A novel dehydrin-like protein from Aspergillus fumigatus regulates freezing tolerance

Heterologous overexpression of Tawzy1-2 gene encoding an SK3 dehydrin enhances multiple abiotic stress tolerance in Escherichia coli and Nicotiania benthamiana

MAIN CONCLUSION

The nuclear localized TaWZY1-2 helps plants resist abiotic stress by preserving the cell's ability to remove reactive oxygen species and decrease lipid oxidation under such conditions. In light of the unpredictable environmental conditions in which food crops grow, precise strategies must be developed by crops to effectively cope with abiotic stress and minimize damage over their lifespan. A key component in this endeavor is the group II of late embryogenesis abundant (LEA) proteins, known as dehydrins, which play crucial roles in enhancing the tolerance of plants to abiotic stress. Tawzy1-2 is a dehydrin-encoding gene which is constitutively expressed in various tissues of wheat. However, the biological function of TaWZY1-2 is not yet fully understood. In this study, TaWZY1-2 was isolated and identified in the wheat genome, and its functional role in conferring tolerance to abiotic stresses was detected in both prokaryotic and eukaryotic cells. Results showed that TaWZY1-2 is a nuclear localized hydrophilic protein that accumulates in response to multiple stresses. Escherichia coli cells expressing TaWZY1-2 showed enhanced tolerance to multiple stress conditions. Overexpression of TaWZY1-2 in Nicotiania benthamiana improved growth, germination and survival rate of the transgenic plants exposed to four kinds of abiotic stress conditions. Our results show that Tawzy1-2 transgenic plants exhibit improved capability in clearing reactive oxygen species and reducing lipid degradation, thereby enhancing their resistance to abiotic stress. This demonstrates a significant role of TaWZY1-2 in mitigating abiotic stress-induced damage. Consequently, these findings not only establish a basis for future investigation into the functional mechanism of TaWZY1-2 but also contribute to the expansion of functional diversity within the dehydrin protein family. Moreover, they identify potential candidate genes for crop optimization.

Distinct transcriptional responses to fludioxonil in Aspergillus fumigatus and its ΔtcsC and Δskn7 mutants reveal a crucial role for Skn7 in the cell wall reorganizations triggered by this antifungal

BACKGROUND

Aspergillus fumigatus is a major fungal pathogen that causes severe problems due to its increasing resistance to many therapeutic agents. Fludioxonil is a compound that triggers a lethal activation of the fungal-specific High Osmolarity Glycerol pathway. Its pronounced antifungal activity against A. fumigatus and other pathogenic molds renders this agent an attractive lead substance for the development of new therapeutics. The group III hydride histidine kinase TcsC and its downstream target Skn7 are key elements of the multistep phosphorelay that represents the initial section of the High Osmolarity Glycerol pathway. Loss of tcsC results in resistance to fludioxonil, whereas a Δskn7 mutant is partially, but not completely resistant.

RESULTS

In this study, we compared the fludioxonil-induced transcriptional responses in the ΔtcsC and Δskn7 mutant and their parental A. fumigatus strain. The number of differentially expressed genes correlates well with the susceptibility level of the individual strains. The wild type and, to a lesser extend also the Δskn7 mutant, showed a multi-faceted stress response involving genes linked to ribosomal and peroxisomal function, iron homeostasis and oxidative stress. A marked difference between the sensitive wild type and the largely resistant Δskn7 mutant was evident for many cell wall-related genes and in particular those involved in the biosynthesis of chitin. Biochemical data corroborate this differential gene expression that does not occur in response to hyperosmotic stress.

CONCLUSIONS

Our data reveal that fludioxonil induces a strong and TcsC-dependent stress that affects many aspects of the cellular machinery. The data also demonstrate a link between Skn7 and the cell wall reorganizations that foster the characteristic ballooning and the subsequent lysis of fludioxonil-treated cells.

AGC/AKT Protein Kinase SCH9 Is Critical to Pathogenic Development and Overwintering Survival in Magnaporthe oryzae

Primary inoculum that survives overwintering is one of the key factors that determine the outbreak of plant disease. Pathogenic resting structures, such as chlamydospores, are an ideal inoculum for plant disease. Puzzlingly, Magnaporthe oryzae, a devastating fungal pathogen responsible for blast disease in rice, hardly form any morphologically changed resting structures, and we hypothesize that M. oryzae mainly relies on its physiological alteration to survive overwintering or other harsh environments. However, little progress on research into regulatory genes that facilitate the overwintering of rice blast pathogens has been made so far. Serine threonine protein kinase AGC/AKT, MoSch9, plays an important role in the spore-mediated pathogenesis of M. oryzae. Building on this finding, we discovered that in genetic and biological terms, MoSch9 plays a critical role in conidiophore stalk formation, hyphal-mediated pathogenesis, cold stress tolerance, and overwintering survival of M. oryzae. We discovered that the formation of conidiophore stalks and disease propagation using spores was severely compromised in the mutant strains, whereas hyphal-mediated pathogenesis and the root infection capability of M. oryzae were completely eradicated due to MoSch9 deleted mutants’ inability to form an appressorium-like structure. Most importantly, the functional and transcriptomic study of wild-type and MoSch9 mutant strains showed that MoSch9 plays a regulatory role in cold stress tolerance of M. oryzae through the transcription regulation of secondary metabolite synthesis, ATP hydrolyzing, and cell wall integrity proteins during osmotic stress and cold temperatures. From these results, we conclude that MoSch9 is essential for fungal infection-related morphogenesis and overwintering of M. oryzae.

Genetic Diversity and Dispersal of Aspergillus fumigatus in Arctic Soils

Aspergillus fumigatus is a saprophytic mold and an opportunistic pathogen with a broad geographic and ecological distribution. A. fumigatus is the most common etiological agent of aspergillosis, affecting over 8,000,000 individuals worldwide. Due to the rising number of infections and increasing reports of resistance to antifungal therapy, there is an urgent need to understand A. fumigatus populations from local to global levels. However, many geographic locations and ecological niches remain understudied, including soil environments from arctic regions. In this study, we isolated 32 and 52 A. fumigatus strains from soils in Iceland and the Northwest Territories of Canada (NWT), respectively. These isolates were genotyped at nine microsatellite loci and the genotypes were compared with each other and with those in other parts of the world. Though significantly differentiated from each other, our analyses revealed that A. fumigatus populations from Iceland and NWT contained evidence for both clonal and sexual reproductions, and shared many alleles with each other and with those collected from across Europe, Asia, and the Americas. Interestingly, we found one triazole-resistant strain containing the TR₃₄ /L98H mutation in the cyp51A gene from NWT. This strain is closely related to a triazole-resistant genotype broadly distributed in India. Together, our results suggest that the northern soil populations of A. fumigatus are significantly influenced by those from other geographic regions.

The high osmolarity glycerol (HOG) pathway in fungi†

While fungi are widely occupying nature, many species are responsible for devastating mycosis in humans. Such niche diversity explains how quick fungal adaptation is necessary to endow the capacity of withstanding fluctuating environments and to cope with host-imposed conditions. Among all the molecular mechanisms evolved by fungi, the most studied one is the activation of the phosphorelay signalling pathways, of which the high osmolarity glycerol (HOG) pathway constitutes one of the key molecular apparatus underpinning fungal adaptation and virulence. In this review, we summarize the seminal knowledge of the HOG pathway with its more recent developments. We specifically described the HOG-mediated stress adaptation, with a particular focus on osmotic and oxidative stress, and point out some lags in our understanding of its involvement in the virulence of pathogenic species including, the medically important fungi Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, compared to the model yeast Saccharomyces cerevisiae. Finally, we also highlighted some possible applications of the HOG pathway modifications to improve the fungal-based production of natural products in the industry.

The Putative APSES Transcription Factor RgdA Governs Growth, Development, Toxigenesis, and Virulence in Aspergillus fumigatus

Immunocompromised patients are susceptible to infections with the opportunistic human-pathogenic fungus Aspergillus fumigatus. This fungus causes systemic infections such as invasive aspergillosis (IA), which is one of the most life-threatening fungal diseases. To control this serious disease, it is critical to identify new antifungal drug targets. In fungi, the transcriptional regulatory proteins of the APSES family play crucial roles in controlling various biological processes, including mating, asexual sporulation and dimorphic growth, and virulence traits. This study found that a putative APSES transcription factor, RgdA, regulates normal growth, asexual development, conidium germination, spore wall architecture and hydrophobicity, toxin production, and virulence in A. fumigatus. Better understanding the molecular mechanisms of RgdA in human-pathogenic fungi may reveal a novel antifungal target for future drug development.

The APSES transcription factor (TF) in Aspergillus species is known to govern diverse cellular processes, including growth, development, and secondary metabolism. Here, we investigated functions of the rgdA gene (Afu3g13920) encoding a putative APSES TF in the opportunistic human-pathogenic fungus Aspergillus fumigatus. The rgdA deletion resulted in significantly decreased hyphal growth and asexual sporulation. Consistently, transcript levels of the key asexual developmental regulators abaA, brlA, and wetA were decreased in the ΔrgdA mutant compared to those in the wild type (WT). Moreover, ΔrgdA resulted in reduced spore germination rates and elevated transcript levels of genes associated with conidium dormancy. The conidial cell wall hydrophobicity and architecture were changed, and levels of the RodA protein were decreased in the ΔrgdA mutant. Comparative transcriptomic analyses revealed that the ΔrgdA mutant showed higher mRNA levels of gliotoxin (GT)-biosynthetic genes and GT production. While the ΔrgdA mutant exhibited elevated production of GT, ΔrgdA strains showed reduced virulence in the mouse model. In addition, mRNA levels of genes associated with the cyclic AMP (cAMP)-protein kinase A (PKA) signaling pathway and the SakA mitogen-activated protein (MAP) kinase pathway were increased in the ΔrgdA mutant. In summary, RgdA plays multiple roles in governing growth, development, GT production, and virulence which may involve attenuation of PKA and SakA signaling.

IMPORTANCE Immunocompromised patients are susceptible to infections with the opportunistic human-pathogenic fungus Aspergillus fumigatus. This fungus causes systemic infections such as invasive aspergillosis (IA), which is one of the most life-threatening fungal diseases. To control this serious disease, it is critical to identify new antifungal drug targets. In fungi, the transcriptional regulatory proteins of the APSES family play crucial roles in controlling various biological processes, including mating, asexual sporulation and dimorphic growth, and virulence traits. This study found that a putative APSES transcription factor, RgdA, regulates normal growth, asexual development, conidium germination, spore wall architecture and hydrophobicity, toxin production, and virulence in A. fumigatus. Better understanding the molecular mechanisms of RgdA in human-pathogenic fungi may reveal a novel antifungal target for future drug development.

Molecular Mechanisms of Conidial Germination in Aspergillus spp

Aspergilli produce conidia for reproduction or to survive hostile conditions, and they are highly effective in the distribution of conidia through the environment. In immunocompromised individuals, inhaled conidia can germinate inside the respiratory tract, which may result in invasive pulmonary aspergillosis. The management of invasive aspergillosis has become more complex, with new risk groups being identified and the emergence of antifungal resistance. Patient survival is threatened by these developments, stressing the need for alternative therapeutic strategies. As germination is crucial for infection, prevention of this process might be a feasible approach. A broader understanding of conidial germination is important to identify novel antigermination targets. In this review, we describe conidial resistance against various stresses, transition from dormant conidia to hyphal growth, the underlying molecular mechanisms involved in germination of the most common Aspergillus species, and promising antigermination targets. Germination of Aspergillus is characterized by three morphotypes: dormancy, isotropic growth, and polarized growth. Intra- and extracellular proteins play an important role in the protection against unfavorable environmental conditions. Isotropically expanding conidia remodel the cell wall, and biosynthetic machineries are needed for cellular growth. These biosynthetic machineries are also important during polarized growth, together with tip formation and the cell cycle machinery. Genes involved in isotropic and polarized growth could be effective antigermination targets. Transcriptomic and proteomic studies on specific Aspergillus morphotypes will improve our understanding of the germination process and allow discovery of novel antigermination targets and biomarkers for early diagnosis and therapy.

Responses to Hydric Stress in the Seed-Borne Necrotrophic Fungus Alternaria brassicicola

Alternaria brassicicola is a necrotrophic fungus causing black spot disease and is an economically important seed-borne pathogen of cultivated brassicas. Seed transmission is a crucial component of its parasitic cycle as it promotes long-term survival and dispersal. Recent studies, conducted with the Arabidopsis thaliana/A. brassicicola pathosystem, showed that the level of susceptibility of the fungus to water stress strongly influenced its seed transmission ability. In this study, we gained further insights into the mechanisms involved in the seed infection process by analyzing the transcriptomic and metabolomic responses of germinated spores of A. brassicicola exposed to water stress. Then, the repertoire of putative hydrophilins, a group of proteins that are assumed to be involved in cellular dehydration tolerance, was established in A. brassicicola based on the expression data and additional structural and biochemical criteria. Phenotyping of single deletion mutants deficient for fungal hydrophilin-like proteins showed that they were affected in their transmission to A. thaliana seeds, although their aggressiveness on host vegetative tissues remained intact.

Stress-Activated Protein Kinases in Human Fungal Pathogens

The ability of fungal pathogens to survive hostile environments within the host depends on rapid and robust stress responses. Stress-activated protein kinase (SAPK) pathways are conserved MAPK signaling modules that promote stress adaptation in all eukaryotic cells, including pathogenic fungi. Activation of the SAPK occurs via the dual phosphorylation of conserved threonine and tyrosine residues within a TGY motif located in the catalytic domain. This induces the activation and nuclear accumulation of the kinase and the phosphorylation of diverse substrates, thus eliciting appropriate cellular responses. The Hog1 SAPK has been extensively characterized in the model yeast Saccharomyces cerevisiae. Here, we use this a platform from which to compare SAPK signaling mechanisms in three major fungal pathogens of humans, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. Despite the conservation of SAPK pathways within these pathogenic fungi, evidence is emerging that their role and regulation has significantly diverged. However, consistent with stress adaptation being a common virulence trait, SAPK pathways are important pathogenicity determinants in all these major human pathogens. Thus, the development of drugs which target fungal SAPKs has the exciting potential to generate broad-acting antifungal treatments.

Ecology of aspergillosis: insights into the pathogenic potency of Aspergillus fumigatus and some other Aspergillus species

Fungi of the genus Aspergillus are widespread in the environment. Some Aspergillus species, most commonly Aspergillus fumigatus, may lead to a variety of allergic reactions and life-threatening systemic infections in humans. Invasive aspergillosis occurs primarily in patients with severe immunodeficiency, and has dramatically increased in recent years. There are several factors at play that contribute to aspergillosis, including both fungus and host-related factors such as strain virulence and host pulmonary structure/immune status, respectively. The environmental tenacity of Aspergilllus, its dominance in diverse microbial communities/habitats, and its ability to navigate the ecophysiological and biophysical challenges of host infection are attributable, in large part, to a robust stress-tolerance biology and exceptional capacity to generate cell-available energy. Aspects of its stress metabolism, ecology, interactions with diverse animal hosts, clinical presentations and treatment regimens have been well-studied over the past years. Here, we synthesize these findings in relation to the way in which some Aspergillus species have become successful opportunistic pathogens of human- and other animal hosts. We focus on the biophysical capabilities of Aspergillus pathogens, key aspects of their ecophysiology and the flexibility to undergo a sexual cycle or form cryptic species. Additionally, recent advances in diagnosis of the disease are discussed as well as implications in relation to questions that have yet to be resolved.

Fungal spores for dispersion in space and time

Spores are an integral part of the life cycle of the gross majority of fungi. Their morphology and the mode of formation are both highly variable among the fungi, as is their resistance to stressors. The main aim for spores is to be dispersed, both in space, by various mechanisms or in time, by an extended period of dormancy. Some fungal ascospores belong to the most stress-resistant eukaryotic cells described to date. Stabilization is a process in which biomolecules and complexes thereof are protected by different types of molecules against heat, drought, or other molecules. This review discusses the most important compounds that are known to protect fungal spores and also addresses the biophysics of cell protection. It further covers the phenomena of dormancy, breaking of dormancy, and early germination. Germination is the transition from a dormant cell toward a vegetative cell and includes a number of specific changes. Finally, the applied aspects of spore biology are discussed.

Dehydrin-like proteins in the necrotrophic fungus Alternaria brassicicola have a role in plant pathogenesis and stress response

In this study, the roles of fungal dehydrin-like proteins in pathogenicity and protection against environmental stresses were investigated in the necrotrophic seed-borne fungus Alternaria brassicicola. Three proteins (called AbDhn1, AbDhn2 and AbDhn3), harbouring the asparagine-proline-arginine (DPR) signature pattern and sharing the characteristic features of fungal dehydrin-like proteins, were identified in the A. brassicicola genome. The expression of these genes was induced in response to various stresses and found to be regulated by the AbHog1 mitogen-activated protein kinase (MAPK) pathway. A knock-out approach showed that dehydrin-like proteins have an impact mainly on oxidative stress tolerance and on conidial survival upon exposure to high and freezing temperatures. The subcellular localization revealed that AbDhn1 and AbDhn2 were associated with peroxisomes, which is consistent with a possible perturbation of protective mechanisms to counteract oxidative stress and maintain the redox balance in AbDhn mutants. Finally, we show that the double deletion mutant ΔΔabdhn1-abdhn2 was highly compromised in its pathogenicity. By comparison to the wild-type, this mutant exhibited lower aggressiveness on B. oleracea leaves and a reduced capacity to be transmitted to Arabidopsis seeds via siliques. The double mutant was also affected with respect to conidiation, another crucial step in the epidemiology of the disease.

An adenosine kinase in apoplastic location is involved in Magnaporthe oryzae cold acclimation

Cold acclimation is an important process to increase freezing tolerance for over-winter survival in many organisms. The apoplastic area is very important in cold acclimation. Two-dimensional electrophoresis was used to identify apoplastic proteins involved in the cold acclimation process of the filamentous fungus Magnaporthe oryzae, and nine protein spots showed at least 1.5-fold increase during cold treatment. These proteins were further analyzed by matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry. One of these proteins was identified to be an adenosine kinase (MoAK), an ortholog of the adenosine kinase from Saccharomyces cerevisiae. The MoAK gene showed significantly increased in transcription level. Microscopic analyses showed that an MoAK::GFP fusion protein was localized in the apoplastic region. The MoAk protein showed anti-freezing activity when expressed in yeast. These results indicated that cold acclimation is crucial for fungal freezing tolerance and MoAK played an important role in this process in M. oryzae.

Current understanding of HOG-MAPK pathway in Aspergillus fumigatus

Aspergillus fumigatus is an important opportunistic fungal pathogen that causes lethal systemic invasive aspergillosis. It must be able to adapt to stress in the microenvironment during host invasion and systemic spread. The high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) signaling pathway is a key element that controls adaptation to environmental stress. It plays a critical role in the virulence of several fungal pathogens. In this review, we summarize the current knowledge about the functions of different components of the HOG-MAPK pathway in A. fumigatus through mutant analysis or inferences from the genome annotation, focusing on their roles in adaptation to stress, regulation of infection-related morphogenesis, and effect on virulence. We also briefly compare the functions of the HOG pathway in A. fumigatus with those in the model fungi Saccharomyces cerevisiae and Aspergillus nidulans as well as several other human and plant pathogens including Candida albicans, Cryptococcus neoformans, and Magnaporthe oryzae. The genes described in this review mainly include tcsB, fos1, skn7, sho1, pbs2, and sakA whose deletion mutants have already been established in A. fumigatus. Among them, fos1 has been considered a virulence factor in A. fumigatus, indicating that components of the HOG pathway may be suitable as targets for developing new fungicides. However, quite a few of the genes of this pathway, such as sskA (ssk1), sskB, steC, and downstream regulator genes, are not well characterized. System biology approaches may contribute to a more comprehensive understanding of HOG pathway functions with dynamic details.

Proteome analysis of the farnesol-induced stress response in Aspergillus nidulans--The role of a putative dehydrin

The isoprenoid alcohol farnesol represents a quorum-sensing molecule in pathogenic yeasts, but was also shown to inhibit the growth of many filamentous fungi. In order to gain a deeper insight into the antifungal activity of farnesol, we performed 2D-differential gel electrophoretic analysis (2D-DIGE) of Aspergillus nidulans exposed to farnesol. We observed an increased abundance of antioxidative enzymes and proteins involved in protein folding and the ubiquitin-mediated protein degradation. A striking finding was the strong up-regulation of a dehydrin-like protein (DlpA). Expression analyses suggested the involvement of DlpA in the cellular response to oxidative, osmotic and cold stress. In line with these data, we demonstrated that dlpA expression was regulated by the MAP kinase SakA/HogA. The generation of both a dlpA Tet(on) antisense RNA-producing A. nidulans strain (dlpA-inv) and a ΔdlpA deletion mutant indicated a role of DlpA in conidiation and stress resistance of dormant conidia against heat and ROS. Furthermore, the production of the secondary metabolite sterigmatocystin was absent in both strains dlpA-inv and ΔdlpA. Our results demonstrate the complexity of the farnesol-mediated stress response in A. nidulans and describe a farnesol-inducible dehydrin-like protein that contributes to the high tolerance of resting conidia against oxidative and heat stress.