Lipid droplet biogenesis regulated by the FgNem1/Spo7-FgPah1 phosphatase cascade plays critical roles in fungal development and virulence in Fusarium graminearum

The target of rapamycin signaling pathway regulates vegetative development, aflatoxin biosynthesis, and pathogenicity in Aspergillus flavus

The target of rapamycin (TOR) signaling pathway is highly conserved and plays a crucial role in diverse biological processes in eukaryotes. Despite its significance, the underlying mechanism of the TOR pathway in Aspergillus flavus remains elusive. In this study, we comprehensively analyzed the TOR signaling pathway in A. flavus by identifying and characterizing nine genes that encode distinct components of this pathway. The FK506-binding protein Fkbp3 and its lysine succinylation are important for aflatoxin production and rapamycin resistance. The TorA kinase plays a pivotal role in the regulation of growth, spore production, aflatoxin biosynthesis, and responses to rapamycin and cell membrane stress. As a significant downstream effector molecule of the TorA kinase, the Sch9 kinase regulates aflatoxin B1 (AFB1) synthesis, osmotic and calcium stress response in A. flavus, and this regulation is mediated through its S_TKc, S_TK_X domains, and the ATP-binding site at K340. We also showed that the Sch9 kinase may have a regulatory impact on the high osmolarity glycerol (HOG) signaling pathway. TapA and TipA, the other downstream components of the TorA kinase, play a significant role in regulating cell wall stress response in A. flavus. Moreover, the members of the TapA-phosphatase complexes, SitA and Ppg1, are important for various biological processes in A. flavus, including vegetative growth, sclerotia formation, AFB1 biosynthesis, and pathogenicity. We also demonstrated that SitA and Ppg1 are involved in regulating lipid droplets (LDs) biogenesis and cell wall integrity (CWI) signaling pathways. In addition, another phosphatase complex, Nem1/Spo7, plays critical roles in hyphal development, conidiation, aflatoxin production, and LDs biogenesis. Collectively, our study has provided important insight into the regulatory network of the TOR signaling pathway and has elucidated the underlying molecular mechanisms of aflatoxin biosynthesis in A. flavus.

Histone H3 N-Terminal Lysine Acetylation Governs Fungal Growth, Conidiation, and Pathogenicity through Regulating Gene Expression in Fusarium pseudograminearum

The acetylation of histone lysine residues regulates multiple life processes, including growth, conidiation, and pathogenicity in filamentous pathogenic fungi. However, the specific function of each lysine residue at the N-terminus of histone H3 in phytopathogenic fungi remains unclear. In this study, we mutated the N-terminal lysine residues of histone H3 in Fusarium pseudograminearum, the main causal agent of Fusarium crown rot of wheat in China, which also produces deoxynivalenol (DON) toxins harmful to humans and animals. Our findings reveal that all the FpH3K9R, FpH3K14R, FpH3K18R, and FpH3K23R mutants are vital for vegetative growth and conidiation. Additionally, FpH3K14 regulates the pathogen's sensitivity to various stresses and fungicides. Despite the slowed growth of the FpH3K9R and FpH3K23R mutants, their pathogenicity towards wheat stems and heads remains unchanged. However, the FpH3K9R mutant produces more DON. Furthermore, the FpH3K14R and FpH3K18R mutants exhibit significantly reduced virulence, with the FpH3K18R mutant producing minimal DON. In the FpH3K9R, FpH3K14R, FpH3K18R, and FpH3K23R mutants, there are 1863, 1400, 1688, and 1806 downregulated genes, respectively, compared to the wild type. These downregulated genes include many that are crucial for growth, conidiation, pathogenicity, and DON production, as well as some essential genes. Gene ontology (GO) enrichment analysis indicates that genes downregulated in the FpH3K14R and FpH3K18R mutants are enriched for ribosome biogenesis, rRNA processing, and rRNA metabolic process. This suggests that the translation machinery is abnormal in the FpH3K14R and FpH3K18R mutants. Overall, our findings suggest that H3 N-terminal lysine residues are involved in regulating the expression of genes with important functions and are critical for fungal development and pathogenicity.

High-quality genome assembly and genetic transformation system of Lasiodiplodia theobromae strain LTTK16-3, a fungal pathogen of Chinese hickory

Lasiodiplodia theobromae, as one of the causative agents associated with Chinese hickory trunk cankers, has caused huge economic losses to the Chinese hickory industry. Although the biological characteristics of this pathogen and the occurrence pattern of this disease have been well studied, few studies have addressed the related mechanisms due to the poor molecular and genetic study basis of this fungus. In this study, we sequenced and assembled L. theobromae strain LTTK16-3, isolated from a Chinese hickory tree (cultivar of Linan) in Linan, Zhejiang province, China. Phylogenetic analysis and comparative genomics analysis presented crucial cues in the prediction of LTTK16-3, which shared similar regulatory mechanisms of transcription, DNA replication, and DNA damage response with the other four Chinese hickory trunk canker-associated Botryosphaeria strains including, Botryosphaeria dothidea, Botryosphaeria fabicerciana, Botryosphaeria qingyuanensis, and Botryosphaeria corticis. Moreover, it contained 18 strain-specific protein clusters (not conserved in the other L. theobromae strains, AM2As and CITRA15), with potential roles in specific host-pathogen interactions during the Chinese hickory infection. Additionally, an efficient system for L. theobromae protoplast preparation and polyethylene glycol (PEG) -mediated genetic transformation was firstly established as the foundation for its future mechanisms study. Collectively, the high-quality genome data and the efficient transformation system of L. theobromae here set up the possibility of targeted molecular improvements for Chinese hickory canker control.IMPORTANCEFungi with disparate genomic features are physiologically diverse, possessing species-specific survival strategies and environmental adaptation mechanisms. The high-quality genome data and related molecular genetic studies are the basis for revealing the mechanisms behind the physiological traits that are responsible for their environmental fitness. In this study, we sequenced and assembled the LTTK16-3 strain, the genome of Lasiodiplodia theobromae first obtained from a diseased Chinese hickory tree (cultivar of Linan) in Linan, Zhejiang province, China. Further phylogenetic analysis and comparative genomics analysis provide crucial cues in the prediction of the proteins with potential roles in specific host-pathogen interactions during the Chinese hickory infection. An efficient PEG-mediated genetic transformation system of L. theobromae was established as the foundation for the future mechanisms exploration. The above genetic information and tools set up valuable clues to study L. theobromae pathogenesis and assist in Chinese hickory canker control.

The Phosphatase Cascade Nem1/Spo7-Pah1 Regulates Fungal Development, Lipid Homeostasis, and Virulence in Botryosphaeria dothidea

Protein phosphatase complex Nem1/Spo7 plays crucial roles in the regulation of various biological processes in eukaryotes. However, its biological functions in phytopathogenic fungi are not well understood. In this study, genome-wide transcriptional profiling analysis revealed that Nem1 was significantly upregulated during the infection process of Botryosphaeria dothidea, and we identified and characterized the phosphatase complex Nem1/Spo7 and its substrate Pah1 (a phosphatidic acid phosphatase) in B. dothidea. Nem1/Spo7 physically interacted with and dephosphorylated Pah1 to promote triacylglycerol (TAG) and subsequent lipid droplet (LD) synthesis. Moreover, the Nem1/Spo7-dependently dephosphorylated Pah1 functioned as a transcriptional repressor of the key nuclear membrane biosynthesis genes to regulate nuclear membrane morphology. In addition, phenotypic analyses showed that the phosphatase cascade Nem1/Spo7-Pah1 was involved in regulating mycelial growth, asexual development, stress responses, and virulence of B. dothidea. IMPORTANCE Botryosphaeria canker and fruit rot caused by the fungus Botryosphaeria dothidea is one of the most destructive diseases of apple worldwide. Our data indicated that the phosphatase cascade Nem1/Spo7-Pah1 plays important roles in the regulation of fungal growth, development, lipid homeostasis, environmental stress responses, and virulence in B. dothidea. The findings will contribute to the in-depth and comprehensive understanding of Nem1/Spo7-Pah1 in fungi and the development of target-based fungicides for disease management.

The GRAS Salts of Na2SiO3 and EDTA-Na2 Control Citrus Postharvest Pathogens by Disrupting the Cell Membrane

Sodium silicate (Na₂SiO₃) and ethylenediaminetetraacetic acid disodium salt (EDTA-Na₂) are inorganic salts classified as 'Generally Recognized as Safe' (GRAS) compounds with great advantages in controlling various pathogens of postharvest fruits and vegetables. Here, we determined the median effective concentration (EC₅₀) of Na₂SiO₃ (0.06%, 0.05%, 0.07% and 0.08%) and EDTA-Na₂ (0.11%, 0.08%, 0.5%, and 0.07%) against common pathogens affecting postharvest citrus fruit, including Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii, and Colletotrichum gloeosporioides. Na₂SiO₃ and EDTA-Na₂ treatments at the EC₅₀ decreased the spore germination rate, visibly disrupted the spore cell membrane integrity, and significantly increased the lipid droplets (LDs) of the four postharvest pathogens. Moreover, both treatments at EC₅₀ significantly reduced the disease incidence of P. italicum (by 60% and 93.335, respectively) and G. citri-aurantii (by 50% and 76.67%, respectively) relative to the control. Furthermore, Na₂SiO₃ and EDTA-Na₂ treatment resulted in dramatically lower disease severity of the four pathogens, while also demonstrating no significant change in citrus fruit quality compared with the control. Therefore, Na₂SiO₃ and EDTA-Na₂ present a promising approach to control the postharvest diseases of citrus fruit.

A lipid droplet-associated protein Nem1 regulates appressorium function for infection of Magnaporthe oryzae

Lipid droplets are important storages in fungal conidia and can be used by plant pathogenic fungi for infection. However, the regulatory mechanism of lipid droplets formation and the utilization during fungal development and infection are largely unknown. Here, in Magnaporthe oryzae, we identified a lipid droplet-associated protein Nem1 that played a key role in lipid droplets biogenesis and utilization. Nem1 was highly expressed in conidia, but lowly expressed in appressoria, and its encoded protein was localized to lipid droplets. Deletion of NEM1 resulted in reduced numbers of lipid droplets and decreased content of diacylglycerol (DAG) or triacylglycerol (TAG). NEM1 was required for asexual development especially conidia production. The Δnem1 mutant was nearly loss of virulence to host plants due to defects in appressorial penetration and invasive growth. Remarkably, Nem1 was regulated by the TOR signaling pathway and involved in the autophagy process. The Ser303 residue of Nem1 could be phosphorylated by the cAMP-PKA signaling pathway and was important for biological function of Nem1. Together, our study revealed a regulatory mechanism of lipid biogenesis and metabolism during the conidium and appressorium formation of the rice blast fungus.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00098-5.

MoLrp1-mediated signaling induces nuclear accumulation of MoMsn2 to facilitate fatty acid oxidation for infectious growth of the rice blast fungus

Fatty acid β-oxidation is critical for fatty acid degradation and cellular development. In the rice blast fungus Magnaporthe oryzae, fatty acid β-oxidation is reported to be important mainly for turgor generation in the appressorium. However, the role of fatty acid β-oxidation during invasive hyphal growth is rarely documented. We demonstrated that blocking peroxisomal fatty acid β-oxidation impaired lipid droplet (LD) degradation and infectious growth of M. oryzae. We found that the key regulator of pathogenesis, MoMsn2, which we identified previously, is involved in fatty acid β-oxidation by targeting MoDCI1 (encoding dienoyl-coenzyme A [CoA] isomerase), which is also important for LD degradation and infectious growth. Cytological observations revealed that MoMsn2 accumulated from the cytosol to the nucleus during early infection or upon treatment with oleate. We determined that the low-density lipoprotein receptor-related protein MoLrp1, which is also involved in fatty acid β-oxidation and infectious growth, plays a critical role in the accumulation of MoMsn2 from the cytosol to the nucleus by activating the cyclic AMP signaling pathway. Our results provide new insights into the importance of fatty acid oxidation during invasive hyphal growth, which is modulated by MoMsn2 and its related signaling pathways in M. oryzae.

Discovery and Characterization of Putative Glycoprotein-Encoding Mycoviruses in the Bunyavirales

Although segmented negative-sense RNA viruses (SNSRVs) have been frequently discovered in various fungi, most SNSRVs reported only the large segments. In this study, we investigated the diversity of the mycoviruses in the phytopathogenic fungus Fusarium asiaticum using the metatranscriptomic technique. We identified 17 fungal single-stranded RNA (ssRNA) viruses including nine viruses within Mitoviridae, one each in Narnaviridae, Botourmiaviridae, Hypoviridae, Fusariviridae, and Narliviridae, two in Mymonaviridae, and one trisegmented virus temporarily named Fusarium asiaticum mycobunyavirus 1 (FaMBV1). The FaMBV1 genome comprises three RNA segments, large (L), medium (M), and small (S) with 6,468, 2,639, and 1,420 nucleotides, respectively. These L, M, and S segments putatively encode the L protein, glycoprotein, and nucleocapsid, respectively. Phylogenetic analysis based on the L protein showed that FaMBV1 is phylogenetically clustered with Alternaria tenuissima negative-stranded RNA virus 2 (AtNSRV2) and Sclerotinia sclerotiorum negative-stranded RNA virus 5 (SsNSRV5) but distantly related to the members of the family Phenuiviridae. FaMBV1 could be vertically transmitted by asexual spores with lower efficiency (16.7%, 2/42). Comparison between FaMBV1-free and -infected fungal strains revealed that FaMBV1 has little effect on hyphal growth, pathogenicity, and conidium production, and its M segment is dispensable for viral replication and lost during subculture and asexual conidiation. The M and S segments of AtNSRV2 and SsNSRV5 were found using bioinformatics methods, indicating that the two fungal NSRVs harbor trisegmented genomes. Our results provide a new example of the existence and evolution of the segmented negative-sense RNA viruses in fungi. IMPORTANCE Fungal segmented negative-sense RNA viruses (SNSRVs) have been frequently found. Only the large segment encoding RNA-dependent RNA polymerase (RdRp) has been reported in most fungal SNSRVs, except for a few fungal SNSRVs reported to encode nucleocapsids, nonstructural proteins, or movement proteins. Virome analysis of the Fusarium spp. that cause Fusarium head blight discovered a novel virus, Fusarium asiaticum mycobunyavirus 1 (FaMBV1), representing a novel lineage of the family Phenuiviridae. FaMBV1 harbors a trisegmented genome that putatively encodes RdRp, glycoproteins, and nucleocapsids. The putative glycoprotein was first described in fungal SNSRVs and shared homology with glycoprotein of animal phenuivirus but was dispensable for its replication in F. asiaticum. Two other trisegmented fungal SNSRVs that also encode glycoproteins were discovered, implying that three-segment bunyavirus infections may be common in fungi. These findings provide new insights into the ecology and evolution of SNSRVs, particularly those infecting fungi.

Fusarium graminearum GGA protein is critical for fungal development, virulence and ascospore discharge through its involvement in vesicular trafficking

Vesicular trafficking is a conserved material transport process in eukaryotic cells. The GGA family proteins are clathrin adaptors that are involved in eukaryotic vesicle transport, but their functions in phytopathogenic filamentous fungi remain unexplored. Here, we examined the only GGA family protein in Fusarium graminearum, FgGga1, which localizes to both the late Golgi and endosomes. In the absence of FgGga1, the fungal mutant exhibited defects in vegetative growth, DON biosynthesis, ascospore discharge and virulence. Fluorescence microscopy analysis revealed that FgGga1 is associated with trans-Golgi network (TGN)-to-plasma membrane, endosome-to-TGN and endosome-to-vacuole transport. Mutational analysis on the five domains of FgGga1 showed that the VHS domain was required for endosome-to-TGN transport while the GAT^167-248 and the hinge domains were required for both endosome-to-TGN and endosome-to-vacuole transport. Importantly, the deletion of the FgGga1 domains that are required in vesicular trafficking also inhibited vegetative growth and virulence of F. graminearum. In addition, FgGga1 interacted with the ascospore discharge regulator Ca²⁺ ATPase FgNeo1, whose transport to the vacuole is dependent on FgGga1-mediated endosome-to-vacuole transport. Our results suggest that FgGga1 is required for fungal development and virulence via FgGga1-mediated vesicular trafficking, and FgGga1-mediated endosome-to-vacuole transport facilitates ascospore discharge in F. graminearum.

Label-free third harmonic generation imaging and quantification of lipid droplets in live filamentous fungi

We report the utilization of Third-Harmonic Generation microscopy for label-free live cell imaging of lipid droplets in the hypha of filamentous fungus Phycomyces blakesleeanus. THG microscopy images showed bright spherical features dispersed throughout the hypha cytoplasm in control conditions and a transient increase in the number of bright features after complete nitrogen starvation. Colocalization analysis of THG and lipid-counterstained images disclosed that the cytoplasmic particles were lipid droplets. Particle Size Analysis and Image Correlation Spectroscopy were used to quantify the number density and size of lipid droplets. The two analysis methods both revealed an increase from 16 × 10^-3 to 23 × 10^-3 lipid droplets/µm² after nitrogen starvation and a decrease in the average size of the droplets (range: 0.5-0.8 µm diameter). In conclusion, THG imaging, followed by PSA and ICS, can be reliably used for filamentous fungi for the in vivo quantification of lipid droplets without the need for labeling and/or fixation. In addition, it has been demonstrated that ICS is suitable for THG microscopy.

Insights into intracellular signaling network in Fusarium species

Fusarium is a large genus of filamentous fungi including numerous important plant pathogens. In addition to causing huge economic losses of crops, some Fusarium species produce a wide range of mycotoxins in cereal crops that affect human and animal health. The intracellular signaling in Fusarium plays an important role in growth, sexual and asexual developments, pathogenesis, and mycotoxin biosynthesis. In this review, we highlight the recent advances and provide insight into signal sensing and transduction in Fusarium species. G protein-coupled receptors and other conserved membrane receptors mediate recognition of environmental cues and activate complex intracellular signaling. Once activated, the cAMP-PKA and three well-conserved MAP kinase pathways activate downstream transcriptional regulatory networks. The functions of individual signaling pathways have been well characterized in a variety of Fusarium species, showing the conserved components with diverged functions. Furthermore, these signaling pathways crosstalk and coordinately regulate various fungal development and infection-related morphogenesis.

Mitochondrial Porin Is Involved in Development, Virulence, and Autophagy in Fusarium graminearum

Mitochondrial porin, the voltage-dependent anion-selective channel (VDAC), is the most abundant protein in the outer membrane, and is critical for the exchange of metabolites and phospholipids in yeast and mammals. However, the functions of porin in phytopathogenic fungi are not known. In this study, we characterized a yeast porin orthologue, Fgporin, in Fusarium graminearum. The deletion of Fgporin resulted in defects in hyphal growth, conidiation, and perithecia development. The Fgporin deletion mutant showed reduced virulence, deoxynivalenol production, and lipid droplet accumulation. In addition, the Fgporin deletion mutant exhibited morphological changes and the dysfunction of mitochondria, and also displayed impaired autophagy in the non-nitrogen medium compared to the wild type. Yeast two-hybrid and bimolecular fluorescence complementation assays indicated that Fgporin interacted with FgUps1/2, but not with FgMdm35. Taken together, these results suggest that Fgporin is involved in hyphal growth, asexual and sexual reproduction, virulence, and autophagy in F. graminearum.

Two typical acyl-CoA-binding proteins (ACBPs) are required for the asexual development and virulence of Phytophthora sojae

Being found in all eukaryotes investigated, acyl-CoA-binding proteins (ACBPs) participate in lipid metabolism via specifically binding acyl-CoA esters with high affinity. The structures and functions of ACBP family proteins have been extensively described in yeasts, fungi, plants and mammals, but not oomycetes. In the present study, seven ACBP genes named PsACBP1-7 were identified from the genome of Phytophthora sojae, an oomycete pathogen of soybean. CRISPR-Cas9 knockout mutants targeting PsACBP1 and PsACBP2 were created for phenotypic assays. PsACBP1 knockout led to defects in sporangia production and virulence. PsACBP2 knockout mutants exhibited impaired vegetative growth, zoospore production, cyst germination and virulence. Moreover, Nile red staining of PsACBP2 knockout and over-expression lines showed that PsACBP2 is involved in the formation of lipid bodies in P. sojae. Our results demonstrate that two ACBP genes are differently required for growth and development, and both are essential for virulence in P. sojae.

The MoPah1 phosphatidate phosphatase is involved in lipid metabolism, development, and pathogenesis in Magnaporthe oryzae

As with the majority of the hemibiotrophic fungal pathogens, the rice blast fungus Magnaporthe oryzae uses highly specialized infection structures called appressoria for plant penetration. Appressoria differentiated from germ tubes rely on enormous turgor pressure to directly penetrate the plant cell, in which process lipid metabolism plays a critical role. In this study, we characterized the MoPAH1 gene in M. oryzae, encoding a putative highly conserved phosphatidate phosphatase. The expression of MoPAH1 was up-regulated during plant infection. The MoPah1 protein is expressed at all developmental and infection stages, and is localized to the cytoplasm. Disruption of MoPAH1 causes pleiotropic defects in vegetative growth, sporulation, and heat tolerance. The lipid profile is significantly altered in the Mopah1 mutant. Lipidomics assays showed that the level of phosphatidic acid (PA) was increased in the mutant, which had reduced levels of diacylglycerol and triacylglycerol. Using a PA biosensor, we showed that the increased level of PA in the Mopah1 mutant was primarily accumulated in the vacuole. The Mopah1 mutant was blocked in both conidiation and the formation of appressorium-like structures at hyphal tips. It was nonpathogenic and failed to cause any blast lesions on rice and barley seedlings. RNA sequencing analysis revealed that MoPah1 regulates the expression of transcription factors critical for various developmental and infection-related processes. The Mopah1 mutant was reduced in the expression and phosphorylation of Pmk1 MAP kinase and delayed in autophagy. Our study demonstrates that MoPah1 is necessary for lipid metabolism, fungal development, and pathogenicity in M. oryzae.

Phosphorylation-mediated regulation of the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis

The PAH1-encoded phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to produce diacylglycerol, controls the divergence of phosphatidate into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the nuclear/endoplasmic reticulum membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase complex. The phosphorylation of Pah1 by protein kinases, which include casein kinases I and II, Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C, controls its cellular location, catalytic activity, and susceptibility to proteasomal degradation. Nem1 (catalytic subunit) and Spo7 (regulatory subunit), which form a protein phosphatase complex catalyzing the dephosphorylation of Pah1 for its activation, are phosphorylated by protein kinases A and C. In this review, we discuss the functions and interrelationships of the protein kinases in the control of the Nem1-Spo7/Pah1 phosphatase cascade and lipid synthesis.

Mutant phosphatidate phosphatase Pah1-W637A exhibits altered phosphorylation, membrane association, and enzyme function in yeast

The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the dephosphorylation of PA to produce diacylglycerol, controls the bifurcation of PA into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase. We show that the conserved Trp-637 residue of Pah1, located in the intrinsically disordered region, is required for normal synthesis of membrane phospholipids, sterols, triacylglycerol, and the formation of lipid droplets. Analysis of mutant Pah1-W637A showed that the tryptophan residue is involved in the phosphorylation-mediated/dephosphorylation-mediated membrane association of the enzyme and its catalytic activity. The endogenous phosphorylation of Pah1-W637A was increased at the sites of the N-terminal region but was decreased at the sites of the C-terminal region. The altered phosphorylation correlated with an increase in its membrane association. In addition, membrane-associated PA phosphatase activity in vitro was elevated in cells expressing Pah1-W637A as a result of the increased membrane association of the mutant enzyme. However, the inherent catalytic function of Pah1 was not affected by the W637A mutation. Prediction of Pah1 structure by AlphaFold shows that Trp-637 and the catalytic residues Asp-398 and Asp-400 in the haloacid dehalogenase-like domain almost lie in the same plane, suggesting that these residues are important to properly position the enzyme for substrate recognition at the membrane surface. These findings underscore the importance of Trp-637 in Pah1 regulation by phosphorylation, membrane association of the enzyme, and its function in lipid synthesis.

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.

S-adenosyl-L-homocysteine hydrolase FgSah1 is required for fungal development and virulence in Fusarium graminearum

The S-adenosyl-L-homocysteine hydrolase (Sah1) plays a crucial role in methylation and lipid metabolism in yeast and mammals, yet its function remains elusive in filamentous fungi. In this study, we characterized Sah1 in the phytopathogenic fungus F. graminearum by generating knockout and knockout-complemented strains of FgSAH1. We found that the FgSah1-GFP fusion protein was localized to the cytoplasm, and that deletion of FgSAH1 resulted in defects in vegetative growth, asexual and sexual reproduction, stress responses, virulence, lipid metabolism, and tolerance against fungicides. Moreover, the accumulations of S-adenosyl-L-homocysteine (AdoHcy) and S-adenosyl-L-methionine (AdoMet) (the methyl group donor in most methyl transfer reactions) in ΔFgSah1 were seven- and ninefold higher than those in the wild-type strain, respectively. All of these defective phenotypes in ΔFgSah1 mutants were rescued by target gene complementation. Taken together, these results demonstrate that FgSah1 plays essential roles in methylation metabolism, fungal development, full virulence, multiple stress responses, lipid metabolism, and fungicide sensitivity in F. graminearum. To our knowledge, this is the first report on the systematic functional characterization of Sah1 in F. graminearum.

The NDR kinase-MOB complex FgCot1-Mob2 regulates polarity and lipid metabolism in Fusarium graminearum

Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential for cell differentiation and polarized morphogenesis. However, their functions in plant pathogenic fungi are not well understood. Here, we characterized the NDR kinase FgCot1 and its activator FgMob2 in Fusarium graminearum, a major pathogen causing Fusarium head blight (FHB) in wheat. FgCot1 and FgMob2 formed a NDR kinase-MOB protein complex. Localization assays using FgCot1-GFP or FgMob2-RFP constructs showed diverse subcellular localizations, including cytoplasm, septum, nucleus and hyphal tip. ΔFgcot1 and ΔFgmob2 exhibited serious defects in hyphal growth, polarity, fungal development and cell wall integrity as well as reduced virulence in planta. In contrast, lipid droplet accumulation was significantly increased in these two mutants. Phosphorylation of FgCot1 at two highly conserved residues (S462 and T630) as well as five new sites synergistically contributed its role in various cellular processes. In addition, non-synonymous mutations in two MAPK (mitogen-activated protein kinase) proteins, FgSte11 and FgGpmk1, partially rescued the growth defect of ΔFgmob2, indicating a functional link between the FgCot1-Mob2 complex and the FgGpmk1 signalling pathway in regulating filamentous fungal growth. These results indicated that the FgCot1-Mob2 complex is critical for polarity, fungal development, cell wall organization, lipid metabolism and virulence in F. graminearum.

Genome-Wide Analysis of Nutrient Signaling Pathways Conserved in Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal (AM) fungi form a mutualistic symbiosis with a majority of terrestrial vascular plants. To achieve an efficient nutrient trade with their hosts, AM fungi sense external and internal nutrients, and integrate different hierarchic regulations to optimize nutrient acquisition and homeostasis during mycorrhization. However, the underlying molecular networks in AM fungi orchestrating the nutrient sensing and signaling remain elusive. Based on homology search, we here found that at least 72 gene components involved in four nutrient sensing and signaling pathways, including cAMP-dependent protein kinase A (cAMP-PKA), sucrose non-fermenting 1 (SNF1) protein kinase, target of rapamycin kinase (TOR) and phosphate (PHO) signaling cascades, are well conserved in AM fungi. Based on the knowledge known in model yeast and filamentous fungi, we outlined the possible gene networks functioning in AM fungi. These pathways may regulate the expression of downstream genes involved in nutrient transport, lipid metabolism, trehalase activity, stress resistance and autophagy. The RNA-seq analysis and qRT-PCR results of some core genes further indicate that these pathways may play important roles in spore germination, appressorium formation, arbuscule longevity and sporulation of AM fungi. We hope to inspire further studies on the roles of these candidate genes involved in these nutrient sensing and signaling pathways in AM fungi and AM symbiosis.

The RNA binding protein FgRbp1 regulates specific pre-mRNA splicing via interacting with U2AF23 in Fusarium

Precursor messenger RNA (pre-mRNA) splicing is an essential and tightly regulated process in eukaryotic cells; however, the regulatory mechanisms for the splicing are not well understood. Here, we characterize a RNA binding protein named FgRbp1 in Fusarium graminearum, a fungal pathogen of cereal crops worldwide. Deletion of FgRbp1 leads to reduced splicing efficiency in 47% of the F. graminearum intron-containing gene transcripts that are involved in various cellular processes including vegetative growth, development, and virulence. The human ortholog RBM42 is able to fully rescue the growth defects of ΔFgRbp1. FgRbp1 binds to the motif CAAGR in its target mRNAs, and interacts with the splicing factor FgU2AF23, a highly conserved protein involved in 3' splice site recognition, leading to enhanced recruitment of FgU2AF23 to the target mRNAs. This study demonstrates that FgRbp1 is a splicing regulator and regulates the pre-mRNA splicing in a sequence-dependent manner in F. graminearum.

Unique Attributes of the Laurel Wilt Fungal Pathogen, Raffaelea lauricola, as Revealed by Metabolic Profiling

Raffaelea lauricola is the causative agent of laurel wilt, a devastating disease of lauraceous trees. R. lauricola is also an obligate nutritional symbiont of several ambrosia beetle species who act as vectors for the pathogen. Here, we sought to establish the baseline “phenome” of R. lauricola with knowledge concerning its metabolic capability, expanding our understanding of how these processes are impacted by environmental and host nutrients. Phenotypic screening using a microarray of over one thousand compounds was used to generate a detailed profile of R. lauricola substrate utilization and chemical sensitivity. These data revealed (i) relatively restricted carbon utilization, (ii) broad sulfur and phosphate utilization, and (iii) pH and osmotic sensitivities that could be rescued by specific compounds. Additional growth profiling on fatty acids revealed toxicity on C10 substrates and lower, with robust growth on C12–C18 fatty acids. Conditions for lipid droplet (LD) visualization and LD dynamics were examined using a series of lipid dyes. These data provide unique insights regarding R. lauricola metabolism and physiology, and identify distinct patterns of substrate usage and sensitivity which likely reflect important aspects of the host-microbe interface and can be exploited for the development of strategies for mitigating the spread of laurel wilt.

A new atypical short-chain dehydrogenase is required for interfungal combat and conidiation in Trichoderma guizhouense

Hypocrealean Trichoderma are the most extensively studied facultative mycoparasites against phytopathogenic fungi. Aerial hyphae of Trichoderma guizhouense can rapidly proliferate over Fusarium oxysporum hyphae, cause sporadic cell death and arrest the growth of the host. The results of the present study demonstrated that a unique short-chain dehydrogenase/reductase (SDR), designated as TgSDR1, was expressed at a high level in T. guizhouense challenged by the hosts. Similar to other SDRs family members, the TgSDR1 protein contains a cofactor-binding motif and a catalytic site. The subcellular localization assay revealed that the TgSDR1::GFP fusion protein translocated to lipid droplets in mycelia and conidia. The data obtained using reverse genetic approach indicated that TgSDR1 is associated with antifungal ability, plays an important role in providing reducing equivalents in the form of NADPH and regulates the amino sugar and nucleotide sugar metabolism in T. guizhouense upon encountering a host. Moreover, the TgSDR1 deletion mutant was defective in conidiation. Thus, TgSDR1 functions as a key metabolic enzyme in T. guizhouense to regulate mycotrophic interactions, defence against other fungi, such as F. oxysporum, and conidiation.

A review of phosphatidate phosphatase assays

Phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol and regulates the synthesis of membrane phospholipids. There is much interest in this enzyme because it controls the cellular levels of its substrate, phosphatidate (PA), and product, DAG; defects in the metabolism of these lipid intermediates are the basis for lipid-based diseases such as obesity, lipodystrophy, and inflammation. The measurement of PAP activity is required for studies aimed at understanding its mechanisms of action, how it is regulated, and for screening its activators and/or inhibitors. Enzyme activity is determined through the use of radioactive and nonradioactive assays that measure the product, DAG, or Pi However, sensitivity and ease of use are variable across these methods. This review summarizes approaches to synthesize radioactive PA, to analyze radioactive and nonradioactive products, DAG and Pi, and discusses the advantages and disadvantages of each PAP assay.

FgPsd2, a phosphatidylserine decarboxylase of Fusarium graminearum, regulates development and virulence

Phosphatidylserine decarboxylases (Psds) are enzymes regulating phosphatidylethanolamine biosynthesis in prokaryotes and eukaryotes, and have the central role in lipid metabolism. To date, the functions of Psds in plant pathogenic fungi are not fully understood. In this study, we have characterized two yeast Psd orthologues: FgPsd1 and FgPsd2, in Fusarium graminearum. Our results indicate that FgPsd1 and FgPsd2 are localized in mitochondria and Golgi, respectively. In addition, we have determined that FgPsd1 is a lethal gene and deletion of FgPsd2 resulted in a significant reduction of mycelial growth and conidiation. Futhermore, the FgPsd2 deletion mutant (ΔFgPsd2) is defective in ascospore production and virulence in wheat. Our study has also found that the ΔFgPsd2 mutant is more sensitive to osmotic and oxygen stresses. Moreover, deletion of FgPsd2 reduced the formation of lipid droplets and aggravated autophagy in F. graminearum. In summary, our findings indicate that FgPsd2 is important for mycelial growth, sexual and asexual reproduction, virulence, lipid droplet formation and autophagy in F. graminearum.

System-wide characterization of subtilases reveals that subtilisin-like protease FgPrb1 of Fusarium graminearum regulates fungal development and virulence

Subtilases represent the second largest subfamily of serine proteases, and are important for various biological processes. However, the biological function of subtilases has not been systematically characterized in plant pathogens. In present study, 32 subtilases were identified in the genome of wheat scab fungus Fusarium graminearum, a devastating cereal plant pathogen. Deletion mutants of each subtilase were obtained and functionally characterized. Among them, the deletion of FgPrb1 resulted in greatly reduced virulence of F. graminearum. The regulatory mechanisms of FgPrb1 in virulence were investigated in details. Our results showed that the loss of FgPrb1 led to defects in deoxynivalenol (DON) production, responses to environmental stimuli, and lipid metabolism. Additionally, we found that FgPrb1 was involved in autophagy regulation. Taken together, the systematic functional characterization of subtilases showed that the FgPrb1 of F. graminearum is critical for plant infection by regulating multiple different cellular processes.

The Dynamin-Like GTPase FgSey1 Plays a Critical Role in Fungal Development and Virulence in Fusarium graminearum

Fusarium graminearum, the main pathogenic fungus causing Fusarium head blight (FHB), produces deoxynivalenol (DON), a key virulence factor, which is synthesized in the endoplasmic reticulum (ER). Sey1/atlastin, a dynamin-like GTPase protein, is known to be required for homotypic fusion of ER membranes, but the functions of this protein are unknown in pathogenic fungi. Here, we characterized Sey1/atlastin homologue FgSey1 in F. graminearum Like Sey1/atlastin, FgSey1 is located in the ER. The FgSEY1 deletion mutant exhibited significantly reduced vegetative growth, asexual development, DON biosynthesis, and virulence. Moreover, the ΔFgsey1 mutant was impaired in the formation of normal lipid droplets (LDs) and toxisomes, both of which participate in DON biosynthesis. The GTPase, helix bundle (HB), transmembrane segment (TM), and cytosolic tail (CT) domains of FgSey1 are essential for its function, but only the TM domain is responsible for its localization. Furthermore, the mutants FgSey1^K63A and FgSey1^T87A lacked GTPase activity and failed to rescue the defects of the ΔFgsey1 mutant. Collectively, our data suggest that the dynamin-like GTPase protein FgSey1 affects the generation of LDs and toxisomes and is required for DON biosynthesis and pathogenesis in F. graminearum IMPORTANCE Fusarium graminearum is a major plant pathogen that causes Fusarium head blight (FHB) of wheats worldwide. In addition to reducing the plant yield, F. graminearum infection of wheats also results in the production of deoxynivalenol (DON) mycotoxins, which are harmful to humans and animals and therefore cause great economic losses through pollution of food products and animal feed. At present, effective strategies for controlling FHB are not available. Therefore, understanding the regulation mechanisms of fungal development, pathogenesis, and DON biosynthesis is important for the development of effective control strategies of this disease. In this study, we demonstrated that a dynamin-like GTPase protein Sey1/atlastin homologue, FgSey1, is required for vegetative growth, DON production, and pathogenicity in F. graminearum Our results provide novel information on critical roles of FgSey1 in fungal pathogenicity; therefore, FgSey1 could be a potential target for effective control of the disease caused by F. graminearum.

Yck1 casein kinase I regulates the activity and phosphorylation of Pah1 phosphatidate phosphatase from Saccharomyces cerevisiae

The PAH1-encoded phosphatidate phosphatase in Saccharomyces cerevisiae plays a major role in triacylglycerol synthesis and the control of phospholipid synthesis. For its catalytic function on the nuclear/endoplasmic reticulum membrane, Pah1 translocates to the membrane through its phosphorylation/dephosphorylation. Pah1 phosphorylation on multiple serine/threonine residues is complex and catalyzed by diverse protein kinases. In this work, we demonstrate that Pah1 is phosphorylated by the YCK1-encoded casein kinase I (CKI), regulating Pah1 catalytic activity and phosphorylation. Phosphoamino acid analysis coupled with phosphopeptide mapping of the CKI-phosphorylated Pah1 indicated that it is phosphorylated mainly on multiple serine residues. Using site-directed mutagenesis and phosphorylation analysis of Pah1, we identified eight serine residues (i.e. Ser-114, Ser-475, Ser-511, Ser-602, Ser-677, Ser-705, Ser-748, and Ser-774) as the target sites of CKI. Of these residues, Ser-475 and Ser-511 were specific for CKI, whereas the others were shared by casein kinase II (Ser-705), Cdc28-cyclin B (Ser-602), Pho85-Pho80 (Ser-114, Ser-602, and Ser-748), protein kinase A (Ser-667 and Ser-774), and protein kinase C (Ser-677). CKI-mediated phosphorylation of Pah1 stimulated both its phosphatidate phosphatase activity and its subsequent phosphorylation by casein kinase II. However, the CKI-mediated phosphorylation of Pah1 strongly inhibited its subsequent phosphorylation by Pho85-Pho80, protein kinase A, and protein kinase C. In a reciprocal analysis, Pah1 phosphorylation by Pho85-Pho80 inhibited subsequent phosphorylation by CKI. CKI-mediated Pah1 phosphorylation was also inhibited by a peptide containing the Pah1 residues 506-517, including the kinase-specific Ser-511 residue. These findings advance our understanding of how Pah1 catalytic activity and phosphorylation are regulated by multiple protein kinases.

Fusarium graminearum Trichothecene Mycotoxins: Biosynthesis, Regulation, and Management

Fusarium head blight (FHB) of small grain cereals caused by Fusarium graminearum and other Fusarium species is an economically important plant disease worldwide. Fusarium infections not only result in severe yield losses but also contaminate grain with various mycotoxins, especially deoxynivalenol (DON). With the complete genome sequencing of F. graminearum, tremendous progress has been made during the past two decades toward understanding the basis for DON biosynthesis and its regulation. Here, we summarize the current understanding of DON biosynthesis and the effect of regulators, signal transduction pathways, and epigenetic modifications on DON production and the expression of biosynthetic TRI genes. In addition, strategies for controlling FHB and DON contamination are reviewed. Further studies on these biosynthetic and regulatory systems will provide useful knowledge for developing novel management strategies to prevent FHB incidence and mycotoxin accumulation in cereals.