Isolation and sequence analysis of Clpg1, a gene coding for an endopolygalacturonase of the phytopathogenic fungus Colletotrichum lindemuthianum

A SET domain-containing protein involved in cell wall integrity signaling and peroxisome biogenesis is essential for appressorium formation and pathogenicity of Colletotrichum gloeosporioides

The chromatin modulator Set5 plays important regulatory roles in both cell growth and stress responses of Saccharomyces cerevisiae. However, its function in filamentous fungi remains poorly understood. Here, we report the pathogenicity-related gene CgSET5 discovered in a T-DNA insertional mutant M285 of Colletotrichum gloeosporioides. Bioinformatic analysis revealed that CgSET5 encodes a SET domain-containing protein that is a homolog of the budding yeast S. cerevisiae Set5. CgSET5 is important for hyphae growth and conidiation and is necessary for appressorium formation and pathogenicity. CgSet5 regulates appressorium formation in a mitogen-activated protein kinase-independent manner. Inactivation of CgSET5 resulted in a significant reduction in chitin content within the cell wall, indicating CgSet5 plays a vital role in cell wall integrity. CgSet5 is involved in peroxisome biogenesis. We identified CgSet5 as the histone H4 methyltransferase, which methylates the critical H4 lysine residues 5 and 8 in C. gloeosporioides. We carried out a yeast two-hybrid screen to find CgSet5 interacting partners. We found CgSet5 putatively interacts with an inorganic pyrophosphatase named CgPpa1, which co-localized in the cytoplasm with CgSet5. Finally, CgPpa1 was found to strongly interact with CgSet5 in vivo during appressorium formation by bimolecular fluorescence complementation assays. These data corroborate a complex control function of CgSet5 acting as a core pathogenic regulator, which connects cell wall integrity and peroxisome biogenesis in C. gloeosporioides.

ABC protein CgABCF2 is required for asexual and sexual development, appressorial formation and plant infection in Colletotrichum gloeosporioides

ATP-binding cassette (ABC) proteins are exclusively found in both prokaryotes and eukaryotes. In this study, we have characterized a gene from Glomerella leaf spot pathogen Colletotrichum gloeosporioides that encodes an ABC protein, whose function to date remains unknown. We designated this gene as CgABCF2. Deletion of CgABCF2 showed drastic reduction both growing rate and conidial production in C. gloeosporioides. The Δcgabcf2 mutant did not form the appressoria, lost the capability to infect apple and failed to form lesions on the wounded leaves and fruits. The C. gloeosporioides native CgABCF2 fully recovered defect of the Δcgabcf2 mutant. These data indicated that CgABCF2 was required for fungal development and invasion. The transcriptions of six pectolytic enzymes genes (CgPG1, CgPG2, pnl-1, pnl-2, pelA and pelB) significantly reduced in the Δcgabcf2 mutant, indicating that deletion of CgABCF2 impaired the fungal necrotrophic growth. In addition, CgABCF2 mediated sexual development through the positive regulation of the gene MAT1-2-1 expression. These results indicated that CgABCF2 underlies the complex process governing morphogenesis, sexual and asexual reproduction, appressorial formation and pathogenicity in C. gloeosporioides.

The pectate lyase encoded by the pecCl1 gene is an important determinant for the aggressiveness of Colletotrichum lindemuthianum

Colletotrichum lindemuthianum is the causal agent of anthracnose in the common bean, and the genes that encode its cell-wall-degrading enzymes are crucial for the development of the disease. Pectinases are the most important group of cell wall-degrading enzymes produced by phytopathogenic fungi. The pecC1l gene, which encodes a pectate lyase in C. lindemuthianum, was isolated and characterized. Possible cis-regulatory elements and transcription factor binding sites that may be involved in the regulation of genetic expression were detected in the promoter region of the gene. pecCl1 is represented by a single copy in the genome of C. lindemuthianum, though in silico analyses of the genomes of Colletotrichum graminicola and Colletotrichum higginsianum suggest that the genome of C. lindemuthianum includes other genes that encode pectate lyases. Phylogenetic analysis detected two groups that clustered based on different members of the pectate lyase family. Analysis of the differential expression of pecCl1 during different stages of infection showed a significant increase in pecCl1 expression five days after infection, at the onset of the necrotrophic phase. The split-maker technique proved to be an efficient method for inactivation of the pecCl1 gene, which allowed functional study of a mutant with a site-specific integration. Though gene inactivation did not result in complete loss of pectate lyase activity, the symptoms of anthracnose were reduced. Analysis of pectate lyases might not only contribute to the understanding of anthracnose in the common bean but might also lead to the discovery of an additional target for controlling anthracnose.

Spatial distribution of polygalacturonase-inhibiting proteins in Arabidopsis and their expression induced by Stemphylium solani infection

Disease-induced polygalacturonase-inhibiting proteins (PGIPs) are the major defense proteins which play an important role in resistance to infection of pathogens. To date, the AtPGIP expression in Arabidopsis induced by Stemphylium solani (S. solani) was not described. Here the distribution of AtPGIPs and their expression induced by S. solani infection in Arabidopsis was reported. Notably, immunofluorescence localization showed that the AtPGIPs were distributed in leaves, petioles, stems and roots of 5 week old Arabidopsis, but they were mainly localized in epidermis, vascular bundles and vascular cylinder. Further studies indicated that the transcription level of AtPGIP1 and AtPGIP2 was both up-regulated in response to infection with S. solani which caused hypersensitive cell death, but the transcription level of AtPGIP2 was less induced than AtPGIP1. Consistently, the bulk AtPGIPs of Arabidopsis showed a higher activity in leaves infected by S. solani. Taken together, our preliminary results showed that AtPGIPs were spatially distributed and AtPGIP expression might take part in resistance to infection of S. solani. This study might highlight the potential importance of AtPGIPs and plant disease resistance.

Use of the Saccharomyces cerevisiae endopolygalacturonase promoter to direct expression in Escherichia coli

In Saccharomyces cerevisiae, an endopolygalacturonase encoded by the PGL1 gene catalyzes the random hydrolysis of the α-1,4 glycosidic linkages in polygalacturonic acid. To study the regulation of the PGL1 gene, we constructed a reporter vector containing the lacZ gene under the control of PGL1 promoter. Surprisingly, when Escherichia coli DH5α was transformed by this vector, cells harboring the constructed plasmid produced blue colonies. Sequence analysis of this promoter revealed that E. coli consensus sequences required to express an in-frame lacZ alpha product were present. We next decided to investigate how the PGL1 promoter is regulated in E. coli compared to yeast. In this study, we examined the modulation of the PGL1 promoter in E. coli, and the results indicated that its activity is greatly induced by saturated digalacturonic acid and is indirectly regulated by the transcriptional regulators the 2-keto-3-deoxygluconate repressor. Moreover, PGL1 expression is enhanced under aerobic conditions. We found that β-galactosidase activity in E. coli could reach 180 units, which is 40-fold greater than the activity produced in S. cerevisiae, and greater than recombinant protein expression previously reported by other researchers. We thus demonstrate that this vector can be considered as a dual expression plasmid for both E. coli and S. cerevisiae hosts. So far, no modulation of endoPG promoters expressed in E. coli has been reported.

Mapping glycans onto specific N-linked glycosylation sites of Pyrus communis PGIP redefines the interface for EPG-PGIP interactions

Polygalacturonase inhibiting proteins (PGIPs) are members of the leucine rich repeat family of proteins, involved in plant defense against fungal pathogens. PGIPs exhibit a remarkable degree of specificity in terms of their ability to bind and inhibit their target molecules, the endopolygalacturonases (EPGs). This specificity has been attributed for certain EPG/PGIP combinations to differences in primary sequence, but this explanation is unable to account for the full range of binding and inhibitory activities observed. In this paper, we have fully characterized the glycosylation on the PGIP derived from Pyrus communis and demonstrated, using a combination of PNGaseF and PNGaseA in (18)O-water, that the Pyrus communis PGIP utilizes all seven potential sites of N-linked glycosylation. Further, we demonstrate that certain sites appear to be modified only by glycans bearing alpha3-linked core fucosylation, while others are occupied by a mixture of fucosylated and nonfucosylated glycans. Modeling of the carbohydrates onto a homologous structure of PGIP indicates potential roles for glycosylation in mediating the interactions of PGIPs with EPGs.

Molecular characterization and expression profile of pectin-lyase-encoding genes from Penicillium griseoroseum

Penicillium griseoroseum has been studied by our group because of its good pectinase production. Attempts have been done to clone pectinolytic genes, aiming to obtain pectinase-overproducing strains for industrial purposes. Here, two genes coding for pectin lyase were isolated from the P. griseoroseum genome. The plg1 gene has an open reading frame of 1341 bp coding for a putative protein of 374 amino acids with a calculated molecular mass of 40.1 kDa. The plg2 gene is characterized by an open reading frame of 1400 nucleotides and codes for a polypeptide of 383 amino acids. The plg1 gene 5'-flanking region contains putative binding sites for the transcription factors involved in regulation by ambient pH and catabolite repression. The primary structure of Plg1 and Plg2 proteins showed a relatively high homology (varying between 32.4% and 74.8%) to fungal pectin lyases characterized to date. Southern blotting analysis revealed that both genes are present as single copies in the fungus genome. Expression studies revealed a differing pattern of gene expression of plg1 and plg2 when mycelium was cultivated on medium containing different pectic components. Citric pectin followed by apple pectin were the carbon sources that best induced plg1 expression, and transcripts were detected from 24 to 76 h. The expression of the plg2 gene was monitored by reverse transcriptase - polymerase chain reaction, since Northern analysis failed to detect hybridization signals. The differential expression of these genes may provide means for the fungus to adapt to various growth conditions.

Cloning and characterization of a gene rpg1 encoding polygalacturonase of Rhizopus oryzae

The polygalacturonase (PG)-encoding gene (rpg1) of Rhizopus oryzae, the causal pathogen of rhizopus rot of mulberry, was cloned and sequenced. PGs were partially purified from incubation mixture of 2% pectin medium and their N-terminal amino acid sequences were determined by a gas-phase protein sequencer. RT-PCR was performed using degenerate primers designed from the amino acid sequences, which resulted in part of a PG-encoding gene being obtained. By 3'-RACE and TAIL-PCR analyses, the entire region of the PG-encoding gene was cloned and sequenced. The structural gene comprised 1199 bp coding for 383 amino acids with a putative signal peptide of 26 amino acids, and the open reading frame was interrupted by single intron of 47 bp. Phylogenetic analysis using the deduced amino acid sequence revealed that R. oryzae RPG1 belonged to a clade consisting of exo-PGs of ascomycete fungi.

Characterization of pco-1, a newly identified gene which regulates purine catabolism in Neurospora

A new gene of Neurospora crassa, designated pco-1, was characterized and shown to regulate the expression of several genes which encode enzymes required for the catabolism of purines. Unlike the wild type, a pco-1 mutant created by repeat-induced point mutation cannot utilize purines as a nitrogen source. The PCO1 protein contains a Zn(II)2Cys6 binuclear cluster motif near its N-terminus, followed by a putative coiled-coil motif. A chemical crosslinking experiment demonstrated that PCO1 forms homodimers. PCO1 binds to CGG-N6-CCG elements located in the upstream promoter region of four genes encoding purine catabolic enzymes. Northern blot analysis demonstrated that a functional PCO1 protein is required for induction of xdh, which encodes xanthine dehydrogenase. Moreover, PCO1 was required for induction of three different purine catabolic enzymes. Two glutamine-rich domains occur in the C-terminal region of PCO1 and at least one of the glutamine-rich regions is required for PCO1 function, suggesting that they might play a role in transcriptional activation. The PCO1 protein does not interact with the global-acting NIT2 protein or the negative-acting NMR protein that functions in nitrogen catabolite repression. Induction of the xdh gene and synthesis of xanthine dehydrogenase is completely dependent upon PCO1, but does not require the global-acting NIT2 protein, suggesting that it is controlled by a novel regulatory mechanism.

Interaction of Sclerotinia sclerotiorum with Brassica napus: cloning and characterization of endo- and exo-polygalacturonases expressed during saprophytic and parasitic modes

Five major and several minor PG isoenzymes were identified in a Sclerotinia sclerotiorum isolate from Brassica napus by isoelectric focusing and pectin gel overlays. Using a combination of degenerate PCR and expressed sequence tags (ESTs) four endo-polygalacturonase (PG) genes, designated as sspg1d, sspg3, sspg5, and sspg6, and two exo-PG genes, ssxpg1 and ssxpg2, were identified. SSPG1d is a member of the PG gene family previously described by Fraissinet-Tachet et al. [Curr. Genet. 29 (1995) 96]. The mature SSPG1d is a neutral PG, whereas fully processed SSPG3, SSPG5, and SSPG6 are acidic enzymes. Under saprophytic growth conditions, sspg1d, sspg3, sspg5, and ssxpg1 expression was induced by pectin and galacturonic acid and subject to catabolite repression by glucose. Conditions could not be identified under which sspg6 or ssxpg2 were expressed well. Transfer of mycelia from liquid media to solid substrates induced expression of sspg1d suggesting that it may also be regulated by thigmotrophic interactions. Under pathogenic conditions, sspg1d was highly expressed during infection. sspg3 was also expressed during infection, albeit at lower levels than sspg1d, whereas sspg5, sspg6, and ssxpg1 were expressed only weakly.

Production of a cell wall-associated endopolygalacturonase by Colletotrichum lindemuthianum and pectin degradation during bean infection

The bean pathogen Colletotrichum lindemuthianum expresses two endopolygalacturonase genes, CLPG1 and CLPG2, during interaction with its host plant. However, only CLPG1 was found to be secreted to the extracellular medium during saprophytic growth of the fungus on pectin. To localize CLPG2, a FLAG epitope sequence was inserted in the C-terminal sequence of CLPG2 and the modified gene was introduced into C. lindemuthianum. Western blot analysis using a FLAG monoclonal antibody allowed the detection of CLPG2 in intracellular protein extracts and in the cell wall fraction, but not in the culture medium. Indirect immunofluorescence microscopy was performed to detect CLPG2 during saprophytic or parasitic growth. According to the expression pattern of CLPG2, it was found that CLPG2 accumulates in the fungal cell wall during growth on pectin medium and during appressorium formation, both in vitro and during interaction with the plant. Pectin degradation was not detected around the infection peg using the monoclonal antibody JIM7, specific for methyl-esterified galacturonan. However, extensive pectin dissolution was observed during the development of secondary hyphae.

Elicitor activity of a fungal endopolygalacturonase in tobacco requires a functional catalytic site and cell wall localization

CLPG1, an endopolygalacturonase (endoPG) gene of Colletotrichum lindemuthianum, was transferred to tobacco (Nicotiana tabacum) leaves by using the Agrobacterium tumefaciens transient delivery system. The following four constructs were prepared: CLPG1, with or without its signal peptide (SP; PG1, PG1deltaSP); CLPG1 with the tobacco expansin1 SP instead of its own SP (Exp::PG1deltaSP); and a mutated version of the latter on two amino acids potentially involved in the catalytic site of CLPG1 (D202N/D203N). Chlorotic and necrotic lesions appeared 5 to 7 d postinfiltration, exclusively in response to CLPG1 fused to the expansin SP. The lesions were correlated to the production of an active enzyme. Necrosis-inducing activity, as well as endoPG activity, were completely abolished by site-directed mutagenesis. Ultrastructural immunocytolocalization experiments indicated that the expansin SP addressed CLPG1 to the cell wall. Staining of parenchyma cells revealed the progressive degradation of pectic material in junction zones and middle lamella as a function of time after infiltration, ultimately leading to cell separation. A 30% decrease in the GalUA content of the cell walls was simultaneously recorded, thereby confirming the hydrolytic effect of CLPG1 on pectic polysaccharides, in planta. The elicitor activity of CLPG1 was further illustrated by the induction of defense responses comprising active oxygen species and beta-1,3-glucanase activity, before leaf necrosis. Altogether, the data demonstrate that an appropriate SP and a functional catalytic site are required for the proper expression and elicitor activity of the fungal endoPG CLPG1 in tobacco.

A cis-acting sequence homologous to the yeast filamentation and invasion response element regulates expression of a pectinase gene from the bean pathogen Colletotrichum lindemuthianum

Phytopathogenic fungi secrete hydrolytic enzymes that degrade plant cell walls, notably pectinases. The signaling pathway(s) that control pectinase gene expression are currently unknown in filamentous fungi. Recently, the green fluorescent protein coding sequence was used as a reporter gene to study the expression of CLPG2, a gene encoding an endopolygalacturonase of the bean pathogen Colletotrichum lindemuthianum. CLPG2 is transcriptionally induced by pectin in the axenic culture of the fungus and during formation of the appressorium, an infection structure specialized in plant tissue penetration. In the present study, promoter deletion and mutagenesis, as well as gel shift mobility assays, allowed for the first time identification of cis-acting elements that bind protein factors and are essential for the regulation of a pectinase gene. We found that two different adjacent DNA motifs are combined to form an active element that shows a strong sequence homology with the yeast filamentation and invasion response element. The same element is required for the transcriptional activation of CLPG2 by pectin and during appressorium development. This study strongly suggests that the control of virulence genes of fungal plant pathogens, such as pectinases, involves the formation of a complex of transcriptional activators similar to those regulating the invasive growth in yeast.

Ultrastructural Characterization of Infection and Colonization of Maize Leaves by Colletotrichum graminicola, and by a C. graminicola Pathogenicity Mutant

ABSTRACT Observations were made of the ultrastructure of infection and colonization of leaves of a susceptible maize inbred by Colletotrichum graminicola and by a C. graminicola pathogenicity mutant. The mutant causes no symptoms on either maize leaves or stalks. Prior evidence suggested that it is deficient in production of signal peptidase, responsible for cleavage of signal peptides from proteins destined for transport through the endoplasmic reticulum. There was no significant difference in the process of infection or colonization by the mutant and wild-type strains up to 48 h after inoculation. Both the mutant and the wild type produced globose, melanized appressoria within 24 h after inoculation on the host surface. By 36 h, both strains had penetrated the host epidermal cells directly. The host cells frequently formed papillae in response to appressoria, but these were not usually successful in preventing fungal ingress in either case. Penetration was followed by formation of irregularly shaped, swollen infection hyphae. Infection hyphae of both strains grew biotrophically for a relatively short time (less than 12 h). One or more hyphal branches was produced from each infection hypha, and these invaded adjacent mesophyll cells. Both strains of the fungus grew cell-to-cell, setting up new biotrophic interactions in each cell, between 36 and 48 h after inoculation. Papillae were frequently formed by the mesophyll cells, but these were not successful in preventing fungal ingress. The first noticeable difference between the mutant and the wild type was related to their interaction with mesophyll cells. Cells invaded by the wild type died relatively quickly, whereas those infected by the mutant appeared to survive longer. The most dramatic difference between the mutant and wild type occurred when the mutant completely failed to make a transition to necrotrophic growth, while the wild type made that switch at 48 to 72 h after inoculation. The mutant may be unable to secrete sufficient quantities of one or more proteins that are necessary to support the switch between biotrophy and necrotrophy.

Endopolygalacturonase is encoded by a multigene family in the basidiomycete Chondrostereum purpureum

The basidiomycete Chondrostereum purpureum produces several plant cell wall-degrading enzymes, including endopolygalacturonase (endoPG). Degenerate oligonucleotide primers were designed according to conserved regions of endoPG genes from various fungi, plants, and bacteria and used to amplify members of this gene family from C. purpureum. Four different amplification products showed significant similarity to known endoPGs and were used as hybridization probes to screen a library of genomic DNA sequences and to retrieve five full-length endoPG genes (epgA, epgB1, epgB2, epgC, and epgD). The identities between the deduced polypeptides for epgA, epgB1, epgC, and epgD ranged from 61.8 to 80.0%, while the deduced polypeptides for epgB1 and epgB2 shared 97.1% identity. Phylogenetic analysis suggested that the duplication of existing endoPG genes occurred after the divergence of the ascomycetes and basidiomycetes. C. purpureum is the first basidiomycete fungus for which the endoPG gene family has been described.

Colletotrichum: A model genus for studies on pathology and fungal-plant interactions

Species of Colletotrichum use diverse strategies for invading host tissue, ranging from intracellular hemibiotrophy to subcuticular intramural necrotrophy. In addition, these pathogens develop a series of specialized infection structures, including germ tubes, appressoria, intracellular hyphae, and secondary necrotrophic hyphae. Colletotrichum species provide excellent models for studying the molecular basis of infection structure differentiation and fungal-plant interactions. In this review we cover the various stages of the infection processes of Colletotrichum species, including spore adhesion and germination, germ tube and appressorium differentiation and functions, and biotrophic and necrotrophic development. The contribution of molecular, biochemical, and immunological approaches to the identification of genes and proteins relevant to each stage of fungal development will be considered. As well as reviewing results from several groups, we also describe our own work on the hemibiotrophic pathogen, C. lindemuthianum.

Cloning and partial characterization of endopolygalacturonase genes from Botrytis cinerea

Botrytis cinerea is a plant-pathogenic fungus infecting over 200 different plant species. We use a molecular genetic approach to study the process of pectin degradation by the fungus. Recently, we described the cloning and characterization of an endopolygalacturonase (endoPG) gene from B. cinerea (Bcpg1) which is required for full virulence. Here we describe the cloning and characterization of five additional endoPG-encoding genes from B. cinerea SAS56. The identity at the amino acid level between the six endoPGs of B. cinerea varied from 34 to 73%. Phylogenetic analysis, by using a group of 35 related fungal endoPGs and as an outgroup one plant PG, resulted in the identification of five monophyletic groups of closely related proteins. The endoPG proteins from B. cinerea SAS56 could be assigned to three different monophyletic groups. DNA blot analysis revealed the presence of the complete endoPG gene family in other strains of B. cinerea, as well as in other Botrytis species. Differential gene expression of the gene family members was found in mycelium grown in liquid culture with either glucose or polygalacturonic acid as the carbon source.

Use of green fluorescent protein to detect expression of an endopolygalacturonase gene of Colletotrichum lindemuthianum during bean infection

The 5' noncoding region of clpg2, an endopolygalacturonase gene of the bean pathogen Colletotrichum lindemuthianum, was fused to the coding sequence of a gene encoding a green fluorescent protein (GFP), and the construct was introduced into the fungal genome. Detection of GFP accumulation by fluorescence microscopy examination revealed that clpg2 was expressed at the early stages of germination of the conidia and during appressorium formation both in vitro and on the host plant.

Molecular genetic evidence for the involvement of a specific polygalacturonase, P2c, in the invasion and spread of Aspergillus flavus in cotton bolls

Isolates of Aspergillus flavus can be differentiated based on production of the polygalacturonase P2c. One group of isolates produces P2c, whereas the other group does not. In general, the group that produces P2c causes more damage and spreads to a greater extent in cotton bolls than those isolates that do not produce P2c. To determine whether P2c contributes to disease, the expression of pecA, the gene previously determined to encode P2c, was genetically altered. Adding the pecA gene to a strain previously lacking the gene resulted in the ability to cause significantly more damage to the intercarpellary membrane and the ability spread to a greater extent within the adjacent locule compared to the abilities of a control transformant. Conversely, eliminating the expression of pecA by targeted disruption caused a significant reduction in aggressiveness compared to that of a nondisrupted control transformant. These results provide direct evidence that P2c contributes to the invasion and spread of A. flavus during infection of cotton bolls. However, other factors not evaluated in this study also contribute to aggressiveness.

Endopolygalacturonase genes from Colletotrichum lindemuthianum: cloning of CLPG2 and comparison of its expression to that of CLPG1 during saprophytic and parasitic growth of the fungus

Following the previous isolation of CLPG1, a gene encoding an endopolygalacturonase (endoPG) secreted into the culture filtrate of Colletotrichum lindemuthianum, we have isolated and sequenced an additional endoPG gene, CLPG2. This gene is present as a single copy in the genome of the fungus. At the amino acid level, CLPG2 shows 61% identity to CLPG1 and between 37 to 59% identity to other fungal endoPGs. RNA blot analyses of endoPG gene expression were followed with specific probes during in vitro culture of the fungus. When conidia were used to inoculate a synthetic medium containing pectin as sole carbon source, only CLPG1 was found to be expressed after 3 days of culture. However, transferring the mycelium grown on glucose for 4 days to a pectin-containing medium allowed the detection of CLPG1 and CLPG2 transcripts as early as 12 h after transfer on this substrate. Expression of CLPG2 was transient while that of CLPG1 was more prolonged. Immunocytological localization of endoPG in C. lindemuthianum-infected bean tissues with antibodies against CLPG1 confirmed that the protein is produced in planta and is associated with extensive degradation of the host cell wall. Detection of endoPG transcripts by reverse transcription-polymerase chain reaction revealed that CLPG1, but not CLPG2, is expressed at the beginning of the necrotrophic stage of infection. These results show that the two endoPG genes are differentially expressed and that CLPG1 encodes the major secreted endoPG both during saprophytic growth and during plant infection.

Induction by (alpha)-L-Arabinose and (alpha)-L-Rhamnose of Endopolygalacturonase Gene Expression in Colletotrichum lindemuthianum

The production of endopolygalacturonase (endoPG) by Colletotrichum lindemuthianum, a fungal pathogen causing anthracnose on bean seedlings, was enhanced when the fungus was grown in liquid medium with L-arabinose or L-rhamnose as the sole carbon source. These two neutral sugars are present in plant cell wall pectic polysaccharides. The endolytic nature of the enzyme was demonstrated by its specific interaction with the polygalacturonase-inhibiting protein of the host plant as well as by sugar analysis of the products released from its action on oligogalacturonides. Additional characterization of the protein was achieved with an antiserum raised against the pure endoPG of the fungus. Induction by arabinose and rhamnose was more prolonged and led to a level of enzyme activity at least five times higher than that on pectin. Northern blot experiments showed that this effect was correlated to the induction of a 1.6-kb transcript. A dose-response study indicated that the endoPG transcript level was already increased at a concentration of each sugar as low as 2.75 mM in the medium and was maximum at 55 mM arabinose and 28 mM rhamnose. Glucose, the main plant cell wall sugar residue which is also present in the apoplast, prevented endoPG gene expression, partially when added to pectin at concentrations ranging from 5 to 110 mM and totally when added at 55 mM to arabinose. Inhibition by glucose of the rhamnose-induced endoPG was correlated to nonuptake of rhamnose. This is the first report that arabinose and rhamnose stimulate endoPG gene expression in a fungus. The possible involvement of these various sugars on endoPG gene expression during pathogenesis is discussed.