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Cryo-Treatment Enhances the Embryogenicity of Mature Somatic Embryos via the lncRNA–miRNA–mRNA Network in White Spruce

In conifers, somatic embryogenesis is uniquely initiated from immature embryos in a narrow time window, which is considerably hindered by the difficulty to induce embryogenic tissue (ET) from other tissues, including mature somatic embryos. In this study, the embryogenic ability of newly induced ET and DNA methylation levels was detected, and whole-transcriptome sequencing analyses were carried out. The results showed that ultra-low temperature treatment significantly enhanced ET induction from mature somatic embryos, with the induction rate from 0.4% to 15.5%, but the underlying mechanisms remain unclear. The newly induced ET showed higher capability in generating mature embryos than the original ET. DNA methylation levels fluctuated during the ET induction process. Here, WGCNA analysis revealed that OPT4, TIP1-1, Chi I, GASA5, GST, LAX3, WRKY7, MYBS3, LRR-RLK, PBL7, and WIN1 genes are involved in stress response and auxin signal transduction. Through co-expression analysis, lncRNAs MSTRG.505746.1, MSTRG.1070680.1, and MSTRG.33602.1 might bind to pre-novel_miR_339 to promote the expression of WRKY7 genes for stress response; LAX3 could be protected by lncRNAs MSTRG.1070680.1 and MSTRG.33602.1 via serving as sponges for novel_miR_495 to initiate auxin signal transduction; lncRNAs MSTRG.505746.1, MSTRG.1070680.1, and MSTRG.33602.1 might serve as sponges for novel_miR_527 to enhance the expression of Chi I for early somatic embryo development. This study provides new insight into the area of stress-enhanced early somatic embryogenesis in conifers, which is also attributable to practical applications.


Introduction
Conifers are important forest species, which are hard to be propagated via organogenesis due to the difficulty in root induction, while somatic embryogenesis provides an efficient propagation means [1,2]. In order to initiate somatic embryogenesis, immature zygotic embryos were commonly used as explants to induce embryogenic tissue (ET) in conifers. The embryogenicity of embryos declines with the increase in the degree of embryo maturation [3,4]. Though ET could be induced from mature zygotic embryos in very few coniferous species [5,6], the induction rate is generally low. Thus, the window of ET induction is important and limited by suitable embryo developmental stages, which are influenced by the weather in different years.
Cryopreservation is an important technology for the mass production of elite coniferous species [3,4]. Embryogenic materials need to be stored in liquid nitrogen during  In order to detect the embryonic ability of NET, the ET and NET were cultured by pre-maturation and maturation medium at the same time after obtaining the NET from CRSE. As evident in Figure 3b, the embryonic ability of NET (1345 SE/g) was higher than that of the ET (995 SE/g). These results revealed that cryo-treatment could enhance the embryogenicity of mature somatic embryos, and the embryonic ability of NET induced from the cryo-treated mature somatic embryos was improved.

Differentially Expressed Genes (DEGs) and KEGG Analysis during the Induction Process
Non-coding RNAs exert special functions on the biological process via regulating targeted genes; therefore, DEGs were firstly analyzed in this study. Analysis showed that 9486 unigenes were screened by the criteria of FDR < 0.05 and |log2 (fold change)| > 1. Firstly, a sample cluster was applied for verifying repeatability among three biological replications and detecting affinity between different samples, including samples of day 0, 1, and 10 of the induction cultures of CRSE (CR0D, CR1D, CR10D) and RTSE (RT0D, RT1D, RT10D). As Figure 4a indicates, RT10D was firstly clustered together with RT1D, then clustered together with CR1D and CR10D, and lastly clustered together with CR0D (CRSE) and RT0D (RTSE), which corresponded with the change in explants during the induction process (Figures 1 and 2). All these also indicated that the transcriptomic results were reliable.
Hierarchical clustering (HCL) of DEGs analysis suggested that differentially expressed mRNAs were mainly divided into three types of expression profiles (Figure 4b). Obviously, most DEGs were highly expressed in the induction process of CRSE, while 2872 DEGs were highly expressed in the induction process of RTSE. Additionally, there were also 3083 DEGs similarly expressed in the induction process of CRSE and RTSE. Moreover, KEGG analysis results showed that DEGs were highly expressed in "ribosome" and "glycolysis/gluconeogenesis" during the induction process, especially at the stage of RT1D and CR1D, in which the ribosome depolymerized and dedifferentiation process started. Interestingly, DEGs enriched in "inositol phosphate metabolism" were specially expressed in CR10D ( Figure 5). The results revealed that much more genes were activated during the ET induction process, especially genes related to inositol phosphate for signaling. In order to detect the embryonic ability of NET, the ET and NET were cultured by pre-maturation and maturation medium at the same time after obtaining the NET from CRSE. As evident in Figure 3b, the embryonic ability of NET (1345 SE/g) was higher than that of the ET (995 SE/g). These results revealed that cryo-treatment could enhance the embryogenicity of mature somatic embryos, and the embryonic ability of NET induced from the cryo-treated mature somatic embryos was improved.

Differentially Expressed Genes (DEGs) and KEGG Analysis during the Induction Process
Non-coding RNAs exert special functions on the biological process via regulating targeted genes; therefore, DEGs were firstly analyzed in this study. Analysis showed that 9486 unigenes were screened by the criteria of FDR < 0.05 and |log2 (fold change)| > 1. Firstly, a sample cluster was applied for verifying repeatability among three biological replications and detecting affinity between different samples, including samples of day 0, 1, and 10 of the induction cultures of CRSE (CR0D, CR1D, CR10D) and RTSE (RT0D, RT1D, RT10D). As Figure 4a indicates, RT10D was firstly clustered together with RT1D, then clustered together with CR1D and CR10D, and lastly clustered together with CR0D (CRSE) and RT0D (RTSE), which corresponded with the change in explants during the induction process (Figures 1 and 2). All these also indicated that the transcriptomic results were reliable.

Differentially Expressed Genes (DEGs) and KEGG Analysis during the Induction Process
Non-coding RNAs exert special functions on the biological process via regulating targeted genes; therefore, DEGs were firstly analyzed in this study. Analysis showed that 9486 unigenes were screened by the criteria of FDR < 0.05 and |log2 (fold change)| > 1. Firstly, a sample cluster was applied for verifying repeatability among three biological replications and detecting affinity between different samples, including samples of day 0, 1, and 10 of the induction cultures of CRSE (CR0D, CR1D, CR10D) and RTSE (RT0D, RT1D, RT10D). As Figure 4a indicates, RT10D was firstly clustered together with RT1D, then clustered together with CR1D and CR10D, and lastly clustered together with CR0D (CRSE) and RT0D (RTSE), which corresponded with the change in explants during the induction process (Figures 1 and 2). All these also indicated that the transcriptomic results were reliable.

DNA Methylation Level Fluctuated during the Induction Process
Previous studies have shown that embryonic ability was closely related to DNA methylation [36]. In this study, the global DNA methylation at different induction stages was detected by HPLC to confirm whether DNA methylation was correlated with the induction process in white spruce (Figure 3c). DNA methylation level during the induction process with RTSE slightly altered from 17.5% to 19.4%. The DNA methylation level rose to 19.3% on the first day, then slightly reduced to 18.1% on day 10, and then sharply increased to 19.4% on day 20. Unlike RTSE, the trend of DNA methylation level during the induction from CRSE was saddle shaped. The highest level was 22.7% on the 5th day, then the DNA methylation level obviously decreased to 17.5% on the 20th day. The significantly increased DNA methylation level might be attributed to the stress response by cryo-treatment at the early days of the induction process with CRSE and decreased as the callus appeared on the latter days of induction. most DEGs were highly expressed in the induction process of CRSE, while 2872 DEGs were highly expressed in the induction process of RTSE. Additionally, there were also 3083 DEGs similarly expressed in the induction process of CRSE and RTSE. Moreover, KEGG analysis results showed that DEGs were highly expressed in "ribosome" and "glycolysis/gluconeogenesis" during the induction process, especially at the stage of RT1D and CR1D, in which the ribosome depolymerized and dedifferentiation process started. Interestingly, DEGs enriched in "inositol phosphate metabolism" were specially expressed in CR10D ( Figure 5). The results revealed that much more genes were activated during the ET induction process, especially genes related to inositol phosphate for signaling.

DNA Methylation Level Fluctuated during the Induction Process
Previous studies have shown that embryonic ability was closely related to DNA methylation [36]. In this study, the global DNA methylation at different induction stages was detected by HPLC to confirm whether DNA methylation was correlated with the induction process in white spruce ( Figure 3c). DNA methylation level during the induction process with RTSE slightly altered from 17.5% to 19.4%. The DNA methylation level rose to 19.3% on the first day, then slightly reduced to 18.1% on day 10, and then sharply increased to 19.4% on day 20. Unlike RTSE, the trend of DNA methylation level during the induction from CRSE was saddle shaped. The highest level was 22.7% on the 5th day, then the DNA methylation level obviously decreased to 17.5% on the 20th day. The significantly increased DNA methylation level might be attributed to the stress response by cryo-treatment at the early days of the induction process with CRSE and decreased as the callus appeared on the latter days of induction. Differentially expressed DNA methyltransferase genes (DNMTs) chromatinmethylase (CMT), domains rearranged methylase 2 (DRM2), and methyltransferase (MET) were screened during the induction process ( Figure 3d). The expression of DRM2 decreased during the early induction process of CRSE, while the expression of CMT and MET increased from day 1 to day 10, similar to the fluctuation profile of DNA methylation level at the early days of the induction process. These observations indicated that CMT and MET may play important roles in the induction process.
Based on the VENN analysis results, a co-expression network between lncRNA and mRNA belonging to blue modules was constructed. To this end, 235 pairs of lncRNA and mRNA relationships were determined with significant values of Pearson's correlation coefficients (p < 0.05) to generate a network map, containing 6 lncRNAs and 104 mRNAs. The final networks including 4 lncRNAs and 68 mRNAs and 187 relationships are presented in detail in Figure 8C. In the complex co-expression network, it is well known that one lncRNA could control multiple genes, and one gene could be correlated with multiple lncRNAs in different ways. From the network, we found that OPT4, TIP1-1, Chi I, GST, WRKY7, and WIN1 genes were closely related to lncRNA MSTRG.

Expression Profiles of miRNAs and Analysis of Targeted mRNA in the Blue Module
To further construct co-expressed miRNA-mRNA interaction, 995 differentially expressed miRNAs were analyzed. Among them, there were 465 conserved miRNAs and 531 novel miRNAs. Cluster results showed that the expression levels of 782 and 648 miR-NAs were downregulated, respectively, during the cultivation of either CRSE or RTSE, while the expression levels of 213 and 347 miRNAs were upregulated, respectively, during the cultivation of CRSE or RTSE (Figure 10a). This network is more complex, compared with that of the lncRNA-mRNA relationship (Figure 10b). However, there still exists miRNA-hub gene regulation interaction during the process of induction. In our results, all the differentially expressed hub genes in the miRNA-targeted mRNA interaction were WRKY7, LAX3, and Chi I (Figure 10c). Among them, WRKY7 was targeted by pab-miR391c, pab-miR3627d, and novel_miR_339, and LAX3 was regulated by novel_miR_495, and Chi I was related to novel_miR_527. Then, novel_miR_339, novel_miR_495, and novel_miR_527 were verified by qPCR (Figure 9b). The expression of novel_miR_495 and novel_miR_527 decreased during the NET induction, but the expression of novel_miR_339 was not negatively correlated with targeted gene WRKY7.  Based on the VENN analysis results, a co-expression network between lncRNA and mRNA belonging to blue modules was constructed. To this end, 235 pairs of lncRNA and

Expression Profiles of miRNAs and Analysis of Targeted mRNA in the Blue Module
To further construct co-expressed miRNA-mRNA interaction, 995 differential pressed miRNAs were analyzed. Among them, there were 465 conserved miRNA 531 novel miRNAs. Cluster results showed that the expression levels of 782 and 648 NAs were downregulated, respectively, during the cultivation of either CRSE or while the expression levels of 213 and 347 miRNAs were upregulated, respectively ing the cultivation of CRSE or RTSE (Figure 10a). This network is more complex, pared with that of the lncRNA-mRNA relationship (Figure 10b). However, there st ists miRNA-hub gene regulation interaction during the process of induction. In o sults, all the differentially expressed hub genes in the miRNA-targeted mRNA inter were WRKY7, LAX3, and Chi I (Figure 10c). Among them, WRKY7 was targeted by miR391c, pab-miR3627d, and novel_miR_339, and LAX3 was regulated novel_miR_495, and Chi I was related to novel_miR_527. Then, novel_miR novel_miR_495, and novel_miR_527 were verified by qPCR (Figure 9b). The expre of novel_miR_495 and novel_miR_527 decreased during the NET induction, but th pression of novel_miR_339 was not negatively correlated with targeted gene WRKY

Discussion
Somatic embryogenesis is an efficient means for mass propagation of coniferous species once the protocols are optimized as needed. Although CRSE and RTSE were both put on the same induction medium containing plant growth regulators, NET was easily in- Figure 10. Analysis of DEmiRNAs (differentially expressed microRNAs) and targeted genes in blue module of WGCNA results: (a) hierarchical clustering of DEmiRNAs analysis; (b) network of miRNAs and targeted genes of blue module; blue triangle represents miRNAs, while yellow square represents targeted genes; (c) network of miRNAs and targeted hub genes; blue triangle represents miRNAs, and yellow square represents targeted genes.

Discussion
Somatic embryogenesis is an efficient means for mass propagation of coniferous species once the protocols are optimized as needed. Although CRSE and RTSE were both put on the same induction medium containing plant growth regulators, NET was easily induced from CRSE, instead of RTSE, which is mainly due to the liquid nitrogen treatment applied to the mature somatic embryos. It is well known that stress including wounding or exogenous hormones could trigger plant cell division and tissue regeneration [37]. Cells would be injured by the freezing treatment, insulting in the damage of plasma membrane, similar to dehydration or mechanical injury [5].

Application Value of Ultra-Low Temperature in Promoting ET Induction
This is the first report that cryo-treatment could enhance ET induction from mature somatic embryos in conifers. Compared with zygotic embryos, somatic embryos are visible and developing in in vitro conditions without a sampling window; in addition, mature somatic embryos, as sterile explant materials, could be inoculated into the induction medium with no need for sterilization, in contrast to zygotic ones. Furthermore, the genotype of mature somatic embryos is known and identical when the ET of known genotypes is used. On the contrary, zygotic embryos are obtained through pollination in nature, and the genotype of each zygotic embryo is different. Mature somatic embryos can also be preserved in liquid nitrogen for a long time during which elite genotypes can be screened by field test [3,4]. Then, mature somatic embryos of the selected genotype could be used for studies on the differences between genotypes and the propagation of conifers.
The previously reported results also showed that ET could be induced from mature zygotic embryos [5,6], and cold treatment could retain ET initiation capacity from zygotic embryos [38]; therefore, ultra-low temperature treatment might also be applied to mature zygotic embryos, which are cryo-tolerable, for a much better ET induction.

Effects of Cryo-Treatment and Exogenous Hormones on Gene Expression during the Induction from CRSE
The increased DNA methylation level of CRSE indicated that cryo-treatment caused stress to CRSE similarly to drought and salinity [39][40][41] at the early induction stage, which was important in initiating dedifferentiation. The highly expressed genes OPT4, TIP1-1, Chi I, GST, WRKY7, MYBS3, LRR-RLK, PBL7, and WIN1 might be attributed to cryotreatment during the CRSE induction process. LRR-RLK and PBL7, known as cytoplasmic membrane receptors, might function in signaling for subsequently activating plant defense mechanisms and controlling cell development, due to freezing treatment of CRSE [42,43]. Additionally, studies have shown that myo-Inositol phosphates are important for signaling processes in eukaryotes [44], suggesting that "inositol phosphate metabolism" also plays an important role in the signaling of NET induction. After receiving a signal from RLK, MYBS3 and WRKY7 genes regulate gene expression to promote resistance to cold stress [45][46][47][48]. In this study, a series of freeze-induced genes were regulated to enhance resistance. Firstly, specific messenger RNAs WIN1 was largely induced after mechanical wounding due to cryo-treatment [49]; The accumulation of class I chitinases belonging to the GH19 family was highly consistent with WIN1 in plant defense against cold stress [50,51], which play important roles in the early developmental stage of somatic embryos [52]. GST, a soluble cytoplasmic enzyme, also played an important role in this resistant mechanism [53,54]. TIP1-1 was responsible for water loss due to being frozen at liquid nitrogen [55]. OPT4 proteins have transmembrane-spanning a-helical segments (TMSs) [56] and were also involved in stress response via transporting various substrates across the plasma membrane into the cell [57]. All these events might be connected closely with each other and play vital roles in the stress response of wounded somatic embryos.
Meanwhile, the exogenous hormone-induced genes LAX3 were also highly expressed in the cryo-treated somatic embryos. In Arabidopsis, the auxin influx carrier gene LAX3 induced by auxin promoted the development of lateral root primordia and apical hook [58][59][60][61], while GASA5 was localized to cell peripheries and could be upregulated by auxin and cytokinin, playing an important role in flowering time and seeds germination [62,63]. Although CRSE and RTSE were put on the same induction medium, the expression of genes LAX3 and GASA5 were notably lower in RTSE than CRSE, in particular, the expression of the LAX3 gene, which decreased at day 10 in RTSE. Apart from the DNA methylation level profile, the reason why genes LAX3 and GASA5 expressed lower in RTSE might be that wounds resulting from ultra-low temperature treatment also caused epigenetic changes and cell reprogramming in CRSE.

CeRNA Network of lncRNAs, miRNAs, mRNAs during the Induction from CRSE
Non-coding RNAs, as epigenetic factors, could play important roles in regulating gene expression according to the present study. MicroRNAs could inhibit transcription or cleave genes to regulate gene expression level [64], while lncRNAs could protect gene expression by competing for endogenous RNA (ceRNA) for miRNAs, such as serving as miRNA sponges [25,65] or blocking the cleavage of pre-miRNAs [66].
In the current study, the expression level of lncRNA was highly intimated with gene expression, revealing that lncRNAs protected the expression of related genes in the process of NET induction of CRSE, such as OPT4, TIP1-1, Chi I, GST, LAX3, WRKY7, LRR-RLK, PBL7, and WIN1 genes. Combined with miRNAs, the decreased expression level of novel miRNA 495 and the highly expressed LAX3 gene illustrated that pre-novel_miR_495 might be bonded by lncRNAs MSTRG.33602.1 and MSTRG.1070680.1 to block the cleavage by DICER complex and protect the expression of related gene LAX3, increasing endogenous auxin levels and thus activating GASA5 [67]. Similarly, lncRNA MSTRG.33602.1, MSTRG.505746.1, and MSTRG.1070680.1 might block the cleavage of pre-novel_miR_527 to protect the expression of Chi I, promoting the development of early somatic embryos [52]. However, the expression of novel_miR_339 was not negatively correlated with the targeted gene WRKY7, which was still highly expressed during the induction process. This phenomenon might be attributed to lncRNA MSTRG.33602.1, MSTRG.505746.1, and MSTRG.1070680.1, which, as miRNA sponges, could protect the expression of related gene WRKY7; then, the downstream genes of WRKY7 such as WIN1, OPT4, TIP1-1, Chi I, and GST were regulated for stress response. Therefore, the successful induction of NET from CRSE was the result of the ceRNA network of lncRNAs, miRNAs, and mRNAs during the induction process ( Figure 11).  [52]. However, the expression of novel_miR_339 was not negatively correlated with the targeted gene WRKY7, which was still highly expressed during the induction process. This phenomenon might be attributed to lncRNA MSTRG.33602.1, MSTRG.505746.1, and MSTRG.1070680.1, which, as miRNA sponges, could protect the expression of related gene WRKY7; then, the downstream genes of WRKY7 such as WIN1, OPT4, TIP1-1, Chi I, and GST were regulated for stress response. Therefore, the successful induction of NET from CRSE was the result of the ceRNA network of lncRNAs, miRNAs, and mRNAs during the induction process ( Figure 11).

Preparation of Plant Materials
The high-embryogenic cell line WSP17 of white spruce (Picea glauca) was selected and used for this study. The embryogenic tissues (ETs) were cultured on a half-strength modified Litvay (1/2 mLV) medium (1 mg/L 2,4-D, 0.5 mg/L 6-BA, 0.5 g/L hydrolyzed casein, 0.5 g/L glutamine, 10 g/L sucrose, 3 g/L gellan gum, pH 5.8) for maintenance and sub-cultured once two weeks at 23 ± 1 • C [68,69]. The ETs were transferred to liquid maintenance medium without gellan gum to suspend the embryogenic tissues and then moved to a pre-mature medium (1/2 mLV basal medium, 30 µM ABA, 0.4 g/L hydrolyzed casein, 0.5 g/L glutamine, 10 g/L sucrose, pH 5.8) for one week to promote early embryo development. To obtain the mature embryo, the embryogenic tissues were cultured on a mature medium (1/2 mLV basal medium, 45 µM ABA, 0.2 g/L hydrolyzed casein, 0.4 g/L glutamine, 30 g/L sucrose, 10 g/L maltose, 6 g/L gellan gum, pH 5.8) for 28 days. Somatic embryos were sampled and put into 1.5 mL sample tubes and then frozen in liquid nitrogen for 24 h, after which they were directly recovered for 5 min at 38 • C in a water bath. Lastly, the treated and untreated somatic embryos were put on an induction medium (1/2 mLV basal medium, 2 mg/L 2,4-D, 1 mg/L 6-BA, 0.5 g/L hydrolyzed casein, 0.5 g/L glutamine, 10 g/L sucrose, 3 g/L gellan gum, pH 5.8) and sampled at day 0, 1, 5, 10, and 20, respectively.

Microscopic Observation
The induction process was observed under dissecting microscopy (Leica Stereozoom S9i, Leica Co., Ltd., Germany, Wezlar). To further confirm the difference between cryotreated somatic embryos (CRSEs) and normal somatic embryos cultured at room temperature at 23 ± 1 • C (RTSEs), during the induction for embryogenic tissue, samples were embedded in paraffin and dyed with safranine and fast green solution [70]. The DM6B Leica upright microscope (Leica Co., Ltd., Germany, Wezlar) was used for observing stained samples with a LAS X software image capture.

Detection of Induction Rate and Somatic Embryo Maturation Ability
The numbers of somatic embryos generated new embryogenic tissue (NET) and the total somatic embryos were counted for induction rate analysis, according to the following formula: induction rate = (the number of somatic embryos generated NET/the number of somatic embryos) × 100%. Subsequently, the ET and NET obtained from CRSEs were transferred to a liquid maintenance medium, followed by a pre-maturated medium, and finally, were cultured on the mature medium for four weeks. Then, the total number of somatic embryos was counted with 10 biological replicates, respectively. Lastly, we gathered the total number of somatic embryos per gram, which indicates the embryonic ability of ET.

Whole-Transcriptome Sequencing
Samples obtained from day 0, 1, 10 in the induction process of RTSE and CRSE were used for RNA extraction at three biological replicates with RNA Extraction Kit (RN40, Aidlab Biotechnologies, Beijing, China), defined as RT0D, RT1D, RT10D, CR0D, CR1D, and CR10D. After sequencing on an Illumina Hiseq 2500 platform and eliminating the low-quality reads, HISAT2 [72] was used for mapping reads to the Picea glauca genome (GCA_000966675.2_WS77111_v2), and StringTie [73] was used for assembling mapped reads for mRNA and lncRNA analysis. The criteria of |fold change| ≥ 2 &FDR < 0.05 was adopted for differential analysis of mRNA and lncRNA via DESeq2 [74]. For circRNA analysis, clean reads were also aligned to a pre-determined reference genome with BWA, in order to obtain their positions and specific sequence features. For siRNA analysis, Bowtie [75] was used for aligning sequence with Silva, GtRNAdb, Rfam, and RepBase databases, respectively; differential expression analysis was carried out by DESeq2 with |log2(FC)| ≥ 0.58 & p-value ≤ 0.05.

Transcriptomic Co-Expression Network Analysis
The WGCNA R package (ver. 1.70-3) was utilized to construct the weighted gene coexpression network and divide related modules [76]. First, mRNAs with |(fold change)| > 5 were analyzed for correlation, and then, the power formula (adjacency = |(1+ Pearson correlation)/2 | β was adopted for constructing the adjacency matrix. After determining soft threshold parameters β, the adjacency matrix was transformed into a topological overlap matrix, and the corresponding dissimilarity (1-TOM) was calculated. The minimum number of genes in the module was set to 30, and hierarchical clustering was carried out based on the TOM measurement method. Lastly, mRNAs with similar expression spectra were divided into the same module.
The lncRNA-mRNA co-expression network was constructed based on the correlation analysis between the differentially expressed lncRNAs and mRNAs. The expression of differentially expressed lncRNA and mRNAs were analyzed by Pearson's correlation coefficient with coefficient > 0.9 and p-value < 0.01. The targeted mRNAs of miRNAs were predicted by TargetFinder [59]. All the network maps were drawn by Cytoscape software (ver. 3.8.2).

Quantitative PCR Analysis
To validate the gene expression results of transcriptomes, qPCR was employed to determine the expression of genes on Applied Biosystems™ QuantStudio 6™ Real-Time PCR System (ABI, Carlsbad, CA, USA) by using Hieff UNICON Universal Blue SYBR Green Master Mix (Yeason Biotechnology Co., Ltd., Shanghai, China), according to the MIQEB guidelines [77]. The PCR cycling conditions were 40 cycles of 95 • C for 30 s, 95 • C for 3 s, and 60 • C for 20 s. The expression profiles of lncRNAs were verified with Applied Biosystems™ QuantStudio 6™ Real-Time PCR System (ABI, Carlsbad, CA, USA) by using PerfectStart ® Green qPCR SuperMix (+Dye II) (Transgene Biotechnology Co., Ltd., Beijing, China) according to manufacturer's protocol. The PCR cycling conditions were 40 cycles of 94 • C for 30 s, 94 • C for 5 s, and 60 • C for 34 s. The melting curves were analyzed during the reactions to ensure reaction specificity. The average expression level of three reference genes PaEF1, PsACTIN, PsUBQ gene [78][79][80] was used for the normalization of gene expression results (Table S1). PCR primers in Tables S2 and S3 were designed by primer blast (https://www.ncbi.nlm.nih.gov/tools/primer-blast/) with default parameters except for the product size (100-250 bp), and the primers were ordered from Tsingke BiotechCo., Ltd. (Beijing, China) qPCR analysis was performed in biological triplicate. miRNAs were verified through poly (A) RT-qPCR [81] on Applied Biosystems™ QuantStudio 6™ Real-Time PCR System (ABI, Carlsbad, CA, USA) by using PerfectStart ® Green qPCR SuperMix (+Dye II) (Transgene Biotechnology Co., Ltd., Beijing, China). In addition to using Pa5S, PatRNA-H1, PatRNA-R1 [82] as control samples (Table S1), PCR primers in Table S4 were designed by the poly (A) RT-qPCR method. The PCR and calculation methods were similar to those of the lncRNAs.

Statistical Analysis
All experiments were replicated at least three times (N ≥ 3), and all data were subjected to a two-way analysis of variance. The analysis of significant difference (p < 0.05) was carried out on SPSS 20 (IBM, Armonk, NY, USA) software via Duncan's multiple range test.

Conclusions
This is the first report that cryo-treatment of mature SE, as explants, enhanced ET induction in conifers. This method could be beneficial in genoplasm conservation and propagation of elite genotypes in conifers. The 5mC DNA methylation levels were noticeably altered differentially between the cryo-treated materials and the control during ET induction, which indicated that epigenetic factors might play important roles during the induction process. With whole-transcriptome technology and bioinformatic means, we found that ncRNAs, as epigenetic factors, critically affect the regulation of gene expressions such as lncRNAs and miRNAs. In this study, three lncRNAs (MSTRG.33602.1, MSTRG.505746.1, and MSTRG.1070680.1) and three miRNAs (novel_miR_339, novel_miR_495, and novel_miR_527) were selected by co-expression of lncRNA-mRNA and miRNA-mRNA in a blue module, and targeted at hub genes (OPT4, TIP1-1, Chi I, GASA5, GST, LAX3, WRKY7, MYBS3, LRR-RLK, PBL7, and WIN1 genes). Results of this research revealed a probable regulation mechanism in the induction process using CRSE and provide insight into the initiation of early somatic embryogenesis in conifers. Additionally, the ultra-low temperature treatment might also be applied to mature zygotic embryos to increase the ET induction rate.