Combined de novo and genome guided assembly and annotation of the Pinus patula juvenile shoot transcriptome

Abstract

Background

Pines are the most important tree species to the international forestry industry, covering 42 % of the global industrial forest plantation area. One of the most pressing threats to cultivation of some pine species is the pitch canker fungus, Fusarium circinatum, which can have devastating effects in both the field and nursery. Investigation of the Pinus-F. circinatum host-pathogen interaction is crucial for development of effective disease management strategies. As with many non-model organisms, investigation of host-pathogen interactions in pine species is hampered by limited genomic resources. This was partially alleviated through release of the 22 Gbp Pinus taeda v1.01 genome sequence (http://pinegenome.org/pinerefseq/) in 2014. Despite the fact that the fragmented state of the genome may hamper comprehensive transcriptome analysis, it is possible to leverage the inherent redundancy resulting from deep RNA sequencing with Illumina short reads to assemble transcripts in the absence of a completed reference sequence. These data can then be integrated with available genomic data to produce a comprehensive transcriptome resource. The aim of this study was to provide a foundation for gene expression analysis of disease response mechanisms in Pinus patula through transcriptome assembly.

Results

Eighteen de novo and two reference based assemblies were produced for P. patula shoot tissue. For this purpose three transcriptome assemblers, Trinity, Velvet/OASES and SOAPdenovo-Trans, were used to maximise diversity and completeness of assembled transcripts. Redundancy in the assembly was reduced using the EvidentialGene pipeline. The resulting 52 Mb P. patula v1.0 shoot transcriptome consists of 52 112 unigenes, 60 % of which could be functionally annotated.

Conclusions

The assembled transcriptome will serve as a major genomic resource for future investigation of P. patula and represents the largest gene catalogue produced to date for this species. Furthermore, this assembly can help detect gene-based genetic markers for P. patula and the comparative assembly workflow could be applied to generate similar resources for other non-model species.

Background

Pinus species play keystone ecological roles, representing the major component of many forests across all continents [1]. These species are also the predominantly planted trees in the global commercial forestry sector [2]. One of the largest threats to global pine forestry is the pitch canker fungus Fusarium circinatum, especially where susceptible Pinus species are cultivated [3]. Consequent losses caused by this fungus have large economic impacts on commercial forestry [3, 4]. Resistance to F. circinatum varies among Pinus species [5]. Species such as Pinus patula and P. radiata, both of which are important plantation species in southern Africa, are highly susceptible, while species such as P. tecunumanii are more resistant [5]. Very little is known regarding the interaction between F. circinatum and its Pinus hosts at the molecular level. Investigation of the different responses employed by susceptible and resistant hosts, such as P. patula and P. tecunumanii [5], will improve our knowledge of responses necessary for effective defence against F. circinatum.

RNA sequencing (RNA-seq) approaches have opened the way for transcriptome-wide analysis of gene expression [6]. Accurate quantification of gene expression using RNA-seq, however, requires a high quality reference sequence for read mapping. For organisms with well characterised reference genomes, such as Arabidopsis, this requirement is easily met, while organisms lacking a well characterized reference sequence present numerous challenges. Although the P. taeda v1.01 draft genome assembly is available [7], the size and fragmented state of the assembly can limit comprehensive transcriptome analysis [8, 9]. De novo transcriptome assembly can be used to provide a reference sequence for RNA-seq analysis while circumventing potential issues arising from problems in a genome assembly [10]. De novo transcriptome assemblies are available through GenBank and the TreeGenes database [11, 12] for at least ten Pinus species (P. banksiana [13], P. contorta [13], P. lambertiana, P. massoniana, P. monticola [14], P. palustris, P. pinaster, P. radiata, P. sylvestris [15], and P. taeda), at various levels of completion. Of these, the P. taeda transcriptome is the most comprehensive, consisting of data obtained from many different tissues and developmental stages (Mockaitis et al. unpublished).

The aim of this study was to generate a resource for transcriptome profiling in P. patula by assembling the shoot transcriptome of this economically important species. We report a P. patula shoot transcriptome containing 52 112 transcripts, of which 30 844 (60 %) are annotated. This is the largest gene catalogue for P. patula to date and a major genomic resource, which will facilitate functional genomics research in this tropical pine species.

Methods

Plant material

Six month old P. patula seedlings, from a single open pollinated family, were sourced from Top Crop Nursery, South Africa. Seedlings were transferred to, and maintained for the duration of the trial in an environmentally controlled glasshouse at 25–28 °C without supplemental lighting and allowed to acclimatize for two weeks. F. circinatum isolate FCC3579 was cultured on ½ strength potato dextrose agar (½ PDA; Merck) after which spores were harvested by washing with 15 % (v/v) sterile glycerol. Spore concentration was quantified using a haemocytometer and diluted to 5×10⁴ spores/mL by addition of 15 % (v/v) sterile glycerol. Seedlings were inoculated by clipping the apical bud and pipetting 10 μL of diluted spore solution onto the wound [16]. Seedlings inoculated with 10 μL sterile 15 % glycerol served as mock-inoculated control. Shoot tissue was harvested one day post inoculation (dpi) for three biological replicates per group (inoculated and mock-inoculated). Each biological replicate consisted of the top 4 cm of shoot tissue, measured from the wounded apical bud, harvested from 16 seedlings and pooled prior to being frozen using liquid nitrogen. Frozen tissue was stored at −80 °C until use. Disease development was monitored for six weeks post inoculation by measuring lesion and stem length from the wounded apical bud on 52 plants per group. F. circinatum infection was confirmed based on culture morphology on ½ PDA by re-isolation using tissue harvested from inoculated plants 14 dpi.

RNA isolation and sequencing

Frozen samples were homogenised using a high speed grinder (IKA-Werke, Staufen, Germany) and total RNA extracted using a modified version of Lewinsohn’s protocol [17]. Modifications were as follows: All solutions were prepared using diethylpyrolecarbonate (DEPC) treated water. Approximately 5 g homogenised shoot tissue was placed in a 50 mL conical tube containing 150 mg PVP-360 and 300 mg PVPP before adding 15 mL chilled extraction buffer. Tubes were mixed by vortexing, snap frozen in liquid nitrogen and allowed to thaw on ice. Polysaccharides were precipitated by addition of 1/9^th volume 3.3 M sodium acetate and 10 % (v/v) absolute ethanol. Nucleic acids were precipitated at −20 °C for 4 h. The pellet produced from ultracentrifugation was re-suspended in 100 μL DEPC treated water and stored at −80 °C until use. Total RNA samples were treated with RNase-free DNaseI enzyme (Qiagen Inc, Valencia, CA) to digest genomic DNA and purified using the RNeasy® MinElute kit (Qiagen) according to the manufacturer’s instructions. Concentration and integrity of purified RNA samples were evaluated using a Bio-Rad Experion™ automated electrophoresis system (Bio-Rad Laboratories, Hercules, CA, USA).

High quality RNA samples (RNA Integrity Number > 7.5) for both groups were sequenced using Illumina HighSeq 2000 instruments (200 bp insert size, 101PE sequencing, 40 million reads per sample; BGI, Hong Kong). Sequence quality of raw RNA-seq data was assessed using FastQC v0.10.1 [18]. Quality trimming and filtering of data was performed using Sickle v1.210 [19] and all unpaired reads were discarded. Short reads (<40 bp) were removed from the filtered RNA-seq reads using SolexaQA LengthSort [20]. The trimmed and filtered read data for all six samples were combined, resulting in Dataset 1. FastUniq v1.1 [21] was used to reduce PCR artefacts from Dataset 1 by removing duplicate reads, resulting in Dataset 2.

Transcriptome assembly

Multiple k-mer de novo transcriptome assembly

De novo transcriptome assembly was performed using three assemblers; Trinity r2013-11-10 [22], SOAPdenovo-Trans v1.04 [23] and Velvet v1.2.10/ Oases v0.2.08 [24, 25]. Assembly with Trinity was performed on both datasets using default parameters [26], except min_contig_length = 350, and repeated on Dataset 1 with the CuffFly parameter included. Trinity was applied to both Dataset 1 and 2 as Trinity allows for duplicate reads, however, SOAPdenovo-Trans and Velvet/Oases assemblers were used on Dataset 2 only, where duplicates were removed. Assembly with SOAPdenovo-Trans was performed on Dataset 2, for eight different k-mer lengths (23, 25, 31, 39, 47, 55, 63 and 71), with the parameters as follows: max_rd_len = 100, rd_len_cutoff = 100, avg_ins = 200, reverse_seq = 0, asm_flags = 3, pair_num_cutoff = 3, map_len = 35, −f and -F. Assembly with Velvet/Oases was performed on Dataset 2, for seven different k-mer lengths (23, 25, 31, 39, 47, 55, and 63), with the parameters as follows: default parameters for velveth; cov_cutoff = 10, ins_length = 200 and read_trkg = yes for velvetg; cov_cutoff = 10, min_pair_count = 5 and min_trans_lgth = 350 for Oases.

P. taeda v1.01 genome guided transcriptome assembly

Trinity genome guided transcriptome assembly was performed on Datasets 1 and 2 using the P. taeda v1.01 draft genome assembly (ca. 14.4 million scaffolds) with a minimum contig length of 350 bp. GSNAP 2014-02-28 (Genomic Short-read Nucleotide Alignment Program) [27] was used to align reads to the reference genome for transcriptome assembly using the following paramters: −-nofails, −-novelsplicing = 1, −-localsplicedist = 250000, −-npaths = 20. Transrate v0.3.1 [27] was used to calculate assembly quality metrics.

Decreasing redundancy across assemblies

The de novo and genome guided transcriptome assemblies were combined to form a redundant over-assembly. The tr2aacds pipeline, from the EvidentialGene package [28], was used to reduce redundancy in the over-assembly by selecting for the ‘optimal’ set of assembled transcripts based on coding potential. The pipeline consists of five steps: (1) prediction of coding DNA sequence (CDS) and amino acid sequences for each transcript, (2) removal of redundant sequences based on coding potential among identical sequences, (3) substring de-replication to remove sequence fragments, (4) clustering of highly similar sequences into loci and (5) classification of transcripts as ‘primary’ or ‘alternate’ and discarding of low scoring ‘drop’ transcripts. The primary assembled transcripts were used for further assessments.

Annotation

Local alignments to the National Centre for Biotechnology Information (NCBI) non-redundant (nr) and plant protein databases were generated for the primary assembled transcripts from the tr2aacds pipeline using uBLAST (Edgar RC, unpublished) [29]. Parameters used for local alignments were: −evalue 1e-10, −weak_evalue 1e-4, −id 0.9, −weak_id 0.8. Local alignment sequence descriptions were used to remove non-pine origin sequences, sequences with significant alignments to prevalent fungal, bacterial, viral and insect sequences, from the assembly to produce the P. patula v1.0 draft transcriptome assembly. Blast2GO® v2.7.2 [30] was used to predict protein domains through InterProScan 5 [31] as well as to perform Gene Ontology (GO) and Enzyme Code (EC) mapping. The P. patula transcriptome GO distribution was compared to the P. taeda v1.01 draft genome annotation using CateGOrizer [32]. Gene family memberships among species were visualized using custom scripts and Venn diagrams (http://bioinformatics.psb.ugent.be/webtools/Venn/).

Identification of orthologous protein groups

Annotated protein sequences for ten different species were retrieved from version 2.5 of the PLAZA protein database [33] and four external proteins sets, from conifer species, were also included (Table 1). The complete set of predicted P. patula v1.0 proteins from the assembled transcriptome were included. Each of the 15 protein sets were clustered to 90 % identity within species and combined. Gene families were identified and annotated for the 442 372 sequences using the approach described in [8]. Pfam domains [34] were assigned to the P. patula sequences using InterProScan 5.7 [31]. Identified gene families unique to P. patula with fewer than 5 members were discarded as these could result from assembly artefacts.

Table 1 Protein sets used for analysis of orthologous genes

Assembly validation

The Core Eukaryotic Genes Mapping Approach (CEGMA) pipeline [35] as well as the Benchmarking Universal Single-Copy Orthologs (BUSCO) v1.1b1 tool [36] were used to identify putative core eukaryotic genes (CEGs) and universal single copy orthologs (USCOs) in the assembly as a measure of the completeness and contiguity. BUSCO analysis was performed using the early access plant dataset. In addition, conditional reciprocal best BLAST (CRBB) analysis of the P. patula draft transcriptome assembly, the P. taeda v1.01 gene models and the P. taeda draft transcriptome assembly was implemented with two different sets of reference sequences using Transrate [27]. Reference sets used were as follows: the P. taeda v1.01 predicted gene models available through the TreeGenes Database [11, 12] and the 87 P. patula protein sequences available through the NCBI and TrEMBL databases.

Sequence alignments against the P. taeda v1.01 draft genome assembly were generated to compare transcript to genome mapping of the P. patula v1.0 transcriptome assembly to that of other Pinus transcriptomes. Comparative alignments were produced using transcriptome data for seven other Pinus spp. available from the TreeGenes database [11, 12]: P. taeda (83 285 sequences), P. banksiana (21 675), P. contorta (14 375), P. pinaster (14 130), P. palustris (14 228), P. lambertiana (48 891), and P. radiata (4 742). Transcript sequences were aligned to the reference genome using GMAP 2014–02-28 (Genomic Mapping and Alignment Program) [37] with the following parameters: −-intronlength = 350000, −-no-chimeras, −-canonical-mode = 1, −-cross-species. The ‘--cross-species’ parameter was excluded for alignment of the P. taeda transcriptome. Sequence alignments were examined at two different cut-offs, the first (95 % identity, 95 % coverage) to compare mapping between species and the second (95 % identity, 50 % coverage) to account for possible effects due to genome fragmentation. The P. patula v1.0 transcriptome assembly was further validated by alignment to full-length Sanger sequenced P. taeda cDNA reference sequences present in NCBI and obtained through the TreeGenes database [11]. The 188 cDNA sequences were clustered to 90 % identity. CRBB analysis to the P. patula v1.0 transcriptome was performed using Transrate [27].

Differential expression analysis

RNA-seq read mapping to the P. patula v1.0 transcriptome and expression quantification was performed through RSEM v1.2.23 (RNA-Seq by Expectation-Maximum) [38] using Bowtie2 v2.2.5 [39]. Differential expression testing was performed with EBSeq v1.10.0 [40] using median normalization (FDR < 0.05).

Results and discussion

Data generation and pre-processing

Due to the expected size of the P. patula genome (ca. 22 Gb) [41], sequencing and assembly of the genome would be a costly and challenging endeavour. Therefore, transcriptome assembly was employed to generate a P. patula reference sequence. RNA-seq of shoot tissue harvested 1 dpi for inoculated and mock-inoculated samples yielded between 21 and 43 million read pairs per sample and a total of ca. 440 million reads (Table 2). Quality filtering removed ca. 13 % of reads and duplicate filtering removed a further 35 % of reads. Thus Dataset 1 consisted of ca. 36 Gb of sequence data and Dataset 2 consisted of ca. 23 Gb that passed through quality filtering and were subsequently used for transcriptome assembly.

Table 2 Quality statistics for RNA sequencing data

Comparison of assembler output

The completeness and quality of an assembled transcriptome is affected by the assembly program used as well as the assembly parameters used [42–48]. Comparative studies have also shown that the effectiveness of assembly programs can vary by input data set, with no assembler consistently outperforming any other [42, 43]. Due to this variability among assembler outputs, each variant assembly is likely to contain more accurate and complete assemblies at different loci. Therefore, in an effort to maximise diversity of assembled transcripts, we produced 18 de novo and two genome guided transcriptome assemblies using; Trinity, SOAPdenovo-Trans and Velvet/Oases. As expected from previous studies, large variation in the number, length and redundancy of contigs assembled was observed within and between assemblers (Fig. 1).

Fig. 1

Summarised assembly statistics for all preliminary assemblies. Pipt = Pinus patula (a) – Assembly size and length statistics. (b) –Transcript N statistics and GC ratio for all assemblies. In each case the right hand y-axis only applies to the dashed line. The first three Trinity assemblies were de novo assemblies using Dataset 1 with (50 k) and without (75 k) CuffFly, and using Dataset 2 (df). The last two Trinity assemblies represent reference guided assemblies using Dataset 1 (gg) and Dataset 2 (dfgg). For Velvet/Oases and SOAPdenovo-Trans, the numbers indicated the k-mer value used. ORF = open reading frame

Trinity exhibited the most uniformity among assemblies compared to the variation among assemblies from Velvet/Oases and SOAPdenovo-Trans. An inverse relationship has been shown to exist between the number of contigs assembled and k-mer length used for assembly [43]. Therefore, the greater uniformity in assembled contig number between Trinity assemblies can likely be attributed to the program’s implementation of a fixed k-mer length for all assemblies. For each value of k used in assembly, SOAPdenovo-Trans resulted in the highest number of assembled contigs followed by Velvet/Oases and lastly, Trinity. In a comparative study, Trinity consistently assembled more contigs than Velvet/Oases and Trinity assemblies consistently had a higher N50 statistic [45]. Although the present study used newer versions of Velvet/Oases and Trinity, the difference in trends obtained illustrates the difference in performance of assemblers under different conditions, supporting the need for use of multiple assemblers during transcriptome reconstruction.

Trinity genome guided assemblies displayed lower GC ratios, as well as fewer predicted open reading frames (ORFs) compared to other assemblies (Fig. 1). This was ascribed to fragmentation of the P. taeda v1.01 genome. Nevertheless, the two genome guided Trinity assemblies were included in downstream analysis. In total, 3 447 807 assembled transcripts were used as input for the EvidentialGene tr2aacds pipeline.

The EvidentialGene pipeline selects the ‘best’ transcripts based on coding potential, thus selecting for the best ORFs assembled. Open reading frames were successfully predicted for ca. 2.7 million (77 %) of the input transcripts. Of these, 49 % were classified as redundant and 51 % were classified as differing in CDS (non-redundant). A further 55 % of non-redundant sequences were classified as perfect fragments of other longer CDS, leaving 23 % of the predicted 2.7 million CDS as informative. Of the informative CDS, 60 % were assigned to the ‘drop’ category and discarded. Overall, this brought about a 14-fold reduction in assembled transcript number, with only 7 % of the original input sequences maintained. The resulting merged assembly contained 247 035 transcripts grouped into 66 377 predicted loci (Additional file 1: Table S1). This assembly was compared to the average assembly statistics across assemblies for each assembly program respectively (Fig. 2; Additional file 2: Table S2). Despite the decrease in transcript number, the proportion of transcripts containing a predicted ORF in the merged assembly was 10–40 % higher compared to the average ORFs per assembly for all three assemblers. This indicates that a higher proportion of transcripts in the merged set have been accurately assembled to near-full or full length. The average length among the 1 000 longest predicted proteins in the merged assembly was 1 425 amino acids. Due to the high resource expenditure required to produce these long proteins they are often well-conserved and a biological maximum has been observed for their average length [28]. For plants this maximum observed average is ca. 1 500 amino acids (based on the average between Arabidopsis, banana, cacao and poplar) [28]. This indicates that these proteins are well assembled in the merged assembly, although there is room for improvement.

Fig. 2

Assembly statistics for tr2aacds pipeline merged assembly compared to average assembly statistics for each assembler. Assembly size and length statistics. The dashed y-axis only applies to the dashed line. Unfiltered output assemblies from Trinity, Velvet/Oases and SOAPdenovo-Trans were used

Annotation

Sequence homology searches against the NR and plant protein databases successfully obtained significant local alignments for 32 416 loci. Of these, 14 255 were classified as non-pine origin transcripts and filtered from the transcriptome assembly. The majority of non-pine origin transcripts (71.62 %) aligned to transcripts from Fusarium species (Additional file 3: Table S3), with most aligning to either F. fujikuroi (4 698 transcripts) or F. oxysporum (3 290 transcripts). A further 22.81 % of non-pine origin transcripts aligned to Bipolaris maydis (1 309 transcripts), Pyrenophora tritici-repentis (992 transcripts) and Leptosphaeria maculans (949 transcripts), while the remaining 5.57 % (798 transcripts) aligned to 146 different species.

Removal of non-pine origin transcripts resulted in 52 112 putative loci, classified as the P. patula v1.0 shoot transcriptome. The current estimates for conifer gene numbers lie between ca. 32 000 for Picea glauca [49] and ca. 50 000 for P. taeda and P. pinaster [8, 50]. Roughly 60 % of assembled P. patula transcript sequences were successfully annotated, representing a wide array of molecular functions, biological processes and cellular compartment GO terms. The remaining 40 % of the assembled transcript sequences could not be annotated through similarity searches, however, each sequence contained an ORF predicted by the EvidentialGene pipeline and could potentially be expressed. Thus, these sequences were not removed from the assembly as they could represent uncharacterised or conifer specific genes. The top molecular function terms for P. patula v1.0 transcriptome were protein binding, transferase activity and nucleic acid binding (Additional file 4: Figure S1), similar to what has been observed for P. taeda, P. glauca and Picea mariana [8, 51].

Identification of putative NB-ARC defence related gene families

Orthologous protein groups were identified by comparing 41 956 clustered P. patula protein sequences from the assembled transcriptome to the 400 416 clustered protein sequences from 14 other plant species [52] (Table 1). Tribe-MCL analysis [53] resulted in 21,492 unique gene families, with an average of 18 members per family (Additional file 5: Table S4). Gene families were identified for 396 684 (89.6 %) sequences and ranged in size from 6 258 members from 15 species to 2 members from one species. Genes from the P. patula v1.0 transcriptome assembly initially clustered to 9 677 gene families (35 433 genes). This was reduced to 8 743 families (33 367 genes) by removing P. patula specific gene families with less than 5 members. While there are likely valid families in the removed set, these families were removed as most are likely to have arisen due to the remaining heterozygosity in the assembly. Of the total gene families, 2 165 were unique to conifers (Fig. 3). Although this is higher than the 1 554 reported by the P. taeda genome project [7, 8], it is a similar increase from the 1 021 reported by the P. abies genome project [54]. Included in the conifer-specific gene families are 130 that were unique to P. patula. The largest family identified in P. patula (1 794 members) contained leucine rich repeat (LRR), toll/interleukin-1 receptor (TIR), nucleotide binding domain with an ARC motif (NB-ARC), golgi transport complex 5 (COG5) and poxvirus A32 protein motifs. This gene family was also one of the largest observed for P. taeda and had low representation among the angiosperms while representation in the moss species differed. In total, 35 NB-ARC families were identified, of which 13 were present in conifers. NB-ARC gene families with higher representation of angiosperm genes had little to no representation from the conifers and vice versa (Fig. 4). The NB-ARC family of genes are associated with disease resistance as the majority of resistance proteins (R proteins) characterized are members of the NB-ARC and NB-LRR families [55]. Thus this difference could result from divergent R gene evolution between the plant lineages.

Fig. 3

Unique orthologous protein groups identified through Tribe-MCL analysis. Left Comparison of protein family counts for all identified orthologous protein groups between five different plant classifications. Right Comparison of conifer specific protein counts between four conifer species. Dicots = Arabidopsis thaliana, Glycine max, Populus trichocarpa, Ricinus communis, Theobroma cacao, Vitis vinifera. Mosses = Selaginella moellendorffii, Physcomitrella patens. Monocots = Oryza sativa, Zea mays. Gymnosperms = Picea abies, Picea sitchensis, Pinus patula, Pinus taeda. Basal = Amborella trichopoda

Fig. 4

Number of proteins per species for the eight most populated NB-ARC motif containing gene families identified. Gene families were identified using Tribe-MCL. Left – NB-ARC families most prominent in conifers. Right – NB-ARC families most prominent in angiosperms. Each color represents a different gene family (Additional file 2: Table S2). Family 2 (green bars) for P. taeda and P. patula had 852 and 1 794 members respectively

Assembly validation

Many well established metrics exist for assessment of genome assembly quality, the majority of which are based on size, such as contig and N50 size. Size based metrics such as N50 have been used in the past as a measure of transcriptome completeness [56], yet these metrics have no real biological relevance and are ineffective without prior knowledge of the actual size distribution in the sequenced data set. These metrics are also highly sensitive to assembly parameters and assembled isoforms (Fig. 1), which can bias quality assessment. For this reason, three reference based metrics were used to assess the transcriptome assembly; completeness, contiguity and accuracy [10]. Completeness and contiguity are closely linked. Completeness is the percentage of a reference set that has been assembled. Contiguity is the percentage of assembled reference sequences covered to X%, where X is an arbitrary minimum threshold [56]. In this study, contiguity and completeness of the P. patula transcriptome assembly was measured by comparison against four data sets.

Comparison to the CEGMA core eukaryotic proteins identified 217 (88 % completeness) of the 248 core genes, whereas 203 (82 % completeness) were identified in the P. taeda v1.01 genome [7]. At the same time, of the identified core genes, 91 % showed full length alignments to the P. taeda v1.01 genome, while 93 % (contiguity) of those from the P. patula v1.0 shoot transcriptome were full length. The higher completeness and contiguity obtained for CEGs in the P. patula transcriptome assembly compared to the P. taeda v1.01 genome can most likely be attributed to genome fragmentation. This illustrates the value of de novo transcriptome assembly for analysing genes missing from an incomplete genome. CEGMA analysis also identified multiple orthologs for 90 % of the identified CEGs. This is likely due to the presence of high allelic variation in the data used for assembly arising from the pooled nature of the samples (pooled RNA from seedlings) used for sequencing.

BUSCO analysis against the early access plant data set identified 850 (88 %) complete BUSCOs, out of 956 groups searched, of which 307 (32 %) were duplicated. A further 26 fragmented BUSCOs were identified. The high amount of duplicated complete BUSCOs further indicate the presence of assembled haplotypes still present in the transcriptome [36].

CRBB analysis to the P. patula reference proteins showed a similar pattern as above when comparing completeness (49 %) and contiguity (92 %) of the P. patula transcriptome to that of the P. taeda transcriptome (43; 92 %). This indicates higher completeness of the P. patula transcriptome for P. patula origin proteins as would be expected. The completeness (46 %) of the P. taeda gene models was intermediate between the transcriptomes, however, its contiguity (13 %) was notably lower. This low contiguity most likely arose due to the presence of partial genes in the high confidence gene models. The completeness and contiguity of the P. patula transcriptome assembly was also investigated through CRBB analysis to the P. taeda v1.0 transcriptome assembly using the gene models extracted from the P. taeda v1.01 genome assembly and the 87 available P. patula protein sequences as the reference sets. Compared to the P. taeda transcriptome, the P. patula transcriptome covered a higher proportion of the P. taeda gene models (at 95 % coverage), had a higher proportion of reference sequences with a CRBB result and had the lowest reciprocal best hit (RBH) ratio (Table 3). Overall CRBB statistics for comparison to the P. patula proteins were higher for the P. patula transcriptome compared to both the P. taeda transcriptome and gene models (Table 3).

Table 3 Conditional reciprocal best BLAST (CRBB) comparisons^a of assembled Pinus patula transcripts to available Pinus taeda gene models and transcripts

The third metric assessed was accuracy, defined as the percentage of correctly assembled bases in an assembly compared to a reference [10]. This was estimated through high-identity mapping of the assembled P. patula transcriptome, along with seven other pine transcriptomes, to the P. taeda v1.01 genome (Table 4). Mapping to the genome precluded calculation of completeness and contiguity, due to genome fragmentation and lack of exact gene number and location. At 95 % sequence identity and query coverage thresholds a total of 64 % of P. patula sequences mapped. The highest total mapping rates were observed for P. banksiana and P. contorta, while the lowest mapping rate was obtained for P. lambertiana, as expected from their phylogenetic relationship and previous studies [8]. Mapping rates obtained for the P. patula transcriptome were similar to the mapping rates obtained for the P. radiata and P. taeda transcriptome assemblies. These alignment metrics serve as a measure of transcriptome accuracy. Lowering the minimum coverage threshold to 50 % increased mapping by between 2 % and 15 %. The P. banksiana (3.8 %), P. contorta (2.5 %) and P. patula (2.8 %) transcriptomes were the least affected, while the transcriptomes for P. radiata (15.1 %), P. sylvestris (10.3 %) and P. taeda (13.1 %) showed the largest increase in mapping rates, suggesting that these transcriptomes have a higher content of genes that were fragmented in the genome assembly. Comparison of accuracy metrics between assemblies should be done with care, however, as even though the P. taeda transcriptome showed a lower accuracy (57 %) than P. patula, the size of the transcriptome means that it still contains ca. 10 000 more accurately assembled sequences. This illustrates the importance of considering assembly size when comparing between datasets, such as the high mapping rates to the P. taeda v1.01 genome obtained for P. contorta, P. pinaster and P. radiata (Table 4). Still, more than 33 000 (64 %) of the assembled P. patula sequences were shown to be accurately assembled and this number is expected to increase as fragmentation in the genome decreases.

Table 4 Mapping statistics to the P. taeda v1.01 genome

The assembled P. patula transcripts were further compared to corresponding P. taeda complete CDS sequences to ascertain the quality of the assembly against experimentally validated data (Additional file 6: Table S5). Of the 121 cDNA sequences, 89 (73.5 %) mapped to the P. patula transcriptome with greater than 89 % identity and 80 % subject coverage (Additional file 6: Table S5). Of the mapped sequences, 47 had a query coverage of more than 80 % with an average sequence identity of 98.4 ± 1.9 % and an average coverage of 97.9 ± 3.5 % and 91.5 ± 6.8 % for the subject and query sequences respectively (Additional file 7: Figure S2). Thus, of the P. taeda cDNA sequences present in the assembled P. patula transcriptome, 52.8 % have been assembled to near full-length.

Differential expression analysis

Comparison of inoculated and mock-inoculated data sets using EBSeq identified 166 transcripts as differentially expressed between conditions (Additional file 8: Table S6). The small number of detectable differentially expressed transcripts is likely a reflection of the very early time-point investigated, where small amounts of pathogen would have been in contact with the host tissue.

Ten transcripts were up-regulated (log₂(fold change) > 1) in the inoculated set, relative to mock-inoculated, while 156 transcripts were down-regulated (log₂(fold change) < −0.25; 77 had log₂(fold change) < −1). Among the up-regulated genes four had putative annotations (Additional file 9: Table S7). Two of these genes are involved in folate metabolism (methylenetetrahydrofolate dehydrogenase) and stomatal closure (PF03595), while the other two are linked to sugar metabolism (PREDICTED: alpha-galactosidase-like; sucrose synthase-like protein).

In the down-regulated set 83 transcripts had putative annotations (Additional file 9: Table S7). Some of these are related to plant defence such as a putative WRKY76 encoding transcript, implicated in susceptibility against Magnoporthe oryzae but increased tolerance to cold in rice [57] and a putative phenylalanine ammonia-lyase (PAL) encoding transcript. PAL is an important enzyme for salicylic acid production and is a key enzyme in the phenylpropanoid pathway, shown to be induced in response to wounding and fungal infection in Pinus sylvestris [58]. A putative map kinase 4 is also down-regulated. In Arabidopsis, map kinase 4 is known to regulate the salicylic acid and jasmonic acid/ ethylene defence signaling [59]. Although it is tempting to speculate that the down-regulation of such important transcripts in defence may, in part, contribute to susceptibility against F. circinatum, a detailed time-course of infection in P. patula is necessary to determine the full suite of host responses during this susceptible interaction.

Conclusions

This study presents the first transcriptome sequencing and assembly analysis for Pinus patula. The P. patula v1.0 transcriptome assembly constitutes the largest gene catalogue for this economically important species to date. More than 23 Gb of data was used to assemble 52 112 sequences with a total length of 52 Mb and an average coverage of more than 200×. Of these sequences, 30 844 could be assigned annotations. This transcriptome represents a major genomic resource for future studies on this tropical Pinus species, and will be used as the basis for further investigation of the host pathogen interaction between P. patula and F. circinatum. The workflow used for transcriptome assembly can in future be reapplied and altered as new sequencing data becomes available for P. patula to produce a more comprehensive and complete assembly. Furthermore, the workflow implemented during this study could be applied to other species where a high quality genome sequence is not available. One species to which the workflow could be applied in future is P. tecunumanii, a species that is closely related to P. patula [60] but which displays resistance to F. circinatum. Assembly of the P. tecunumanii transcriptome would thus allow for further investigation of the mechanisms differentiating resistance and susceptibility through comparison of defence responses in these closely related species.

Availability of supporting data

The data sets supporting the results of this article are available through the NCBI BioProject repository, [PRJNA301922; http://www.ncbi.nlm.nih.gov/bioproject/301922].