Identification of a fungal cutinase promoter that is inducible by a plant signal via a phosphorylated trans-acting factor

The Multifaceted Roles of Fungal Cutinases during Infection

Cuticles cover the aerial epidermis cells of terrestrial plants and thus represent the first line of defence against invading pathogens, which must overcome this hydrophobic barrier to colonise the inner cells of the host plant. The cuticle is largely built from the cutin polymer, which consists of C16 and C18 fatty acids attached to a glycerol backbone that are further modified with terminal and mid-chain hydroxyl, epoxy, and carboxy groups, all cross-linked by ester bonds. To breach the cuticle barrier, pathogenic fungal species employ cutinases-extracellular secreted enzymes with the capacity to hydrolyse the ester linkages between cutin monomers. Herein, we explore the multifaceted roles that fungal cutinases play during the major four stages of infection: (i) spore landing and adhesion to the host plant cuticle; (ii) spore germination on the host plant cuticle; (iii) spore germ tube elongation and the formation of penetrating structures; and (iv) penetration of the host plant cuticle and inner tissue colonisation. Using previous evidence from the literature and a comprehensive molecular phylogenetic tree of cutinases, we discuss the notion whether the lifestyle of a given fungal species can predict the activity nature of its cutinases.

Production, purification and application of Cutinase in enzymatic scouring of cotton fabric isolated from Acinetobacter baumannii AU10

Conventional cotton scouring in the textile industry using alkali results in huge environmental impact which can be overcome by using enzymes. Pectinase along with cutinase gives enhanced bioscouring results. Cutin was extracted from tomato peels and was used as substrate in the microbial media. The strain isolated from tomato peel was identified as Acinetobacter baumannii AU10 by 16S rDNA sequencing. The cutinase production was optimized by Placket-Burman and Response Surface Methodology (RSM) and the maximum production of 82.75 U/mL obtained at sucrose 6.68% (w/v), gelatin 2.74 g/L at a temperature of 35.93 °C. Cutinase was purified by ammonium sulfate precipitation, hydrophobic interaction chromatography and ion exchange chromatography with a recovery of 25.6% and specific activity of 38030 U/mg. The confirmation test for the purity of cutinase was analyzed by RP-HPLC. The molecular mass of cutinase was determined as 28.9 kDa by SDS-PAGE technique. Scanning electron microscopic analysis showed a rough and open primary wall surface on the cutinase bioscoured fabric which confirmed its activity on cutin present in the cotton fabric. Additionally, the cutinase-bioscoured samples showed better absorbency than the untreated samples. Therefore, enzymatic scouring increases wetting capacity of scoured cotton and also helps to reduce environmental pollution.

Dissection of Ramularia Leaf Spot Disease by Integrated Analysis of Barley and Ramularia collo-cygni Transcriptome Responses

Ramularia leaf spot disease (RLS), caused by the ascomycete fungus Ramularia collo-cygni, has emerged as a major economic disease of barley. No substantial resistance has been identified, so far, among barley genotypes and, based on the epidemiology of the disease, a quantitative genetic determinacy of RLS has been suggested. The relative contributions of barley and R. collo-cygni genetics to disease infection and epidemiology are practically unknown. Here, we present an integrated genome-wide analysis of host and pathogen transcriptome landscapes identified in a sensitive barley cultivar following infection by an aggressive R. collo-cygni isolate. We compared transcriptional responses in the infected and noninfected leaf samples in order to identify which molecular events are associated with RLS symptom development. We found a large proportion of R. collo-cygni genes to be expressed in planta and that many were also closely associated with the infection stage. The transition from surface to apoplastic colonization was associated with downregulation of cell wall-degrading genes and upregulation of nutrient uptake and resistance to oxidative stresses. Interestingly, the production of secondary metabolites was dynamically regulated within the fungus, indicating that R. collo-cygni produces a diverse panel of toxic compounds according to the infection stage. A defense response against R. collo-cygni was identified in barley at the early, asymptomatic infection and colonization stages. We found activation of ethylene signaling, jasmonic acid signaling, and phenylpropanoid and flavonoid pathways to be highly induced, indicative of a classical response to necrotrophic pathogens. Disease development was found to be associated with gene expression patterns similar to those found at the onset of leaf senescence, when nutrients, possibly, are used by the infecting fungus. These analyses, combining both barley and R. collo-cygni transcript profiles, demonstrate the activation of complex transcriptional programs in both organisms.

Adaptation to the Host Environment by Plant-Pathogenic Fungi

Many fungi can live both saprophytically and as endophyte or pathogen inside a living plant. In both environments, complex organic polymers are used as sources of nutrients. Propagation inside a living host also requires the ability to respond to immune responses of the host. We review current knowledge of how plant-pathogenic fungi do this. First, we look at how fungi change their global gene expression upon recognition of the host environment, leading to secretion of effectors, enzymes, and secondary metabolites; changes in metabolism; and defense against toxic compounds. Second, we look at what is known about the various cues that enable fungi to sense the presence of living plant cells. Finally, we review literature on transcription factors that participate in gene expression in planta or are suspected to be involved in that process because they are required for the ability to cause disease.

The transcription factor SlSHINE3 modulates defense responses in tomato plants

The cuticle plays an important role in plant interactions with pathogens and with their surroundings. The cuticle acts as both a physical barrier against physical stresses and pathogens and a chemical deterrent and activator of the plant defense response. Cuticle production in tomato plants is regulated by several transcription factors, including SlSHINE3, an ortholog of the Arabidopsis WIN/SHN3. Here we used a SlSHINE3-overexpressing (SlSHN3-OE) and silenced (Slshn3-RNAi) lines and a mutant in SlCYP86A69 (Slcyp86A69)--a direct target of SlSHN3--to analyze the roles of the leaf cuticle and cutin content and composition in the tomato plant's defense response to the necrotrophic foliar pathogen Botrytis cinerea and the biotrophic bacterial pathogen Xanthomonas campestris pv. vesicatoria. We showed that SlSHN3, which is predominantly expressed in tomato fruit epidermis, also affects tomato leaf cuticle, as morphological alterations in the SlSHN3-OE leaf tissue resulted in shiny, stunted and permeable leaves. SlSHN3-OE leaves accumulated 38% more cutin monomers than wild-type leaves, while Slshn3-RNAi and Slcyp86A69 plants showed a 40 and 70% decrease in leaf cutin monomers, respectively. Overexpression of SlSHN3 resulted in resistance to B. cinerea infection and to X. campestris pv. vesicatoria, correlated with cuticle permeability and elevated expression of pathogenesis-related genes PR1a and AOS. Further analysis revealed that B. cinerea-infected Slshn3-RNAi plants are more sensitive to B. cinerea and produce more hydrogen peroxide than wild-type plants. Cutin monomer content and composition differed between SlSHN3-OE, Slcyp86A69, Slshn3-RNAi and wild-type plants, and cutin monomer extracted from SlSHN3-OE plants altered the expression of pathogenesis-related genes in wild-type plants.

Overexpression of AtSHN1/WIN1 provokes unique defense responses

The plant cell cuticle serves as the first barrier protecting plants from mechanical injury and invading pathogens. The cuticle can be breached by cutinase-producing pathogens and the degradation products may activate pathogenesis signals in the invading pathogens. Cuticle degradation products may also trigger the plant’s defense responses. Botrytis cinerea is an important plant pathogen, capable of attacking and causing disease in a wide range of plant species. Arabidopsis thaliana shn1-1D is a gain-of-function mutant, which has a modified cuticular lipid composition. We used this mutant to examine the effect of altering the whole-cuticle metabolic pathway on plant responses to B. cinerea attack. Following infection with B. cinerea, the shn1-1D mutant discolored more quickly, accumulated more H₂O₂, and showed accelerated cell death relative to wild-type (WT) plants. Whole transcriptome analysis of B. cinerea-inoculated shn1-1D vs. WT plants revealed marked upregulation of genes associated with senescence, oxidative stress and defense responses on the one hand, and genes involved in the magnitude of defense-response control on the other. We propose that altered cutin monomer content and composition of shn1-1D plants triggers excessive reactive oxygen species accumulation and release which leads to a strong, unique and uncontrollable defense response, resulting in plant sensitivity and death.

Overexpression of AtSHN1/WIN1 provokes unique defense responses

The plant cell cuticle serves as the first barrier protecting plants from mechanical injury and invading pathogens. The cuticle can be breached by cutinase-producing pathogens and the degradation products may activate pathogenesis signals in the invading pathogens. Cuticle degradation products may also trigger the plant's defense responses. Botrytis cinerea is an important plant pathogen, capable of attacking and causing disease in a wide range of plant species. Arabidopsis thaliana shn1-1D is a gain-of-function mutant, which has a modified cuticular lipid composition. We used this mutant to examine the effect of altering the whole-cuticle metabolic pathway on plant responses to B. cinerea attack. Following infection with B. cinerea, the shn1-1D mutant discolored more quickly, accumulated more H2O2, and showed accelerated cell death relative to wild-type (WT) plants. Whole transcriptome analysis of B. cinerea-inoculated shn1-1D vs. WT plants revealed marked upregulation of genes associated with senescence, oxidative stress and defense responses on the one hand, and genes involved in the magnitude of defense-response control on the other. We propose that altered cutin monomer content and composition of shn1-1D plants triggers excessive reactive oxygen species accumulation and release which leads to a strong, unique and uncontrollable defense response, resulting in plant sensitivity and death.

Detection of potential suberinase-encoding genes in Streptomyces scabiei strains and other actinobacteria

Streptomyces scabiei causes common scab, an economically important disease of potato tubers. Some authors have previously suggested that S. scabiei penetration into host plant tissue is facilitated by secretion of esterase enzymes degrading suberin, a lipidic biopolymer of the potato periderm. In the present study, S. scabiei EF-35 showed high esterase activity in suberin-containing media. This strain also exhibited esterase activity in the presence of other biopolymers, such as lignin, cutin, or xylan, but at a much lower level. In an attempt to identify the esterases involved in suberin degradation, translated open reading frames of S. scabiei 87-22 were examined for the presence of protein sequences corresponding to extracellular esterases of S. scabiei FL1 and of the fungus Coprinopsis cinerea VTT D-041011, which have previously been shown to be produced in the presence of suberin. Two putative extracellular suberinase genes, estA and sub1, were identified. The presence of these genes in several actinobacteria was investigated by Southern blot hybridization, and both genes were found in most common-scab-inducing strains. Moreover, reverse transcription - polymerase chain reaction performed with S. scabiei EF-35 showed that estA was expressed in the presence of various biopolymers, including suberin, whereas the sub1 gene appeared to be specifically expressed in the presence of suberin and cutin.

Analysis of the promoter region of the gene LIP1 encoding triglyceride lipase from Fusarium graminearum

Triglyceride lipases catalyze the reversible degradation of glycerol esters with long-chain fatty acids into fatty acids and glycerol. In silico analysis of 5'-end flanking sequence of the gene LIP1 encoding a triglyceride lipase from the wheat head blight pathogen Fusarium graminearum revealed the presence of several cis-regulatory elements. To delineate the function of these regulatory elements, we constructed a series of deletion mutants in the LIP1 promoter region fused to the open reading frame of a green fluorescent protein (GFP) and assayed the promoter activity. Analysis of GFP expression levels in mutants indicated that a 563-bp promoter sequence was sufficient to drive the expression of LIP1 and regulatory elements responsible for the gene induction were located within the 563-372bp region. To further investigate the regulatory elements, putative cis-acting elements spanned within the 563-372bp region were mutated using a targeted mutagenesis approach. A CCAAT box, a CreA binding site, and a fatty acid responsive element (FARE) were identified and confirmed to be required for the basal expression of LIP1, glucose suppression and fatty acid induction, respectively.

Molecular cloning, characterization, and expression of a redox-responsive cutinase from Monilinia fructicola (Wint.) Honey

cDNA clones encoding a cutinase expressed in cutin-induced cultures of the plant pathogen Monilinia fructicola were isolated using a protein-based strategy. The largest cDNA (Mfcut1) was found to contain an open reading frame of 603 bp that predicted a 20.2-kDa protein of 201 amino acids with a 20-amino-acid secretory signal peptide and a pI of 8.4. The predicted protein contained cutinase/lipase consensus sequences with active site serines and potential protein kinase phosphorylation sites. Comparison of the deduced amino sequence from Mfcut1 with other fungal cutinase sequences revealed new features, which include conserved cysteines, C-terminal aromatic residues, and a novel histidine substitution in the D-H active site motif. The presence in the growth medium of antioxidants, such as caffeic acid, suppressed mRNA accumulation and enzyme activity of a cutinase from M. fructicola. MFCUT1 was expressed at high levels as a His-tagged fusion protein in Pichia pastoris and purified to apparent homogeneity in a single step by Ni(2+)-nitrilotriacetic acid affinity chromatography. Analysis of variant MFCUT1 mutants in which the novel serine and histidine residues were replaced by site-directed mutagenesis indicated that these residues had an important effect on enzyme activity.

Regulation of constitutively expressed and induced cutinase genes by different zinc finger transcription factors in Fusarium solani f. sp. pisi (Nectria haematococca)

Cutin monomers, generated by the low levels of constitutively expressed cutinase, induce high levels of cutinase that can help pathogenic fungi to penetrate into the host through the cuticle whose major structural polymer is cutin. We cloned three highly homologous cutinase genes, cut1, cut2, and cut3, from Fusarium solani f. pisi (Nectria haematococca). Amino acid sequence deduced from the nucleotide sequence of cut1 and cut2/3 matched with that of the peptides from cutinase 1 and cutinase 2, respectively, isolated from F. solani pisi grown on cutin as the sole carbon source. Induction of beta-glucuronidase gene fused to the promoters of the cutinases integrated into F. solani pisi genome indicates that cut2 is constitutively expressed and induced under starvation, whereas cut1 is highly induced by cutin monomers. A palindrome binding protein (PBP) previously cloned binds only to palindrome 1 of cut1 promoter but not palindrome 1 of cut2/3 which contains two base substitutions. PBP is thought to interfere with the binding of CTF1 alpha, the transcription factor involved in induction, to cut1 promoter and thus keep cut1 gene repressed until induced by cutin monomers. Because PBP cannot bind palindrome 1 of cut2, this gene is not repressed. CTF1 alpha does not transactivate cut2 promoter. A new Cys(6)Zn(2) motif-containing transcription factor, CTF1 beta, that binds palindrome 2 was cloned and sequenced. In yeast, CTF1 beta transactivates cut2 promoter but not cut1 promoter unless its palindrome 1 is mutated, unlike CTF1 alpha which transactivates cut1. Thus, CTF1 beta is involved in the constitutive expression of cut2 that causes production of low levels of cutin monomers that strongly induce cut1 using CTF1 alpha as the transcription factor.

Polyesters in higher plants

Polyesters occur in higher plants as the structural component of the cuticle that covers the aerial parts of plants. This insoluble polymer, called cutin, attached to the epidermal cell walls is composed of interesterified hydroxy and hydroxy epoxy fatty acids. The most common chief monomers are 10,16-dihydroxy C16 acid, 18-hydroxy-9,10 epoxy C18 acid, and 9,10,18-trihydroxy C18 acid. These monomers are produced in the epidermal cells by omega hydroxylation, in-chain hydroxylation, epoxidation catalyzed by P450-type mixed function oxidase, and epoxide hydration. The monomer acyl groups are transferred to hydroxyl groups in the growing polymer at the extracellular location. The other type of polyester found in the plants is suberin, a polymeric material deposited in the cell walls of a layer or two of cells when a plant needs to erect a barrier as a result of physical or biological stress from the environment, or during development. Suberin is composed of aromatic domains derived from cinnamic acid, and aliphatic polyester domains derived from C16 and C18 cellular fatty acids and their elongation products. The polyesters can be hydrolyzed by pancreatic lipase and cutinase, a polyesterase produced by bacteria and fungi. Catalysis by cutinase involves the active serine catalytic triad. The major function of the polyester in plants is as a protective barrier against physical, chemical, and biological factors in the environment, including pathogens. Transcriptional regulation of cutinase gene in fungal pathogens is being elucidated at a molecular level. The polyesters present in agricultural waste may be used to produce high value polymers, and genetic engineering might be used to produce large quantities of such polymers in plants.

Requirement for either a host- or pectin-induced pectate lyase for infection of Pisum sativum by Nectria hematococca

Fungal pathogens usually have multiple genes that encode extracellular hydrolytic enzymes that may degrade the physical barriers in their hosts during the invasion process. Nectria hematococca, a plant pathogen, has two inducible pectate lyase (PL) genes (pel) encoding PL that can help degrade the carbohydrate barrier in the host. pelA is induced by pectin, whereas pelD is induced only in planta. We show that the disruption of either the pelA or pelD genes alone causes no detectable decrease in virulence. Disruption of both pelA and pelD drastically reduces virulence. Complementation of the double disruptant with pelD gene, or supplementation of the infection droplets of the double disruptant with either purified enzyme, PLA, or PLD, caused a recovery in virulence. These results show that PL is a virulence factor. Thus, we demonstrate that disruption of all functionally redundant genes is required to demonstrate the role of host barrier-degrading enzymes in pathogenesis and that dismissal of the role of such enzymes based on the effects of single-gene disruption may be premature.

Early expression of the calmodulin gene, which precedes appressorium formation in Magnaporthe grisea, is inhibited by self-inhibitors and requires surface attachment

Fungal conidia contain chemicals that inhibit germination and appressorium formation until they are well dispersed in a favorable environment. Recently, such self-inhibitors were found to be present on the conidia of Magnaporthe grisea, and plant surface waxes were found to relieve this self-inhibition. To determine whether the self-inhibitors suppress the expression of early genes involved in the germination and differentiation of conidia, the calmodulin gene was chosen as a representative early gene, because it was found to be expressed early in Colletotrichum gloeosporioides and Colletotrichum trifolii differentiation. After calmodulin cDNA and genomic DNA from M. grisea were cloned, the promoter of the calmodulin gene was fused to a reporter gene, that for green fluorescent protein (GFP), and transformed into the M. grisea genome. Confocal microscopic examination and quantitation of expression of GFP green fluorescence showed (i) that the expression of the calmodulin gene decreased significantly when self-inhibition of M. grisea appressorium formation occurred because of high conidial density or addition of exogenous self-inhibitors and (ii) that the expression level of this gene was restored when self-inhibition was relieved by the addition of plant surface waxes. The increase in fluorescence correlated with the percentage of conidia that formed appressoria. The induction of calmodulin was also confirmed by RNA blotting. Concanavalin A inhibited surface attachment of conidia, GFP expression, and appressorium formation without affecting germination. The high correlation between GFP expression and appressorium formation strongly suggests that calmodulin gene expression and appressorium formation require surface attachment.

Depression of the xylanase-encoding cgxA gene of Chaetomium gracile in Aspergillus nidulans

Regulation of the Chaetomium gracile xylanase A gene (cgxA) was investigated using Aspergillus nidulans as an intermediate host. Deletion of a 185 bp DNA fragment from its promoter region led to higher levels of the cgxA gene expression, indicating that the 185 bp DNA fragment contains an element involved in repression of the gene. A nuclear extract was assayed for proteins which bind to the 185 bp DNA fragment. A protein designated AnRP bound sequence specifically to the DNA fragment. The minimum sequence required for AnRP binding, 5'TTGACAAAT-3', was determined by means of gel mobility shift assays with various double-stranded oligonucleotides. Furthermore, this sequence repressed the expression of the cgxA gene when inserted at the 5' end of the cgxA gene on pXAH, which was deleted for the repressive element from the promoter region.

Regulation of the action of steroid/thyroid hormone receptors by medium-chain fatty acids

Triiodothyronine (T3) causes a 30-fold increase in transcription of the malic enzyme gene in chick embryo hepatocytes; medium-chain fatty acids (MCFAs) inhibit this increase. T3 action is mediated by T3 receptors (TRs) that bind to T3 response elements (T3REs) in this gene's 5'-flanking DNA. In transiently transfected hepatocytes, fragments of 5'-flanking DNA of the malic enzyme gene or artificial T3REs that conferred T3 stimulation also conferred MCFA inhibition to linked reporter genes. Thus, MCFA inhibition may be mediated through cis-acting T3REs and trans-acting TRs, distinguishing MCFA action from that of other fatty acids which act through unique sequence elements. Using binding assays and overexpression of TR, we showed that MCFAs inhibited the transactivating but not the silencing function of TR and did not alter binding of T3 to TR or of TR to T3RE. The C-terminal ligand-binding domain of TR was sufficient to confer stimulation by T3, but not inhibition by MCFA. Inhibition of transactivation by MCFA was specific: ligand-stimulated transcription from T3 or estrogen response elements was inhibited, but that from glucocorticoid or cyclic AMP response elements was not. We propose that MCFAs or metabolites thereof influence the activity of a factor(s) that interacts with the T3 and estrogen receptors to inhibit ligand-stimulated transcription.

Functional expression in yeast and characterization of a clofibrate-inducible plant cytochrome P-450 (CYP94A1) involved in cutin monomers synthesis

The chemical tagging of a cytochrome P-450-dependent lauric acid omega-hydroxylase from clofibrate-treated Vicia sativa seedlings with [1-14C]11-dodecynoic acid allowed the isolation of a full-length cDNA designated CYP94A1. We describe here the functional expression of this novel P-450 in two Saccharomyces cerevisiae strains overproducing their own NADPH-cytochrome P-450 reductase or a reductase from Arabidopsis thaliana. The results show a much higher efficiency of the yeast strain overproducing the plant reductase compared with the yeast strain overproducing its own reductase for expressing CYP94A1. The methyl end of saturated (from C-10 to C-16) and unsaturated (C18:1, C18:2 and C18:3) fatty acids was mainly oxidized by CYP94A1. Both E/Z and Z/E configurations of 9, 12-octadecadienoic acids were omega-hydroxylated. Lauric, myristic and linolenic acids were oxidized with the highest turnover rate (24 min-1). The strong regioselectivity of CYP94A1 was clearly shifted with sulphur-containing substrates, since both 9- and 11-thia laurate analogues were sulphoxidized. Similar to animal omega-hydroxylases, this plant enzyme was strongly induced by clofibrate treatment. Rapid CYP94A1 transcript accumulation was detected less than 20 min after exposure of seedlings to the hypolipidaemic drug. The involvement of CYP94A1 in the synthesis of cutin monomers and fatty acid detoxification is discussed.

Cloning and expression of cDNA encoding a mitogen-activated protein kinase from a phytopathogenic filamentous fungus

We have cloned a mitogen-activated protein kinase (MAPK) designated Fusarium solani f. sp. pisi mitogen-activated protein kinase (FsMAPK) from the phytopathogenic filamentous fungus F. solani f. sp. pisi T8 strain. A single open reading frame (ORF) of 1068 bp encoding a polypeptide of 355 amino acids (aa) with a predicted molecular mass of 41,194 Da was found in the cloned 1583-bp cDNA insert. FsMAPK is highly homologous to SPK1 of fission yeast, FUS3 of budding yeast, MsERK1 of alfalfa, Sur-1 of nematode, and hERK1 of human. That this gene is expressed in F. solani f. sp. pisi was shown by the finding that immunoblot of the fungal extracts with anti-FsMAPK antibodies (Ab) raised in a rabbit against the FsMAPK, expressed in Escherichia coli (E. coli), detected the corresponding protein. DNA blot analysis indicated that Fsmapk is present as a single copy in the fungal genome.

Cloning of cutinase transcription factor 1, a transactivating protein containing Cys6Zn2 binuclear cluster DNA-binding motif

Hydroxy fatty acids from plant cutin were shown previously to induce the expression of the cutinase gene via a palindromic sequence located at -159 base pairs of the cutinase gene in Fusarium solani f. sp. pisi (Nectria hematococca mating type VI). Of the two overlapping palindromes in this sequence, palindrome 2 was found to be essential for the inducibility of cutinase by hydroxy fatty acids. Screening of a phage expression library with the concatenated palindrome 2 as probe detected a distinct cDNA clone encoding a polypeptide designated cutinase transcription factor 1alpha (CTF1alpha) with a calculated molecular weight of 101,109. This protein contains a Cys6Zn2 binuclear cluster motif sharing homology to the Cys6Zn2 binuclear cluster DNA-binding domains of transcription factors from Saccharomyces cerevisiae, S. carlsbergensis, Kluyveromyces lactis, Neurospora crassa, Aspergillus nidulans, and A. flavus. CTF1alpha, expressed in Escherichia coli, showed specific binding to the palindrome 2 DNA fragment but not to palindrome 1 or mutant palindrome 2 DNA fragments, suggesting specific binding of CTF1alpha to palindrome 2. When CTF1alpha was expressed as a fusion protein with the nuclear localization sequence of SV40 in yeast, it transactivated the native cutinase promoter fused to the chloramphenicol acetyl transferase (cat) gene. Mutation of palindrome 2 but not palindrome 1 abolished this transactivation. Thus, CTF1alpha positively acts in vivo by binding selectively to palindrome 2 of the cutinase gene promoter.

Effect of disruption of a cutinase gene (cutA) on virulence and tissue specificity of Fusarium solani f. sp. cucurbitae race 2 toward Cucurbita maxima and C. moschata

A 3.9-kb genomic DNA fragment from the cucurbit pathogen Fusarium solani f. sp. cucurbitae race 2 was cloned. Sequence analysis revealed an open reading frame of 690 nucleotides interrupted by a single 51-bp intron. The nucleotide and predicted amino acid sequences showed 92 and 98% identity, respectively, to those of the cutA gene of the pea pathogen F. solani f. sp. pisi. A gene replacement vector was constructed and used to generate cutA- mutants that were detected with a polymerase chain reaction (PCR) assay. Seventy-one cutA- mutants were identified among the 416 transformants screened. Vector integration was assessed by Southern analysis in 23 of these mutants. PCR and Southern analysis data showed the level of homologous integration was 14%. Disruption of the cutA locus in mutants was confirmed by RNA gel blot hybridization. Neither virulence on Cucurbita maxima cv. Delica at any of six different inoculum concentrations, nor pathogenicity on intact fruit of four different species or cultivars of cucurbit or hypocotyl tissue of C. maxima cv. Crown, was found to be affected by disruption of the cutA gene.

Signal pathways and appressorium morphogenesis

Fungal pathogens have evolved elaborate strategies to gain access to plant tissues. For many pathogens, following attachment of spores to the leaf surface, germ tubes emerge and grow across the surface, often in response to particular environmental cues and to a specific location. At an appropriate site, polar elongation of the germ tube ceases, the tip attaches to the surface and swells to form an appressorium, a uniquely organized infection structure. Following a period of maturation, a hypha then emerges at the plant interface and penetrates into the plant tissues. This chapter discusses recent developments that provide new insight into the molecular mechanism regulating induction and function of appressoria. Topics include attachment to the leaf surface; environmental cues that signal germ-tube growth and appressorium formation; mechanisms for sensing environmental cues; endogenous signaling pathways; and mechanisms of penetration from the appressorium.

Disruption of the serine proteinase gene (sep) in Aspergillus flavus leads to a compensatory increase in the expression of a metalloproteinase gene (mep20)

The serine proteinase gene (sep) in Aspergillus flavus was disrupted by homologous recombination with a hygromycin resistance gene as the marker. The gene-disrupted mutant GR-2 contained a single-copy insertion of the marker gene and did not express the sep gene. Serine proteinase activity, 36-kDa protein labeled by 3H-diisopropylfluorophosphate, and immunologically detectable proteinase were not detected in the culture fluid of GR-2. Despite the absence of the serine proteinase, the total elastinolytic activity levels in the mutant and the wild-type A.flavus were comparable. Immunoblots revealed that the mutant secreted greater amounts of an elastinolytic metalloproteinase gene (mep20) product than did the wild type. Furthermore, mep20 mRNA levels, measured by RNase protection assay, in the mutant were higher than those in the wild type. Inhibition of the serine proteinase by Streptomyces subtilisin inhibitor (SSI) in the culture medium of wild-type A.flavus also resulted in an elevation of mep20 gene products. Although no serine proteinase activity could be detected, the level of elastinolytic activity of the SSI-treated culture was comparable to that of the control. Immunoblots revealed that the addition of SSI caused an elevation in the levels of metalloproteinase and its mRNA. These results suggest that the expression of the genes encoding serine and metalloproteinases are controlled by a common regulatory system and the fungus has a mechanism to sense the status of extracellular proteolytic activities.

Host signals in fungal gene expression involved in penetration into the host

Fungal spores, on contact with their hosts, perceive the plant signals and consequently initiate gene expression that enables the fungus to penetrate through the host barriers. Germination and appressorium formation by Colletotrichum gloeosporioides spore is induced by host surface wax on the growing avocado (Persea americana) fruits and, at ripening of the fruit, ethylene induces multiple appressorium formation. Both the wax and ethylene may use phosphorylation of 29- and 43-kDa proteins in the signal transduction. Unique genes that are expressed during appressorium formation induced by the host signal were cloned and sequenced. These include cap3 and cap5 that encode cysteine-rich small proteins, cap22 that encodes a secreted glycoprotein found in the appressorial wall, and cap20 whose disruption drastically decreases virulence. Disruption of cutinase gene drastically reduces the virulence of Fusarium solani pisi on pea (Pisum sativum L.). The promoter elements in cutinase gene involved in the induction of this gene by the hydroxy fatty acid monomers of cutin were identified and transcription factors that bind these elements were cloned. One of them, that binds to a palindrome, essential for cutinase induction, was found to be phosphorylated. Several proteins kinases from F. solani pisi were cloned. Key words: appressorium, cutin, cutinase, ethylene, gene disruption, protein phosphorylation, protein kinase, transcription factor.

Cloning and expression of cDNA encoding a protein that binds a palindromic promoter element essential for induction of fungal cutinase by plant cutin

Previous studies showed that a palindromic sequence located at -159 base pairs is essential for induction of cutinase gene in Fusarium solani f. sp. pisi (Nectria haematococca mating type VI) by the hydroxy fatty acids from plant cutin and that a 50-kDa nuclear protein binds to a promoter that contains this element. Screening of a phage lambda gt11 expression library with the concatenated palindromic sequence as the probe identified a cDNA encoding a palindrome-binding protein (PBP). Nucleotide sequence of this cDNA revealed an open reading frame that would code for PBP with a calculated molecular weight of 49,847. This PBP contains a putative nuclear localization signal and a zinc finger motif sharing homology with the zinc finger DNA binding domains of transcription factors from mammals, Saccharomyces cerevisiae, Neurospora crassa, and Ustilago maydis. A highly basic region immediately adjacent to the carboxyl side of the zinc finger was also observed. PBP expressed in Escherichia coli showed specific binding to the palindromic DNA fragment.

Isolation and characterization of genes expressed uniquely during appressorium formation by Colletotrichum gloeosporioides conidia induced by the host surface wax

Appressorium formation in germinating Colletotrichum gloeosporioides is induced by the surface wax of the host, the avocado fruit. To elucidate the mechanism by which differentiation of appressorium formation is induced, the fungal genes specifically activated by this host signal were sought. From a cDNA library of the transcripts present in appressorium-forming conidia, the clones representing nongerminating conidia were removed by hybridization with cDNAs synthesized from the nongerminating conidia. From this subtracted library, clones that hybridized with cDNA for transcripts from appressorium-forming conidia and not with cDNA for transcripts from germinating conidia were selected. Three such clones were isolated and sequenced. The genes for these three transcripts were also cloned and sequenced. Northern blot analysis showed that transcripts that hybridized with these three clones were expressed in the conidium only during the process of appressorium formation induced by avocado surface wax, and that these transcripts were not detectable when appressorium formation was prevented even in the presence of avocado wax. Nucleotide sequences of the clones revealed that one clone, cap3, contained an open reading frame (ORF) that would code for a 26-amino acid, cysteine-rich peptide with significant homology to Neurospora crassa copper metallothionein. Another clone, cap5, contained an ORF that would code for a 27-amino acid cysteine-rich peptide with less homology to metallothioneins. Cu2+ and Cd2+ also induced the expression of these genes at lower levels. The histochemical analysis of transformants containing the cap5 promoter fused to the beta-glucuronidase (GUS) gene showed that the cap5 gene promoter caused GUS expression exclusively during appressorium formation and most of the gus activity was in the appressorium. The cap22 clone contained an ORF coding for a 227-amino acid polypeptide of 22 kDa, which did not show significant homology to any known proteins. Recombinant CAP22 protein was produced using a pET-19b expression system in Escherichia coli, purified, and used to prepare rabbit antibodies. Western blot analysis of proteins from the appressorium-forming conidia revealed a major cross-reacting protein at 43 kDa and a minor band at 68 kDa, indicating that the potential glycosylation sites found in the primary translation product were probably glycosylated. Results of immunogold localization showed that CAP22 protein was located on the wall of the appressorium.

Surface signaling in pathogenesis

Surface signaling plays a major role in fungal infection. Topographical features of the plant surface and chemicals on the surface can trigger germination of fungal spores and differentiation of the germ tubes into appressoria. Ethylene, the fruit-ripening hormone, triggers germination of conidia, branching of hyphae, and multiple appressoria formation in Colletotrichum, thus allowing fungi to time their infection to coincide with ripening of the host. Genes uniquely expressed during appressoria formation induced by topography and surface chemicals have been isolated. Disruption of some of them has been shown to decrease virulence on the hosts. Penetration of the cuticle by the fungus is assisted by fungal cutinase secreted at the penetration structure of the fungus. Disruption of cutinase gene in Fusarium solani pisi drastically decreased its virulence. Small amounts of cutinase carried by spores of virulent pathogens, upon contact with plant surface, release small amounts of cutin monomers that trigger cutinase gene expression. The promoter elements involved in this process in F. solani pisi were identified, and transcription factors that bind these elements were cloned. One of them, cutinase transcription factor 1, expressed in Escherichia coli, is phosphorylated. Several protein kinases from F. solani pisi were cloned. The kinase involved in phosphorylation of specific transcription factors and the precise role of phosphorylation in regulating cutinase gene transcription remain to be elucidated.

Cloning of a gene expressed during appressorium formation by Colletotrichum gloeosporioides and a marked decrease in virulence by disruption of this gene

Appressorium formation in germinating Colletotrichum gloeosporioides is induced by the surface wax of its host. One of the genes expressed uniquely in C. gloeosporioides during appressorium formation induced by the host signal has been designated cap20, and this gene and its cDNA were cloned and sequenced. Nucleotide sequences of both revealed an open reading frame that could encode a 183-amino acid polypeptide that did not have significant homology with any known proteins. Reverse transcriptase-polymerase chain reaction detected cap20 gene transcripts at the infection front on the surface and within tomato fruits infected by C. gloeosporioides. Gene-disrupted mutants incapable of expressing cap20 showed a drastically decreased virulence on avocado and tomato fruits. These results suggest that cap20 plays a significant role in the infection of the host.

Analysis of transcriptional activation of a cyclic AMP response element by 2,6,10,14-tetramethylpentadecane (pristane) in JB6 mouse epidermal cells

Pristane is a naturally occurring isoprenoid that is believed to be derived from the phytyl moiety of chlorophyll. Thus, it is not surprising that pristane is present in many common fruits and vegetables. Furthermore, pristane can be detected in the tissues of fish and mammals. In animal models using rodents, pristane can function as a potent tumor promoter. At the molecular level, pristane can induce changes in the plasma membrane, alter the conformation of chromatin, and selectively activate gene expression. Addition of pristane to a mouse epidermal cell line (JB6 P+) allows these cells to grow in an anchorage-independent manner. In contrast, JB6 P-cells are not transformed by pristane. Our study was undertaken to correlate transformation of P+ cells with changes induced by pristane. Transcriptional activation of a cyclic AMP response element (CRE) was induced by pristane in P+ and P-cells. Point mutations in the CRE abolished activation by pristane, thus indicating that an intact CRE was necessary for pristane activation. In P+ cells, pristane repressed phosphodiesterase activity. However, protein kinase A was activated by pristane in P+ and P-cells. Taken together, these results indicated pristane induced novel changes in P+ cells that in turn may facilitate neoplastic transformation.

Isolation and analysis of a novel inducible pectate lyase gene from the phytopathogenic fungus Fusarium solani f. sp. pisi (Nectria haematococca, mating population VI)

A pectate lyase produced by Fusarium solani f. sp. pisi (Nectria haematococca, mating population VI) was previously shown to be essential for host infection (M. S. Crawford and P. E. Kolattukudy, Arch. Biochem. Biophys. 258:196-205, 1987). Pectate lyase genes have not been cloned from any phytopathogenic fungi. A gene, designated pelA, encoding an inducible pectate lyase was isolated from F. solani f. sp. pisi. A probe was synthesized by polymerase chain reaction with oligonucleotide primers based on the known amino acid sequences of two regions of the mature protein and first-strand cDNA as template. Both cDNA and the gene were isolated and sequenced. That the cloned cDNA represents the previously purified pectate lyase is shown by the complete match of the sequences of the N-terminal 38 amino acid residues and the 20 amino acid residues of an internal peptide with the sequence deduced from the cDNA sequence. This lyase sequence shows little homology to those of other pectolytic enzymes. The pelA gene shows standard characteristics with respect to promoter, intron, and polyadenylation sequences. As determined by primer extension and nuclease S1 analysis of the origin of the transcription, there are multiple initiation sites clustered in a region of 12 nucleotides located about 55 bp upstream of the start codon. Northern (RNA) blot analysis showed a single band of mRNA at about 1 kb. The pelA gene mRNA was detected only when F. solani f. sp. pisi was grown with pectin, and there was no detectable transcript accumulation when the fungus was grown with glucose as the sole carbon source. When both carbon sources were present, the pelA gene was transcribed only after glucose was completely depleted, indicating carbon catabolite repression. Moreover, the levels of transcription decreased rapidly prior to maximal enzyme accumulation, suggesting a mechanism of self catabolite repression.