Genome-wide analysis of the Dof transcription factor gene family reveals soybean-specific duplicable and functional characteristics.

The Dof domain protein family is a classic plant-specific zinc-finger transcription factor family involved in a variety of biological processes. There is great diversity in the number of Dof genes in different plants. However, there are only very limited reports on the characterization of Dof transcription factors in soybean (Glycine max). In the present study, 78 putative Dof genes were identified from the whole-genome sequence of soybean. The predicted GmDof genes were non-randomly distributed within and across 19 out of 20 chromosomes and 97.4% (38 pairs) were preferentially retained duplicate paralogous genes located in duplicated regions of the genome. Soybean-specific segmental duplications contributed significantly to the expansion of the soybean Dof gene family. These Dof proteins were phylogenetically clustered into nine distinct subgroups among which the gene structure and motif compositions were considerably conserved. Comparative phylogenetic analysis of these Dof proteins revealed four major groups, similar to those reported for Arabidopsis and rice. Most of the GmDofs showed specific expression patterns based on RNA-seq data analyses. The expression patterns of some duplicate genes were partially redundant while others showed functional diversity, suggesting the occurrence of sub-functionalization during subsequent evolution. Comprehensive expression profile analysis also provided insights into the soybean-specific functional divergence among members of the Dof gene family. Cis-regulatory element analysis of these GmDof genes suggested diverse functions associated with different processes. Taken together, our results provide useful information for the functional characterization of soybean Dof genes by combining phylogenetic analysis with global gene-expression profiling.

Introduction

The transcriptional regulation of gene expression influences or controls many important cellular processes, such as signal transduction, morphogenesis, and environmental stress responses [1]. Transcription factors (TFs) are a group of proteins that control cellular processes by regulating the expression of downstream target genes [2]. Therefore, the identification and functional characterization of TFs is essential for the reconstruction of transcriptional regulatory networks [3]. In plants, ~60 families of TFs have been identified based on bioinformatics analysis and manual inspection [4,5]. The genome codes for at least 1533 TFs, which account for about 5.9% of its estimated total number of genes [1]. As for soybean (), ~12.2% of the 46,430 predicted protein-coding loci have been identified to encode 5,671 putative TFs [6].

The Dof (DNA binding with one finger) TF family belongs to a class of plant-specific TFs that are not found in other eukaryotes such as yeast, , , fish or humans [7]. Bioinformatics analysis predicts 36 Dof genes in the genome and 30 in the rice genome [8], while 41 have been described in poplar [9], 31 in wheat [10], and 28 in sorghum [11]. Dof protein is characterized by an N-terminal Dof domain of 50-52 amino-acid residues structured as a Cys2/Cys2 (C2/C2) zinc finger that recognizes a cis-regulatory element containing the common core sequence 5’-

(T/A)AAAG-3’ [12-14]. The Dof domain is bifunctional, mediating both DNA-protein and protein-protein interactions. Different Dof TFs may form homo- and/or hetero-dimeric complexes through the Dof domain in a given cell type and have various functions, acting as positive or negative regulators of their targets [15,16]. Other than the conserved Dof domain, diversified transcriptional regulation domains are also located at the C-terminal regions of Dof proteins. The conserved Dof domain might endow all Dof domain proteins with similar characteristics, while the diversified regions outside the Dof domain might be linked to the different functions of distinct Dof domain proteins [14].

Dof TFs are associated with many plant-specific physiological processes related to stress responses, photosynthesis, growth and development [17-27]. In , some of the well-characterized Dof genes include DAG1 and DAG2 which are associated with seed germination [17,28], and CDF1, CDF2 and CDF3 which are involved in the photoperiodic control of flowering [19]. Some of the Dof TF genes (AtDof2.4, AtDof5.8 and AtDof5.6/HCA2) are reported to be expressed specifically in cells at an early stage of vascular tissue development [18,29]. In rice, OsDof3 is involved in gibberellins-regulated expression [30]. Maize Dof1 and Dof2 are activators of gene expression associated with carbohydrate metabolism, including the gene encoding phosphoenolpyruvate carboxylase [25,27]. In wheat, the Dof TF gene WPBF functions both during seed development and other growth and development processes [31]. A Dof gene, StDof1, which is expressed in epidermal fragments highly-enriched in guard cells, interacts in a sequence-specific manner with a KST1 promoter fragment containing the TAAAG motif in tomato [12]. Some Dof TF genes also take part in the stress and defense responses of plants. Previous study showed that the RNA expression levels of three Dof genes (OBP1, OBP2, and OBP3) increase following treatment with auxin, salicylic acid or cycloheximide, while the OBP proteins have similar in vitro DNA-binding properties and are able to interact with OBF4, a bZIP transcription factor [32]. In response to drought treatment, some TaDof genes are down-regulated and two of them (TaDof14 and TaDof15) are significantly upregulated, indicating that these genes may be involved in drought adaptation [10].

Although quite a few Dof TFs have been functionally characterized in the model plant and others, the functions of most members of the Dof family remain unknown. Especially in soybean, the typical legume species, there are only very limited reports on the functional characterization of Dof TFs. Wang et al. (2006) identified 28 GmDof proteins with recognizable Dof domain from 39 putative unigenes for the Dof gene family after analysis of their Expressed Sequence Tags (ESTs) in soybean [33,34] and detailed study of two GmDof genes suggested they increased the content of total fatty-acids and lipids in transgenic by upregulating genes that were associated with fatty-acid biosynthesis [34]. Completion of the soybean genome greatly facilitated the identification of gene families at the whole-genome level [6]. In the present study, a genome-wide identification of Dof domain TFs in soybean was performed and revealed an expanded Dof family with 78 members.

Detailed analysis of the sequence phylogeny, genome organization, gene structure, conserved motifs, duplication status, expression profiling, and cis-elements was performed. It is noteworthy that nearly all of the GmDof genes (38 pairs) were preferentially-retained duplicates located in duplicated regions of the genome, indicating soybean-specific duplicable characteristics of the Dof gene family in this species. The putative soybean-specific functions of the predicted GmDof genes were investigated by analyzing the expression profiles using RNA-seq data and cis-regulatory elements associated with these genes in the promoter region. Our data provide a basis for the further evolutionary and functional characterization of the Dof gene family in soybean.

Materials and Methods

Database search and sequence retrieval

The Dof sequences of and were downloaded from the genome TAIR release 9.0 (http://www.arabidopsis.org/) and the rice genome annotation database (http://rice.plantbiology.msu.edu/, release 5.0). The amino-acid sequence of the Dof domain was used to search for potential Dof-domain homolog hits in the whole-genome sequence of with BLASTP at the Phytozome database (http:/www.phytozome.net) [35]. All non-redundant hits with expected values <1E-5 were collected and compared with the Dof family in PlantTFDB (http://planttfdb.cbi.edu.cn/) [5] and LegumeTFDB (http://legumetfdb.psc.riken.jp/) [36]. As for the incorrectly-predicted genes, manual re-annotation was performed using the on-line web server GENSCAN (http://genes.mit.edu/GENSCAN.html) [37] and/or RT-PCR cloning. The re-annotated sequences were further manually analyzed to confirm the presence of the Dof domain using the InterProScan program (http://www.ebi.ac.uk/Tools/InterProScan/) [38].

Protein Alignment and Phylogenetic Analysis

Multiple sequence alignments of the full-length deduced amino-acid sequences of Dof proteins were performed by Clustal X (version 1.83) [39]. The distribution of amino-acid residues at the corresponding positions in domain profiles for the conserved Dof domains of GmDofs were created using WebLogo [40]. Unrooted phylogenetic trees were constructed with MEGA 4.0 using the Neighbor-Joining (NJ) method and the bootstrap test carried out with 1000 iterations [41]. The pairwise gap deletion mode was used to ensure that the more divergent C-terminal domains could contribute to the topology of the NJ tree.

Genomic structure and chromosomal location

The Gene Structure Display Server program [42] was used to illustrate the exon/intron organization for individual Dof genes by comparison of the coding sequences with their corresponding genomic DNA sequences from Phytozome (http://www.phytozome.net/gmax). The chromosomal locations of soybean Dofs were mapped to the duplicated blocks using the CViT (Chromosome Visualization Tool) genome search and synteny viewer at the Legume Information System (http://comparative-legumes.org/) [43,44]. The deduced amino-acid sequences of all GmDofs were used to search against the soybean genome and the results were displayed using CViT.

Calculation of Ks and Ka to date duplication events

Clustal X (version 1.83) was used to make pairwise alignments of the paralogous nucleotide sequences [39]. Ks (synonymous substitution rate) and Ka (non-synonymous substitution rate) were estimated using the program DnaSp v5 [45]. The Ks values were then used to calculate the approximate date of duplication event (T = Ks/2λ), assuming a clock-like rate (λ) of synonymous substitution of 6.1×10−9 substitutions/synonymous site/year for soybean [6,46,47].

Identification of conserved motifs

The deduced amino-acid sequences of the 78 GmDofs were analyzed by MEME (Multiple EM for Motif Elicitation) version 4.9.0 (http://meme.nbcr.net/meme/cgi-bin/meme.cgi) [48] for motif analysis. To identify conserved motifs in these sequences, selection of the maximum number of motifs was set to 30 with a minimum width of 6 and a maximum width of 200 amino-acids, while other factors were set at default values. Structural motif annotation was performed using the SMART (http://smart.embl-heidelberg.de) [49] and Pfam (http://pfam.sanger.ac.uk) databases [50].

Expression analysis of soybean Dof genes

The genome-wide transcriptome data from seeds during several stages of development and throughout the soybean life cycle (obtained with high-throughput sequencing) were downloaded from the NCBI database (http://www.ncbi.nlm.nih.gov; accession numbers SRX062325–SRX062334). The transcript data were obtained from seeds at five stages of development (globular, heart, cotyledon, early-maturation, and dry seeds), vegetative tissue (leaves, roots, stems, and whole seedlings), and reproductive tissue (floral buds). All transcript data were analyzed with Cluster 3.0 [51] and the heat map was viewed in Java Treeview [52].

Cis-regulatory element analysis

For promoter analysis, 1000-bp sequences upstream from the initiation codon of the putative GmDofs were retrieved. These sequences were then subjected to search in the PLACE database (http://www.dna.affrc.go.jp/PLACE/signalscan.html) [53] to identify cis-regulatory elements.

Results and Discussion

Identification of Dof-encoding gene family in soybean

In order to identify the Dof gene family in the soybean genome, the amino-acid sequence of the conserved Dof domain was used to perform a BLAST search against the v1.1 genome (http://www.phytozome.net). A total of 79 non-redundant Dof transcription factor-encoding genes were identified from the whole genome. The presence of the conserved Dof domain in the predicted GmDof protein was a typical feature for consideration as a member of the Dof TF family. To verify the reliability of our results, all of the putative Dof protein sequences were subjected to functional analysis by InterProScan. A typical zinc-finger Dof-type profile was found in all GmDof-encoding genes except for one, annotated as Glyma08g12230, which appears to be a pseudogene owing to a stop codon within the Dof domain.

The 78 soybean Dof genes were numbered from GmDof01.1 to GmDof20.2 following the nomenclature proposed for and according to their positions on different chromosomes. The identified GmDof genes encode peptides ranging from 147 to 555 amino-acids in length with an average of 335. The detailed information of the Dof family genes in soybean, including accession numbers and similarities to their orthologs, as well as nucleotide and protein sequences, are listed in Table 1 and Additional Table S1. The Dof gene family in soybean is largest compared with the estimates for other plant species, which range from ~36 in [13], ~30 in rice [8], ~28 in sorghum [11] and ~27 in [54]. The member of Dof genes in soybean is roughly 2.4-fold that in , which is consistent with the ratio of 1.4-1.6 putative homologs for each gene, based on comparative genomics studies [9]. This ratio is almost consistent with that among all the putative protein coding genes of these three species, although the genome size of soybean (1,115 Mb) is almost 9.7 times that of (115 Mb) and 2.3 times that of (480 Mb) [6,55,56].

Table 1

Summary of Dof family members in soybean.

Gene Symbol	Gene Locus	Gene Location	Amino Acids	Introns	Score	E-value
GmDof01.1	Glyma01g02610	Gm01: 2137617-2139436	337	0	106.4	8.00E-24
GmDof01.2	Glyma01g05960	Gm01: 5750259-5754433	479	1	92.0	4.00E-20
GmDof01.3	Glyma01g38970	Gm01: 50951027-50952807	336	0	104.4	3.10E-23
GmDof02.1	Glyma02g06970	Gm02: 5595711-5596415	234	0	96.7	5.50E-21
GmDof02.2	Glyma02g10250	Gm02: 8123065-8125204	371	1	101.3	2.30E-22
GmDof02.3	Glyma02g12081	Gm02: 10302501-10306472	485	1	95.9	1.00E-20
GmDof02.4	Glyma02g35296	Gm02: 40034736-40035659	307	0	102.1	1.60E-22
GmDof03.1	Glyma03g01030	Gm03: 756237-758785	472	1	92.8	9.20E-20
GmDof03.2	Glyma03g41980	Gm03: 47319684-47321893	257	0	105.1	1.70E-23
GmDof04.1	Glyma04g31690	Gm04: 35880682-35882596	341	0	99.8	8.00E-22
GmDof04.2	Glyma04g33410	Gm04: 39029262-39032664	470	1	100.5	4.30E-22
GmDof04.3	Glyma04g35650	Gm04: 42048974-42051454	344	1	110.2	5.50E-25
GmDof04.4	Glyma04g41170	Gm04: 47030349-47032300	297	1	105.1	1.80E-23
GmDof04.5	Glyma04g41830	Gm04: 47667211-47668500	289	0	110.5	4.30E-25
GmDof05.1	Glyma05g00970	Gm05: 586599-589518	473	1	98.2	2.00E-21
GmDof05.2	Glyma05g02220	Gm05: 1636697-1639230	330	1	105.5	1.30E-23
GmDof05.3	Glyma05g07460	Gm05: 7516304-7518205	292	0	104.8	2.00E-23
GmDof05.4	Glyma05g29090	Gm05: 34760928-34763043	165	1	92.0	1.60E-19
GmDof06.1	Glyma06g12950	Gm06: 10094214-10095083	289	0	112.1	1.40E-25
GmDof06.2	Glyma06g13671	Gm06: 10805902-10807867	206	1	104.8	2.40E-23
GmDof06.3	Glyma06g19330	Gm06: 15557061-15559563	353	1	108.2	2.00E-24
GmDof06.4	Glyma06g20950	Gm06: 17335571-17338829	458	1	100.9	2.90E-22
GmDof06.5	Glyma06g22797	Gm06: 19579399-19580371	303	1	99.8	6.80E-22
GmDof07.1	Glyma07g01461	Gm07: 936400-938618	211	0	98.6	1.40E-21
GmDof07.2	Glyma07g05950	Gm07: 4649017-4651265	281	0	107.1	4.90E-24
GmDof07.3	Glyma07g31340	Gm07: 36361704-36363720	332	0	97.1	4.70E-21
GmDof07.4	Glyma07g31860	Gm07: 36820811-36821677	288	0	93.2	7.60E-20
GmDof07.5	Glyma07g31870	Gm07: 36829670-36831859	348	1	103.2	6.90E-23
GmDof07.6	Glyma07g35690	Gm07: 41004726-41008389	479	1	97.1	5.20E-21
GmDof08.1	Glyma08g20840	Gm08: 15829658-15831897	213	0	93.6	5.80E-20
GmDof08.2	Glyma08g24591	Gm08: 18749907-18753887	463	1	95.1	1.70E-20
GmDof08.3	Glyma08g37530	Gm08: 36252447-36254191	403	0	105.9	9.00E-24
GmDof08.4	Glyma08g47290	Gm08: 46169187-46171177	367	1	108.6	1.50E-24
GmDof09.1	Glyma09g33350	Gm09: 39841007-39842035	342	0	105.9	9.00E-24
GmDof09.2	Glyma09g37170	Gm09: 42705807-42709793	503	1	91.7	2.00E-19
GmDof10.1	Glyma10g10142	Gm10: 9742414-9743975	309	0	102.4	1.10E-22
GmDof10.2	Glyma10g31700	Gm10: 40190913-40205863	324	1	103.2	6.80E-23
GmDof11.1	Glyma11g06300	Gm11: 4474891-4476607	339	0	104.0	3.70E-23
GmDof11.2	Glyma11g14920	Gm11: 10654917-10656815	288	1	104.0	4.30E-23
GmDof11.3	Glyma11g15761	Gm11: 11423453-11425703	310	1	101.7	2.10E-22
GmDof12.1	Glyma12g06880	Gm12: 4679868-4681949	307	1	104.0	3.40E-23
GmDof12.2	Glyma12g07710	Gm12: 5322929-5325618	305	1	107.8	2.90E-24
GmDof13.1	Glyma13g05480	Gm13: 5801463-5804791	488	1	96.3	7.60E-21
GmDof13.2	Glyma13g24600	Gm13: 27964926-27967177	353	1	102.1	1.50E-22
GmDof13.3	Glyma13g24611	Gm13: 27973342-27974271	309	0	96.7	6.50E-21
GmDof13.4	Glyma13g25120	Gm13: 28389200-28391375	336	0	97.1	4.80E-21
GmDof13.5	Glyma13g30331	Gm13: 33007956-33010080	147	1	86.3	8.00E-18
GmDof13.6	Glyma13g31100	Gm13: 33571320-33573635	357	1	103.2	6.30E-23
GmDof13.7	Glyma13g31110	Gm13: 33583810-33584763	317	0	102.1	1.40E-22
GmDof13.8	Glyma13g31560	Gm13: 33969725-33970600	278	0	93.2	6.00E-20
GmDof13.9	Glyma13g40420	Gm13: 40913246-40915457	285	1	104.0	3.80E-23
GmDof13.10	Glyma13g41031	Gm13: 41429101-41431274	269	1	102.4	1.10E-22
GmDof13.11	Glyma13g42820	Gm13: 42682406-42684307	212	0	103.2	5.80E-23
GmDof15.1	Glyma15g02620	Gm15: 1777967-1779680	211	0	103.2	7.00E-23
GmDof15.2	Glyma15g04430	Gm15: 3099789-3101706	304	1	102.8	8.70E-23
GmDof15.3	Glyma15g04980	Gm15: 3568928-3571019	285	1	101.3	2.50E-22
GmDof15.4	Glyma15g07730	Gm15: 5453626-5455994	285	0	93.2	6.70E-20
GmDof15.5	Glyma15g08230	Gm15: 5800695-5803209	313	0	102.1	1.40E-22
GmDof15.6	Glyma15g08250	Gm15: 5817356-5819506	353	1	109.8	6.50E-25
GmDof15.7	Glyma15g08860	Gm15: 6264258-6266252	153	1	86.3	8.00E-18
GmDof15.8	Glyma15g29870	Gm15: 32718091-32721358	464	1	93.2	7.10E-20
GmDof16.1	Glyma16g02550	Gm16: 2119565-2121907	276	0	107.1	4.90E-24
GmDof16.2	Glyma16g26030	Gm16: 30193624-30194977	236	0	94.7	2.00E-20
GmDof17.1	Glyma17g08950	Gm17: 6612406-6614430	300	0	99.4	9.30E-22
GmDof17.2	Glyma17g09710	Gm17: 7203819-7206839	330	1	108.6	1.70E-24
GmDof17.3	Glyma17g10920	Gm17: 8207249-8210723	471	1	99.4	0.0
GmDof17.4	Glyma17g21540	Gm17: 20917544-20919496	352	0	105.5	1.30E-23
GmDof18.1	Glyma18g26870	Gm18: 30922106-30923215	369	0	104.4	2.90E-23
GmDof18.2	Glyma18g38560	Gm18: 46153747-46155733	363	1	102.8	9.20E-23
GmDof18.3	Glyma18g49520	Gm18: 58916821-58920915	501	1	95.1	1.70E-20
GmDof18.4	Glyma18g52661	Gm18: 61211505-61213733	363	1	102.4	1.20E-22
GmDof19.1	Glyma19g02710	Gm19: 2647356-2650816	385	1	97.1	4.90E-21
GmDof19.2	Glyma19g29610	Gm19: 37285687-37288840	483	1	90.9	3.00E-19
GmDof19.3	Glyma19g38660	Gm19: 45513027-45514071	271	0	104.0	4.00E-23
GmDof19.4	Glyma19g38750	Gm19: 45606704-45607516	270	0	99.4	8.40E-22
GmDof19.5	Glyma19g44670	Gm19: 50031772-50033750	252	0	102.8	7.40E-23
GmDof20.1	Glyma20g04600	Gm20: 4815565-4819043	482	1	95.5	1.20E-20
GmDof20.2	Glyma20g35910	Gm20: 44105729-44107846	300	1	103.2	5.70E-23

To investigate the features of the homologous domain sequences, and the frequency of the most prevalent amino-acids at each position within the soybean Dof domain, multiple-alignment analysis using the amino-acid sequences of the Dof domains from 78 GmDofs was performed. In general, the basic regions of the Dof domains had 52 basic residues. The distribution of amino-acid residues at the corresponding positions of the soybean Dof domains also revealed that it was very similar to that of , as expected from the evolutionary distances among plants (Figure 1). The Dof domain of soybean revealed highly-conserved sequences and 26 out of 52 amino-acids were 100% conserved in all GmDof proteins, including four absolutely-conserved cysteine residues that presumably coordinate zinc ion. Other highly conserved residues in the soybean Dof domains were Pro-4, Arg-5, Ser-8, Thr-11, Lys-12, Phe-13, Cys-14, Tyr-15, Asn-17, Asn-18, Tyr-19, Gln-23, Pro-24, Arg-25, Arg-33, Trp-35, Thr-36, Gly-38, Gly-39, Arg-42, Gly-47 and Gly-49. These highly-conserved residues were also nearly identical to the Dof domain proteins of other plants such as sorghum and tomato [11,57]. Moreover, five other amino-acid residues showed variation in less than three sequences among all GmDofs.

Figure 1

Dof domains are highly conserved across all Dof proteins in soybean.

The sequence logos are based on alignments of all soybean Dof domains. Multiple alignment analysis of 78 typical soybean Dof domains was performed with ClustalW. The bit score indicates the information content for each position in the sequence. Asterisks indicate the conserved cysteine residues (Cys) in the Dof domain.

Phylogenetic Relationships and Gene Structure of Soybean Dof Genes

To examine the phylogenetic relationships among the Dof domain proteins in soybean, an unrooted tree was constructed from alignments of the full-length amino-acid sequences of all GmDof proteins (Figure 2A). The observed sequence similarity and phylogenetic tree topology allowed us to classify the soybean Dof gene family into nine subgroups (subgroups I-IX). Each subgroup had 4-19 members and the very high bootstrap value in each subgroup suggested a common origin for the Dof genes in each subgroup. Inspection of the phylogenetic tree topology revealed several pairs of Dof proteins with a high degree of homology in the terminal nodes of each subgroup, suggesting that they are putative paralogous pairs (Figure 2A). A total of 38 pairs of putative paralogous Dof proteins were identified, accounting for nearly the entire family (except for GmDof17.4 and GmDof05.4), with sequence identity ranging from 72% to 97% (see Additional Table S2 for details). So many putative paralogous Dof proteins supported the hypothesis that they evolved from a recent soybean genome duplication event [58].

Figure 2

Phylogenetic relationships and gene structure of soybean Dof genes.

(A) The phylogenetic tree of soybean Dof proteins constructed from a complete alignment of 78 GmDof proteins using MEGA 4.0 by the neighbor-joining method with 1,000 bootstrap replicates. Percentage bootstrap scores >50% are indicated on the nodes. The nine major phylogenetic subgroups designated I to IX are indicated. (B) Exon/intron structures of Dof genes from soybean. Exons are represented by green boxes and introns by black lines. The sizes of exons and introns can be estimated using the scale below.

It is well known that gene structural diversity is a possible mechanism for the evolution of multigene families. In order to gain further insight into the structural diversity of Dof genes, we compared the exon/intron organization in the coding sequences of individual Dof genes in soybean. A detailed illustration of the exon/intron structures is shown in Figure 2B. According to their predicted structures, 35 of the GmDof genes have no introns whereas 38 contain one intron generally placed up-stream of the Dof domain, except for five (GmDof10.2, GmDof20.2, GmDof13.5, GmDof15.7, and GmDof05.4) with a down-stream intron. These exon/intron structures are similar to those of , rice, and other plants [8,11,54]. The most closely-related members in the same subgroup generally showed the same exon/intron pattern, with the position and length of the intron almost completely conserved within most subgroups (Figure 2). For instance, the Dof genes in subgroups II, IV, VII and VIII all lacked an intron, while all members of subgroups III and IX contained one intron. In contrast, the gene structure appeared to be more variable in subgroups I, V and VI, which had the largest numbers of exon/intron structural variants with striking distinctions.

Chromosomal location and duplication of soybean Dof genes

Genome chromosomal location analyses revealed that GmDofs were non-randomly distributed on 19 of the 20 chromosomes (Figure 3). Nearly all GmDof genes were distributed on the chromosome arms while none were on the heterochromatin regions around the centromeric repeats. Among these chromosomes, chromosome 13 contained the largest number of eleven Dof genes followed by eight on chromosome 15. In contrast, no Dof genes were found on chromosome 14 and only two occurred on six chromosomes (chromosome 03, 09, 10, 12, 16, and 20). Substantial clustering of Dof genes was evident on several chromosomes, especially on those with high densities of the genes. For example, GmDof07.4 and GmDof07.5 located in an 8.8-kb segment on chromosome 07, while GmDof15.5 and GmDof15.6 located within a 19-kb segment on chromosome 15. Similarly, four genes (GmDof13.2 and 13.3, and GmDof13.6 and 13.7) were arranged in two clusters in 10-kb and 13-kb segments on chromosome 13 respectively (Figure 3).

Figure 3

Chromosomal locations, region duplications, and predicted clusters for soybean Dof genes.

The schematic diagram of genome-wide chromosome organization and segmental duplication arising from the genome duplication event in soybean was derived from the CViT genome search and synteny viewer at the Legume Information System (http://comparative-legumes.org). Colored blocks to the left of each chromosome show duplications with chromosomes of the same color. For example, the gray blocks at the bottom of Gm10 correspond with regions on the brown Gm20, and vice versa. The chromosomal positions of all Dof genes in soybean were mapped on each chromosome. The locations of centromeric repeats are shown as black rectangles over the chromosomes. The chromosome numbers are indicated at the top of each bar and sizes of chromosomes are represented by the vertical scale.

Segmental duplication, tandem duplication, and transposition events are the main causes of gene-family expansion. Two or more genes located on the same chromosome confirms a tandem duplication event, while gene duplication on different chromosomes is designated a segmental duplication event [59]. Previous studies revealed that the soybean genome has undergone at least two rounds of genome-wide duplication followed by multiple segmental duplication, tandem duplication, and transposition events such as retroposition and replicative transposition [58]. To detect a potential relationship between putative paralogous pairs of soybean Dofs and potential segmental duplications, the Dof genes were mapped to the duplicated blocks using the CViT genome search and synteny viewer at the Legume Information System (http://comparative-legumes.org/) [43,44]. The distributions of Dof genes relative to the corresponding duplicate genomic blocks are illustrated in Figure 3. Within the duplicated blocks associated with a duplication event, 22 out of 38 putative paralogous pairs were preferentially-retained duplicates that were located in a segmental duplication of a long fragment (>1 Mb), and 13 putative paralogous pairs were located in a segmental duplication of a short fragment (<1 Mb) (Table 2). Another two putative paralogous pairs lacked the corresponding duplicates and only one putative paralogous pair (GmDof19.3/19.4) was possibly due to tandem duplication in the same orientation. These results implied that segmental duplication was predominant for Dof gene evolution in soybean, and that tandem duplication was involved. This relationship between soybean Dofs and potential segmental duplications suggests that dynamic changes occurred following segmental duplication, leading to loss of some of the genes.

Table 2

Duplicated Dof genes in soybean and the dates of the duplication blocks.

Gene 1	Gene 2	Fragment Duplication	Ka	Ks	Ka/Ks	Date (Mya)
GmDof07.3	GmDof13.4	Small	0.0313	0.1010	0.3099	8.28
GmDof07.5	GmDof13.2	Small	0.0662	0.1355	0.4886	11.11
GmDof13.6	GmDof15.6	Large	0.0556	0.0951	0.5846	7.80
GmDof07.4	GmDof13.3	Small	0.0916	0.1079	0.8489	8.84
GmDof13.7	GmDof15.5	Large	0.0441	0.1205	0.3660	9.88
GmDof02.2	GmDof18.4	Small	0.0498	0.0938	0.5309	7.69
GmDof13.10	GmDof15.2	Large	0.0555	0.1133	0.4898	9.29
GmDof08.3	GmDof18.1	None	0.1244	0.3315	0.3753	27.17
GmDof13.11	GmDof15.1	Large	0.0424	0.1295	0.3274	10.61
GmDof10.2	GmDof20.2	Large	0.0615	0.1561	0.3940	12.80
GmDof04.4	GmDof06.2	Large	0.0496	0.1395	0.3556	11.43
GmDof11.3	GmDof12.2	Small	0.0369	0.1188	0.3106	9.74
GmDof13.9	GmDof15.3	Large	0.0379	0.1148	0.3301	9.41
GmDof05.2	GmDof17.2	Large	0.0406	0.1156	0.3512	9.48
GmDof04.1	GmDof06.5	None	0.0811	0.2524	0.3213	20.69
GmDof04.5	GmDof06.1	Large	0.0807	0.2125	0.3798	17.42
GmDof02.4	GmDof10.1	Small	0.0410	0.1334	0.3073	10.93
GmDof03.1	GmDof19.2	Small	0.0503	0.1633	0.3080	13.39
GmDof08.2	GmDof15.8	Small	0.0901	0.1474	0.6113	12.08
GmDof07.6	GmDof20.1	Small	0.0458	0.1444	0.3172	11.84
GmDof05.1	GmDof17.3	Large	0.0448	0.0732	0.6120	6.00
GmDof13.1	GmDof19.1	Large	0.0633	0.1013	0.6249	8.30

In order to trace the dates of the duplication blocks, the DnaSP program was used to estimate the Ks and Ka distances, as well as the Ka/Ks ratios. The approximate dates of duplication events were calculated using Ks. Table 2 shows the results of analysis of segmental and tandem duplication blocks. The segmental duplications of the Dof genes in soybean originated from 6.0 Mya (million years ago, Ks = 0.0732) to 27.17 Mya (Ks = 0.2018), with the mean of 11.90 Mya (Ks = 0.1452); the Ks of tandem duplication of GmDof19.3 and GmDof19.4 was 0.0111, dating the duplication event at 0.91 Mya. Since the soybean genome underwent two polyploidy events at 13 and 58 Mya, all the segmental duplications of the GmDof genes occurred around 13 Mya when -specific duplication occurred in the soybean genome. The Ka/Ks ratios of 15 segmental duplication pairs and one tandem duplication pair were <0.3, while the ratios of the other 22 segmental duplication pairs were all >0.3, suggesting that significant functional divergence of some GmDof genes might have occurred after the duplication events.

Phylogenetic analysis of the Dof gene family in soybean, , and rice

To investigate the molecular evolution and phylogenetic relationships among the Dof domain proteins in soybean, , and rice, the 78 predicted GmDof proteins were subjected to multiple sequence alignment along with 36 and 30 rice Dof proteins, and an unrooted phylogenetic tree was constructed using the NJ method, based on the alignment of all the Dof amino-acid sequences (Figure 4, Additional Table S3). The NJ tree showed that all the Dof family proteins from the three higher plants were divided into four Major Clusters of Orthologous Groups (MCOG A, B, C, and D) and nine well-supported clades (Figure 4), similar to previous reports [8,13]. Among these, group C constituted the largest clade, containing 47 members and accounting for 32.6% of the total Dof genes, and the other three groups contained 25 (Group A), 30 (Group B), and 42 (Group D) members, respectively. In general, the Dof members demonstrated an interspersed distribution in most subfamilies, indicating that the expansion of Dof genes occurred before the divergence of soybean, , and rice. Based on the phylogenetic tree, several putative orthologs (GmDof06.3/AtDof5.6, OsDof-2/GmDof07.6 (GmDof09.2), AtDof1.6/OsDof-10, or AtDof2.4/OsDof-16/GmDof13.10 (GmDof15.2)) and paralogs (AtDof5.7/AtDof4.7, OsDof-13/OsDof-30, GmDof03.1/GmDof19.2) were also identified.

Figure 4

Phylogenetic tree of all Dof domain containing proteins from soybean, , and rice.

The deduced full-length amino-acid sequences of 78 soybean, 36 and 30 rice Dof genes were aligned by Clustal X 1.83 and the phylogenetic tree was constructed using MEGA 4.0 by the neighbor-joining method with 1,000 bootstrap replicates. Each Dof subgroup is indicated by a specific color.

Moreover, since most of the Dof genes with similar functions showed a tendency to fall into one subgroup, soybean Dof genes in the same subgroup may have similar functions. In subgroup A, eight soybean Dof genes clustered with the Dof genes AtDof2.4, AtDof4.7, AtDof5.7 and AtDof3.6(OBP3) in subgroup B1, and these have been identified to be involved in tissue differentiation (vascular development, floral organ abscission, leaf blade polarity and growth regulation) [20,29,32,60,61]. About 19 GmDofs showed maximum similarity with AtDof5.5(CDF1), AtDof5.2(CDF2), AtDof3.3(CDF3), AtDof2.3(CDF4), AtDof1.10(CDF5), and AtDof1.5(COG1) of representing subgroup D1, which are basically CDF (Cycling Dof Factor) proteins associated with the regulation of photoperiodic flowering time by repressing the CONSTANS gene [19,62]. Specifically, the Dof proteins AtDof4.2, 4.3, 4.4 and 4.5 constitute the distinct subgroup C3 and OsDof-13, 24, 25, 30 constitute the distinct subgroup D3, similar to what has been reported in and rice clusters C3 and D3 [8]. These sets of Dof genes might be exclusively present in Arabidopsis/rice as no apparent counterpart in soybean as well as other plants.

Conserved motifs outside the Dof domain

To reveal the diversification of Dof genes in soybean, putative motifs were predicted by the program MEME (Multiple Em for Motif Elicitation), and a total of 30 conserved motifs were found in all the 78 Dof proteins (Figure 5). Motif 1 was uniformly present in all the Dof proteins and represents the conserved Dof domain. Moreover, a number of common motifs were found in all soybean Dofs (the amino-acid consensus sequence of each motif is listed in Additional Table S4). As expected, most of the closely-related members in the phylogenetic tree had common motif compositions. For example, there were no conserved motifs outside the Dof domain in Subgroup I, while motifs 2, 3, 4, 5, 6, 7, 9, 10, 12, 17, and 22 appeared in nearly all the members of subgroup IX. In other subgroups, motifs 8 and 15 were specific to subgroup III, motifs 20 and 24 were specific to subgroup IV, motifs 18 and 29 were specific to subgroup V, motifs 11, 21, 19, 23, and 30 were specific to subgroup VI, motif 13 was specific to subgroup VII, and motifs 25, 26 and 27 were specific to subgroup VIII. These similarities in motif patterns might be related to similar functions of the Dof proteins within the same subgroup.

Figure 5

Schematic distributions of the conserved motifs among defined gene clusters.

Motifs were identified by means of MEME software using the deduced amino-acid sequences of the 78 GmDofs. The relative position of each identified motif in all Dof proteins is shown. Multilevel consensus sequences for the MEME defined motifs are listed in Table S4.

Expression pattern of Dof genes in soybean

Since high-throughput sequencing and gene expression analyses have been performed on many soybean tissues at various developmental stages, publicly-available RNA-Seq data is thought to be a useful resources for studying gene expression profiles. Distinct transcript abundance patterns were readily identifiable in the RNA-Seq dataset at NCBI. Nearly all Dof genes (except for three: GmDof02.4, GmDof13.1, and GmDof19.3) have sequence reads in at least one tissue, their universal expression also indicating the importance of Dof TFs. The expression profiles of the 75 Dof genes were analyzed as shown in Figure 6. Most of the Dof genes showed distinct tissue-specific expression patterns across the ten tissues examined. All of the GmDofs having expression profiles were clustered into nine groups based on their expression patterns. The genes in clusters A-I were mainly expressed in root/floral bud, root, root/globular embryo, floral bud/globular embryo, leaf/floral bud, floral bud, cotyledon/early-maturation embryo, heart/cotyledon embryo, and dry seed.

Figure 6

Heatmap of expression profiles for soybean Dof genes across different tissues.

The genome-wide transcriptome data of soybean were generated from the NCBI database (accession numbers SRX062325–SRX062334). The expression data were gene-wise normalized and hierarchically clustered. The relative expression level of a particular gene in each row was normalized against the mean value. The color scale below represents expression values, green indicating low levels and red indicating high levels of transcript abundance. The sources of the samples were as follows: SDLG (whole seedlings 6 days after imbibition), LEAF (leaves), ROOT (roots), STEM (stems), FBUD (floral buds), GLOB (globular-stage embryos), HRT (heart-stage embryos), COT (cotyledon-stage embryos), EM (early maturation stage embryos), and DRY (dry soybean seeds).

Detailed analysis of the expression patterns of GmDofs showed that some of the genes clustered in the same subgroup of the phylogenetic tree (Figure 2) had similar expression patterns, also indicating the existence of redundancy among the Dof genes in these subgroups. For example, all of the GmDofs in subgroup VII were mainly expressed in floral buds while all of genes in subgroup V were mainly expressed in root and/or globular embryo. Most of the genes in subgroup IX had dominant expression patterns in floral buds and/or globular embryo. However, some Dof members in the same subgroups also had totally different expression patterns, even among paralogous genes with high identity of amino-acid sequences. In subgroup I of the phylogenetic tree (Figure 2), there were five kinds of expression patterns among all eight GmDof members. Three of four pairs of paralogous genes (GmDof07.3/13.4, GmDof07.5/13.2, and GmDof13.6/15.6) had different expression patterns and one pair (GmDof13.8/15.4) was mainly expressed in floral buds and globular embryo. The genes in the same subgroup with different expression pattern, especially paralogous genes, also revealed their functional diversity despite these Dof genes had highly similar amino-acid sequences.

The transcription rate of a gene is determined by trans-acting TFs that bind to cis-regulatory elements in promoters, additional co-factors, and chromatin accessibility [63]. A common approach to identify functional cis-acting promoter elements is to discover over-represented motifs in co-expressed genes. It is assumed that promoter motifs conserved in clusters of co-expressed and functionally-related genes may be involved in mediating coordinated gene activity [64,65]. The promoter regions of the GmDof genes (1000-bp sequences upstream from the translational start site) were analyzed using the PLACE database to identify putative cis-elements. According to the PLACE results, many similar cis-acting regulatory DNA elements associated with root, leaf, flower, seed, nodulin, abiotic or biotic stress, and hormone (Additional Table S5) occurred in the promoter regions of the 78 GmDof genes. For example, cis-elements related to root-specific (ROOTMOTIFTAPOX1), leaf-specific (CACTFTPPCA1), and flower-specific (POLLEN1LELAT52) were present in all soybean GmDof promoters (Additional Table S5). Especially, all of the GmDof promoters contained Dof elements (DOFCOREZM) ranging from 4 to 37 copies, indicating the important role of Dof TFs in regulating themselves. Furthermore, the differences in common cis-elements across these promoter regions, including both number and distance from the start codon (Additional Table S5), indicated that the number of cis-elements and their distance from the start site affect the responsiveness of GmDofs to the environment and development.

Conclusions

Transcriptional regulation is an important mechanism underlying gene expression. The number, position and interaction between different cis-elements and the TFs at a given gene promoter determine the gene expression pattern. These TFs can be classified into gene families according to the presence of a particular DNA-binding domain. In this study, a comprehensive analysis was conducted and a multitude of Dof gene family members were identified in the soybean genome. Genome-wide analysis revealed the existence of 78 full-length Dof genes, and multiple sequence alignment of the GmDof proteins showed strong conservation of four cysteine residues and the other amino-acid residues in the Dof domains. Phylogenetic analysis revealed that all GmDofs were clustered into nine distinct subgroups. The exon/intron structure and motif composition of the Dofs were highly conserved in each subfamily, indicating their functional conservation. The Dof genes were non-randomly distributed within and across 19 chromosomes, and a high proportion of GmDofs were preferentially-retained duplicates located on duplicated blocks. Soybean-specific segmental duplications of the genome contributed significantly to the expansion of the soybean Dof gene family. The comparative phylogenetic analysis of soybean Dof proteins with and rice Dof proteins revealed four Major Clusters of Orthologous Groups and nine well-supported clades. The global expression profile analysis provided insight into the soybean-specific functional divergence among members of the Dof gene family. A majority of GmDofs showed specific temporal and spatial expression patterns, based on RNA-seq data analyses. The expression patterns of duplicate genes were partially redundant or divergent. The cis-regulatory element analysis of the predicted Dof genes revealed differences in common cis-elements across these promoter regions including both their number and distance from the start codon. The results presented here provide information useful for the functional characterization of soybean gene families by combining phylogenetic analysis with global gene expression profiling.

Complete list of soybean The list comprises 78 GmDof gene sequences. The amino-acid sequences were deduced from their corresponding coding sequences; the genomic DNA sequences were obtained from Phytozome. Most of the transcripts were based on the Glycine max v1.1 annotation and some were from v1.0. Some of the Dof genes were re-annotated based on GENESCAN, paralogous genes, and/or RT-PCR.

(XLS)

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Pairwise identities between homologous pairs of Pairwise identities and sequence alignments of the 38 homologous pairs identified from the soybean Dof family.

(XLS)

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List of The Dof sequences of and were downloaded from genome TAIR release 9.0 (http://www.Arabidopsis.org/) and those of from the rice genome annotation database (http://rice.plantbiology.msu.edu/, release 5.0). The nomenclature is according to previous reports [8,13].

(XLS)

Click here for additional data file.

Multilevel consensus sequences for the MEME-defined motifs found among different Dof proteins from soybean. Consensus amino-acid sequences obtained from analysis of the 78 soybean Dof proteins with MEME software. The motif numbers are equivalent to those described in Figure 5. Motif 1 corresponds to the Dof DNA-binding domain.

(XLS)

Click here for additional data file.

The The motifs of the soybean GmDof promoters were predicted by PLACE (http://www.dna.affrc.go.jp/PLACE/). The numbers show the occurrence frequency of the motifs in one promoter. The sequences were from the 1-kb sequence upstream of the ATG.

(XLS)

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