Characterization and disruption of a gene in the maize pathogen Cochliobolus carbonum encoding a cellulase lacking a cellulose binding domain and hinge region

Comparative genomic analysis reveals cellulase plays an important role in the pathogenicity of Setosphaeria turcica f. sp. zeae

Setosphaeria turcica f. sp. zeae and S. turcica f. sp. sorghi, the two formae speciales of S. turcica, cause northern leaf blight disease of corn and sorghum, respectively, and often cause serious economic losses. They have obvious physiological differentiation and show complete host specificity. Host specificity is often closely related to pathogen virulence factors, including secreted protein effectors and secondary metabolites. Genomic sequencing can provide more information for understanding the virulence mechanisms of pathogens. However, the complete genomic sequence of S. turcica f. sp. sorghi has not yet been reported, and no comparative genomic information is available for the two formae speciales. In this study, S. turcica f. sp. zeae was predicted to have fewer secreted proteins, pathogen-host interaction (PHI) genes and carbohydrate-active enzymes (CAZys) than S. turcica f. sp. sorghi. Fifteen and 20 polyketide synthase (PKS) genes were identified in S. turcica f. sp. zeae and S. turcica f. sp. sorghi, respectively, which maintained high homology. There were eight functionally annotated effector protein-encoding genes specifically in S. turcica f. sp. zeae, among which the encoding gene StCEL2 of endo-1, 4-β-D-glucanase, an important component of cellulase, was significantly up-regulated during the interaction process. Finally, gluconolactone inhibited cellulase activity and decreased infection rate and pathogenicity, which indicates that cellulase is essential for maintaining virulence. These findings demonstrate that cellulase plays an important role in the pathogenicity of S. turcica f. sp. zeae. Our results also provide a theoretical basis for future research on the molecular mechanisms underlying the pathogenicity of the two formae speciales and for identifying any associated genes.

Host Cell Wall Damage during Pathogen Infection: Mechanisms of Perception and Role in Plant-Pathogen Interactions

The plant cell wall (CW) is a complex structure that acts as a mechanical barrier, restricting the access to most microbes. Phytopathogenic microorganisms can deploy an arsenal of CW-degrading enzymes (CWDEs) that are required for virulence. In turn, plants have evolved proteins able to inhibit the activity of specific microbial CWDEs, reducing CW damage and favoring the accumulation of CW-derived fragments that act as damage-associated molecular patterns (DAMPs) and trigger an immune response in the host. CW-derived DAMPs might be a component of the complex system of surveillance of CW integrity (CWI), that plants have evolved to detect changes in CW properties. Microbial CWDEs can activate the plant CWI maintenance system and induce compensatory responses to reinforce CWs during infection. Recent evidence indicates that the CWI surveillance system interacts in a complex way with the innate immune system to fine-tune downstream responses and strike a balance between defense and growth.

Comparative Transcriptomic Analysis of Race 1 and Race 4 of Fusarium oxysporum f. sp. cubense Induced with Different Carbon Sources

The fungal pathogen Fusarium oxysporum f. sp. cubense causes Fusarium wilt, one of the most destructive diseases in banana and plantain cultivars. Pathogenic race 1 attacks the "Gros Michel" banana cultivar, and race 4 is pathogenic to the Cavendish banana cultivar and those cultivars that are susceptible to Foc1. To understand the divergence in gene expression modules between the two races during degradation of the host cell wall, we performed RNA sequencing to compare the genome-wide transcriptional profiles of the two races grown in media containing banana cell wall, pectin, or glucose as the sole carbon source. Overall, the gene expression profiles of Foc1 and Foc4 in response to host cell wall or pectin appeared remarkably different. When grown with host cell wall, a much larger number of genes showed altered levels of expression in Foc4 in comparison with Foc1, including genes encoding carbohydrate-active enzymes (CAZymes) and other virulence-related genes. Additionally, the levels of gene expression were higher in Foc4 than in Foc1 when grown with host cell wall or pectin. Furthermore, a great majority of genes were differentially expressed in a variety-specific manner when induced by host cell wall or pectin. More specific CAZymes and other pathogenesis-related genes were expressed in Foc4 than in Foc1 when grown with host cell wall. The first transcriptome profiles obtained for Foc during degradation of the host cell wall may provide new insights into the mechanism of banana cell wall polysaccharide decomposition and the genetic basis of Foc host specificity.

Fungal cellulase is an elicitor but its enzymatic activity is not required for its elicitor activity

Plant-pathogenic fungi produce cellulases. However, little information is available on cellulase as an elicitor in plant-pathogen interactions. Here, an endocellulase (EG1) was isolated from Rhizoctonia solani. It contains a putative protein of 227 amino acids with a signal peptide and a family-45 glycosyl hydrolase domain. Its aspartic acid (Asp) residue at position 32 was changed to alanine (Ala), resulting in full loss of its catalytic activity. Wild-type and mutated forms of the endoglucanase were expressed in yeast and purified to homogeneity. The purified wild-type and mutant forms induced cell death in maize, tobacco and Arabidopsis leaves, and the transcription of three defence marker genes in maize and tobacco and 10 genes related to defence responses in maize. Moreover, they also induced the accumulation of reactive oxygen species (ROS), medium alkalinization, Ca(2+) accumulation and ethylene biosynthesis of suspension-cultured tobacco cells. Similarly, production of the EG1 wild-type and mutated forms in tobacco induced cell death using the Potato virus X (PVX) expression system. In vivo, expression of EG1 was also related to cell death during infection of maize by R. solani. These results provide direct evidence that the endoglucanase is an elicitor, but its enzymatic activity is not required for its elicitor activity.

Enzymatic cocktails produced by Fusarium graminearum under submerged fermentation using different lignocellulosic biomasses

Fusarium graminearum was grown on four lignocellulosic substrates (corn cobs, wheat bran, hop cell walls, and birchwood) and glucose as the sole carbon source. Proteomic studies performed on the resulting enzymatic cocktails highlighted a great diversity in the number and type of proteins secreted. The cell wall-degrading enzymes (CWDE) proportion varied greatly from 20% to 69%. Only one of the 57 CWDEs detected in this study was common to the five proteomes. In contrast, 35 CWDEs were specific to one proteome only. The polysaccharide-degradation activities were different depending on the cocktail and the polysaccharide used. F. graminearum strongly modifies the enzymatic cocktail it secretes as a function of the biomass used for growth.

The characterisation of xyloglucanase inhibitors from Humulus lupulus

Phytopathogenic fungi secrete a powerful arsenal of enzymes that are potentially active against each polysaccharide component of the plant cell wall. To defend themselves, plants synthetise a variety of molecules that inhibit the activity of cell wall-degrading enzymes. Xyloglucan-specific endoglucanase inhibitor proteins (XEGIPs) act specifically against the members of fungal glycoside hydrolase family 12 (GH12 in the CAZy database). In the present study, we describe the identification of three XEGIP homologues from hop (Humulus lupulus L.). When incubating each of the recombinant inhibitors with an enzymatic cocktail from Aspergillus aculeatus (Viscozyme®), the xyloglucan-degrading endoglucanase activity decreased to 15% and 5% for HlXEGIP1 and HlXEGIP2, respectively, whereas no inhibition of the Viscozyme® enzymes was observed for the third (also called HlXEGIP homologue 3, or HlXEGIPh3). Fungal enzymatic cocktails from 20 different species also showed xyloglucan-degrading endoglucanase activities, and most of them were inhibited by HlXEGIP1 and -2. Furthermore, a real time RT-PCR analysis revealed variations in the spatial distribution of the genes encoding the three inhibitors and differential expression during development and (a) biotic stress. The role of XEGIPs in the plant-fungus interaction is discussed, and a model suggesting a distinct role of these XEGIP homologues is proposed: HlXEGIP1 may act in cases of abiotic stress, while HlXEGIP2 reacts to biotic stress, and physiological development may be influenced by HlXEGIPh3.

Genome-wide transcriptional responses of Fusarium graminearum to plant cell wall substrates

We report a genome-wide transcriptomic study of Fusarium graminearum grown on four different substrates based on plant cell wall components. About 5% of the genes were differentially expressed in at least one condition. Analysis of upregulated cell wall-degrading enzymes highlights a sharp growth medium-specific adaptation process. In particular, the nature of the polysaccharides available for fungal growth induced a specific transcriptional response aiming at the targeted enzymatic degradation of the given polysaccharides.

Comparative genomics of a plant-pathogenic fungus, Pyrenophora tritici-repentis, reveals transduplication and the impact of repeat elements on pathogenicity and population divergence

Pyrenophora tritici-repentis is a necrotrophic fungus causal to the disease tan spot of wheat, whose contribution to crop loss has increased significantly during the last few decades. Pathogenicity by this fungus is attributed to the production of host-selective toxins (HST), which are recognized by their host in a genotype-specific manner. To better understand the mechanisms that have led to the increase in disease incidence related to this pathogen, we sequenced the genomes of three P. tritici-repentis isolates. A pathogenic isolate that produces two known HSTs was used to assemble a reference nuclear genome of approximately 40 Mb composed of 11 chromosomes that encode 12,141 predicted genes. Comparison of the reference genome with those of a pathogenic isolate that produces a third HST, and a nonpathogenic isolate, showed the nonpathogen genome to be more diverged than those of the two pathogens. Examination of gene-coding regions has provided candidate pathogen-specific proteins and revealed gene families that may play a role in a necrotrophic lifestyle. Analysis of transposable elements suggests that their presence in the genome of pathogenic isolates contributes to the creation of novel genes, effector diversification, possible horizontal gene transfer events, identified copy number variation, and the first example of transduplication by DNA transposable elements in fungi. Overall, comparative analysis of these genomes provides evidence that pathogenicity in this species arose through an influx of transposable elements, which created a genetically flexible landscape that can easily respond to environmental changes.

A key enzyme of the Leloir pathway is involved in pathogenicity of Leptosphaeria maculans toward oilseed rape

Agrobacterium tumefaciens-mediated random insertional mutagenesis was used to investigate pathogenicity determinants in Leptosphaeria maculans. One tagged nonpathogenic mutant, termed m186, is analyzed in detail here. Microscopic analyses of infected plant tissues revealed that m186 is specifically blocked at the invasive growth phase after an unaffected initial penetration stage and is unable to switch to the necrotrophic lifestyle. In addition, m186 exhibits an altered cell wall and seems to be affected in its ability to produce cell-wall-degrading enzymes. The T-DNA insertion occurs in the intergenic region between two head-to-tail genes, leading to a constitutive upregulation of their expression. Complementation experiments showed that only one of these two genes, Lmepi, fully accounts for the mutant phenotype. Bioinformatics and expression analyses along with functional studies suggested that the Lmepi gene encodes for the highly conserved UDP-glucose-4-epimerase, a key enzyme of the Leloir pathway involved in galactose metabolism. For the third time, this study highlights the intimate connection between primary metabolism and pathogenicity in L. maculans. This finding, along with similar data obtained from the related species Stagonospora nodorum, indicates the importance of in planta nutrition for the success of infection of plants by fungi belonging to class Dothideomycete.

The yeast signal sequence trap identifies secreted proteins of the hemibiotrophic corn pathogen Colletotrichum graminicola

The hemibiotroph Colletotrichum graminicola is the causal agent of stem rot and leaf anthracnose on Zea mays. Following penetration of epidermal cells, the fungus enters a short biotrophic phase, followed by a destructive necrotrophic phase of pathogenesis. During both phases, secreted fungal proteins are supposed to determine progress and success of the infection. To identify genes encoding such proteins, we constructed a yeast signal sequence trap (YSST) cDNA-library from RNA extracted from mycelium grown in vitro on corn cell walls and leaf extract. Of the 103 identified unigenes, 50 showed significant similarities to genes with a reported function, 25 sequences were similar to genes without a known function, and 28 sequences showed no similarity to entries in the databases. Macroarray hybridization and quantitative reverse-transcriptase polymerase chain reaction confirmed that most genes identified by the YSST screen are expressed in planta. Other than some genes that were constantly expressed, a larger set showed peaks of transcript abundances at specific phases of pathogenesis. Another set exhibited biphasic expression with peaks at the biotrophic and necrotrophic phase. Transcript analyses of in vitro-grown cultures revealed that several of the genes identified by the YSST screen were induced by the addition of corn leaf components, indicating that host-derived factors may have mimicked the host milieu.

MoSNF1 regulates sporulation and pathogenicity in the rice blast fungus Magnaporthe oryzae

The protein kinase Snf1 is a major component of the glucose derepression pathway in yeast and a regulator of gene expression for the cell wall degrading enzyme (CWDE) in some plant pathogenic fungi. To address the molecular function of Snf1 in Magnaporthe oryzae, which causes the rice blast disease, MoSNF1 was cloned and functionally characterized using gene knock-out strategies. MoSNF1 functionally complemented the growth defect of the yeast snf1 mutant on a non-fermenting carbon source. However, the growth rate of the Deltamosnf1 mutant on various carbon sources was reduced independent of glucose, and the expression of the CWDE genes in the mutant was induced during derepressing condition like the wild type. The pre-penetration stage including conidial germination and appressorium formation of the Deltamosnf1 was largely impaired, and the pathogenicity of the Deltamosnf1 was significantly reduced. Most strikingly, the Deltamosnf1 mutant produced only a few conidia and had a high frequency of abnormally shaped conidia compared to the wild type. Our results suggest that MoSNF1 is a functional homolog of yeast Snf1, but its contribution to sporulation, vegetative growth and pathogenicity is critical in M. oryzae.

Xyr1 regulates xylanase but not cellulase formation in the head blight fungus Fusarium graminearum

Fusarium graminearum is a plant pathogen that causes severe economical losses by infecting numerous agriculturally important plants and until now most culture plants have only low levels of Fusarium resistance. The plant cell wall can be assumed as the first target that has to be overcome by plant pathogens. Therefore pathogenic organisms are known to produce a complex cocktail of plant cell wall lytic enzymes. Xylanases are besides cellulases the most prominent enzymes secreted by Fusarium during growth on plant cell walls. We identified a putative regulator of xylanase production with high similarity to the Aspergillus niger XlnR and the Trichoderma reesei Xyr1 proteins. Disruptant strains of F. graminearum were heavily impaired in xylose utilization and xylanase production on wheat cell walls. In contrast to other filamentous fungi the lack of this transcriptional activator had no effect on the induction of cellulases.

Comparative proteomics of extracellular proteinsin vitro andin planta from the pathogenic fungusFusarium graminearum

High-throughput MS/MS was used to identify proteins secreted by Fusarium graminearum (Gibberella zeae) during growth on 13 media in vitro and in planta during infection of wheat heads. In vitro secreted proteins were collected from the culture filtrates, and in planta proteins were collected by vacuum infiltration. A total of 289 proteins (229 in vitro and 120 in planta) were identified with high statistical confidence. Forty-nine of the in planta proteins were not found in any of the in vitro conditions. The majority (91-100%) of the in vitro proteins had predicted signal peptides, but only 56% of the in planta proteins. At least 13 of the nonsecreted proteins found only in planta were single-copy housekeeping enzymes, including enolase, triose phosphate isomerase, phosphoglucomutase, calmodulin, aconitase, and malate dehydrogenase. The presence of these proteins in the in planta but not in vitro secretome might indicate that significant fungal lysis occurs during pathogenesis. On the other hand, several of the proteins lacking signal peptides that were found in planta have been reported to be potent immunogens secreted by animal pathogenic fungi, and therefore could be important in the interaction between F. graminearum and its host plants.

Fusarium graminearum on plant cell wall: no fewer than 30 xylanase genes transcribed

The transcription of a set of 32 putative xylanase genes from Fusarium graminearum was examined by quantitative PCR after growth on different carbon sources (hop cell wall, xylan, xylose, or carboxymethylcellulose). Growing on plant cell wall medium, this fungus displays a great diversity of expression of xylan-related genes, with 30 being induced. A second level of diversity exists because expression patterns can be very different for loci encoding enzymes with the same activity (the same EC number). The wealth of xylan-degrading enzymes and the differential expression confer on the fungus a great flexibility of reaction to variation in its environment.

Diversity of the exoproteome of Fusarium graminearum grown on plant cell wall

The exoproteome of the fungus Fusarium graminearum grown on glucose and on hop (Humulus lupulus, L.) cell wall has been investigated. The culture medium was found to contain a higher quantity of proteins and the proteins are more diverse when the fungus is grown on cell wall. Using both 1D and 2D electrophoresis followed by mass spectrometry analysis and protein identification based on similarity searches, 84 unique proteins were identified in the cell wall-grown fungal exoproteome. Many are putatively implicated in carbohydrate metabolism, mainly in cell wall polysaccharide degradation. The predicted carbohydrate-active enzymes fell into 24 different enzymes classes, and up to eight different proteins within a same class are secreted. This indicates that fungal metabolism becomes oriented towards synthesis and secretion of a whole arsenal of enzymes able to digest almost the complete plant cell wall. Cellobiohydrolase is one of the only four proteins found both after growth on glucose and on plant cell wall and we propose that this enzyme could act as a sensor of the extracellular environment. Extensive knowledge of this very diverse F. graminearum exoproteome is an important step towards the full understanding of Fusarium/plants interactions.

Expression patterns and phylogenetic analysis of two xylanase genes (htxyl1 and htxyl2) from Helminthosporium turcicum, the cause of northern leaf blight of maize

A xylanase gene (htxyl2) was cloned from Helminthosporium turcicum, the cause of northern leaf blight of maize by screening the genomic library from the fungus using a approximately 500 bp PCR fragment of the gene as a probe. The gene is a second xylanase gene cloned from the fungus and it is different from the previously cloned xylanase gene (htxyl1). The two genes are grouped into separate clades in a phylogenetic tree based on DNA sequences. The patterns of htxyl1 and htxyl2 transcript accumulation were studied in vitro with different carbon and nitrogen sources and during infection of maize by the pathogen. In culture high htxyl1 transcript was detected in mycelium obtained from medium containing birchwood xylan and birchwood xylan plus xylose as sole carbon sources. On the other hand htxyl2 transcript was detected in a medium containing birchwood xylan plus xylose but not in the medium where only birchwood xylan was used as the sole carbon source. Addition of glucose to the basal inducing media had inhibitory effect to the expression of both genes. The htxyl1 transcript accumulation declined sharply with an increase in concentration of ammonium sulphate and glutamic acid, while increased htxyl2 transcript was observed with an increase in concentrations of the two nitrogen sources. Abundant htxyl2 transcript was detected at early and late stages of infection of maize by Helminthosporium turcicum. On the other hand no detectable htxyl1 transcript was found in northern blot under the test conditions. This study demonstrated that in H. turcicum xylanolytic system htxyl1 and htxyl2 are differentially expressed and might play important roles in the saprophytic and pathogenic phases of the fungus, respectively.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

Characterization of Alternaria alternata glucanase genes expressed during infection of resistant and susceptible persimmon fruits

Summary Preharvest treatment with gibberellic acid (GA(3)) or its inhibitor paclobutrazol (PBZ) can reduce or increase, respectively, the susceptibility of persimmon fruits to Alternaria alternata. This was suggested to be the result of the ability of the fungus and produced endoglucanases to induce symptom development. To evaluate the importance of glucanases during A. alternata attack, five glucanase genes, corresponding to the C, F, and K families, were cloned from A. alternata using 'family-specific' oligonucleotide primers. The genes, present in a single copy, encode for exoglucanases AaC1 and AaC2, endoxylanase AaF1, endoglucanase AaK1, and the mixed-linked glucanase AaMLG1. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis of RNA extracted from persimmon fruits, 2 and 4 days post-infection with A. alternata, showed the expression of all five glucanase genes in GA3- and PBZ-treated fruits. However, transcription levels and enzyme production of the endoglucanase (AaK1) and one exoglucanase (AaC1) were enhanced during A. alternata growth on cell walls from susceptible PBZ-treated fruits, whereas the expression of these genes and their enzyme production were significantly reduced in resistant GA(3)-treated fruits. The present results suggest the involvement of endo- and exoglucanase in symptom development caused by A. alternata in resistant and susceptible persimmon fruits.

Two pectin lyase genes, pnl-1 and pnl-2, from Colletotrichum gloeosporioides f. sp. malvae differ in a cellulose-binding domain and in their expression during infection of Malva pusilla

Two pectin lyase genes, designated pnl-1 and pnl-2, were cloned from Colletotrichum gloeosporioides f. sp. malvae, a pathogen of round-leaved mallow (Malva pusilla). pnl-1 was isolated using cDNA from infected plant material; pnl-2 was isolated using cDNA from 3-day-old mycelia grown in mallow-cell-wall extract (MCWE) broth. pnl-1 is the first pectinase gene described thus far to encode a cellulose-binding domain (CBD), which is common in cellulases and xylanases, whereas pnl-2 encodes a pectin lyase that lacks a CBD. In pure culture, pnl-1 expression could be detected when purified pectin or glucose was the sole carbon source, but not when MCWE was the sole carbon source. The lack of pnl-1 expression appeared to be due to gene repression by some unknown factor(s) in the cell-wall extract. In contrast, expression of pnl-2 was detected in cultures when MCWE, but not when purified pectin or glucose, was the sole carbon source. In infected tissue, detection of pnl-1 expression by Northern-blot hybridization and by RT-PCR began with the onset of the necrotrophic phase of infection. Expression ofpnl-2 was not detectable by Northern-blot hybridization, but was observed byRT-PCR in both the biotrophic and necrotrophic phases of infection. The differences between pnl-1 and pnl-2 (i.e. pnl-1 encoding a CBD and differences in the expression patterns of both genes) may be related to the requirements of C. gloeosporioides f. sp. malvae to be able to grow in host tissue under the different conditions present during the biotrophic and necrotrophic phases of infection.

Botrytis cinerea endopolygalacturonase genes are differentially expressed in various plant tissues

Botrytis cinerea, the causal agent of blight, rot, and gray mold on many plant species, secretes various endopolygalacturonases during all stages of infection. The expression pattern of the encoding genes (Bcpg 1-6) was studied on four hosts: tomato, broad bean, apple, and courgette (also known as zucchini). All gene family members are differentially expressed, depending on the stage of infection and the host. Bcpg1 is expressed in all tissues tested although differences in transcript levels occur. Bcpg2 expression is detected early in the infection of three of four plant tissues tested. Bcpg3 and Bcpg5 are expressed in apple fruit tissue, although probably as a result of different regulatory mechanisms. The expression patterns of Bcpg4 and 6 are in agreement with their inducibility by monogalacturonic acid. The pattern of Bcpg gene expression indicates that B. cinerea is equipped with a flexible enzymatic pectate degradation machinery. The studies pinpoint new targets for gene disruption studies.

Molecular cloning and characterization of cel2 from the fungus Cochlibolus carbonum

A new cellulase gene, cel2, from the filamentous fungus Cochliobolus carbonum was cloned by using egl-1 of Trichoderma reesei as a heterologous probe. DNA blot analysis of cel2 showed that this gene is present as a single copy. The gene contains one 49-bp- intron. cel2 encodes a predicted protein (Cel2p) of 423 amino acids with a molecular mass of 45.8 kDa. The predicted pI is 4.96. It shows similarity to other endoglucanases from various fungi. From the comparison with other cellulase genes, cel2 belongs to family 7 of glucohydrolases. cel2 is located on a 2.5-Mb chromosome in C. carbonum and its expression is repressed by sucrose. A cel2 mutant of C. carbonum was created by transformation-mediated gene disruption. The pathogenicity of the mutant was indistinguishable from the wild type, indicating that cel2 by itself is not important for pathogenicity.

Cloning, characterization and chromosomal location of three genes encoding host-cell-wall-degrading enzymes in Leptosphaeria maculans, a fungal pathogen of Brassica spp

The ascomycete, Leptosphaeria maculans, causes blackleg disease of oilseed Brassica spp. such as canola (Brassica napus). We have cloned a gene encoding endopolygalacturonase, pg1, and two genes encoding cellulases, cel1 and cel2, in L. maculans. These genes are not clustered in the genome, as they are located on different chromosomes. The deduced amino acid sequences of all three genes predict an N-terminal signal sequence, as is common for secreted fungal enzymes that degrade plant cell walls. The endopolygalacturonase encoded by pg1 shows the highest similarity (54% amino acid identity) to endopolygalacturonase 4 from Botrytis cinerea. Both cel1 and cel2 appear to encode cellobiohydrolase, and neither gene encodes a recognizable cellulose-binding domain or linker region. Transcription of pg1 is induced in cultures containing 1% polygalacturonic acid or pectin, and cel1 is induced in 1% cellulose or carboxymethylcellulose, as shown by Northern analysis. Glucose represses the induction of cel1 caused by cellulose and carboxymethylcellulose, but does affect transcription of pg1. Transcription of cel2 (but not cel1 or pg1) is detectable during infection of B. napus and B. juncea cotyledons and leaves using reverse transcription-PCR.

The endopolygalacturonase gene Bcpg1 is required for full virulence of Botrytis cinerea

Botrytis cinerea, a fungus that causes diseases in over 200 plant species, secretes a number of endopolygalacturonases that have been suggested to be involved in pathogenesis. However, so far the corresponding genes have not been isolated from this fungus. We cloned Bcpg1, encoding endopolygalacturonase, with the pgaII gene from Aspergillus niger as a heterologous probe. The Bcpg1 gene is expressed to similar levels in liquid cultures of B. cinerea containing either 1% polygalacturonic acid or 1% sucrose, and is expressed during infection of tomato leaves. The Bcpg1 gene was eliminated by partial gene replacement, and the resulting mutants were tested for virulence on tomato leaves and fruits, as well as on apple fruits. Although the mutants were still pathogenic and displayed similar primary infections when compared with control strains, a significant decrease in secondary infection, i.e., growth of the lesion beyond the inoculation spot, was observed on all three host tissues. These results indicate that the Bcpg1 gene is required for full virulence.

Phanerochaete chrysosporium cellobiohydrolase and cellobiose dehydrogenase transcripts in wood

The transcripts of structurally related cellobiohydrolase genes in Phanerochaete chrysosporium-colonized wood chips were quantified. The transcript patterns obtained were dramatically different from the transcript patterns obtained previously in defined media. Cellobiose dehydrogenase transcripts were also detected, which is consistent with the hypothesis that such transcripts play an important role in cellulose degradation.

Targeted mutants of Cochliobolus carbonum lacking the two major extracellular polygalacturonases

The filamentous fungus Cochliobolus carbonum produces endo-alpha 1,4-polygalacturonase (endoPG), exo-alpha 1,4-polygalacturonase (exoPG), and pectin methylesterase when grown in culture on pectin. Residual activity in a pgn1 mutant (lacking endoPG) was due to exoPG activity, and the responsible protein has now been purified. After chemical deglycosylation, the molecular mass of the purified protein decreased from greater than 60 to 45 kDa. The gene that encodes exoPG, PGX1, was isolated with PCR primers based on peptide sequences from the protein. The product of PGX1, Pgx1p, has a predicted molecular mass of 48 kDa, 12 potential N-glycosylation sites, and 61% amino acid identity to an exoPG from the saprophytic fungus Aspergillus tubingensis. Strains of C. carbonum mutated in PGX1 were constructed by targeted gene disruption and by gene replacement. Growth of pgx1 mutant strains on pectin was reduced by ca. 20%, and they were still pathogenic on maize. A double pgn1/pgx1 mutant strain was constructed by crossing. The double mutant grew as well as the pgx1 single mutant on pectin and was still pathogenic despite having less than 1% of total wild-type PG activity. Double mutants retained a small amount of PG activity with the same cation-exchange retention time as Pgn1p and also pectin methylesterase and a PG activity associated with the mycelium. Continued growth of the pgn1/pgx1 mutant on pectin could be due to one or more of these residual activities.

Cloning and targeted disruption of MLG1, a gene encoding two of three extracellular mixed-linked glucanases of Cochliobolus carbonum

Mixed-linked glucanases (MLGases), which are extracellular enzymes able to hydrolyze beta 1,3-1,4-glucans (also known as mixed-linked glucans or cereal beta-glucans), were identified in culture filtrates of the plant-pathogenic fungus Cochliobolus carbonum. Three peaks of MLGase activity, designated Mlg1a, Mlg1b, and Mlg2, were resolved by cation-exchange and hydrophobic-interaction high-performance liquid chromatography (HPLC). Mlg1a and Mlg1b also hydrolyze beta 1,3-glucan (laminarin), whereas Mlg2 does not degrade beta 1,3-glucan but does degrade beta 1,4-glucan to a slight extent. Mlg1a, Mlg1b, and Mlg2 have monomer molecular masses of 33.5, 31, and 29.5 kDa, respectively. The N-terminal amino acid sequences of Mlg1a and Mlg1b are identical (AAYNLI). Mlg1a is glycosylated, whereas Mlg1b is not. The gene encoding Mlg1b, MLG1, was isolated by using PCR primers based on amino acid sequences of Mlg1b. The product of MLG1 has no close similarity to any known protein but does contain a motif (EIDI) that occurs at the active site of MLGases from several prokaryotes. An internal fragment of MLG1 was used to create mlg1 mutants by transformation-mediated gene disruption. The total MLGase and beta 1,3-glucanase activities in culture filtrates of the mutants were reduced by approximately 50 and 40%, respectively. When analyzed by cation-exchange HPLC, the mutants were missing the two peaks of MLGase activity corresponding to Mlg1a and Mlg1b. Together, the data indicate that Mlg1a and Mlg1b are products of the same gene, MLG1. The growth of mlg1 mutants in culture medium supplemented with macerated maize cell walls or maize bran and the disease symptoms on maize were identical to the growth and disease symptoms of the wild type.

A fatty acid synthase gene in Cochliobolus carbonum required for production of HC-toxin, cyclo(D-prolyl-L-alanyl-D-alanyl-L-2-amino-9, 10-epoxi-8-oxodecanoyl)

The fungal maize pathogen Cochliobolus carbonum produces a phytotoxic and cytostatic cyclic peptide, HC-toxin, of structure cyclo(D-prolyl-L-alanyl-D-alanyl-L-Aeo), in which Aeo stands for 2-amino-9,10-epoxi-8-oxodecanoic acid. Here we report the isolation of a gene, TOXC, that is present only in HC-toxin-producing (Tox2+) fungal strains. TOXC is present in most Tox2+ strains in three functional copies, all of which are on the same chromosome as the gene encoding HC-toxin synthetase. When all copies of TOXC are mutated by targeted gene disruption, the fungus grows and sporulates normally in vitro but no longer makes HC-toxin and is not pathogenic, indicating that TOXC has a specific role in HC-toxin production and hence virulence. The TOXC mRNA is 6.5 kb and the predicted product has 2,080 amino acids and a molecular weight of 233,000. The primary amino acid sequence is highly similar (45 to 47% identity) to the beta subunit of fatty acid synthase from several lower eukaryotes, and contains, in the same order as in other beta subunits, domains predicted to encode acetyl transferase, enoyl reductase, dehydratase, and malonyl-palmityl transferase. The most plausible function of TOXC is to contribute to the synthesis of the decanoic acid backbone of Aeo.

Cel1, probably encoding a cellobiohydrolase lacking the substrate binding domain, is expressed in the initial infection phase of Claviceps purpurea on Secale cereale

At the host-pathogen interface of hyphae penetrating host cell walls in the rye ovary, a lack of cellulase-gold labeling of beta-1, 4-glucan in host cell walls indicates that enzymatic degradation of cellulose might be an important factor during the infection of rye by Claviceps purpurea. Using cbh1 from Trichoderma reesei as a probe, a putative cellulase gene (cel1) was isolated from a genomic library of the C. purpurea strain T5. The coding region of 1,616 bp contains two introns and a putative signal peptidase cleavage site, leaving a coding capacity of 437 amino acids for the mature protein. The derived amino acid sequence shares significant homology with other fungal cellobiohydrolases and lacks the substrate binding domain. Expression analysis using reverse transcriptase-polymerase chain reaction (RT-PCR) shows that cel1 is induced during the first days of infection of rye by C. purpurea. It may be involved in the penetration and degradation of host cell walls by depolymerizing plant beta-1, 4-glucan and, therefore, play a role in the infection process.

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.

Cloning, disruption, and expression of two endo-beta 1, 4-xylanase genes, XYL2 and XYL3, from Cochliobolus carbonum

In culture, the filamentous fungus Cochliobolus carbonum, a pathogen of maize, makes three cationic xylanases, XYL1, which encodes the major endoxylanase (Xyl1), was earlier cloned and shown by gene disruption to encode the first and second peaks of xylanase activity (P. C. Apel, D. G. Panaccione, F. R. Holden, and J. D. Walton, Mol. Plant-Microbe Interact. 6:467-473, 1993). Two additional xylanase genes, XYL2 and XYL3, have now been cloned from C. carbonum. XYL2 and XYL3 are predicted to encode 22-kDa family G xylanases similar to Xyl1. Xyl2 and Xyl3 are 60% and 42% identical, respectively, to Xyl1, and Xyl2 and Xyl3 are 39% identical. XYL1 and XYL2 but not XYL3 mRNAs are present in C. carbonum grown in culture, and XYL1 and XYL3 but not XYL2 mRNAs are present in infected plants. Transformation-mediated gene disruption was used to construct strains mutated in XYL1, XYL2, and XYL3. Xyl1 accounts for most of the total xylanase activity in culture, and disruption of XYL2 or XYL3 does not result in the further loss of any xylanase activity. In particular, the third peak of cationic xylanase activity is still present in a xyl1 xyl2 xyl3 triple mutant, and therefore this xylanase must be encoded by yet a fourth xylanase gene. A minor protein of 22 kDa that can be detected immunologically in the xyl1 mutant disappears in the xyl2 mutant and is therefore proposed to be the product of XYL2. The single xylanase mutants were crossed with each other to obtain multiple xylanase disruptions within the same strain. Strains disrupted in combinations of two and in all three xylanases were obtained. The triple mutant grows at the same rate as the wild type on xylan and on maize cell walls. The triple mutant is still fully pathogenic on maize with regard to lesion size, morphology, and rate of lesion development.

A putative cyclic peptide efflux pump encoded by the TOXA gene of the plant-pathogenic fungus Cochliobolus carbonum

Race 1 isolates of Cochliobolus carbonum are pathogenic on certain maize lines due to production of a host-selective cyclic tetrapeptide, HC-toxin. Flanking HTS1, which encodes the central enzyme in HC-toxin biosynthesis, a gene was identified and named TOXA. Like HTS1, TOXA occurred only in isolates of the fungus that make HC-toxin and was present as two linked copies in most toxin-producing isolates. HTS1 and TOXA were transcribed in the opposite orientation and their transcriptional start sites were 386 bp apart. The predicted product of TOXA was a 58 kDa hydrophobic protein with 10-13 membrane-spanning regions. The sequence was highly similar to several members of the major facilitator superfamily that confer resistance to tetracycline, methylenomycin, and other antibiotics. Although it was possible to mutate one copy or the other of TOXA by targeted gene disruption, numerous attempts to disrupt both copies in a single strain were unsuccessful, suggesting that TOXA is an essential gene in strains that synthesize HC-toxin. On the basis of its presence only in HC-toxin-producing strains, its proximity to HTS1 and its predicted amino acid sequence, we propose that TOXA encodes an HC-toxin efflux pump which contributes to self-protection against HC-toxin and/or the secretion of HC-toxin into the extracellular milieu.

Morphogenesis and mechanisms of penetration by plant pathogenic fungi

Infection structures of phytopathogenic fungi are modified hyphae specialized for the invasion of plant tissues. Initial events are adhesion to the cuticle and directed growth of the germ tube on the plant surface. At the site of penetration, appressoria are often formed that may have melanized walls and develop high turgor pressure to support the penetration process. The penetration hypha accumulates components of the cytoskeleton in the tip and secretes a variety of cell wall-degrading enzymes in a highly regulated fashion in order to penetrate the cuticle and the plant cell wall. This article reviews recent papers on the cytology, physiology, and molecular biology of the penetration process.

Induction of a Cryphonectria parasitica cellobiohydrolase I gene is suppressed by hypovirus infection and regulated by a GTP-binding-protein-linked signaling pathway involved in fungal pathogenesis

Extracellular cellulase activity is readily induced when the chestnut blight fungus Cryphonectria parasitica is grown on cellulose substrate as the sole carbon source. However, an isogenic C. parasitica strain rendered hypovirulent due to hypovirus infection failed to secrete detectable cellulase activity when grown under parallel conditions. Efforts to identify C. parasitica cellulase-encoding genes resulted in the cloning of a cellobiohydrolase (exoglucanase, EC 3.2.1.91) gene designated chb-1. Northern blot analysis revealed an increase in cbh-1 transcript accumulation in a virus-free virulent C. parasitica strain concomitant with the induction of extracellular cellulase activity. In contrast, induction of cbh-1 transcript accumulation was suppressed in an isogenic hypovirus-infected strain. Significantly, virus-free C. parasitica strains rendered hypovirulent by transgenic cosuppression of a GTP-binding protein alpha subunit were also found to be deficient in the induction of cbh-1 transcript accumulation.