Polyamine metabolism in fungi with emphasis on phytopathogenic species

Intracellular polyamines regulate redox homeostasis with cAMP-PKA signalling during sexual mating/filamentation and pathogenicity of Sporisorium scitamineum

Sugarcane smut caused by Sporisorium scitamineum seriously impairs sugarcane production and quality. Sexual mating/filamentation is a critical step of S. scitamineum pathogenesis, yet the regulatory mechanisms are not fully understood. In this study, we identified the SsAGA, SsODC, and SsSAMDC genes, which are involved in polyamine biosynthesis in S. scitamineum. Deletion of SsODC led to complete loss of filamentous growth after sexual mating, and deletion of SsAGA or SsSAMDC caused reduced filamentation. Double deletion of SsODC and SsSAMDC resulted in auxotrophy for putrescine (PUT) and spermidine (SPD) when grown on minimal medium (MM), indicating that these two genes encode enzymes that are critical for PUT and SPD biosynthesis. We further showed that low PUT concentrations promoted S. scitamineum filamentation, while high PUT concentrations suppressed filamentation. Disrupted fungal polyamine biosynthesis also resulted in a loss of pathogenicity and reduced fungal biomass within infected plants at the early infection stage. SPD formed a gradient from the diseased part to nonsymptom parts of the cane stem, suggesting that SPD is probably favourable for fungal virulence. Mutants of the cAMP-PKA (SsGPA3-SsUAC1-SsADR1) signalling pathway displayed up-regulation of the SsODC gene and elevated intracellular levels of PUT. SsODC directly interacted with SsGPA3, and sporidia of the ss1uac1ΔodcΔ mutant displayed abundant pseudohyphae. Furthermore, we found that elevated PUT levels caused accumulation of intracellular reactive oxygen species (ROS), probably by suppressing transcription of ROS-scavenging enzymes, while SPD played the opposite role. Overall, our work proves that polyamines play important roles in the pathogenic development of sugarcane smut fungus, probably by collaboratively regulating intracellular redox homeostasis with the cAMP-PKA signalling pathway.

Control of white mold (Sclerotinia sclerotiorum) through plant-mediated RNA interference

The causative agent of white mold, Sclerotinia sclerotiorum, is capable of infecting over 600 plant species and is responsible for significant crop losses across the globe. Control is currently dependent on broad-spectrum chemical agents that can negatively impact the agroecological environment, presenting a need to develop alternative control measures. In this study, we developed transgenic Arabidopsis thaliana (AT1703) expressing hairpin (hp)RNA to silence S. sclerotiorum ABHYDROLASE-3 and slow infection through host induced gene silencing (HIGS). Leaf infection assays show reduced S. sclerotiorum lesion size, fungal load, and ABHYDROLASE-3 transcript abundance in AT1703 compared to wild-type Col-0. To better understand how HIGS influences host-pathogen interactions, we performed global RNA sequencing on AT1703 and wild-type Col-0 directly at the site of S. sclerotiorum infection. RNA sequencing data reveals enrichment of the salicylic acid (SA)-mediated systemic acquired resistance (SAR) pathway, as well as transcription factors predicted to regulate plant immunity. Using RT-qPCR, we identified predicted interacting partners of ABHYDROLASE-3 in the polyamine synthesis pathway of S. sclerotiorum that demonstrate co-reduction with ABHYDROLASE-3 transcript levels during infection. Together, these results demonstrate the utility of HIGS technology in slowing S. sclerotiorum infection and provide insight into the role of ABHYDROLASE-3 in the A. thaliana-S. sclerotiorum pathosystem.

Safety evaluation of FAD2 RNAi transgenic Brassica napus L. based on microbial diversity and metabonomic analysis

Oleic acid desaturase (FAD2) is the key enzyme that produces polyunsaturated fatty acids in rapeseed (Brassica napus L), which is one of the main oil crops. RNA interference (RNAi) is an emerging technique that provides new opportunities for the generation of new traits in plants. To increase oleic acid content and reduce linoleic and linolenic acid content in rapeseed, we constructed an ihpRNA plant expression vector of the FAD2 gene and obtained transgenic plants for multiple generations by stable inheritance. In this study, third-generation transgenic plants (T3), seventh-generation transgenic plants (T7), and wild-type plants (WT) were used. The differences in microbial community diversity between transgenic plants and wild-type plants and the up- and downregulation of rhizosphere metabolite contents were investigated. In conclusion, the results showed that the soil microbial community structure was stable, the general microbial community structure was not changed by the transgenic rhizosphere exudates, and no significant harmful root exudate of transgenic rapeseed on the environment was found through the microbial community and metabolomics analysis. This work may provide an understanding of the impact of RNAi on plant metabolites and a safety evaluation method for transgenic plants and a reference for rapeseed breeding.

Difluoromethylornithine (DFMO) and AMXT 1501 inhibit capsule biosynthesis in pneumococci

Polyamines are small cationic molecules that have been linked to various cellular processes including replication, translation, stress response and recently, capsule regulation in Streptococcus pneumoniae (Spn, pneumococcus). Pneumococcal-associated diseases such as pneumonia, meningitis, and sepsis are some of the leading causes of death worldwide and capsule remains the principal virulence factor of this versatile pathogen. α-Difluoromethyl-ornithine (DFMO) is an irreversible inhibitor of the polyamine biosynthesis pathway catalyzed by ornithine decarboxylase and has a long history in modulating cell growth, polyamine levels, and disease outcomes in eukaryotic systems. Recent evidence shows that DFMO can also target arginine decarboxylation. Interestingly, DFMO-treated cells often escape polyamine depletion via increased polyamine uptake from extracellular sources. Here, we examined the potential capsule-crippling ability of DFMO and the possible synergistic effects of the polyamine transport inhibitor, AMXT 1501, on pneumococci. We characterized the changes in pneumococcal metabolites in response to DFMO and AMXT 1501, and also measured the impact of DFMO on amino acid decarboxylase activities. Our findings show that DFMO inhibited pneumococcal polyamine and capsule biosynthesis as well as decarboxylase activities, albeit, at a high concentration. AMXT 1501 at physiologically relevant concentration could inhibit both polyamine and capsule biosynthesis, however, in a serotype-dependent manner. In summary, this study demonstrates the utility of targeting polyamine biosynthesis and transport for pneumococcal capsule inhibition. Since targeting capsule biosynthesis is a promising way for the eradication of the diverse and pathogenic pneumococcal strains, future work will identify small molecules similar to DFMO/AMXT 1501, which act in a serotype-independent manner.

Spermidine Is Critical for Growth, Development, Environmental Adaptation, and Virulence in Fusarium graminearum

Putrescine, spermidine, and spermine are the most common natural polyamines. Polyamines are ubiquitous organic cations of low molecular weight and have been well characterized for the cell function and development processes of organisms. However, the physiological functions of polyamines remain largely obscure in plant pathogenic fungi. Fusarium graminearum causes Fusarium head blight (FHB) and leads to devastating yield losses and quality reduction by producing various kinds of mycotoxins. Herein, we genetically analyzed the gene function of the polyamine biosynthesis pathway and evaluated the role of the endogenous polyamines in the growth, development, and virulence of F. graminearum. Our results found that deletion of spermidine biosynthesis gene FgSPE3 caused serious growth defects, reduced asexual and sexual reproduction, and increased sensitivity to various stresses. More importantly, ΔFgspe3 exhibited significantly decreased mycotoxin deoxynivalenol (DON) production and weak virulence in host plants. Additionally, the growth and virulence defects of ΔFgspe3 could be rescued by exogenous application of 5 mM spermidine. Furthermore, RNA-seq displayed that FgSpe3 participated in many essential biological pathways including DNA, RNA, and ribosome synthetic process. To our knowledge, these results indicate that spermidine is essential for growth, development, DON production, and virulence in Fusarium species, which provides a potential target to control FHB.

Polyamines Upregulate Cephalosporin C Production and Expression of β-Lactam Biosynthetic Genes in High-Yielding Acremonium chrysogenum Strain

The high-yielding production of pharmaceutically significant secondary metabolites in filamentous fungi is obtained by random mutagenesis; such changes may be associated with shifts in the metabolism of polyamines. We have previously shown that, in the Acremonium chrysogenum cephalosporin C high-yielding strain (HY), the content of endogenous polyamines increased by four- to five-fold. Other studies have shown that the addition of exogenous polyamines can increase the production of target secondary metabolites in highly active fungal producers, in particular, increase the biosynthesis of β-lactams in the Penicillium chrysogenum Wis 54-1255 strain, an improved producer of penicillin G. In the current study, we demonstrate that the introduction of exogenous polyamines, such as spermidine or 1,3-diaminopropane, to A. chrysogenum wild-type (WT) and HY strains, leads to an increase in colony germination and morphological changes in a complete agar medium. The addition of 5 mM polyamines during fermentation increases the production of cephalosporin C in the A. chrysogenum HY strain by 15-20% and upregulates genes belonging to the beta-lactam biosynthetic cluster. The data obtained indicate the intersection of the metabolisms of polyamines and beta-lactams in A. chrysogenum and are important for the construction of improved producers of secondary metabolites in filamentous fungi.

Polyamines: double agents in disease and plant immunity

Polyamines (PAs) are ubiquitous amine molecules found in all living organisms. In plants, beside their role in signaling and protection against abiotic stresses, there is increasing evidence that PAs have a major role in the interaction between plants and pathogens. Plant PAs are involved in immunity against pathogens, notably by amplifying pattern-triggered immunity (PTI) responses through the production of reactive oxygen species (ROS). In response, pathogens use phytotoxins and effectors to manipulate the levels of PAs in the plant, most likely to their own benefit. It also appears that pathogenic microorganisms produce PAs during infection, sometimes in large quantities. This may reflect different infectious strategies based on the selective exploitation of these molecules and the functions they perform in the cell.

Ornithine decarboxylase of the fungal pathogen Colletotrichum higginsianum plays an important role in regulating global metabolic pathways and virulence

Colletotrichum higginsianum is an important fungal pathogen causing anthracnose disease of cruciferous plants. In this study, we characterized a putative orthologue of yeast SPE1 in C. higginsianum, named ChODC. Deletion mutants of ChODC were defective in hyphal and conidial development. Importantly, deletion of ChODC significantly affected appressorium-mediated penetration in C. higginsianum. However, polyamines partially restore appressorium function and virulence indicating that loss of ChODC caused significantly decreased virulence by the crosstalk between polyamines and other metabolic pathways. Subsequently, transcriptomic and metabolomic analyses demonstrated that ChODC played an important role in metabolism of various carbon and nitrogen compounds including amino acids, carbohydrates and lipids. Along with these clues, we found deletion of ChODC affected glycogen and lipid metabolism, which were important for conidial storage utilization and functional appressorium formation. Loss of ChODC affected the mTOR signalling pathway via modulation of autophagy. Interestingly, cAMP treatment restored functional appressoria to the ΔChODC mutant, and rapamycin treatment also stimulated formation of functional appressoria in the ΔChODC mutant. Overall, ChODC was associated with the polyamine biosynthesis pathway, as a mediator of cAMP and mTOR signalling pathways to regulate appressorium function. Our study provides evidence of a link between ChODC and the cAMP signalling pathway and defines a novel mechanism by which ChODC regulates infection-associated autophagy and plant infection by fungi.

Metabolomics of Dry Versus Reanimated Antarctic Lichen-Dominated Endolithic Communities

Cryptoendolithic communities are almost the sole life form in the ice-free areas of the Antarctic desert, encompassing among the most extreme-tolerant organisms known on Earth that still assure ecosystems functioning, regulating nutrient and biogeochemical cycles under conditions accounted as incompatible with active life. If high-throughput sequencing based studies are unravelling prokaryotic and eukaryotic diversity, they are not yet characterized in terms of stress adaptations and responses, despite their paramount ecological importance. In this study, we compared the responses of Antarctic endolithic communities, with special focus on fungi, both under dry conditions (i.e., when dormant), and after reanimation by wetting, light, and optimal temperature (15 °C). We found that several metabolites were differently expressed in reanimated opposite sun exposed communities, suggesting a critical role in their success. In particular, the saccharopine pathway was up-regulated in the north surface, while the spermine/spermidine pathway was significantly down-regulated in the shaded exposed communities. The carnitine-dependent pathway is up-regulated in south-exposed reanimated samples, indicating the preferential involvement of the B-oxidation for the functioning of TCA cycle. The role of these metabolites in the performance of the communities is discussed herein.

High-Yielding Lovastatin Producer Aspergillus terreus Shows Increased Resistance to Inhibitors of Polyamine Biosynthesis

The biosynthesis of pharmaceutically significant secondary metabolites in filamentous fungi is a multistep process that depends on a wide range of various factors, one of which is the intracellular content of polyamines. We have previously shown that in Aspergillus terreus lovastatin high-yielding strain (HY) exogenous introduction of polyamines during fermentation can lead to an increase in the production of lovastatin by 20–45%. However, the molecular mechanisms of this phenomenon have not been elucidated. In this regard, we carried out an inhibitory analysis at the key stage of polyamine biosynthesis, the conversion of L-ornithine to putrescine by the enzyme ornithine decarboxylase (ODC). A. terreus HY strain showed upregulation of genes for biosynthesis of polyamines, 3–10-fold, and increased resistance compared to the original wild-type strain upon inhibition of ODC on synthetic medium with 5 mM α-difluoromethylornithine (DFMO), by 20–25%, and 5 mM 1-aminooxy-3-aminopropane (APA), by 40–45%. The data obtained indicate changes in the metabolism of polyamines in A. terreus HY strain. The observed phenomenon may have a universal character among fungal producers of secondary metabolites improved by classical methods, since previously the increased resistance to ODC inhibitors was also shown for Acremonium chrysogenum, a high-yielding producer of cephalosporin C.

FvSTUA is a Key Regulator of Sporulation, Toxin Synthesis, and Virulence in Fusarium verticillioides

Fusarium verticillioides is one of the most important pathogens of maize, causing rot and producing fumonisin mycotoxins during infection. Ingestion of fumonisin-contaminated corn causes underperformance and even fatal toxicity in livestock and is associated with neural tube birth defects, growth stunting in children, and some cancers. StuA, an APSES-class transcription factor, is a major developmental transcriptional regulator in fungi. It has been shown to regulate crucial developmental processes, such as sporulation, virulence, and mycotoxin synthesis among others. In this study, the role of FvSTUA in F. verticillioides was examined by characterizing ∆FvstuA deletion mutants functionally and transcriptomally. The deletion mutants exhibited reduced vegetative growth, stunted aerial hyphae, and significant reductions in microconidiation. Macroconidiation and hydrophobicity of the deletion strains were reduced as well. Additionally, fumonisin production and virulence of the deletion mutants were greatly reduced. Transcriptomic analysis revealed downregulation of expression of several genes in the fumonisin and fusarin C biosynthetic clusters and differential expression of genes involved in conidiation and virulence. Nuclear localization of FvSTUA supported its likely function as a transcription factor. Together, our results indicate that FvSTUA plays a global role in transcriptional regulation in F. verticillioides influencing morphogenesis, toxin production, and virulence.

Molecular Mechanisms Controlling the Disease Cycle in the Vascular Pathogen Verticillium dahliae Characterized Through Forward Genetics and Transcriptomics

The soil-borne pathogen Verticillium dahliae has a worldwide distribution and a plethora of hosts of agronomic value. Molecular analysis of virulence processes can identify targets for disease control. In this work, we compared the global gene transcription profile of random T-DNA insertion mutant strain D-10-8F, which exhibits reduced virulence and alterations in microsclerotium formation and polar growth, with that of the wild-type strain. Three genes identified as differentially expressed were selected for functional characterization. To produce deletion mutants, we developed an updated version of one-step construction of Agrobacterium-recombination-ready plasmids (OSCAR) that included the negative selection marker HSVtk (herpes simplex virus thymidine kinase gene) to prevent ectopic integration of the deletion constructs. Deletion of VdRGS1 (VDAG_00683), encoding a regulator of G protein signaling (RGS) protein and highly upregulated in the wild type versus D-10-8F, resulted in phenotypic alterations in development and virulence that were indistinguishable from those of the random T-DNA insertion mutant. In contrast, deletion of the other two genes selected, vrg1 (VDAG_07039) and vvs1 (VDAG_01858), showed that they do not play major roles in morphogenesis or virulence in V. dahliae. Taken together the results presented here on the transcriptomic analysis and phenotypic characterization of D-10-8F and ∆VdRGS1 strains provide evidence that variations in G protein signaling control the progression of the disease cycle in V. dahliae. We propose that G protein-mediated signals induce the expression of multiple virulence factors during biotrophic growth, whereas massive production of microsclerotia at late stages of infection requires repression of G protein signaling via upregulation of VdRGS1 activity.

Citrus Polyamines: Structure, Biosynthesis, and Physiological Functions

Polyamines (PAs) are ubiquitous biogenic amines found in all living organisms from bacteria to Archaea, and Eukaryotes including plants and animals. Since the first description of putrescine conjugate, feruloyl-putrescine (originally called subaphylline), from grapefruit leaves and juice, many research studies have highlighted the importance of PAs in growth, development, and other physiological processes in citrus plants. PAs appear to be involved in a wide range of physiological processes in citrus plants; however, their exact roles are not fully understood. Accordingly, in the present review, we discuss the biosynthesis of PAs in citrus plants, with an emphasis on the recent advances in identifying and characterizing PAs-biosynthetic genes and other upstream regulatory genes involved in transcriptional regulation of PAs metabolism. In addition, we will discuss the recent metabolic, genetic, and molecular evidence illustrating the roles of PAs metabolism in citrus physiology including somatic embryogenesis; root system formation, morphology, and architecture; plant growth and shoot system architecture; inflorescence, flowering, and flowering-associated events; fruit set, development, and quality; stomatal closure and gas-exchange; and chlorophyll fluorescence and photosynthesis. We believe that the molecular and biochemical understanding of PAs metabolism and their physiological roles in citrus plants will help citrus breeding programs to enhance tolerance to biotic and abiotic stresses and provide bases for further research into potential applications.

Novel Aspects on The Interaction Between Grapevine and Plasmopara viticola: Dual-RNA-Seq Analysis Highlights Gene Expression Dynamics in The Pathogen and The Plant During The Battle For Infection

Mgaloblishvili, a Vitis vinifera cultivar, exhibits unique resistance traits against Plasmopara viticola, the downy mildew agent. This offers the unique opportunity of exploring the molecular responses in compatible and incompatible plant-pathogen interaction. In this study, whole transcriptomes of Mgaloblishvili, Pinot noir (a V. vinifera susceptible cultivar), and Bianca (a resistant hybrid) leaves, inoculated and non-inoculated with the pathogen, were used to identify P. viticola effector-encoding genes and plant susceptibility/resistance genes. Multiple effector-encoding genes were identified in P. viticola transcriptome, with remarkable expression differences in relation to the inoculated grapevine cultivar. Intriguingly, five apoplastic effectors specifically associated with resistance in V. vinifera. Gene coexpression network analysis identified specific modules and metabolic changes occurring during infection in the three grapevine cultivars. Analysis of these data allowed, for the first time, the detection in V. vinifera of a putative P. viticola susceptibility gene, encoding a LOB domain-containing protein. Finally, the de novo assembly of Mgaloblishvili, Pinot noir, and Bianca transcriptomes and their comparison highlighted novel candidate genes that might be at the basis of the resistant phenotype. These results open the way to functional analysis studies and to new perspectives in molecular breeding of grapevine for resistance to P. viticola.

Bioremediation of Dichlorodiphenyltrichloroethane (DDT)-Contaminated Agricultural Soils: Potential of Two Autochthonous Saprotrophic Fungal Strains

DDT (dichlorodiphenyltrichloroethane) was used worldwide as an organochlorine insecticide to control agricultural pests and vectors of several insect-borne human diseases. It was banned in most industrialized countries; however, due to its persistence in the environment, DDT residues remain in environmental compartments, becoming long-term sources of exposure. To identify and select fungal species suitable for bioremediation of DDT-contaminated sites, soil samples were collected from DDT-contaminated agricultural soils in Poland, and 38 fungal taxa among 18 genera were isolated. Two of them, Trichoderma hamatum FBL 587 and Rhizopus arrhizus FBL 578, were tested for tolerance in the presence of 1-mg liter^-1 DDT concentration by using two indices based on fungal growth rate and biomass production (the tolerance indices Rt:Rc and TI), showing a clear tolerance to DDT. The two selected strains were studied to evaluate catabolic versatility on 95 carbon sources with or without DDT by using the Phenotype MicroArray system and to investigate the induced oxidative stress responses. The two strains were able to use most of the substrates provided, resulting in both high metabolic versatility and ecological functionality in the use of carbon sources, despite the presence of DDT. The activation of specific metabolic responses with species-dependent antioxidant enzymes to cope with the induced chemical stress has been hypothesized, since the presence of DDT promoted a higher formation of reactive oxygen species in fungal cells than the controls. The tested fungi represent attractive potential candidates for bioremediation of DDT-contaminated soil and are worthy of further investigations.IMPORTANCE The spread and environmental accumulation of DDT over the years represent not only a threat to human health and ecological security but also a major challenge because of the complex chemical processes and technologies required for remediation. Saprotrophic fungi, isolated from contaminated sites, hold promise for their bioremediation potential toward toxic organic compounds, since they might provide an environment-friendly solution to contamination. Once we verified the high tolerance of autochthonous fungal strains to high concentrations of DDT, we showed how fungi from different phyla demonstrate a high metabolic versatility in the presence of DDT. The isolates showed the singular ability to keep their functionality, despite the DDT-induced production of reactive oxygen species.

Polyamine Catabolism in Plants: A Universal Process With Diverse Functions

Polyamine (PA) catabolic processes are performed by copper-containing amine oxidases (CuAOs) and flavin-containing PA oxidases (PAOs). So far, several CuAOs and PAOs have been identified in many plant species. These enzymes exhibit different subcellular localization, substrate specificity, and functional diversity. Since PAs are involved in numerous physiological processes, considerable efforts have been made to explore the functions of plant CuAOs and PAOs during the recent decades. The stress signal transduction pathways usually lead to increase of the intracellular PA levels, which are apoplastically secreted and oxidized by CuAOs and PAOs, with parallel production of hydrogen peroxide (H2O2). Depending on the levels of the generated H2O2, high or low, respectively, either programmed cell death (PCD) occurs or H2O2 is efficiently scavenged by enzymatic/nonenzymatic antioxidant factors that help plants coping with abiotic stress, recruiting different defense mechanisms, as compared to biotic stress. Amine and PA oxidases act further as PA back-converters in peroxisomes, also generating H2O2, possibly by activating Ca2+ permeable channels. Here, the new research data are discussed on the interconnection of PA catabolism with the derived H2O2, together with their signaling roles in developmental processes, such as fruit ripening, senescence, and biotic/abiotic stress reactions, in an effort to elucidate the mechanisms involved in crop adaptation/survival to adverse environmental conditions and to pathogenic infections.

Polyamines levels increase in smut teliospores after contact with sugarcane glycoproteins as a plant defensive mechanism

Previous studies have already highlighted the correlation between Sporisorium scitamineum pathogenicity and sugarcane polyamine accumulation. It was shown that high infectivity correlates with an increase in the amount of spermidine, spermine and cadaverine conjugated to phenols in the sensitive cultivars whereas resistant plants mainly produce free putrescine. However, these previous studies did not clarify the role of these polyamides in the disorders caused to the plant. Therefore, the purpose of this research is to clarify the effect of polyamines on the development of smut disease. In this paper, commercial polyamines were firstly assayed on smut teliospores germination. Secondly, effects were correlated to changes in endogenous polyamines after contact with defense sugarcane glycoproteins. Low concentrations of spermidine significantly activated teliospore germination, while putrescine had no activating effect on germination. Interestingly, it was observed that the diamine caused nuclear decondensation and breakage of the teliospore cell wall whereas the treatment of teliospores with spermidine did not induce nuclear decondensation or cell wall breakdown. Moreover, the number of polymerized microtubules increased in the presence of 7.5 mM spermidine but it decreased with putrescine which indicates that polyamines effects on Sporisorium scitamineum teliospore germination could be mediated through microtubules interaction. An increased production of polyamines in smut teliospores has been related to sugarcane resistance to the disease. Teliospores incubation with high molecular mass glycoproteins (HMMG) from the uninoculated resistant variety of sugarcane, Mayari 55-14, caused an increase of the insoluble fraction of putrescine, spermidine and spermine inside the teliospore cells. Moreover, the level of the soluble fraction of spermidine (S fraction) increased inside teliospores and the excess was released to the medium. The HMMG glycoproteins purified from Mayarí 55-14 plants previously inoculated with the pathogen significantly increased the levels of both retained and secreted soluble putrescine and spermidine. Polyamines levels did not increase in teliospores after incubation with HMMG produced by non resistant variety Barbados 42231 which could be related to the incapacity of these plants to defend themselves against smut disease. Thus, a hypothesis about the role of polyamines in sugarcane-smut interaction is explained.

Contribution of Maize Polyamine and Amino Acid Metabolism Toward Resistance Against Aspergillus flavus Infection and Aflatoxin Production

Polyamines (PAs) are ubiquitous polycations found in plants and other organisms that are essential for growth, development, and resistance against abiotic and biotic stresses. The role of PAs in plant disease resistance depends on the relative abundance of higher PAs [spermidine (Spd), spermine (Spm)] vs. the diamine putrescine (Put) and PA catabolism. With respect to the pathogen, PAs are required to achieve successful pathogenesis of the host. Maize is an important food and feed crop, which is highly susceptible to Aspergillus flavus infection. Upon infection, the fungus produces carcinogenic aflatoxins and numerous other toxic secondary metabolites that adversely affect human health and crop value worldwide. To evaluate the role of PAs in aflatoxin resistance in maize, in vitro kernel infection assays were performed using maize lines that are susceptible (SC212) or resistant (TZAR102, MI82) to aflatoxin production. Results indicated significant induction of both PA biosynthetic and catabolic genes upon A. flavus infection. As compared to the susceptible line, the resistant maize lines showed higher basal expression of PA metabolism genes in mock-inoculated kernels that increased upon fungal infection. In general, increased biosynthesis and conversion of Put to Spd and Spm along with their increased catabolism was evident in the resistant lines vs. the susceptible line SC212. There were higher concentrations of amino acids such as glutamate (Glu), glutamine (Gln) and γ-aminobutyric acid (GABA) in SC212. The resistant lines were significantly lower in fungal load and aflatoxin production as compared to the susceptible line. The data presented here demonstrate an important role of PA metabolism in the resistance of maize to A. flavus colonization and aflatoxin contamination. These results provide future direction for the manipulation of PA metabolism in susceptible maize genotypes to improve aflatoxin resistance and overall stress tolerance.

Computational analysis of the Plasmodiophora brassicae genome: mitochondrial sequence description and metabolic pathway database design

Plasmodiophora brassicae is an obligate biotrophic pathogenic protist responsible for clubroot, a root gall disease of Brassicaceae species. In addition to the reference genome of the P. brassicae European e3 isolate and the draft genomes of Canadian or Chinese isolates, we present the genome of eH, a second European isolate. Refinement of the annotation of the eH genome led to the identification of the mitochondrial genome sequence, which was found to be bigger than that of Spongospora subterranea, another plant parasitic Plasmodiophorid phylogenetically related to P. brassicae. New pathways were also predicted, such as those for the synthesis of spermidine, a polyamine up-regulated in clubbed regions of roots. A P. brassicae pathway genome database was created to facilitate the functional study of metabolic pathways in transcriptomics approaches. These available tools can help in our understanding of the regulation of P. brassicae metabolism during infection and in response to diverse constraints.

Leveraging single-cell genomics to expand the fungal tree of life

Environmental DNA surveys reveal that most fungal diversity represents uncultured species. We sequenced the genomes of eight uncultured species across the fungal tree of life using a new single-cell genomics pipeline. We show that, despite a large variation in genome and gene space recovery from each single amplified genome (SAG), ≥90% can be recovered by combining multiple SAGs. SAGs provide robust placement for early-diverging lineages and infer a diploid ancestor of fungi. Early-diverging fungi share metabolic deficiencies and show unique gene expansions correlated with parasitism and unculturability. Single-cell genomics holds great promise in exploring fungal diversity, life cycles and metabolic potential.

The Aspergillus flavus Spermidine Synthase (spds) Gene, Is Required for Normal Development, Aflatoxin Production, and Pathogenesis During Infection of Maize Kernels

Aspergillus flavus is a soil-borne saprophyte and an opportunistic pathogen of both humans and plants. This fungus not only causes disease in important food and feed crops such as maize, peanut, cottonseed, and tree nuts but also produces the toxic and carcinogenic secondary metabolites (SMs) known as aflatoxins. Polyamines (PAs) are ubiquitous polycations that influence normal growth, development, and stress responses in living organisms and have been shown to play a significant role in fungal pathogenesis. Biosynthesis of spermidine (Spd) is critical for cell growth as it is required for hypusination-mediated activation of eukaryotic translation initiation factor 5A (eIF5A), and other biochemical functions. The tri-amine Spd is synthesized from the diamine putrescine (Put) by the enzyme spermidine synthase (Spds). Inactivation of spds resulted in a total loss of growth and sporulation in vitro which could be partially restored by addition of exogenous Spd. Complementation of the Δspds mutant with a wild type (WT) A. flavus spds gene restored the WT phenotype. In WT A. flavus, exogenous supply of Spd (in vitro) significantly increased the production of sclerotia and SMs. Infection of maize kernels with the Δspds mutant resulted in a significant reduction in fungal growth, sporulation, and aflatoxin production compared to controls. Quantitative PCR of Δspds mutant infected seeds showed down-regulation of aflatoxin biosynthetic genes in the mutant compared to WT A. flavus infected seeds. Expression analyses of PA metabolism/transport genes during A. flavus-maize interaction showed significant increase in the expression of arginine decarboxylase (Adc) and S-adenosylmethionine decarboxylase (Samdc) genes in the maize host and PA uptake transporters in the fungus. The results presented here demonstrate that Spd biosynthesis is critical for normal development and pathogenesis of A. flavus and pre-treatment of a Δspds mutant with Spd or Spd uptake from the host plant, are insufficient to restore WT levels of pathogenesis and aflatoxin production during seed infection. The data presented here suggest that future studies targeting spermidine biosynthesis in A. flavus, using RNA interference-based host-induced gene silencing approaches, may be an effective strategy to reduce aflatoxin contamination in maize and possibly in other susceptible crops.

Chemical suppressors of mlo-mediated powdery mildew resistance

Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2',3'-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.

In silico characterization of DNA motifs associated with the differential expression of the ornithine decarboxylase gene during in vitro cystocarp development in the red seaweed Grateloupia imbricata

To gain a better understanding of the regulatory mechanism(s) modulating expression of the ornithine decarboxylase gene ODC during cystocarp development in the red seaweed Grateloupia imbricata, DNA motifs found in the 5'-upstream region of the gene were identified by in silico analysis. In addition, when infertile G. imbricata thalli were treated with ethylene, methyl jasmonate, or light as an elicitor of cystocarp development, different ODC expression patterns were observed. ODC expression correlated with (i) the elicitation (treatment) period and the period post-treatment just prior to observation of the first visible developing cystocarps (disclosure period), and (ii) the type of elicitor. Ethylene and light activated ODC expression during the elicitation period, and methyl jasmonate activated its expression during the disclosure period, suggesting that initiation and cystocarp development may involve more than one signaling pathway. In addition, expression of ODC was 450-fold greater when thalli were stimulated by ethylene compared with untreated control thalli, suggesting that G. imbricata mounts an efficient response to sense and activate ethylene-responsive signaling pathways. The patterns of differential ODC expression induced by the different elicitors during cystocarp development might provide an useful tool for characterizing the precise transcriptional regulation of ODC in G. imbricata.

A Fusarium graminearum strain-comparative proteomic approach identifies regulatory changes triggered by agmatine

Plant pathogens face different environmental clues depending on the stage of the infection cycle they are in. Fusarium graminearum infects small grain cereals producing trichothecenes type B (TB) that act as virulence factor in the interaction with the plant and have important food safety implications. This study addresses at the proteomic level the effect of an environmental stimulus (such as the presence of a polyamine like agmatine) possibly encountered by the fungus when it is already within the plant. Because biological diversity affects the proteome significantly, a multistrain (n=3) comparative approach was used to identify consistent effects caused on the fungus by the nitrogen source (agmatine or glutamic acid). Proteomics analyses were performed by the use of 2D-DIGE. Results showed that agmatine augmented TB production but not equally in all strains. The polyamine reshaped drastically the proteome of the fungus activating specific pathways linked to the translational control within the cell. Chromatin restructuring, ribosomal regulations, protein and mRNA processing enzymes were modulated by the agmatine stimulus as well as metabolic, structural and virulence-related proteins, suggesting the need to reshape specifically the fungal cell for TB production, a key step for the pathogen spread within the spike.

BIOLOGICAL SIGNIFICANCE

Induction of toxin synthesis by plant compounds plays a crucial role in toxin contamination of food and feed, in particular trichothecenes type B produced mainly by F. graminearum on wheat. This work describes the level of diversity of 3 strains facing 2 toxin inducing plant derived compounds. This knowledge is of use for the research community on toxigenic Fusarium strains in cereals for understanding the role of fungal diversity in toxin inducibility. This work also suggests that environmental clues that can be found within the plant during infection (like different nitrogen compounds) are crucial stimuli for reshaping the proteome profile and consequently the specialization profiling of the fungus, ultimately leading to very different toxin contamination levels in the plant.

The effect of agmatine on trichothecene type B and zearalenone production in Fusarium graminearum, F. culmorum and F. poae

Agmatine and other putrescines are known for being strong inducers of deoxynivalenol (DON) production in Fusarium graminearum. Other important species produce DON and/or other trichothecene type B toxins (3 acetylated DON, 15 acetylated DON, Fusarenon-X, Nivalenol), such as F. culmorum and F. poae. In order to verify whether the mechanism of the regulation of trichothecene type B induction by agmatine is shared by different species of Fusarium, we tested the hypothesis on 19 strains belonging to 3 Fusarium species (F. graminearum, F. culmorum, F. poae) with diverse genetic chemotypes (3ADON, 15ADON, NIV) by measuring trichothecene B toxins such as DON, NIV, Fusarenon-X, 3ADON and 15ADON. Moreover, we tested whether other toxins like zearalenone were also boosted by agmatine. The trichothecene type B boosting effect was observed in the majority of strains (13 out of 19) in all the three species. Representative strains from all three genetic chemotypes were able to boost toxin production after agmatine treatment. We identified the non-responding strains to the agmatine stimulus, which may contribute to deciphering the regulatory mechanisms that link toxin production to agmatine (and, more generally, polyamines).

Genomic Patterns of Positive Selection at the Origin of Rust Fungi

Understanding the origin and evolution of pathogenicity and biotrophic life-style of rust fungi has remained a conundrum for decades. Research on the molecular mechanisms responsible for rust fungi evolution has been hampered by their biotrophic life-style until the sequencing of some rust fungi genomes. With the availability of multiple whole genomes and EST data for this group, it is now possible to employ genome-wide surveys and investigate how natural selection shaped their evolution. In this work, we employed a phylogenomics approach to search for positive selection and genes undergoing accelerated evolution at the origin of rust fungi on an assembly of single copy genes conserved across a broad range of basidiomycetes. Up to 985 genes were screened for positive selection on the phylogenetic branch leading to rusts, revealing a pervasive signal of positive selection throughout the data set with the proportion of positively selected genes ranging between 19.6–33.3%. Additionally, 30 genes were found to be under accelerated evolution at the origin of rust fungi, probably due to a mixture of positive selection and relaxation of purifying selection. Functional annotation of the positively selected genes revealed an enrichment in genes involved in the biosynthesis of secondary metabolites and several metabolism and transporter classes.

Disruption of the GABA shunt affects mitochondrial respiration and virulence in the cereal pathogen Fusarium graminearum

The cereal pathogen Fusarium graminearum threatens food and feed production worldwide. It reduces the yield and poisons the remaining kernels with mycotoxins, notably deoxynivalenol (DON). We analyzed the importance of gamma-aminobutanoic acid (GABA) metabolism for the life cycle of this fungal pathogen. GABA metabolism in F. graminearum is partially regulated by the global nitrogen regulator AreA. Genetic disruption of the GABA shunt by deletion of two GABA transaminases renders the pathogen unable to utilize the plant stress metabolites GABA and putrescine. The mutants showed increased sensitivity against oxidative stress, GABA accumulation in the mycelium, downregulation of two key enzymes of the TCA cycle, disturbed potential gradient in the mitochondrial membrane and lower mitochondrial oxygen consumption. In contrast, addition of GABA to the wild type resulted in its rapid turnover and increased mitochondrial steady state oxygen consumption. GABA concentrations are highly upregulated in infected wheat tissues. We conclude that GABA is metabolized by the pathogen during infection increasing its energy production, whereas the mutants accumulate GABA intracellularly resulting in decreased energy production. Consequently, the GABA mutants are strongly reduced in virulence but, because of their DON production, are able to cross the rachis node.

Transcriptomic analysis of the GCN5 gene reveals mechanisms of the epigenetic regulation of virulence and morphogenesis in Ustilago maydis

Chromatin in the eukaryotic nucleus is highly organized in the form of nucleosomes where histones wrap DNA. This structure may be altered by some chemical modifications of histones, one of them, acetylation by histone acetyltransferases (HATs) that originates relaxation of the nucleosome structure, providing access to different transcription factors and other effectors. In this way, HATs regulate cellular processes including DNA replication, and gene transcription. Previously, we isolated Ustilago maydis mutants deficient in the GCN5 HAT that are avirulent, and grow constitutively as mycelium. In this work, we proceeded to identify the genes differentially regulated by GCN5, comparing the transcriptomes of the mutant and the wild type using microarrays, to analyse the epigenetic control of virulence and morphogenesis. We identified 1203 genes, 574 positively and 629 negatively regulated in the wild type. We found that genes belonging to different categories involved in pathogenesis were downregulated in the mutant, and that genes involved in mycelial growth were negatively regulated in the wild type, offering a working hypothesis on the epigenetic control of virulence and morphogenesis of U. maydis. Interestingly, several differentially regulated genes appeared in clusters, suggesting a common regulation. Some of these belonged to pathogenesis or secondary metabolism.

Common and metal-specific proteomic responses to cadmium and zinc in the metal tolerant ericoid mycorrhizal fungus Oidiodendron maius Zn

Although adaptive metal tolerance may arise in fungal populations in polluted soils, the mechanisms underlying metal-specific tolerance are poorly understood. Comparative proteomics is a powerful tool to identify variation in protein profiles caused by changing environmental conditions, and was used to investigate protein accumulation in a metal tolerant isolate of the ericoid mycorrhizal fungus Oidiodendron maius exposed to zinc and cadmium. Two-dimensional gel electrophoresis and shotgun proteomics followed by mass spectrometry lead to the identification of common and metal-specific proteins and pathways. Proteins selectively induced by cadmium exposure were molecular chaperons of the Hsp90 family, cytoskeletal proteins and components of the translation machinery. Zinc significantly up-regulated metabolic pathways related to energy production and carbohydrates metabolism, likely mirroring zinc adaptation of this fungal isolate. Common proteins induced by the two metal ions were the antioxidant enzyme Cu/Zn superoxide dismutase and ubiquitin. In mycelia exposed to zinc and cadmium, both proteomic techniques also identified agmatinase, an enzyme involved in polyamine biosynthesis. This novel finding suggests that, like plants, polyamines may have important functions in response to abiotic environmental stress in fungi. Genetic evidence also suggests that the biosynthesis of polyamines via an alternative metabolic pathway may be widespread in fungi.

Potential mechanisms underlying response to effects of the fungicide pyrimethanil from gene expression profiling in Saccharomyces cerevisiae

Pyrimethanil is a fungicide mostly applied in vineyards. When misused, residue levels detected in grape must or in the environment may be of concern. The present work aimed to analyze mechanisms underlying response to deleterious effects of pyrimethanil in the eukaryotic model Saccharomyces cerevisiae. Pyrimethanil concentration-dependent effects at phenotypic (inhibition of growth) and transcriptomic levels were examined. For transcriptional profiling, analysis focused on two sublethal exposure conditions that inhibited yeast growth by 20% or 50% compared with control cells not exposed to the fungicide. Gene expression modifications increased with the magnitude of growth inhibition, in numbers and fold-change of differentially expressed genes and in diversity of over-represented functional categories. These included mostly biosynthesis of arginine and sulfur amino acids metabolism, as well as energy conservation, antioxidant response, and multidrug transport. Several pyrimethanil-responsive genes encoded proteins sharing significant homology with proteins from phytopathogenic fungi and ecologically relevant higher eukaryotes.

Stress and polyamine metabolism in fungi

Fungi, as well as the rest of living organisms must deal with environmental challenges such as stressful stimuli. Fungi are excellent models to study the general mechanisms of the response to stress, because of their simple, but conserved, signal-transduction and metabolic pathways that are often equivalent to those present in other eukaryotic systems. A factor that has been demonstrated to be involved in these responses is polyamine metabolism, essentially of the three most common polyamines: putrescine, spermidine and spermine. The gathered evidences on this subject suggest that polyamines are able to control cellular signal transduction, as well as to modulate protein-protein interactions. In the present review, we will address the recent advances on the study of fungal metabolism of polyamines, ranging from mutant characterization to potential mechanism of action during different kinds of stress in selected fungal models.

Molecular mechanisms of paraquat-induced acute lung injury: a current review

Paraquat is an organic heterocyclic herbicide that is widely used in agriculture, especially in Asian countries. The prevalence of paraquat poisonings has increased dramatically in the past two decades in China. Nearly all paraquat poisonings resulted from intentional or accidental oral administration leading to acute lung injury and, ultimately, acute respiratory distress syndrome. The mortality rate has been reported to be greater than 90%. However, the exact toxic mechanism remains unclear. Herein, we reviewed and summarized the most recent publications related to the molecular mechanisms of paraquat-induced acute lung injury.

Transcriptomic analysis of the dimorphic transition of Ustilago maydis induced in vitro by a change in pH

Dimorphism is the property of fungi to grow as budding yeasts or mycelium, depending on the environmental conditions. This phenomenon is important as a model of differentiation in eukaryotic organisms, and since a large number of fungal diseases are caused by dimorphic fungi, its study is important for practical reasons. In this work, we examined the transcriptome during the dimorphic transition of the basidiomycota phytopathogenic fungus Ustilago maydis using microarrays, utilizing yeast and mycelium monomorphic mutants as controls. This way, we thereby identified 154 genes of the fungus that are specifically involved in the dimorphic transition induced by a pH change. Of these, 82 genes were up-regulated, and 72 were down-regulated. Differential categorization of these genes revealed that they mostly belonged to the classes of metabolism, cell cycle and DNA processing, transcription and protein fate, transport and cellular communication, stress, cell differentiation and biogenesis of cellular components, while a significant number of them corresponded to unclassified proteins. The data reported in this work are important for our understanding of the molecular bases of dimorphism in U. maydis, and possibly of other fungi.

Candida albicans flu1-mediated efflux of salivary histatin 5 reduces its cytosolic concentration and fungicidal activity

Histatin 5 (Hst 5) is a salivary human antimicrobial peptide that is toxic to the opportunistic yeast Candida albicans. Fungicidal activity of Hst 5 requires intracellular translocation and accumulation to a threshold concentration for it to disrupt cellular processes. Previously, we observed that total cytosolic levels of Hst 5 were gradually reduced from intact cells, suggesting that C. albicans possesses a transport mechanism for efflux of Hst 5. Since we identified C. albicans polyamine transporters responsible for Hst 5 uptake, we hypothesized that one or more polyamine efflux transporters may be involved in the efflux of Hst 5. C. albicans FLU1 and TPO2 were found to be the closest homologs of Saccharomyces cerevisiae TPO1, which encodes a major spermidine efflux transporter, indicating that the products of these two genes may be involved in efflux of Hst 5. We found that flu1Δ/Δ cells, but not tpo2Δ/Δ cells, had significant reductions in their rates of Hst 5 efflux and had significantly higher cytoplasmic Hst 5 and Hst 5 susceptibilities than did the wild type. We also found that flu1Δ/Δ cells had reduced biofilm formation compared to wild-type cells in the presence of Hst 5. Transcriptional levels of FLU1 were not altered over the course of treatment with Hst 5; therefore, Hst 5 is not likely to induce FLU1 gene overexpression as a potential mechanism of resistance. Thus, Flu1, but not Tpo2, mediates efflux of Hst 5 and is responsible for reduction of its toxicity in C. albicans.