Introduction

The transition from vegetative to reproductive growth is a crucial event in the life cycle of plants. An intricate genetic circuitry has evolved to control the onset of flowering in response to diverse endogenous pathways and exogenous stimuli1. In plants, the complexity of regulation ensures that bolting and flowering is initiated at a condition that is suitable and favorable for successful reproduction. The molecular and genetic analysis in Arabidopsis thaliana has revealed that there are five major interactive pathways involved in flowering control including photoperiod pathway, vernalization pathway, autonomous pathway, gibberellin (GA) pathway and age pathway1,2,3. During the past two decades, a set of genes associated with the transition to flowering have been identified and integrated into these cross-regulating pathways1,4. Although the biological functions of some key flowering genes have been extensively characterized in many plant species4, the molecular mechanisms of flowering-time regulation are still not fully understood in some root vegetable crops including radish (Raphanus sativus L.).

MicroRNAs (miRNAs) are a class of endogenous non-coding small RNAs with approximately 21 to 24 nucleotides (nt) in animals and plants5. In plants, miRNAs are processed from long primary transcripts cleaved by Dicer-like 1 RNase (DCL1) and subsequently incorporated into the RNA-induced silencing complex (RISC), where miRNAs are complementary to specific target mRNAs and regulate their expressions post-transcriptionally by guiding the cleavage of complementary mRNAs6,7. Many plant miRNAs are evolutionally conserved among many species8,9. Recent evidences have indicated that miRNAs could play pivotal roles in various plant growth and developmental processes, such as hormone homeostasis10,11, root development12,13, leaf morphogenesis14,15, flower development11,16,17, embryogenesis18,19 and stress responses20,21.

Recently, an increasing number of studies have revealed miRNAs as master regulators of gene expressions to be implicated in the developmental transition from vegetative growth to flowering4,8,22. With the development of high-throughput sequencing (next-generation sequencing, NGS) technology, a large number of miRNAs related to flowering and flower development have been identified and characterized in several important species, such as trifoliate orange23, rice24, hickory17, Xanthoceras sorbifolia25 and poplar26. Several miRNAs, such as miR156 and miR172, two major players in flowering regulation, have been shown to regulate the floral transition and are also integrated into flowering pathways22,27. Age pathway is a newly defined endogenous flowering pathway and mediated by miR156, which targets Squamosa Promoter Binding Protein Like (SPL) transcription factors and decreases with increasing age of the plant2,3. Overexpression of miR156 could delay flowering and prolong vegetative stage in many species including Arabidopsis28, rice29 and tomato30. The miR172, targeting APETALA2 (AP2)-like transcription factors, has the opposite effect to the miR156 on the regulation of flowering time and increases with phase development in Arabidopsis31,32, which could induce floral organogenesis and promote flowering32,33. In addition, some miRNAs including miR15934,35, miR16716, miR16936, miR39337 and miR39938, were found to be involved in regulation of flowering time. For instance, GA pathway is governed by miR159, which represses the regulation of MYB transcription factors4,22,34. The overexpression of miR159 could delay flowering and its roles in manipulating flowering time control have been clarified in Arabidopsis34 and Sinningia speciosa35. These studies showed that miRNAs play important roles in a number of developmental processes and pathways which control flowering time. However, there is little information about miRNA-mediated gene expressions and flowering-time regulatory networks, especially in vegetable crops.

Radish (2n = 2x = 18), belonging to the Brassicaceae family, is one of the most economically important root vegetable crops worldwide. In the entire life cycle of radish, bolting and flowering are integrated steps. Premature bolting is a destructive problem in radish production as it causes significant reduction in yield and quality of the economic product in Brassicaceae crops. The optimum timing of bolting and flowering is vital for economy organ growth in preventing radish bolting and flowering prematurely. Therefore, elucidating the miRNA-directed genetic networks of radish bolting- and flowering-time modulation is imperative. Although using high-throughput sequencing, some tissue-specific as well as heavy metal-responsive miRNAs have been extensively identified in radish19,20,21,39. Nevertheless, there are no reports on systematic identification and characterization of bolting-related miRNAs in radish. To explore the roles of miRNAs in bolting- and flowering-time regulation, two small RNA libraries (NAU-VS and NAU-RS) from radish leaves at vegetative stage and reproductive stage were constructed and sequenced using Solexa sequencing. The objectives of this study were to identify known and potential novel miRNAs from radish leaves and to validate target genes of bolting- and flowering-related miRNAs. These results revealed complex miRNA-mediated flowering-time regulatory network in radish and provided insights into clarification of the molecular mechanisms underlying the regulation of plant bolting and flowering.

Results

High-throughput sequencing of transcriptome and small RNAs in radish leaf

To obtain a comprehensive overview of the radish leaf transcriptome, an mRNA library from equally mixed radish leaves at vegetative and reproductive stages was sequenced using Solexa sequencing, with the generation of 58.6 million raw reads. After removing poly (A) tails, low-quality tags and adaptor contaminants, a total of 54.6 million clean reads were obtained. Then all high-quality clean reads were assembled into 111,167 contigs. After further pair-end annotation and gap filling, these contigs were assembled into 53,642 unigenes, with an average length of 802 nt and a N50 length of 1035 nt. Our radish leaf transcriptome library integrated with the available genomic survey sequences (GSS) and expressed sequence tag (EST) sequences were used as the radish reference sequences for the identification of known and novel miRNAs in radish.

To identify miRNAs during radish bolting and flowering, two sRNA libraries, ‘NAU-VS’ and ‘NAU-RS’, were constructed from radish leaves at vegetative and reproductive stages, respectively. In total, 36,841,873 raw reads representing 7,497,083 unique sequences were obtained from the two sRNA libraries (Table 1; Supplementary Fig. S1). After filtering out the low quality reads, contaminants and adapter sequences, 18,948,210 (representing 4,992,584 unique sequences) and 17,893,663 (representing 3,491,761 unique sequences) clean reads were obtained from NAU-VS and NAU-RS libraries, respectively (Table 1). The length of total sRNA ranged from 18 to 30 nt, with 21-nt sRNAs being the most abundant in both libraries, which accounted for 33.33% and 52.97% in NAU-VS and NAU-RS libraries, respectively (Fig. 1). By comparing with the NCBI GenBank and Rfam databases, these sRNA sequences that matched non-coding sRNAs included tRNAs, rRNAs, snoRNAs and snRNAs were removed (Supplementary Table S2). Thereafter, 61,574 (NAU-VS) and 50,222 (NAU-RS) miRNA analogous sequences were annotated and obtained for known miRNA identification. The remaining 4,846,072 (NAU-VS) and 3,375,354 (NAU-RS) unannotated sRNAs were used to predict potentially novel miRNAs for subsequent analysis.

Table 1 Statistical analysis of sequencing reads from NAU-VS and NAU-RS libraries of radish leaves.
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Figure 1
figure 1

Length distribution and frequency percent of small RNA sequences in NAU-VS and NAU-RS libraries in radish.

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Identification of known miRNAs in radish leaf

To identify known miRNAs in radish leaves at vegetative and reproductive stages, the unique sequences were aligned to the mature sequences of known miRNAs deposited in miRBase. A total of 75 conserved miRNAs representing 21 miRNA families were identified in the both libraries (Table 2; Supplementary Table S3). Among these some miRNA families, miR156/157 and miR165/166 families had 11 and 9 members, respectively; whereas five families, miR171, miR393, miR395, miR397 and miR408 had only one member (Fig. 2A). Moreover, 19 non-conserved miRNAs belonging to 13 miRNA families were also identified in this study, which comprised fewer members as compared to conserved miRNAs (Fig. 2A). For these non-conserved miRNA families, eight miRNA families had only one member, whereas four miRNA families (miR1885, miR2111, miR400 and miR824) and miR394 had two and three members, respectively. In addition, the secondary structures of these known miRNA precursors were predicted as showed in Supplementary Fig. S2. The average minimal folding free energy (MFE) value of these miRNA precursors was −58.6 kcal mol−1 and these precursors length ranged from 69 nt to 689 nt, with an average length of 141 nt (Supplementary Table S3).

Table 2 Known miRNA families and their abundance identified from NAU-VS and NAU-RS libraries.
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Figure 2
figure 2

Members and abundances of known miRNA families identified in radish.

(A) Distribution of known miRNA family member. (B) Abundance of each known miRNA family.

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In this study, the read numbers of 34 identified known miRNA families varied greatly (Table 2). Of these families, the expression levels of several conserved miRNA families, such as miR156/157, miR165/166, miR167 and miR168, were extremely high (Fig. 2B). miR167 (407,770 reads) and miR408 (740,985 reads) were the most abundant in NAU-VS and NAU-RS libraries, accounting for 23.91% and 43.57% of all known miRNA reads, respectively. On the contrary, most of non-conserved miRNA families showed relatively lower read levels as compared to conserved miRNAs. For instance, miR394 and miR400 families had less than 500 reads in both libraries. Moreover, a few non-conserved miRNA families including miR535, miR5227, miR6273 and miR6284, were identified and observed in only one library. Furthermore, different members of each miRNA family displayed drastically different expression levels (Supplementary Table S3).

Identification of potential novel miRNAs in radish leaf

A total of 44 potential novel miRNAs belonging to 28 novel miRNA families were identified (Table 3; Supplementary Table S4). Among them, one member was detected for 24 novel miRNA families; whereas four families, rsa-miRn2, rsa-miRn7, rsa-miRn15 and rsa-miRn26 were generated from two loci. Secondary structures of these novel miRNA precursors were predicted and identified (Supplementary Fig. S2), with the average MFE value of −46.09 kcal mol−1. The average length of these miRNA precursors was 134 nt, with a range of 72 nt to 340 nt. According to the strict criteria of the presence of miRNA* being a prerequisite for predicting novel miRNAs40, 12 novel miRNAs were identified with complementary miRNA*s (Supplementary Table S4), which provide strong evidences for these predicted novel miRNAs in radish.

Table 3 Novel miRNAs and their abundance identified from NAU-VS and NAU-RS libraries.
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Identification of bolting related miRNAs

To identify differentially regulated miRNAs related to regulation of bolting and flowering time in radish, a total of 94 known and 44 novel miRNAs were subjected to differential expression analysis between NAU-VS and NAU-RS libraries. In all, 59 differentially expressed miRNAs which included 42 known and 17 novel miRNAs were identified and considered as bolting related miRNAs (Supplementary Table S5). These miRNAs showed significantly differential expression between the two libraries. Among these miRNAs, 31 miRNAs including 19 known and 12 novel miRNAs were up-regulated in NAU-RS library, whereas 28 miRNAs containing 23 known and five novel miRNAs showed down-regulated patterns (Fig. 3). Furthermore, the expression profiles of different members in the same miRNA family were not identical. Two miRNAs, miR535b (13.93-fold) and rsa-miRn1 (11.52-fold), exhibited the greatest change in expression levels. For differentially expressed novel miRNAs, 11 of 17 were detected in only one library (Fig. 3B; Supplementary Table S5), indicating that these miRNAs might display stage-specific expressions in particular developmental process of radish. These results suggest that differentially regulated miRNAs might be important regulators in the process of radish bolting and flowering.

Figure 3
figure 3

Validation and comparative relative expression of differentially expressed known (A) and novel (B) miRNAs between NAU-VS and NAU-RS libraries.

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Target prediction of bolting and flowering related miRNAs

Target validation is a prerequisite to characterize functionally the biological roles of miRNAs in plants. The psRNATarget server has been proven to be a useful approach to systematically identify target transcripts for known and novel miRNAs41. In this study, a total of 3,335 target transcripts with 3,829 target locations were predicted for all the identified miRNAs in radish by the psRNATarget server41 (Supplementary Table S6). To better understand the biological functions of miRNAs, the annotation of these target transcripts was performed by Blast2GO analysis. GO analysis revealed that these target genes could be classified into 20 biological processes, 11 cellular components and nine molecular functions (Fig. 4). For biological processes, cellular, metabolic and single-organism processes were the three most dominant GO categories. With regard to cellular components, the three most abundant GO terms were cell, cell part and organelle. The three most dominant GO terms in molecular functions were binding, catalytic activity and nucleic acid binding transcription factor activity. Additionally, GO enrichment analysis demonstrated that these target transcripts were involved in various metabolic and reproductive processes (Supplementary Fig. S3).

Figure 4
figure 4

GO classification of target genes for all identified miRNAs in radish.

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In total, 154 target transcripts related to bolting and flowering were identified as the putative targets of 50 bolting-related miRNAs including 27 conserved, 11 non-conserved and 12 novel miRNAs (Supplementary Table S7). Blast2GO analysis of flowering-related miRNAs revealed that these targets could be enriched in 16 biological process, 10 cellular component and nine molecular function categories (Supplementary Fig. S4). Compared with the enriched GO terms of all targets in Fig. 4, the GO analysis results of flowering related targets showed that the great majority of enriched GO categories were consistent with those of all putative targets. The three most abundant GO categories of biological processes and molecular functions were similar to those of all miRNA targets; whereas the ‘cell part’ term which was second enriched in cellular components for all targets, was not found in flowering related targets. In addition, 137 targets were also predicted for unclassified non-conserved miRNAs (Supplementary Table S8).

Many target genes for known miRNAs encoded transcription factors (TFs), such as squamosa promoter binding protein (SBP), NAC domain transcription factor, auxin response factor (ARF) and MYB transcription factor (Table 4). Moreover, some target genes including SPLs (targeted by miR156/157)2,3, TOC1 (targeted by miR156)42, AP2 (targeted by miR172)31,33, SPY (targeted by miR167)43, AGL16 (targeted by miR824)44 and VRN1 (targeted by miR5227)45, were identified to be related to bolting and flowering processes in radish. A few target sequences were also found to be implicated in biotic and abiotic stress response (Supplementary Table S7), such as aluminum-activated malate transporter 9 (ALMT9) (Unigene466, miR160) and TIR-NBS-LRR class disease resistance protein (CL9922.Contig3 and Unigene1615, miR1885), which implied that these potential target genes were involved in a broad range of physiological functions and biological processes.

Table 4 Identified targets for conserved miRNAs in radish.
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RT-qPCR validation of miRNAs and their target genes

To validate the quality of Solexa sequencing and study the expression patterns of miRNAs in radish, the expression levels of 12 selected miRNAs, miR156a, miR159a, miR164a, miR167a, miR172a, miR408-5p, miR535b, miR824, miR5227, rsa-miRn3, rsa-miRn9 and rsa-miRn11, at vegetative and reproductive phases were validated by RT-qPCR analysis and were further compared with the results from Solexa sequencing (Fig. 5). The comparisons showed that the expression patterns of most miRNAs shared a similar tendency between the RT-qPCR and NGS sequencing, indicating that the data from sRNA sequencing are reliable and could represent relative expression levels of miRNAs in radish.

Figure 5
figure 5

Comparison of relative expression levels of miRNAs between RT-qPCR and Solexa sequencing in radish.

Each bar shows the mean ± SE of triplicate assays.

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To investigate the expression patterns of bolting-related miRNAs and their target genes during different developmental stages and in various tissues of radish, 12 miRNAs were selected for RT-qPCR analysis (Fig. 6). As expected, the expression levels of miR156a, miR159a, miR164a, miR167a, miR172a, miR824 and miR5227 peaked at four-leaf stage (FS) and declined from vegetative stage (VS) to reproductive stage (RS) (Fig. 6A); whereas miR408-5p, miR535b, rsa-miRn9 and rsa-miRn11 exhibited similarly higher expression patterns at reproductive stage. As shown in Fig. 6B, miR172a, miR164a and miR167a were most abundant in leaf, whereas miR535b, miR824, miR5227, rsa-miRn3 and rsa-miRn11 had highest expression levels in flower. Moreover, the dynamic expression patterns of 12 corresponding target genes were validated: SPL6-Unigene3780, SPL9-EX886942, SPL13-CL2234 and TOC1-FD977501 for miR156a; AP2-FD572123 for miR172a; MYB65-CL9825 for miR159a; SPY-CL1667 and ARF8-Unigene18528 for miR167a; ARF16-FD576484 and ARF17-EX896877 for rsa-miRn11; AGL16-Unigene1577 for miR824; VRN1-CL5009 for miR5227. The results revealed that the expression of different genes varied at different developmental stages of radish (Fig. 7). For example, four target genes, SPL13, SPL9, SPL6 and TOC1, being targeted by miR156a showed up-regulated patterns at vegetative stage (Fig. 7). The expression patterns of SPL13, SPL9 and SPL6 from vegetative stage to reproductive stage showed negative correlation to the down-regulated miR156a, whereas TOC1 declined at bolting stage (BS) and remained at relatively higher levels at reproductive stage. These results implied that the different target genes of bolting-related miRNA might play different roles during the transition from vegetative stage to flowering. However, most of miRNAs could negatively regulate their corresponding target genes, which might play important roles during the bolting and flowering processes of radish.

Figure 6
figure 6

Validation of temporal and spatial expression patterns of miRNAs by RT-qPCR.

(A) The relative expression levels of miRNAs during different development stages (‘FS’, four-leaf stage; ‘VS’, vegetative stage; ‘BS’, bolting stage; ‘RS’, reproductive stage). (B) The relative expression levels of miRNAs in various tissues (root, leaf, flower, floral bud and pod). Each bar shows the mean ± SE of triplicate assays.

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Figure 7
figure 7

RT-qPCR validation of putative target genes at different development stages.

Each bar shows the mean ± SE of triplicate assays.

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Discussion

The timely transition from vegetative growth to reproductive development is regulated by a series of flowering-related genes involved in a complex gene-network, which is controlled by diverse environmental and endogenous cues coordinately1,4,22. Previous evidences have suggested that miRNAs were important players in gene regulatory networks involved in plant growth and developmental timing22,46,47. Notably, a number of miRNAs and target genes associated with flowering-time regulation and flower development have been recently identified in some species17,23,24. However, few studies on the miRNA-directed flowering regulatory mechanism in vegetable crops were conducted and there has been no report on the identification and comparative profiling of bolting- and flowering-related miRNAs and targets in radish.

Overview of sRNA sequencing in radish leaf

Progress in the next-generation sequencing technology has offered a powerful tool to identify a comprehensive set of miRNAs at certain tissues or during particular stages and to explore the molecular basis of miRNA-mediated flowering-time regulation8,9. In this study, two sRNA libraries from radish leaves were constructed during vegetative and reproductive stage and sequenced by NGS sequencing. 21-nt sRNA groups were predominant, which was consistent with the reports on tissue-specific miRNAs in B. oleracea48, wild rice24 and castor bean49, implying that 21 nt miRNAs may be canonical miRNAs in plants8,24.

Recently, many studies have shown that most of known miRNAs in the plant kingdom are evolutionarily conserved and highly or moderately expressed17,19,20. In the current study, some conserved miRNA families including miR156/157 and miR168, were sequenced more than 10,000 or even 100,000 times per library, whereas a few non-conserved miRNAs showed relatively lower reads with less than 100, which was in agreement with the previous observations that conserved miRNAs represented higher expression abundance as compared to non-conserved miRNAs17,20,23. In addition, many novel miRNAs were identified in both libraries and the read numbers of most novel miRNAs were much lower than those of conserved miRNAs, implying that highly conserved miRNAs might play important roles in plant growth and developmental processes17.

Characteristics of bolting-related miRNAs and target genes in radish

High-throughput sequencing in combination with bioinformatics analysis facilitate the identification of differentially expressed miRNAs and functional analysis of miRNAs, which contribute greatly to the elucidation of underlying mechanisms in regulating biological processes and developmental pathways8,50. Recently, an increasing number of studies reported that many flowering-related miRNAs have been identified in some species, which could facilitate the exploration of miRNA roles in flowering control in plants23,24. In wild rice, most of identified flowering-related miRNAs, including miR156/157, miR164 and miR167, were found to be down-regulated during the phase change from vegetative stage to flowering stage, whereas miR319 and miR408 showed up-expressed patterns24. In trifoliate orange, miR156/157 and miR159 were significantly down-expressed during adult development process in the mutant plant23. Several miRNAs related to flower development have also been detected in hickory17, X. sorbifolia25 and andromonoecious poplar26, which enriched our knowledge of the regulatory roles of miRNAs. In this study, a set of known and novel miRNAs were identified to be bolting- and flowering-related miRNAs (Supplementary Table S5). The expression patterns of several selected miRNAs which were validated by RT-qPCR analysis, suggested that some previously reported flowering-related miRNAs also showed significantly differential expression in the process of radish bolting and flowering. As expected, the expression level of miR156a was declined from four-leaf stage to reproductive stage and the level was also relatively low at different tissues during radish flowering phase, which was consistent with several previous studies2,3,51,52. In addition, RT-qPCR analysis revealed that several conserved miRNAs including miR156, miR164 and miR167, were down-regulated, which agreed with the results from wild rice24; however, their expressions were partly different from the observations in hickory17, which maybe result from that hickory belongs to perennial plants which could initiate flowering more than once during their life cycle, whereas it should undergo a long juvenile phase before flowering. Moreover, the majority of selected miRNAs expression patterns exhibited inverse correlation with their corresponding mRNAs, indicating that these miRNAs could negatively regulate their corresponding target genes19,21,24.

Previous studies showed that plant miRNAs could play certain roles in almost all developmental processes by regulating their corresponding target genes11, most of which encode key regulatory proteins or TFs that are involved in multiple plant biological processes8. In this study, a series of rsa-miRNAs were found to target the genes of TF families, such as SPLs, NACs, ARFs, WRKYs and MYBs, most of which have been verified to play vital roles in the regulation of plant flowering time. ARFs were confirmed to play important roles in plant growth and flower development53. Previous studies demonstrated that ARF8 played essential roles in plant hormone homeostasis54 and could promote floral development16,17. In this investigations, three members of ARF gene family, ARF8, ARF16 and ARF17, were identified as putative targets of rsa-miR167, rsa-miR160 and rsa-miRn11, respectively, implying that they could participate in the process of radish flowering and flower development. In addition, miR393 have been shown to be correlated with auxin-signaling pathway, which increased level displayed hyposensitivity to auxin and promoted flowering in rice37. In this study, rsa-miR393 was predicted to target two genes TIR1 and AFB3, which encoded auxin receptor proteins and could be involved in auxin signal transduction and floral transition. Furthermore, these identified target genes for bolting- and flowering-related miRNAs have shown that some genes could not only influence the floral transition, but also be associated with a wide range of biological functions including signal transduction, defense responses and circadian clock regulation. The result suggested that these bolting- and flowering-related miRNAs and potential target genes were involved in diverse biological and developmental processes in radish.

MiRNA-mediated flowering-time regulatory networks in radish

As major regulators of gene expressions, miRNAs regulate the expressions of specific target genes mainly at the post-transcriptional level6,7. Recently, several flowering related miRNAs and their target genes have been shown to play important roles in the developmental switch from vegetative growth to flowering4,22,27. In this study, several crucial bolting and flowering related genes, such as SPLs, AP2s and MYBs, were identified and involved in the complex genetic networks of bolting and flowering time control. Based on the identification and characterization of R.sativus miRNAs and corresponding target genes, a putative schematic model of miRNA-mediated bolting- and flowering-time regulatory networks was proposed (Fig. 8). Considerable reports showed that miR156 targeted 11 SPL genes in Arabidopsis, which controlled the newly defined age pathway and acted in regulating flowering time by the crosstalk with photoperiodic, GA and vernalization pathways2,3. The roles of miR156-SPLs node in controlling flowering time could be pronounced by the evidence that the overexpression of miR156 resulted in delayed flowering in several species28,29,30. Remarkably, the expression of miR156 was temporally regulated by plant age2,51, which was higher in embryos and seedlings and displayed a lower level at reproductive phase52. In agreement with previous studies, both NGS sequencing and RT-qPCR validation revealed that rsa-miR156 was down-expressed at reproductive stage of radish, which was found to target a series of SPL genes including SPL3, SPL5, SPL6, SPL9, SPL13 and SPL15 (Table 4), inferring that rsa-miR156 and its SPL targets could play important roles in determining bolting and flowering time in radish (Fig. 8).

Figure 8
figure 8

The putative schematic model of miRNA-mediated bolting and flowering regulatory network in radish.

‘V’ and ‘R’: The vegetative and reproductive stage of radish, respectively. The up- and down-regulated miRNAs are in red and green, respectively. All the identified targets are represented in black.

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Interestingly, a vital floral integrator miR172, which is important in regulating the developmental transition of flowering and the determination of floral organ identity, was down-regulated by miR156 via SPL927,46,51. The putative target gene of miR172 was a small group of AP2-like genes, including AP2, TARGET OF EAT1 (TOE1), TOE2, TOE3, SCHLAFMUTZE (SMZ) and SCHNARCHZAPFEN (SNZ), which mainly function as flowering repressors31,32,33. In this study, rsa-miR172 was identified and predicted to target three flowering genes, AP2, TOE2 and RAP2.7, which would be down-regulated by rsa-miR172 and regulate flowering time (Fig. 8).

Functions in flowering time control have been shared by miR159 and its target MYB transcription factors via the GA pathway. It had been shown that GA treatment could direct the degradation of DELLA proteins that cause an increase in miR159 level22,47. MiR159-MYB module played vital roles in flowering time manipulation, which had been demonstrated in Arabidopsis34 and Sinningia speciosa35. Our results also showed that rsa-miR159 was predicted to target two MYB genes, MYB65 and MYB101. Moreover, two flowering related genes, AGL16 and VRN1, belonging to MADS-box transcription factors were identified for rsa-miR824 and rsa-miR5227, respectively. MiR824 targeted AGL16 gene could modulate the flowering time of Arabidopsis by interacting with FLOWERING LOCUS C (FLC), SHORT VEGETATIVE PHASE (SVP) and FLOWERING LOCUS T (FT)44; while VRN1 as flowering activator repressed the expression of floral repressor FLC45 and modulated the level of FT in a rhythmic manner55. These evidences indicated that rsa-miR824 and rsa-miR5227 might be essential components of gene regulatory network that orchestrate developmental transition of flowering in radish (Fig. 8). These findings showed that the expression of bolting- and flowering-related genes including SPLs, AP2s, MYBs, AGL16 and VRN1 mediated by corresponding rsa-miRNAs played vital roles in inhibiting or promoting the initiation of radish bolting and flowering, indicating that these identified crucial miRNA-target modules could serve to dissect the genetic networks of miRNA-mediated bolting- and flowering-time control in radish.

In conclusion, bolting-related miRNAs and their targets between vegetative and reproductive stages were first identified and comparatively profiled at transcriptome-wide level using small RNA sequencing in radish. A total of 28 conserved miRNAs, 14 non-conserved miRNAs and 17 novel miRNAs were significantly differentially expressed and identified as bolting- and flowering-related miRNAs in radish. RT-qPCR analysis suggested that some differentially regulated miRNAs displayed differential temporal and spatial expressions. GO categorization and functional analysis suggested that several bolting- and flowering-related miRNAs, such as miR156, miR172, miR159, miR824 and miR5227, targeted some key transcription factors and regulatory proteins which were involved in radish bolting and flowering control. The outcomes of this study provided insights into the miRNA-mediated flowering-time regulatory networks in radish and advanced the understanding of molecular mechanisms associated with the control of bolting and flowering time in Brassicaceae crops.

Methods

Plant Materials

Seeds of an advanced inbred line of radish, ‘NAU-LU127’ (late bolting and flowering), were sown in plastic pots and cultured in greenhouse at 16 h light (25 °C)/8 h dark (16 °C). To examine the temporal and spatial expression patterns of miRNAs, radish leaves were collected during four-leaf stage (FS), vegetative stage (VS), bolting stage (BS) and reproductive stage (RS), respectively. The root, leaf, flower, floral bud and pod were harvested separately at reproductive stage. All the samples were collected from three randomly selected individual plants and immediately frozen in liquid nitrogen and stored at −80 °C for further use.

High-throughput sequencing of transcriptome and small RNAs

The equal amounts of radish leaves from three independent biological replicates at vegetative and reproductive stages were pooled and used for a radish leaf transcriptome library construction. Total RNA was isolated using Trizol reagent (Invitrogen) according to the manufacturer’s protocol. The transcriptome library was prepared from the mixed leaves using an Illumina TruSeq RNA Sample PrepKit following the manufacturer’s instructions. Two small RNA (sRNA) libraries from leaves at vegetative stage (NAU-VS) and reproductive stage (NAU-RS) were constructed following previously reported procedures, respectively19,20. Briefly, the sRNAs sized at 18–30 nt were separated and gel-purified on a 15% polyacrylamide denaturing gel from the total RNAs of the two samples. Then the isolated sRNAs were ligated to 5′- and 3′-RNA adaptors by T4 RNA ligase (TaKaRa) and transcribed to single-stranded cDNA using One Step PrimerScript miRNA cDNA Synthesis Kit (TaKaRa). Both small RNAs and transcriptome were sequenced using Illumina HiSeq™ 2000 at Beijing Genomics Institute (BGI), Shenzhen, China.

Bioinformatic analysis of small RNA sequencing data

After removing the low quality and contaminated reads from raw reads, clean reads were generated from two independent sRNA libraries. The unique small RNAs that ranged from 18 to 30 nt in length were mapped to the radish reference sequences, which were composed of our radish leaf transcriptome library, the available GSS and EST sequences deposited in NCBI databases. These unique sRNA sequences were compared with non-coding sRNAs deposited in the NCBI GenBank and Rfam (RNA family) databases by BLASTn search. Thereafter, these matched non-coding sRNAs including ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nucleolar RNAs (snoRNAs) and small nuclear RNAs (snRNAs) were filtered and removed. The remaining matched unique sRNA sequences were then aligned with known miRNA sequences from other plant species deposited in miRBase 21 (http://www.mirbase.org/index.shtml) with a maximum of two mismatches for known miRNA identification. The secondary structures of all known miRNA precursors were obtained by Mfold software56.

Identification of novel miRNAs

After mapping the clean reads to NCBI, Rfam and miRBase databases, the unannotated sRNAs were used for novel miRNAs identification. These sRNA sequences were subjected to MIREAP software (https://sourceforge.net/projects/mireap/) to predict novel miRNA candidates according to the parameters as follows: the allowed miRNA sequence length ranged from 18–25 bp, while the range of reference sequence length was from 20–23 bp. The previously reported basic criteria including the presence of corresponding miRNA* sequences, minimum 16 bp in the miRNA and miRNA* pairs, maximum −18 kcal mol–1 for a miRNA precursor free energy, no more than four bulge and asymmetry between miRNA and miRNA*, were also followed in novel miRNA prediction40. In addition, according to the instructions for high confidence miRNAs in miRBase 21, the predicted novel miRNAs were further screened and validated in this study. The secondary structures of pre-miRNA for all novel miRNA candidates were constructed by Mfold software56.

Differential expression analysis of bolting related miRNAs

The expression abundance of all the identified miRNAs was normalized to one million against total clean reads of miRNAs in each library (normalized expression = actual miRNA count/total count of clean reads × 1,000,000). The differential expression of bolting-related miRNAs between the two libraries was calculated as: fold-change = log2 (miRNA normalized reads in ‘NAU-RS’/miRNA normalized reads in ‘NAU-VS’). The P-value was calculated according to previously established methods17,24,57. The miRNAs with fold-change ≥ 1.0 or ≤ −1.0 and P ≤ 0.05 were determined as up- or down-expressed miRNAs during radish bolting and flowering, respectively.

Prediction and annotation of target genes for miRNAs

The potential target prediction of all the identified miRNAs was conducted using the plant small RNA target server (psRNATarget; http://plantgrn.noble.org/psRNATarget/)41. The publicly radish EST and GSS sequences in NCBI database and our mRNA transcriptome sequences were used as the radish reference sequences. To investigate the potential functions of these predicted target transcripts, Blast2GO program was performed for Gene Ontology (GO) annotation and enrichment analysis. Candidate targets were analyzed by BLASTX search against the NCBI Nr database with the default parameters. In addition, KEGG Orthology Based Annotation System (KOBAS 2.0; http://kobas.cbi.pku.edu.cn/home.do/) was employed to further understand the biological functions of genes19,58.

RT-qPCR validation

Reverse transcription quantitative real-time PCR (RT-qPCR) was performed to validate the quality of high-throughput sequencing and the relative expression levels of miRNAs and targets according to previous methods20,59. Total RNAs from all samples were isolated and transcribed to cDNA as described above following the manufacturer’s protocol. Each reaction was performed in a total volume of 20 μl containing 10 μl 2 × SYBR green reaction mix, 2.0 μl diluted cDNA and 0.2 μM of each primer. The amplification reactions were carried out on a BioRad iQ5 sequence detection system (BioRad, USA) following the reported protocol20. Based on the mature miRNA sequences, the specific forward miRNA primers were designed and the reverse primers were universal for miRNA. The specific primers of target genes for RT-qPCR were designed using Beacon Designer 7.0 (Premier Bio-soft International, USA). All the primer sequences of PCRs are listed in Supplementary Table S1. For each condition, three replicates were performed and expression levels were normalized according to that of the internal control. The relative gene expression data were analyzed using the 2−∆∆CT method60. The 5.8S rRNA was used as the reference gene59.

Additional Information

How to cite this article: Nie, S. et al. Identification of bolting-related microRNAs and their targets reveals complex miRNA-mediated flowering-time regulatory networks in radish (Raphanus sativus L.). Sci. Rep. 5, 14034; doi: 10.1038/srep14034 (2015).