Analysis of the Genome, Transcriptome and Secretome Provides Insight into Its Pioneer Colonization Strategies of Wood

Summary

The wood decay fungus Phlebiopsis gigantea degrades all components of plant cell walls and is uniquely able to rapidly colonize freshly exposed conifer sapwood. However, mechanisms underlying its conversion of lignocellulose and resinous extractives have not been explored. We report here analyses of the genetic repertoire, transcriptome and secretome of P. gigantea. Numerous highly expressed hydrolases, together with lytic polysaccharide monooxygenases were implicated in P. gigantea's attack on cellulose, and an array of ligninolytic peroxidases and auxiliary enzymes were also identified. Comparisons of woody substrates with and without extractives revealed differentially expressed genes predicted to be involved in the transformation of resin. These expression patterns are likely key to the pioneer colonization of conifers by P. gigantea.

Introduction

The most abundant source of terrestrial carbon is plant biomass, composed primarily of cellulose, hemicellulose, and lignin. Numerous microbes utilize cellulose and hemicellulose, but a much smaller group of filamentous fungi has the capacity to degrade lignin, the most recalcitrant component of plant cell walls. Uniquely, such ‘white-rot’ fungi efficiently depolymerize lignin to access cell wall carbohydrates for carbon and energy sources. As such, white-rot fungi play a key role in the carbon cycle.

White-rot basidiomycetes may differ in their substrate preference and morphological patterns of decay (for review see [1], [2]). The majority of lignin-degrading fungi, including Phanerochaete chrysosporium and Ceriporiopsis subvermispora, are unable to colonize freshly cut wood unless inhibitory compounds (extractives) are removed or transformed [2]–[5]. A few basidiomycetes, including Phlebiopsis gigantea, are pioneer colonizers of softwood because they tolerate and utilize resinous extractives (e.g., resin acids, triglycerides, long chain fatty acids, see Figure 1) which cause pitch deposits in paper pulp manufacturing [6]. It is this unusual capability that also led to the development of P. gigantea as a biocontrol agent against subsequent colonization of cut stumps by the root rot pathogen Heterobasidium annosum sensu lato (now considered several species) [7], [8] and of harvested wood by blue stain fungi [9], [10]. It seems likely that when applied to freshly cut wood, P. gigantea is able to rapidly metabolize accessible extractives and hemicellulose. As the hyphae continue to invade tracheids and ray parenchyma cells, the more recalcitrant cell wall polymers (cellulose, lignin; Figure 1) are eroded. Little is known of how some white-rot fungi degrade conifer extractives [11], [12] or interact with other fungi such as H. annosum [13].

White-rot fungi degrade major cell wall polymers through concerted action of hydrolytic and oxidative enzymes (reviewed in [14], [15]). Cellulose is attacked by a combination of exo-cellobiohydrolases and endoglucanases assigned to glycoside hydrolase families GH5, GH6, GH7 and possibly GH9, GH12, GH44 and GH45 [16], [17]. In addition to these hydrolases, recent evidence strongly supports the involvement of lytic polysaccharide monooxygenases (LPMOs) in cellulose degradation [18]–[20]. Lignin degradation is catalyzed by an array of oxidative enzymes, especially lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase (VP) belonging to class II of the plant-fungal-prokaryotic peroxidase superfamily. Recent genome investigations reveal that all efficient lignin degraders possess some combination of these class II ligninolytic peroxidases [21], [22]. In P. gigantea, four MnP sequences were previously identified [23].

In addition to peroxidases, laccases have been implicated in lignin degradation [24]–[26]. To date, multiple laccase isozymes and/or the corresponding genes have been characterized from most white-rot fungi except P. chrysosporium, an efficient lignocellulose degrader that lacks such enzymes [27]–[29]. The mechanism(s) by which laccases might degrade lignin remain unclear as the enzyme lacks sufficient oxidation potential to cleave non-phenolic linkages within the polymer. Interestingly, laccase activity has not been reported in P. gigantea.

Additional ‘auxiliary activities’ [30] commonly ascribed to ligninolytic systems include extracellular enzymes capable of generating H₂O₂. These enzymes may be physiologically coupled to peroxidases. Among them, aryl-alcohol oxidase (AAO), methanol oxidase (MOX), pyranose 2-oxidase (P2O), and copper radical oxidases (such as glyoxal oxidase, GLX) have been extensively studied. With the exception of P2O [31], none of these activities have been reported in P. gigantea cultures. In short, the repertoire of extracellular enzymes produced by P. gigantea is largely unknown, and its mechanism(s) for cell wall degradation remain unexplored.

Beyond extracellular systems, the complete degradation of lignin requires many intracellular enzymes for the complete mineralization of monomers to CO₂ and H₂O. Examples of enzymes that have been characterized from P. chrysosporium include cytochromes P450 (CYPs) [32]–[34], glutathione transferases [35], and aryl alcohol dehydrogenase (AAD) [36]. The role of such enzymes in P. gigantea, if any, is unknown.

Herein, we report analysis of the P. gigantea draft genome. Gene annotation, transcriptome analyses and secretome profiles identified numerous genes involved in lignocellulose degradation and in the metabolism of conifer extractives.

Results

Genome assembly and annotation

Following an assessment of wood decay properties (Figure 2), P. gigantea single basidiospore strain 5–6 was selected for sequencing using Illumina reads assembled with AllPathsLG. Genome size was estimated to be approximately 30 Mbp (Text S1), somewhat lower than closely related members of the ‘Phlebia clade’ [23], [37] such as C. subvermispora (39 Mbp) and P. chrysosporium (35 Mbp) [22], [27]. Aided by 17,915 mapped EST clusters, the JGI annotation pipeline predicted 11,891 genes. Proteins were assigned to 6412, 5615, 6932 and 2253 KOG categories, GO terms, pfam domains and EC numbers, respectively. Significant synteny with P. chrysosporium was observed (Figure S1). Detailed information on the assembly and annotations is available via the JGI portal MycoCosm [38].

Gene families

Principal component analysis (PCA), based on 73 and 12 families of carbohydrate active enzymes (CAZys, [16]) and auxiliary activities (AAs), [30]), respectively, clustered P. gigantea with other efficient lignin degraders ([39], Figures 3A and S2). Gene numbers were extracted from 21 fungal genomes and excluded genes encoding putative GMC oxidases such as methanol oxidase, alcohol oxidase and glucose oxidase (Dataset S1). Highest contribution of PC1 (50% of variance separating white-rot and brown-rot fungi) and PC2 (13.0% of variance)) values were those genes associated with degradation of plant cell wall polysaccharides and lignin, respectively (Text S1). Hierarchical clustering analysis with this dataset also categorized P. gigantea into a clade of white-rot fungi that included the polypore P. chrysosporium. The precise number and distribution of P. gigantea genes likely involved in lignocellulose degradation were similar, but not identical, to other polypores such as P. chrysosporium and C. subvermispora (Figure 4). Like P. chrysosporium and Phanerochaete flavido-alba, P. gigantea had no laccase sensu stricto genes. Interestingly, while both P. gigantea and the white-rot Russulales H. annosum are adapted to colonization of conifers, significant numbers of laccase sensu stricto genes were only observed in H. annosum (Figure 4). This important conifer pathogen also lacked GLX, LiP and representatives of GH5 subfamiles 15 and 31.

With regard to hemicellulose degradation, the genomes of conifer-adapted P. gigantea and H. annosum revealed increased numbers of genes involved in pectin degradation such as GH28 polygalacturonase, CE8 pectin methylesterase and CE12 rhamnogalacturonan acetylesterase (Figure 4). The major hemicellulose of conifer is galactoglucomannan ([40], Figure 1) but, in the case of mannan degradation, no significant increase in genes encoding GH2 β-mannosidase, GH5_7 endo-mannanase and GH27 α-galactosidase was observed relative to other wood decay fungi (Figure 4). Similarly, no significant differences in the number of genes involved in arabinoglucuronoxylan hydrolysis were identified, except for two transcriptionally convergent GH11 genes present in P. gigantea (Text S1). Encoding putative endo-1,4-β-xylanases, wood decay fungi typically harbor one or no GH11 genes. Auricularia delicata is another exception with three of these endoxylanases. Also unusual among white-rot fungi, none of the P. gigantea protein models were assigned to GH95 (Dataset S1). This family includes 1,2-α-fucosidases that hydrolyze the α-Fuc-1,2-Gal linkages in plant xyloglucans.

The P. gigantea genome includes representatives for all the peroxidase families reported in basidiomycetes, including LiP, MnP, heme-thiolate peroxidases, and dye-decolorizing type peroxidases (DyP), with the only exception of VP (Text S1; Figures S8–S13). MnP gene expansion is similar to that found in the C. subvermispora and H. annosum genomes. Beyond class II peroxidases and multicopper oxidases (MCOs), genes encoding auxiliary enzymes involved in ligninolysis were also found such as GMC oxidoreductases (Figures S14–S19; Table S5) and copper radical oxidases (CRO, Figure 4; Table S4). Among the latter group, GLX is coupled to P. chrysosporium LiPs via extracellular H₂O₂ generation [41]. Consistent with this physiological connection, the P. gigantea genome features both GLX- and LiP-encoding genes. GMC genes encoding putative AAO, MOX and glucose oxidase (GOX) may also be involved in H₂O₂ production by oxidation of low molecular weight aliphatic and aromatic alcohols. The P2O gene (protein model Phlgi1_130349) lies immediately adjacent to a putative pyranosone dehydratase (Phlgi1_16096) gene. This arrangement is conserved in several wood decay fungi and, in addition to peroxide generation, suggests a route for conversion of glucose to the pyrone antibiotic, cortalcerone [42], [43]. Genes encoding AAD, members of the zinc-type alcohol dehydrogenase superfamily [44], are also abundant in P. gigantea. Relatively few genes were predicted to encode CYPs which are generally considered important in the intracellular metabolism of lignin derivatives and related aromatic compounds (Figure S19; Dataset S2).

The repertoire of P. gigantea genes contrasts sharply with that of brown-rot polypores, such as Postia placenta [45], which lack ligninolytic class II peroxidases, cellobiohydrolases (GH6, GH7), and endoglucanases fused to cellulose binding modules [21], [46] (Figure 4). Unlike P. gigantea and other white-rot fungi, brown-rot fungi often lack genes encoding cellobiose dehydrogenase (CDH) and have relatively few lytic polysaccharide monooxygenase genes (LPMOs). Formerly classified as GH61 ‘hydrolases’, the LPMOs are now known to be copper-dependent monooxygenases [18]–[20] capable of enhancing cellulose attack by CDH and cellobiohydrolase (CBH) [47], [48]. With the exception of Gloeophyllum trabeum, genes encoding GH74 enzymes have not been found in brown-rot fungi. Two such xyloglucanase genes were identified in P. gigantea (Text S1).

In contrast to analysis of genes involved in lignocellulose degradation (Figure 3A), white-rot and brown-rot fungi were not clearly separated by principal component analysis of 14 enzymes involved in lipid metabolism (Figures 3B and S3). However, P. gigantea was grouped near B. adusta and P. carnosa. These associations seem in line with the preferential colonization of softwood substrates by P. carnosa [49] and with the efficient degradation of conifer extractives by B. adusta culture supernatants [50].The highest contribution to PC1 (26.0% variance) and PC2 (6.8% variance) were aldehyde dehydrogenase and long chain fatty acid CoA ligase, respectively (Figures 3A and S3, Text S1). Also potentially involved in intracellular lipid metabolism, CYP52 and CYP505 clans of cytochrome P450s are associated with degradation of fatty acids and alkanes. Relative to other white-rot fungi, P. gigantea had a slightly greater number of CYP52-encoding genes whereas CYP505 gene numbers were similar (Figure 4; Dataset S1; Figures S31, S32; Tables S13–S15).

P. gigantea also diverges from other Agaricomycetes with respect to genes encoding proteins that are more distantly connected to lignocellulose degradation, including hydrophobins (Figures S33 and S34; Tables S17–S19), transporters (Table S20) and non laccase MCOs (Figure S20). Detailed analyses are provided for CAZys (Tables S7–S10; Figures S22–S30; Dataset S1), peroxidases (Figures S8–S13), auxiliary proteins, cytochrome P450s (Figures S31–S32; Table S13–S15), potential regulatory genes (Figures S4–S7; Tables S3, S11–S12) and genes involved in secondary metabolite synthesis (Table S16).

Differential gene expression of P. gigantea in response to substrate

Transcript levels were determined in cultures in which the sole carbon source was glucose (Glc), freshly harvested loblolly pine wood (Pinus taeda; LP) extracted with acetone (ELP), or freshly harvested but not extracted loblolly pine wood (NELP) (Text S1). GC-MS analysis [51] identified the major extract components as resin acids (46%), triglycerides (13%) and fatty acids (11%) (Text S1; Figure S35; Table S21).

Excluding genes with relatively low transcript levels (RPKM values <10) in LP-containing media, transcripts of 187 genes were increased>2-fold (p<0.05) in NELP or ELP relative to Glc. Of those Glc-derived transcripts with RPKM values>10, 146 genes had higher transcripts in Glc relative to NELP or ELP (Figure 5; Dataset S2).

Mass spectrometry (nanoLC-MS/MS) identified extracellular peptides corresponding to a total of 319 gene products in NELP and ELP cultures (Dataset S2). Most proteins were observed in both NELP and ELP culture filtrates, which contained 294 and 268 proteins, respectively. Approximate protein abundance, expressed as the exponentially modified protein abundance index (emPAI) [52], varied substantially within samples. As expected, gene products with predicted secretion signals and high transcript levels were often detected. Other detected proteins (e.g. MOX model Phlgi1_120749; [53]) may be loosely associated with cell walls and/or secreted via ‘non-classical’ mechanisms ([54]; www.cbs.dtu.dk/services/SecretomeP). Still other peptides correspond to true intracellular proteins released by cell lysis, e.g. ribosomal proteins (Dataset S2).

Glycoside hydrolase gene expression was heavily influenced by media composition with transcripts corresponding to 76 genes increasing>2-fold in NELP- or ELP-containing media relative to glucose medium (Figure 6). Some of these genes were highly expressed with RPKM values well over 100. For example, transcript and peptide levels matching GH7 cellobiohydrolase (CBH1; model Phlgi1_34136) were among the ten most highly expressed genes (Table 1). Indicative of a complete cellulolytic system, this CBH1 was accompanied by upregulated transcripts and extracellular proteins corresponding to another CBH1 (Phlgi1_13298), a GH6 family member CBH2 (Phlgi1_17701) and GH5_5 β-1,4 endoglucanases (EGs; Phlgi1_86144, Phlgi1_84111), all of which feature a family 1 carbohydrate binding module (CBM1). Also highly expressed were putative β-glucosidases (Phlgi1_127564, Phlgi1_18210) and a GH12 (Phlgi1_34479). Other glycoside hydrolases likely involved in degradation of cell wall hemicelluloses include GH5_7 endomannanases (Phlgi1_97727, Phlgi1_110296), a GH74 xyloglucanase (Phlgi1_98770), a GH27 α-galactosidase (Phlgi1_72848) and a GH10 endoxylanase (Phlgi1_85016).

Expression of oxidative enzymes implicated in lignocellulose degradation was also influenced by growth on LP-media (NELP or ELP) relative to Glc-containing media. Transcripts corresponding to five LPMO-encoding genes showed significant regulation (P<0.01) in LP-medium, and three LPMO proteins were detected (Phlgi1_227588, Phlgi1_227560, Phlgi1_37310). An AAD-like oxidoreductase (Phlgi1_30343), possibly involved in the transformation of lignin metabolites, was also upregulated. However, we did not observe high expression of class II peroxidases under the conditions tested (Dataset S2). On the other hand, a DyP (Phlgi1_85295) was significantly upregulated in certain LP-containing media (Table 1). The importance of these peroxidases is further supported by the high protein levels of another DyP, Phlgi1_122124. Specifically, the latter protein showed emPAI values>17 after 5 days growth on LP media and, relative to Glc medium, its transcript ratios were>5-fold higher (p<0.04) (Dataset S2). High DyP gene expression has been observed in white-rot fungi Trametes versicolor and Dichomitus squalens [21], but no genes for these proteins are present in P. chrysosporium and C. subvermispora (Figure 4). The P. gigantea DyP (Phlgi1_122124) was also abundant in media containing microcrystalline cellulose (Avicel) as the sole carbon source (Dataset S2).

To identify enzymes involved in tolerance to and/or degradation of extractives, comparisons were made of gene expression in ground loblolly pine wood that had been extensively extracted with acetone (ELP) versus non-extracted loblolly pine wood (NELP) (Figure 7A). In general, this treatment had little impact on gene expression. For example, glycoside hydrolase transcript and protein patterns showed only minor differences (Figure 8). Nevertheless, transcripts corresponding to 22 genes showed significantly increased levels (>4-fold; p<0.01) in NELP relative to ELP (Figure 7B; Table 2). Of particular interest were genes potentially involved in metabolism of resin acids (e.g. CYPs; [55]), in altering the accessibility of cell wall components (e.g., an endoxylanase), and in regulating gene expression (e.g. proteins containing putative Zn finger domains or HMG-Box transcription factors). Integration of transcript profiles of genes involved in intracellular lipid and oxalate metabolism, together with TCA and glyoxylate cycles, strongly supports a central role for β-oxidation in triglyceride and terpenoid transformation by P. gigantea (Figure 9).

Relaxing the transcript fold-change threshold (>2-fold; p<0.01) and focusing on mass spectrometry-identified proteins revealed 14 additional genes potentially involved in metabolism and/or tolerance to loblolly pine wood extractives (Table 3).Among these extract-induced genes, lipases Phlgi1_19028 and Phlgi1_36659 likely hydrolyze the significant levels of triglycerides. The substrate specificity of aldehyde dehydrogenases such as Phlgi1_115040 is difficult to assess based on sequence, although several have been implicated in the degradation of pine wood resins by bacteria [56]. Secretome patterns in media containing microcrystalline cellulose (Avicel) as sole carbon source generally supported the importance of the same proteins in the metabolism of pine wood extractives (Table 3, Dataset S2). Specifically, lipases Phlgi1_19028 and Phlgi1_36659 and aldehyde dehydrogenase Phlg1_115040 were more abundant in loblolly pine wood and in Avicel media relative to the same media without extractives. The role of peroxiredoxin (Phlgi1_95619) and glutathione S-transferase (Phlgi1_113065) are less clear, but transformations involving H₂O₂ reduction and glutathione conjugation are possible. A single MCO (Phlgi1_129839) and its corresponding transcripts, were observed to be upregulated in ELP relative to NELP. Although lacking the L2 signature common to laccases [57], the MCO4 protein may have iron oxidase activity provided that an imperfectly aligned Glu residue serves in catalysis (Text S1; Figures S20 and S21; Table S6).

Discussion

The distinctive repertoire and regulation of P. gigantea genes underlie a unique and efficient system for degrading all components of conifer sapwood. Transcriptome and proteome analyses demonstrate an active system of hydrolases and LPMOs involved in the complete deconstruction of cellulose and hemicellulose. The overall enzymatic strategy is therefore similar to most cellulolytic microbes, but unlike closely related brown-rot decay Agaricomycetes such as P. placenta.

With regard to ligninolysis, key genes were identified including LiPs, MnPs, CROs and GMC oxidoreductases. As in P. chrysosporium, the presence of both LiP- and GLX-encoding genes is consistent with a close physiological connection involving peroxide generation [41]. We also annotated non-class II peroxidases HTPs and DyPs some of which have been implicated in metabolism of lignin derivatives [58], [59]. The distribution and expression of DyP-encoding genes are notable; with no genes present in P. chrysosporium and C. subvermispora but several highly expressed genes in T. versicolor, D. squalens [21] and P. gigantea (Table 2). Physiological roles of DyP are likely diverse, but oxidation of lignin-related aromatic compounds has been demonstrated [59].

In addition to lignin, oxidative mechanisms likely play a central role in P. gigantea cellulose attack. Of 15 LPMO-encoding genes, transcripts of six genes were regulated (>2-fold; p<0.01) and peptides corresponding to three were unambiguously identified in NELP- or ELP-containing media. Our inability to detect any LPMO proteins in Avicel media (Dataset S2) suggests induction by substrates other than cellulose [60]. Beyond this, the CDH gene was highly expressed (transcripts and protein) in LP media. The observed coordinate expression of CDH and LPMO may reflect oxidative ‘boosting’ as recently demonstrated [19], [20], [47], [61]. However, we did not detect elevated transcripts or peptides corresponding to the two P. gigantea aldose 1-epimerase genes even though these were previously observed in culture filtrates of various white-rot fungi [21], [62], including Bjerkandera adusta, Ganoderma sp, and Phlebia brevispora [17]. Thus, it seems unlikely that enzymatic conversion of oligosaccharides to their β-anomers is necessary for efficient CDH catalysis.

Softwood hemicellulose composition typically includes 15-20% galactoglucomannan while hardwoods contain 15–30% glucuronoxylan [40]. Consistent with an adaption to conifer hemicellulose, GH5_7 β-mannanases were highly expressed in both NELP and ELP cultures, together with a GH27 α-galactosidase (Table 1). GH11 endoxylanase and CE carbohydrate esterase peptides were also detected in the pine wood-containing media, but not in Avicel cultures (Dataset S2). In aggregate, these results demonstrate P. gigantea adaptation to conifer hemicellulose degradation.

P. gigantea's gene expression patterns reveal multiple strategies for overcoming the challenging composition of resinous sapwood. Tolerance to monoterpenes may be mediated in part by a putative ABC efflux transporter (Phlbi1_130987, Figure 9). Of the 51 ABC proteins of P. gigantea, this protein is most closely related to the GcABC-G1 gene of the ascomycete Grosmannia clavigera, a pathogen of Pinus contorta [63]. The GcABC-G1 gene is upregulated in response to various terpenes and appears to be a key element against the host defenses. Consistent with a similar function, our analysis showed the P. gigantea homolog to be upregulated>4.9-fold (p = 0.02) in NELP relative to ELP media (Dataset S2). Other transcripts accumulating in NELP-derived mycelia included three CYPs (Table 2) potentially involved in the hydroxylation of diterpenoids and related resin acids [55]. Differential regulation also implicates glutathione S-transferase, aldehyde dehydrogenase and peroxiredoxin in the transformation and detoxification of extractives (Table 2). Three putative transcription regulators were similarly regulated (Table 3). Aldehyde dehydrogenase- and AAD-encoding genes, some of which are upregulated in P. gigantea LP cultures relative to Glc cultures (Tables 1), are induced by aromatic compounds in P. chrysosporium [64], [65].

Predicted to degrade triglycerides, a total of nine lipase-encoding genes were identified in the P. gigantea genome and four of these were upregulated>2-fold (p<0.01) in LP media compared to Glc medium (Dataset S2). Two lipases displayed similar patterns of transcript and protein upregulation on NELP relative to ELP (Table 3), and the pine wood extractive also induced accumulation of these lipases in Avicel media (Table 3). Further metabolism of triglycerides is uncertain, although a putative glycerol uptake facilitator (Phlbi1_99331) was highly expressed (RPKM value of 2532) and significantly (p<0.02) upregulated (2.1-fold) in NELP compared to ELP (Dataset S2). Fatty acids activation and β-oxidation can be inferred by the expression of fatty acid CoA ligase (Phlgi1_107548, Phlgi1_126556, Phlgi1_89325), β-ketothiolase (Phlgi1_27649, Phlgi1_130767), and fatty acid desaturase (Phlgi1_100083, Phlgi1_115799). Upregulation of a mitochondrial malate dehydrogenase (Phlgi1_22176, Table 3), together with relatively high transcript levels of other TCA cycle components (citrate synthases Phlgi1_126205, Phlgi1_100215; 2-oxoglutarate dehydrogenase, Phlgi1_126652) may complete fatty acid oxidation. In this connection, we also observed high expression of isocitrate lyase (Phlgi1_21482, Phlgi1_93159) and malate synthase (Phlgi1_27815), which partially explain oxalate accumulation [66] and strongly support an active glyoxylate shunt [45], [67] (Figure 9). Upregulation of glycoside hydrolases, transcription factors, cyclophilins, ATP synthase and ribonuclease may also reflect broad shifts in metabolism or reduced accessibility of the unextracted substrate (Tables 2 and 3).

Beyond genetic regulation, certain constitutively expressed genes are also likely involved in the degradation of all plant cell wall components, including complex resins and triglycerides. For example MOX (Phlgi1_120749) is among the most abundant transcripts in both NELP and ELP (Dataset S2), suggesting an important role in H₂O₂ production associated with lignin demethylation [53]. Extracellular peroxide generation is key to peroxidase activity, and MOX fulfills this role along with CRO, AAO, and P2O. Along these lines, we also observed high extracellular protein levels of DyP (Phlgi1_122124) under all culture conditions.

Most problematic, many P. gigantea genes and proteins exhibited little or no homology to NCBI NR or Swiss-Prot entries. Some of these ‘hypothetical’ or ‘uncharacterized’ proteins are undoubtedly important, particularly those that are highly expressed, regulated and/or secreted. For example, of 92 genes upregulated (>2-fold; p<0.01) in NELP relative to ELP, 51 were designated as hypothetical (Table 2; Dataset S2). Three of these featured predicted secretion signals and peptides were detected in one case. In the absence of biochemical characterization and/or genetic evidence, assigning function to these genes represents a major undertaking. Nevertheless, high throughput transcript and secretome profiling substantially filtered the number of potential targets from a genome-wide estimate of 4744 ‘hypothetical’ genes to the more manageable numbers reported here. More broadly, the results advance understanding of the early and exclusive colonization of coniferous wood by P. gigantea and also provide a framework for developing effective wood protection strategies, improving biocontrol agents and identifying useful enzymes [6], [9], [10].

Methods

Wood colonization assays

Wood wafers (1 cm by 1 cm by 2 mm) were cut from the sapwood of aspen (Populus tremuloides), pine (P. taeda) and spruce (Picea glauca) and sterilized by autoclaving. Following inoculation by contact with mycelium growing on malt extract agar (15 g malt extract [Difco, Detroit, MI] and 15 g agar per liter of water) in Petri dishes, colonized wafers were harvested 30, 60 and 90 days. Noninoculated wood wafers placed on the same media in Petri dishes served as controls. Wafers were removed 30, 60 or 90 days later, weighed and percent weight loss was determined. Additional wafers were removed at the same time period, immediately frozen to −20°C and prepared for scanning electron microscopy as previously described [68].

Sequencing and annotation

The genome was sequenced using Illumina and annotated using the JGI Annotation Pipeline [69]. Assembly and annotations are available from JGI portal MycoCosm [38] and deposited to DDBJ/EMBL/GenBank under accession AZAG00000000. The version described in this paper is version AZAG01000000. The completeness of the P. gigantea genome was assessed by finding 99.1% of CEGMA proteins conserved across sequenced genomes of eukaryotes [70](Text S1; Tables S1, S2).

RNA-seq

Mycelium was derived from triplicate cultures of 250 ml basal salts containing: i. 1.25 g freshly-harvested, ground (1mm mesh) loblolly pine wood that had been ‘spiked’ with acetone and thoroughly dried (NELP); or ii. the same material following extended acetone extraction in a Soxhlet apparatus and drying (ELP). The composition of the extract (Text S1) was determined by GC-MS [51]. Duplicate cultures of basal salts medium with glucose as sole carbon source served as a reference. After 5 days incubation, total RNA was purified from frozen mycelium as described [22], [71]. Multiplexed libraries were constructed and sequenced on an Illumina HiSeq2000. DNAStar Inc (Madison, WI) modules SeqNGen and Qseq were used for mapping reads and statistical analysis. Transcriptome data was deposited to the NCBI Gene Expression Omnibus (GEO) database and assigned accession GSE53112 (Reviewer access via http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?token=ilovmswixtajjez&acc=GSE53112). Experimental details are provided in Text S1 and all transcriptome analyses are summarized in Dataset S2.

Secretome analysis

With minor modification, NanoLC-MS/MS analysis identified extracellular proteins in culture filtrates as described [22], [72]. For each of the two woody substrates (e.g NELP and ELP), cultures were harvested after 5, 7 and 9 days. Mass spectrometric protein identifications were accepted if they could be established at greater than 95.0% probability within 0.9% False Discovery Rate and contained at least two identified peptides. Protein probabilities were assigned by the Protein Prophet algorithm [73]. To verify the effects of pine wood extractives in a well-defined substrate, media containing microcrystalline cellulose (Avicel) were also employed [22], [45], [74]. Filtrates from these cultures, with or without addition of loblolly pine wood acetone extract, were collected after 5 days and analyzed. Approximate protein abundance in each of the cultures was expressed as the number of unique peptide and the exponentially modified protein abundance index (emPAI) value [52] (See Text S1 for detailed methods).

Supporting Information

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