and penetration peg formation and hence facilitates host penetration. Cutinase2 serves as an upstream activator of cAMP/PKA and DAG/PKC signalling cascades and is essential for full virulence. Here, we speculate on the role of rice blast hydrophobins as surface interactors facilitating fungal cutinase activity.

The powdery mildews: a review of the world's most familiar (yet poorly known) plant pathogens

The past decade has seen fundamental changes in our understanding of powdery mildews (Erysiphales). Research on molecular phylogeny demonstrated that Erysiphales are Leotiomycetes (inoperculate discomycetes) rather than Pyrenomycetes or Plectomycetes. Life cycles are surprisingly variable, including both sexual and asexual states, or only sexual states, or only asexual states. At least one species produces dematiaceous conidia. Analyses of rDNA sequences indicate that major lineages are more closely correlated with anamorphic features such as conidial ontogeny and morphology than with teleomorph features. Development of molecular clock models is enabling researchers to reconstruct patterns of coevolution and host-jumping, as well as ancient migration patterns. Geographic distributions of some species appear to be increasing rapidly but little is known about species diversity in many large areas, including North America. Powdery mildews may already be responding to climate change, suggesting they may be useful models for studying effects of climate change on plant diseases.

Genetics of avirulence genes in Blumeria graminis f.sp. hordei and physical mapping of AVR(a22) and AVR(a12)

Powdery mildew fungi are parasites that cause disease on a wide range of important crops. Plant resistance (R) genes, which induce host defences against powdery mildews, encode proteins that recognise avirulence (AVR) molecules from the parasite in a gene-for-gene manner. To gain insight into how virulence evolves in Blumeria graminis f.sp. hordei, associations between segregating AVR genes were established. As a prerequisite to the isolation of AVR genes, two loci were selected for further analysis. AVR(a22) is located in a tightly linked cluster comprising AVR(a10) and AVR(k1) as well as up to five other AVR genes. The ratio between physical and genetic distance in the cluster ranged between 0.7 and 35 kB/cM. The AVR(a22) locus was delimited by the previously isolated gene AVR(a10) and two cleaved amplified polymorphic sequence (CAPS) markers, 19H12R and 74E9L. By contrast, AVR(a12) was not linked to other AVR genes in two crosses. Bulk segregant analysis of over 100,000 AFLP fragments yielded two markers, ETAMTG-285 and PAAMACT-473, mapping 10 and 2cM from AVR(a12), respectively, thus delimiting AVR(a12) on one side. All markers obtained for AVR(a12) mapped proximal to it, indicating that the gene is located at the end of a chromosome. Three more AVR(a10) paralogues were identified at the locus interspersed among genes for metabolic enzymes and abundant repetitive elements, especially those homologous to the CgT1 class of retrotransposons. The flanking and close markers obtained will facilitate the isolation of AVR(a22) and AVR(a12) and provide useful tools for studies of the evolution of powdery mildew fungi in agriculture and nature.

Magnaporthe grisea cutinase2 mediates appressorium differentiation and host penetration and is required for full virulence

The rice blast fungus Magnaporthe grisea infects its host by forming a specialized infection structure, the appressorium, on the plant leaf. The enormous turgor pressure generated within the appressorium drives the emerging penetration peg forcefully through the plant cuticle. Hitherto, the involvement of cutinase(s) in this process has remained unproven. We identified a specific M. grisea cutinase, CUT2, whose expression is dramatically upregulated during appressorium maturation and penetration. The cut2 mutant has reduced extracellular cutin-degrading and Ser esterase activity, when grown on cutin as the sole carbon source, compared with the wild-type strain. The cut2 mutant strain is severely less pathogenic than the wild type or complemented cut2/CUT2 strain on rice (Oryza sativa) and barley (Hordeum vulgare). It displays reduced conidiation and anomalous germling morphology, forming multiple elongated germ tubes and aberrant appressoria on inductive surfaces. We show that Cut2 mediates the formation of the penetration peg but does not play a role in spore or appressorium adhesion, or in appressorial turgor generation. Morphological and pathogenicity defects in the cut2 mutant are fully restored with exogenous application of synthetic cutin monomers, cAMP, 3-isobutyl-1-methylxanthine, and diacylglycerol (DAG). We propose that Cut2 is an upstream activator of cAMP/protein kinase A and DAG/protein kinase C signaling pathways that direct appressorium formation and infectious growth in M. grisea. Cut2 is therefore required for surface sensing leading to correct germling differentiation, penetration, and full virulence in this model fungus.

Multiple displacement amplification, a powerful tool for molecular genetic analysis of powdery mildew fungi

Powdery mildew fungi (Erysiphales) are probably the largest group of plant pathogens that remain uncharacterized from genetic and molecular points of view, with the only exception of the powdery mildew of cereals, Blumeria graminis. Their nature as obligate biotrophic parasites and consequent inability to grow on culture media has significantly hampered research. A common bottleneck to the molecular genetic analysis of powdery mildew fungi is the availability of genomic DNA of suitable quality and in sufficient quantity. The so-called whole genome amplification technology has the potential to overcome this limitation. Here we present the application of phi29 DNA polymerase-mediated multiple displacement amplification (MDA) to amplify the whole genome of Podosphaera fusca, the main causal agent of powdery mildew in cucurbits, to address this problem. The genome coverage and fidelity of the MDA process was evaluated by PCR amplification and sequencing of two genetics markers: the nuclear rDNA internal transcribed spacer (ITS) regions and the mitochondrial cytochrome b gene (CYTB). Our results show that MDA is a valuable tool for molecular genetic analysis of powdery mildew fungi that can be used for a number of downstream applications in different fields, such as epidemiology and population genetics or systematics.

Nutrition acquisition strategies during fungal infection of plants

In host-pathogen interactions, efficient pathogen nutrition is a prerequisite for successful colonization and fungal fitness. Filamentous fungi have a remarkable capability to adapt and exploit the external nutrient environment. For phytopathogenic fungi, this asset has developed within the context of host physiology and metabolism. The understanding of nutrient acquisition and pathogen primary metabolism is of great importance in the development of novel disease control strategies. In this review, we discuss the current knowledge on how plant nutrient supplies are utilized by phytopathogenic fungi, and how these activities are controlled. The generation and use of auxotrophic mutants have been elemental to the determination of essential and nonessential nutrient compounds from the plant. Considerable evidence indicates that pathogen entrainment of host metabolism is a widespread phenomenon and can be accomplished by rerouting of the plant's responses. Crucial fungal signalling components for nutrient-sensing pathways as well as their developmental dependency have now been identified, and were shown to operate in a coordinate cross-talk fashion that ensures proper nutrition-related behaviour during the infection process.

Tête à tête inside a plant cell: establishing compatibility between plants and biotrophic fungi and oomycetes

'Compatibility' describes the complementary relationship between a plant species and an adapted pathogen species that underlies susceptibility and which ultimately results in disease. Owing to elaborate surveillance systems and defence mechanisms on the plant side and a common lack of adaptation of many microbial pathogens, resistance is the rule and compatibility the exception for most plant-microbe combinations. While there has been major scientific interest in 'resistance' in the past decade, which has revealed many of its underlying molecular components, the analysis of 'compatibility', although intimately intertwined with 'resistance', has not been pursued with a similar intensity. Various recent studies, however, provide a first glimpse of the pivotal players and potential molecular mechanisms essential for compatibility in both the plant and parasite partners. In this review we highlight these findings with a particular emphasis on obligate biotrophic and hemibiotrophic fungal and oomycete pathogens and discuss novel strategies that might help to uncover further the molecular principles underlying compatibility to these highly specialized pathogens.