Agrobacterium tumefaciens-Mediated Transformation of the Lichen Fungus, Umbilicaria muehlenbergii

Abstract

Transformation-mediated mutagenesis in both targeted and random manners has been widely applied to decipher gene function in diverse fungi. However, a transformation system has not yet been established for lichen fungi, severely limiting our ability to study their biology and mechanism underpinning symbiosis via gene manipulation. Here, we report the first successful transformation of the lichen fungus, Umbilicaria muehlenbergii, via the use of Agrobacterium tumefaciens. We generated a total of 918 transformants employing a binary vector that carries the hygromycin B phosphotransferase gene as a selection marker and the enhanced green fluorescent protein gene for labeling transformants. Randomly selected transformants appeared mitotically stable, based on their maintenance of hygromycin B resistance after five generations of growth without selection. Genomic Southern blot showed that 88% of 784 transformants contained a single T-DNA insert in their genome. A number of putative mutants affected in colony color, size, and/or morphology were found among these transformants, supporting the utility of Agrobacterium tumefaciens-mediated transformation (ATMT) for random insertional mutagenesis of U. muehlenbergii. This ATMT approach potentially offers a systematic gene functional study with genome sequences of U. muehlenbergii that is currently underway.

About the Authors:

Sook-Young Park

Contributed equally to this work with: Sook-Young Park, Min-Hye Jeong

Affiliations Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea, Dept. of Agricultural Biotechnology, Fungal Bioinformatics Laboratory, Center for Fungal Genetic Resources, and Center for Fungal Pathogenesis, Seoul National University, Seoul, Korea

Min-Hye Jeong

Contributed equally to this work with: Sook-Young Park, Min-Hye Jeong

Affiliations Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea, Dept. of Biology, Sunchon National University, Sunchon, Korea

Hai-Ying Wang

Affiliation: College of Life Sciences, Shandong Normal University, Jinan, China

Jung A. Kim

Affiliation: Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea

Nan-Hee Yu

Affiliations Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea, Dept. of Biology, Sunchon National University, Sunchon, Korea

Sungbeom Kim

Affiliation: Dept. of Agricultural Biotechnology, Fungal Bioinformatics Laboratory, Center for Fungal Genetic Resources, and Center for Fungal Pathogenesis, Seoul National University, Seoul, Korea

Yong Hwa Cheong

Affiliation: Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea

Seogchan Kang

Affiliation: Dept. of Plant Pathology & Environmental Microbiology, The Pennsylvania State University, University Park, Pennsylvania, United States of America

Yong-Hwan Lee

* E-mail: [email protected] (JSH); [email protected] (Y-HL)

Affiliation: Dept. of Agricultural Biotechnology, Fungal Bioinformatics Laboratory, Center for Fungal Genetic Resources, and Center for Fungal Pathogenesis, Seoul National University, Seoul, Korea

Jae-Seoun Hur

* E-mail: [email protected] (JSH); [email protected] (Y-HL)

Affiliation: Korean Lichen Research Institute, Sunchon National University, Sunchon, Korea

Introduction

Availability of an efficient transformation system is crucial for the experimental study of gene function. Polyethylene glycol (PEG)-mediated introduction of DNA to protoplasts has been widely utilized for fungal transformation. Generation of good protoplasts through efficient digestion of the fungal cell wall using a mixture of hydrolytic enzymes is critical for high transformation efficiency [1]–[3]. Accordingly, this approach is not readily applicable to fungi that are recalcitrant to cell wall degrading enzymes.

Agrobacterium tumefaciens has been found to introduce and integrate engineered T-DNA efficiently into the genome of Saccharomyces cerevisiae [4] and several filamentous fungi [5]. A. tumefaciens-mediated transformation (ATMT) works well with several intact fungal tissues (e.g., spores, mycelia, gill tissues from mushroom), offering an alternative means for transforming fungi that do not readily produce protoplasts. In addition, the transformation efficiency of Aspergillus awamori via ATMT was up to 600-fold higher than by PEG-mediated transformation [5]. Since these initial reports of fungal ATMT, this method has been successfully applied to transform phylogenetically diverse fungi. Many binary vectors with different features and utilities have been developed to support molecular genetic studies of fungi via ATMT [6]–[9]. Random insertional mutagenesis of the fungal genome via ATMT has been successfully applied to several fungi to identify many genes essential for their life cycle and pathogenicity [8], [10]–[13]. ATMT also facilitates efficient targeted gene manipulation via homologous recombination [4], [14], [15].

More than 19% of all known fungi form lichens [16], [17], and nearly 98% of them belong to the phylum Ascomycota. Lichens are mutualistic organisms producing a myriad of potentially useful metabolites [18] that exhibit antitumor [19]–[21], antimicrobial [22]–[38], anti-inflammatory [39], [40], and antioxidant activities [30]–[32], [40]. Although lichens consist of a fungal partner (the mycobioint) and a photosynthetic alga, cyanobacterium, or both (the photobiont) [41], it is generally thought that most secondary metabolites of lichens are produced by the mycobioint [38], [42], [43]. Heterologous expression of genes of lichen fungi in surrogate fungi such as Aspergillus species has been evaluated [44], [45]. This heterologous expression offers an alternative approach to explore lichen polyketide biosynthesis. However, this approach still remains to be validated the production of identical substances that generated by lichen thallus.

With efficient transformation methods, direct mutagenesis of candidate genes in lichen fungi via random insertional mutagenesis or targeted deletion offers a more direct approach to discover and study such genes. Unfortunately, however, no such method has been established in lichen fungi. This is mainly because axenically cultured lichen fungi typically grow extremely slowly and have a tough cell well [46], [47]. Recently, we found a relatively fast growing and dimorphic lichen fungus, Umbilicaria muehlenbergii (Figure 1). Relative to other lichen fungi, U. muehlenbergii could be grown easily and quickly in liquid nutrient media. Moreover, polysaccharides from Umbilicaria species showed antitumor and anti-HIV activities [48]–[50]. Establishment of a transformation system for U. muehlenbergii will offer a valuable tool for studying its genes as well as those in other lichen fungi. We therefore tested whether ATMT could be applied to U. muehlenbergii, this has led to the generation of mitotically stable transformants. To our knowledge, this is the first time transformation of a lichen fungus is reported. This method may be applicable to other lichen fungi, potentially enabling direct manipulations of their genes and genomes.

[Figure omitted. See PDF.]

Figure 1. Morphologies of U. muehlenbergii.

Images of the hyphae phase (A) and the yeast phase (B) were obtained using a stereoscope (left) and a scanning electron microscope (right).

https://doi.org/10.1371/journal.pone.0083896.g001

Results

Isolation of U. muehelenbergii

Lichen thalli of U. muehlenbergii were collected from a rock at Mt. Tulaopoding, Jilin province, China in 2012. The lichen was identified by Dr. Wang in Shandong Normal University (Jinan 250014, China). A voucher specimen was deposited in the herbarium of the university and duplicated in the Korean Lichen Research Institute (KoLRI) at Sunchon National University (Sunchon 540–950, Korea). Lichen-forming fungus (LFF) of U. muehlenbergii was isolated by the tissue culture method [51] from the lichen thalli. Hyphal and yeast-like growth forms of the LFF were isolated. A number of conidia were produced both on solid PDA medium and in liquid PDB medium. Analysis of internal transcribed spacer (ITS) sequence indicated that the two growth forms of LFF were genetically identical to the originally isolated U. muehlenbergii strain.

Agrobacterium Tumefaciens-mediated Transformation of U. muehlenbergii

In order to test whether A. tumefaciens can transfer T-DNA to U. muehlenbergii (Figure 1), we used the binary vector pSK1044 (Figure 2 and Information S1). First, we evaluated the sensitivity of U. muehlenbergii to hygromycin B to determine an optimal concentration for selecting transformants. The fungus was highly sensitive to hygromycin B, with complete growth inhibition at 20 µg/ml (Figure 3A). Therefore this concentration was used for selecting transformants.

[Figure omitted. See PDF.]

Figure 2. Map of binary vector pSK1044.

pBHt2 contains the hygromycin B resistance gene under the control of the Aspergillus nidulans trpC promoter [8] was used to construct pSK1044 by insertion of the eGFP gene cassette between the EcoRI and HindIII sites in the multi-cloning site of pBHt2.

https://doi.org/10.1371/journal.pone.0083896.g002

[Figure omitted. See PDF.]

Figure 3. Growth, transformation efficiency, and Southern analysis.

(A) Sensitivity of a wild-type U. muehlenbergii strain to hygromycin B was evaluated by culturing it on PDA amended with 0, 5, 10 and 20 µg/ml of hygromycin B. (B) Membranes overlaid with only fungal cells (1st on the left), and a mix of A. tumefaciens and fungal cells (2nd, 3rd and 4th) were placed on PDA containing 20 µg/ml hygromycin B after culturing on co-cultivation medium for 24, 36 and 48 hrs (2nd, 3rd and 4th). The pictures were taken after 28 days of culture on this selection medium. (C) Effect on the transformation efficiency by increasing the co-cultivation time. Data presented as the average of eight plates per treatments. Error bars indicates standard error. (D) Distribution of T-DNA copy number among transformants of different time interval of co-cultivation. Genomic DNAs were digested with HindIII, a restriction enzyme that does not cut the hph cassette (see Figure 2). The digested DNAs, fractionated using 0.7% agarose gel and transferred to a nylon membrane, were probed with a labeled hph cassette. (E) Representative result of Southern blot analysis from 15 randomly selected transformants and a wild-type strain. (F) Distribution of T-DNA copy numbers among 784 transformants.

https://doi.org/10.1371/journal.pone.0083896.g003

Co-cultivation of A. tumefaciens cells with yeast-like conidia of U. muehlenbergii in the presence of acetosyringone (AS), a phenolic compound that is secreted from wounded plants, led to the appearance of hygromycin B-resistant colonies approximately one month after transfer to selection medium. To test the effect of co-cultivation time on transformation efficiency, we evaluated 24, 36, and 48 hrs of co-cultivation. The longer co-cultivation at 48 hrs led to >64-fold increase in the number of transformants, resulting in 27 to 98 transformants per 1×106 conidia (Figure 3B and 3C). However, increasing the co-cultivation time decreased the frequency of transformants that contained a single copy of T-DNA (Figure 3D). Therefore, we choose 36 hrs of co-cultivation time to generate transformants for U. muehlenbergii.

Assessment of the mitotic stability of 24 transformants showed that all maintained hygromycin B resistance after being cultured for five generations in the absence of hygromycin B (data not shown).

Generation of a Pool of T-DNA Tagged Mutants and Expression of Green Fluorescent Protein in U. muehlenbergii

We conducted insertional mutagenesis of U. muehlenbergii via ATMT, resulting in 918 hygromycin B-resistant mutants. Southern analysis of 784 transformants revealed that 88.0% (690 transformants) appeared to have a single copy T-DNA insertion (Figure 3E and 3F). Small fractions of transformants showed two (9.9%), three (1.8%), and four or more (0.3%) copies of T-DNA inserted in their genome (Figure 3F).

We also tested the transformants for green fluorescence, as pSK1044 contains the eGFP gene under the control of Cochliobolus heterostrophus GAPD gene promoter in its T-DNA. Thirteen randomly selected transformants were screened using fluorescence microscopy (Table 1). All of them consistently displayed green fluorescence, although fluorescence in five transformants, including UmT-009, UmT-143, UmT-151, UmT-152 and UmT-156, was stronger than in the others (Figure 4). Differential expression of eGFP among these transformants suggested that the level of eGFP expression might vary depending on the sites of T-DNA integration (i.e., positional effect).

[Figure omitted. See PDF.]

Figure 4. Expression of eGFP in 11 transformants. DIC, differential interference contrast images; GFP, fluorescence image.

https://doi.org/10.1371/journal.pone.0083896.g004

[Figure omitted. See PDF.]

Table 1. Results from BlastX searches of the NCBI database with U. muehlenbergii genomic sequences flanking inserted T-DNA as queries.

https://doi.org/10.1371/journal.pone.0083896.t001

Identification of Genomic Sequences Flanking Inserted T-DNA

Thermal asymmetry interlaced-PCR (TAIL-PCR) and subsequent sequencing were used to identify the genomic locations of T-DNA insertions and the context of flanking sequences in selected transformants. To increase the efficiency of TAIL-PCR in amplifying the flanking regions, we employed a mix of arbitrary degenerate primers (AD-1,2,3,4, and 6 = 1∶1:1∶1:1) with one specific primer for the primary (RB1), secondary (RB2-1) and tertiary (RB3) PCR reactions. This approach led to successful amplification of genomic sequences flanking the right border of the inserted T-DNA from all 50 transformants analyzed (Figure 5, Table 1). The size of the PCR products ranged from 0.2 kb to 4 kb. Products from primary PCR reactions typically displayed similar band patterns, whereas more specific bands appeared in secondary and tertiary PCR reactions (Figure 5). Sequence comparison among 50 flanking regions (Information S2) suggests that insertion of T-DNA appears to be without preferential sequence contexts (Figure 6 and Information S2). Unlike in plants or other fungi tested [52]–[55], we did not find any truncated T-DNA in these 50 transformants (Figure 6 and Information S2).

[Figure omitted. See PDF.]

Figure 5. Agarose gel analysis of products resulted from TAIL-PCR of eight randomly selected transformants.

The gels from the top to the bottom show products from the primary, secondary, and tertiary reactions, respectively.

https://doi.org/10.1371/journal.pone.0083896.g005

[Figure omitted. See PDF.]

Figure 6. Genomic sequences of U. muehlenbergii flanking the right border of inserted T-DNA.

All sequences are shown 5′→3′. Bold letters correspond to the 30 bp border sequences.

https://doi.org/10.1371/journal.pone.0083896.g006

Since the genome of U. muehlenbergii has not yet been sequenced, sequences of individual flanking regions were used to search the GenBank protein database using BLASTX. No significant match was found with sequences from 30 transformants, whereas flanking sequences from the remaining 20 transformants matched sequences corresponding to known or hypothetical gene products (Table 1).

Phenotypic Characterization of Transformants

Phenotypic screening of transformants led to the identification of putative mutants that differ from the wild-type strain in colony color, size, and/or morphology (Figure 7). Compared with the round, white colonies of the wild-type U. muehlenbergii typically formed on PDA, colonies of some transformants displayed differential morphological features including heavily pigmented (UmT-263 and UmT-272), reduced size (UmT-400 and UmT-272), and/or different morphologies (UmT-263, UmT-272 and UmT-270) (Figure 7). One putative mutant, UmT-270, exhibited pseudohyphal growth with yellow pigmentation (Figure 7). However, sequences flanking T-DNA in these putative mutants did not exhibit significant similarities to any previously known gene products in the GenBank database.

[Figure omitted. See PDF.]

Figure 7. Colonies of four putative mutants of U. muehlenbergii generated via ATMT.

From left to right, colonies of a wild-type strain and four mutants are shown. These strains were grown for 28 days on PDA.

https://doi.org/10.1371/journal.pone.0083896.g007

Discussion

Lichens have colonized a wide range of ecosystems, including extreme environments such as deserts and arctic regions. As summarized in the Introduction, many lichens are known to produce pharmacologically active metabolites [18]–[40]. Lack of a transformation system for lichen fungi has hampered efforts to study the genetic mechanisms underpinning important biological and ecological processes associated with lichens and their mycobionts. Success in ATMT of diverse fungi in both the Ascomycota and the Basidiomycota [5], [8], [56]–[58] motivated us to apply ATMT to U. muehlenbergii. Successful transformation of U. muehlenbergii now makes it possible to manipulate its genes and also opens the possibility of using this fungus as a surrogate (i.e., heterologous expression system) for studying the function of genes from other fungi.

The observed efficiency of transformation, 27 to 98 transformants per 1×106 conidia, is not very high compared to those of other fungi [5], [8], [48]. Several studies showed that the efficiency of fungal ATMT is affected by several factors, including a) the concentration of acetosyringone (AS), b) A. tumefaciens strain and its growth conditions, c) numbers of fungal and bacterial cells mixed for co-cultivation [4], [5], [8], [10], [57]. Since AS functions as a signal for activating a set of virulence (vir) genes in A. tumefaciens that are required for T-DNA transfer [59], the presence of AS is essential for ATMT of fungi [5], [8], [57]. Adjustments of AS concentration during the induction and co-cultivation stages may help improve the efficiency. The choice of an A. tumefaciens strain is another factor that can be explored, as some strains of A. tumefaciens worked better than others in transforming fungi (S. Kang, unpublished result). In addition, the growth conditions of A. tumefaciens cells prior to co-cultivation also affected transformation efficiency [8], [48], potentially offering another control point that can be improved and the ratio of bacterial to fungal cells can also be adjusted optimally.

Since many of these factors may also affect the number of T-DNA copies inserted in individual transformants [8], [13], [57], simply employing conditions that provide the greatest number of transformants may not be desirable for certain applications, such as insertional mutagenesis and targeted gene disruption. A key advantage of insertional mutagenesis via ATMT over chemical or radiation mutagenesis is that inserted T-DNA serves as a convenient tag for cloning and identifying mutated genes (e.g., Figure 6 and Table 1). Insertion of T-DNA at multiple loci in a mutant, therefore, can cause a substantial nuisance when it comes to identifying the gene causing a phenotype of interest. For such mutants from fungi with a sexual stage, a genetic cross can be performed to determine which T-DNA segregates with the mutant phenotype before cloning the mutated gene. However, for fungi like U. muehlenbergii that require a relatively long time to complete the sexual cycle, genomic sequences flanking all T-DNA inserts need to be functionally validated (e.g., complementation, targeted mutation or both) to characterize the gene of interest. Thus, for efficient insertional mutagenesis and subsequent functional analyses of mutated genes, employing transformation conditions that ensure a high percentage of transformants possessing a single copy of T-DNA is highly desirable. Fortunately, with 36 hrs of co-cultivation, only one copy of T-DNA was inserted in the genome of 88% of the U. muehlenbergii transformants (Figure 3E), which is higher than in other fungi [5], [57], [60].

Isolation of three different types of putative morphological mutants from a pool of 918 transformants supports the utility of ATMT for mutagenizing U. muehlenbergii via random insertion of T-DNA and subsequently identifying the mutated genes (Figure 7). Pseudohyphal growth of UmT-270 (Figure 7) suggests that a gene required for the regulation of a dimorphic switch may be disrupted. Identification of this gene as well as those tagged in other mutants will help understand how growth is controlled in U. muehlenbergii. Unfortunately, amplified genomic sequences that flank the right border of T-DNA in these mutants did not exhibit significant similarity to any known or hypothetical gene products (Table 1). It is possible that the genes tagged in these mutants may be unique to U. muehlenbergii. Alternatively, T-DNA insertion may be more indirectly, e.g. by inactivating a promoter or destabilizing mRNA.

In addition, to examine homologous recombination, we conducted amplification of four conserved MAP kinase genes in eukaryotes [61], Ste7, Fus3, Hog1, and Ste12, with degenerated primer sets to acquire sequence information of the genes. However, no amplified fragment was obtained from Ste7 and Ste12 and no sequence matches were obtained for Fus3 and Hog1, suggesting that the orthologs in U. muehlenbergii may have unique start and/or end sequences. Therefore, genome sequence of U. muehlenbergii will be needed to carry out homologous recombination.

Since genome sequencing of U. muehlenbergii is currently underway, we will integrate T-DNA insertion mutants and phenotype data into U. muehlenbergii genome data. In addition, we will apply this system for homologous recombination of the U. muehlenbergii and other lichen-forming fungi. This ATMT system will open an era of systematic gene functional studies for lichen fungi.

Materials and Methods

Fungal Cultures and Growth Conditions

A culture of U. muehlenbergii KoLRI_LF000956, obtained from the Korean Lichen Center at Sunchon National University (Sunchon 540–742, Korea), was stored in the form of a conidial suspension in 15% glycerol at –80°C. It was revitalized by shake culturing in 100 ml of potato dextrose broth (PDB, Difco Laboratories) at 150 rpm and 15°C in the dark. For transformation, after harvesting yeast-like cells in 14 days old culture were harvested by centrifugation at 5,000 rpm for 5 min and washed three times with sterilized distilled water to remove polysaccharides.

Amplification of ITS Region and Sequencing

The ITS region was amplified from the two growth forms of LFF and the thallus of U. muehlenbergii with the primers ITS4 and ITS5 (Table 2) using i-StarMAXII PCR master mix system (iNtRON Biotechnology, Sungnam city, Korea). A Takara PCR thermal cycler MP (Takara, Tokyo, Japan) was employed for 30 cycles of PCR reaction. The amplified PCR products were purified using MEGAquick-spin Total Fragment DNA Purification Kit (iNtRON Biotechnology) and sequenced on both strands with the same primers used for the PCR amplification. ITS sequences were analyzed through NCBI (http://www.ncbi.nlm.nih.gov/).

[Figure omitted. See PDF.]

Table 2. Primers used in this study.

https://doi.org/10.1371/journal.pone.0083896.t002

Plasmid Construction

Fluorescent protein constructs were made by using eGFP, enhanced green fluorescent protein gene. The primers used to amplify the promoter, eGFP coding sequences and terminator are shown in Table 2. Each primer contains a restriction enzyme site at the 5′ end to facilitate subsequent construction. PCR amplifications were performed using the FailSafe PCR system (Epicentre, Madison, WI, USA). The PCR products were isolated from gels, purified by QIA quick columns (Qiagen) and then cloned in pGEM-T Easy (Promega Corp.). All clones were verified by sequencing. BamHI and SphI fragment of eGFP coding sequence (720 bp) and a SphI and HindIII fragment of terminator from Neurospora crassa trpC (260 bp) under the control of the EcoRI-BamHI fragment of Cochlioborus heterostrophus GAPD promoter (466bp) was ligated into EcoRI and HindIII site from the multi-cloning site of pGEM-3ZF (Promega Corp.). Subsequently, the EcoRI and HindIII fragments were cloned between the EcoRI and HindIII sites of binary vector pBHt2 [8], named pSK1044 (Fig. 2 and Information S1).

ATMT of U. muehlenbergii

Initially, A. tumefaciens strain AGL-1 was transformed with binary vector pSK1044 (Figure 2). ATMT was carried out as previously described [57] with a few minor modifications. Cells of A. tumefaciens were grown at 28°C for 2 days in 1 ml minimal medium (MM) [62] supplemented with kanamycin (50 µg/ml). After transferring 100 µl bacterial cells into 900 µl of induction medium (IM) [4] containing 200 µM acetosyringone (AS), they were grown for 6 hrs at 28°C. For co-cultivation, freshly harvested cells of U. muehlenbergii were used. After mixing equal volumes of U. muehlenbergii (1×107 conidia/ml) and A. tumefaciens cells, 200 µl of the mix was spread on sterilized 0.45 µm pore cellulose membrane (cellulose nitrate, 47 mm diameter, Whatman Ltd, Maidstone, UK) overlaid on co-cultivation medium amended with 200 µM AS. Following co-cultivation at 23°C for 24 hrs or 36 hrs, the membranes were transferred to PDA amended with 20 µg/ml hygromycin B and 250 µg/ml cefatoxim to select transformants. Approximately one month later, individual colonies were transferred to 24-well plates (SPL, Korea) containing PDA. For isolation of pure culture, method for isolation of bacterial colonies was applied. All transformants were cultured in PDB, mixed with glycerol to make a 15% solution, and stored at –80°C for long-term storage. Colony color, size, and morphology of individual transformants were examined by culturing them on PDA.

Genomic DNA Isolation and Southern Analysis

Transformants and the wild-type strain were grown in 30 ml of PD broth at 15°C in an orbital shaker (200 rpm) for 7 days. Genomic DNA was extracted using DNAeasy mini kit. One microgram of DNA from each strain was digested with HindIII, a restriction enzyme that can be used to determine single copy integration of T-DNA, and fractionated in 0.7% agarose gels at 40 V for 6 hrs in 0.5% Tris-Acetate-EDTA buffer. Fractionated DNAs were transferred to Hybond N+ membrane (Amersham International, Little Chalfont, England) using 10× SSC (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate).

Hybridization of Southern blots was conducted using the hygromycin B resistance gene as a probe. The probe was prepared by PCR amplification of a 1.4 kb fragment from pBHt2 [8] using a primer pair, HYG_5F and HYG_3R (Table 2). The resulting PCR product was isolated from a gel using QIAquick spin columns (Qiagen, Valencia, CA, U.S.A.) to produce a probe, which was labeled with 32P-dCTP by random priming (Rediprime II DNA labeling system, GE Heathcare Life Sciences). Hybridization was performed at 65°C overnight in 6× SSPE (1× SSPE: 0.18 M NaCl, 1 mM EDTA, and 10 mM sodium phosphate (pH 7.4)) containing 1% sodium dodecyl sulfate (SDS) and 100 µg of denatured salmon sperm DNA per ml. After hybridization, blots were washed twice in 2× SSPE and 0.1% SDS for 5 min at 65°C. Signals were detected by autoradiography using BAS-MS imaging plate (Fuji Film).

Microscopy

To screen for expression of eGFP in transformants, conidial suspensions from randomly chosen transformants were dropped onto a slide glass and observed using a Zeiss Axio Imager A1 fluorescence microscope (Carl Zeiss, Oberkochen, Germany). A filter set with excitation at 470/40 nm and emission at 525/50 nm was used.

Scanning electron microscope observations of U. muehlenbergii hyphae and yeast-like cells (Figure 1) were carried out by fixing cultures with 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer (PB, pH7.2) for overnight at 4°C and then treating them with 1% OsO4 in PB for 1 hr at 4°C. The fixed specimens were dehydrated in an ascending series of ethyl alcohol and subsequently sputter-coated with OsO4. The specimens were imaged using a field emission scanning electron microscope (FE-SEM; S-4800; Hitachi High-Technologies, Tokyo, Japan) operating at 15–20 kV.

TAIL-PCR and Sequencing

TAIL-PCR was performed to isolate genomic DNA sequences flanking inserted T-DNA as described previously [52]. The secondary or tertiary PCR products were treated with using ExoSAP-IT® (USB, Cleveland, OH, USA) according to the manufacturer’s instruction and sequenced using primer RB3. Resulting sequences were analyzed through NCBI (http://www.ncbi.nlm.nih.gov/) to search for protein homology at the insertion site of T-DNA in each transformant.

Mitotic Stability of Transformants

To determine the mitotic stability of transformants, 24 randomly selected transformants were cultured on PDA without hygromycin B. Following five successive transfers, colonies were tested for growth on fresh PDA amended with hygromycin B (20 µg/ml).

Supporting Information

[Figure omitted. See PDF.]

Information S1.

pSK1044.

https://doi.org/10.1371/journal.pone.0083896.s001

(PDF)

Information S2.

Fifty full genome sequences with RB-border sequences are generated by TAIL-PCR.

https://doi.org/10.1371/journal.pone.0083896.s002

(PDF)

Author Contributions

Conceived and designed the experiments: SYP HYW JAK YHC S. Kang YHL JSH. Performed the experiments: SYP MHJ JAK NHY S. Kim. Analyzed the data: SYP YHC S. Kang YHL JSH. Contributed reagents/materials/analysis tools: SYP MHJ JAK NHY. Wrote the paper: SYP S. Kang YHL JSH.

Citation: Park S-Y, Jeong M-H, Wang H-Y, Kim JA, Yu N-H, Kim S, et al. (2013) Agrobacterium tumefaciens-Mediated Transformation of the Lichen Fungus, Umbilicaria muehlenbergii. PLoS ONE 8(12): e83896. https://doi.org/10.1371/journal.pone.0083896

References

1. Daboussi MJ, Djeballi A, Gerlinger C, Blaiseau PL, Bouvier I, et al. (1989) Transformation of seven species of filamentous fungi using the nitrate reductase gene of Aspergillus nidulans. Curr Genet 15: 453–456.

2. Leung H, Lehtinen U, Karjalainen R, Skinner D, Tooley P, et al. (1990) Transformation of the rice blast fungus Magnaporthe grisea to hygromycin B resistance. Curr Genet 17: 409–411.

3. Marmeisse R, Gay G, Debaud JC, Casselton LA (1992) Genetic transformation of the symbiotic basidiomycete fungus Hebeloma cylindrosporum. Curr Genet 22: 41–45.

4. Bundock P, den Dulk-Ras A, Beijersbergen A, Hooykaas PJ (1995) Trans-kingdom T-DNA transfer from Agrobacterium tumefaciens to Saccharomyces cerevisiae. EMBO J 14: 3206–3214.

5. de Groot MJ, Bundock P, Hooykaas PJ, Beijersbergen AG (1998) Agrobacterium tumefaciens-mediated transformation of filamentous fungi. Nat Biotechnol 16: 839–842.

6. Frandsen RJN (2011) A guide to binary vectors and strategies for targeted genome modification in fungi using Agrobacterium tumefaciens-mediated transformation. J of Microbiol Methods 87: 247–262.

7. Khang C, Park S, Lee Y, Kang S (2005) A dual selection based, targeted gene replacement tool for Magnaporthe grisea and Fusarium oxysporum. Fungal Genet Biol 42: 483–492.

8. Mullins E, Romaine CP, Chen X, Geiser D, Raina R, et al. (2001) Agrobacterium tumefaciens-mediated transformation of Fusarium oxysporum: An efficient tool for insertional mutagenesis and gene transfer. Phytopathology 91: 173–180.

9. Paz Z, Garcia-Pedrajas MD, Andrews DL, Klosterman SJ, Baeza-Montanez L, et al. (2011) One step construction of Agrobacterium-recombination-ready-plasmids (OSCAR), an efficient and robust tool for ATMT based gene deletion construction in fungi. Fungal Genet Biol 48: 677–684.

10. Islam MN, Nizam S, Verma PK (2012) A highly efficient Agrobacterium mediated transformation system for chickpea wilt pathogen Fusarium oxysporum f. sp. ciceri using DsRed-Express to follow root colonisation. Microbiol Res 167: 332–338.

11. Jeon J, Park SY, Chi MH, Choi J, Park J, et al. (2007) Genome-wide functional analysis of pathogenicity genes in the rice blast fungus. Nat Genet 39: 561–565.

12. Liu H, Bao X (2009) Overexpression of the chitosanase gene in Fusarium solani via Agrobacterium tumefaciens-mediated transformation. Curr Microbiol 58: 279–282.

13. Munch S, Ludwig N, Floss DS, Sugui JA, Koszucka AM, et al. (2011) Identification of virulence genes in the corn pathogen Colletotrichum graminicola by Agrobacterium tumefaciens-mediated transformation. Mol Plant Pathol 12: 43–55.

14. Kim HS, Park SY, Lee S, Adams EL, Czymmek K, et al. (2011) Loss of cAMP-dependent protein kinase A affects multiple traits important for root pathogenesis by Fusarium oxysporum. Mol Plant-Microbe Interact 24: 719–732.

15. Gouka RJ, Gerk C, Hooykaas PJ, Bundock P, Musters W, et al. (1999) Transformation of Aspergillus awamori by Agrobacterium tumefaciens-mediated homologous recombination. Nat Biotechnol 17: 598–601.

16. Hawksworth DL, Kirk PM, Sutton BC, Pegler DN (1995) Dictionary of the Fungi. Wallingford: CAB.

17. Lutzoni F, Pagel M, Reeb V (2001) Major fungal lineages are derived from lichen symbiotic ancestors. Nature 411: 937–940.

18. Huneck S (1999) The significance of lichens and their metabolites. Naturwissenschaften 86: 559–570.

19. Nishikawa Y, Ohno H (1981) Studies on the water-soluble constituents of lichens. IV. Effect of antitumor lichen-glucans and related derivatives on the phagocytic activity of the reticuloendothelial system in mice. Chem Pharm Bull (Tokyo) 29: 3407–3410.

20. Rezanka T, Dembitsky VM (2006) The colleflaccinosides, two chiral bianthraquinone glycosides with antitumor activity from the lichen Collema flaccidum collected in Israel and Russia. Nat Prod Res 20: 969–980.

21. Watanabe M, Iwai K, Shibata S, Takahashi K, Narui T, et al. (1986) Purification and characterization of mouse alpha 1-acid glycoprotein and its possible role in the antitumor activity of some lichen polysaccharides. Chem Pharm Bull (Tokyo) 34: 2532–2541.

22. Candan M, Yilmaz M, Tay T, Erdem M, Turk AO (2007) Antimicrobial activity of extracts of the lichen Parmelia sulcata and its salazinic acid constituent. Z Naturforsch C 62: 619–621.

23. Candan M, Yilmaz M, Tay T, Kivanc M, Turk H (2006) Antimicrobial activity of extracts of the lichen Xanthoparmelia pokornyi and its gyrophoric and stenosporic acid constituents. Z Naturforsch C 61: 319–323.

24. Cansaran D, Kahya D, Yurdakulola E, Atakol O (2006) Identification and quantitation of usnic acid from the lichen Usnea species of Anatolia and antimicrobial activity. Z Naturforsch C 61: 773–776.

25. Gambichler T, Skrygan M, Tigges C, Kobus S, Glaser R, et al. (2009) Significant upregulation of antimicrobial peptides and proteins in lichen sclerosus. Br J Dermatol 161: 1136–1142.

26. Garcia Rowe J, Garcia Gimenez MD, Saenz Rodriguez MT (1999) Some lichen products have antimicrobial activity. Z Naturforsch C 54: 605–609.

27. Gollapudi SR, Telikepalli H, Jampani HB, Mirhom YW, Drake SD, et al. (1994) Alectosarmentin, a new antimicrobial dibenzofuranoid lactol from the lichen, Alectoria sarmentosa. J Nat Prod 57: 934–938.

28. Ingolfsdottir K, Bloomfield SF, Hylands PJ (1985) In vitro evaluation of the antimicrobial activity of lichen metabolites as potential preservatives. Antimicrob Agents Chemother 28: 289–292.

29. Ivanova V, Backor M, Dahse HM, Graefe U (2010) Molecular structural studies of lichen substances with antimicrobial, antiproliferative, and cytotoxic effects from Parmelia subrudecta. Prep Biochem Biotechnol 40: 377–388.

30. Manojlovic NT, Vasiljevic PJ, Maskovic PZ, Juskovic M, Bogdanovic-Dusanovic G (2012) Chemical Composition, Antioxidant, and Antimicrobial Activities of Lichen Umbilicaria cylindrica (L.) Delise (Umbilicariaceae). Evid Based Complement Alternat Med 2012: 452431.

31. Mitrovic T, Stamenkovic S, Cvetkovic V, Tosic S, Stankovic M, et al. (2011) Antioxidant, antimicrobial and antiproliferative activities of five lichen species. Int J Mol Sci 12: 5428–5448.

32. Rankovic B, Rankovic D, Maric D (2010) Antioxidant and antimicrobial activity of some lichen species. Mikrobiologiia 79: 812–818.

33. Tay T, Turk AO, Yilmaz M, Turk H, Kivanc M (2004) Evaluation of the antimicrobial activity of the acetone extract of the lichen Ramalina farinacea and its (+)-usnic acid, norstictic acid, and protocetraric acid constituents. Z Naturforsch C 59: 384–388.

34. Turk AO, Yilmaz M, Kivanc M, Turk H (2003) The antimicrobial activity of extracts of the lichen Cetraria aculeata and its protolichesterinic acid constituent. Z Naturforsch C 58: 850–854.

35. Turk H, Yilmaz M, Tay T, Turk AO, Kivanc M (2006) Antimicrobial activity of extracts of chemical races of the lichen Pseudevernia furfuracea and their physodic acid, chloroatranorin, atranorin, and olivetoric acid constituents. Z Naturforsch C 61: 499–507.

36. Yilmaz M, Tay T, Kivanc M, Turk H, Turk AO (2005) The antimicrobial activity of extracts of the lichen Hypogymnia tubulosa and its 3-hydroxyphysodic acid constituent. Z Naturforsch C 60: 35–38.

37. Yilmaz M, Turk AO, Tay T, Kivanc M (2004) The antimicrobial activity of extracts of the lichen Cladonia foliacea and its (-)-usnic acid, atranorin, and fumarprotocetraric acid constituents. Z Naturforsch C 59: 249–254.

38. Harmala P, Hiltunen R, Oksman-Caldentey KM, Laakso T, Kauppinen V (1992) Isolation and in vitro cultivation of lichen algae and their antimicrobial properties. Fitoterapia 58: 217.

39. Bugni TS, Andjelic CD, Pole AR, Rai P, Ireland CM, et al. (2009) Biologically active components of a Papua New Guinea analgesic and anti-inflammatory lichen preparation. Fitoterapia 80: 270–273.

40. Tanas S, Odabasoglu F, Halici Z, Cakir A, Aygun H, et al. (2010) Evaluation of anti-inflammatory and antioxidant activities of Peltigera rufescens lichen species in acute and chronic inflammation models. J Nat Med 64: 42–49.

41. Nash ITH (1996) Lichen Biology; Nash THI, editor. Cambridge: Cambridge University Press.

42. Culberson CF, Armaleo D (1992) Induction of a complete secondary-product pathway in a cultured lichen fungus. Exp Mycol 16: 52–63.

43. Huneck S, Yaoshimura I (1996) Identification of Lichen Substances: Springer-Verlaq.

44. Gagunashvili AN, Davidsson SP, Jonsson ZO, Andresson OS (2009) Cloning and heterologous transcription of a polyketide synthase gene from the lichen Solorina crocea. Mycol Res 113: 354–363.

45. Simmenmann SJ, Andresson OS, Brown DW, Miao VPW (2000) Cloning and heterologous expression of Solorina crocea pyrG. Curr Genet 2000: 333–338.

46. Molina MC, Crespo A (2000) Comparison of development of axenic cultures of five species of lichen-forming fungi. Mycol Res 104: 595–602.

47. McDonald TR, Mueller O, Dietrich FS, Lutzoni F (2013) High-throughput genome sequencing of lichenizing fungi to assess gene loss in the ammonium transporter/ammonia permease gene family. BMC Genomics 14: 225.

48. Hirabayashi K, Iwata S, Ito M, Shigeta S, Narui T, et al. (1989) Inhibitory effect of a lichen polysaccharide sulfate, GE-3-S, on the replication of human immunodeficiency virus (HIV) in vitro. Chem Pharm Bull (Tokyo) 37: 2410–2412.

49. Nishikawa Y, Tanaka M, Shibata S, Fukuoka F (1970) Polysaccharides of lichens and fungi. IV. Antitumour active O-acetylated pustulan-type glucans from the lichens of Umbilicaria species. Chem Pharm Bull (Tokyo) 18: 1431–1434.

50. Stepanenko LS, Maksimov OB, Fedoreev SA, Miller GG (1998) Polysaccharides of lichens and their sulfated derivatives: antivial activity. Chem Nat Compd 34: 337–338.

51. Yamammoto Y, Kinoshita Y, Yoshimura I (2002) Culture of thallus fragments and redifferentiation of lichen; Kranner I, Beckett RP, Varma AK, editors. Berlin: Springer. 34–46 p.

52. Choi J, Park J, Jeon J, Chi MH, Goh J, et al. (2007) Genome-wide analysis of T-DNA integration into the chromosomes of Magnaporthe oryzae. Mol Microbiol 66: 371–382.

53. Bundock P, Hooykaas PJ (1996) Integration of Agrobacterium tumefaciens T-DNA in the Saccharomyces cerevisiae genome by illegitimate recombination. Proc Natl Acad Sci U S A 93: 15272–15275.

54. Meza TJ, Stangeland B, Mercy IS, Skarn M, Nymoen DA, et al. (2002) Analyses of single-copy Arabidopsis T-DNA-transformed lines show that the presence of vector backbone sequences, short inverted repeats and DNA methylation is not sufficient or necessary for the induction of transgene silencing. Nucleic Acids Res 30: 4556–4566.

55. Sha Y, Li S, Pei Z, Luo L, Tian Y, et al. (2004) Generation and flanking sequence analysis of a rice T-DNA tagged population. Theor Appl Genet 108: 306–314.

56. Marchand G, Fortier E, Neveu B, Bolduc S, Belzile F, et al. (2007) Alternative methods for genetic transformation of Pseudozyma antarctica, a basidiomycetous yeast-like fungus. J Microbiol Methods 70: 519–527.

57. Rho HS, Kang S, Lee YH (2001) Agrobacterium tumefaciens-mediated transformation of the plant pathogenic fungus, Magnaporthe grisea. Mol Cells 12: 407–411.

58. Chen EC, Su YH, Kanagarajan S, Agrawal DC, Tsay HS (2009) Development of an activation tagging system for the basidiomycetous medicinal fungus Antrodia cinnamomea. Mycol Res 113: 290–297.

59. Winans SC (1992) Two-way chemical signaling in Agrobacterium-plant interactions. Microbiol Rev 56: 12–31.

60. Maruthachalam K, Klosterman SJ, Kang S, Hayes RJ, Subbarao KV (2011) Identification of pathogenicity-related genes in the vascular wilt fungus Verticillium dahliae by Agrobacterium tumefaciens-mediated T-DNA insertional mutagenesis. Mol Biotechnol 49: 209–221.

61. Gustin MC, Albertyn J, Alexander M, Davenport K (1998) MAP kinase pathways in the yeast Saccharomyces cerevisiae. Microbiol Mol Biol Rev 62: 1264–1300.

62. Hooykaas PJJ, Roobol C, Schilperoort RA (1979) Regulation of the transfer of Ti-plasmids of Agrobacterium tumefaciens. J Gen Microbiol 110: 99–109.