Isolation of a Fusarium solani mutant reduced in cutinase activity and virulence

Review of nomenclature and methods of analysis of polyethylene terephthalic acid hydrolyzing enzymes activity

Enzymatic degradation of polyethylene terephthalic acid (PET) has been gaining increasing importance. This has resulted in a significant increase in the search for newer enzymes and the development of more efficient enzyme-based systems. Due to the lack of a standard screening process, screening new enzymes has relied on other assays to determine the presence of esterase activity. This, in turn, has led to various nomenclatures and methods used to describe them and measure their activity. Since all PET-hydrolyzing enzymes are α/β hydrolases, they catalyze a serine nucleophilic attack and cleave an ester bond. They are lipases, esterases, cutinases and hydrolases. This has been used interchangeably, leading to difficulties while comparing results and evaluating progress. This review discusses the varied enzyme nomenclature being adapted, the different assays and analysis methods reported, and the strategies used to increase PET-hydrolyzing enzyme efficiency. A section on the various ways to quantify PET hydrolysis is also covered.

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.

Cutinases: Characteristics and Insights in Industrial Production

Cutinases (EC 3.1.1.74) are serin esterases that belong to the α/β hydrolases superfamily and present in the Ser-His-Asp catalytic triad. They show characteristics between esterases and lipases. These enzymes hydrolyze esters and triacylglycerols and catalyze esterification and transesterification reactions. Cutinases are synthesize by plant pathogenic fungi, but some bacteria and plants have been found to produce cutinases as well. In nature they facilitate a pathogen’s invasion by hydrolyzing the cuticle that protects plants, but can be also used for saprophytic fungi as a way to nourish themselves. Cutinases can hydrolyze a wide range of substrates like esters, polyesters, triacylglycerols and waxes and that makes this enzyme very attractive for industrial purposes. This work discusses techniques of industrial interest such as immobilization and purification, as well as some of the most important uses of cutinases in industries.

A Verticillium dahliae Extracellular Cutinase Modulates Plant Immune Responses

Cutinases have been implicated as important enzymes during the process of fungal infection of aerial plant organs. The function of cutinases in the disease cycle of fungal pathogens that invade plants through the roots has been less studied. Here, functional analysis of 13 cutinase (carbohydrate esterase family 5 domain-containing) genes (VdCUTs) in the highly virulent vascular wilt pathogen Verticillium dahliae Vd991 was performed. Significant sequence divergence in cutinase family members was observed in the genome of V. dahliae Vd991. Functional analyses demonstrated that only VdCUT11, as purified protein, induced cell death and triggered defense responses in Nicotiana benthamiana, cotton, and tomato plants. Virus-induced gene silencing showed that VdCUT11 induces plant defense responses in Nicotiana benthamania in a BAK1 and SOBIR-dependent manner. Furthermore, coinfiltration assays revealed that the carbohydrate-binding module family 1 protein (VdCBM1) suppressed VdCUT11-induced cell death and other defense responses in N. benthamiana. Targeted deletion of VdCUT11 in V. dahliae significantly compromised virulence on cotton plants. The cutinase VdCUT11 is an important secreted enzyme and virulence factor that elicits plant defense responses in the absence of VdCBM1.

Optimization of medium parameters by response surface methodology (RSM) for enhanced production of cutinase from Aspergillus sp. RL2Ct

Cutinases are hydrolytic enzymes which catalyzes esterification and trans-esterification reactions that make them highly potential industrial biocatalyst. In the present investigation microorganisms showing cutinase activity were isolated from plant samples. The strain showing maximum cutinase activity was identified by 18S rDNA sequencing as Aspergillus sp. RL2Ct and was selected for further studies. To achieve maximum enzyme production, the medium components affecting cutinase production were screened by Plackett-Burman followed by central composite design. The results obtained suggested that cutin, temperature and CaCl₂ have influenced the cutinase production significantly with very high confidence levels. Cutinase production was maximum (663 U/mg protein) when using cutin prepared from orange peel as sole source of carbon. An overall 4.33-fold increase in the production of cutinase was observed after optimization of culture conditions (including 2.5-fold increase using RSM) during 24 h of incubation. The production time of Aspergillus sp. RL2Ct cutinase is significantly lower than the most of the earlier reported cutinase-producing fungus.

Genome-wide identification, classification and expression analysis in fungal-plant interactions of cutinase gene family and functional analysis of a putative ClCUT7 in Curvularia lunata

Cutinase is described as playing various roles in fungal-plant pathogen interactions, such as eliciting host-derived signals, fungal spore attachment and carbon acquisition during saprophytic growth. However, the characteristics of the cutinase genes, their expression in compatible interactions and their roles in pathogenesis have not been reported in Curvularia lunata, an important leaf spot pathogen of maize in China. Therefore, a cutinase gene family analysis could have profound significance. In this study, we identified 13 cutinase genes (ClCUT1 to ClCUT13) in the C. lunata genome. Multiple sequence alignment showed that most fungal cutinase proteins had one highly conserved GYSQG motif and a similar DxVCxG[ST]-[LIVMF](3)-x(3)H motif. Gene structure analyses of the cutinases revealed a complex intron-exon pattern with differences in the position and number of introns and exons. Based on phylogenetic relationship analysis, C. lunata cutinases and 78 known cutinase proteins from other fungi were classified into four groups with subgroups, but the C. lunata cutinases clustered in only three of the four groups. Motif analyses showed that each group of cutinases from C. lunata had a common motif. Real-time PCR indicated that transcript levels of the cutinase genes in a compatible interaction between pathogen and host had varied expression patterns. Interestingly, the transcript levels of ClCUT7 gradually increased during early pathogenesis with the most significant up-regulation at 3 h post-inoculation. When ClCUT7 was deleted, pathogenicity of the mutant decreased on unwounded maize (Zea mays) leaves. On wounded maize leaves, however, the mutant caused symptoms similar to the wild-type strain. Moreover, the ClCUT7 mutant had an approximately 10 % reduction in growth rate when cutin was the sole carbon source. In conclusion, we identified and characterized the cutinase family genes of C. lunata, analyzed their expression patterns in a compatible host-pathogen interaction, and explored the role of ClCUT7 in pathogenicity. This work will increase our understanding of cutinase genes in other fungal-plant pathogens.

Recognition of seven species in the Cryptococcus gattii/Cryptococcus neoformans species complex

Phylogenetic analysis of 11 genetic loci and results from many genotyping studies revealed significant genetic diversity with the pathogenic Cryptococcus gattii/Cryptococcus neoformans species complex. Genealogical concordance, coalescence-based, and species tree approaches supported the presence of distinct and concordant lineages within the complex. Consequently, we propose to recognize the current C. neoformans var. grubii and C. neoformans var. neoformans as separate species, and five species within C. gattii. The type strain of C. neoformans CBS132 represents a serotype AD hybrid and is replaced. The newly delimited species differ in aspects of pathogenicity, prevalence for patient groups, as well as biochemical and physiological aspects, such as susceptibility to antifungals. MALDI-TOF mass spectrometry readily distinguishes the newly recognized species.

Carbohydrate-related enzymes of important Phytophthora plant pathogens

Carbohydrate-Active enZymes (CAZymes) form particularly interesting targets to study in plant pathogens. Despite the fact that many CAZymes are pathogenicity factors, oomycete CAZymes have received significantly less attention than effectors in the literature. Here we present an analysis of the CAZymes present in the Phytophthora infestans, Ph. ramorum, Ph. sojae and Pythium ultimum genomes compared to growth of these species on a range of different carbon sources. Growth on these carbon sources indicates that the size of enzyme families involved in degradation of cell-wall related substrates like cellulose, xylan and pectin is not always a good predictor of growth on these substrates. While a capacity to degrade xylan and cellulose exists the products are not fully saccharified and used as a carbon source. The Phytophthora genomes encode larger CAZyme sets when compared to Py. ultimum, and encode putative cutinases, GH12 xyloglucanases and GH10 xylanases that are missing in the Py. ultimum genome. Phytophthora spp. also encode a larger number of enzyme families and genes involved in pectin degradation. No loss or gain of complete enzyme families was found between the Phytophthora genomes, but there are some marked differences in the size of some enzyme families.

Expression of fungal cutinase and swollenin in tobacco chloroplasts reveals novel enzyme functions and/or substrates

In order to produce low-cost biomass hydrolyzing enzymes, transplastomic lines were generated that expressed cutinase or swollenin within chloroplasts. While swollenin expressing plants were homoplasmic, cutinase transplastomic lines remained heteroplasmic. Both transplastomic lines showed interesting modifications in their phenotype, chloroplast structure, and functions. Ultrastructural analysis of chloroplasts from cutinase- and swollenin-expressing plants did not show typical lens shape and granal stacks. But, their thylakoid membranes showed unique scroll like structures and chloroplast envelope displayed protrusions, stretching into the cytoplasm. Unusual honeycomb structures typically observed in etioplasts were observed in mature chloroplasts expressing swollenin. Treatment of cotton fiber with chloroplast-derived swollenin showed enlarged segments and the intertwined inner fibers were irreversibly unwound and fully opened up due to expansin activity of swollenin, causing disruption of hydrogen bonds in cellulose fibers. Cutinase transplastomic plants showed esterase and lipase activity, while swollenin transplastomic lines lacked such enzyme activities. Higher plants contain two major galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), in their chloroplast thylakoid membranes that play distinct roles in their structural organization. Surprisingly, purified cutinase effectively hydrolyzed DGDG to MGDG, showing alpha galactosidase activity. Such hydrolysis resulted in unstacking of granal thylakoids in chloroplasts and other structural changes. These results demonstrate DGDG as novel substrate and function for cutinase. Both MGDG and DGDG were reduced up to 47.7% and 39.7% in cutinase and 68.5% and 67.5% in swollenin expressing plants. Novel properties and functions of both enzymes reported here for the first time should lead to better understanding and enhanced biomass hydrolysis.

Pea-Fusarium solani interactions contributions of a system toward understanding disease resistance

This mini-review points to the usefulness of the pea-Fusarium solani interaction in researching the biochemical and molecular aspects of the nonhost resistance components of peas. This interaction has been researched to evaluate the resistance roles of the phytoalexin, pisatin, the cuticle barrier, and the activation of the nonhost resistance response. Concurrently, evaluations of associated signaling processes and the tools possessed by the pathogen to contend with host obstacles were included. The properties of some pathogenesis-related genes of pea and their regulation and contribution to resistance are discussed. A proposed action of two biotic elicitors on both chromatin conformation and the architectural transcription factor, HMG A, is presented and includes time lines of events within the host immune response.

Pseudozyma jejuensis sp. nov., a novel cutinolytic ustilaginomycetous yeast species that is able to degrade plastic waste

An ustilaginomycetous anamorphic yeast, isolated from orange leaves on Jeju island in South Korea, represents a novel Pseudozyma species according to morphologic and physiologic findings and molecular taxonomic analysis using the D1/D2 domains of the large subunit (26S) rRNA gene and the internally transcribed spacer (ITS) 1+2 regions. The name Pseudozyma jejuensis sp. nov. is proposed for this novel species, with OL71(T) (=KCTC 17482(T)=CBS 10454(T)) as type strain. In the present study, we have also demonstrated that Pseudozyma jejuensis OL71 is capable of producing cutinase and degrading polycaprolactone. These results suggest that Pseudozyma jejuensis or its cutinase may be useful for the biological degradation of plastic waste.

Mixture design as a first step for optimization of fermentation medium for cutinase production from Colletotrichum lindemuthianum

Cutinase enzymes from fungi have found diverse applications in industry. However, most of the available literature on cutinase production is related to the cultivation of genetically engineered bacteria or yeast cells. In the present study, we use mixture design experiments to evaluate the influence of six nutrient elements on production of cutinase from the fungus Colletotrichum lindemuthianum. The nutritional elements were starch, glucose, ammonium sulfate, yeast extract, magnesium sulfate, and potassium phosphate. In the experimental design, we imposed the constraints that exactly one factor must be omitted in each set of experiments and no factor can account for more than one third of the mixture. Thirty different sets of experiments were designed. Results obtained showed that while starch is found to have negative influence on the production of the enzyme, yeast extract and potassium phosphate have a strong positive influence. Magnesium sulfate, ammonium sulfate, and glucose have low positive influence on the enzyme production. Contour plots have also been created to obtain information concerning the interaction effects of the media components on enzyme production.

Enhanced degradation and toxicity reduction of dihexyl phthalate by Fusarium oxysporum f. sp. pisi cutinase

AIMS

This research aims to investigate the efficiency of two lipolytic enzymes--fungal cutinase and yeast esterase--upon the biodegradation of dihexyl phthalate (DHP).

METHOD AND RESULTS

During the enzymatic degradation of DHP dissolved in methanol, several degradation products were detected and their time-course changes were monitored using GC/MS. The DHP-degradation rate of cutinase was surprisingly high; i.e. almost 70% of the initial DHP (500 mg l(-1)) was decomposed within 4.5 h. Although the same amount of esterase was employed, more than 85% of the DHP remained after 3 days. Almost all the DHP was converted by cutinase into 1,3-isobenzofurandione (IBF), whereas hexyl methyl phthalate and IBF were abundantly produced by esterase. In addition, the toxicities of the DHP-degraded products by esterase were evaluated using various recombinant bioluminescent bacteria, which caused oxidative and protein damage, whereas the hydrolysis products from cutinase never caused any cellular damage in the methanol-containing reaction system.

CONCLUSIONS

Cutinase starts to act as a DHP-degrader much earlier and faster than esterase, with high stability in ester-hydrolytic activity, therefore a plausible approach to the practical application of cutinase for DHP degradation in the DHP-contaminated environments may be possible.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study describes the enhanced degradation and detoxification of DHP using Fusarium oxysporum f. sp. pisi cutinase.

Comparative analysis of Phytophthora genes encoding secreted proteins reveals conserved synteny and lineage-specific gene duplications and deletions

Comparative analysis of two Phytophthora genomes revealed overall colinearity in four genomic regions consisting of a 1.5-Mb sequence of Phytophthora sojae and a 0.9-Mb sequence of P. ramorum. In these regions with conserved synteny, the gene order is largely similar; however, genome rearrangements also have occurred. Deletions and duplications often were found in association with genes encoding secreted proteins, including effectors that are important for interaction with host plants. Among secreted protein genes, different evolutionary patterns were found. Elicitin genes that code for a complex family of highly conserved Phytophthora-specific elicitors show conservation in gene number and order, and often are clustered. In contrast, the race-specific elicitor gene Avrlb-1 appeared to be missing from the region with conserved synteny, as were its five homologs that are scattered over the four genomic regions. Some gene families encoding secreted proteins were found to be expanded in one species compared with the other. This could be the result of either repeated gene duplications in one species or specific deletions in the other. These different evolutionary patterns may shed light on the functions of these secreted proteins in the biology and pathology of the two Phytophthora spp.

Extensive colonization of apples by smut anamorphs causes a new postharvest disorder

Colonization of apples by ballistoconidium-forming fungi causes a new disorder, here named 'white haze'. White haze may occur in mild form in the field, but only becomes problematic after Ultra-Low Oxygen storage, and, therefore, may be considered as a postharvest disorder. All isolates, obtained using the spore-fall method, were morphologically identified as anamorphs of smut fungi belonging to the genus Tilletiopsis. Sequence analysis of the D1/D2 and the ITS domains of the rDNA revealed nine novel taxa scattered among the Exobasidiomycetidae (Ustilaginomycetes). Field experiments confirmed the erratic incidence of white haze over the years, and the development of the disorder seems to be enhanced at lower temperatures and a high relative humidity. Several scab fungicide treatments showed diminishing effects on the incidence of white haze.

Biodegradation of dipropyl phthalate and toxicity of its degradation products: a comparison of Fusarium oxysporum f. sp. pisi cutinase and Candida cylindracea esterase

The efficiency of two lypolytic enzymes (fungal cutinase, yeast esterase) in the degradation of dipropyl phthalate (DPrP) was investigated. The DPrP-degradation rate of fungal cutinase was surprisingly high, i.e., almost 70% of the initial DPrP (500 mg/l) was decomposed within 2.5 h and nearly 50% of the degraded DPrP disappeared within the initial 15 min. With the yeast esterase, despite the same concentration, more than 90% of the DPrP remained even after 3 days of treatment. During the enzymatic degradation of DPrP, several DPrP-derived compounds were detected and time-course changes in composition were also monitored. The final chemical composition after 3 days was significantly dependent on the enzyme used. During degradation with fungal cutinase, most DPrP was converted into 1,3-isobenzofurandione (IBF) by diester hydrolysis. However, in the degradation by yeast esterase, propyl methyl phthalate (PrMP) was produced in abundance in addition to IBF. The toxic effects of the final degradation products were investigated using various recombinant bioluminescent bacteria. As a result, the degradation products (including PrMP) from yeast esterase severely caused oxidative stress and damage to protein synthesis in bacterial cells, while in the fungal cutinase processes, DPrP was significantly degraded to non-toxic IBF after the extended period (3 days).

Biodegradation and detoxification of organophosphate insecticide, malathion by Fusarium oxysporum f. sp. pisi cutinase

Efficiencies of two lypolytic enzymes (fungal cutinase and yeast esterase) in malathion degradation were investigated. Surprisingly, degradation rate of malathion by fungal cutinase was very high, i.e. almost 60% of initial malathion (500 mg l(-1)) was decomposed within 0.5 h, and nearly 50% of the degraded malathion disappeared within initial 15 min. With the yeast esterase, despite the same concentration, more than 65% of malathion remained even after 2-day treatment. During enzymatic degradation of malathion, two malathion-derived compounds were detected, and time-course changes in composition were also monitored. In the degradation by both fungal cutinase and yeast esterase, two additional organic chemicals were produced from malathion: malathion monoacid (MMA) and malathion diacid (MDA) by ester hydrolysis. Final chemical composition after 2 d was significantly dependent on the enzyme used. Fungal cutinase produced MDA as a major degradation compound. However in the malathion degradation by yeast esterase, an isomer of MMA was produced in abundance in addition to MDA. Toxic effects of malathion and its final degradation products were investigated using various recombinant bioluminescent bacteria. As a result, the degradation products (including MMA) by esterase severely caused membrane damage and inhibition of protein synthesis in bacterial cells, while in the fungal cutinase processes, malathion was significantly degraded to non-toxic MDA after the extended period (2 days).

The yeast flora of the coast redwood, Sequoia sempervirens

Only four yeast species could be isolated from young and perannual shoots of the coast redwood tree, Sequoia sempervirens, and from soil beneath the trees, viz. both varieties of Debaryomyces hansenii, Trichosporon pullulans, T. porosum and an unidentified red basidiomycetous yeast.

Molecular evidence that the extracellular cutinase Pbc1 is required for pathogenicity of Pyrenopeziza brassicae on oilseed rape

Recent evidence has suggested that cutinase is required for cuticular penetration and, therefore, is essential for pathogenicity of Pyrenopeziza brassicae, the causal organism of light leaf spot disease of oilseed rape and other brassicas. In order to acquire molecular evidence for the role of cutinase in pathogenesis, the single-copy P. brassicae cutinase gene Pbc1 was disrupted by a transformation-mediated approach. Southern hybridization analysis revealed that in one mutant, NH10-1224, the disruption was due to a tandem insertion of two copies of the disruption vector into the 5' coding region of Pbc1. In contrast to the wild type, no expression of Pbc1 was detected during in planta growth or in cutin-induced mycelium of NH10-1224 and no cutinase activity was detected in culture supernatants from NH10-1224 using p-nitrophenyl butyrate as substrate. Scanning electron microscopy of Brassica napus cotyledons infected with wild-type P. brassicae confirmed that entry into the host is by direct penetration of the cuticle. In contrast, the cutinase-deficient mutant NH10-1224 failed to penetrate the cuticular layer and was unable to develop disease symptoms. This evidence is consistent with the hypothesis that Pbc1 is required for P. brassicae to penetrate the plant cuticle. Demonstration that complementation of NH10-1224 with the Pbc1 wild-type gene restores both cutinase activity and pathogenicity will be required to definitively establish that cutinase is required for successful pathogenesis of brassicas by P. brassicae.

Enhanced degradation of an endocrine-disrupting chemical, butyl benzyl phthalate, by Fusarium oxysporum f. sp. pisi cutinase

Compared to yeast esterase, fungal cutinase degraded butyl benzyl phthalate (BBP) far more efficiently; i.e., almost 60% of the BBP disappeared within 7.5 h. Also, the final chemical composition significantly depended on the enzyme used. Toxicity monitoring using bioluminescent bacteria showed that butyl methyl phthalate, a major product of degradation by esterase, was an oxidative toxic hazard.

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.

Cutinase: from molecular level to bioprocess development

This review analyzes the role of cutinases in nature and their potential biotechnological applications. The cloning and expression of a fungal cutinase, Fusarium solani f. pisi, in Escherichia coli and Saccharomyces cerevisiae hosts are described. The three-dimensional structure of this cutinase is also analyzed and its function as a lipase is discussed and compared with other lipases. The biocatalytic applications of cutinase are described taking into account the preparation of different cutinase forms and the media in which the different types of reactions have been performed, namely hydrolysis, esterification, transesterification, and resolution of racemic mixtures. The stability of cutinase preparations is discussed and, in particular, the cutinase stability in anionic reversed micelles is analyzed considering the role of hexanol as a substrate, a cosurfactant, and a stabilizer. Process development, based on the operation of cutinase reactors, is also reviewed.

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.

Fusarium polycaprolactone depolymerase is cutinase

Polycaprolactone (PCL), a synthetic polyester, is degraded by a variety of microorganisms, including some phytopathogens. Many phytopathogens secrete cutinase, a serine hydrolase that degrades cutin, the structural polymer of the plant cuticle. We compared wild-type strains and a cutinase-negative gene replacement mutant strain of Fusarium solani f. sp. pisi (D. J. Stahl and W. Schäfer, Plant Cell 4:621-629, 1992) and a wild-type strain of Fusarium moniliforme to show that Fusarium cutinase is a PCL depolymerase. The wild-type strains, but not the mutant strain, (i) degraded PCL and used it as a source of carbon and energy, (ii) showed induction of secreted PCL depolymerase and an esterase activity of cutinase when grown in the presence of cutin, and (iii) showed induction of PCL depolymerase and an esterase activity of cutinase when grown in the presence of a hydrolysate of PCL, which contains PCL oligomers that are structurally similar to the natural inducers of cutinase. These results together with other details of regulation and conditions for optimal enzyme activity indicate that the Fusarium PCL depolymerase, required for degradation and utilization of PCL, is cutinase.

Role of cutinase in the invasion of plants

Cutin as the polymeric component of plant cuticles has to be breached during the direct penetration of plant surfaces by invading fungal pathogens. Several lines of evidence had suggested that cutinases produced under saprophytic growth conditions with cutin as sole carbon source were also involved in the penetration of host surfaces during plant infection. However, retained pathogenicity of cutinase gene disrupted mutants of Magnaporthe grisea and Nectria haematococca was not in support of the model. The controversial involvement of cutinase in plant infection was investigated with Alternaria brassicicola. The fungus produced two cutinase isozymes during saprophytic growth on cutin. The disruption of the single gene CUTAB1 abolished the expression of both isozymes and gene-disrupted transformants lost their trait of utilizing cutin as saprophytic carbon source. However, small amounts of two serine esterases with cutinolytic activities and molecular masses of 31 and 19 kDa continued to be secreted into the extracellular fluid. These two esterases were also expressed on leaves of cabbage during host invasion. In contrast, the two cutinase isozymes expressed by the wild-type strain under saprophytic conditions were not detected on host surfaces. The results indicate that different cutinase isozymes are expressed during saprophytic and parasitic stages of the fungus. Key words: cutinase, cuticle penetration, Alternaria brassicicola, plant infection, saprophyte.

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.

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.

Cutinase gene disruption in Fusarium solani f sp pisi decreases its virulence on pea

Fusarium solani f sp pisi (Nectria haematococca) isolate 77-2-3 with one cutinase gene produced 10 to 20% of the cutinase produced by isolate T-8 that has multiple cutinase genes, whereas cutinase gene-disrupted mutant 77-102 of isolate 77-2-3 did not produce cutinase. On the surface of pea stem segments, lesion formation was most frequent and most severe with T-8, less frequent and less severe with 77-2-3, and much less frequent and much milder with the gene-disrupted mutant. Microscopic examination of the lesions caused by the mutant strongly suggest that it penetrated the host mostly via the stomata. In seedling assays, 77-2-3 caused severe lesions on every seedling and stunted growth, whereas the mutant showed very mild lesions on one-third of the seedlings with no stunting. Thus, cutinase gene disruption resulted in a significant decrease in the pathogenicity of F. s. pisi on pea.

Cutinase is not required for fungal pathogenicity on pea

Cutinase, a fungal extracellular esterase, has been proposed to be crucial in the early events of plant infection by many pathogenic fungi. To test the long-standing hypothesis that cutinase of Nectria haematococca (Fusarium solani f sp pisi) is essential to pathogenicity, we constructed cutinase-deficient mutants by transformation-mediated gene disruption of the single cutinase gene of a highly virulent N. haematococca strain. Four independent mutants were obtained lacking a functional cutinase gene, as confirmed by gel blot analyses and enzyme assays. Bioassays of the cutinase-deficient strains showed no difference in pathogenicity and virulence on pea compared to the wild type and a control transformant. We conclude that the cutinase of N. haematococca is not essential for the infection of pea.

Disruption of a Magnaporthe grisea cutinase gene

Using a one-step strategy to disrupt CUT1, a gene for cutinase, cut1- mutants were generated in two strains of Magnaporthe grisea. One strain, pathogenic on weeping lovegrass and barley and containing the arg3-12 mutation, was transformed with a disruption vector in which the Aspergillus nidulans ArgB+ gene was inserted into CUT1. Prototrophic transformants were screened by Southern hybridization, and 3 of 53 tested contained a disrupted CUT1 gene (cut1::ArgB+). A second strain, pathogenic on rice, was transformed with a disruption vector in which a gene for hyg B resistance was inserted into CUT1. Two of the 57 transformants screened by Southern hybridization contained a disrupted CUT1 gene (cut1::Hyg). CUT1 mRNA was not detectable in transformants that contained a disrupted gene. Transformants with a disrupted CUT1 gene failed to produce a cutin-inducible esterase that is normally detected by activity staining on non-denaturing polyacrylamide gels. Enzyme activity, measured either with tritiated cutin or with p-nitrophenyl butyrate as a substrate, was reduced but not eliminated in strains with a disrupted CUT1 gene. The infection efficiency of the cut1- disruption transformants was equal to that of the parent strains on all three host plants. Lesions produced by these mutants had an appearance and a sporulation rate similar to those produced by the parent strains. We conclude that the M. grisea CUT1 gene is not required for pathogenicity.