Exploring infection of wheat and carbohydrate metabolism in Mycosphaerella graminicola transformants with differentially regulated green fluorescent protein expression

Biology, taxonomy, genetics, and management of Zymoseptoria tritici: the causal agent of wheat leaf blotch

Septoria tritici blotch or Septoria leaf blotch has been used for long time, but leaf blotch is a correct disease name. Moreover, Lb resistant gene is the correct name, but, not Stb gene. It has sexual and asexual parts on the mycelia, known as heterothallic fungi. Its pathogenic diversity ranged from 40% to 93% and has produced a wide variety of AvrLb6 haplotypes. M. graminicola has a plasmogamy and karyogamy sexual process. The pathogen can use macropycnidiospores, micropycnidiospores, and pycnidia vegetative growths for infection and overwintering. Synthetic M3, Kavkaz-K4500, Synthetic 6×, and TE9111 wheat genotypes have horizontal resistance. Avirulence (Avr) genes in Z. tritici and their matching wheat (R) genes indicate gene for gene mechanisms of resistance. Twenty-two R genes (vertical resistance) have been identified. In both horizontal and vertical resistance, different Lb genes have been broken down due to new Z.tritici virulent gene and currently Lb19 resistant gene is being recommended. Mixing of resistant and susceptible cultivars is also the most effective management strategy. Moreover, different cultural practices and biological control have been proposed. Lastly, different fungicides are also available. However, in developing countries cultivar mixture, isolates diversity, biological control, and epidemic studies have been greatly missed.

A Laminarin-Based Formulation Protects Wheat Against Zymoseptoria tritici via Direct Antifungal Activity and Elicitation of Host Defense-Related Genes

The present study aimed to evaluate the potential of the laminarin-based formulation Vacciplant to protect and induce resistance in wheat against Zymoseptoria tritici, a major pathogen on this crop. Under greenhouse conditions, a single foliar spraying of the product 2 days before inoculation with Z. tritici reduced disease severity and pycnidium density by 42 and 45%, respectively. Vacciplant exhibited a direct antifungal activity on Z. tritici conidial germination both in vitro and in planta. Moreover, it reduced in planta substomatal colonization as well as pycnidium formation on treated leaves. Molecular investigations revealed that Vacciplant elicits but did not prime the expression of several wheat genes related to defense pathways, including phenylpropanoids (phenylalanine ammonia-lyase and chalcone synthase), octadecanoids (lipoxygenase and allene oxide synthase), and pathogenesis-related proteins (β-1,3-endoglucanase and chitinase). By contrast, it did not modulate the expression of oxalate oxidase gene involved in the reactive oxygen species metabolism. Ultrahigh-performance liquid chromatography-mass spectrometry analysis indicated limited changes in leaf metabolome after product application in both noninoculated and inoculated conditions, suggesting a low metabolic cost associated with induction of plant resistance. This study provides evidence that the laminarin-based formulation confers protection to wheat against Z. tritici through direct antifungal activity and elicitation of plant defense-associated genes.

Blocked at the Stomatal Gate, a Key Step of Wheat Stb16q-Mediated Resistance to Zymoseptoria tritici

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is among the most threatening wheat diseases in Europe. Genetic resistance remains one of the main environmentally sustainable strategies to efficiently control STB. However, the molecular and physiological mechanisms underlying resistance are still unknown, limiting the implementation of knowledge-driven management strategies. Among the 22 known major resistance genes (Stb), the recently cloned Stb16q gene encodes a cysteine-rich receptor-like kinase conferring a full broad-spectrum resistance against Z. tritici. Here, we showed that an avirulent Z. tritici inoculated on Stb16q quasi near isogenic lines (NILs) either by infiltration into leaf tissues or by brush inoculation of wounded tissues partially bypasses Stb16q-mediated resistance. To understand this bypass, we monitored the infection of GFP-labeled avirulent and virulent isolates on Stb16q NILs, from germination to pycnidia formation. This quantitative cytological analysis revealed that 95% of the penetration attempts were unsuccessful in the Stb16q incompatible interaction, while almost all succeeded in compatible interactions. Infectious hyphae resulting from the few successful penetration events in the Stb16q incompatible interaction were arrested in the sub-stomatal cavity of the primary-infected stomata. These results indicate that Stb16q-mediated resistance mainly blocks the avirulent isolate during its stomatal penetration into wheat tissue. Analyses of stomatal aperture of the Stb16q NILs during infection revealed that Stb16q triggers a temporary stomatal closure in response to an avirulent isolate. Finally, we showed that infiltrating avirulent isolates into leaves of the Stb6 and Stb9 NILs also partially bypasses resistances, suggesting that arrest during stomatal penetration might be a common major mechanism for Stb-mediated resistances.

Root microbiome assembly of As-hyperaccumulator Pteris vittata and its efficacy in arsenic requisition

The assemblage of root-associated microorganisms plays important roles in improving their capability to adapt to environmental stress. Metal(loid) hyperaccumulators exhibit disparate adaptive capability compared to that of non-hyperaccumulators when faced with elevated contents of metal(loid)s. However, knowledge of the assemblage of root microbes of hyperaccumulators and their ecological roles in plant growth is still scarce. The present study used Pteris vittata as a model plant to study the microbial assemblage and its beneficial role in plant growth. We demonstrated that the assemblage of microbes from the associated bulk soil to the root compartment was based on their lifestyles. We used metagenomic analysis and identified that the assembled microbes were primarily involved in root-microbe interactions in P. vittata root. Notably, we identified that the assembled root microbiome played an important role in As requisition, which promoted the fitness and growth of P. vittata. This study provides new insights into the root microbiome and potential valuable knowledge to understand how the root microbiome contributes to the fitness of its host.

Conditional promoters to investigate gene function during wheat infection by Zymoseptoria tritici

The fungus Zymoseptoria tritici causes Septoria tritici leaf blotch, which poses a serious threat to temperate-grown wheat. Recently, we described a raft of molecular tools to study the biology of this fungus in vitro. Amongst these are 5 conditional promoters (Pnar1, Pex1A, Picl1, Pgal7, PlaraB), which allow controlled over-expression or repression of target genes in cells grown in liquid culture. However, their use in the host-pathogen interaction in planta was not tested. Here, we investigate the behaviour of these promoters by quantitative live cell imaging of green-fluorescent protein-expressing cells during 6 stages of the plant infection process. We show that Pnar1 and Picl1 are repressed in planta and demonstrate their suitability for studying essential gene expression and function in plant colonisation. The promoters Pgal7 and Pex1A are not fully-repressed in planta, but are induced during pycnidiation. This indicates the presence of inducing galactose or xylose and/or arabinose, released from the plant cell wall by the activity of fungal hydrolases. In contrast, the PlaraB promoter, which normally controls expression of an α-l-arabinofuranosidase B, is strongly induced inside the leaf. This suggests that the fungus is exposed to L-arabinose in the mesophyll apoplast. Taken together, this study establishes 2 repressible promoters (Pnar1 and Picl1) and three inducible promoters (Pgal7, Pex1A, PlaraB) for molecular studies in planta. Moreover, we provide circumstantial evidence for plant cell wall degradation during the biotrophic phase of Z. tritici infection.

Quantification of In Planta Zymoseptoria tritici Progression Through Different Infection Phases and Related Association with Components of Aggressiveness

In planta growth of Zymoseptoria tritici, causal agent of Septoria tritici blotch of wheat, during the infection process has remained an understudied topic due to the long symptomless latent period before the emergence of fruiting bodies. In this study, we attempted to understand the relationship between in planta growth of Z. tritici relative to the primary components of aggressiveness, i.e., latent period and pycnidia coverage in regard to contrasting host resistance. We tested isolates collected from Ireland against the susceptible cultivar Gallant and cultivar Stigg, which has strong partial resistance. A clear isolate-host interaction effect (F = 3.018; P = 0.005, and F = 6.008; P < 0.001) for latent period and pycnidia coverage, respectively, was identified. Furthermore, during the early infection phase of latency from 5 to 11 days postinoculation (dpi), in planta growth rate of fungal biomass was significantly (F = 30.06; P < 0.001) more affected by host resistance than isolate specificity (F = 1.27; P = 0.27), indicating the importance of host resistance in the early infection phase. In planta Z. tritici growth rates in cultivar Gallant spiked between 11 and 16 dpi followed by a continuous fall onward, whereas in cultivar Stigg it was slowly progressive in nature. From correlation and regression analysis, we found that the in planta growth rate preceding the average latent period of cultivar Gallant has more influence on latency duration and pycnidia production. Likewise, correlation between component of aggressiveness and in planta growth rate of pathogen supports our understanding of aggressiveness to be driven by the pathogen's multiplication capacity within host tissue.

Does the Latent Period of Leaf Fungal Pathogens Reflect Their Trophic Type? A Meta-Analysis of Biotrophs, Hemibiotrophs, and Necrotrophs

We performed a meta-analysis to search for a relation between the trophic type and latent period of fungal pathogens. The pathogen incubation period and the level of resistance of the hosts were also investigated. This ecological knowledge would help us to more efficiently regulate crop epidemics for different types of pathogens. We gathered latent period data from 103 studies dealing with 51 fungal pathogens of the three major trophic types (25 biotrophs, 15 hemibiotrophs, and 11 necrotrophs), representing 2,542 mean latent periods. We show that these three trophic types display significantly different latent periods. Necrotrophs exhibited the shortest latent periods (<100 degree-days [DD]), biotrophs had intermediate ones (between 100 and 200 DD), and hemibiotrophs had the longest latent periods (>200 DD). We argue that this relation between trophic type and latent period points to two opposing host exploitation strategies: necrotrophs mount a rapid destructive attack on the host tissue, whereas biotrophs and hemibiotrophs avoid or delay the damaging phase. We query the definition of hemibiotrophic pathogens and discuss whether the length of the latent period is determined by the physiological limits inherent to each trophic type or by the adaptation of pathogens of different trophic types to the contrasting conditions experienced in their interaction with the host.

sRNA Profiling Combined With Gene Function Analysis Reveals a Lack of Evidence for Cross-Kingdom RNAi in the Wheat - Zymoseptoria tritici Pathosystem

Cross-kingdom small RNA (sRNA) silencing has recently emerged as a mechanism facilitating fungal colonization and disease development. Here we characterized RNAi pathways in Zymoseptoria tritici, a major fungal pathogen of wheat, and assessed their contribution to pathogenesis. Computational analysis of fungal sRNA and host mRNA sequencing datasets was used to define the global sRNA populations in Z. tritici and predict their mRNA targets in wheat. 389 in planta-induced sRNA loci were identified. sRNAs generated from some of these loci were predicted to target wheat mRNAs including those potentially involved in pathogen defense. However, molecular approaches failed to validate targeting of selected wheat mRNAs by fungal sRNAs. Mutant strains of Z. tritici carrying deletions of genes encoding key components of RNAi such as Dicer-like (DCL) and Argonaute (AGO) proteins were generated, and virulence bioassays suggested that these are dispensable for full infection of wheat. Nonetheless, our results did suggest the existence of non-canonical DCL-independent pathway(s) for sRNA biogenesis in Z. tritici. dsRNA targeting essential fungal genes applied in vitro or generated from an RNA virus vector in planta in a procedure known as HIGS (Host-Induced Gene Silencing) was ineffective in preventing Z. tritici growth or disease. We also demonstrated that Z. tritici is incapable of dsRNA uptake. Collectively, our data suggest that RNAi approaches for gene function analyses in this fungal species and potentially also as a control measure may not be as effective as has been demonstrated for some other plant pathogenic fungi.

A Review of the Interactions between Wheat and Wheat Pathogens: Zymoseptoria tritici, Fusarium spp. and Parastagonospora nodorum

Zymoseptoria tritici is a hemibiotrophic pathogen which causes Septoria leaf blotch in wheat. The pathogenesis of the disease consists of a biotrophic phase and a necrotrophic phase. The pathogen infects the host plant by suppressing its immune response in the first stage of infection. Hemibiotrophic pathogens of the genus Fusarium cause Fusarium head blight, and the necrotrophic Parastagonosporanodorum is responsible for Septoria nodorum blotch in wheat. Cell wall-degrading enzymes in plants promote infections by necrotrophic and hemibiotrophic pathogens, and trichothecenes, secondary fungal metabolites, facilitate infections caused by fungi of the genus Fusarium. There are no sources of complete resistance to the above pathogens in wheat. Defense mechanisms in wheat are controlled by many genes encoding resistance traits. In the wheat genome, the characteristic features of loci responsible for resistance to pathogenic infections indicate that at least several dozen genes encode resistance to pathogens. The molecular interactions between wheat and Z. tritici, P. nodorum and Fusarium spp. pathogens have been insufficiently investigated. Most studies focus on the mechanisms by which the hemibiotrophic Z. tritici suppresses immune responses in plants and the role of mycotoxins and effector proteins in infections caused by P. nodorum and Fusarium spp. fungi. Trichothecene glycosylation and effector proteins, which are involved in defense responses in wheat, have been described at the molecular level. Recent advances in molecular biology have produced interesting findings which should be further elucidated in studies of molecular interactions between wheat and fungal pathogens. The Clustered Regularly-Interspaced Short Palindromic Repeats/ CRISPR associated (CRISPR/Cas) system can be used to introduce targeted mutations into the wheat genome and confer resistance to selected fungal diseases. Host-induced gene silencing and spray-induced gene silencing are also useful tools for analyzing wheat-pathogens interactions which can be used to develop new strategies for controlling fungal diseases.

Apoplastic recognition of multiple candidate effectors from the wheat pathogen Zymoseptoria tritici in the nonhost plant Nicotiana benthamiana

The fungus Zymoseptoria tritici is a strictly apoplastic, host-specific pathogen of wheat leaves and causal agent of septoria tritici blotch (STB) disease. All other plants are considered nonhosts, but the mechanism of nonhost resistance (NHR) to Z. tritici has not been addressed previously. We sought to develop Nicotiana benthamiana as a system to study NHR against Z. tritici. Fluorescence microscopy and quantitative reverse transcription polymerase chain reactions were used to establish the interaction between Z. tritici and N. benthamiana. Agrobacterium-mediated transient expression was used to screen putative Z. tritici effector genes for recognition in N. benthamiana, and virus-induced gene silencing (VIGS) was employed to determine the role of two receptor-like kinases (RLKs), NbBAK1 and NbSOBIR1, in Z. tritici effector recognition. Numerous Z. tritici putative effectors (14 of 63 tested) induced cell death or chlorosis in N. benthamiana. For most, phenotypes were light-dependent and required effector secretion to the leaf apoplastic space. Moreover, effector-induced host cell death was dependent on NbBAK1 and NbSOBIR1. Our results indicate widespread recognition of apoplastic effectors from a wheat-infecting fungal pathogen in a taxonomically distant nonhost plant species presumably by cell surface immune receptors. This suggests that apoplastic recognition of multiple nonadapted pathogen effectors may contribute to NHR.

A Dual-Color Imaging System for Sugarcane Smut Fungus Sporisorium scitamineum

The life cycle of the sugarcane smut fungus Sporisorium scitamineum is a multistep process. Haploid sporidia of compatible (MAT-1 versus MAT-2) mating types fuse to generate pathogenic dikaryotic hyphae to infect the host. Within the host tissues, diploid teliospores are formed and induce a characteristic sorus that looks like a black whip. The diploid teliospores germinate to form haploid sporidia by meiosis. In order to monitor fungal development throughout the whole life cycle, we expressed the green fluorescent protein (GFP) and red fluorescent protein (RFP) in S. scitamineum MAT-1 and MAT-2 sporidia, respectively. Observation by epifluorescence microscope showed that conjugation tube formation and sporidia fusion occurred at 4 to 8 h, and formation of dikaryotic filaments was detected at 12 h after mating. The resultant teliospores, with diffused GFP and RFP, underwent meiosis as demonstrated by septated hypha with single fluorescent signal. We demonstrated that GFP- and RFP-tagged strains can be used to study the life cycle development of the fungal pathogen S. scitamineum, including the sexual mating and meiosis events. This dual-color imaging system would be a valuable tool for investigation of biotic and abiotic factors that might affect the fungal life cycle development and pathogenesis.

A codon-optimized green fluorescent protein for live cell imaging in Zymoseptoria tritici

Fluorescent proteins (FPs) are powerful tools to investigate intracellular dynamics and protein localization. Cytoplasmic expression of FPs in fungal pathogens allows greater insight into invasion strategies and the host-pathogen interaction. Detection of their fluorescent signal depends on the right combination of microscopic setup and signal brightness. Slow rates of photo-bleaching are pivotal for in vivo observation of FPs over longer periods of time. Here, we test green-fluorescent proteins, including Aequorea coerulescens GFP (AcGFP), enhanced GFP (eGFP) from Aequorea victoria and a novel Zymoseptoria tritici codon-optimized eGFP (ZtGFP), for their usage in conventional and laser-enhanced epi-fluorescence, and confocal laser-scanning microscopy. We show that eGFP, expressed cytoplasmically in Z. tritici, is significantly brighter and more photo-stable than AcGFP. The codon-optimized ZtGFP performed even better than eGFP, showing significantly slower bleaching and a 20-30% further increase in signal intensity. Heterologous expression of all GFP variants did not affect pathogenicity of Z. tritici. Our data establish ZtGFP as the GFP of choice to investigate intracellular protein dynamics in Z. tritici, but also infection stages of this wheat pathogen inside host tissue.

Yeast recombination-based cloning as an efficient way of constructing vectors for Zymoseptoria tritici

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Yeast recombination-based cloning (YRBC) is a reliable and inexpensive way of generating plasmids.

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We provide 4 vectors for YRBC that a cover different resistance genes.

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Using this technique promises rapid generation of molecular tools to study Z. tritici.

Many pathogenic fungi are genetically tractable. Analysis of their cellular organization and invasion mechanisms underpinning virulence determinants profits from exploiting such molecular tools as fluorescent fusion proteins or conditional mutant protein alleles. Generation of these tools requires efficient cloning methods, as vector construction is often a rate-limiting step. Here, we introduce an efficient yeast recombination-based cloning (YRBC) method to construct vectors for the fungus Zymoseptoria tritici. This method is of low cost and avoids dependency on the availability of restriction enzyme sites in the DNA sequence, as needed in more conventional restriction/ligation-based cloning procedures. Furthermore, YRBC avoids modification of the DNA of interest, indeed this potential risk limits the use of site-specific recombination systems, such as Gateway cloning. Instead, in YRBC, multiple DNA fragments, with 30 bp overlap sequences, are transformed into Saccharomyces cerevisiae, whereupon homologous recombination generates the vector in a single step. Here, we provide a detailed experimental protocol and four vectors, each encoding a different dominant selectable marker cassette, that enable YRBC of constructs to be used in the wheat pathogen Z. tritici.

Conditional promoters for analysis of essential genes in Zymoseptoria tritici

Development of new fungicides, needed for sustainable control of fungal plant pathogens, requires identification of novel anti-fungal targets. Essential fungal-specific proteins are good candidates, but due to their importance, gene deletion mutants are not viable. Consequently, their cellular role often remains elusive. This hindrance can be overcome by the use of conditional mutants, where expression is controlled by an inducible/repressible promoter. Here, we introduce 5 inducible/repressible promoter systems to study essential genes in the wheat pathogen Zymoseptoria tritici. We fused the gene for enhanced green-fluorescent protein (egfp) to the promoter region of Z. tritici nitrate reductase (Pnar1; induced by nitrogen and repressed by ammonium), 1,4-β-endoxylanase A (Pex1A; induced by xylose and repressed by maltodextrin), l-arabinofuranosidase B (PlaraB; induced by arabinose and repressed by glucose), galactose-1-phosphate uridylyltransferase 7 (Pgal7; induced by galactose and repressed by glucose) and isocitrate lyase (Picl1; induced by sodium acetate and repressed by glucose). This was followed by quantitative analysis of cytoplasmic reporter fluorescence under induced and repressed conditions. We show that Pnar1, PlaraB and Pex1A drive very little or no egfp expression when repressed, but induce moderate protein production when induced. In contrast, Pgal7 and Picl1 show considerable egfp expression when repressed, and were strongly induced in the presence of their inducers. Normalising the expression levels of all promoters to that of the α-tubulin promoter Ptub2 revealed that PlaraB was the weakest promoter (∼20% of Ptub2), whereas Picl1 strongly expressed the reporter (∼250% of Ptub2). The use of these tools promises a better understanding of essential genes, which will help developing novel control strategies that protect wheat from Z. tritici.

The severity of wheat diseases increases when plants and pathogens are acclimatized to elevated carbon dioxide

Wheat diseases present a constant and evolving threat to food security. We have little understanding as to how increased atmospheric carbon dioxide levels will affect wheat diseases and thus the security of grain supply. Atmospheric CO₂ exceeded the 400 ppmv benchmark in 2013 and is predicted to double or even treble by the end of the century. This study investigated the impact of both pathogen and wheat acclimation to elevated CO₂ on the development of Fusarium head blight (FHB) and Septoria tritici blotch (STB) disease of wheat. Here, plants and pathogens were cultivated under either 390 or 780 ppmv CO₂ for a period (two wheat generations, multiple pathogen subcultures) prior to standard disease trials. Acclimation of pathogens and the wheat cultivar Remus to elevated CO₂ increased the severity of both STB and FHB diseases, relative to ambient conditions. The effect of CO₂ on disease development was greater for FHB than for STB. The highest FHB disease levels and associated yield losses were recorded for elevated CO₂ -acclimated pathogen on elevated CO₂ -acclimated wheat. When similar FHB experiments were conducted using the disease-resistant cultivar CM82036, pathogen acclimation significantly enhanced disease levels and yield loss under elevated CO₂ conditions, thereby indicating a reduction in the effectiveness of the defence pathways innate to this wheat cultivar. We conclude that acclimation to elevated CO₂ over the coming decades will have a significant influence on the outcome of plant-pathogen interactions and the durability of disease resistance.

Exploring the utility of Brachypodium distachyon as a model pathosystem for the wheat pathogen Zymoseptoria tritici

BACKGROUND

Zymoseptoria tritici, the causative organism of Septoria tritici blotch disease is a prevalent biotic stressor of wheat production, exerting substantial economic constraints on farmers, requiring intensive chemical control to protect yields. A hemibiotrophic pathogen with a long asymptomless phase of up to 11 days post inoculation (dpi) before a rapid switch to necrotrophy; a deficit exists in our understanding of the events occurring within the host during the two phases of infection. Brachypodium distachyon has demonstrated its potential as a model species for the investigation of fungal disease resistance in cereal and grass species. The aim of this study was to assess the physical interaction between Z. tritici (strain IPO323) and B. distachyon and examine its potential as a model pathosystem for Z. tritici.

RESULTS

Septoria tritici blotch symptoms developed on the wheat cultivar Riband from 12 dpi with pycnidial formation abundant by 20 dpi. Symptoms on B. distachyon ecotype Bd21-1 were visible from 1 dpi: characteristic pale, water soaked lesions which developed into blotch-like lesions by 4 dpi. These lesions then became necrotic with chlorotic regions expanding up to 7 dpi. Sporulation on B. distachyon tissues was not observed and no evidence of fungal penetration could be obtained, indicating that Z. tritici was unable to complete its life cycle within B. distachyon ecotypes. However, observation of host responses to the Z. tritici strain IPO323 in five B. distachyon ecotypes revealed a variation in resistance responses, ranging from immunity to a chlorotic/necrotic phenotype.

CONCLUSIONS

The observed interactions suggest that B. distachyon is an incompatible host for Z. tritici infection, with STB symptom development on B. distachyon comparable to that observed during the early infection stages on the natural host, wheat. However first visible symptoms occurred more rapidly on B. distachyon; from 1 dpi in comparison to 12 dpi in wheat. Consequently, we propose that the interaction between B. distachyon and Z. tritici as observed in this study could serve as a suitable model pathosystem with which to investigate mechanisms underpinning an incompatible host response to Z. tritici.

The wheat-Septoria conflict: a new front opening up?

In the utopic absence of abiotic and/or biotic stressors, attaining the predicted increase (up to 70%) in wheat demand by 2050 in response to global population trends is a challenge. This objective becomes daunting, however, when one factors in the continuous constraint on global wheat production posed by Septoria tritici blotch (STB) disease. This is because, despite resistant loci being identified, a deficit of commercially relevant STB-resistant wheat germplasm remains. The issue is further compounded for growers by the emergence and prevalence of fungicide-resistant/insensitive strains of the causative pathogen Zymoseptoria tritici (formerly known as Mycosphaerella graminicola/Septoria tritici). However, biotechnology-based research is providing new opportunities in this struggle. As the exome response of wheat to STB attack begins to be deciphered, genes intrinsic to resistant and susceptible phenotypes will be identified. Combined with the application of genome-editing techniques and a growing appreciation of the complexity of wheat's and the dynamism of Z. tritici's genome, the generation of resulting STB-resistant wheat varieties will counter the prevalent threat of STB disease in wheat-production systems.

Real-time PCR to study the effect of timing and persistence of fungicide application and wheat varietal resistance on Mycosphaerella graminicola and its sterol 14α-demethylation-inhibitor-resistant genotypes

BACKGROUND

Sterol 14α-demethylase inhibitors (DMIs) have been widely used for more than 20 years against wheat Septoria leaf blotch. However, resistance towards DMIs has increased in recent years. The objective of this study was to evaluate the effect of fungicide timing and persistence and wheat resistance varietal on Mycosphaerella graminicola and its DMI-resistant genotypes.

RESULTS

Using qPCR, M. graminicola was detected 2 weeks later in the resistant cultivar than in the susceptible cultivar. A high proportion of DMI-moderate resistant genotypes (≥94%) was found in all samples, with an average of 74.2, 0.1 and 19.7% for R6, R7- and R7+ genotypes, respectively. Distribution of DMI-resistant genotypes was neither affected by different wheat cultivars nor by analysis dates. Electron microscopy coupled with qPCR analysis showed that the DMI fungicide prothioconazole had a significant inhibitive effect against spore germination and post-germination. However, the preventive treatment was the most effective, but it was affected strongly by fungicide persistence.

CONCLUSION

Preventive fungicide applications are more effective against Septoria leaf blotch than the curative treatments, so persistence and wheat varietal resistance should be taken into account in the management of this disease. It would seem that none of the studied factors affect the frequency of DMI-resistant genotypes.

Transcriptional reprogramming of wheat and the hemibiotrophic pathogen Septoria tritici during two phases of the compatible interaction

The disease septoria leaf blotch of wheat, caused by fungal pathogen Septoria tritici, is of worldwide concern. The fungus exhibits a hemibiotrophic lifestyle, with a long symptomless, biotrophic phase followed by a sudden transition to necrotrophy associated with host necrosis. Little is known about the systematic interaction between fungal pathogenicity and host responses at specific growth stages and the factors triggering the transition. In order to gain some insights into global transcriptome alterations in both host and pathogen during the two phases of the compatible interaction, disease transition was monitored using pathogenesis-related gene markers and H₂O₂ signature prior to RNA-Seq. Transcriptome analysis revealed that the slow symptomless growth was accompanied by minor metabolic responses and slightly suppressed defences in the host, whereas necrotrophic growth was associated with enhanced host responses involving energy metabolism, transport, signalling, defence and oxidative stress as well as a decrease in photosynthesis. The fungus expresses distinct classes of stage-specific genes encoding potential effectors, probably first suppressing plant defence responses/facilitating the symptomless growth and later triggering life style transition and inducing host necrosis/facilitating the necrotrophic growth. Transport, signalling, anti-oxidative stress mechanisms and apoplastic nutrient acquisition play important roles in the entire infection process of S. tritici. Our findings uncover systematic S. tritici-induced expression profiles of wheat related to specific fungal infection strategies and provide a transcriptome resource for studying both hosts and pathogens in plant-Dothideomycete interactions.

Battle through signaling between wheat and the fungal pathogen Septoria tritici revealed by proteomics and phosphoproteomics

The fungus Septoria tritici causes the disease septoria tritici blotch in wheat, one of the most economically devastating foliar diseases in this crop. To investigate signaling events and defense responses in the wheat-S. tritici interaction, we performed a time-course study of S. tritici infection in resistant and susceptible wheat using quantitative proteomics and phosphoproteomics, with special emphasis on the initial biotrophic phase of interactions. Our study revealed an accumulation of defense and stress-related proteins, suppression of photosynthesis, and changes in sugar metabolism during compatible and incompatible interactions. However, differential regulation of the phosphorylation status of signaling proteins, transcription and translation regulators, and membrane-associated proteins was observed between two interactions. The proteomic data were correlated with a more rapid or stronger accumulation of signal molecules, including calcium, H2O2, NO, and sugars, in the resistant than in the susceptible cultivar in response to the infection. Additionally, 31 proteins and 5 phosphoproteins from the pathogen were identified, including metabolic proteins and signaling proteins such as GTP-binding proteins, 14-3-3 proteins, and calcium-binding proteins. Quantitative PCR analysis showed the expression of fungal signaling genes and genes encoding a superoxide dismutase and cell-wall degrading enzymes. These results indicate roles of signaling, antioxidative stress mechanisms, and nutrient acquisition in facilitating the initial symptomless growth. Taken in its entirety, our dataset suggests interplay between the plant and S. tritici through complex signaling networks and downstream molecular events. Resistance is likely related to several rapidly and intensively triggered signal transduction cascades resulting in a multiple-level activation of transcription and translation processes of defense responses. Our sensitive approaches and model provide a comprehensive (phospho)proteomics resource for studying signaling from the point of view of both host and pathogen during a plant-pathogen interaction.

Correlation of cytological and biochemical parameters with resistance and tolerance to Mycosphaerella graminicola in wheat

This study investigated the infection process of Mycosphaerella graminicola and enzyme activities related to reactive oxygen species (ROS) or oxylipin biosynthesis in four French wheat cultivars with variable resistance to M. graminicola infection. At field level, cultivars Caphorn, Maxyl and Gen11 were susceptible, whereas Capnor showed high levels of quantitative resistance. Moreover, Capnor and Gen11 were tolerant, i.e., their yield was less affected by infection compared to non-tolerant Maxyl and Caphorn. These four cultivars were inoculated under laboratory conditions with the M. graminicola IPO323 reference strain. Cytological and biochemical responses were studied on collected first plantlet leaves and several features discriminated between cultivars. However, resistance and tolerance had no impact on the fungal infection process. Levels of lipoxygenase (LOX), peroxidase (PO) and glutathione-S-transferase (GST) activities were also compared with regard to cultivar resistance or tolerance to M. graminicola. LOX, PO and GST activities did not discriminate resistance and tolerance profiles, although a low level of PO in inoculated and non-inoculated plants could be associated with tolerance. In addition, cell necrosis correlated positively with LOX in non-tolerant cultivars, while mycelia surrounding stomata were positively correlated with PO in the resistant cultivar. GST activity presented correlations between cytological and biochemical parameters only for susceptible cultivars. Stomatal and direct penetration were positively correlated with GST activity in the susceptible non-tolerant cultivars, while these correlations were negative in the tolerant cultivar. When combining cytological and biochemical observations with resistance and tolerance profiles, for each cultivar and at each time point, cultivars could be classified in tight accordance with their previous field characterisation. Moreover, tolerance allowed us to distinguish susceptible cultivars when both biochemical and cytological parameters were considered together.

Living colors in the gray mold pathogen Botrytis cinerea: codon-optimized genes encoding green fluorescent protein and mCherry, which exhibit bright fluorescence

The green fluorescent protein (GFP) and its variants have been widely used in modern biology as reporters that allow a variety of live-cell imaging techniques. So far, GFP has rarely been used in the gray mold fungus Botrytis cinerea because of low fluorescence intensity. The codon usage of B. cinerea genes strongly deviates from that of commonly used GFP-encoding genes and reveals a lower GC content than other fungi. In this study, we report the development and use of a codon-optimized version of the B. cinerea enhanced GFP (eGFP)-encoding gene (Bcgfp) for improved expression in B. cinerea. Both the codon optimization and, to a smaller extent, the insertion of an intron resulted in higher mRNA levels and increased fluorescence. Bcgfp was used for localization of nuclei in germinating spores and for visualizing host penetration. We further demonstrate the use of promoter-Bcgfp fusions for quantitative evaluation of various toxic compounds as inducers of the atrB gene encoding an ABC-type drug efflux transporter of B. cinerea. In addition, a codon-optimized mCherry-encoding gene was constructed which yielded bright red fluorescence in B. cinerea.

Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control

BACKGROUND

Banana (Musa spp.) is grown throughout the tropical and subtropical regions of the world. The fruits are a key staple food in many developing countries and a source of income for subsistence farmers. Bananas are also a major, multibillion-dollar export commodity for consumption primarily in developed countries, where few banana cultivars are grown. The fungal pathogen Mycosphaerella fijiensis causes black leaf streak disease (BLSD; aka black Sigatoka leaf spot) on the majority of edible banana cultivars grown worldwide. The fact that most of these cultivars are sterile and unsuitable for the breeding of resistant lines necessitates the extensive use of fungicides as the primary means of disease control. BLSD is a significant threat to the food security of resource-poor populations who cannot afford fungicides, and increases the environmental and health hazards where large-acreage monocultures of banana (Cavendish subgroup, AAA genome) are grown for export.

TAXONOMY

Mycosphaerella fijiensis M. Morelet is a sexual, heterothallic fungus having Pseudocercospora fijiensis (M. Morelet) Deighton as the anamorph stage. It is a haploid, hemibiotrophic ascomycete within the class Dothideomycetes, order Capnodiales and family Mycosphaerellaceae. Its taxonomic placement is based on DNA phylogeny, morphological analyses and cultural characteristics.

DISEASE SYMPTOMS AND HOST RANGE

Mycosphaerella fijiensis is a leaf pathogen that causes reddish-brown streaks running parallel to the leaf veins, which aggregate to form larger, dark-brown to black compound streaks. These streaks eventually form fusiform or elliptical lesions that coalesce, form a water-soaked border with a yellow halo and, eventually, merge to cause extensive leaf necrosis. The disease does not kill the plants immediately, but weakens them by decreasing the photosynthetic capacity of leaves, causing a reduction in the quantity and quality of fruit, and inducing the premature ripening of fruit harvested from infected plants. Although Musa spp. are the primary hosts of M. fijiensis, the ornamental plant Heliconia psittacorum has been reported as an alternative host.

NEW OPPORTUNITIES

Several valuable tools and resources have been developed to overcome some of the challenges of studying this host-pathogen system. These include a DNA-mediated fungal transformation system and the ability to conduct targeted gene disruptions, reliable quantitative plant bioassays, diagnostic probes to detect and differentiate M. fijiensis from related pathogens and to distinguish strains of different mating types, and a genome sequence that has revealed a wealth of gene sequences and molecular markers to be utilized in functional and population biology studies.

USEFUL WEBSITES

http://bananas.bioversityinternational.org/, http://genome.jgi-psf.org/Mycfi2/Mycfi2.home.html, http://www.isppweb.org/names_banana_pathogen.asp#fun, http://www.promusa.org/.

The potential for pathogenicity was present in the ancestor of the Ascomycete subphylum Pezizomycotina

BACKGROUND

Previous studies in Ascomycetes have shown that the function of gene families of which the size is considerably larger in extant pathogens than in non-pathogens could be related to pathogenicity traits. However, by only comparing gene inventories in extant species, no insights can be gained into the evolutionary process that gave rise to these larger family sizes in pathogens. Moreover, most studies which consider gene families in extant species only tend to explain observed differences in gene family sizes by gains rather than by losses, hereby largely underestimating the impact of gene loss during genome evolution.

RESULTS

In our study we used a selection of recently published genomes of Ascomycetes to analyze how gene family gains, duplications and losses have affected the origin of pathogenic traits. By analyzing the evolutionary history of gene families we found that most gene families with an enlarged size in pathogens were present in an ancestor common to both pathogens and non-pathogens. The majority of these families were selectively maintained in pathogenic lineages, but disappeared in non-pathogens. Non-pathogen-specific losses largely outnumbered pathogen-specific losses.

CONCLUSIONS

We conclude that most of the proteins for pathogenicity were already present in the ancestor of the Ascomycete lineages we used in our study. Species that did not develop pathogenicity seemed to have reduced their genetic complexity compared to their ancestors. We further show that expansion of gained or already existing families in a species-specific way is important to fine-tune the specificities of the pathogenic host-fungus interaction.

Effects of beta-1,3-glucan from Septoria tritici on structural defence responses in wheat

The accumulation of the pathogenesis-related (PR) proteins beta-1,3-glucanase and chitinase and structural defence responses were studied in leaves of wheat either resistant or susceptible to the hemibiotrophic pathogen Septoria tritici. Resistance was associated with an early accumulation of beta-1,3-glucanase and chitinase transcripts followed by a subsequent reduction in level. Resistance was also associated with high activity of beta-1,3-glucanase, especially in the apoplastic fluid, in accordance with the biotrophic/endophytic lifestyle of the pathogen in the apoplastic spaces, thus showing the highly localized accumulation of defence proteins in the vicinity of the pathogen. Isoform analysis of beta-1,3-glucanase from the apoplastic fluid revealed that resistance was associated with the accumulation of an endo-beta-1,3-glucanase, previously implicated in defence against pathogens, and a protein with identity to ADPG pyrophosphatase (92%) and germin-like proteins (93%), which may be involved in cell wall reinforcement. In accordance with this, glycoproteins like extensin were released into the apoplast and callose accumulated to a greater extent in cell walls, whereas lignin and polyphenolics were not found to correlate with defence. Treatment of a susceptible wheat cultivar with purified beta-1,3-glucan fragments from cell walls of S. tritici gave complete protection against disease and this was accompanied by increased gene expression of beta-1,3-glucanase and the deposition of callose. Collectively, these data indicate that resistance is dependent on a fast, initial recognition of the pathogen, probably due to beta-1,3-glucan in the fungal cell walls, and this results in the accumulation of beta-1,3-glucanase and structural defence responses, which may directly inhibit the pathogen and protect the host against fungal enzymes and toxins.

Role of hydrogen peroxide during the interaction between the hemibiotrophic fungal pathogen Septoria tritici and wheat

Hydrogen peroxide (H(2)O(2)) is reported to inhibit biotrophic but benefit necrotrophic pathogens. Infection by necrotrophs can result in a massive accumulation of H(2)O(2) in hosts. Little is known of how pathogens with both growth types are affected (hemibiotrophs). The hemibiotroph, Septoria tritici, infecting wheat (Triticum aestivum) is inhibited by H(2)O(2) during the biotrophic phase, but a large H(2)O(2) accumulation occurs in the host during reproduction. Here, we infiltrated catalase, H(2)O(2) or water into wheat during the biotrophic or the necrotrophic phase of S. tritici and studied the effect of infection on host physiology to get an understanding of the survival strategy of the pathogen. H(2)O(2) removal by catalase at both early and late stages made plants more susceptible, whereas H(2)O(2) made them more resistant. H(2)O(2) is harmful to S. tritici throughout its life cycle, but it can be tolerated. The late accumulation of H(2)O(2) is unlikely to result from down-regulation of photosynthesis, but probably originates from damage to the peroxisomes during the general tissue collapse, which is accompanied by release of soluble sugars in a susceptible cultivar.

The MAP kinase-encoding gene MgFus3 of the non-appressorium phytopathogen Mycosphaerella graminicola is required for penetration and in vitro pycnidia formation

SUMMARY In eukaryotes, a family of serine/threonine protein kinases known as mitogen-activated protein kinases (MAPKs) is involved in the transduction of a variety of extracellular signals and in the regulation of growth and development. We identified a MAPK-encoding gene in Mycosphaerella graminicola strain IPO323 with high homology to the orthologous Fus3 gene of Saccharomyces cerevisiae and designated it MgFus3. Early colony development of the MgFus3 mutants during in vitro growth was similar to those of the wild-type and ectopic controls, but at the later stages of growth MgFus3 mutants did not become melanized, showed altered polarized growth and were unable to produce aerial mycelia. The MgFus3 mutants were non-pathogenic, and detailed microscopic analyses revealed that they failed to colonize the mesophyll tissue owing to the inability to penetrate stomata. Unlike the wild-type strain, MgFus3 mutants were unable to differentiate pycnidia on plant-derived media. Thus, in addition to the crucial role of MgFus3 in the regulation of penetration, it may also be involved in regulating asexual fructification. Hence, MgFus3 can be regarded as a multifunctional pathogenicity factor of M. graminicola.

Histopathological studies of sclerotia of phytopathogenic fungi parasitized by a GFP transformed Trichoderma virens antagonistic strain

The gfp gene from the jellyfish Aequorea victoria, coding for the Green Fluorescent Protein (GFP), was used as a reporter gene to transform a Trichoderma virens strain I10, characterized as having a promising biocontrol activity against a large number of phytopathogenic fungi. On the basis of molecular and biological results, a stable GFP transformant was selected for further experiments. In order to evaluate the effects of GFP transformation on mycoparasitic ability of T. virens I10, sclerotia of Sclerotium rolfsii, Sclerotinia sclerotiorum and S. minor were inoculated with the T. virens strain I10 GFP transformant or the wild type strain. Statistical analysis of percentages of decayed sclerotia showed that the transformation of the antagonistic isolate with the GFP reporter gene did not modify mycoparasitic activity against sclerotia. Sclerotium colonization was followed by fluorescent microscopy revealing intracellular growth of the antagonist in the cortex (S. rolfsii) and inter-cellular growth in the medulla (S. rolfsii, and S. sclerotiorum). The uniformly distributed mycelium of T. virens just beneath the rind of sclerotia of both S. rolfsii and S. sclerotiorum suggests that the sclerotia became infected at numerous randomly distributed locations without any preferential point of entry.

Sequence diversity of beta-tubulin (tubA) gene in Phaeosphaeria nodorum and P. avenaria

Full-length coding sequences of the beta-tubulin gene (tubA) were PCR-amplified and sequenced from 42 Phaeosphaeria isolates, including 16 P. nodorum and 23 P. avenaria species from cereals, two Polish isolates from rye (Secale cereale L.), and one isolate from dallis grass (Paspalum dilatatum Poir). A tubA gene of size 1556bp was identified in wheat- and barley-biotype P. nodorum (PN-w and PN-b), P. avenaria f. sp. avenaria (Paa), homothallic P. avenaria f. sp. triticea (P.a.t.) (Pat1) and the P.a.t. isolate (Pat3) from the State of Washington. The tubA gene length polymorphisms were detected in two P.a.t. isolates (Pat2) from foxtail barley (Hordeum jubatum L.), one from dallis grass and two Polish isolates from rye. These size differences were due to the variation of intron lengths among these three Phaeosphaeria species. All Phaeosphaeria isolates have identical 1344bp exons that can be translated into a 447 amino acid beta-tubulin. Like glyceraldehyde-3-phosphate dehydrogenase, the beta-tubulin amino acid sequence was identical in all Phaeosphaeria species used in this study, with the exception of the two Pat2 isolates. Six amino acid differences were evident in the beta-tubulin of these Pat2 isolates.

Metabolic and stress adaptation by Mycosphaerella graminicola during sporulation in its host revealed through microarray transcription profiling

SUMMARY Pathogenic microbes must successfully adapt to the host environment, acquiring nutrients and tolerating immune/defence responses. Studies on host-pathogen interactions at the transcriptome level have predominantly investigated host responses. Here we present a broad-scale transcriptional analysis on a fungal pathogen during sporulation within its host environment. Septoria leaf blotch is an important fungal disease of cultivated wheat and is caused by the ascomycete fungus Septoria tritici (teleomorph Mycosphaerella graminicola). A cDNA microarray containing 2563 unigenes was generated and then used to compare fungal nutrition and development in vitro under nutrient-rich and nutrient-limiting conditions and in vivo at a late stage of plant infection. The data obtained provided clear insights into metabolic adaptation in all three conditions and an elevated stress adaptation/tolerance specifically in the host environment. We conclude that asexual sporulation of M. graminicola during the late stage of plant infection occurs in a rich nutritional environment involving adaptation to stresses imposed in part by the presence of reactive oxygen species.

Gene expression profiles of Blumeria graminis indicate dynamic changes to primary metabolism during development of an obligate biotrophic pathogen

cDNA microarrays of Blumeria graminis f sp hordei transcript profiles during the asexual development cycle reveal the dynamics of global gene expression as the fungus germinates, penetrates, feeds on its host, and produces masses of conidia for dispersal. The expression profiles of genes encoding enzymes involved in primary metabolism show that there is a striking degree of coordinate regulation of some of the genes in the same pathway. In one example, genes encoding several glycolytic enzymes are significantly upregulated as mature appressoria form and also in infected epidermis, which contain fungal haustoria. In another example, mRNAs for lipid degrading enzymes are initially expressed at high levels in the conidia and the early germination stages and decrease significantly later. We discuss these results and draw inferences on the metabolic status of this obligate biotrophic fungus as it infects its host and completes its life cycle.

Use of Green Fluorescent Protein and Image Analysis to Quantify Proliferation of Trichoderma harzianum in Nonsterile Soil

ABSTRACT One drawback of traditional methods for fungal biomass measurement is the inability to distinguish biomass of an introduced fungus from that of the indigenous microbial community in nonsterile soil. We quantified biomass of a specific fungal biological control agent in nonsterile soil using epifluorescence microscopy and image analysis of green fluorescent protein (GFP)-expressing Trichoderma harzianum (ThzID1-M3). Numbers of colony forming units on a semiselective medium were compared with biomass estimates from image analysis, after ThzID1-M3 was incubated in soil that either remained moist (-0.05 MPa) for 14 to 21 days or remained moist for approximately 5 days and then was allowed to dry to <-3.0 MPa. Recovery of significant numbers of ThzID1-M3 propagules lagged approximately 3 days behind initiation of hyphal growth. Reductions in both colony counts and biomass were observed over time when soil was allowed to dry. However, in soil that remained moist, colony counts increased over a 14- to 21-day period even though biomass declined after approximately 3 to 5 days. Our results confirm that use of GFP, along with epifluorescence microscopy, is a useful tool to distinguish active hyphal biomass, the form of the fungus that is functional for biological control, from inactive propagules such as conidia or chlamydospores that are enumerated by plate counts.

Arabidopsis pathology breathes new life into the necrotrophs-vs.-biotrophs classification of fungal pathogens

SUMMARY Fungal plant pathologists have for many decades attempted to classify pathogens into groups called necrotrophs, biotrophs and, more recently, hemibiotrophs. Although these terms are well known and frequently used, disagreements about which pathogens fall into which classes, as well as the precise definition of these terms, has conspired to limit their usefulness. Dogmas concerning the properties of the classes have been progressively eroded. However, the genetic analysis of disease resistance, particularly in the model plant Arabidopsis thaliana, has provided a biologically meaningful division based on whether defence against fungal pathogens is controlled via the salicylate or jasmonate/ethylene pathways. This mode-of-defence division distinguishes necrotrophs and biotrophs but it limits the biotroph class to pathogens that possess haustoria. The small number and limited range of pathogens that infect Arabidopsis means that several interesting questions are still unanswered. Do hemibiotrophs represents a distinct class or a subclass of the necrotrophs? Does the division apply to other plant families and particularly to cereals? and does this classification help us understand the intricacies of either fungal pathogenicity or plant defence?

Green Fluorescent Detection of Fungal Colonization and Endopolygalacturonase Gene Expression in the Interaction of Alternaria citri with Citrus

ABSTRACT Alternaria citri, a postharvest pathogen, produces endopolygalacturonase (endoPG) and causes black rot on citrus fruit. We previously described that an endoPG-disrupted mutant of Alternaria citri was significantly reduced in its ability to macerate plant tissue and cause black rot symptoms on citrus. In order to investigate colonization of citrus fruit tissues by Alternaria citri, pTEFEGFP carrying a green fluorescent protein (GFP) gene was introduced into wild-type Alternaria citri and its endoPG-disrupted mutant (M60). Green fluorescence was observed in spores, germ tubes, appressoria, and infection hyphae of transformants G1 (derived from wild type) and GM4 (derived from M60). Hyphae of G1 but not GM4 vertically penetrated the peel, but the hyphae of both G1 and GM4 spread equally in the juice sac area of citrus fruit. Green fluorescence of Alternaria citri transformant EPG7 carrying a GFP gene under control of the endoPG gene promoter of Alternaria citri was induced by pectin in the peel during the infection stage, but repressed completely in the juice sac area, likely by carbon catabolite repression by sugars in the juice.

Wanted: pathogenesis-related marker molecules for Fusarium oxysporum

Although Fusarium oxysporum pathogens cause severe wilts in about 80 botanical species, the mechanisms of pathogenicity and symptom induction are poorly understood. Knowledge about the genetic and biochemical pathways involved in the pathogenesis of F. oxysporum would be invaluable in getting targets for both fungicide development and search for biocontrol agents. In this respect, we described the main approaches that have been developed to identify some mechanisms underlying the pathogenesis of F. oxysporum. During the last decades, the potential functions triggering of F. oysporum pathogenicity have mainly been investigated by comparing soilborne pathogenic strains with nonpathogenic ones with regards to the analysis of the pre- and infection stages and of the resulting plant-fungus interactions. The relatively recent progress in the molecular biology of this fungus has allowed complementary approaches to be developed in order to identify key factors involved in F. oxysporum pathogenicity. Screening mutants of F. oxysporum for loss of virulence led to the successful identification of some pathogenesis-related factors, such as hydrophobicity or attachment of germlings. Taken together, the strategies described above support the idea that changes in fungal metabolism is also of importance in triggering of F. oxysporum pathogenesis.

Regulation of the MPG1 hydrophobin gene in the rice blast fungus Magnaporthe grisea

The hydrophobin-encoding gene MPG1 of the rice blast fungus Magnaporthe grisea is highly expressed during the initial stages of host plant infection and targeted deletion of the gene results in a mutant strain that is reduced in virulence, conidiation, and appressorium formation. The green fluorescent protein-encoding allele sGFP was used as a reporter to investigate regulatory genes that control MPG1 expression. The MAP kinase-encoding gene PMK1 and the wide domain regulators of nitrogen source utilization, NPR1 and NUT1, were required for full expression of MPG1 in response to starvation stress. The CPKA gene, encoding the catalytic subunit of protein kinase A, was required for repression of MPG1 during growth in rich nutrient conditions. During appressorium morphogenesis, high-level MPG1 expression was found to require the CPKA and NPR1 genes. Expression of a destabilized GFP allele indicated that de novo MPG1 expression occurs during appressorium formation. Three regions of the MPG1 promoter were identified which are required for high-level expression of MPG1 during appressorium formation and are necessary for the biological activity of the MPG1 hydrophobin during spore formation and plant infection.

Quantitative PCR monitoring of the effect of azoxystrobin treatments on Mycosphaerella graminicola epidemics in the field

Quantitative PCR and visual monitoring of Mycosphaerella graminicola epidemics were performed to investigate the effect of curative and preventative applications of azoxystrobin in wheat field crops. A non-systemic protectant and a systemic curative fungicide, chlorothalonil and epoxiconazole, respectively, were used as references. PCR diagnosis detected leaf infection by M graminicola 3 weeks before symptom appearance, thereby allowing a clear distinction between curative and preventative treatments. When applied 1 week after the beginning of infection, azoxystrobin curative activity was intermediate between chlorothalonil (low effect) and epoxiconazole. When applied preventatively, none of the fungicides completely prevented leaf infection. There was some indication that azoxystrobin preventative treatments may delay fungal DNA increase more than epoxiconazole at the beginning of leaf infection. Both curative and preventative treatments increased the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Azoxystrobin only slightly decreased the speed of necrotic area increase compared with epoxiconazole. Hence, azoxystrobin activity toward M graminicola mainly resides in lengthening the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Information generated in this way is useful for optimal positioning of azoxystrobin treatments on M graminicola.

Mycosphaerella graminicola: latent infection, crop devastation and genomics

summary Mycosphaerella graminicola is an important pathogen of wheat, causing septoria leaf blotch disease. This review summarizes the current knowledge on disease development and control of the fungus, and discusses how molecular tools and genomics are being employed to uncover the genetic basis of pathogenicity.

TAXONOMY

Mycosphaerella graminicola (Fuckel) J. Schröt. in Cohn (anamorph: Septoria tritici Roberge in Desmaz.). Kingdom Fungi, Phylum Ascomycota, Class Loculoascomycetes (filamentous ascomycetes), Order Dothideales, Genus Mycosphaerella, Species graminicola.

HOST RANGE

Bread and durum wheat (Triticum aestivum L. and T. turgidum ssp. durum L.) Disease symptoms: Initiating with the appearance of chlorotic flecks on leaves that develop into irregular sunken necrotic lesions peppered with tiny black spots (pycnidia). In addition to the necrotic tissue, the disease results in early leaf tissue senescence and therefore reduced photosynthetic capacity.

USEFUL WEBSITES

<http://cogeme.ex.ac.uk>, <http://www.plant.wageningen-ur.nl>, <http://www.paragen.com>.

Microscopic analysis of the effect of azoxystrobin treatments on Mycosphaerella graminicola infection using green fluorescent protein (GFP)-expressing transformants

Green fluorescent protein (GFP)-expressing transformants were used to investigate the effects of strobilurin fungicide azoxystrobin on Mycosphaerella graminicola infection. Azoxystrobin treatments (125 or 250 g AI ha-1) were applied at various stages of the infection process under controlled conditions. GFP transformants showed conserved in vitro sensitivity to azoxystrobin and pathogenicity. Azoxystrobin controlled over 90% of M graminicola infections when applied before or during penetration of the pathogen (15% of the incubation phase). Azoxystrobin also impaired the growth of intercellular hyphae in M graminicola post-penetration infection stages when applied at up to 50% of the incubation phase. Incubating infections observed in treated leaves were viable, but their growth was impaired and they did not induce necrosis under controlled conditions. Reduction by half of azoxystrobin dosage had little or no effect on azoxystrobin efficiency in controlling M graminicola. The contribution of post-penetration fungistatic effect to azoxystrobin curative properties toward M graminicola in a field situation is discussed.