FvatfA regulates growth, stress tolerance as well as mycotoxin and pigment productions in Fusarium verticillioides.

FvatfA from the maize pathogen Fusarium verticillioides putatively encodes the Aspergillus nidulans AtfA and Schizasaccharomyces pombe Atf1 orthologous bZIP-type transcription factor, FvAtfA. In this study, a ΔFvatfA deletion mutant was constructed and then genetically complemented with the fully functional FvatfA gene. Comparing phenotypic features of the wild-type parental, the deletion mutant and the restored strains shed light on the versatile regulatory functions played by FvAtfA in (i) the maintenance of vegetative growth on Czapek-Dox and Potato Dextrose agars and invasive growth on unwounded tomato fruits, (ii) the preservation of conidiospore yield and size, (iii) the orchestration of oxidative (H2O2, menadione sodium bisulphite) and cell wall integrity (Congo Red) stress defences and (iv) the regulation of mycotoxin (fumonisins) and pigment (bikaverin, carotenoid) productions. Expression of selected biosynthetic genes both in the fumonisin (fum1, fum8) and the carotenoid (carRA, carB) pathways were down-regulated in the ΔFvatfA strain resulting in defected fumonisin production and considerably decreased carotenoid yields. The expression of bik1, encoding the polyketide synthase needed in bikaverin biosynthesis, was not up-regulated by the deletion of FvatfA meanwhile the ΔFvatfA strain produced approximately ten times more bikaverin than the wild-type or the genetically complemented strains. The abolishment of fumonisin production of the ΔFvatfA strain may lead to the development of new-type, biology-based mycotoxin control strategies. The novel information gained on the regulation of pigment production by this fungus can be interesting for experts working on new, Fusarium-based biomass and pigment production technologies. Key points • FvatfA regulates vegetative and invasive growths of F. verticillioides. • FvatfA also orchestrates oxidative and cell wall integrity stress defenses. • The ΔFvatfA mutant was deficient in fumonisin production. • FvatfA deletion resulted in decreased carotenoid and increased bikaverin yields.

Signals of Systemic Immunity in Plants: Progress and Open Questions.

Systemic acquired resistance (SAR) is a defence mechanism that induces protection against a wide range of pathogens in distant, pathogen-free parts of plants after a primary inoculation. Multiple mobile compounds were identified as putative SAR signals or important factors for influencing movement of SAR signalling elements in Arabidopsis and tobacco. These include compounds with very different chemical structures like lipid transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE1), methyl salicylate (MeSA), dehydroabietinal (DA), azelaic acid (AzA), glycerol-3-phosphate dependent factor (G3P) and the lysine catabolite pipecolic acid (Pip). Genetic studies with different SAR-deficient mutants and silenced lines support the idea that some of these compounds (MeSA, DIR1 and G3P) are activated only when SAR is induced in darkness. In addition, although AzA doubled in phloem exudate of tobacco mosaic virus (TMV) infected tobacco leaves, external AzA treatment could not induce resistance neither to viral nor bacterial pathogens, independent of light conditions. Besides light intensity and timing of light exposition after primary inoculation, spectral distribution of light could also influence the SAR induction capacity. Recent data indicated that TMV and CMV (cucumber mosaic virus) infection in tobacco, like bacteria in Arabidopsis, caused massive accumulation of Pip. Treatment of tobacco leaves with Pip in the light, caused a drastic and significant local and systemic decrease in lesion size of TMV infection. Moreover, two very recent papers, added in proof, demonstrated the role of FMO1 (FLAVIN-DEPENDENT-MONOOXYGENASE1) in conversion of Pip to N-hydroxypipecolic acid (NHP). NHP systemically accumulates after microbial attack and acts as a potent inducer of plant immunity to bacterial and oomycete pathogens in Arabidopsis. These results argue for the pivotal role of Pip and NHP as an important signal compound of SAR response in different plants against different pathogens.

Detection of Lipid Peroxidation-Derived Free Azelaic Acid, a Biotic Stress Marker and Other Dicarboxylic Acids in Tobacco by Reversed-Phase HPLC-MS Under Non-derivatized Conditions.

Azelaic acid (AzA, 1,9-nonadienoic acid) is a nine-carbon chain (C9) dicarboxylic acid with multiple and diverse functions in humans and plants. In plants this compound was suggested as a marker for lipid peroxidation under biotic and abiotic stress conditions and an inducer (priming agent) of plant immunity (acquired resistance). Detection methods for AzA in plants include a wide range of methodological approaches. This new and simple reversed-phase HPLC-MS protocol describes the measurement of AzA and other dicarboxylic acids either from tobacco leaf tissue or petiolar exudates (vascular sap) of plants under non-derivatized conditions.

Near-Isogenic Barley Lines Show Enhanced Susceptibility to Powdery Mildew Infection Following High-Temperature Stress.

Barley cultivation is adversely affected by high-temperature stress, which may modulate plant defense responses to pathogens such as barley powdery mildew (Blumeria graminis f. sp. hordei, Bgh). Earlier research focused mainly on the influence of short-term heat stress (heat shock) of barley on Bgh infection. In this study, our aim was to investigate the effects of both short- and long-term heat stress (35 °C from 30 s to 5 days) on Bgh infection in the barley cultivar Ingrid and its near-isogenic lines containing different powdery mildew resistance genes (Mla12, Mlg, and mlo5) by analyzing symptom severity and Bgh biomass with RT-qPCR. The expression of selected barley defense genes (BAX inhibitor-1, Pathogenesis- related protein-1b, Respiratory burst oxidase homologue F2, and Heat shock protein 90-1) was also monitored in plants previously exposed to heat stress followed by inoculation with Bgh. We demonstrated that pre-exposure to short- and long-term heat stress negatively affects the resistance of all resistant lines manifested by the appearance of powdery mildew symptoms and increased Bgh biomass. Furthermore, prolonged heat stress (48 and 120 h) enhanced both Bgh symptoms and biomass in susceptible wild-type Ingrid. Heat stress suppressed and delayed early defense gene activation in resistant lines, which is a possible reason why resistant barley became partially susceptible to Bgh.

Adenylyl cyclase regulates heavy metal sensitivity, bikaverin production and plant tissue colonization in Fusarium proliferatum.

A homologue of the adenylyl cyclase (AC) gene of Neurospora crassa, named Fpacy1 was cloned from the genomic library of Fusarium proliferatum ITEM 2287 by screening the library with a DNA fragment amplified by using PCR primers designed from conserved sequences of the catalytic domain of AC genes from other fungi. The deduced FPACY1 protein had 53-77% identity with the AC proteins of other fungi. DeltaFpacy1 mutants obtained by targeted gene disruption showed retarded vegetative growth, increased conidiation and delayed conidial germination. Colonization capability of the mutants, assessed on maize seedlings and tomato fruits also was adversely affected. In sexual crosses the AC mutants retained full male fertility, but their female fertility decreased significantly. Disruption of Fpacy1 abolished vegetative self-incompatibility, suggesting that the AC gene is involved in multiple developmental processes related to vegetative growth, as well as sexual and parasexual events. The elevated thermo- and H(2)O(2)-tolerance of the DeltaFpacy1 mutants was coupled to an increased sensitivity towards Cd and Cu, indicating that the cAMP signaling pathway may have both negative and positive regulatory roles on the stress response mechanisms of fungal cells. When grown under nitrogen limitation conditions, the DeltaFpacy1 mutants produced an average of approximately 274 microg g(-1) bikaverin, whereas only traces of this metabolite was detected in the wild type. This finding provides further evidence of the role of the cAMP-PKA pathway in regulating bikaverin production.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance.

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Toxicity of abiotic stressors to Fusarium species: differences in hydrogen peroxide and fungicide tolerance.

Stress sensitivity of three related phytopathogenic Fusarium species (Fusarium graminearum, Fusarium oxysporum and Fusarium verticillioides) to different oxidative, osmotic, cell wall, membrane, fungicide stressors and an antifungal protein (PAF) were studied in vitro. The most prominent and significant differences were found in oxidative stress tolerance: all the three F. graminearum strains showed much higher sensitivity to hydrogen peroxide and, to a lesser extent, to menadione than the other two species. High sensitivity of F. verticillioides strains was also detectable to an azole drug, Ketoconazole. Surprisingly, no or limited differences were observed in response to other oxidative, osmotic and cell wall stressors. These results indicate that fungal oxidative stress response and especially the response to hydrogen peroxide (this compound is involved in a wide range of plant-fungus interactions) might be modified on niche-specific manner in these phylogenetically related Fusarium species depending on their pathogenic strategy. Supporting the increased hydrogen peroxide sensitivity of F. graminearum, genome-wide analysis of stress signal transduction pathways revealed the absence one CatC-type catalase gene in F. graminearum in comparison to the other two species.

Adenylyl cyclase plays a regulatory role in development, stress resistance and secondary metabolism in Fusarium fujikuroi.

The ascomycete fungus Fusarium fujikuroi (Gibberella fujikuroi MP-C) produces secondary metabolites of biotechnological interest, such as gibberellins, bikaverin, and carotenoids. Production of these metabolites is regulated by nitrogen availability and, in a specific manner, by other environmental signals, such as light in the case of the carotenoid pathway. A complex regulatory network controlling these processes is recently emerging from the alterations of metabolite production found through the mutation of different regulatory genes. Here we show the effect of the targeted mutation of the acyA gene of F. fujikuroi, coding for adenylyl cyclase. Mutants lacking the catalytic domain of the AcyA protein showed different phenotypic alterations, including reduced growth, enhanced production of unidentified red pigments, reduced production of gibberellins and partially derepressed carotenoid biosynthesis in the dark. The phenotype differs in some aspects from that of similar mutants of the close relatives F. proliferatum and F. verticillioides: contrary to what was observed in these species, ΔacyA mutants of F. fujikuroi showed enhanced sensitivity to oxidative stress (H(2)O(2)), but no change in heavy metal resistance or in the ability to colonize tomato tissue, indicating a high versatility in the regulatory roles played by cAMP in this fungal group.

The MAT1-2-1 mating-type gene upregulates photo-inducible carotenoid biosynthesis in Fusarium verticillioides.

Filamentous ascomycetes, including mitotic holomorphs, have constitutively transcribed MAT (mating type) genes. These genes encode transcription factors considered to be the major regulators of sexual communication. The proven targets of the MAT transcription factors are pheromone precursor and pheromone receptor genes. However, recent studies demonstrated that MAT proteins may also affect other genes not involved directly in the mating process. When grown in the light, Fusarium verticillioides produces the acidic xanthophyll neurosporoxanthin and lower amounts of nonpolar precursor carotenes, such as phytoene, torulene, ß-carotene, and γ-carotene. Depending on the illumination conditions, a drastic decrease or the absence of light-inducible carotenoid accumulation was detected in three independent ΔFvMAT1-2-1 knockout mutants of F. verticillioides as compared with the parental wild-type strain. Transcript levels of the carB, carRA, and carT genes, encoding key enzymes of the carotenoid biosynthetic pathway, were also significantly reduced in the mutants. The downregulation of these genes in the ΔFvMAT1-2-1 mutant indicates that MAT genes play a role in the control of carotenogenesis in Fusarium. The finding that mating-type genes regulate important processes unrelated to sex helps to understand the presence of functional MAT genes in asexually reproducing fungus populations.

Time-lapse analysis of cell death in mammalian and fungal cells.

Time-lapse video microscopy was designed to follow the movement of single cells for an unlimited period of time under physiological conditions. The system is based on two inverted microscopes located in a CO(2) incubator and equipped with charge-coupled device cameras connected to the computer. Frames were recorded every minute and the subsequent video sequence was converted to database form. The system was applied to describe the movements of normal HaCaT cells and Pb-treated cells causing the so-called apoptotic dance during cell death. The apoptotic movement was also followed in high-osmolarity glycerol-type mitogen-activated protein kinase (MAPK) null mutant of Fusarium proliferatum, a filamentous fungus, during osmotic stress. The shortest (20 min) and most vigorous death movements were observed in apoptotic fungal cells subjected to salt stress. The necrotic process at higher Pb concentration (50 microM) took 2-3 h, whereas the apoptotic process at lower Pb concentrations lasted from minutes to days.

N-starvation stress induced FUM gene expression and fumonisin production is mediated via the HOG-type MAPK pathway in Fusarium proliferatum.

During cultivation of a wild type strain of Fusarium proliferatum on ammonium dihydrogen phosphate containing defined medium, expression levels of FUM1 and FUM8, members of the fumonisin biosynthesis gene cluster significantly increased when ammonium ion concentration of the culture medium decreased below 10 mM, indicating that N-depletion triggers the fumonisin biosynthesis genes. Deletion of Fphog1, a HOG-type MAP kinase gene resulted in further increases in FUM1 and FUM8 expression under nitrogen starvation (absence of any N-source) conditions. Fumonisin B1 (FB1) production paralleled with increased FUM gene expression: significant amounts of FB1 were measured in culture filtrates of the DeltaFphog1 deleted mutant after five days culturing, whereas only traces of FB1 could be detected in filtrates of the wild type and the restored strain (R1) complemented with the wild-type Fphog1-24 gene. N-starvation strongly retarded the growth of the DeltaFphog1 mutant in comparison to wild type. The up-regulation of fumonisin biosynthesis genes in the DeltaFphog1 mutant could be explained by the increased sensitivity of these strains to N-starvation stress that appears in the absence of an intact HOG-type MAPK pathway.

Fphog1, a HOG-type MAP kinase gene, is involved in multistress response in Fusarium proliferatum.

Delta Fphog1 mutants of Fusarium proliferatum obtained by targeted gene disruption of Fphog1, an orthologue of the Saccharomyces cerevisiae hog1 MAPK gene showed increased sensitivity towards different abiotic stressors including UV-irradiation, heat, salt, osmotic and hydrogen peroxide treatments. Incubation of the Delta Fphog1 mutants under hyperosmotic conditions was accompanied with prolonged growth arrest, inhibition of conidial germination, morphological abnormalities and time-dependent increase of the cell death rate. The wild type Fphog1 gene, under the control of its own promoter, was able to rescue the multistress sensitivity of the mutant strain. Real time qPCR data demonstrated that under salt and sorbitol stress conditions the Fphog1 gene is not subject of transcriptional regulation. Levels of reactive oxygen species (ROS), mitochondrial membrane permeability transition, nuclear disintegration and DNA fragmentation, indicators of programmed cell death (PCD) all showed significant increases under osmotic stress conditions in the Delta Fphog1 mutant in comparison to the wild type strain. These results suggest that an important function of Fphog1 is attenuating apoptotic phenotypes under salt and sorbitol stressors.