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Overproduction of mycotoxin biosynthetic enzymes triggers Fusarium toxisome-shaped structure formation via endoplasmic reticulum remodeling [1]

['Minhui Wang', 'State Key Laboratory Of Rice Biology', 'Key Laboratory Of Biology Of Crop Pathogens', 'Insects', 'Institute Of Biotechnology', 'Zhejiang University', 'Hangzhou', 'People S Republic Of China', 'Ningjie Wu', 'Zhejiang Research Institute Of Chemical Industry']

Date: 2024-02

Mycotoxin deoxynivalenol (DON) produced by the Fusarium graminearum complex is highly toxic to animal and human health. During DON synthesis, the endoplasmic reticulum (ER) of F. graminearum is intensively reorganized, from thin reticular structure to thickened spherical and crescent structure, which was referred to as “DON toxisome”. However, the underlying mechanism of how the ER is reorganized into toxisome remains unknown. In this study, we discovered that overproduction of ER-localized DON biosynthetic enzyme Tri4 or Tri1, or intrinsic ER-resident membrane proteins FgHmr1 and FgCnx was sufficient to induce toxisome-shaped structure (TSS) formation under non-toxin-inducing conditions. Moreover, heterologous overexpression of Tri1 and Tri4 proteins in non-DON-producing fungi F. oxysporum f. sp. lycopersici and F. fujikuroi also led to TSS formation. In addition, we found that the high osmolarity glycerol (HOG), but not the unfolded protein response (UPR) signaling pathway was involved in the assembly of ER into TSS. By using toxisome as a biomarker, we screened and identified a novel chemical which exhibited high inhibitory activity against toxisome formation and DON biosynthesis, and inhibited Fusarium growth species-specifically. Taken together, this study demonstrated that the essence of ER remodeling into toxisome structure is a response to the overproduction of ER-localized DON biosynthetic enzymes, providing a novel pathway for management of mycotoxin contamination.

The mycotoxin deoxynivalenol (DON) produced by the Fusarium graminearum complex is the most frequently detected mycotoxin in cereal grains worldwide. DON is synthesized in the compartmentalized organelle named “DON toxisome” that is highly remodeled, organized smooth endoplasmic reticulum (OSER). In this study, we demonstrated that ER remodeling into toxisome structure in F. graminearum is a response to overproduction of ER-localized DON biosynthetic enzymes, including Tri1 and Tri4. We further found that the HOG signaling pathway is important for the toxisome-shaped structure assembly. Moreover, using the Tri1-GFP labeled toxisome as a biomarker, we identified a novel small chemical exhibiting high inhibitory activity on toxisome assembly and DON biosynthesis. Our study elucidates the intrinsic mechanism of ER remodeling into toxisomes upon DON induction, and blocking the ER remodeling would provide a new avenue for management of FHB and mycotoxin contamination.

Funding: This research was supported by the National Natural Science Foundation of China (31930088 to ZM, 32102145 to MW, U21A20219 to ZM), National Key Research and Development Program of China (2022YFD1400100 to ZM) and China Agriculture Research System (CARS-03-29 to ZM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

The objective of this study was to explore the underlying mechanism of toxisome formation under DON-inducing conditions. Our results showed that overproduction of ER-localized SM enzymes Tri4 or Tri1, or intrinsic ER-resident membrane proteins FgHmr1 and FgCnx was sufficient to induce toxisome-shaped structure (TSS) formation, even under non-toxin-inducing conditions. Interestingly, heterologous overexpression of Tri1 and Tri4 proteins in trichothecene non-production fungi F. oxysporum f. sp. lycopersici and F. fujikuroi also leads to TSS formation. In addition, the UPR signaling pathway appears to be unnecessary for the remodeling of ER into TSS, whereas the HOG signaling pathway is important for the TSS assembly. Importantly, using the Tri1-GFP labeled toxisome as a biomarker, we identified a novel compound ZJU212 with high inhibitory activity on toxisome assembly and DON biosynthesis, providing new ways for management of DON contamination and FHB.

Enzymes for fungal secondary metabolite (SM) synthesis are often compartmentalized at conserved subcellular sites, which plays important roles in precursor channeling, concentration of biosynthetic components, sequestering and trafficking pathway intermediates and products, and promoting pathway efficiency [ 14 , 18 ]. In Aspergillus, aflatoxin biosynthetic enzymes Nor-1, Ver-1 and Vbs initially reside in cytoplasm and then relocate to motile vesicles termed aflatoxisomes for aflatoxin biosynthesis [ 19 , 20 , 21 ]. In Penicillium chrysogenum, the biosynthesis site of penicillin shifts from cytoplasm to the peroxisome for the formation of final product [ 22 ]. In A. fumigatus and A. nidulans, melanin biosynthetic enzymes involved in the early steps are recruited to endosomes to facilitate melanogenesis, while late melanin enzymes accumulate in the cell wall [ 23 ]. Recent studies have suggested that subcellular compartmentalization also occurs during DON biosynthesis in F. graminearum [ 24 , 25 ]. Menke and colleagues first observed that under DON-inducing conditions, two cytochrome P-450 oxygenases (Tri4 and Tri1) responsible for catalyzing early and late steps in trichothecene biosynthesis in F. graminearum were co-localized to ∼3 μm spherical organelles called “toxisomes”, which were presumed to be the site of trichothecene assembly [ 26 ]. Later, it was discovered that toxisomes are highly remodeled, organized smooth endoplasmic reticulum (OSER) with pronounced expansion at perinuclear- and peripheral positions [ 24 ]. Interestingly, under non-DON-inducing conditions, the native perinuclear and peripheral ER in F. graminearum appeared reticulate and thin as determined by ER marker proteins GFP-HDEL, Hmr1-GFP and Sec22-GFP, while the ER was remodeled to highly thickened spherical, crescent and ovoid toxisomes upon DON induction, indicating a striking remodeling of the ER structure under toxin inducing conditions [ 24 ]. The remodeling of ER (e.g. toxisome formation) appears to be critical for trichothecene production since inhibition of toxisome formation by the fungicide phenamacril, which targets the motor protein myosin I, leads to significant reduction in DON accumulation [ 25 ]. However, the molecular mechanism of how the ER is reorganized into toxisomes remains elusive.

Trichothecenes are a large family of toxic sesquiterpenoid secondary metabolites (SMs) produced by certain species of Fusarium and other fungal genera [ 11 ]. Trichothecenes produced by F. graminearum include DON, nivalenol (NIV), and acetylated derivatives 15-ADON and 3-ADON [ 4 ]. The trichothecene biosynthetic gene (TRI) cluster is one of the most studied SM gene clusters in fungi. In F. graminearum, the biosynthesis of trichothecene involves 15 TRI genes, which are located on three different chromosomes in F. graminearum: a 12-gene core TRI cluster on chromosome 2, two genes at the TRI1-TRI16 locus on chromosome 1, and the single-gene TRI101 locus on chromosome 3 [ 4 , 12 , 13 ]. Trichothecene biosynthesis begins with the cyclization of the primary metabolite farnesyl pyrophosphate, which is catalyzed by the trichodiene synthase Tri5, resulting in the product trichodiene (TDN). TDN is subsequently oxidized by cytochrome P450 monooxygenase Tri4 to yield isotrichotriol. Further reactions sequentially catalyzed by Tri101, Tri11 and Tri3 convert isotrichotriol to calonectrin, which is hydroxylated by cytochrome P450 monooxygenase Tri1 to generate 7,8-dihydroxycalonectrin (7,8-DHC). The following transformations convert 7,8-DHC to 3-ADON or 15-ADON, which is deacetylated by Tri8, leading to the formation of DON [ 4 , 12 , 14 ]. In addition to genes encoding trichothecene biosynthetic enzymes, the F. graminearum TRI cluster also encodes a predicted major facilitator superfamily (MFS) transporter Tri12, and two cluster-specific transcription regulators Tri6 and Tri10 [ 15 , 16 , 17 ]. Because trichothecene is a secondary metabolite, the TRI cluster genes are not expressed in toxin non-inducing conditions, while their expressions are highly induced during incubation in toxin-inducing medium or during infection on plants [ 4 ].

Fusarium graminearum is an aggressive fungal pathogen causing Fusarium head blight (FHB), which is an economically devastating disease of cereal crops especially wheat [ 1 , 2 ]. Due to global warming and changes in cultural practices, FHB has frequently reached epidemic levels in wheat-growing regions worldwide during the past 20 years, resulting in enormous yield losses across millions of hectares [ 3 , 4 , 5 ]. In addition to severe yield and economic losses, F. graminearum produces various mycotoxins during infection of wheat, such as trichothecenes (including deoxynivalenol (DON) and its acetylated derivatives) and zearalenone, thus raising food safety risks and posing a great threat to human and animal health [ 4 ]. Among these mycotoxins, DON is the most frequently detected mycotoxin in cereal grains all over the world, with an average incidence rate more than 50% [ 4 ]. DON inhibits eukaryotic protein synthesis by binding to the ribosome and may cause emesis, diarrhea, anorexia and immunedysregulation [ 6 , 7 ]. Consequently, many countries have set maximum permissible levels for DON in cereals and cereal products to protect consumers from mycotoxicosis [ 8 , 9 ]. In addition, DON is a key virulence factor that promotes F. graminearum infection on wheat plants [ 10 ]. Therefore, understanding the biosynthesis and regulation of DON in F. graminearum is critical for combatting FHB and mycotoxin contamination.

Results

Overexpressed Tri proteins induce toxisome-shaped structure formation under toxin non-inducing conditions In F. graminearum, the expression of trichodiene (Tri4) and calonectrin oxygenase (Tri1) enzymes that catalyze early and late steps of the trichothecene biosynthetic pathway is highly upregulated in trichothecene biosynthesis inducing (TBI) medium [4]. Both enzymes co-localized to DON toxisomes when F. graminearum was cultured in TBI medium for 48 h, when accumulation of Tri1 and Tri4 proteins reached peak levels [25]. However, at 20–24 h of incubation in TBI medium, expression of Tri1 and Tri4 protein was low, and Tri1-GFP or Tri4-GFP was observed as faint reticulate GFP pattern of the ER and no toxisome structures were observed [24]. Based on these observations, we hypothesized that remodeling of the ER into DON toxisome structure correlates with the overexpression of Tri1 and Tri4 proteins. To test this, Tri4-GFP and Tri1-GFP constructs with a strong constitutive promoter (the gpda promoter from Aspergillus nidulans) were generated and transformed into ΔTri4 and ΔTri1 mutants, respectively. The corresponding GFP fusion constructs with the native promoter (np) were used as controls. To visualize the ER structure, the ER marker RFP-HDEL expression vector was also constructed and transformed into the ΔTri4 and ΔTri1 strains. As shown in Fig 1A and 1B (left panels), Tri4- and Tri1-GFP driven by native promoters displayed no fluorescence signals in the DON non-inducing medium YEPD (yeast extract peptone dextrose) after 48 hours of incubation, while the ER labeled by RFP-HDEL showed a thin reticular structure. Whereas Tri4- and Tri1-GFP were highly induced and localized at the thickened spherical and crescent structures (toxisomes) in the TBI medium after incubation for 48 hours, and the toxisomes co-localized with the RFP-HDEL labelled ER (Fig 1A and 1B left panels). When Tri4- and Tri1-GFP were driven under the gpda promoter, the two proteins displayed toxisome localizations even in the YEPD medium after 48 h growth and localized to the remodeled ER (Fig 1A and 1B right panel). In the TBI medium, Tri4- and Tri1-GFP under the gpda promoter were similarly localized to toxisomes (Fig 1A and 1B right panel). Western blotting assay further showed that expression of the Tri4- or Tri1-GFP proteins driven by the native promoter strains in TBI medium was similar to those by the gpda promoter strains in YEPD medium (Fig 1C and 1D). Due to the ability of strains expressed under gpda promoter to form toxisome structure in YEPD, we next examined the DON production of these strains under DON non-inducing (YEPD) and inducing (TBI) conditions by LC-MS. Results showed that although overexpression of Tri4- and Tri1-GFP under gpda promoter induced the ER remodeling into spherical and crescent structures in YEPD medium (Fig 1A and 1B), the two strain ΔTri4::gpda-Tri4-GFP::RFP-HDEL and ΔTri1::gpda-Tri1-GFP::RFP-HDEL did not produce any DON toxin in YEPD medium, while produced DON normally in TBI (S1 Fig). Therefore, these Tri protein overexpression-induced spherical and crescent structures formed in non-inducing medium were not a functional “toxisome” with the ability for DON production, and thus were designated as “toxisome-shaped structure” (TSS). In addition, we also assessed the formation of TSS in another two commonly used nutrient-rich liquid media for F. graminearum growth, namely complete medium (CM) and potato dextrose broth (PDB). TSS was also observed in the strain ΔTri4::gpda-Tri4-GFP and ΔTri1::gpda-Tri1-GFP (under gpda promoter) after 48 h of incubation in CM and PDB media (S2 Fig), suggesting that formation of TSS is not related to the medium, but resulted from overexpression of Tri proteins. These results indicate that overproduction of Tri4 or Tri1 protein is sufficient for induction of ER remodeling to form TSS under DON non-inducing conditions. PPT PowerPoint slide

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TIFF original image Download: Fig 1. Overproduction of Tri4 and Tri1 induced toxisome-shaped structure (TSS) formation under DON non-inducing conditions. (A-B) Localization of Tri4-GFP (A) and Tri1-GFP (B) under native promoter (left panels) and gpda promoter (right panels) in YEPD medium or TBI medium. RFP-HDEL is used as the ER marker. Images of each strain were taken after incubation at 28°C for 48 h. DIC indicates differential interference contrast. Bar = 10 μm. (C-D) The protein abundance of Tri4-GFP (C) and Tri1-GFP (D) isolated from the same set of samples used in A and B was determined by western blot assay with the anti-GFP antibody. The protein abundance of H3 of each sample served as a loading control. The intensities of the western blot bands were quantified using the Image J software, and numbers below the bands represent relative intensity of GFP normalized to H3. Native and gpda represent Tri proteins that are expressed under native and gpda promoters, respectively. https://doi.org/10.1371/journal.ppat.1011913.g001

The transmembrane domain of Tri4 is essential for Tri4-marked TSS formation Tri4 and Tri1 are cytochrome P450 monooxygenases, each with a p450 catalytic domain at their C-terminus (Fig 2A). As predicted by InterPro [27], both Tri4 and Tri1 contain a putative short transmembrane domain (TMD) at their N-terminus (Fig 2A). To determine the role of TMD and p450 catalytic domains in toxisome-shaped structure formation, two truncated Tri4 constructs: the Tri4ΔTMD-GFP fusion construct (a truncated Tri4 lacking 13–35 aa fused with GFP) and Tri4Δp450-GFP fusion construct (a truncated Tri4 lacking 47–520 aa fused with GFP) under native promoter were generated and transformed into ΔTri4, respectively. The complemented strain ΔTri4::Tri4-GFP with full-length Tri4 was used as positive control. After 48 hours of incubation in TBI medium, the full-length Tri4-GFP was localized to the toxisomes, while the truncated Tri4ΔTMD-GFP protein was detected as diffuse fluorescent signals in the cytoplasm without any toxisome localization (Fig 2B). Interestingly, in the dual-labelled strain ΔTri4::Tri4ΔTMD-GFP::Tri1-RFP under native promoter, although the transmembrane-deleted Tri4ΔTMD-GFP protein was still localized dispersedly in the cytoplasm outside of toxisome, Tri1-RFP was localized to the toxisome-shaped structures (TSS) after 48 h incubation in TBI (S3 Fig), indicating that the transmembrane domain of Tri4 only affects the TSS labelled by Tri4, but not other protein like Tri1-induced TSS. The Tri4Δp450-GFP was partially localized to TSS, with some GFP signals simultaneously observed in cytoplasm (Fig 2B). However, the number of Tri4-marked TSS formed in the ΔTri4::Tri4Δp450-GFP strain decreased significantly as compared with the ΔTri4::Tri4-GFP strain (Fig 2B and 2C). These results indicate that the transmembrane domain of Tri4 is essential for Tri4-marked TSS formation while the p450 catalytic domain is partially required for Tri4-marked TSS formation. Consistent with TSS formation labelled by Tri4, the ΔTri4::Tri4ΔTMD-GFP strain was nearly unable to produce DON (Fig 2D), indicating that localization of Tri4 to the toxisome is critical for DON production. PPT PowerPoint slide

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TIFF original image Download: Fig 2. An essential role of the transmembrane domain of Tri4 in the formation of Tri4-marked TSS. (A) Domain architecture of Tri4 and Tri1. The number at the bottom indicates the deduced amino acid positions of corresponding domains. (B) The TSS formation patterns in truncated Tri4 proteins. Each strain was grown in TBI medium for 48 h. Bar = 10 μm. (C) The average number of toxisome-shaped structures in an examination field of 135 μm × 135 μm. (D) DON production of the truncated Tri4 complemented strains. After growth in TBI for 7 d, each strain was determined for DON production. Data represent the mean ± s.d. from three independent experiments. Different letters indicate a significant difference (P < 0.05) based on one-way ANOVA followed by Tukey’s multiple comparison test. https://doi.org/10.1371/journal.ppat.1011913.g002

Overexpression of ER-resident proteins FgHmr1 and FgCnx is able to induce ER remodeling under toxin non-inducing conditions It has been reported that a few dozen ER resident enzymes can induce ER hypertrophy and reorganization when expressed at elevated levels [24]. Given that overexpression of ER-localized oxygenases Tri4 and Tri1 induced ER remodeling (Fig 1), we speculated whether overexpression of other ER-resident proteins would have similar effect. To test this, two ER-resident proteins FgHmr1 and FgCnx were selected for further investigation. Hmr1 (HMG-CoA reductase, a key enzyme in the isoprenoid biosynthetic pathway) and Cnx (calnexin, molecular chaperone of the ER) are two highly conserved integral membrane protein of ER in eukaryotes [28,29], and have been frequently used as ER markers in fungi and animals [30,31,32]. Different from Tri4 and Tri1, which are expressed only under specific induction conditions (such as TBI medium), Hmr1 and Cnx are constitutive expressed ER proteins. To determine effect of overproduction of these two proteins on ER remodeling, FgHmr1 and FgCnx were tagged with GFP at their C-terminal in situ to generate the strains np-FgHmr1-GFP and np-FgCnx-GFP driven by their native promoter (np), in which the np was further replaced with gpda strong promoter, yielding the overexpression strains gpda-FgHmr1-GFP and gpda-FgCnx-GFP. After incubation for 48 h in YEPD medium, fluorescent signal of FgHmr1-GFP or FgCnx-GFP under np was faint and observed at thin spherical (presumably perinuclear) and reticulate peripheral structures (Fig 3A and 3B, upper panel); while GFP fluorescent signals in the gpda-FgHmr1-GFP and gpda-FgCnx-GFP strains were significantly higher with thickened spherical and crescent structures (Fig 3A and 3B, lower panel), indicating a dramatic remodeling in ER structure. Western blotting assay further confirmed that the amount of FgCnx-GFP protein increased about 7 folds under the strong gpda promoter as compared to that under the np (Fig 3D). Consistently, the FgHmr1-GFP protein also increased significantly under the gpda promoter (Fig 3C). Interestingly, overexpression of another ER-localized membrane protein 14-α-demethylase FgCyp51A [33] in F. graminearum by the gpda promoter failed to induce ER remodeling into TSS (S4 Fig). Together, these results revealed that remodeling of ER into TSS can also be directly triggered by overproduction of certain ER-resident proteins in F. graminearum. PPT PowerPoint slide

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TIFF original image Download: Fig 3. Overexpression of ER-resident proteins FgHmr1 and FgCnx triggers the ER remodeling into TSS. (A-B) Localization of FgHmr1-GFP (A) and FgCnx-GFP (B) under native promoter (upper panels) and gpda promoter (lower panels) in YEPD. Each strain was grown in YEPD medium at 28°C for 48 h. Bar = 10 μm. (C-D) The protein abundance of FgHmr1-GFP (C) and FgCnx-GFP (D) under native and gpda promoters was determined by western blot assay using the anti-GFP antibody. The protein H3 was used as a reference. The intensities of the western blot bands were quantified using the Image J software, and numbers below the bands represent relative intensity of GFP normalized to H3. https://doi.org/10.1371/journal.ppat.1011913.g003

ER remodeling in other fungi that do not produce DON To further verify that formation of TSS in F. graminearum is triggered by overproduction of the DON biosynthetic enzymes, we preformed heterologous expression of Tri1 and Tri4 proteins in other filamentous fungi that do not produce DON. The Tri4-GFP or Tri1-GFP fusion construct under the strong gpda promoter identical to Fig 1 was transformed into Fusarium oxysporum f. sp. lycopersici (Fol) (causal agents of wilt disease of tomato) and Fusarium fujikuroi (Ff) (causal agents of bakanae of rice), respectively. It should be noted that these two phytopathogenic fungi don’t contain the DON biosynthetic gene cluster in their genomes. Results showed that both Tri4-GFP and Tri1-GFP were observed to be localized on the TSS in Fol and Ff (Fig 4A and 4B), indicating that high-level heterologous expression of the DON biosynthesis enzymes Tri1 or Tri4 in Fol and Ff is sufficient for induction of the ER remodeling to form TSS. PPT PowerPoint slide

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TIFF original image Download: Fig 4. Remodeling of the ER in other fungi that do not produce DON. (A-B) Heterologous overexpression of Tri4-GFP (A) and Tri1-GFP (B) in F. oxysporum f. sp. lycopersici (Fol) (left panels) and F. fujikuroi (Ff) (right panels) induced the formation of TSS. Each strain was incubated in YEPD at 28°C for 48 h. Bar = 10 μm. (C) Localization of ER-resident protein Hmr1 under its native promoter in Fol (upper panel) and Ff (lower panel). Each strain was incubated in YEPD medium at 28°C for 48 h. Bar = 10 μm. (D) Upon overexpression of the endogenous Hmr1 in Fol (upper panel) and Ff (lower panel) by gpda promoter, the ER was reorganized into TSS. Images of each strain were taken after incubation in YEPD medium at 28°C for 48 h. Bar = 10 μm. https://doi.org/10.1371/journal.ppat.1011913.g004 This observation also promoted us to ask whether homologous overproduction of ER-resident proteins in these two fungi would result in ER remodeling as well. To test this, endogenous ER-localized proteins FolHmr1 in Fol and FfHmr1 in Ff were fused with GFP at their C-terminals in situ to generate the strains np-FolHmr1-GFP and np-FfHmr1-GFP under their native promoters (np). Concurrently, the overexpression strains gpda-FolHmr1-GFP and gpda-FfHmr1-GFP, in which FolHmr1 and FfHmr1 were driven by the gpda promoter were also generated. When Hmr1-GFP was expressed under the np, both np-FolHmr1-GFP and np-FfHmr1-GFP displayed faint signals and localized at a thin perinuclear and reticulate peripheral ER (Fig 4C). Upon overexpression of Hmr1-GFP driven by the gpda promoter, thicker spheres and crescent ER structures were observed in both gpda-FolHmr1-GFP and gpda-FfHmr1-GFP strains (Fig 4D). These results demonstrated that homologous overproduction of the endogenous ER-resident protein Hmr1 in Fol and Ff can induce ER remodeling to form TSS.

Regulation of the Tri4-marked TSS formation by the positive regulators in TRI cluster Next, we attempted to explore the regulators for TSS formation. We first focused on the effect of the TRI cluster on the formation of toxisome since many fungal secondary metabolite (SM) biosynthesis gene clusters generally possess a self-regulation role. The TRI cluster contains 15 TRI genes, which are located at three different loci on different chromosomes in F. graminearum. Among them, the TRI16 is only functional in F. sporotrichioides which produces T-2 toxin (a type of trichothecenes), while in F. graminearum the TRI16 homologue is nonfunctional due to multiple insertions and deletions in its coding region [12]. Thus the remaining 13 TRI genes were knocked out individually in the np-Tri4-GFP/RFP-HDEL dual labeled strain to determine the function of TRI cluster on toxisome-shaped structure formation. Since TRI gene cluster hardly expressed under normal nutrient-rich medium, TSS formation in mycelia of these TRI gene deletion mutants harboring the tagged Tri4-GFP/RFP-HDEL was examined after incubation of each strain in TBI medium. All the TRI gene deletion mutants except for TRI6 and TRI10 produced typical TSS, and the Tri4-GFP labeled TSS co-localized with RFP-HDEL labeled ER (Fig 5A). In the ΔTri6 and ΔTri10 mutants, the Tri4-GFP fluorescent signals decreased dramatically and no typical TSS were observed in the mycelia, while the RFP-HDEL labeled ER was also faint (Fig 5A). Both TRI6 and TRI10 have been identified as positive regulatory genes in the TRI cluster for regulating the expression of trichothecene biosynthetic enzyme-encoding genes [15]. Consistently, the amounts of Tri4-GFP protein in ΔTri6 and Δtri10 mutants were significantly reduced or undetectable as compared with that in the wild-type strain after 48 h induction in TBI medium (Fig 5B). Whereas, except for TRI6 and TRI10, the protein levels of Tri4-GFP in the remaining TRI gene deletion mutants were similar to or slightly higher than that in the wild-type strain (Fig 5B). These results indicate that in the TRI cluster, TRI6 and TRI10 are essential for the Tri4-marked TSS formation, which may result from the significantly decreased amount of Tri4 protein in these two gene deletion mutants. PPT PowerPoint slide

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TIFF original image Download: Fig 5. Regulation of the Tri4-marked TSS formation by the TRI cluster. (A) The effect of gene deletion in the TRI cluster on the formation of TSS under DON inducing conditions. The wild-type strain and TRI gene deletion mutants were grown in TBI medium at 28°C for 48 h. RFP-HDEL is used as the ER marker. Bar = 10 μm. (B) Accumulation of Tri4-GFP proteins in TRI gene deletion mutants was determined by western blot with the anti-GFP antibody (left panel). The protein abundance of H3 in each sample served as a loading control. The relative intensity of Tri4-GFP in each strain was shown in right panel, and was calculated by determining the intensity of Tri4-GFP band normalized to the intensity of H3 band. The relative intensity of the wild-type strain PH-1 was normalized to 1. Data represent the mean ± s.d. from three independent experiments. https://doi.org/10.1371/journal.ppat.1011913.g005

The UPR signaling pathway is not involved in TSS formation in F. graminearum During formation of toxisome in TBI medium, F. graminearum cells are subjected to many stress conditions, such as nutrient deprivation, acidic pH, reactive oxygen species (ROS), which are potential inducers provoking ER stress [34,35]. In addition, ER-localized Tri proteins (Tri4 and Tri1) were highly induced during DON induction [4,25], which might also result in ER stress. To mitigate ER stress, eukaryotic cells activate a signal transduction pathway called the UPR, which maintains ER homeostasis by regulating the expression of numerous genes encoding ER chaperones, folding enzymes and other proteins [34]. In Saccharomyces cerevisiae, the UPR pathway is composed of the kinase Ire1 and the bZIP transcription factor Hac1. Upon ER stress, Ire1 is activated and then removes the unconventional intron of the HAC1 mRNA via its endoribonuclease domain in a spliceosome-independent manner. Spliced HAC1 mRNA is subsequently translated to produce an active Hac1 protein to upregulate expression of UPR target genes [34]. To determine whether the UPR signaling pathway contributes to F. graminearum toxisome formation, we first tried to knock out the homologue of Ire1 (FGRAMPH1_01G18735) and Hac1 (FGRAMPH1_01G11295) in F. graminearum. However, this attempt failed after screening over 100 ectopic transformants from at least three independent transformation experiments, indicating that these two genes are likely to be essential in F. graminearum due to high homologous recombination efficiency in this fungus [36,37]. F. graminearum HAC1 was predicted to contain a 20-nt non-canonical intron spliced by Ire1 [38], with a conserved Ire1 cleavage motif CNG’CNGN at the 5’ and 3’ boundary of the intron (Fig 6A, upper panel). To further investigate whether the UPR pathway regulates toxisome formation, we checked the FgHAC1 mRNA splicing pattern under DON inducing (TBI medium) and non-inducing (YEPD medium) conditions with RT-PCR. The YEPD medium supplemented with dithiothreitol (DTT), an ER stress agent, was used as a positive control. Under normal nutrient conditions (YEPD), two bands: one unspliced (FgHAC1U) and the other spliced (FgHAC1S), were detected (Fig 6A, lower panel), indicating that even without an ER-stressing stimulus, the UPR pathway was partially activated. Upon ER stress (YEPD+DTT), the splicing of FgHAC1 was triggered intensively, indicating a significant activation of the UPR signaling pathway under DTT treatment (Fig 6A, lower panel). DNA sequencing of the two bands confirmed that the 20-nt non-canonical intron (Fig 6A, upper panel) was excised in the spliced band. Unexpectedly, the splicing band patterns of FgHAC1 in TBI conditions during the time course of 0–60 hours were similar to that in YEPD medium (Fig 6A, lower panel). Therefore, it is reasonable to speculate that the UPR signaling pathway is not further activated under DON inducing condition as comparison to that under the DON non-inducing condition. PPT PowerPoint slide

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TIFF original image Download: Fig 6. The HOG, but not the UPR signaling pathway, is involved in TSS assembly in F. graminearum. (A) The FgHAC1 mRNA splicing pattern in TBI and YEPD medium. DNA sequence alignment of the spliced and unspliced FgHAC1 (upper panel). The 20 bp non-canonical intron is indicated with lowercase blue letters (upper panel). The FgHAC1 mRNA splicing pattern was analyzed by RT-PCR using the total RNA of PH-1 after incubation for the indicated times in TBI and YEPD medium (lower panel). PH-1 cultured in YEPD for 12 h and then treated with the ER stress agent DTT (10 mM) for 2 h was used as the positive control. Unspliced and spliced FgHAC1 transcripts yielded 211 and 191-bp products, respectively. The transcripts of FgACTIN gene (FGRAMPH1_01G24551) served as the loading control. (B) DTT induces an aggregation of ER membrane. The strain PH-1::GFP-HDEL was used to visualize the ER and cultured in YEPD medium for 12 h followed by treatment with DTT for the indicated times. Bar = 10 μm. (C) The FgHog1 deletion mutant failed to induce the TSS formation under DON non-inducing conditions (YEPD medium). Images of each strain were taken after incubation in YEPD at 28°C for 48 h. Bar = 10 μm. (D) Time course analysis of the phosphorylation of FgHog1 in TBI. After growing in TBI medium at 28°C for the indicated times, mycelia of PH-1 were harvested for protein extraction in western blot analysis. FgHog1 and phosphorylated FgHog1 were detected using anti-Hog1 antibody and Anti-phospho-p38 antibody, respectively. https://doi.org/10.1371/journal.ppat.1011913.g006 Given that the ER stress inducer DTT is able to intensively activate the UPR pathway in F. graminearum (Fig 6A), we wondered whether the ER would be reorganized with DTT treatment, a condition inducing the UPR pathway. Using the ER marker GFP-HDEL, we found that DTT treatment induced globular aggregation structures linked with the perinuclear- and peripheral ER, which was more apparent with prolong treatment time (Fig 6B). However, this ER remodeling mediated by DTT was clearly distinct from TSS, with strongly thickened spherical and crescent structures around nuclear (Figs 1A, 1B, 3A and 3B). These results suggest that the UPR signaling pathway is not required for the toxisome formation in F. graminearum.

The HOG signaling pathway is important for TSS formation To further identify potential signal pathways that are involved in TSS formation, we focused on the three major mitogen-activated protein kinase (MAPK) signal transduction pathways: the cell wall integrity (CWI) signaling pathway, the high osmolarity glycerol (HOG) pathway, and the penetration pathway, which have all been reported to regulate F. graminearum DON production and expression of the TRI genes [4]. Three central components of these pathways: FgGpmk1 (MAPK of the penetration pathway), FgMgv1 (MAPK of the CWI pathway), and FgHog1 (MAPK of the HOG pathway) were thus knocked out in the strain PH-1::gpda-Tri1-GFP. We used this strain due to that the expression of Tri1-GFP protein was driven by the gpda promoter, which could maintain at a relatively stable level to avoid the effect of disruption of these kinases on the expression of TRI1-GFP gene. Results showed that the FgGpmk1 and FgMgv1 deletion mutants produced normal TSS as the wild-type strain in YEPD medium, whereas the FgHog1 deletion mutant failed to form the typical toxisome-shaped structures (Fig 6C), suggesting that the HOG signaling pathway is required for the ER remodeling into TSS. To further verify that the HOG pathway is involved in TSS formation, we examined the phosphorylation pattern of FgHog1 (a marker for the activation of the HOG pathway) under DON inducing conditions. During the time course of DON induction in TBI medium, phosphorylation level of FgHog1 continuously increased between 12–48 hours, reached peak level at 48 h, and then decreased at 60 h (Fig 6D). This trend of phosphorylation of FgHog1 during DON induction highly correlated with the formation of toxisome structure, which also reached its peak at 48 h under DON inducing conditions [25]. These results indicate that the HOG pathway is involved in regulating toxisome formation during DON induction in F. graminearum.

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[1] Url: https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1011913

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