Hog1 Mitogen-activated Protein Kinase Research Articles

Dissecting the role of mitogen-activated protein kinase Hog1 in yeast flocculation.

Living organisms are frequently exposed to multiple biotic and abiotic stress forms during their lifetime. Organisms cope with stress conditions by regulating their gene expression programs. In response to different environmental stress conditions, yeast cells activate different tolerance mechanisms, many of which share common signaling pathways. Flocculation is one of the key mechanisms underlying yeast survival under unfavorable environmental conditions, and the Tup1-Cyc8 corepressor complex is a major regulator of this process. Additionally, yeast cells can utilize different mitogen-activated protein kinase (MAPK) pathways to modulate gene expression during stress conditions. Here, we show that the high osmolarity glycerol (HOG) MAPK pathway is involved in the regulation of yeast flocculation. We observed that the HOG MAPK pathway was constitutively activated in flocculating cells, and found that the interaction between phosphorylated Hog1 and the FLO genes promoter region increased significantly upon sodium chloride exposure. We found that treatment of cells with cantharidin decreased Hog1 phosphorylation, causing a sharp reduction in the expression of FLO genes and the flocculation phenotype. Similarly, deletion of HOG1 in yeast cells reduced flocculation. Altogether, our results suggest a role for HOG MAPK signaling in the regulation of FLO genes and yeast flocculation.

The Mitogen-Activated Protein Kinase Hog1 Regulates Fungal Development, Pathogenicity, and Stress Response in Botryosphaeria dothidea.

The high-osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway plays a central role in environmental stress adaptation in eukaryotes. However, the biological function of the HOG-MAPK pathway varies in different fungi. In this study, we investigated the HOG-MAPK pathway by inactivation of the core element Hog1 in Botryosphaeria dothidea, the causal agent of Botryosphaeria canker and apple ring rot. Targeted deletion of BdHOG1 resulted in the loss of conidiation ability and significant reduction of virulence. In addition, the ΔBdHog1 mutant exhibited hypersensitivity to osmotic stress but resistance to phenylpyrrole and dicarboximide fungicides. Comparative transcriptome analysis revealed that inactivation of BdHog1 influenced multiple metabolic pathways in B. dothidea. Taken together, our results suggest that BdHog1 plays a crucial role in development, virulence, and stress tolerance in B. dothidea, which provides a theoretical basis for the development of target-based fungicides.

Discovery of N-Benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a Potential Antifungal Agent against Sporidia Growth and Teliospore Germination of Sporisorium scitamineum.

The cultivation of sugar cane using perennial roots is the primary planting method, which is one of the reasons for the serious occurrence of sugar cane smut disease caused by the basidiomycetous fungus Sporisorium scitamineum in the sugar cane perennial root planting area. Consequently, it is crucial to eliminate pathogens from perennial sugar cane buds. In this study, we found that MAP kinase Hog1 is necessary for heat stress resistance. Subsequent investigations revealed a significant reduction in the expression of the heat shock protein 104-encoding gene, SsHSP104, in the ss1hog1Δ mutant. Additionally, the overexpression of SsHSP104 partially restored colony growth in the ss1hog1Δ strain following heat stress treatment, demonstrating the crucial role of SsHsp104 in SsHog1-mediated heat stress tolerance. Hence, we constructed the ss1hsp104:eGFP fusion strain in the wild type of S. scitamineum to identify small-molecule compounds that could inhibit the heat stress response, leading to the discovery of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a potential compound that targets the SsHog1 mediation SsHsp104 pathway during heat treatment. Furthermore, the combination of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine and warm water treatment (45 °C for 15 min) inhibits the growth of S. scitamineum and teliospore germination, thereby reducing the occurrence of sugar cane smut diseases and indicating its potential for eliminating pathogens from perennial sugar cane buds. In conclusion, these findings suggest that N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine is promising as a targeted compound for the SsHog1-mediated SsHsp104 pathway and may enable the reduction of hot water treatment duration and/or temperature, thereby limiting the occurrence of sugar cane smut diseases caused by S. scitamineum.

Proline-directed yeast and human MAP kinases phosphorylate the Dot1p/DOT1L histone H3K79 methyltransferase.

Disruptor of telomeric silencing 1 (Dot1p) is an exquisitely conserved histone methyltransferase and is the sole enzyme responsible for H3K79 methylation in the budding yeast Saccharomyces cerevisiae. It has been shown to be highly phosphorylated in vivo; however, the upstream kinases that act on Dot1p are almost entirely unknown in yeast and all other eukaryotes. Here, we used in vitro and in vivo kinase discovery approaches to show that mitogen-activated protein kinase HOG1 (Hog1p) is a bona fide kinase of the Dot1p methyltransferase. In vitro kinase assays showed that Hog1p phosphorylates Dot1p at multiple sites, including at several proline-adjacent sites that are consistent with known Hog1p substrate preferences. The activity of Hog1p was specifically enhanced at these proline-adjacent sites on Dot1p upon Hog1p activation by the osmostress-responsive MAP kinase kinase PBS2 (Pbs2p). Genomic deletion of HOG1 reduced phosphorylation at specific sites on Dot1p in vivo, providing further evidence for Hog1p kinase activity on Dot1p in budding yeast cells. Phenotypic analysis of knockout and phosphosite mutant yeast strains revealed the importance of Hog1p-catalysed phosphorylation of Dot1p for cellular responses to ultraviolet-induced DNA damage. In mammalian systems, this kinase-substrate relationship was found to be conserved: human DOT1L (the ortholog of yeast Dot1p) can be phosphorylated by the proline-directed kinase p38β (also known as MAPK11; the ortholog of yeast Hog1p) at multiple sites in vitro. Taken together, our findings establish Hog1p and p38β as newly identified upstream kinases of the Dot1p/DOT1L H3K79 methyltransferase enzymes in eukaryotes.

The bZIP transcription factor ATF1 regulates blue light and oxidative stress responses in Trichoderma guizhouense

AbstractIn several filamentous fungi, incident light and environmental stress signaling share the mitogen‐activated protein kinase (MAPK) HOG (SAK) pathway. It has been revealed that short‐term illumination with blue light triggers the activation of the HOG pathway in Trichoderma spp. In this study, we demonstrate the crucial role of the basic leucine zipper transcription factor ATF1 in blue light responses and signaling downstream of the MAPK HOG1 in Trichoderma guizhouense. The lack of ATF1 severely impaired photoconidiation and delayed vegetative growth and conidial germination. Upon blue light or H2O2 stimuli, HOG1 interacted with ATF1 in the nucleus. Genome‐wide transcriptome analyses revealed that 61.8% (509 out of 824) and 85.2% (702 out of 824) of blue light‐regulated genes depended on ATF1 and HOG1, respectively, of which 58.4% (481 out of 824) were regulated by both of them. Our results also show that blue light promoted conidial germination and HOG1 and ATF1 played opposite roles in controlling conidial germination in the dark. Additionally, the lack of ATF1 led to reduced oxidative stress resistance, probably because of the downregulation of catalase‐encoding genes. Overall, our results demonstrate that ATF1 is the downstream component of HOG1 and is responsible for blue light responses, conidial germination, vegetative growth, and oxidative stress resistance in T. guizhouense.

The sucrose non-fermenting-1 kinase Snf1 is involved in fludioxonil resistance via interacting with the high osmolarity glycerol MAPK kinase Hog1 in Fusarium

Fusarium head blight (FHB) caused by Fusarium graminearum complex is a worldwide devastating disease of wheat, barley, maize, and other cereals. In the field, application of fungicides is one of the main strategies for management of FHB. With the long-time usage of fungicides, resistant pathogen populations have become a new challenge for disease management. Application of new pesticide is necessary for sustainable control of this disease. The phenylpyrrole fungicide fludioxonil has been registered recently for management of FHB. However, the resistance mechanisms of F. graminearum to this compound are largely unknown. Here we isolated a biocontrol bacterium Burkholderia pyrrocina W1, which produced the antifungal compound pyrrolnitrin and showed greatly antagonistic activity towards FHB. Spontaneous mutants of pyrrolnitrin-resistant F. graminearum were induced and re-sequenced. Single base mutations were identified in the genes encoding the osmoregulation MAP kinase Hog1 and the AMP dependent kinase Snf1 in pyrrolnitrin-resistant mutants. Snf1 was further confirmed to interact with Hog1 and involved in the response of this fungus to pyrrolnitrin and its derivate, the fungicide fludioxonil. This study reveals that the Snf1 interacts with Hog1 to regulate fludioxonil resistance in a pathogenic fungus.

Physiological and transcriptomic insights into sugar stress resistance in osmophilic yeast Zygosaccharomyces rouxii

The osmophilic yeast Zygosaccharomyces rouxii has attracted increasing attention for its ability to survive and grow in extremely high sugar environments. This trait determines its role in fermentation process and results in contamination in the food industry. However, the behavior of Z. rouxii in regulating cell metabolism to combat high sugar stress and the corresponding mechanism have not been completely elucidated. Here, the resistance strategies of Z. rouxii against high glucose stress were explored by physiological analysis at cell membrane level and transcriptomic analysis. Physiological analysis showed that under high glucose stress, colony transparency increased, cell volume decreased, which was accompanied by reduction in permeability and integrity of cell membrane and subsequent gradual recovering. Additionally, the proportion of ergosterol and unsaturated fatty acids in cell membrane significantly increased under high glucose stress. A comparison of transcriptome data showed that most of the obtained differentially expressed genes (DEGs) involved in ergosterol and linoleic acid synthesis pathways as well as cell wall integrity (CWI) and high osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathways, which was in line with the results of physiological data. Our results provided a theoretic basis to develop the process control for the production of high sugar foods.

Genome-wide fitness profiling reveals molecular mechanisms that bacteria use to interact with Trichoderma atroviride exometabolites.

Trichoderma spp. are ubiquitous rhizosphere fungi capable of producing several classes of secondary metabolites that can modify the dynamics of the plant-associated microbiome. However, the bacterial-fungal mechanisms that mediate these interactions have not been fully characterized. Here, a random barcode transposon-site sequencing (RB-TnSeq) approach was employed to identify bacterial genes important for fitness in the presence of Trichoderma atroviride exudates. We selected three rhizosphere bacteria with RB-TnSeq mutant libraries that can promote plant growth: the nitrogen fixers Klebsiella michiganensis M5aI and Herbaspirillum seropedicae SmR1, and Pseudomonas simiae WCS417. As a non-rhizosphere species, Pseudomonas putida KT2440 was also included. From the RB-TnSeq data, nitrogen-fixing bacteria competed mainly for iron and required the siderophore transport system TonB/ExbB for optimal fitness in the presence of T. atroviride exudates. In contrast, P. simiae and P. putida were highly dependent on mechanisms associated with membrane lipid modification that are required for resistance to cationic antimicrobial peptides (CAMPs). A mutant in the Hog1-MAP kinase (Δtmk3) gene of T. atroviride showed altered expression patterns of many nonribosomal peptide synthetase (NRPS) biosynthetic gene clusters with potential antibiotic activity. In contrast to exudates from wild-type T. atroviride, bacterial mutants containing lesions in genes associated with resistance to antibiotics did not show fitness defects when RB-TnSeq libraries were exposed to exudates from the Δtmk3 mutant. Unexpectedly, exudates from wild-type T. atroviride and the Δtmk3 mutant rescued purine auxotrophic mutants of H. seropedicae, K. michiganensis and P. simiae. Metabolomic analysis on exudates from wild-type T. atroviride and the Δtmk3 mutant showed that both strains excrete purines and complex metabolites; functional Tmk3 is required to produce some of these metabolites. This study highlights the complex interplay between Trichoderma-metabolites and soil bacteria, revealing both beneficial and antagonistic effects, and underscoring the intricate and multifaceted nature of this relationship.

Loss of function of Hog1 improves glycerol assimilation in Saccharomyces cerevisiae

We previously isolated a mutant of Saccharomyces cerevisiae strain 85_9 whose glycerol assimilation was improved through adaptive laboratory evolution. To investigate the mechanism for this improved glycerol assimilation, genome resequencing of the 85_9 strain was performed, and the mutations in the open reading frame of HOG1, SIR3, SSB2, and KGD2 genes were found. Among these, a frameshift mutation in the HOG1 open reading frame was responsible for the improved glycerol assimilation ability of the 85_9 strain. Moreover, the HOG1 gene disruption improved glycerol assimilation. As HOG1 encodes a mitogen-activated protein kinase (MAPK), which is responsible for the signal transduction cascade in response to osmotic stress, namely the high osmolarity glycerol (HOG) pathway, we investigated the effect of the disruption of PBS2 gene encoding MAPK kinase for Hog1 MAPK on glycerol assimilation, revealing that PBS2 disruption can increase glycerol assimilation. These results indicate that loss of function of Hog1 improves glycerol assimilation in S. cerevisiae. However, single disruption of the SSK2, SSK22 and STE11 genes encoding protein kinases responsible for Pbs2 phosphorylation in the HOG pathway did not increase glycerol assimilation, while their triple disruption partially improved glycerol assimilation in S. cerevisiae. In addition, the HOG1 frameshift mutation did not improve glycerol assimilation in the STL1-overexpressing RIM15 disruptant strain, which was previously constructed with high glycerol assimilation ability. Furthermore, the effectiveness of the HOG1 disruptant as a bioproduction host was validated, indicating that the HOG1 CYB2 double disruptant can produce L-lactic acid from glycerol.

White Collar 1 Modulates Oxidative Sensitivity and Virulence by Regulating the HOG1 Pathway in Fusarium asiaticum.

Fusarium asiaticum is an epidemiologically important pathogen of cereal crops in east Asia, accounting for both yield losses and mycotoxin contamination problems in food and feed products. FaWC1, a component of the blue-light receptor White Collar complex (WCC), relies on its transcriptional regulatory zinc finger domain rather than the light-oxygen-voltage domain to regulate pathogenicity of F. asiaticum, although the downstream mechanisms remain obscure. In this study, the pathogenicity factors regulated by FaWC1 were analyzed. It was found that loss of FaWC1 resulted in higher sensitivity to reactive oxygen species (ROS) than in the wild type, while exogenous application of the ROS quencher ascorbic acid restored the pathogenicity of the ΔFawc1 strain to the level of the wild type, indicating that the reduced pathogenicity of the ΔFawc1 strain is due to a defect in ROS tolerance. Moreover, the expression levels of the high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) pathway genes and their downstream genes encoding ROS scavenging enzymes were downregulated in the ΔFawc1 mutant. Upon ROS stimulation, the FaHOG1-green fluorescent protein (GFP)-expressing signal driven by the native promoter was inducible in the wild type but negligible in the ΔFawc1 strain. Overexpressing Fahog1 in the ΔFawc1 strain could recover the ROS tolerance and pathogenicity of the ΔFawc1 mutant, but it remained defective in light responsiveness. In summary, this study dissected the roles of the blue-light receptor component FaWC1 in regulating expression levels of the intracellular HOG-MAPK signaling pathway to affect ROS sensitivity and pathogenicity in F. asiaticum. IMPORTANCE The well-conserved fungal blue-light receptor White Collar complex (WCC) is known to regulate virulence of several pathogenic species for either plant or human hosts, but how WCC determines fungal pathogenicity remains largely unknown. The WCC component FaWC1 in the cereal pathogen Fusarium asiaticum was previously found to be required for full virulence. The present study dissected the roles of FaWC1 in regulating the intracellular HOG MAPK signaling pathway to affect ROS sensitivity and pathogenicity in F. asiaticum. This work thus extends knowledge of the association between fungal light receptors and the intracellular stress signaling pathway to regulate oxidative stress tolerance and pathogenicity in an epidemiologically important fungal pathogen of cereal crops.

Positive feedback induces switch between distributive and processive phosphorylation of Hog1

Cellular decision making often builds on ultrasensitive MAPK pathways. The phosphorylation mechanism of MAP kinase has so far been described as either distributive or processive, with distributive mechanisms generating ultrasensitivity in theoretical analyses. However, the in vivo mechanism of MAP kinase phosphorylation and its activation dynamics remain unclear. Here, we characterize the regulation of the MAP kinase Hog1 in Saccharomyces cerevisiae via topologically different ODE models, parameterized on multimodal activation data. Interestingly, our best fitting model switches between distributive and processive phosphorylation behavior regulated via a positive feedback loop composed of an affinity and a catalytic component targeting the MAP kinase-kinase Pbs2. Indeed, we show that Hog1 directly phosphorylates Pbs2 on serine 248 (S248), that cells expressing a non-phosphorylatable (S248A) or phosphomimetic (S248E) mutant show behavior that is consistent with simulations of disrupted or constitutively active affinity feedback and that Pbs2-S248E shows significantly increased affinity to Hog1 in vitro. Simulations further suggest that this mixed Hog1 activation mechanism is required for full sensitivity to stimuli and to ensure robustness to different perturbations.

Construction of Candida albicans Strains with ATP-Analog-Sensitive Protein Kinase A and Hog1.

Candida albicans is an opportunistic human fungal pathogen and a member of the mucosal microbiota. To survive in the host and cause disease, C. albicans utilizes several virulence traits, including the ability to respond and adapt to diverse stressors, as well as the morphogenetic switch between yeast and filamentous morphologies. While complex cellular circuitry governs these virulence attributes, the following two kinase-mediated signaling pathways play particularly critical roles in controlling these processes: the Hog1 mitogen-activated protein kinase (MAPK) cascade and the protein kinase A (PKA) pathway. Here, we describe the construction of C. albicans strains harboring substitutions in the ATP-binding pockets of Hog1 and the catalytic subunits of PKA, Tpk1, and Tpk2 to render their activities sensitive to the addition of bulky ATP analogs. Specifically, inhibition by the ATP analog 1NM-PP1 resulted in phenotypes characteristic of the corresponding homozygous deletion mutants for each kinase gene. These strains represent a toolset for the rapid and specific inhibition of PKA and Hog1 kinase activity to further understand their roles in regulating C. albicans morphogenesis and stress responses. IMPORTANCE As an opportunistic pathogen in humans, the fungus Candida albicans relies on virulence traits to cause disease. They include the ability to transition from yeast to filamentous morphologies and the ability to grow in diverse environmental stress conditions, including nutrient limitation, as well as osmotic and heat shock. Previous work identified the following two kinases that play a critical role in regulating these responses: Hog1 and PKA. Here, we generated versions of each kinase that are sensitive to inhibition by a bulky ATP analog, 1NM-PP1. In the presence of the analog, kinase activity is inhibited rapidly and specifically, facilitating the analysis of both kinases in regulating C. albicans morphogenesis and stress responses. Together, these strains represent an important toolset to further our understanding of C. albicans biology and virulence.

Modification of Phosphorylation Sites in the Yeast Lysine Methyltransferase Set5 Exerts Influences on the Mitogen-Activated Protein Kinase Hog1 under Prolonged Acetic Acid Stress.

Responses to acetic acid toxicity in the budding yeast Saccharomyces cerevisiae have widespread implications in the biorefinery of lignocellulosic biomass and food preservation. Our previous studies revealed that Set5, the yeast lysine methyltransferase and histone H4 methyltransferase, was involved in acetic acid stress tolerance. However, it is still mysterious how Set5 functions and interacts with the known stress signaling network. Here, we revealed that elevated phosphorylation of Set5 during acetic acid stress is accompanied by enhanced expression of the mitogen-activated protein kinase (MAPK) Hog1. Further experiments uncovered that the phosphomimetic mutation of Set5 endowed yeast cells with improved growth and fermentation performance and altered transcription of specific stress-responsive genes. Intriguingly, Set5 was found to bind the coding region of HOG1 and regulate its transcription, along with increased expression and phosphorylation of Hog1. A protein-protein interaction between Set5 and Hog1 was also revealed. In addition, modification of Set5 phosphosites was shown to regulate reactive oxygen species (ROS) accumulation, which is known to affect yeast acetic acid stress tolerance. The findings in this study imply that Set5 may function together with the central kinase Hog1 to coordinate cell growth and metabolism in response to stress. IMPORTANCE Hog1 is the yeast homolog of p38 MAPK in mammals that is conserved across eukaryotes, and it plays crucial roles in stress tolerance, fungal pathogenesis, and disease treatments. Here, we provide evidence that modification of Set5 phosphorylation sites regulates the expression and phosphorylation of Hog1, which expands current knowledge on upstream regulation of the Hog1 stress signaling network. Set5 and its homologous proteins are present in humans and various eukaryotes. The newly identified effects of Set5 phosphorylation site modifications in this study benefit an in-depth understanding of eukaryotic stress signaling, as well as the treatment of human diseases.

HOG1 Mitogen-Activated Protein Kinase Pathway-Related Autophagy Induced by H2O2 in Lentinula edodes Mycelia.

Mycelial ageing is associated with ROS and autophagy in Lentinula edodes. However, the underlying cellular and molecular mechanisms between ROS and autophagy remain obscure. This study induced autophagy in L. edodes mycelia through exogenous H2O2 treatment. Results showed that 100 μM H2O2 treatment for 24 h significantly inhibited mycelial growth. H2O2 caused the depolarisation of MMP and accumulation of TUNEL-positive nuclei, which was similar to the ageing phenotype of L. edodes mycelia. Transcriptome analysis showed that differentially expressed genes were enriched in the mitophagic, autophagic, and MAPK pathways. LeAtg8 and LeHog1 were selected as hub genes. RNA and protein levels of LeATG8 increased in the H2O2-treated mycelia. Using fluorescent labelling, we observed for the first time the classic ring structure of autophagosomes in a mushroom, while 3D imaging suggested that these autophagosomes surrounded the nuclei to degrade them at specific growth stages. Phospho-LeHOG1 protein can translocate from the cytoplasm to the nucleus to regulate mycelial cells, resisting ROS-induced oxidative stress. Furthermore, LeATG8 expression was suppressed when LeHOG1 phosphorylation was inhibited. These results suggest that the LeATG8-dependent autophagy in L. edodes mycelial is closely associated with the activity or even phosphorylation of LeHOG1.

Two activating phosphorylation sites of Pbs2 MAP2K in the yeast HOG pathway are differentially dephosphorylated by four PP2C phosphatases Ptc1–Ptc4

To cope with an increased external osmolarity, thebudding yeastSaccharomyces cerevisiaeactivates the Hog1 mitogen-activated protein kinase (MAPK) through the high-osmolarity glycerol (HOG) pathway, which governs adaptive responses to osmostress. In the HOG pathway, two apparently redundant upstream branches, termed SLN1 and SHO1, activate cognate MAP3Ks (MAPKK kinase) Ssk2/22 and Ste11, respectively. These MAP3Ks, when activated, phosphorylate and thus activate the Pbs2 MAP2K (MAPK kinase), which in turn phosphorylates and activates Hog1. Previous studies have shown that protein tyrosine phosphatases and the serine/threonine protein phosphatases type 2C negatively regulate the HOG pathway to prevent its excessive and inappropriate activation, which is detrimental to cell growth. The tyrosine phosphatases Ptp2 and Ptp3 dephosphorylate Hog1 at Tyr-176, whereas the protein phosphatase type 2Cs Ptc1 and Ptc2 dephosphorylate Hog1 at Thr-174. In contrast, the identities of phosphatases that dephosphorylate Pbs2 remained less clear. Here, we examined the phosphorylation status of Pbs2 at the activating phosphorylation sites Ser-514 and Thr-518 (S514 and T518) in various mutants, both in the unstimulated and osmostressed conditions. Thus, we found that Ptc1-Ptc4 collectively regulate Pbs2 negatively, but each Ptc acts differently to the two phosphorylation sites in Pbs2. T518 is predominantly dephosphorylated by Ptc1, while S514 can be dephosphorylated by any of Ptc1-4 to an appreciable extent. We also show that Pbs2 dephosphorylation by Ptc1 requires the adaptor protein Nbp2 that recruits Ptc1 to Pbs2, thus highlighting the complex processes involved in regulating adaptive responses to osmostress.

The defective gut colonization of Candida albicans hog1 MAPK mutants is restored by overexpressing the transcriptional regulator of the white opaque transition WOR1

ABSTRACT The transcriptional master regulator of the white opaque transition of Candida albicans WOR1 is important for the adaptation to the commensal lifestyle in the mammalian gut, a major source of invasive candidiasis. We have generated cells that overproduce Wor1 in mutants defective in the Hog1 MAP kinase, defective in several stress responses and unable to colonize the mice gut. WOR1 overexpression allows hog1 to be established as a commensal in the murine gut in a commensalism model and even compete with wild-type C. albicans cells for establishment. This increased fitness correlates with an enhanced ability to adhere to biotic surfaces as well as increased proteinase and phospholipase production and a decrease in filamentation in vitro. We also show that hog1 WOR1OE are avirulent in a systemic candidiasis model in mice.

The Transcription Factor CsAtf1 Negatively Regulates the Cytochrome P450 Gene CsCyp51G1 to Increase Fludioxonil Sensitivity in Colletotrichum siamense.

Previous studies have shown that the high-osmolarity glycerol mitogen-activated protein kinase (HOG MAPK) signaling pathway and its downstream transcription factor CsAtf1 are involved in the regulation of fludioxonil sensitivity in C. siamense. However, the downstream target genes of CsAtf1 related to the fludioxonil stress response remain unclear. Here, we performed chromatin immunoprecipitation sequencing (ChIP-Seq) and high-throughput RNA-sequencing (RNA-Seq) to identify genome-wide potential CsAtf1 target genes. A total of 3809 significantly differentially expressed genes were predicted to be directly regulated by CsAtf1, including 24 cytochrome oxidase-related genes. Among them, a cytochrome P450-encoding gene, designated CsCyp51G1, was confirmed to be a target gene, and its transcriptional expression was negatively regulated by CsAtf1, as determined using an electrophoretic mobility shift assay (EMSA), a yeast one-hybrid (Y1H) assay, and quantitative real-time PCR (qRT-PCR). Moreover, the overexpression mutant CsCYP51G1 of C. siamense exhibited increased fludioxonil tolerance, and the CsCYP51G1 deletion mutant exhibited decreased fludioxonil resistance, which revealed that CsCyp51G1 is involved in fludioxonil sensitivity regulation in C. siamense. However, the cellular ergosterol content of the mutants was not consistent with the phenotype of fludioxonil sensitivity, which indicated that CsCyp51G1 regulates fludioxonil sensitivity by affecting factors other than the ergosterol level in C. siamense. In conclusion, our data indicate that the transcription factor CsAtf1 negatively regulates the cytochrome P450 gene CsCyp51G1 to increase fludioxonil sensitivity in C. siamense.

GTPase Rac Regulates Conidiation, AFB1 Production and Stress Response in Pathogenic Fungus Aspergillus flavus.

As a member of the Rho family, Rac plays important roles in many species, including proliferation, differentiation, apoptosis, DNA damage responses, metabolism, angiogenesis, and immunosuppression. In this study, by constructing Rac-deleted mutants in Aspergillus flavus, it was found that the deletion of Rac gene led to the decline of growth and development, conidia production, AFB1 toxin synthesis, and seed infection ability of A. flavus. The deletion of Rac gene also caused the disappearance of A. flavus sclerotium, indicating that Rac is required for sclerotium formation in A. flavus. The sensitivity of Rac-deficient strains responding to cell wall stress and osmotic pressure stress increased when compared to A. flavus WT. The Western blot result showed that mitogen-activated serine/threonine-protein kinase Slt2 and mitogen-activated protein kinase Hog1 proteins were no longer phosphorylated in Rac-deficient strains of A. flavus, showing that Rac may be used as a molecular switch to control the Slt2-MAPK cascade pathway and regulate the osmotic Hog-MAPK cascade pathway in A. flavus in response to external stress. Altogether, these results indicated that Rac was involved in regulating the growth and development, conidia formation and AFB1 synthesis, and response to cell wall stress and osmotic pressure stress in A. flavus.

Kinase Hog1 and Adr1 Opposingly Regulate Haploid Cell Morphology by Controlling Vacuole Size in Sporisorium scitamineum.

Morphogenesis is a strictly regulated efficient system in eukaryotes for adapting to environmental changes. However, the morphogenesis regulatory mechanism in smut fungi is not clear. This study reports a relationship between MAP kinase Hog1 and cAMP-dependent protein kinase A catalytic subunit (Adr1) for the morphological regulation in the sugarcane pathogen Sporisorium scitamineum. The results demonstrated that MAP kinase Hog1 and cAMP/PKA signaling pathways are essential for the morphological development of S. scitamineum. Interestingly, MAP kinase Hog1 and cAMP/PKA signaling pathways’ defective mutants exhibit an opposite morphological phenotype. The morphology of cAMP/PKA defective mutants is recovered by deleting the SsHOG1 gene. However, MAP kinase Hog1 and cAMP-dependent protein kinase catalytic subunit Adr1 do not interfere with each other. Further investigations showed that kinase Hog1 and Adr1 antagonistically regulates the vacuolar size, which contributes to the cell size and determines the cellular elongation rates. Kinase Hog1 and Adr1 also antagonistically balanced the cell wall integrity and permeability. Taken together, kinase Hog1- and Adr1-based opposing morphogenesis regulation of S. scitamineum by controlling the vacuolar size and cell wall permeability is established during the study.

Mechanism of HOG-MAPK pathway in regulating mycotoxins formation under environmental stresses

In order to adapt to different environmental stresses including heat stress, oxidative stress, osmotic pressure stress and ultraviolet stress in the process of growing and infecting agricultural products, fungi have developed a set of high osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway to alleviate the environmental stresses. This pathway plays an important role in the growth, development, mycotoxin production and pathogenicity of fungi. There are two branches in the HOG-MAPK pathway, among which the SLN1 branch is more sensitive to osmotic stress than another branch (SHO1 branch), and is able to respond to high osmotic pressure and high salt concentration. The SHO1 branch is involved in a variety of signal transduction in response to oxidative stress and thermal stress. This paper reviews the functions of key genes sln1, sho1, ste11, ssk2, pbs2 and hog1 in the HOG-MAPK pathway of phytopathogenic fungi in responses to different environmental stresses such as osmotic stress and oxidative stress. We show that the HOG-MAPK pathway can respond to a variety of environmental signals and is involved in regulating the growth of phytopathogenic fungi such as Aspergillus flavus and Aspergillus ochraceus, and the production of mycotoxins such as aflatoxins and ochratoxins. Understanding the mechanism of how HOG-MAPK pathway regulates mycotoxins' formation under different environmental stresses that provides a theoretical basis and guidance for the prevention and control of mycotoxins in agricultural products such as food and feed.