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Department of Plant Pathology and Microbiology, Program for the Biology of Filamentous Fungi, Texas A&M University, College Station, TX 77843-2132, USA
Correspondence
Won-Bo Shim
wbshim{at}tamu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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gap1 : : HYG) and GAG8 (gap1 : : GEN) exhibited restricted growth, with more aerial hyphae than their wild-type progenitor on solid media. No defect in mycelial mass or filamentous growth was observed when the GAM126 and GAG8 strains were grown in liquid media under shaking conditions. When grown in suspended conditions, GAM126 and GAG8 strains produced significantly fewer conidia and produced comparatively densely branched hyphae. Concanavalin A staining indicated that the GAP1 deletion altered the cell wall carbohydrate composition/deposition process. Deletion of GAP1 did not affect the production level of FB1 or F. verticillioides virulence on maize seedlings and stalks. Complementation of GAM126 with the wild-type GAP1 gene restored growth, conidiation and cell wall abnormality phenotypes. The results suggest that GAP1 is associated with growth, development and conidiation in F. verticillioides, but not with pathogenicity or regulation of FB1.
The GenBank/EMBL/DDBJ accession no. for the 350 bp GAP1 gene sequence from F. verticillioides is DQ458798.
Figures showing the F. verticillioides seedling rot assay on hybrid sweetcorn, and Northern analysis of GAP2 and GAP3 expression in F. verticillioides, are available as supplementary data with the online version of this paper.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). Stalk rot of maize is widespread in dry, warm climates (White, 1999), and can cause losses in several different ways, such as stalk breakage, lodging, interruption of the normal grain-filling process, and sometimes premature death of the plant (Dodd, 1980; Michaelson, 1957; White, 1999). On maize ears, the fungus can produce polyketide-derived carcinogenic mycotoxins called fumonisins (Nelson et al., 1993; Marasas, 2001). Since the discovery of fumonisins, several efforts have been made to understand the regulation of fumonisin biosynthesis in the F. verticillioides–maize system (reviewed by Sagaram et al., 2006). Briefly, fumonisins are synthesized by a cluster of 22 FUM genes, of which 15 are co-regulated (Proctor et al., 1999, 2003). Recently, a putative regulatory gene within the FUM cluster, FUM21, has been identified and characterized (Brown et al., 2007). However, FUM21 gene deletion is not sufficient to completely block fumonisin biosynthesis, suggesting that transcriptional regulation of FUM genes is also affected by regulatory genes outside the cluster. Fumonisin biosynthesis is also impacted by several environmental factors, including available nitrogen, ambient pH and maize kernel constitution environment (reviewed by Sagaram et al., 2006).
The first regulatory gene identified by the restriction enzyme-mediated integration (REMI) strategy was FCC1, which encodes a C-type cyclin (Shim & Woloshuk, 2001). Mutation in FCC1 leads to a severe reduction in conidiation and fumonisin production. Nitrogen source and pH strongly influence the conidiation and expression of the fumonisin biosynthetic gene (FUM1) in a fcc1 strain, indicating a regulatory role for FCC1 in fumonisin biosynthesis and fungal development (Shim & Woloshuk, 2001). Subsequent genomic approaches, such as construction of cDNA subtraction libraries using a wild-type strain and fcc1 strains (Shim & Woloshuk, 2001), and microarray analysis (Pirttilä et al., 2004), have identified additional regulatory genes for fumonisin biosynthesis. As a result, several genes, e.g. FCK1, a C-type cyclin-dependent kinase, PAC1, a pH regulatory gene, ZFR1, a Zn(II)2Cys6 gene, and AMY1, a gene encoding a putative 
-amylase (reviewed by Sagaram et al., 2006), have been identified and characterized.
During the screening of F. verticillioides EST index (TIGR) and suppression subtractive hybridization (SSH) cDNA libraries (Shim & Woloshuk, 2001), a 350 bp EST was identified in the wild-type SSH library that encodes a protein sequence with high homology to glycosylphosphatidylinositol (GPI)-anchored β-1,3-glucanosyltransferases that belong to a class of GPI-anchored glycoproteins. We designated this gene GAP1 (GPI-anchored protein 1). GPI-anchored cell wall proteins (GPI-CWPs) are among the most common proteins in eukaryotic cell walls (De Groot et al., 2005) that are covalently linked to β-1,6-glucan, which can be further linked to β-1,3-glucan or chitin, resulting in a strong anchorage of these proteins to cell walls (Kapteyn et al., 1996, 1997; Kollar et al., 1997). The β-1,3-glucanosyltransferases are involved in the elongation of β-1,3-glucan side chains of the cell wall (Mouyna et al., 2000). These enzymes split the β-1,3-glucan molecule and transfer the newly produced reducing end to the non-reducing end of another β-1,3-glucan molecule (Mouyna et al., 2000). GPI-anchored β-1,3-glucanosyltransferases have been shown to play a critical role in fungal cell wall biogenesis (Mouyna et al., 2000), morphogenesis and virulence in opportunistic fungal pathogens of humans (Muhlschlegel & Fonzi, 1997; DeBernardis et al., 1998; Ghannoum et al., 1995; Mouyna et al., 2005). For example, GEL1 and GEL2 are GPI-anchored β-1,3-glucanosyltransferase-encoding genes characterized in Aspergillus fumigatus (Mouyna et al., 2000, 2005). Disruption of GEL1 does not result in a phenotype, while disruption of GEL2, and disruption of both GEL1 and GEL2, result in altered cell wall composition, slower growth, unusual conidiogenesis and reduced invasive aspergillosis in a murine model (Mouyna et al., 2005). Significantly, a β-1,3-glucanosyltransferase-encoding gene, GAS1, is also involved in virulence in the plant-pathogenic fungus Fusarium oxysporum (Caracuel et al., 2005), suggesting that GAP1 may be associated with fungal virulence in F. verticillioides. Caracuel et al. (2005) have also reported that F. oxysporum GAS1 expression is pH-independent, in contrast to Candida albicans PHR1 and PHR2 (Muhlschlegel & Fonzi, 1997).
The fact that the GAP1 EST was isolated in the F. verticillioides wild-type SSH cDNA library (Shim & Woloshuk, 2001) prompted us initially to hypothesize that GAP1 is primarily associated with fumonisin biosynthesis. However, we also argued that GAP1 could be primarily associated with fungal development and conidiation (Mouyna et al., 2000; Shim & Woloshuk, 2001), which in turn may affect fungal virulence (Caracuel et al., 2005). Thus, we formulated a hypothesis that GAP1 regulates fungal development and virulence in F. verticillioides. To test this hypothesis, we generated GAP1 deletion mutants of F. verticillioides and investigated growth, conidiation and cell wall biogenesis. In addition, we analysed fungal virulence on maize and fumonisin production. In this study, we demonstrate that GAP1 is required for proper growth and conidiation on solid synthetic media and conidiation in liquid media. Our study revealed that GAP1 influences cell wall structure by organizing the carbohydrate deposition in the cell walls when the fungus is growing on solid synthetic media. We also show that GAP1 is not involved in F. verticillioides virulence on maize, and does not positively regulate fumonisin biosynthesis.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Nucleic acid manipulation.
Bacterial plasmid DNA was isolated with the Wizard miniprep DNA purification system (Promega). Fungal genomic DNA was extracted by methods described previously (Shim & Woloshuk, 2001). Total RNA was extracted with an RNeasy Plant Mini kit (Qiagen) or Trizol reagent (Invitrogen) as per the manufacturers' protocols. Southern and Northern analyses were performed following standard procedures (Sambrook & Russell, 2001). For Southern analysis, genomic DNA (10 µg) was digested with EcoRI and subjected to electrophoresis on a 1 % agarose gel. The DNA was transferred onto a nylon membrane and probed with a 32P-labelled DNA fragment amplified from F. verticillioides genomic DNA with primers FvGAS-F4t and FvGAS-R3 (Table 1). For Northern analysis, total RNA was subjected to electrophoresis on a 1.2 % denaturing agarose gel, transferred onto a nylon membrane and hybridized with 32P-labelled gene-specific probes. The probes used in all hybridization experiments were 32P-labelled with a Prime-It Random Primer Labeling kit (Stratagene).
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. PCR amplification of DNA was performed in 25 or 50 µl total volumes with Taq DNA polymerase (Promega) or Expand Long Polymerase (Roche). The PCR conditions were 2 min of pre-denaturation at 94 °C followed by 30 cycles of 30 s of denaturation at 94 °C, 30 s of annealing at 55–57 °C, and 1–2 min of extension at 72 °C for Taq DNA polymerase and 68 °C for Expand Long Polymerase.
Deletion and complementation vector construction.
Two deletion constructs were generated by the double-joint PCR strategy (Yu et al., 2004) to replace the GAP1 ORF in F. verticillioides strain 7600. First, 1150 bp 5' and 1050 bp 3' flanking regions of the GAP1 gene were amplified from F. verticillioides genomic DNA using Taq DNA polymerase. The primers FvGAS-F2 and FvGAS-R2t were used to amplify the 5' flanking region, and primers FvGAS-F4t and FvGAS-R4 were used to amplify the 3' flanking region. Simultaneously, the hygromycin phosphotransferase (HYG)-encoding gene and the geneticin (G418)-resistance gene (GEN) were amplified from plasmid vectors pBP15 and pBS-G, respectively, using the primers M13-F and M13-R. The drug-resistance markers were amplified using Expand Long Polymerase, which has proof-reading activity. Subsequently, the three amplicons were mixed in a single tube in a 1 : 3 : 1 (5' fragment : marker : 3' fragment) molar ratio and joined by PCR without using any primers. Finally, nested primers FvGAS-F5 and FvGAS-R4 were used to amplify the 3.9 kb amplicon carrying the HPH or GEN marker fused to the GAP1 flanking regions. These fused products, named GAS1DV2-H and GAS1DV2-G, which harbour HYG and GEN, respectively, were used as GAP1 disruption constructs.
The GAP1 deletion mutant strain GAM126 (gap1 : : HYG) was complemented with a wild-type GAP1 gene that was fused to GEN. GEN was amplified with primers M13-F and M13-R from pBS-G, and GAP1 [including 1530 bp 5' untranslated region (UTR) and 1200 bp 3' UTR] was amplified from genomic DNA with FvGAS-F7t and FvGAS-R3 using Expand Long Polymerase. GEN and GAP1 were fused by a single-joint PCR strategy to generate the complementation construct (Shim et al., 2006; Yu et al., 2004). The joined-PCR product GAS1CV1 was amplified with the primers M13-F and FvGAS-R4 using Expand Long Polymerase and used for transformation to complement GAM126.
Fungal transformation.
Fungal protoplasts were prepared and transformed as described previously (Shim & Woloshuk, 2001), except that mureinase (2 mg ml–1) was replaced by Driselase (5 mg ml–1) (Sigma), and 1 g wet mycelium was digested in 20 ml enzyme solution [10 mM NaH2PO4 (pH 5.8), 20 mM CaCl2, β-glucuronidase (1343 U ml–1; Sigma), Driselase (5 mg ml–1; Sigma), 1.2 M KCl]. Transformants were selected on regeneration medium (343 g sucrose l–1, 0.2 g yeast extract l–1, 10 g agar l–1) containing 150 µg ml–1 hygromycin B (Calbiochem) or G-418 (Cellgro).
Mycelial staining, microscopy and cell wall assays.
For mycelial staining, 105 spores were inoculated on potato dextrose agar (PDA) and allowed to grow. After 2 days, the mycelia were scraped and collected in a 1.5 ml tube. Concanavalin A (ConA) type VI conjugated to FITC (Sigma, catalogue no. C7642) was used at a final concentration of 100 µg ml–1 using previously published conditions (Shaw & Hoch, 1999). ConA in a 100 µl volume was added to fungal mycelia and incubated for 45 min. Later, samples were washed twice with 100 µl water by successive centrifugation and resuspension. Finally, the samples were resuspended in 100 µl water before observation under the microscope. Microscopic imaging used an Olympus BX51 microscope (Olympus America) fitted with Uplanapo objectives and an Olympus DP70 cooled charge-coupled device (CCD) digital camera. Further details of this imaging system have recently been described (Upadhyay & Shaw, 2006). Images were acquired using DP70-BSW software (version 01.01) and prepared for publication with Adobe Photoshop. For FITC fluorescence visualization, an Olympus U-MNIBA2 (Olympus) filter cube was used with excitation wavelengths from 470 to 480 nm, emission wavelengths from 510 to 550 nm, and a dichroic mirror at 505 nm FITC. Susceptibility assays with Congo red (CR) (Matheson Coleman & Bell) and Calcofluor white (CFW) (fluorescent brightener F3453, Sigma) were conducted as described in Ram & Klis (2006), with few modifications. Ten drops 5 M KOH was added to dissolve CFW. The assays were conducted on PDA with initial pH adjusted to 8.0.
Stalk rot and seedling rot assay.
Stalk rot assays were performed on 8-week-old plants of the B73 corn line. Internodal regions of the stalk were punctured (2 mm deep) with a sterile needle and 104 spores were inoculated into the punctured hole. Plants were incubated in a growth chamber at 25 °C and 40 % humidity. Stalks were split open longitudinally after 21 days and the disease symptoms were photographed. For seedling rot assays, the B73 corn line and Golden Cross Bantam hybrid sweetcorn (Ed Hume Seeds) were surface-sterilized with 100 % bleach for 15 min, washed twice with sterile water, and soaked in sterile water for 1 h. The seeds were placed in between two layers of moist filter paper in a sterile glass saucer (30x15 cm) and allowed to germinate. After 7 days, the top filter paper was removed and the ungerminated seeds were discarded. Seeds were inoculated at the embryo region with 104 spores in a 5 µl volume. Eight seeds were inoculated for each fungal strain. The glass saucer was covered with Saran wrap and incubated at room temperature (22–23 °C) for 8 days. The experiments were repeated twice with similar results.
FB1 analysis.
FB1 analysis was performed as described previously, with some modifications (Shim & Woloshuk, 1999). Fungal strains were grown on cracked corn (B73 line; 1 g dry weight) medium for 15 and 25 days at room temperature (22–23 °C). FB1 was extracted with acetonitrile : water (1 : 1, v/v) for 48 h. The crude extracts were passed over equilibrated PrepSep SPE C18 columns (Fisher Scientific) to eliminate unwanted metabolites coming from corn and fungus. The FB1 concentration of samples was analysed by HPLC using the Shimadzu LC-20AT HPLC system (Shimadzu Scientific Instruments) equipped with an analytical Zorbax ODS column (4.6x150 mm) (Agilent Technologies) and a Shimadzu SPD-20A Prominence UV/Vis detector (335 nm). The HPLC system was operated following the protocol described by Shim & Woloshuk (1999). FB1 was quantified by comparing HPLC peak areas with those of FB1 standards (Sigma). Four biological replicates were included in the experiment and the experiment was repeated twice.
F. verticillioides sexual cross experiments.
Sexual crosses were performed as described in Shim et al. (2006), with modifications. Fresh carrots (400 g) were washed, diced and boiled for 20 min in 400 ml distilled water. The carrots were macerated and the mixture was passed through cheesecloth. Agar (20 g) was added to the filtrate and the final volume was brought to 1 l with distilled water. The carrot agar medium was autoclaved for 20 min and poured into 100 mm diameter Petri plates. Both mating types were grown on V8 agar prior to mating. F. verticillioides 7600, GAM126, GAG8 and GASC1 strains (all with genotype MAT1-1) were crossed with F. verticillioides strain 7598 (genotype MAT1-2) (Fungal Genetics Stock Center) on carrot agar plates. The growth conditions were maintained at 25 °C, with a 14 h light and 10 h dark cycle.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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) that shared high similarity to Saccharomyces cerevisiae GAS protein (E=1x10–33) and C. albicans pH responsive protein (E=3x10–33). The complete GAP1 sequence was obtained from a search of the F. verticillioides 7600 strain genome (Broad Institute of Harvard and MIT; http://www.broad.mit.edu). GPI-anchored surface proteins are known to be differentially expressed with changes in ambient pH (Saporito-Irwin et al., 1995; Muhlschlegel & Fonzi, 1997). In F. verticillioides, GAP1 expression was repressed at pH 8.5 and induced proportionately at acidic pH (Fig. 1). The GAP1 gene is 1669 bp long with a single putative intron (49 bp) and encodes a putative protein of 540 aa. The nucleotide sequence and putative translated protein sequence were submitted to GenBank (accession no. DQ458798). A database search for conserved domains in the Conserved Domain Database (CDD; National Center for Biotechnology Information) revealed the presence of a characteristic glycolipid-anchored surface (GAS) protein (GAS1) domain at the N-terminal region of the protein (Marchler-Bauer et al., 2005). Amino acid comparison using BLASTP analysis indicated that Gap1 shares high homology with predicted protein sequences of other filamentous fungi and yeast (Altschul et al., 1997). Sequence accession numbers of homologues in other fungi with percentage identity to putative Gap1 are: F. oxysporum AAX78216 (94 %); Fusarium graminearum EAA69544 (81 %); Magnaporthe grisea XP_362679 (53 %); Neurospora crassa CAD70754 (51 %); Aspergillus nidulans XP_661334 (50 %) and A. fumigatus EAL84955 (49 %).
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gap1 : : HYG) and GAG8 (gap1 : : GEN), using a double homologous recombination strategy. After protoplast transformation, hygromycin-resistant colonies were selected and screened for gene deletion by PCR (data not shown). The homologous recombination event was verified by Southern analysis (Fig. 2d). The wild-type strain produced a 5 kb band, whereas the deletion mutant GAM126 produced a 3 kb band, indicating that the GAP1 gene was replaced with the disruption construct GAS1DV2-H (Fig. 2b, d). To further validate the GAM126 phenotype, we generated GAG8, a GAP1 knockout strain harbouring a different selectable marker. The deletion of GAP1 in GAG8 was also confirmed by Southern analysis (not shown). Northern analysis showed that the GAP1 expression was completely abolished in both GAM126 and GAG8 strains, whereas the wild-type strain produced a 2.7 kb transcript (Fig. 3).
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; Davydenko et al., 2005; Kottom et al., 2001; Mouyna et al., 2005), and therefore one of the key biological functions of GAP1 that we were interested in was its involvement in cell wall biogenesis. We noted a brighter CFW staining pattern for the GAP1 deletion strains GAM126 and GAG8 compared to that for the wild-type and the complemented strain GASC1 (data not shown). To better visualize this difference, we assayed the staining pattern of all four strains incubated in ConA conjugated to the fluorophore FITC. ConA is a lectin that selectively binds to glucan and mannan moieties (Shaw & Hoch, 1999). The GAM126 and GAG8 deletion strains each exhibited a significantly altered and brighter staining pattern than that of the wild-type and GASC1 when observed with identical staining protocols and camera settings (Fig. 4). Deletion strains each exhibited uniformly brightly stained cell walls punctuated by regularly spaced discrete binding foci. The wild-type and the GASC1 strain both lacked significant ConA binding.
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; Madaule et al., 1987). RHO1 of F. verticillioides was identified by using the nucleotide sequence of rho1 of F. oxysporum (AY884607) as a query in BLASTN analysis of the F. verticillioides genome sequence (http://www.broad.mit.edu). The putative translated RHO1 of F. verticillioides is 98 % identical to RHO1 of F. oxysporum. A 500 bp DNA fragment that spans the RHO1 gene was amplified from F. verticillioides genomic DNA using primers FvRhoGase-D1 and FvRhoGase-U1, and used as a probe for Northern analysis. Since GAP1 itself is pH-regulated, we decided to test whether the GAP1-mediated regulation of RHO1 is affected by different pHs. Since PACC-group transcriptional regulators control pH-responsive genes such as PHR1 and PHR2 in C. albicans (Muhlschlegel & Fonzi, 1997; Porta et al., 1999; Saporito-Irwin et al., 1995), we included an F. verticillioides PAC1-deletion strain to test if PAC1 has a direct effect on GAP1 and/or RHO1. Surprisingly, RHO1 transcript levels in GAM126 and GAG8 mutants were similar to those of the wild-type under both pH conditions tested (Fig. 3). Our results also indicated that GAP1 expression is induced under alkaline conditions in a PAC1-deletion mutant, whereas RHO1 expression is unaffected (Fig. 3). These results suggest that the deletion of GAP1 in F. verticillioides might alter the composition of the cell wall through a mechanism that does not involve RHO1, unlike in F. oxysporum (Caracuel et al., 2005). Alternatively, it can be hypothesized that the involvement of Rho1 in cell wall biogenesis in F. verticillioides is post-transcriptional.
We also examined the effect of cell wall stressors such as CR and CFW on the susceptibility of GAM126 and GAG8 (Ram & Klis, 2006). Surprisingly, the deletion strains were resistant to these compounds compared to their wild-type progenitor. The wild-type strain produced significantly less radial growth on plates containing CR (25 and 50 µg ml–1) in comparison to a negative control (Fig. 5). In contrast, there was no noticeable difference in the growth of GAM126 and GAG8 in the presence of CR (Fig. 5). The resistant phenotype was not obvious in the first few days after inoculation, but was clearly expressed when observed 5 and 7 days after inoculation (not shown). The complemented strain GASC1 showed a phenotype similar to that of the wild-type. Similar results were observed with CFW at 25 µg ml–1 (not shown). At CFW concentrations of 50 µg ml–1 or higher, all the strains were highly susceptible, and as a result no growth was observed (not shown).
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, Fig. 6). When grown on agar medium, the growth of both the GAM126 strain and the GAG8 strain was restricted when compared to that of the wild-type. The deletion mutants produced a compact mass of mycelium with restricted edges that tended to grow into the agar medium, while their wild-type progenitor produced thin and spreading borders on the surface of the medium (Fig. 6a). Microscopic examination of spore germination revealed that deletion strains produced short and highly branched hyphae, whereas the wild-type strain produced long and less-branched hyphae that extended on the surface of agar (Fig. 6b). However, we observed no defect in deletion strains with regard to mycelial mass production (Table 2) or filamentous growth (Fig. 6c) when grown in liquid media under shaking conditions. In YEPD suspension, when compared to their wild-type progenitor, GAM126 and GAG8 produced thin and densely branched hyphae that tended to remain submerged (not shown). In PDB suspension, GAM126 and GAG8 produced densely branched hyphae, but hyphal diameter was not noticeably different from that of the wild-type (not shown).
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). We speculated that the production of fewer conidia on agar medium might be due to the restricted growth of deletion strains. Hence, we tested the microconidia production in a liquid medium (PDB). Under this condition, GAM126 and GAG8 produced approximately half the microconidia produced by their wild-type progenitor (Table 2).
Deletion of GAP1 did not influence pathogenicity and FB1 production
Previous reports showing that GAS proteins in filamentous fungi are involved in virulence (Caracuel et al., 2005; Martinez-Lopez et al., 2004; Mouyna et al., 2005) prompted us to examine the involvement of GAP1 in F. verticillioides virulence. In the maize stalk rot assay, we tested the ability of deletion strains to infect and invade stalk tissue. As observed 21 days after infection, the deletion strains successfully colonized and rotted the tissue in a similar manner to the wild-type (Fig. 7a). In the maize seedling assay, the deletion strains were as efficient as the wild-type in infecting and colonizing seedlings (Fig. 7b). After 8 days, all the strains colonized the kernels completely, and invaded the radicle and plumule regions of the seedlings. Similar results were observed in both the maize lines tested (Fig. 7b, Supplementary Fig. S1). Based on these results, we concluded that GAP1 is not involved in virulence in F. verticillioides.
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Gene complementation
To reconfirm that restricted growth and the reduced conidiation phenotype in GAM126 and GAG8 strains is due to the deletion of GAP1, we complemented GAM126 with a wild-type copy of GAP1. Geneticin-resistant transformants were analysed for a functional GAP1 gene by PCR (data not shown) and Southern analysis (Fig. 2c, d). The complemented strain GASC1 contains a 4 kb fragment, indicative of complementation construct insertion at an ectopic location, in addition to a 3 kb fragment resulting from GAS1DV2-H. While we observed that the restoration of GAP1 expression in GASC1 was not to that of the wild-type (lane 4, Fig. 3a and b), all the phenotypes observed in GAM126 and GAG8 strains, such as conidiation, growth and cell wall phenotypes, were completely restored to wild-type levels (Figs 4, 5 and 6), providing evidence that GAP1 deletion was the direct cause of GAM126 and GAG8 mutant phenotypes.
Effect of GAP1 deletion on F. verticillioides sexual development
We demonstrated that the GAP1 deletion had no impact on sexual development in F. verticillioides. Wild-type, GAM126, GAG8 and GASC1 strains were crossed to the opposite mating type wild-type strain 7598, and after 10 days' incubation at 25 °C with a 14 h light and 10 h dark cycle we observed the development of perithecia as black spots embedded in the mycelium (not shown). Later, we observed viable ascospore formation in all the crosses performed (not shown).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). The F. verticillioides GAP1 gene was isolated from the wild-type SSH cDNA library, suggesting that GAP1 is downstream of FCC1 in the signalling pathway that regulates fumonisin biosynthesis or conidiation, or both. In silico analysis showed that GAP1 encodes a putative protein that shares high structural similarity to a family of GPI-anchored glycoproteins that are known to be involved in cell development in filamentous fungi (Chabane et al., 2006; Mouyna et al., 2000, 2005). Therefore, we hypothesized that GAP1 is primarily associated with conidiation and fungal development. While earlier studies have shown that osmotic stabilization with 1.2 M sorbitol partially remediates the restricted colony phenotype in a gas1 mutant (Caracuel et al., 2005), the restricted colony growth phenotypes in GAM126 and GAG8 were not remediated upon addition of osmotic stabilizers, including 1.2 M KCl and 1.2 M sorbitol (not shown).
When mycelia of wild-type, GAM126 and GAG8 were stained with CFW, we noticed that the cell walls of the GAM126 and GAG8 strains were more brightly stained than those of the wild-type, indicating either that the mutant strains produced more chitin and β-glucans in their cell walls, or that these moieties were more readily stained by CFW in the mutant (not shown). When stained with ConA–FITC, GAM126 and GAG8 exhibited uniform binding of the lectin to cell walls, punctuated by discrete sites of greater binding (Fig. 4). In contrast, neither the wild-type nor GASC1 revealed any significant ConA binding sites. ConA is routinely used to assay the presence of glucan and mannan moieties in fungal cell walls (Shaw & Hoch, 1999), and is generally believed to bind to terminal -mannan and -glucan (Smith & Goldstein, 1967). One might have hypothesized that the GAP1 mutants would exhibit fewer ConA binding sites than the wild-type, since the GAP1 protein is thought to function in the linking of β-linked glucan groups. Our result is not surprising, however, since the alteration of the cell wall by lack of GAP1 function may lead to a more amorphous cell wall that is less cross-linked, therefore leading to exposure of more ConA binding sites. While it is difficult to unambiguously explain this ConA staining pattern, it is clear from our results that the GAP1 mutant cell wall is significantly altered from that of the wild-type. In this study, we also observed that GAM126 and GAG8 strains were resistant to the cell wall-stressing agents CR and CFW. This result contrasts with those obtained with S. cerevisiae gas1 mutants, which are sensitive to CFW (Popolo et al., 1993). Interestingly, resistance to CR and CFW has also been observed in S. cerevisiae mutants that are defective in chitin synthesis (Imai et al., 2005; Roncero et al., 1988). In general, the cell wall mutants that are disrupted in the synthesis of β-1,3 or β-1,6 glucans, GPI-anchored glycoprotein biosynthesis or mannosylation of mannoproteins have elevated chitin levels and are hypersensitive to CR and CFW (Imai et al., 2005; Ram et al., 1994). However, in F. verticillioides, we hypothesize that deletion of GAP1 results in reduced chitin production that renders the mutants resistant to CR and CFW.
The expression of pH-responsive genes in filamentous fungi is regulated by key transcriptional regulators such as pacC in A. nidulans (Tilburn et al., 1995) and PRR1 in C. albicans (Porta et al., 1999). For example, the expression of PHR1 and PHR2, the β-1,3-glucanotransferase-encoding genes in C. albicans, is regulated by changes in the external pH via PRR1 (Muhlschlegel & Fonzi, 1997; Porta et al., 1999; Saporito-Irwin et al., 1995). In contrast, GEL1 and GEL2, the two characterized β-1,3-glucanotransferase-encoding genes in A. fumigatus, are constitutively expressed over a range of growth conditions (Mouyna et al., 2000). Interestingly, the expression of GAS1 in F. oxysporum is not controlled by ambient pH, or by the responsive regulator PacC (Caracuel et al., 2005). In contrast to the results of F. oxysporum studies, the expression of GAP1 in F. verticillioides is induced under acidic conditions (Fig. 1) and seems to be dependent upon the pH-responsive transcription factor PAC1 (Fig. 3). Our Northern analysis revealed that GAP1 expression is upregulated under alkaline conditions in a PAC1 deletion mutant compared to that of the wild-type (Fig. 3b, lane 5). We propose that under alkaline conditions, induction of PAC1 suppresses the expression of GAP1 and vice versa. However, further studies are needed to confirm the regulation of GAP1 by PAC1. Since ambient pH plays a critical role in the expression of GAP1, and GAP1 plays an important role in growth, we decided to check if the pH variation affects the growth of GAP1 deletion strains. Unlike the C. albicans deletion strains (Muhlschlegel & Fonzi, 1997; Saporito-Irwin et al., 1995), differences in pH (4.5 and 7.5) did not affect the growth of the GAM126 and GAG8 strains in either solid or liquid media (not shown).
In addition to investigating the developmental phenotypes of GAM126 and GAG8, we studied the impact of gap1 on key biological features that are linked to the FCC1 in F. verticillioides. In our study we observed a decrease in production of conidia due to the deletion of GAP1 (Table 2). Flaherty et al. (2003) have shown that PAC1 is critical to maintain proper growth and conidiation under alkaline conditions. However, the ambient pH did not have any impact on growth in the GAM126 and GAG8 strains. Hence, we speculate that the reduction in microconidia production in our deletion mutants was due to a developmental defect similar to the abnormal conidiophores observed in A. fumigatus gel2 and gel2gel1 mutants (Mouyna et al., 2005), rather than molecular regulation of conidiation genes. Overall, very little information is available regarding GPI-anchored proteins in relation to conidiation. Identification and characterization of developmental-related regulatory genes in F. verticillioides are necessary to clarify this relationship. We were intrigued that GAP1 deletion did not have a negative impact on fumonisin production, which was in contrast to the conidiation data. Rather, we observed an increased level of FB1 production in the GAG8 strain. These data provide indirect evidence that the primary functional role of GAP1 is to regulate proper hyphal development and conidiation in F. verticillioides. The discovery of additional genes in the FCC1-mediated signalling pathways will allow us to investigate the epistatic relationship between the genes and clarify the pathway–phenotype association.
In this report we also demonstrated that GAP1 in F. verticillioides is not essential for invasion and colonization of maize stalks and seedlings. In a way it is interesting that the GAM126 and GAG8 strains, which are severely impaired in hyphal development on synthetic solid surfaces, can grow as effectively as the wild-type in maize stalks and on seedlings that in theory can be considered to be solid surfaces. Based on our observations it is unlikely that GAP1 plays a signalling role in the interaction between maize and F. verticillioides. Our finding is significant, based on the fact that supposedly orthologous proteins (94 % identical) performed dissimilar functions (Caracuel et al., 2005; this study), although both fungi might have different modes of infection. However, our finding is not surprising, since the fungus might have a compensatory mechanism to overcome the loss of GAP1. Similar results have been observed in other filamentous fungi. For example, deletion of GEL1 in A. fumigatus does not have an impact on virulence, but deletion of its homologue GEL2 affects morphogenesis and virulence (Mouyna et al., 2005). Surprisingly, GEL1 and GEL2 of A. fumigatus both complement a GAS1 deletion strain of S. cerevisiae, indicating that both proteins can act as functional homologues in yeast (Mouyna et al., 2005). In addition, under in vitro conditions, Gel2p exhibits the same enzymic activity as Gel1p, reconfirming the redundancy in function (Mouyna et al., 2005). The regulation of expression of structurally similar but functionally different genes could be under the control of external signals such as pH, nutrient source, etc. On the other hand, PHR1 and PHR2 are GPI-anchored proteins in C. albicans that structurally and functionally exhibit opposing relationships between pH and morphology: PHR1 and PHR2 mutants are morphologically abnormal at alkaline and acidic pH, respectively (Muhlschlegel & Fonzi, 1997; Saporito-Irwin et al., 1995). The fact that deletion of GAP1 had no effect on pathogenicity, whereas its structural homologue in F. oxysporum has a significant impact on virulence (Caracuel et al., 2005), further indicates that the function of GPI proteins is specific to each organism.
Analyses of publicly available fungal genomes revealed that more than one GAS protein family gene member is present in filamentous fungi, and as expected two additional GPI-anchored glycoprotein-encoding genes were identified in the F. verticillioides genome. This might indicate that β-1,3-glucanotransferases are functionally homologous or that they perform a different function. To address this question, we first studied the expression of the other two GAP protein-encoding genes in F. verticillioides. The complete GAP2 (supercontig 3; locus FVEG_02799.3) and GAP3 (supercontig 5; locus FVEG_04666.3) sequences were obtained from a genome search of the F. verticillioides 7600 strain (Broad Institute of Harvard and MIT; http://www.broad.mit.edu). Our Northern analysis indicated that GAP2 expression is also dependent on ambient pH (Supplementary Fig. S2). In contrast to GAP1 expression, GAP2 is induced under alkaline conditions (Supplementary Fig. S2). No detectable GAP3 transcript levels were observed under the conditions tested (Supplementary Fig. S2). Our studies also revealed that GAP2 expression is unaltered in the GAM126 and GAM8 strains under both pH (3.0 and 8.0) conditions tested (not shown). This suggests that GAS proteins (at least Gap1 and Gap2) in F. verticillioides are regulated by ambient pH, probably through pH transcription factor Pac1, and might perform their functions independently of one other. Further mutational studies are needed to better understand the role of GAP2 and GAP3 proteins in F. verticillioides.
Finally, it is evident that β-1,3-glucanotransferases share a common function of involvement in cell wall biogenesis. In spite of their homology, GPI proteins of fungi are regulated in different ways and perform different cellular functions, probably to meet the specific requirements of an organism.
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ACKNOWLEDGEMENTS |
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Edited by: J.-R. Xu
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REFERENCES |
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Received 1 March 2007;
revised 3 May 2007;
accepted 9 May 2007.
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