Publications by authors named "Li-Hung Chen"

15 Publications

  • Page 1 of 1

A diverse member of the fungal Avr4 effector family interacts with de-esterified pectin in plant cell walls to disrupt their integrity.

Sci Adv 2021 May 7;7(19). Epub 2021 May 7.

Department of Plant Pathology, University of California, Davis, Davis, CA, USA.

Effectors are small, secreted proteins that promote pathogen virulence. Although key to microbial infections, unlocking the intrinsic function of effectors remains a challenge. We have previously shown that members of the fungal Avr4 effector family use a carbohydrate-binding module of family 14 (CBM14) to bind chitin in fungal cell walls and protect them from host chitinases during infection. Here, we show that gene duplication in the Avr4 family produced an Avr4-2 paralog with a previously unknown effector function. Specifically, we functionally characterize Avr4-2, a paralog of Avr4 in the tomato pathogen , and show that although it contains a CBM14 domain, it does not bind chitin or protect fungi against chitinases. Instead, Avr4-2 interacts with highly de-esterified pectin in the plant's middle lamellae or primary cell walls and interferes with Ca-mediated cross-linking at cell-cell junction zones, thus loosening the plant cell wall structure and synergizing the activity of pathogen secreted endo-polygalacturonases.
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http://dx.doi.org/10.1126/sciadv.abe0809DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8104879PMC
May 2021

Targeted Delivery of Gene Silencing in Fungi Using Genetically Engineered Bacteria.

J Fungi (Basel) 2021 Feb 9;7(2). Epub 2021 Feb 9.

Department of Plant Pathology, University of California Davis, Davis, CA 95616, USA.

Exploiting RNA interference (RNAi) in disease control through non-transformative methods that overcome the hurdle of producing transgenic plants has attracted much attention over the last years. Here, we explored such a method and used non-pathogenic bacteria as a versatile system for delivering RNAi to fungi. Specifically, the RNaseIII-null mutant strain of HT115(DE3) was transformed with two plasmid vectors that enabled the constitutive or IPTG-inducible production of double-stranded RNAs (dsRNAs) against genes involved in aflatoxins production in () or virulence of (). To facilitate the release of the dsRNAs, the bacterial cells were further genetically engineered to undergo a bacteriophage endolysin R-mediated autolysis, following a freeze-thaw cycle. Exposure under in vitro conditions of or to living bacteria or their whole-cell autolysates induced silencing of and in a bacteria concentration-dependent manner, and instigated a reduction in aflatoxins production and mycelial growth, respectively. In planta applications of the living bacteria or their crude whole-cell autolysates produced similar results, thus creating a basis for translational research. These results demonstrate that bacteria can produce biologically active dsRNA against target genes in fungi and that bacteria-mediated RNAi can be used to control fungal pathogens.
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http://dx.doi.org/10.3390/jof7020125DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7914413PMC
February 2021

CaNRT2.1 Is Required for Nitrate but Not Nitrite Uptake in Chili Pepper Pathogen .

Front Microbiol 2020 5;11:613674. Epub 2021 Jan 5.

Department of Plant Pathology, National Chung Hsing University, Taichung, Taiwan.

Chili peppers are an important food additive used in spicy cuisines worldwide. However, the yield and quality of chilis are threatened by anthracnose disease caused by Despite the impact of on chili production, the genes involved in fungal development and pathogenicity in this species have not been well characterized. In this study, through T-DNA insertional mutagenesis, we identified a mutant strain termed B7, which is defective for the growth of on a minimal nutrient medium. Our bioinformatics analysis revealed that a large fragment DNA (19.8 kb) is deleted from the B7 genome, thus resulting in the deletion of three genes, including encoding a glycosylphosphatidyl-inisotol (GPI)-anchored protein, encoding a membrane-bound nitrate/nitrite transporter, and encoding a RecQ helicase protein. In addition, T-DNA is inserted upstream of the gene encoding a hypothetical protein. Functional characterization of , , and by targeted gene disruption and bioassays indicated that is responsible for the growth-defective phenotype of B7. Both B7 and mutant strains cannot utilize nitrate as nitrogen sources, thus restraining the fungal growth on a minimal nutrient medium. In addition to , our results showed that CaGpiP1 is a cell wall-associated GPI-anchored protein. However, after investigating the functions of and in fungal pathogenicity, growth, development and stress tolerance, we were unable to uncover the roles of these two genes in Collectively, in this study, our results identify the growth-defective strain B7 via T-DNA insertion and reveal the critical role of CaNRT2.1 in nitrate transportation for the fungal growth of .
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http://dx.doi.org/10.3389/fmicb.2020.613674DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7813687PMC
January 2021

Cloning of the Cytochrome Gene From the Tomato Powdery Mildew Fungus Reveals High Levels of Allelic Variation and Heteroplasmy for the G143A Mutation.

Front Microbiol 2019 10;10:663. Epub 2019 Apr 10.

Department of Plant Pathology, University of California, Davis, Davis, CA, United States.

is a major pathogen of tomato and several other crops that can cause substantial yield losses in favorable conditions for the fungus. Quinone outside inhibitor fungicides (QIs) are routinely used for the control of the pathogen in tomato fields across California, but their recurrent use could lead to the emergence of resistance against these compounds. Here, we partially cloned the cytochrome gene from () and searched within populations of the fungus collected from tomato fields across California for mutations that confer resistance to QIs. A total of 21 single nucleotide polymorphisms (SNPs) were identified within a 704 bp fragment of the gene analyzed, of which five were non-synonymous substitutions. Among the most frequent SNPs encountered within field populations of the pathogen was the G143A substitution that confers high levels of resistance against QIs in several fungi. The other four amino acid substitutions were novel mutations, whose effect on QI resistance is currently unknown. Sequencing of the gene from individual single-cell conidia of the fungus further revealed that most SNPs, including the one leading to the G143A substitution, were present in a heteroplasmic state, indicating the co-existence of multiple mitotypes in single cells. Analysis of the field samples showed that the G143A substitution is predominantly heteroplasmic also within field populations of in California, suggesting that QI resistance in this fungus is likely to be quantitative rather than qualitative.
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http://dx.doi.org/10.3389/fmicb.2019.00663DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6467933PMC
April 2019

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 .

Front Microbiol 2018 18;9:2229. Epub 2018 Sep 18.

Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.

relies on the ability to produce a host-selective toxin and to detoxify reactive oxygen species to successfully colonize the host. An 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. 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 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). 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 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 . Northern blotting analysis revealed that the expression of was suppressed by xenobiotics. The current studies indicate that the Yap1-mediated transporter plays a role in resistance to toxic oxidants and fungicides in . The relationships of MFS transporters with other regulatory components conferring stress resistance and pathogenesis are also discussed.
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http://dx.doi.org/10.3389/fmicb.2018.02229DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6153361PMC
September 2018

Structure of the Cladosporium fulvum Avr4 effector in complex with (GlcNAc)6 reveals the ligand-binding mechanism and uncouples its intrinsic function from recognition by the Cf-4 resistance protein.

PLoS Pathog 2018 08 27;14(8):e1007263. Epub 2018 Aug 27.

Department of Chemistry, University of California, Davis, Davis, California, United States of America.

Effectors are microbial-derived secreted proteins with an essential function in modulating host immunity during infections. CfAvr4, an effector protein from the tomato pathogen Cladosporium fulvum and the founding member of a fungal effector family, promotes parasitism through binding fungal chitin and protecting it from chitinases. Binding of Avr4 to chitin is mediated by a carbohydrate-binding module of family 14 (CBM14), an abundant CBM across all domains of life. To date, the structural basis of chitin-binding by Avr4 effector proteins and of recognition by the cognate Cf-4 plant immune receptor are still poorly understood. Using X-ray crystallography, we solved the crystal structure of CfAvr4 in complex with chitohexaose [(GlcNAc)6] at 1.95Å resolution. This is the first co-crystal structure of a CBM14 protein together with its ligand that further reveals the molecular mechanism of (GlcNAc)6 binding by Avr4 effector proteins and CBM14 family members in general. The structure showed that two molecules of CfAvr4 interact through the ligand and form a three-dimensional molecular sandwich that encapsulates two (GlcNAc)6 molecules within the dimeric assembly. Contrary to previous assumptions made with other CBM14 members, the chitohexaose-binding domain (ChBD) extends to the entire length of CfAvr4 with the reducing end of (GlcNAc)6 positioned near the N-terminus and the non-reducing end at the C-terminus. Site-directed mutagenesis of residues interacting with (GlcNAc)6 enabled the elucidation of the precise topography and amino acid composition of Avr4's ChBD and further showed that these residues do not individually mediate the recognition of CfAvr4 by the Cf-4 immune receptor. Instead, the studies highlighted the dependency of Cf-4-mediated recognition on CfAvr4's stability and resistance against proteolysis in the leaf apoplast, and provided the evidence for structurally separating intrinsic function from immune receptor recognition in this effector family.
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http://dx.doi.org/10.1371/journal.ppat.1007263DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6128652PMC
August 2018

Silencing of the Mitogen-Activated Protein Kinases (MAPK) and in Reduces Growth and Virulence on Host Plants.

Front Plant Sci 2018 13;9:291. Epub 2018 Mar 13.

International Institute of Tropical Agriculture, Nairobi, Kenya.

, causal agent of the black Sigatoka disease (BSD) of spp., has spread globally since its discovery in Fiji 1963 all the banana and plantain growing areas across the globe. It is becoming the most damaging and economically important disease of this crop. The identification and characterization of genes that regulate infection processes and pathogenicity in will provide important knowledge for the development of disease-resistant cultivars. In many fungal plant pathogens, the and are reported to be essential for pathogenicity. regulates filamentous-invasion pathways including the formation of infection structures, sporulation, virulence, and invasive and filamentous growth, whereas is involved in the cell-wall integrity pathway, virulence, invasive growth, and colonization in host tissues. Here, we used RNAi-mediated gene silencing to investigate the role of the and homologs in in pathogen invasiveness, growth and pathogenicity. The and silenced transformants showed significantly lower gene expression and reduced virulence, invasive growth, and lower biomass in infected leaf tissues of East African Highland Banana (EAHB). This study suggests that and MAPK signaling pathways play important roles in plant infection and pathogenic growth of fungal pathogens. The silencing of these vital fungal genes through host-induced gene silencing (HIG) could be an alternative strategy for developing transgenic banana and plantain resistant to BSD.
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http://dx.doi.org/10.3389/fpls.2018.00291DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5859377PMC
March 2018

-Mediated Transformation of to Determine the Role of in Osmotic Stress Regulation and Virulence Modulation.

Front Microbiol 2017 16;8:830. Epub 2017 May 16.

International Institute of Tropical AgricultureNairobi, Kenya.

Black Sigatoka disease, caused by is a serious constraint to banana production worldwide. The disease continues to spread in new ecological niches and there is an urgent need to develop strategies for its control. The high osmolarity glycerol (HOG) pathway in is well known to respond to changes in external osmolarity. HOG pathway activation leads to phosphorylation, activation and nuclear transduction of the HOG1 mitogen-activated protein kinases (MAPKs). The activated HOG1 triggers several responses to osmotic stress, including up or down regulation of different genes, regulation of protein translation, adjustments to cell cycle progression and synthesis of osmolyte glycerol. This study investigated the role of the MAPK-encoding gene on osmotic stress adaptation and virulence of nsis. RNA interference-mediated gene silencing of significantly suppressed growth of on potato dextrose agar media supplemented with 1 M NaCl, indicating that regulates osmotic stress. In addition, virulence of the -silenced mutants of on banana was significantly reduced, as observed from the low rates of necrosis and disease development on the infected leaves. Staining with lacto phenol cotton blue further confirmed the impaired mycelial growth of the in the infected leaf tissues, which was further confirmed with quantification of the fungal biomass using absolute- quantitative PCR. Collectively, these findings demonstrate that plays a critical role in osmotic stress regulation and virulence of on its host banana. Thus, could be an interesting target for the control of black Sigatoka disease in banana.
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http://dx.doi.org/10.3389/fmicb.2017.00830DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5432539PMC
May 2017

A Major Facilitator Superfamily Transporter-Mediated Resistance to Oxidative Stress and Fungicides Requires Yap1, Skn7, and MAP Kinases in the Citrus Fungal Pathogen Alternaria alternata.

PLoS One 2017 6;12(1):e0169103. Epub 2017 Jan 6.

Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.

Major Facilitator Superfamily (MFS) transporters play an important role in multidrug resistance in fungi. We report an AaMFS19 gene encoding a MFS transporter required for cellular resistance to oxidative stress and fungicides in the phytopathogenic fungus Alternaria alternata. AaMFS19, containing 12 transmembrane domains, displays activity toward a broad range of substrates. Fungal mutants lacking AaMFS19 display profound hypersensitivities to cumyl hydroperoxide, potassium superoxide, many singlet oxygen-generating compounds (eosin Y, rose Bengal, hematoporphyrin, methylene blue, and cercosporin), and the cell wall biosynthesis inhibitor, Congo red. AaMFS19 mutants also increase sensitivity to copper ions, clotrimazole, fludioxonil, and kocide fungicides, 2-chloro-5-hydroxypyridine (CHP), and 2,3,5-triiodobenzoic acid (TIBA). AaMFS19 mutants induce smaller necrotic lesions on leaves of a susceptible citrus cultivar. All observed phenotypes in the mutant are restored by introducing and expressing a wild-type copy of AaMFS19. The wild-type strain of A. alternata treated with either CHP or TIBA reduces radial growth and formation and germination of conidia, increases hyphal branching, and results in decreased expression of the AaMFS19 gene. The expression of AaMFS19 is regulated by the Yap1 transcription activator, the Hog1 and Fus3 mitogen-activated protein (MAP) kinases, the 'two component' histidine kinase, and the Skn7 response regulator. Our results demonstrate that A. alternata confers resistance to different chemicals via a membrane-bound MFS transporter.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0169103PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5218470PMC
August 2017

Structural Analysis of an Avr4 Effector Ortholog Offers Insight into Chitin Binding and Recognition by the Cf-4 Receptor.

Plant Cell 2016 08 8;28(8):1945-65. Epub 2016 Jul 8.

Department of Plant Pathology, University of California Davis, Davis, California 95616

Chitin is a key component of fungal cell walls and a potent inducer of innate immune responses. Consequently, fungi may secrete chitin-binding lectins, such as the Cf-Avr4 effector protein from the tomato pathogen Cladosporium fulvum, to shield chitin from host-derived chitinases during infection. Homologs of Cf-Avr4 are found throughout Dothideomycetes, and despite their modest primary sequence identity, many are perceived by the cognate tomato immune receptor Cf-4. Here, we determined the x-ray crystal structure of Pf-Avr4 from the tomato pathogen Pseudocercospora fuligena, thus providing a three-dimensional model of an Avr4 effector protein. In addition, we explored structural, biochemical, and functional aspects of Pf-Avr4 and Cf-Avr4 to further define the biology of core effector proteins and outline a conceptual framework for their pleiotropic recognition by single immune receptors. We show that Cf-Avr4 and Pf-Avr4 share functional specificity in binding (GlcNAc)6 and in providing protection against plant- and microbial-derived chitinases, suggesting a broader role beyond deregulation of host immunity. Furthermore, structure-guided site-directed mutagenesis indicated that residues in Pf-Avr4 important for binding chitin do not directly influence recognition by Cf-4 and further suggested that the property of recognition is structurally separated or does not fully overlap with the virulence function of the effector.
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http://dx.doi.org/10.1105/tpc.15.00893DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5006696PMC
August 2016

How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1.

PLoS One 2016 10;11(2):e0149153. Epub 2016 Feb 10.

Department of Plant Pathology, College of Agriculture and Natural Resources, National Chung-Hsing University, Taichung, Taiwan.

The tangerine pathotype of Alternaria alternata is a necrotrophic fungal pathogen causing brown spot disease on a number of citrus cultivars. To better understand the dynamics of signal regulation leading to oxidative and osmotic stress response and fungal infection on citrus, phenotypic characterization of the yeast SSK1 response regulator homolog was performed. It was determined that SSK1 responds to diverse environmental stimuli and plays a critical role in fungal pathogenesis. Experiments to determine the phenotypes resulting from the loss of SSK1 reveal that the SSK1 gene product may be fulfilling similar regulatory roles in signaling pathways involving a HOG1 MAP kinase during ROS resistance, osmotic resistance, fungicide sensitivity and fungal virulence. The SSK1 mutants display elevated sensitivity to oxidants, fail to detoxify H2O2 effectively, induce minor necrosis on susceptible citrus leaves, and displays resistance to dicarboximide and phenylpyrrole fungicides. Unlike the SKN7 response regulator, SSK1 and HOG1 confer resistance to salt-induced osmotic stress via an unknown kinase sensor rather than the "two component" histidine kinase HSK1. SSK1 and HOG1 play a moderate role in sugar-induced osmotic stress. We also show that SSK1 mutants are impaired in their ability to produce germ tubes from conidia, indicating a role for the gene product in cell differentiation. SSK1 also is involved in multi-drug resistance. However, deletion of the yeast SHO1 (synthetic high osmolarity) homolog resulted in no noticeable phenotypes. Nonetheless, our results show that A. alternata can sense and react to different types of stress via SSK1, HOG1 and SKN7 in a cooperative manner leading to proper physiological and pathological functions.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0149153PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4749125PMC
July 2016

Resistance to oxidative stress via regulating siderophore-mediated iron acquisition by the citrus fungal pathogen Alternaria alternata.

Microbiology (Reading) 2014 May 28;160(Pt 5):970-979. Epub 2014 Feb 28.

Department of Plant Pathology, IFAS, University of Florida, Gainesville, FL 32611, USA.

The ability of the necrotrophic fungus Alternaria alternata to detoxify reactive oxygen species (ROS) is crucial for pathogenesis to citrus. We report regulation of siderophore-mediated iron acquisition and ROS resistance by the NADPH oxidase (NOX), the redox activating yes-associated protein 1 (YAP1) regulator, and the high-osmolarity glycerol 1 (HOG1) mitogen-activated protein kinase (MAPK). The A. alternata nonribosomal peptide synthetase (NPS6) is essential for the biosynthesis of siderophores, contributing to iron uptake under low-iron conditions. Fungal strains impaired for NOX, YAP1, HOG1 or NPS6 all display increased sensitivity to ROS. Exogenous addition of iron at least partially rescues ROS sensitivity seen for NPS6, YAP1, HOG1, and NOX mutants. Importantly, expression of the NPS6 gene and biosynthesis of siderophores are regulated by NOX, YAP1 and HOG1, supporting a functional link among these regulatory pathways. Although iron fully rescues H2O2 sensitivity seen in mutants impaired for the response regulator SKN7, neither expression of NPS6 nor biosynthesis of siderophores is controlled by SKN7. Our results indicate that the acquisition of environmental iron has profound effects on ROS detoxification.
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http://dx.doi.org/10.1099/mic.0.076182-0DOI Listing
May 2014

Chloroplast phosphoglycerate kinase is involved in the targeting of Bamboo mosaic virus to chloroplasts in Nicotiana benthamiana plants.

Plant Physiol 2013 Dec 23;163(4):1598-608. Epub 2013 Oct 23.

Graduate Institute of Biotechnology, National Chung Hsing University, Taichung 402, Taiwan.

The Bamboo mosaic virus (BaMV) is a positive-sense, single-stranded RNA virus. Previously, we identified that the chloroplast phosphoglycerate kinase (chl-PGK) from Nicotiana benthamiana is one of the viral RNA binding proteins involved in the BaMV infection cycle. Because chl-PGK is transported to the chloroplast, we hypothesized that chl-PGK might be involved in viral RNA localization in the chloroplasts. To test this hypothesis, we constructed two green fluorescent protein (GFP)-fused mislocalized PGK mutants, the transit peptide deletion mutant (NO TRANSIT PEPTIDE [NOTP]-PGK-GFP) and the nucleus location mutant (nuclear location signal [NLS]-PGK-GFP). Using confocal microscopy, we demonstrated that NOTP-PGK-GFP and NLS-PGK-GFP are localized in the cytoplasm and nucleus, respectively, in N. benthamiana plants. When NOTP-PGK-GFP and NLS-PGK-GFP are transiently expressed, we observed a reduction in BaMV coat protein accumulation to 47% and 27% that of the wild-type PGK-GFP, respectively. To localize viral RNA in infected cells, we employed the interaction of NLS-GFP-MS2 (phage MS2 coat protein) with the modified BaMV RNA containing the MS2 coat protein binding sequence. Using confocal microscopy, we observed that BaMV viral RNA localizes to chloroplasts. Furthermore, elongation factor1a fused with the transit peptide derived from chl-PGK or with a Rubisco small subunit can partially restore BaMV accumulation in NbPGK1-knockdown plants by helping BaMV target chloroplasts.
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http://dx.doi.org/10.1104/pp.113.229666DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3846135PMC
December 2013

A nonribosomal peptide synthetase mediates siderophore production and virulence in the citrus fungal pathogen Alternaria alternata.

Mol Plant Pathol 2013 Jun 26;14(5):497-505. Epub 2013 Feb 26.

Citrus Research and Education Center, Institute of Food and Agricultural Sciences-IFAS, University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, USA.

Alternaria species produce and excrete dimethyl coprogen siderophores to acquire iron. The Alternaria alternata gene AaNPS6, encoding a polypeptide analogous to fungal nonribosomal peptide synthetases, was found to be required for the production of siderophores and virulence on citrus. Siderophores purified from culture filtrates of the wild-type strain did not induce any phytotoxicity on the leaves of citrus. Fungal strains lacking AaNPS6 produced little or no detectable extracellular siderophores and displayed an increased sensitivity to H₂O₂, superoxide-generating compounds (KO₂ and menadione) and iron depletion. Δnps6 mutants were also defective for the production of melanin and conidia. The introduction of a wild-type AaNPS6 under the control of its endogenous promoter to a Δnps6 null mutant at least partially restored siderophore production and virulence to citrus, demonstrating a functional link between iron uptake and fungal pathogenesis. Elevated sensitivity to H₂O₂, seen for the Δnps6 null strain could be relieved by exogenous application of ferric iron. The expression of the AaNPS6 gene was highly up-regulated under low-iron conditions and apparently controlled by the redox-responsive yeast transcriptional regulator YAP1. Hence, the maintenance of iron homeostasis via siderophore-mediated iron uptake also plays an important role in resistance to toxic reactive oxygen species (ROS). Our results demonstrate further the critical role of ROS detoxification for the pathogenicity of A. alternata in citrus.
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http://dx.doi.org/10.1111/mpp.12021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6638914PMC
June 2013

Roles for SKN7 response regulator in stress resistance, conidiation and virulence in the citrus pathogen Alternaria alternata.

Fungal Genet Biol 2012 Oct 10;49(10):802-13. Epub 2012 Aug 10.

Citrus Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Lake Alfred, FL 33850, USA.

"Two-component" histidine kinase (HSK1) is the primary regulator of resistance to sugar osmotic stress and sensitivity to dicarboximide or phenylpyrrole fungicides in the citrus fungal pathogen Alternaria alternata. On the other hand, the mitogen-activated protein kinase HOG1 confers resistance solely to salts and oxidative stress. We report here independent and shared functions of the SKN7-mediated signaling pathway with HSK1 and HOG1. SKN7, a putative transcription downstream regulator of HSK1, is primarily required for cellular resistance to oxidative and sugar-induced osmotic stress. SKN7, perhaps acting in parallel with HOG1, is required for resistance to H(2)O(2), tert-butyl hydroperoxide, and cumyl peroxide, but not to the superoxide-generating compounds - menadione, potassium superoxide, and diamide. Because of phenotypic commonalities, SKN7 is likely involved in resistance to sugar-induced osmotic stress via the HSK1 signaling pathway. However, mutants lacking SKN7 displayed wild-type sensitivity to NaCl and KCl salts. SKN7 is constitutively localized in the nucleus regardless of H(2)O(2) treatment. When compared to the wild type, skn7 mutants exhibited lower catalase, peroxidase, and superoxide dismutase activities and induced significantly fewer necrotic lesions on the susceptible citrus cultivar. The skn7 mutant exhibited fungicide resistance at levels between the hsk1 and the hog1 mutant strains. Skn7/hog1 double mutants exhibited fungicide resistance, similar to the strain with a single AaHSK1 gene mutation. Moreover, the A. alternata SKN7 plays a role in conidia formation. Conidia produced by the skn7 mutant are smaller and have fewer transverse septae than those produced by wild type. All altered phenotypes in the mutant were restored by introducing and expressing a wild-type copy of SKN7 under control of the endogenous promoter.
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http://dx.doi.org/10.1016/j.fgb.2012.07.006DOI Listing
October 2012