Genome-wide identification and characterization, phylogenetic comparison and expression profiles of SPL transcription factor family in B. juncea (Cruciferae)

SQUAMOSA promoter-binding protein-like (SPL), as plant specific transcription factors, is involved in many plant growth and development processes. However, there is less systematical study for SPL transcription factor in B. juncea (Cruciferae). Here, a total of 59 SPL genes classified into eight phylogenetic groups were identified in B. juncea, highly conserved within each ortholog were also found based on gene structure, conserved motif, as well as clustering level. In addition, clustering of SPL domain showed that two zinc finger-like structures and NLS segments were identified in almost of BjuSPLs. Analyzed of putative cis-elements for BjuSPLs demonstrated that SPL transcription factors were involved in adverse environmental changes, such as light, plant stresses and phytohormones response. Expression analysis showed that differentially expressed SPL genes were identified in flower and stem development of Cruciferae; such as BjuSPL3a-B, BjuSPL2b_B and BjuSPL2c_A were significantly expressed in flower; BjuSPL 3b_B and BjuSPL10a_A were significantly expressed in stem node (VP: vegetative period). Moreover, 28 of the 59 BjuSPLs were found involved in their posttranscriptional regulation targeted by miR156. We demonstrated that miR156 negatively regulated BjuSPL10a_A and BjuSPL3b_B to act for stem development in B. juncea.


Introduction
Plant SQUAMOSA promoter-binding protein-like (SPL), acts as fundamental roles in plant growth and development, and are defined by the SBP domains (for SQUAMOSA-PROMO-TER BINDING PROTEIN) that have a highly conserved regions of 76 amino acids in length. SPL contains a conserved SBP (SQUAMOSA promoter binding protein) domain having two zinc-binding sites consisted of Cys-Cys-Cys-His and Cys-Cys-His-Cys, respectively, and a nuclear localization signal (NLS) motif located at the C-terminal of the SBP domain [1]. PLOS  Researches have reported that the SBP domain can act as both nuclear import and sequencespecific DNA binding to a consensus-binding site through GTAC core motif and gene-specific flanking regions [2,3]. SPL genes are widely distributed in gymnosperms, mosses, single-celled green algae and angiosperms [4], SPL genes were firstly detected in Antirrhinum majus that were involved in the control of early flower development by binding to the promoter of the floral meristem identity gene SQUAMOSA (SQUA). In recent years, numerous studies have characterized SPL/SBP-box gene family based on genome-wide identification in many plant species [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Sixteen SBP-LIKE (SPL) genes were clustered into eight clades following their conserved SBP domain in Arabidopsis [1]. Of those, 10 SPL genes belong to five clades are targeted by miR156 [2,3], which might highly conserved and acted as important regulators in vegetative phase change in plants [4,5].
Moreover, SPL genes played vital regulatory roles in diversified plant developmental processes in different plant species, such as vegetative phase change [6], male fertility [7], GA biosynthesis [8], plant architecture [9] and response to stress [9] in plant. However, genome-wide analysis of SPL genes is scarcely in B. juncea owing to lack of genome information, compared with other plant species.
B. juncea (Cruciferae), the tumorous stem mustard, was famous as the raw material for Fuling mustard in China. Currently, the genetics breeding, physiology, biochemistry and classification of B. juncea (Cruciferae) have been extensively studied, but little work has been done at the molecular level. Transcription factors SPL and their regulation roles in B. juncea are poorly understood. Here, genome-wide analysis and molecular dissection of the BjuSPL gene family were performed in this study, together with their chromosomal locations. In addition, we constructed phylogenetic tree for SPL gene family collected from A. thaliana, B. juncea and B. napa. Moreover, cis-acting elements, conserved motif, gene structure and expression pattern of all identified BjuSPL genes in B. juncea were also systematically analyzed. This study will give insight into the structure and evolution of the SPL genes family in B. juncea, thereby further helpful for molecular function analysis of those SPL genes in the future.

Identification and distribution of SPL transcription factor family in B. juncea
SBP domain (PF03110) for SPL transcription factor was downloaded from Pfam [11] (Pfam; http://pfam.xfam.org/), thereby exploiting for the identification of all possible SPL genes from B. juncea using HMMER (v3.1b2) [12] (http://hmmer.org) through the e-value <1e-10. All non-redundant hits with expected values were collected and then compared with the SPL family in PlnTFDB (http://plntfdb.bio.uni-potsdam.de/v3.0/) and PlantTFDB (http://planttfdb. cbi.pku.edu.cn). After that, each putative SPL gene was confirmed to the presence of SBP domain using SMART [13] (http://smart.embl-heidelberg.de/), CDD [14] (http://www.ncbi. nlm.nih.gov/Structure/cdd/wrpsb.cgi) and Inter-ProSca (http://www. ebi.ac.uk/Tools/Inter-ProScan/). The theoretical MW (molecular weight) and PI (isoelectric point) of BjuSPLs were investigated through Expasy57 (http://web.expasy.org/protparam/). We fetched the physical location information of the each SPL from the corresponding GFF files. Subsequently, we used mg2c (http://mg2c.iask.in/mg2c_v2.0/) to depict their distribution in each B. juncea chromosome. In addition, we used BLAST-searching [15] for BjuSPLs against each other to identify the duplicated BjuSPLs genes, which were defined when both their identity and query coverage was > 80% of their partner sequence [16]. Tandem-duplicated genes were identified as an array of two or more homologous genes within a distance of 100 kb. A chromosome region containing more than two genes within 200 kb was defined as a gene cluster [17]. Moreover, comparison and annotation of orthologous gene clusters among B. juncea and B. napa were conducted using orthoVenn software [18].

Phylogenetic analysis of BjuSPLs
For phylogenetic analysis of multiple sequence, full-length protein sequences of BjuSPLs were performed by ClustalW alignment, and then a phylogenetic tree was constructed with MEGA 7.0 software [14] (https://mega.nz/) by the Neighbor-Joining (NJ) method, carried out with 1000 replicates boot-strap test.

Sample collection and RNA isolation
The B. juncea 'YA1', 'YA2' and 'YA3' were cultivated in a greenhouse at the experimental farm of the Yihe (Yangtze normal university experiment base) in 2018. Firstly, we sowed seeds of B. juncea 'YA1', 'YA2' and 'YA3' in sterilized soil for 2 weeks under normal growth conditions (23˚C, 16 h light/8 h dark). After that, 2-week-old plants were transferred and kept for 15 days in the cold room (5 ± 1˚C, 12 h light/12 h dark) for vernalization treatment. After the vernalization periods, the plants were grown in a normal growth room under normal growth conditions (23˚C, 16 h light/8 h dark). At least three independent biological replicates for leaf of seedling stage (SS), leaf and stem of flowing period (FP), leaf and stem of mature period (MP) from B. juncea 'YA1', 'YA2' and 'YA3' were collected for RNA-Seq, as well as qRT-PCR for the candidate differential expression BjuSPLs with three replicates. Of those, samples comprised of seedling, leaf (vegetable periods), stem nodule (vegetable periods), stem nodule (Flowing periods), flower and legume from 'YA1' were collected for qRT-PCR for miRNAs and their related targets with three replicates. All harvested tissue were immediately frozen in liquid nitrogen and stored at -80˚C for qRT-PCR respectively. Subsequently, using the mirVana™miRNA Isolation Kit (Ambion) and Trizol Reagent (Invitrogen, Nottingham, UK) kit following the manufacturer's instructions, small RNA and total RNA was isolated from each sample.

Expression pattern analysis of candidate BjuSPLs in B. juncea
For analyze the expression patterns of BjuSPLs genes, our private available RNA expression profile data including leaf of Seedling stage (SS), leaf and stem of flowing period (FP), leaf and stem of mature period (MP) from YA1 (yonganxiaoye1), YA2 (yonganxiaoye2) and YA3 (yon-ganxiaoye3) were selected respectively (S2 Table). We used Log2 (FPKM) for calculating the expression levels of BjuSPL genes and their expression patterns were clustered by hierarchical clustering model and illustrated using homemade R language.

Prediction of BjuSPLs targeted by miR156
BjuSPLs targeted by miR156 collected from literature were predicted using psRNATarget [22] (http://plantgrn.noble.org/psRNATarget/?function.3), and the parameters were set as follows: with the maximum expectation of 3 and the target accessibility (UPE) of 50.

Validation of candidate BjuSPLs in B.juncea using real-time qRT-PCR
To monitor the candidate BjuSPLs in B. juncea, ten candidate BjuSPLs (BjuSPL10b_A, BjuS-PL10a_A, BjuSPL10b_B, BjuSPL11a_B, BjuSPL11b_B, BjuSPL13a_B, BjuSPL2a_A, BjuSPL2b_ B, BjuSPL2c_B, BjuSPL3c_A, BjuSPL7a_B and BjuSPL9b_B H) were selected for qRT-PCR (ABI 7500 real-time PCR System, United States) based on their expression patterns (S2 Table). The primers are designed by Primer 5.0 software for qRT-PCR experiments and B. juncea gene (Actin) is used as a standard control (S3 Table). First-strand cDNA synthesis was performed with 1 μg total RNA from 'Yonganxiaoye 1' B. juncea using a M-MLV reverse transcriptase (Promega). The amplification programs were performed according to the standard protocol of the ABI7500 system, and conducted in triplicate as mentioned by Jian et al. [23]. The relative quantitative method (2 -ΔΔCT ) was used to calculate the fold change in the expression levels of target genes [24].

Validate the Candidate miRNAs by qRT-PCR
To validate of putative miRNAs targeted the candidate BjuSPLs, miR156 was selected based on their corresponding BjuSPLs comprised of BjuSPL10a_A, BjuSPL10b_B and BjuSPL3b_B. The primers are designed by Primer 5.0 software for qRT-PCR experiments and radish gene (Actin) is used as a standard control (S2 Table). Firstly, using One Step miRNA 1st cDNA Synthesis Kit (Shenggong, Chengdu, China), microRNA reverse transcription reactions were performed and incubated in an Eppendorf Mastercycler (Eppendorf North America, Westbury, NY) for 60 min at 37˚C, followed by 5 min at 95˚C, and then 4˚C until further use. The qRT-PCR reactions were performed in a 10 μL volume containing 1 μL diluted reverse transcription product, 1 uL PCR buffer, 0.2 mM dNTPs, 2.0 U EasyTaq DNA polymerase (Trans-Gen Biotech, Beijing, China), and 0.5 μM specific primer of miRNA156 (5-TTGACAGAAGA GAGTGAGCAC-3) and universal primer (5-TTACCTAGCGTATCGTTGAC-3) on Eppendorf Mastercycler. The amplification programs were performed according to the standard protocol of the ABI7500 system, and conducted in triplicate as mentioned by Jian et al. [23]. The relative quantitative method (2 -ΔΔCT ) was used to calculate the fold change in the expression levels of target genes [24].

Genome-wide identification and distribution analysis of the SPL gene family in B. juncea
We used SBP domain (PF03110) to search against protein of B. juncea using HMMER software to identify SPL genes in B. juncea [12]. All non-redundant hits with expected values were collected and then compared with the SPL family in PlnTFDB (http://plntfdb.bio.uni-potsdam. de/v3.0/) and PlantTFDB (http://planttfdb.cbi.pku.edu.cn). In addition, we verified the presence of the SBP domain for the candidate SPL genes through SMART, Prosca and CDD [13,14]. Total of 59 SPL genes were identified in B. juncea (S4 Table). Of those, we accurately named BjuSPL genes following their closest orthologs in A. thaliana and we coded their different paralogs as a, b, c, and so on, together with their order of the homologous chromosomes. The results showed that lengths and predicted molecular weight (Mw) of 59 SPL genes encoding protein ranged from 106 to 1,038 amino acids, as well as 12.161 to 114.796 kDa in B. juncea respectively (S4 Table). In addition, chromosome location showed that those BjuSPL genes were distributed in 17 of 18 chromosomes present in the B. juncea genome (A1-A10, B01-B04, B06-B08), as well as 5 contigs comprised of Contig533, Contig431, Contig4056, Contig1788, Contig137_156605_433750). We found that the numbers of BjuSPL genes mapped on each chromosome were uneven and ranged from 0 to 8. Of those, 8 BjuSPL genes were distributed in B08 chromosomes in B. juncea, but we did not detect SPL gene in B05 of chromosome (Fig  1, S4 Table). In other species, segmental duplication, tandem duplication, and polyploidization were identified determined the genomic locations of the SPL gene family. We identified 17 pairs of segmentally duplicated and three pairs of tandem-duplicated SPL genes in the B. juncea genome (S1 Fig). In addition, relationship between BjuSPLs with its homologs in B. napus were analyzed using OrthoVenn2 software, the results showed that the species of SPLs were identified obtained from 26 clusters, of those, 12 orthologous clusters were found shared between B. juncea and B. napus, expect for 14 single-copy gene clusters (S5 Table).

Phylogenetic analysis of SPL gene family in B. juncea
Total of 135 SPLs (comprised of 59 obtained from this research in B. juncea, 17 from A. thaliana and 59 from B. napa) were used to construct a Neighbour-Joining (NJ) phylogenetic tree using MEGA 7.0 software. We found those SPL genes were clustered into 8 sub-groups named (I to VIII), and we found at least one protein was obtained from two species (A. thaliana and B. napa) for each group, with differently distinguished in B. juncea for their SPL orthologs. Mustard SPL6, SPL7, SPL8 and SPL13 were categorized into group III, VII, VIII and IV, respectively. Mustard SPL5 and SPL9 were categorized into group II, SPL3, SPL4 and SPL5 were categorized into group VI. Mustard SPL2, SPL10 and SPL11 were categorized into group I, and mustard SPL1, SPL14 and SPL16 belong to group V. However, SPL12 were not identified in B. juncea in this study (Fig 2).

Gene structure and conserved motif analysis of SPLs in B. juncea
To demonstrate the structural diversity of the B. juncea SPL genes, conserved motif, exon/ intron structure and putative cis-acting elements from BjuSPLs promoters were analyzed. We constructed unrooted NJ tree only using protein sequences of 59 BjuSPLs (Fig 3A), as well as their gene structures were analyzed by GSDS 2.0 and displayed in Fig 3B. The results showed that introns number varied from 2 to 12 for those 59 BjuSPLs. Of those, 20 BjuSPLs genes had 5 introns (5 BjuSPL2, 3 BjuSPL6, 4 BjuSPL8, 2 BjuSPL9, 3 BjuSPL13 and 3 BjuSPL15); 6 BjuSPL10 and 4 BjuSPL11 had 6 introns, expect for BjuSPL10b_A, which contained 10 introns owing to segment duplication of some introns; More interestingly, BjuSPL3, BjuSPL4 and BjuSPL5 contained 2-3 introns, and we found BjuSPL1, BjuSPL14 and BjuSPL16 contained 12 introns with an ankyrin (ANK) domain along with an SBP domain, suggesting that these proteins represented different functions compared with other groups, which may play a role by interacting with other proteins in plant cells. We revealed that different BjuSPLs orthologs were similar to Arabidopsis orthologs and exhibited different exon-intron structures.  from different species (B. juncea, A. thaliana and B. napa). The SBP domain was identified and the phylogenetic tree was constructed using MEGA 7, and the maximum likelihood method with 1000 bootstraps. Different groups of the SBP family are divided with different colors. Further, protein motifs were identified for BjuSPLs through full-length proteins to clarify the characteristics of the SBP domain in B. Juncea (S1 Fig). The results showed that 20 motifs with the diversity of sequence structures recognized in BjuSPLs proteins among BjuSPLs, named motifs 1 to 20 were identified (S2 Fig). The lengths of 29 (motif 7) and 50 amino acids (motif1-6 et.al) were identified in those conserved motifs. The number of the conserved motifs varied from 3 for BjuSPL2b_B to 17 for BjuSPL1a_B respectively. Of those, we identified that 59 BjuSPLs proteins covered two Zn-finger like structure motif, and 56 BjuSPLs covered nuclear localization signal motif, except for BjuSPL4b_A, BjuSPL4c_A and BjuSPL2_B. Interestingly, most of BjuSPL proteins contained more motifs in SBP domain, expect for BjuSPL15b_A (had motif 1, 2 and 15), such as BjuSPL2 (6-8 motifs), BjuSPL7 (6-7 motifs), BjuSPL14 (9-10 motifs) and BjuSPL1 (11-13 motifs).
Moreover, we presented the SBP domain structures by constructing the multiple alignments of all 59 BjuSPLs proteins using MAFFT version 7. The results revealed that two zinc finger-like structures (Cys3His-type, Cys2HisCys), together with NLS segments were identified in all BjuSPLs, except for three SPLs (BjuSPL4b_A, BjuSPL4c_A and BjuSPL2B2_B) without NLS (S3 Fig). We demonstrated that two Zn-finger structures and NLS section in the sequences SBP domain of BjuSPLs were conserved in B. juncea, resulted from a similar number and type of SBP motifs identified in each of BjuSPL orthologs. We concluded that similar gene structure, motif architecture of BjuSPL orthologs were significantly clustered into the same phylogenetic tree.

The cis-acting elements analysis of BjuSPLs gene promoter regions
To investigate putative functional of 59 BjuSPL genes in B. juncea, we collected their upstream sequences (2,500 bp upstream of the initiation codon) and then used for cis-acting element prediction by PlantCARE. More than 45 types of putative cis-elements were identified present in the promoters of BjuSPLs, such as light responsive elements, stress responsive elements, phytohormone response element, gibberellin, salicylic acid (TCA-element) and MeJA (S4 Fig, S6  Table). This suggests the most of BjuSPLs genes were involved in different biological processes, including response to abiotic stresses (defense and stress) and phytohormones in B. juncea.

Expression profiles of SPL genes in B. juncea
The expression profiles of all the identified 59 BjuSPLs were analyzed available from our private available RNA expression profile data. The expression of BjuSPLs was constructed by heat map using homemade R and showed in Fig 4. More interestingly, only BjuSPL10b_A and BjuSPL10c_A were found highly expressed in all tissues comprised of leaf of seedling stage (SS), leaf and stem of flowing period (FP), leaf and stem of mature period (MP) from YA1 (yonganxiaoye1), YA2 (yonganxiaoye2) and YA3 (yonganxiaoye3) respectively. Especially for BjuSPL10c_A, which were found highly accumulated in stem of flowing period (FP). In addition, BjuSPL3c_A, BjuSPL3a_B and BjuSPL3b_B were found specifically expressed in leaf of seedling stage (SS), flowing period (FP), mature period (MP) from YA2 (yonganxiaoye2) and YA3 (yonganxiaoye3), expect for YA1 (yonganxiaoye1), especially for BjuSPL3b_B. However, other BjuSPL genes were no significantly detected in all tested tissues. Expression patterns of BjuSPL genes with great alteration showed that BjuSPL3a-B, BjuSPL2b_B and BjuSPL2c_A were significantly expressed in flower; BjuSPL 3b_B and BjuSPL10a_A were significantly expressed in stem node (VP: vegetative period), but only BjuSPL10b_B were found cumulative in seedling (Fig 5).

MiR156-mediated posttranscriptional regulation of BjuSPLs
To understand the miR156-mediated posttranscriptional regulation of the BjuSPL genes, we searched the coding regions and 3 0 UTRs of all BjuSPLs for the targets of mustard miR156a-miR156h. The results showed that 28 BjuSPL genes, belonged to all groups expect for BjuSPL9 that are complementary association with the Bju-miR156 mature sequences (Fig 6), suggesting that Bju-miR156 may specifically regulated these BjuSPL genes in B. juncea. In addition, Bju-

Fig 4. Expression patterns of BjuSPL genes and validate of candidate BjuSPLs in different tissues and cultivars of B. juncea. A.
Expression patterns of BjuSPL genes in different tissues and cultivars of B. juncea. The color represents BjuSPLs expression levels: Log2 (FPKM). The phylogenetic relationship was showed on the left. RNA expression profile data were selected as follows: leaf of seedling stage (SS), leaf and stem of flowing period (FP), leaf and stem of mature period (MP) from YA1 (yonganxiaoye1), YA2 (yonganxiaoye2) and YA3 (yonganxiaoye3) respectively. B. Validate of candidate BjuSPLs in 'YA1' using qRT-PCR and then correlation between RNAseq and qRT-PCR data were conducted. Each RNA-seq expression data was plotted against that from quantitative real-time PCR and fit into a linear regression. Both x-and y-axes were shown in log2 scale and each colour represented a different gene. C. Validate of candidate BjuSPLs in 'YA2' using qRT-PCR and then correlation between RNA-seq and qRT-PCR data were conducted. D. Validate of candidate BjuSPLs in 'YA3' using qRT-PCR and then correlation between RNA-seq and qRT-PCR data were conducted.
https://doi.org/10.1371/journal.pone.0224704.g004 miR156 was found acted on the location in the coding region of BjuSPL10, BjuSPL11 and BjuSPL2 belong to group I, BjuSPL1, BjuSPL14 and BjuSPL16 belong to group V and BjuSPL3, BjuSPL4 and BjuSPL5 belong to group VI, respectively. These results showed that miR156 conserved in plants by mediated posttranscriptional regulation of the SPLs.

Discussions
In recent years, researchers have focused on SPL transcription factors owing to their conservation and specially functions in plants. In addition, the number of SPLs have been identified varies from species to species through genome-wide analysis in plants, including 15 SPLs in potato [25] and tomato [26], 17-20 in rice and Arabidopsis [2,27], as well as 27-30 in Chinese cabbage [28]. However, there is no report about SPL gene family in B. juncea based on genome-wide identification, owing to lack of sequenced genome information. In this research, we firstly reported systematically identification of SPL genes and investigation of their functional structure in B. juncea. Based on our results, 59 SPL genes were identified in B. juncea (S1 Table). Here, phylogenetic tree of SPL proteins in B. juncea, Arabidopsis and B. napus demonstrated that all SPL genes were categorized into eight groups (I-VIII) (Fig 2). Among 59 mustard SPLs, each kind of orthologs from 10 Arabidopsis SPLs orthologs was clustered together. In recent years, researches have demonstrated that segmental duplications were existed and play vital roles in the gene expansion of SPL transcription factor gene family in many plants [29,30]. Here, 17 pairs of segmentally duplicated and three pairs of tandem-duplicated SPL genes were identified in the B. juncea genome. In addition, there were 12 ortholog pairs in syntenic regions between B. juncea and B. napa (S5 Table).
By conducting GSDS structure of SPLs gene in mustard, different gene structures were found in different SPL orthologs contained with the diverse number of introns, including 5 introns in BjuSPL2, BjuSPL8, BjuSPL9, BjuSPL13 and BjuSPL15; 6 introns in BjuSPL10 and BjuSPL11; 3 introns in BjuSPL3, BjuSPL4 and BjuSPL5, as well as 12 introns in BjuSPL1, BjuSPL14 and BjuSPL16. In addition, similar motifs were found shared in different cotton SPL orthologs through MEME analyze. 59 BjuSPL proteins were identified contained two Zn-finger like structure motif, and 56 BjuSPLs contained nuclear localization signal motif, except for BjuSPL4b_A, BjuSPL4c_A and BjuSPL2B2_B. Interestingly, most of BjuSPL proteins contained more motifs in SBP domain, expect for BjuSPL15b_A (had motif 1, 2 and 15), such as BjuSPL2 (6-8 motifs), BjuSPL7 (6-7 motifs), BjuSPL14 (9-10 motifs) and BjuSPL1 (11-13 motifs). Moreover, we found BjuSPL1, BjuSPL14 and BjuSPL16 contained 12 introns with an ankyrin (ANK) domain, suggesting that these proteins represented different functions compared with other groups. We speculated that similar function were probably shared among different SPL orthologs in mustard, based on their similar conserved motifs and exon/intron structure, expect for some specially SPL gene, such as BjuSPL1, BjuSPL14 and BjuSPL16 with an ankyrin (ANK) domain, BjuSPL4b_A, BjuSPL4c_A and BjuSPL2B2_B without nuclear localization signal, NLS.
Functional analyzed of cis-acting elements for BjuSPLs showed that SPL transcription factors may be involved in light, phytohormones and stresses responsiveness. RNA-seq profiles demonstrated that several of putative identified SPL genes show a tissue-dependent and development-dependent expression patterns (Fig 4). Of those, we found that BjuSPL10b_A and BjuSPL10c_A were significantly accumulated in the developmental stage and all tested tissues. BjuSPL3c_A, BjuSPL3a_B and BjuSPL3b_B were found specifically expressed in leaf of Seedling stage (SS), flowing period (FP), mature period (MP) from YA2 (yonganxiaoye2) and YA3 (yonganxiaoye3), expect for YA1 (yonganxiaoye1), especially for BjuSPL3b_B. This result suggests that only several of mustard SPL gene family may play vital roles in leaf and stem development, and sub-functionalization, gained new functions and lost functions possibly existed in paralog genes of SPL gene family. To date, less reports about expression and functional analysis of SPL genes were found in mustard, such as SPL gene family were found to be down-regulated in male-sterility lines [31] and SPL13 was found to be cold-inducible in early stages of development. In addition, the target genes of miR156 such as CAT, ABCB, CBS, and SPL showed their involvement in stress response and temporal expression changes during leaf development in B. juncea [32][33][34].
We proposed that miR156 might also involve in SPL-regulated gene networks in B. juncea. The previous showed that 10 SPLs in Arabidopsis were reported to be potential targets of miR156/157 [5], as well as 17 SPLs in soybean [35], 18 SPLs in Populus [30] and 11 SPLs in rice [36]. Here, we also demonstrated that 28 SPLs were putative targeted by miR156. More importantly, we found that a potential target site (motif 7) was found located in those SPLs targeted by the miR156. These findings were also identified in rice and Arabidopsis, all miR156-targeted SPLs were also found miR156 recognition element located in motif [36]. In Arabidopsis, researches have demonstrated that the most of AtSPLs have been acted as diverse function, such as SPL2, SPL9, SPL10, SPL11, SPL13 and SPL15 were specifically association with shoot development [37], SPL3, SPL4 and SPL5 primarily control flowering time [38]. SPL3, SPL9 and SPL10 specifically expressed in lateral root [39]. SPL1 and SPL12 acted as important regulator at the reproductive stage [40]. SPL7 were found involved in the Cu deficiency response [41] and SPL8 conferred to male fertility, as well as regulated gynoecium differential patterning [7,42]. In our study, we found BjuSPL2c_A, BjuSPL3a_B and BjuSPL2b_B were found specifically expressed in flower in mustard and BjuSPL10b_B targeted by miR156 were found highly accumulated in seedling, together with BjuSPL3b_b and BjuSPL10a_A in stem node (VP). More interestingly, we demonstrated that BjuSPL3b_b and BjuSPL10a_A were negatively regulated by miR156 that specially involved in stem node (VP) development. However, the detailed expression patterns and function of those tissue-dependent and development-dependent SPL transcription factor, as well as their regulators miR156 remains to be further investigated in mustard. This is firstly report about identification of SPL gene family in mustard; it would be helpful for well functional analysis of tissue-dependent and development-dependent SPL transcription factor in the future.