Thioredoxin and Glutaredoxin Systems Required for Oxidative Stress Resistance, Fungicide Sensitivity, and Virulence of Alternaria alternata

Comparative proteomics of sugarcane smut fungus - Sporisorium scitamineum unravels dynamic proteomic alterations during the dimorphic transition

Life cycle of the dimorphic sugarcane smut fungi, Sporisorium scitamineum, involves recognition and mating of compatible saprophytic yeast-like haploid sporidia (MAT-1 and MAT-2) that upon fusion, develop into infective dikaryotic mycelia. Although the dimorphic transition is intrinsically linked with the pathogenicity and virulence of S. scitamineum, it has never been studied using a proteomic approach. In the present study, an iTRAQ-based comparative proteomic analysis of three distinct stages was carried out. The stages were: the dimorphic transition period - haploid sporidial stage (MAT-1 and MAT-2); the transition phase (24 h post co-culturing (hpc)) and the dikaryotic mycelial stage (48 hpc). Functional categorization of differentially abundant proteins showed that the most altered biological processes were energy production, primary metabolism, especially, carbohydrate, amino acid, fatty acid, followed by translation, post-translation and protein turnover. Several differentially abundant proteins (DAPs), especially in the dikaryotic mycelial stage were predicted as effectors. Taken together, key molecular mechanisms underpinning the dimorphic transition in S. scitamineum at the proteome level were highlighted. The catalogue of stage-specific and dimorphic transition-associated-proteins and potential effectors identified herein represents a list of potential candidates for defective mutant screening to elucidate their functional role in the dimorphic transition and pathogenicity in S. scitamineum. BIOLOGICAL SIGNIFICANCE: Being the first comparative proteomics analysis of S. scitamineum, this study comprehensively examined three pivotal life cycle stages of the pathogen: the non-pathogenic haploid phase, the transition phase, and the pathogenic dikaryotic mycelial stage. While previous studies have reported the sugarcane and S. scitamineum interactions, this study endeavored to specifically identify the proteins responsible for pathogenicity. By analyzing the proteomic alterations between the haploid and dikaryotic mycelial phases, the study revealed significant changes in metabolic pathway-associated proteins linked to energy production, notably oxidative phosphorylation, and the citrate cycle. Furthermore, this study successfully identified key metabolic pathways that undergo reprogramming during the transition from the non-pathogenic to the pathogenic stage. The study also deciphered the underlying mechanisms driving the morphological and physiological alterations crucial for the S. scitamineum virulence. By studying its life cycle stages, identifying the key metabolic pathways and stage-specific proteins, it provides unprecedented insights into the pathogenicity and potential avenues for intervention. As proteomics continues to advance, such studies pave the way for a deeper understanding of plant-pathogen interactions and the development of innovative strategies to mitigate the impact of devastating pathogens like S. scitamineum.

Antifungal efficiency and mechanisms of ethyl ferulate against postharvest pathogens

Postharvest loss caused by a range of pathogens necessitates exploring novel antifungal compounds that are safe and efficient in managing the pathogens. This study evaluated the antifungal activity of ethyl ferulate (EF) and explored its mechanisms of action against Alternaria alternata, Aspergillus niger, Botrytis cinerea, Penicillium expansum, Penicillium digitatum, Geotrichum candidum and evaluated its potential to inhibit postharvest decay. The results demonstrated that EF exerts potent antifungal activity against a wide board of postharvest pathogens. Results also revealed that its antifungal mechanism is multifaceted: EF may be involved in binding to and disturbing the integrity of the fungal plasma membrane, causing leakage of intracellular content and losing normal morphology and ultrastructure. EF also induced oxidative stress in the pathogen, causing membrane lipid peroxidation and malondialdehyde accumulation. EF inhibited the critical gene expression of the pathogen, affecting its metabolic regulation, antioxidant metabolism, and cell wall degrading enzymes. EF exhibited antifungal inhibitory activity when applied directly into peel wounds or after incorporation with chitosan coating. Due to its wide board and efficient antifungal activity, EF has the potential to provide a promising alternative to manage postharvest decay.

Antioxidant Systems of Plant Pathogenic Fungi: Functions in Oxidative Stress Response and Their Regulatory Mechanisms

During the infection process, plant pathogenic fungi encounter plant-derived oxidative stress, and an appropriate response to this stress is crucial to their survival and establishment of the disease. Plant pathogenic fungi have evolved several mechanisms to eliminate oxidants from the external environment and maintain cellular redox homeostasis. When oxidative stress is perceived, various signaling transduction pathways are triggered and activate the downstream genes responsible for the oxidative stress response. Despite extensive research on antioxidant systems and their regulatory mechanisms in plant pathogenic fungi, the specific functions of individual antioxidants and their impacts on pathogenicity have not recently been systematically summarized. Therefore, our objective is to consolidate previous research on the antioxidant systems of plant pathogenic fungi. In this review, we explore the plant immune responses during fungal infection, with a focus on the generation and function of reactive oxygen species. Furthermore, we delve into the three antioxidant systems, summarizing their functions and regulatory mechanisms involved in oxidative stress response. This comprehensive review provides an integrated overview of the antioxidant mechanisms within plant pathogenic fungi, revealing how the oxidative stress response contributes to their pathogenicity.

Sulfur metabolism-mediated fungal glutathione biosynthesis is essential for oxidative stress resistance and pathogenicity in the plant pathogenic fungus Fusarium graminearum

IMPORTANCE

Fusarium graminearum is a destructive fungal pathogen that causes Fusarium head blight (FHB) on a wide range of cereal crops. To control fungal diseases, it is essential to comprehend the pathogenic mechanisms that enable fungi to overcome host defenses during infection. Pathogens require an oxidative stress response to overcome host-derived oxidative stress. Here, we identify the underlying mechanisms of the Fgbzip007-mediated oxidative stress response in F. graminearum. ChIP-seq and subsequent genetic analyses revealed that the role of glutathione in pathogenesis is not dependent on antioxidant functions in F. graminearum. Altogether, this study establishes a comprehensive framework for the Fgbzip007 regulon on pathogenicity and oxidative stress responses, offering a new perspective on the role of glutathione in pathogenicity.

Screening Universal Stress-Response Terpenoids and Their Biosynthetic Genes via Volatile and Transcriptomic Profiling in Citrus

Volatile terpenoids accumulate in citrus and play important roles in plant defense against various stressors. However, the broad-spectrum response of terpenoid biosynthesis to ubiquitous stressors in citrus has not been comparatively investigated. In this study, volatile terpenoids were profiled under six stressors: high temperature, citrus miner, citrus red mite, citrus canker, Alternaria brown spot, and huanglongbing (HLB). Significant content changes in 15 terpenoids, including β-ocimene, were observed in more than four of the six stressors, implying their possibly universal stress-response effects. Notably, the emission of terpenoids, including β-caryophyllene, β-ocimene, and nerolidol glucoside, was significantly increased by HLB in HLB-tolerant "Shatian" pomelo leaves. The upregulation of CgTPS1 and CgTPS2 and their characterization in vivo identified them as mono- or sesquiterpenoid biosynthetic genes. This study provides a foundation for determining stress resistance mechanisms in citrus and biopesticide designations for future industrial applications.

PeAP1-mediated oxidative stress response plays an important role in the growth and pathogenicity of Penicillium expansum

Penicillium expansum is the causal agent of post-harvest blue mold in various fruits and serves as a model for understanding fungal pathogenicity and mycotoxin production. The relevance of oxidative stress response in the growth and virulence of P. expansum has been largely unexplored. Here, we identify the transcriptional factor PeAP1 as a regulator of oxidative stress response in P. expansum. Gene expression and protein abundance of PeAP1, as well as its nuclear localization, are specifically induced by H2O2. Deletion of PeAP1 results in increased sensitivity to H2O2, and PeAP1 mutants exhibit a variety of defects in hyphal growth and virulence. PeAP1 prevents the accumulation of both intracellular H2O2 during vegetative growth and host-derived H2O2 during biotrophic growth. Application of an antioxidant glutathione and a NADPH oxidase inhibitor, diphenylene iodonium, to the PeAP1 mutant partially restored fungal growth and virulence. RNA sequencing analysis revealed 144 H2O2-induced PeAP1 target genes, including four antioxidant-related genes, PeGST1, PePrx1, PePrx2, and PeTRX2, that were also demonstrated to be involved in oxidative stress response and/or virulence. Collectively, our results demonstrate the global regulatory role of PeAP1 in response to oxidative stress and provide insights into the critical role of the PeAP1-mediated oxidative stress response to regulate growth and virulence of P. expansum. IMPORTANCE Reactive oxygen species are the core of host plant defense and also play a vital role in the successful invasion of host plants by pathogenic fungi. Despite its importance, the relevance of oxidative stress response in fungal growth and virulence is poorly understood in P. expansum. In this study, we reveal that the transcription factor PeAP1 acts as a central regulator of oxidative stress response in P. expansum and that there is a major link between PeAP1-mediated oxidative stress response and fungal growth and virulence. To explore the underlying mechanisms, we performed comparative transcriptomic studies and identified a number of H2O2-induced PeAP1 target genes, including four novel ones, PePrx1, PePrx2, PeGST1, and PeTRX2, whose functions were linked to PeAP1 and pathogenicity. These findings provide novel insights into the regulation mechanism of PeAP1 on growth and virulence, which might offer promising targets for control of blue mold and patulin contamination.

Exogenous melatonin mediates radish (Raphanus sativus) and Alternaria brassicae interaction in a dose-dependent manner

Radish (Raphanus sativus L.) is an economically important vegetable worldwide, but its sustainable production and breeding are highly threatened by blight disease caused by Alternaria brassicae. Melatonin is an important growth regulator that can influence physiological activities in both plants and microbes and stimulate biotic stress resistance in plants. In this study, 0-1500 μM melatonin was exogenously applied to healthy radish seedlings, in vitro incubated A. brassicae, and diseased radish seedlings to determine the effects of melatonin on host, pathogen, and host-pathogen interaction. At sufficient concentrations (0-500 μM), melatonin enhanced growth and immunity of healthy radish seedlings by improving the function of organelles and promoting the biosynthesis of antioxidant enzymes, chitin, organic acid, and defense proteins. Interestingly, melatonin also improved colony growth, development, and virulence of A. brassicae. A strong dosage-dependent effect of melatonin was observed: 50-500 μM promoted host and pathogen vitality and resistance (500 μM was optimal) and 1500 μM inhibited these processes. Significantly less blight was observed on diseased seedlings treated with 500 μM melatonin, indicating that melatonin more strongly enhanced the growth and immunity of radish than it promoted the development and virulence of A. brassicae at this treatment concentration. These effects of MT were mediated by transcriptional changes of key genes as identified by RNA-seq, Dual RNA-seq, and qRT-PCR. The results from this work provide a theoretical basis for the application of melatonin to protect vegetable crops against pathogens.

Peroxisome Proliferator FpPEX11 Is Involved in the Development and Pathogenicity in Fusarium pseudograminearum

Fusarium crown rot (FCR) of wheat, an important soil-borne disease, presents a worsening trend year by year, posing a significant threat to wheat production. Fusarium pseudograminearum cv. b was reported to be the dominant pathogen of FCR in China. Peroxisomes are single-membrane organelles in eukaryotes that are involved in many important biochemical metabolic processes, including fatty acid β-oxidation. PEX11 is important proteins in peroxisome proliferation, while less is known in the fungus F. pseudograminearum. The functions of FpPEX11a, FpPEX11b, and FpPEX11c in F. pseudograminearum were studied using reverse genetics, and the results showed that FpPEX11a and FpPEX11b are involved in the regulation of vegetative growth and asexual reproduction. After deleting FpPEX11a and FpPEX11b, cell wall integrity was impaired, cellular metabolism processes including active oxygen metabolism and fatty acid β-oxidation were significantly blocked, and the production ability of deoxynivalenol (DON) decreased. In addition, the deletion of genes of FpPEX11a and FpPEX11b revealed a strongly decreased expression level of peroxisome-proliferation-associated genes and DON-synthesis-related genes. However, deletion of FpPEX11c did not significantly affect these metabolic processes. Deletion of the three protein-coding genes resulted in reduced pathogenicity of F. pseudograminearum. In summary, FpPEX11a and FpPEX11b play crucial roles in the growth and development, asexual reproduction, pathogenicity, active oxygen accumulation, and fatty acid utilization in F. pseudograminearum.

Pexophagy is critical for fungal development, stress response, and virulence in Alternaria alternata

Alternaria alternata can resist high levels of reactive oxygen species (ROS). The protective roles of autophagy or autophagy-mediated degradation of peroxisomes (termed pexophagy) against oxidative stress remain unclear. The present study, using transmission electron microscopy and fluorescence microscopy coupled with a GFP-AaAtg8 proteolysis assay and an mCherry tagging assay with peroxisomal targeting tripeptides, demonstrated that hydrogen peroxide (H₂ O₂ ) and nitrogen depletion induced autophagy and pexophagy. Experimental evidence showed that H₂ O₂ triggered autophagy and the translocation of peroxisomes into the vacuoles. Mutational inactivation of the AaAtg8 gene in A. alternata led to autophagy impairment, resulting in the accumulation of peroxisomes, increased ROS sensitivity, and decreased virulence. Compared to the wild type, ΔAaAtg8 failed to detoxify ROS effectively, leading to ROS accumulation. Deleting AaAtg8 down-regulated the expression of genes encoding an NADPH oxidase and a Yap1 transcription factor, both involved in ROS resistance. Deleting AaAtg8 affected the development of conidia and appressorium-like structures. Deleting AaAtg8 also compromised the integrity of the cell wall. Reintroduction of a functional copy of AaAtg8 in the mutant completely restored all defective phenotypes. Although ΔAaAtg8 produced wild-type toxin levels in axenic culture, the mutant induced a lower level of H₂ O₂ and smaller necrotic lesions on citrus leaves. In addition to H₂ O₂ , nitrogen starvation triggered peroxisome turnover. We concluded that ΔAaAtg8 failed to degrade peroxisomes effectively, leading to the accumulation of peroxisomes and the reduction of the stress response. Autophagy-mediated peroxisome turnover could increase cell adaptability and survival under oxidative stress and starvation conditions.

Genome-wide identification of the Penicillium digitatum bZIP gene family and the roles of one key member, PdatfA

Penicillium digitatum is the most common cause of postharvest decay in citrus fruits around the world. Previous studies revealed that the bZIP gene family plays crucial roles in development, stress adaptation, and pathogenicity in fungi. However, little is known about the bZIP genes in P. digitatum. In this study, we systematically identified the bZIP family in 23 Penicillium species and analyzed their evolutionary relationships. We found that gene loss and gene duplication shaped the evolution of the Penicillium bZIP family. P. digitatum experienced 3 bZIP gene loss events, but with no gene duplication. We subsequently characterized the biological functions of one important member, PdatfA in P. digitatum by constructing the deletion mutant. Results showed that ΔPdatfA exhibited a moderate growth defect, reduced pigmentation, and slightly increased resistance to fungicides iprodione and fludioxonil. However, ΔPdatfA displayed similar rot symptoms to that of the wild type. The ΔPdatfA mycelia were not affected in response to oxidative stress while its conidia showed enhanced resistance due to the upregulation of catalases. Our results provide new insights into the evolution and functions of the bZIP gene family in Penicillium.

Histone Methylation Is Required for Virulence, Conidiation, and Multi-Stress Resistance of Alternaria alternata

Histone methylation, which is critical for transcriptional regulation and various biological processes in eukaryotes, is a reversible dynamic process regulated by histone methyltransferases (HMTs) and histone demethylases (HDMs). This study determined the function of 5 HMTs (AaDot1, AaHMT1, AaHnrnp, AaSet1, and AaSet2) and 1 HDMs (AaGhd2) in the phytopathogenic fungus Alternaria alternata by analyzing targeted gene deletion mutants. The vegetative growth, conidiation, and pathogenicity of ∆AaSet1 and ∆AaSet2 were severely inhibited indicating that AaSet1 and AaSet2 play critical roles in cell development in A. alternata. Multiple stresses analysis revealed that both AaSet1 and AaSet2 were involved in the adaptation to cell wall interference agents and osmotic stress. Meanwhile, ∆AaSet1 and ∆AaSet2 displayed serious vegetative growth defects in sole carbon source medium, indicating that AaSet1 and AaSet2 play an important role in carbon source utilization. In addition, ∆AaSet2 colony displayed white in color, while the wild-type colony was dark brown, indicating AaSet2 is an essential gene for melanin biosynthesis in A. alternata. AaSet2 was required for the resistance to oxidative stress. On the other hand, all of ∆AaDot1, ∆AaHMT1, and ∆AaGhd2 mutants displayed wild-type phenotype in vegetative growth, multi-stress resistance, pathogenicity, carbon source utilization, and melanin biosynthesis. To explore the regulatory mechanism of AaSet1 and AaSet2, RNA-seq of these mutants and wild-type strain was performed. Phenotypes mentioned above correlated well with the differentially expressed genes in ∆AaSet1 and ∆AaSet2 according to the KEGG and GO enrichment results. Overall, our study provides genetic evidence that defines the central role of HMTs and HDMs in the pathological and biological functions of A. alternata.

Transcriptional changes of proteins of the thioredoxin and glutathione systems in Acanthamoeba spp. under oxidative stress - an RNA approach

The thioredoxin (Trx) and the glutathione (GSH) systems represent important antioxidant systems in cells and in particular thioredoxin reductase (TrxR) has been shown to constitute a promising drug target in parasites. For the facultative protozoal pathogen Acanthamoeba, it was demonstrated that a bacterial TrxR as well as a TrxR, characteristic of higher eukaryotes, mammals and humans is expressed on the protein level. However, only bacterial TrxR is strongly induced by oxidative stress in Acanthamoeba castellanii. In this study, the impact of oxidative stress on key enzymes involved in the thioredoxin and the glutathione system of A. castellanii under different culture conditions and of clinical Acanthamoeba isolates was evaluated on the RNA level employing RT-qPCR. Additionally, the effect of auranofin, a thioredoxin reductase inhibitor, already established as a potential drug in other parasites, on target enzymes in A. castellanii was investigated. Oxidative stress induced by hydrogen peroxide led to significant stimulation of bacterial TrxR and thioredoxin, while diamide had a strong impact on all investigated enzymes. Different strains displayed distinct transcriptional responses, rather correlating to sensitivity against the respective stressor than to respective pathogenic potential. Culture conditions appear to have a major effect on transcriptional changes in A. castellanii. Treatment with auranofin led to transcriptional activation of the GSH system, indicating its role as a potential backup for the Trx system. Altogether, our data provide more profound insights into the complex redox system of Acanthamoeba, preparing the ground for further investigations on this topic.

Histone Acetyltransferases and Deacetylases Are Required for Virulence, Conidiation, DNA Damage Repair, and Multiple Stresses Resistance of Alternaria alternata

Histone acetylation, which is critical for transcriptional regulation and various biological processes in eukaryotes, is a reversible dynamic process regulated by HATs and HDACs. This study determined the function of 6 histone acetyltransferases (HATs) (Gcn5, RTT109, Elp3, Sas3, Sas2, Nat3) and 6 histone deacetylases (HDACs) (Hos2, Rpd3, Hda1, Hos3, Hst2, Sir2) in the phytopathogenic fungus Alternaria alternata by analyzing targeted gene deletion mutants. Our data provide evidence that HATs and HDACs are both required for mycelium growth, cell development and pathogenicity as many gene deletion mutants (ΔGcn5, ΔRTT109, ΔElp3, ΔSas3, ΔNat3, ΔHos2, and ΔRpd3) displayed reduced growth, conidiation or virulence at varying degrees. In addition, HATs and HDACs are involved in the resistance to multiple stresses such as oxidative stress (Sas3, Gcn5, Elp3, RTT109, Hos2), osmotic stress (Sas3, Gcn5, RTT109, Hos2), cell wall-targeting agents (Sas3, Gcn5, Hos2), and fungicide (Gcn5, Hos2). ΔGcn5, ΔSas3, and ΔHos2 displayed severe growth defects on sole carbon source medium suggesting a vital role of HATs and HDACs in carbon source utilization. More SNPs were generated in ΔGcn5 in comparison to wild-type when they were exposed to ultraviolet ray. Moreover, ΔRTT109, ΔGcn5, and ΔHos2 showed severe defects in resistance to DNA-damaging agents, indicating the critical role of HATs and HDACs in DNA damage repair. These phenotypes correlated well with the differentially expressed genes in ΔGcn5 and ΔHos2 that are essential for carbon sources metabolism, DNA damage repair, ROS detoxification, and asexual development. Furthermore, Gcn5 is required for the acetylation of H3K4. Overall, our study provides genetic evidence to define the central role of HATs and HDACs in the pathological and biological functions of A. alternata.

Genome-Wide Identification and Functional Characterization of GATA Transcription Factor Gene Family in Alternaria alternata

In the present study, we identified six GATA transcription factors (AaAreA, AaAreB, AaLreA, AaLreB, AaNsdD, and AaSreA) and characterized their functions in response to environmental stress and virulence in the tangerine pathotype of Alternaria alternata. The targeted gene knockout of each of the GATA-coding genes decreased the growth to varying degrees. The mutation of AaAreA, AaAreB, AaLreB, or AaNsdD decreased the conidiation. All the GATA transcription factors were found to be required for tolerance to cumyl hydroperoxide and tert-butyl-hydroperoxide (oxidants) and Congo red (a cell-wall-destructing agent). Pathogenicity assays assessed on detached citrus leaves revealed that mutations of AaAreA, AaLreA, AaLreB, or AaNsdD significantly decreased the fungal virulence. A comparative transcriptome analysis between the ∆AreA mutant and the wild-type strain revealed that the inactivation of AaAreA led to alterations in the expression of genes involved in a number of biological processes, including oxidoreductase activity, amino acid metabolism, and secondary metabolite biogenesis. Taken together, our findings revealed that GATA-coding genes play diverse roles in response to environmental stress and are important regulators involved in fungal development, conidiation, ROS detoxification, as well as pathogenesis. This study, for the first time, systemically underlines the critical role of GATA transcription factors in response to environmental stress and virulence in A. alternata.

Distinct and essential roles of bZIP transcription factors in the stress response and pathogenesis in Alternaria alternata

The ability to cope with environmental abiotic stress and biotic stress is crucial for the survival of plants and microorganisms, which enable them to occupy multiple niches in the environment. Previous studies have shown that transcription factors play crucial roles in regulating various biological processes including multiple stress tolerance and response in eukaryotes. This work identified multiple critical transcription factor genes, metabolic pathways and gene ontology (GO) terms related to abiotic stress response were broadly activated by analyzing the transcriptome of phytopathogenic fungus Alternaria alternata under metal ions stresses, oxidative stress, salt stresses, and host-pathogen interaction. We investigated the biological functions and regulatory roles of the bZIP transcriptional factor (TF) genes in the phytopathogenic fungus A.alternata by analyzing targeted gene disrupted mutants. Morphological analysis provides evidence that the bZIP transcription factors (Gcn4, MeaB, Atf1, the ER stress regulator Hac1, and the all development altered-1 gene Ada1) are required for morphogenesis as the colony morphology of these gene deletion mutants was significantly different from that of the wild-type. In addition, bZIPs are involved in the resistance to multiple stresses such as oxidative stress (Ada1, Yap1, MetR) and virulence (Hac1, MetR, Yap1, Ada1) at varying degrees. Transcriptome data demonstrated that the inactivation of bZIPs (Hac1, Atf1, Ada1 and Yap1) significantly affected many genes in multiple critical metabolism pathways and gene ontology (GO) terms. Moreover,the ΔHac1 mutants displayed reduced aerial hypha and are hypersensitivity to endoplasmic reticulum disruptors such as tunicamycin and dithiothreitol. Transcriptome analysis showed that inactivation of Hac1 significantly affected the proteasome process and its downstream unfolded protein binding, indicating that Hac1 participates in the endoplasmic reticulum stress response through the conserved unfolded protein response. Taken together, our findings reveal that bZIP transcription factors function as key regulators of fungal morphogenesis, abiotic stress response and pathogenesis, and expand our understanding of how microbial pathogens utilize these genes to deal with environmental stresses and achieve successful infection in the host plant.

Proteomics study reveals the molecular mechanisms underlying cryotolerance induced by mild sublethal stress in human sperm

The preconditioning of human sperm with sublethal nitrosative stress before cryopreservation can potentially improve the thawed sperm quality. However, the underlying mechanisms behind this protective strategy are not entirely understood. We compared the cryosurvival of human sperm exposed to 0.01 μM nitric oxide (NO) throughout the cryopreservation and used multiplexed quantitative proteomics approach to identify changes in the proteome profile of preconditioned sperm cells. Semen samples were obtained from 30 normospermia donors and then each sample was divided into three equal parts: fresh (F), frozen-control (C), and frozen exposed to nitric oxide (NO). The sperm undergoing mild sublethal stress showed higher values for motility and viability compared to the frozen control sperm. Moreover, out of 2912 identified proteins, 248 proteins were detected as differentially abundant proteins (DAPs) between cryopreserved groups and fresh group (F) (p < 0.05). Gene ontology (GO) analysis of differentially abundant proteins indicated that the abundance of proteins associated with glycolysis, gluconeogenesis, and fertilization processes was reduced while oxidative phosphorylation pathway was increased in abundance in cryopreserved sperm compared to the fresh sperm. Moreover, redox protein such as thioredoxin 17 was increased in abundance in the NO group compared to the control freezing group. Therefore, the pre-conditioning of sperm prior to cryopreservation may play an important role in maintaining the redox balance in mitochondria of sperm after freezing. Overall, our results indicate that arylsulfatase A (ARSA), serine protease 37 (PRSS37), and sperm surface protein (SP17) may potentially serve as protein biomarkers associated with screening the fertilization potential of the thawed sperm.

Sclerotinia sclerotiorum Thioredoxin1 (SsTrx1) is required for pathogenicity and oxidative stress tolerance

Sclerotinia sclerotiorum infects host plant tissues by inducing necrosis to source nutrients needed for its establishment. Tissue necrosis results from an enhanced generation of reactive oxygen species (ROS) at the site of infection and apoptosis. Pathogens have evolved ROS scavenging mechanisms to withstand host-induced oxidative damage. However, the genes associated with ROS scavenging pathways are yet to be fully investigated in S. sclerotiorum. We selected the S. sclerotiorum Thioredoxin1 gene (SsTrx1) for our investigations as its expression is significantly induced during S. sclerotiorum infection. RNA interference-induced silencing of SsTrx1 in S. sclerotiorum affected the hyphal growth rate, mycelial morphology, and sclerotial development under in vitro conditions. These outcomes confirmed the involvement of SsTrx1 in promoting pathogenicity and oxidative stress tolerance of S. sclerotiorum. We next constructed an SsTrx1-based host-induced gene silencing (HIGS) vector and mobilized it into Arabidopsis thaliana (HIGS-A) and Nicotiana benthamiana (HIGS-N). The disease resistance analysis revealed significantly reduced pathogenicity and disease progression in the transformed genotypes as compared to the nontransformed and empty vector controls. The relative gene expression of SsTrx1 increased under oxidative stress. Taken together, our results show that normal expression of SsTrx1 is crucial for pathogenicity and oxidative stress tolerance of S. sclerotiorum.

The bZIP Transcription Factor LtAP1 Modulates Oxidative Stress Tolerance and Virulence in the Peach Gummosis Fungus Lasiodiplodia theobromae

Lasiodiplodia theobromae is one of the primary causal agents in peach gummosis disease, leading to enormous losses in peach production. In our previous study, a redox-related gene, LtAP1, from the fungus was significantly upregulated in peach shoots throughout infection. Here, we characterized LtAP1, a basic leucine zipper transcription factor, during peach gummosis progression using the CRISPR-Cas9 system and homologous recombination. The results showed that LtAP1-deletion mutant had slower vegetative growth and increased sensitivity to several oxidative and nitrosative stress agents. LtAP1 was highly induced by exogenous oxidants treatment in the L. theobromae wild-type strain. In a pathogenicity test, the deletion mutant showed decreased virulence (reduced size of necrotic lesions, less gum release, and decreased pathogen biomass) on infected peach shoots compared to the wild-type strain. The mutant showed severely reduced transcription levels of genes related to glutaredoxin and thioredoxin in L. theobroame under oxidative stress or during infection, indicating an attenuated capacity for reactive oxygen species (ROS) detoxification. When shoots were treated with an NADPH oxidase inhibitor, the pathogenicity of the mutant was partially restored. Moreover, ROS production and plant defense response were strongly activated in peach shoots infected by the mutant. These results highlight the crucial role of LtAP1 in the oxidative stress response, and further that it acts as an important virulence factor through modulating the fungal ROS-detoxification system and the plant defense response.

Population genomics provides insights into the genetic basis of adaptive evolution in the mushroom-forming fungus Lentinula edodes

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We detected three subgroups of L. edodes with robust phenotypic differentiation.

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The three subgroups were diverged 36,871 generations ago.

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L. edodes cultivars in China might originate from the vicinity of Northeast China.

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We dissected the genetic basis of adaptive evolution in L. edodes.

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Genes related to fruiting body development are involved in adaptive evolution.

Introduction

Mushroom-forming fungi comprise diverse species that develop complex multicellular structures. In cultivated species, both ecological adaptation and artificial selection have driven genome evolution. However, little is known about the connections among genotype, phenotype and adaptation in mushroom-forming fungi.

Objectives

This study aimed to (1) uncover the population structure and demographic history of Lentinula edodes, (2) dissect the genetic basis of adaptive evolution in L. edodes, and (3) determine if genes related to fruiting body development are involved in adaptive evolution.

Methods

We analyzed genomes and fruiting body-related traits (FBRTs) in 133 L. edodes strains and conducted RNA-seq analysis of fruiting body development in the YS69 strain. Combined methods of genomic scan for divergence, genome-wide association studies (GWAS), and RNA-seq were used to dissect the genetic basis of adaptive evolution.

Results

We detected three distinct subgroups of L. edodes via single nucleotide polymorphisms, which showed robust phenotypic and temperature response differentiation and correlation with geographical distribution. Demographic history inference suggests that the subgroups diverged 36,871 generations ago. Moreover, L. edodes cultivars in China may have originated from the vicinity of Northeast China. A total of 942 genes were found to be related to genetic divergence by genomic scan, and 719 genes were identified to be candidates underlying FBRTs by GWAS. Integrating results of genomic scan and GWAS, 80 genes were detected to be related to phenotypic differentiation. A total of 364 genes related to fruiting body development were involved in genetic divergence and phenotypic differentiation.

Conclusion

Adaptation to the local environment, especially temperature, triggered genetic divergence and phenotypic differentiation of L. edodes. A general model for genetic divergence and phenotypic differentiation during adaptive evolution in L. edodes, which involves in signal perception and transduction, transcriptional regulation, and fruiting body morphogenesis, was also integrated here.

Genetic response to nitrogen starvation in the aggressive Eucalyptus foliar pathogen Teratosphaeria destructans

Teratosphaeria destructans is one of the most aggressive foliar pathogens of Eucalyptus. The biological factors underpinning T. destructans infections, which include shoot and leaf blight on young trees, have never been interrogated. Thus, the means by which the pathogen modifies its host environment to overcome host defences remain unknown. By applying transcriptome sequencing, the aim of this study was to compare gene expression in a South African isolate of T. destructans grown on nitrogen-deficient and complete media. This made it possible to identify upregulated genes in a nitrogen-starved environment, often linked to the pathogenicity of the fungus. The results support the hypothesis that nitrogen starvation in T. destructans likely mirrors an in planta genetic response. This is because 45% of genes that were highly upregulated under nitrogen starvation have previously been reported to be associated with infection in other pathogen systems. These included several CAZymes, fungal effector proteins, peptidases, kinases, toxins, lipases and proteins associated with detoxification of toxic compounds. Twenty-five secondary metabolites were identified and expressed in both nitrogen-deficient and complete conditions. Additionally, the most highly expressed genes in both growth conditions had pathogenicity-related functions. This study highlights the large number of expressed genes associated with pathogenicity and overcoming plant defences. As such, the generated baseline knowledge regarding pathogenicity and aggressiveness in T. destructans is a valuable reference for future in planta work.

Peroxisomes Implicated in the Biosynthesis of Siderophores and Biotin, Cell Wall Integrity, Autophagy, and Response to Hydrogen Peroxide in the Citrus Pathogenic Fungus Alternaria alternata

Little is known about the roles of peroxisomes in the necrotrophic fungal plant pathogens. In the present study, a Pex6 gene encoding an ATPase-associated protein was characterized by analysis of functional mutations in the tangerine pathotype of Alternaria alternata, which produces a host-selective toxin. Peroxisomes were observed in fungal cells by expressing a mCherry fluorescent protein tagging with conserved tripeptides serine-lysing-leucine and transmission electron microscopy. The results indicated that Pex6 plays no roles in peroxisomal biogenesis but impacts protein import into peroxisomes. The number of peroxisomes was affected by nutritional conditions and H₂O₂, and their degradation was mediated by an autophagy-related machinery termed pexophagy. Pex6 was shown to be required for the formation of Woronin bodies, the biosynthesis of biotin, siderophores, and toxin, the uptake and accumulation of H₂O₂, growth, and virulence, as well as the Slt2 MAP kinase-mediated maintenance of cell wall integrity. Adding biotin, oleate, and iron in combination fully restored the growth of the pex6-deficient mutant (Δpex6), but failed to restore Δpex6 virulence to citrus. Adding purified toxin could only partially restore Δpex6 virulence even in the presence of biotin, oleate, and iron. Sensitivity assays revealed that Pex6 plays no roles in resistance to H₂O₂ and superoxide, but plays a negative role in resistance to 2-chloro-5-hydroxypyridine (a hydroxyl radical-generating compound), eosin Y and rose Bengal (singlet oxygen-generating compounds), and 2,3,5-triiodobenzoic acid (an auxin transport inhibitor). The diverse functions of Pex6 underscore the importance of peroxisomes in physiology, pathogenesis, and development in A. alternata.

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

The transcription regulator ACTR controls ACT-toxin biosynthesis and pathogenicity in the tangerine pathotype of Alternaria alternata

The host-selective ACT toxin is essential for the pathogenesis of the citrus fungal pathogen Alternaria alternata. However, the mechanism of ACT-toxin gene clusters ACT-toxin biosynthesis regulated by is still poorly understood. The biosynthesis of ACT toxin is mainly regulated by multiple ACT toxin genes located in the secondary metabolite gene cluster. In this study, we reported a transcription regulator ACTR contributes ACT toxin biosynthesis through mediating ACT toxin synthesis gene ACTS4 in Alternaria alternata. We generated ACTR-disrupted and -silenced mutants in the tangerine pathotype of A. alternata. Phenotype analysis showed that the ACTR mutants displayed a significant loss of ACT toxin production and a decreased virulence on citrus leaves whereas the vegetative growth and sporulation were not affected, indicating an essential role of ACTR in both ACT toxin biosynthesis and pathogenicity. To elucidate the transcription network of ACTR, we performed RNA-Seq experiments on wild-type and ACTR null mutant and identified genes that were differentially expressed between two genotypes. Transcriptome profiling and RT-qPCR analysis demonstrated that the ACT toxin biosynthetic gene ACTS4 is down-regulated in ACTR mutant. We generated ACTS4 knock-down mutant and found that the pathogenicity of ACTS4 mutant was severely impaired. Interestingly, both ACTR and ACTS4 are not involved in the response to different abiotic stresses including oxidative stress, salt stress, cell-wall disrupting regents, and metal ion stress, indicating the function of these two genes is highly specific. In conclusion, our results highlight the important regulatory role of ACTR in ACT toxin biosynthesis through mediating ACT toxin synthesis gene ACTS4 and underline the essential role of in the tangerine pathotype of A. alternata.

Triple-Smart Eco-Friendly Chili Anthracnose Control Agro-Nanocarrier

Pesticide leaching and soil contamination are major issues in the present agriculture formulations. Hence, here 2D graphene oxide in combination with cationic, anionic, or nonionic polymers were tested for runoff resistance and targeted release behavior. Cationic polymer supplemented the binding of rGO on leaf surface by 30% more than control and reduced off-target leaching in soil by 45% more than control. Further, to enhance the fruit rot control caused by Colletotrichum capsici in chili crop, the rGO was decorated with Cu_2-xSe nanocrystals, which provided combined disease control with captan. The chitosan coating in the nanocomposite added targeted pH-responsive fungal inhibition behavior and could reduce the C. capsici growth by ∼1/2 times compared to captan control.

SsCat2 encodes a catalase that is critical for the antioxidant response, QoI fungicide sensitivity, and pathogenicity of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a destructive necrotrophic fungal pathogen with worldwide distribution. The metabolism of reactive oxygen species (ROS) is critical for the development and infection process of this economically important pathogen. Hydrogen peroxide (H₂O₂) is converted into water and dioxygen by catalases, which are major ROS scavengers in cells. Several genes have been predicted to encode the catalases of S. sclerotiorum, but the critical ones that function in the ROS stress response are still unknown. In this research, a catalase gene called SsCat2 was found to contribute to the predominant catalase activity at the stages of hyphae growth and sclerotial development. SsCat2 transcripts were induced under oxidative stress, and the target deletion of SsCat2 led to significant sensitivity to H₂O₂, suggesting that SsCat2 is critical in dealing with the oxidative stress. SsCat2-deletion strains were sensitive to hyperosmotic stresses and cell membrane-perturbing agents, suggesting impairment in cell integrity due to the inactivation of SsCat2. The expression of the alternative oxidase-encoding gene was upregulated in the SsCat2-deletion strains, which showed decreased sensitivity to QoI fungicides. SsCat2-deletion strains showed impaired virulence in different hosts, and more H₂O₂ accumulation was detected during the infect processes. In summary, these results indicate that SsCat2 encodes a catalase that is related to the oxidative stress response, QoI fungicide sensitivity, and pathogenicity of S. sclerotiorum.

The critical role of MetR/MetB/MetC/MetX in cysteine and methionine metabolism, fungal development and virulence of Alternaria alternata

Methionine is a unique sulfur-containing amino acid, which plays an important role in biological protein synthesis and various cellular processes. Here, we characterized the biological functions of AaMetB, AaMetC, and AaMetX in the tangerine pathotype of Alternaria alternata Morphological analysis showed that the mutants lacking AaMetB, AaMetC, or AaMetX resulted in less aerial hypha and fewer conidia in artificial media. Pathogenicity analysis showed that AaMetB, AaMetC, and AaMetX are required for full virulence. The defects in vegetative growth, conidiation and virulence of ΔMetB, ΔMetC, and ΔMetX can be restored by exogenous methionine and homocysteine, indicating that AaMetB, AaMetC, and AaMetX are required for methionine biosynthesis. However, exogenous cysteine only restored the growth and virulence defects of ΔMetR but not ΔMetB/C/X, suggesting that AaMetR is essential for cysteine biosynthesis. Oxidant sensitivity assay showed that only ΔMetR is sensitive to H₂O₂ and many ROS-generating compounds, indicating that AaMetR is essential for oxidative tolerance. Interestingly, fungicides indoor bioassays showed that only the ΔMetR mutants are susceptive to chlorothalonil, a fungicide that could bind to the cysteine of glyceraldehyde-3-phosphate dehydrogenase. Comparative transcriptome analysis showed that the inactivation of MetB, MetC, MetX, or MetR significantly affected the expression of methionine metabolism-related genes. Moreover, the inactivation of AaMetR significantly affected the expression of many genes related to glutathione metabolism, which is essential for ROS tolerance. Taken together, our study provides genetic evidence to define the critical roles of AaMetB, AaMetC, AaMetX, and AaMetR in cysteine and methionine metabolism, fungal development and virulence of Alternaria alternata IMPORTANCE The transcription factor METR regulating methionine metabolism is essential for reactive oxygen species (ROS) tolerance and virulence in many phytopathogenic fungi. However, the underlying regulatory mechanism of METR involved in this process is still unclear. In the present study, we generated AaMetB, AaMetC and AaMetX deletion mutants and compared these mutants with AaMetR disrupted mutants. Interestingly, we found that AaMetB, AaMetC and AaMetX are required for vegetative growth, conidiation, and pathogenicity in Alternaria alternata, but not for ROS tolerance and cysteine metabolism. Furthermore, we found that METR is involved in the biosynthesis of cysteine, which is an essential substrate for the biosynthesis of methionine and glutathione. This study emphasizes the critical roles of MetR, MetB, MetC, MetX in the regulation of cysteine and methionine metabolism, as well as the cross-link with glutathione-mediated ROS tolerance in phytopathogenic fungi, which provides a foundation for future investigations.

Proteomic Analysis of Stationary Growth Stage Adaptation and Nutritional Deficiency Response of Brucella abortus

Brucellosis, an important bacterial zoonosis caused by Brucella species, has drawn increasing attention worldwide. As an intracellular pathogen, the ability of Brucella to deal with stress within the host cell is closely related to its virulence. Due to the similarity between the survival pressure on Brucella within host cells and that during the stationary phase, a label-free proteomics approach was used to study the adaptive response of Brucella abortus in the stationary stage to reveal the possible intracellular adaptation mechanism in this study. A total of 182 downregulated and 140 upregulated proteins were found in the stationary-phase B. abortus. B. abortus adapted to adverse environmental changes by regulating virulence, reproduction, transcription, translation, stress response, and energy production. In addition, both exponential- and stationary-phase B. abortus were treated with short-term starvation. The exponential B. abortus restricted cell reproduction and energy utilization and enhanced material transport in response to nutritional stress. Compared with the exponential phase, stationary Brucella adjusted their protein expression to a lesser extent under starvation. Therefore, B. abortus in the two growth stages significantly differed in the regulation of protein expression in response to the same stress. Overall, we outlined the adaptive mechanisms that B. abortus may employ during growth and compared the differences between exponential- and stationary-phase B. abortus in response to starvation.

The HOG Pathway Plays Different Roles in Conidia and Hyphae During Virulence of Alternaria alternata

The black mold Alternaria alternata causes dramatic losses in agriculture due to postharvest colonization and mycotoxin formation and is a weak pathogen on living plants. Fungal signaling processes are crucial for successful colonization of a host plant. Because the mitogen-activated protein kinase HogA is important for the expression of stress-associated genes, we tested a ∆hogA-deletion strain for pathogenicity. When conidia were used as inoculum, the ∆hogA-deletion strain was largely impaired in colonizing tomato and apple. In comparison, hyphae as inoculum colonized the fruit very well. Hence, HogA appears to be important only in the initial stages of plant colonization. A similar difference between conidial inoculum and hyphal inoculum was observed on artificial medium in the presence of different stress agents. Whereas wild-type conidia adapted well to different stresses, the ∆hogA-deletion strain failed to grow under the same conditions. With hyphae as inoculum, the wild type and the ∆hogA-deletion strain grew in a very similar way. At the molecular level, we observed upregulation of several catalase (catA, -B, and -D) and superoxide dismutase (sodA, -B, and -E) genes in germlings but not in hyphae after exposure to 4 mM hydrogen peroxide. The upregulation required the high osmolarity glycerol (HOG) pathway. In contrast, in mycelia, catD, sodA, sodB, and sodE were upregulated upon stress in the absence of HogA. Several other stress-related genes behaved in a similar way.

Proper Functions of Peroxisomes Are Vital for Pathogenesis of Citrus Brown Spot Disease Caused by Alternaria alternata

In addition to the production of a host-selective toxin, the tangerine pathotype of Alternaria alternata must conquer toxic reactive oxygen species (ROS) in order to colonize host plants. The roles of a peroxin 6-coding gene (pex6) implicated in protein import into peroxisomes was functionally characterized to gain a better understanding of molecular mechanisms in ROS resistance and fungal pathogenicity. The peroxisome is a vital organelle involved in metabolisms of fatty acids and hydrogen peroxide in eukaryotes. Targeted deletion of pex6 had no impacts on the biogenesis of peroxisomes and cellular resistance to ROS. The pex6 deficient mutant (Δpex6) reduced toxin production by 40% compared to wild type and barely induce necrotic lesions on citrus leaves. Co-inoculation of purified toxin with Δpex6 conidia on citrus leaves, however, failed to fully restore lesion formation, indicating that toxin only partially contributed to the loss of Δpex6 pathogenicity. Δpex6 conidia germinated poorly and formed fewer appressorium-like structures (nonmelanized enlargement of hyphal tips) than wild type. Δpex6 hyphae grew slowly and failed to penetrate beyond the epidermal layers. Moreover, Δpex6 had thinner cell walls and lower viability. All of these defects resulting from deletion of pex6 could also account for the loss of Δpex6 pathogenicity. Overall, our results have demonstrated that proper peroxisome functions are of vital importance to pathogenesis of the tangerine pathotype of A. alternata.

The basal transcription factor II H subunit Tfb5 is required for stress response and pathogenicity in the tangerine pathotype of Alternaria alternata

The basal transcription factor II H (TFIIH) is a multicomponent complex. In the present study, we characterized a TFIIH subunit Tfb5 by analysing loss- and gain-of-function mutants to gain a better understanding of the molecular mechanisms underlying stress resistance and pathogenicity in the citrus fungal pathogen Alternaria alternata. Tfb5 deficiency mutants (ΔAatfb5) decreased sporulation and pigmentation, and were impaired in the maintenance of colony surface hydrophobicity and cell wall integrity. ΔAatfb5 increased sensitivity to ultraviolet light, DNA-damaging agents, and oxidants. The expression of Aatfb5 was up-regulated in the wild type upon infection in citrus leaves, implicating the requirement of Aatfb5 in fungal pathogenesis. Biochemical and virulence assays revealed that ΔAatfb5 was defective in toxin production and cellwall-degrading enzymes, and failed to induce necrotic lesions on detached citrus leaves. Aatfb5 fused with green fluorescent protein (GFP) was localized in the cytoplasm and nucleus and physically interacted with another subunit, Tfb2, based on yeast two-hybrid and co-immunoprecipitation analyses. Transcriptome and Antibiotics & Secondary Metabolite Analysis Shell (antiSMASH) analyses revealed the positive and negative roles of Aatfb5 in the production of various secondary metabolites and in the regulation of many metabolic and biosynthetic processes in A. alternata. Aatfb5 may play a negative role in oxidative phosphorylation and a positive role in peroxisome biosynthesis. Two cutinase-coding genes (AaCut2 and AaCut15) required for full virulence were down-regulated in ΔAatfb5. Overall, this study expands our understanding of how A. alternata uses the basal transcription factor to deal with stress and achieve successful infection in the plant host.

Lasiodiplodia theobromae-induced alteration in ROS metabolism and its relation to gummosis development in Prunus persica

Peach gummosis caused by Lasiodiplodia theobromae is one of the most detrimental diseases to peaches in southern China. Reactive oxygen species (ROS) play major roles in plant-pathogen interactions, however, their roles in the pathogenesis of peach gummosis, especially shoot disease in perennials, are largely unknown. In this study, the effects of L. theobromae on ROS production-scavenging systems and on signalling transduction during L. theobromae-induced gummosis in current-year peach shoots were investigated. The infection by L. theobromae led to a ROS burst and activated the plant antioxidant enzyme-dependent scavenging system. With disease progression, the capacity of the plant antioxidant machinery declined, and allowed for ROS accumulation and eventual malondialdehyde production. As for the fungus L. theobromae, the transcripts of genes related to ROS production were significantly repressed, and concomitantly the expression of genes related to antioxidant systems and oxidative stress resistance was markedly upregulated, perhaps to alleviate oxidative stress for successful colonisation. Moreover, genes involved in phytohormones biosynthesis and pathogenesis-related proteins were all markedly promoted, which could contribute to the restriction of disease development in peach shoots. Overall, the results showed that the ROS production-scavenging system in P. persica might affect disease development during peach-L. theobromae interaction. Our findings lay the foundations for future in-depth investigations of the molecular mechanisms and regulatory networks underlying L. theobromae-mediated shoot diseases in terms of ROS production-scavenging systems.

Oxidative stress measurement in different morphological forms of wild-type and mutant cyanobacterial strains: Overcoming the limitation of fluorescence microscope-based method

Monitoring of oxidative stress caused by a wide range of reactive oxygen species (ROS) is essential to have an idea about the fitness and growth of photosynthetic organisms. The imaging-based oxidative stress measurement in cyanobacteria using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) dye has the limitation of small sample size as the only selected number of cells are analyzed to measure the ROS levels. Here, we developed a method for oxidative stress measurement by DCFH-DA and flow cytometer (FCM) using unicellular Synechococcus elongatus PCC 7942 and filamentous Fremyella diplosiphon BK14 cyanobacteria. F. diplosiphon BK14 inherently possess high levels of ROS and showed higher sensitivity to hydrogen peroxide treatment in comparison to S. elongatus PCC 7942. We successfully measured oxidative stress in glutaredoxin lacking strain (Δgrx3) of S. elongatus PCC 7942, and wild-type Synechocystis sp. PCC 6803 using FCM based method. Importantly, ROS were not detected in these two strains of cyanobacteria by fluorescence microscope-based method due to their small spherical morphology. Δgrx3 strain showed high ROS levels in comparison to its wild-type strain. Treatment of abiotic factors such as high PAR in wild-type and Δgrx3 strains of S. elongatus PCC 7942, low PAR or low PAR + UVR in wild-type S. elongatus PCC 7942, and high PAR or high PAR + NaCl in Synechocystis sp. PCC 6803 increased oxidative stress. In summary, the FCM based method can measure ROS levels produced due to physiological conditions associated with genetic changes or abiotic stress in a large population of cells regardless of their morphology. Therefore, the present study shows the usefulness of the method in monitoring the health of organisms in a large scale cultivation system.

Cell-Wall-Degrading Enzymes Required for Virulence in the Host Selective Toxin-Producing Necrotroph Alternaria alternata of Citrus

The necrotrophic fungal pathogen Alternaria alternata attacks many citrus species, causing brown spot disease. Its pathogenic capability depends primarily on the production of host-selective ACT toxin. In the current study a Ste12 transcription factor was characterized to be required for conidial formation and the production of cell-wall-degrading enzymes (CWDEs) in the tangerine pathotype of A. alternata. The Ste12 deficiency strain (ΔSte12) retained wild-type growth, ACT toxin production, and sensitivity to oxidative and osmotic stress. However, pathogenicity tests assayed on detached Dancy leaves revealed a marked reduction in virulence of ΔSte12. Transcriptome and quantitative RT-PCR analyses revealed that many genes associated with Carbohydrate-Active Enzymes (CAZymes) were downregulated in ΔSte12. Two cutinase-coding genes (AaCut3 and AaCut7) regulated by Ste12 were individually and simultaneously inactivated. The AaCut3 or AaCut7 deficiency strain unchanged in cutinase activities and incited wild-type lesions on Dancy leaves. However, the strain carrying an AaCut3 AaCut7 double mutation produced and secreted significantly fewer cutinases and incited smaller necrotic lesions than wild type. Not only is the host-selective toxin (HST) produced by A. alternata required for fungal penetration and lesion formation, but so too are CWDEs required for full virulence. Overall, this study expands our understanding of how A. alternata overcomes citrus physical barriers to carry out successful penetration and colonization.

Sclerotinia sclerotiorum Thioredoxin Reductase Is Required for Oxidative Stress Tolerance, Virulence, and Sclerotial Development

Sclerotinia sclerotiorum is a destructive ascomycete plant pathogen with worldwide distribution. Extensive research on different aspects of this pathogen's capability to cause disease will help to uncover clues about new ways to safely control Sclerotinia diseases. The thioredoxin (Trx) system consists of Trx and thioredoxin reductase (TrxR), which play critical roles in maintenance of cellular redox homeostasis. In this study, we functionally characterized a gene encoding a TrxR (SsTrr1) in S. sclerotiorum. The amino acids of SsTrr1 exhibited high similarity with reported TrxRs in plant pathogens and targeted silencing of SsTrr1 lead to a decrease in TrxR activities of mycelium. SsTrr1 showed high expression levels during hyphae growth, and the levels decreased at the different stages of sclerotial development. SsTrr1 gene-silenced strains produced a smaller number of larger sclerotia on potato dextrose agar medium. The observations were consistent with the inhibitory effects on sclerotial development by the TrxR inhibitor, anrunofin. The expression of SsTrr1 showed a dramatic increase under the oxidative stress and the hyphal growth of gene-silenced strains showed more sensitivity to H2O2. SsTrr1 gene-silenced strains also showed impaired virulence in different hosts. Taken together, our results suggest that SsTrr1 encodes a TrxR that is of great important for oxidative stress tolerance, virulence, and sclerotial development of S. sclerotiorum.

The methionine biosynthesis regulator AaMetR contributes to oxidative stress tolerance and virulence in Alternaria alternata

The tangerine pathotype of A. alternata, which produces a unique host-selective ACT toxin causes brown spots on citrus leaves and fruits. In this study, we report a methionine biosynthesis regulator (MetR), which belong to bZIP transcription factor, is required for methionine metabolism, oxidative stress tolerance and pathogenicity. We generated two ΔAaMetR mutants in the tangerine pathotype of Alternaria alternata and investigated the resulting mutant phenotypes. The ΔAaMetR disruption mutant grew poorly in the absence of methionine and unable to produce conidia. Furthermore, pathogenicity tests have shown that ΔAaMetR mutant on their tangerine host can neither penetrate nor cause disease. These ΔAaMetR mutants exhibit an increased sensitivity to exogenous H₂O₂ and many ROS generating oxidants. To elucidate the transcription network of AaMetR, we performed RNA-Seq experiments on wild-type and ΔAaMetR mutant and identified genes that were differentially expressed between the two genotypes. Transcriptome data demonstrated that AaMetR contributes in many other biological processes including ROS detoxification, sulfur transfer, and amino acid metabolism. Comparative transcriptome analysis indicated that the ΔAaMetR mutant up-regulated several genes involved in cysteine and methionine metabolism. In conclusion, our results highlight the global regulatory role of AaMetR in cysteine and methionine metabolism and provide new insights into the crucial role of ROS detoxification, sporulation and pathogenicity in the tangerine pathotype of A. alternata.

A Major Facilitator Superfamily Transporter Regulated by the Stress-Responsive Transcription Factor Yap1 Is Required for Resistance to Fungicides, Xenobiotics, and Oxidants and Full Virulence in Alternaria alternata

Alternaria alternata relies on the ability to produce a host-selective toxin and to detoxify reactive oxygen species to successfully colonize the host. An A. alternata major facilitator superfamily transporter designated AaMFS54 was functionally characterized by analysis of loss- and gain-of-function mutations to better understand the factors required for fungal pathogenesis. AaMFS54 was originally identified from a wild-type expression library after being subtracted with that of a mutant impaired for the oxidative stress-responsive transcription regulator Yap1. AaMFS54 contains 14 transmembrane helixes. Fungal mutant lacking AaMFS54 produced fewer conidia and increased sensitivity to many potent oxidants (potassium superoxide and singlet-oxygen generating compounds), xenobiotics (2,3,5-triiodobenzoic acid and 2-chloro-5-hydroxypyridine), and fungicides (clotrimazole, fludioxonil, vinclozolin, and iprodione). AaMFS54 mutant induced necrotic lesion sizes similar to those induced by wild-type on leaves of susceptible citrus cultivars after point inoculation with spore suspensions. However, the mutant produced smaller colonies and less fluffy hyphae on the affected leaves. Virulence assays on citrus leaves inoculated by spraying with spores revealed that AaMFS54 mutant induced less severe lesions than wild-type, indicating the requirement of AaMFS54 in pathogenesis. All defective phenotypes were restored in a strain re-acquiring a functional copy of AaMFS54. Northern blotting analysis revealed that the expression of AaMFS54 was suppressed by xenobiotics. The current studies indicate that the Yap1-mediated transporter plays a role in resistance to toxic oxidants and fungicides in A. alternata. The relationships of MFS transporters with other regulatory components conferring stress resistance and A. alternata pathogenesis are also discussed.