Characterization of B-box proteins and their contribution to plant development in Arachis duranensis

B-box (BBX) proteins are important factors involving in the regulation of plant growth and development, and have been identified in many plant species. However, the characteristics and transcription patterns of BBX genes in wild peanut are limited. In the present study, we identified and characterized 24 BBX genes in a wild peanut Arachis duranensis. The AdBBX members distributed on 9 of the 10 chromosomes and chromosome 3 contained the most AdBBX members, with 6 AdBBXs. 16 AdBBX proteins had two distinct BBX domains, 11 members contained one CCT domain, and 7 genes had both BBX and CCT domains. Protein structure analysis revealed that AdBBX were classified into five clades: I (3 genes), II (4 genes), III (4 genes), IV (9 genes) and V (4 genes), on the basis of the diversity of conserved BBX and CCT domains. Moreover, 15 distinct motifs were found in these 24 AdBBX proteins and motif 1 and 5 existed in all the AdBBX proteins. Duplication analysis revealed that 4 interchromosomal duplicated gene pairs were obtained and all of them belonged to group IV. In addition, 95 kinds of cis-acting elements were found in the promoter regions of AdBBXs and 53 types were predicted to have putative functions. The numbers and types of cis-acting elements varied in these AdBBX promoters, as a result, AdBBX genes exhibited distinct expression levels in different tissues. The transcription investigation combined with synteny analysis suggested AdBBX8 might be the key factor involving in flowering time regulation in Arachis duranensis. Conclusion Overall, this study provides a genome-wide identification of BBX genes in a wild peanut Arachis duranensis. Characteristic and transcription pattern analysis revealed their critical roles in plant growth and development. Our study will provide essential information for further functional characteristic investigation of AdBBX genes. Abstract Background: B-box (BBX) proteins are important factors involving in the regulation of plant growth and development, and have been identified in many plant species. However, the characteristics and transcription patterns of BBX genes in wild peanut are limited. Results: In the present study, we identified and characterized 24 BBX genes in a wild peanut Arachis duranensis . The AdBBX members distributed on 9 of the 10 chromosomes and chromosome 3 contained the most AdBBX members, with 6 AdBBXs . 16 AdBBX proteins had two distinct BBX domains, 11 members contained one CCT domain, and 7 genes had both BBX and CCT domains. Protein structure analysis revealed that AdBBX were classified into five clades: I (3 genes), II (4 genes), III (4 genes), IV (9 genes) and V (4 genes), on the basis of the diversity of conserved BBX and CCT domains. Moreover, 15 distinct motifs were found in these 24 AdBBX proteins and motif 1 and 5 existed in all the AdBBX proteins. Duplication analysis revealed that 4 interchromosomal duplicated gene pairs were obtained and all of them belonged to group IV. In addition, 95 kinds of cis -acting elements were found in the promoter regions of AdBBXs and 53 types were predicted to have putative functions. The numbers and types of cis -acting elements varied in these AdBBX promoters, as a result, AdBBX genes exhibited distinct expression levels in different tissues. The transcription investigation combined with synteny analysis suggested AdBBX8 might be the key factor involving in flowering time regulation in Arachis duranensis . Conclusion: Overall, this study provides a genome-wide identification of BBX genes in a wild peanut Arachis duranensis . Characteristic and transcription pattern analysis revealed their critical roles in plant growth and development. Our study will provide essential information for further functional characteristic investigation of AdBBX genes.


Background
Transcription factors are essential elements participating in signal transduction pathways in plants.
They often work as activators or suppressors to bind cis-acting elements in the target promoter regions to regulate downstream gene expressions [1,2]. Various kinds of transcription factors have been found in plants and considered to be involved in different response pathways. Among these gene families, zinc-finger transcription factors, consisting of a conserved domain which can bind metal ions like zinc and interact with DNA, RNA or proteins, are a large transcription factor family and play critical roles in plant growth and development [3,4]. Based on the diversification of protein sequences and structures, the zinc-finger genes are further classified into several distinct subfamilies [3]. A subgroup of zinc-finger proteins containing B-box (BBX) conserved domains, which are considered to be involved in protein-protein interactions, is designated as BBX family and highly conserved across all multicellular species [4][5][6][7].
Two types of BBX domains, B-box1 and B-box2, were found based on their consensus sequences and the spacing of zinc-binding residues [4,[8][9][10]. In Arabidopsis, 21 and 11 of the 32 BBX proteins are found to contain two and one BBX domains, respectively [3,4]. In addition to the conserved BBX domain, some of the BBX members contain some other specific domains, such as CCT (for CONSTANS, CONSTANS-LIKE, TOC1) and VP (valine-proline) domains. 17 Arabidopsis BBX members contained a CCT domain close to their C termini of the protein. BBX proteins are grouped into five clades in Arabidopsis based on the differences of the types and numbers of BBX and CCT domains. Class I and II members have two BBX and one CCT domains and class III contained one BBX and one CCT domains, respectively. Class IV has two different BBX domains and group V only has one BBX domain [3,4,11].
The functions of BBX proteins have been revealed to be involved in the regulation many signal pathways in recent years, including flowering time, circadian clock, seedling photomorphogenesis and abiotic stress [4]. CONSTANS (CO/BBX1) is the first BBX protein identified in Arabidopsis.
Overexpression of CO accelerates flowering time under both long day (LD) and short day (SD) conditions. Mutation of CO shows significantly delayed flowering time under LD, while the flowering time of co is similar as wild type plants under SD [12][13][14]. CO protein directly binds to the COresponsive elements (CORE) and CCAAT-box elements in the promoter region of FLOWERING LOCUS T (FT) to promote FT expression, which is responsible for the acceleration of flowering time [15][16][17].
The CO-FT flowering time regulatory pathway is conserved in some other species [18][19][20][21][22]. For example, Heading date 1 (Hd1), the CO ortholog in rice, participates in the regulation of FT orthologs in rice, Heading date 3a (OsHd3a) and Flowering Locus T1 (OsRFT1) [20,21]. Hd1 promotes flowering 5 time under SD but delayed flowering time under LD [18]. In addition, some CO-like (COL) proteins in Arabidopsis have also been shown to be involved in flowering time or circadian clock regulation, such as BBX2, BBX3, BBX4, BBX6 and BBX7. Moreover, BBX18, BBX24 and BBX32 are considered to be involved in abiotic and biotic stress responses [4].
The emergence of peanut genome database in recent years allows the investigation of peanut gene functions [24][25][26][27][28]. Although much knowledge on BBX functions has been advanced in many species, there is less research on their roles in peanut development. In this study, we identified and characterized 24 BBX proteins from one of the wild peanut species, Arachis duranensis. We investigated many characteristics of these BBX genes, including gene structures, conserved motifs, chromosome localizations, phylogenetic relationships, gene duplications, cis-acting elements in promoter regions and tissue expression profiles. Our study will provide essential information for further functional characterization of BBX proteins in peanut.

Identification of BBX members in Arachis duranensis
The amino acid sequences of the BBX conserved domain (PF00643) and Arabidopsis BBX proteins, downloaded from TAIR (https://www.arabidopsis.org/), were used as blast queries against peanut genome database to search for Arachis duranensis BBX genes (https://www.peanutbase.org/) [24]. All the output candidate genes were analyzed by Pfam database (http://pfam.xfam.org/search) and Pro Scan program (https://www.ebi.ac.uk/interpro/) to confirm the presence of BBX domains and remove genes without conserved BBX domains. The positions of the BBX and CCT domains in each AdBBX protein were analyzed by Pro Scan program. The genomic lengths, CDS lengths and amino acid numbers of these AdBBX genes were obtained from peanut genome database. The GC contents were analyzed using DNASTAR. Moreover, the chemical features of AdBBX proteins, such as molecular weight and theoretical iso-electric points, were investigated by ProtParam 6 (https://web.expasy.org/protparam/).

Sequence alignment and phylogenetic relationship analyses
The amino acid sequences of BBX and CCT domains were aligned by Clustal-X2 and the results were used to generate the alignment map using DNAMAN software (Version 5.2.2, LynnonBiosoft, Canada).
To analyze the evolution relationships of AdBBX genes with the well-studied BBX genes, the full lengths of BBX amino acid sequences from Arachis duranensis, rice and Arabidopsis were aligned by Clustal-X2, and the alignment results were used to construct the phylogenetic tree by MEGA7.0 with Neighbor-Joining method [29].
Gene structure, conserved motif and sequence logo analyses The genomic and CDS sequences of AdBBX genes were downloaded from peanut genome database [24] and used for the analysis of gene structures using Gene Structure Display Server program (GSDS)(http://gsds.cbi.pku.edu.cn/) [30]. To investigate the conserved motifs in these proteins, the full lengths of AdBBX amino acid sequences were obtained from peanut genome database and analyzed using MEME tools (http://meme-suite.org/). In addition, the sequence logos of BBX (B-box1 and B-box2) and CCT conserved domains were investigated using online WebLogo platform (http://weblogo.berkeley.edu/logo.cgi) [31].

Chromosomal localization, synteny and gene duplication analyses
AdBBX genes were mapped to peanut genome to identify the chromosomal localizations and physical positions, and chromosomal distribution map was generated using MapInspect software (http://www.mybiosoftware.com/mapinspect-compare-dis play-linkage-maps.html). To investigate the synteny relationships of the related genome regions in different species, the putative orthologous genes surrounding CO orthologs/homologs from soybean and Arachis duranensis were identified by BLASTP search as described [32,33], and the blast results were used for the generation of the synteny map. To analyze the duplicated gene pairs, we clustered AdBBX genes using OrthoMCL software (https://orthomcl.org/orthomcl/), and the duplicated gene pairs were drawn by Circos software as described [34,35].
Cis-acting element analysis 7 The promoter sequences, 2kb upstream of the initiation code of each gene, were obtained from peanut genome database [24], and used for further cis-acting element analysis using PlantCARE (http://bioinformatics.psb.ugent.be/webtools/plantcare/htm l/) [36]. Then the cis-acting elements were classified based on their putative functions.

Analysis of gene expression in different tissues
To investigate the expression of AdBBX genes in different Arachis duranensis tissues, RNA-seq datasets were downloaded from peanut genome database (https://peanutbase.org/gene_expression/atlas) and used for transcription analysis. Expressions of AdBBX genes were investigated in these 22 different tissues and the related data was used for the construction of the heatmap using MeV 4.9.0 (Multiple Experiment Viewer) [37].

Identification of BBX genes in Arachis duranensis
To identify BBX genes in Arachis duranensis, we used the amino acid sequences of the BBX conserved domain (PF00643) and 32 Arabidopsis BBXs as blast queries against peanut genome database, and then we used Pfam and Pro Scan program to confirm the conserved BBX domains in these candidate genes. A total of 24 BBX genes were found in wild peanut Arachis duranensis. The characteristics of these AdBBX genes were investigated and the detailed information was listed (Table 1). AdBBX genes exhibited diversities in genomic lengths, CDS lengths, amino acid numbers, isoelectric points, molecular weights and GC contents ( Table 1). The genomic lengths of AdBBX genes changed from 467 (AraduF08JS) to 4563bp (AraduBV95P), the CDS ranged from 468 (AraduF08JS) to 1641bp (AraduVV0J1), and the numbers of deduced amino acids varied from 155 to 546 (Table 1). The GC contents represent stability of the genes at some degree, thus we investigated GC contents in these AdBBX genes. The GC contents in AdBBX genes ranged from 32.13% (Aradu1V7PF) to 52.72% (Aradu28KTI). In addition, the isoelectric points of the AdBBX proteins were predicted and varied from 4.16 (AraduF08JS) to 8.85 (Aradu28KTI), and the molecular weight changed from 17020.4 (AraduF08JS) to 61012.69 (AraduVV0J1).

8
To investigate the chromosomal localizations of AdBBXs, we mapped these genes to peanut genome database to obtain their related physical positions. The AdBBX genes were named from AdBBX1 to AdBBX24 based on the chromosomal distributions ( Fig. 1, Table 1). Among these 24 AdBBX genes, 13 members were located on the plus strand and 11 were located on minus strand ( Table 1). 9 of the 10 chromosomes were found to contain AdBBX genes, except for chromosome 2 ( Fig. 1 and Table 1).
Chromosome 3 contained the most AdBBX genes, with 6 AdBBX members, followed by chromosome 6, 7, 9 and 10, with 3 AdBBXs on each, respectively. The AdBBX2, AdBBX3 and AdBBX4 were located closely in chromosome 3, as well as AdBBX5 and AdBBX6. In addition, AdBBX20 and AdBBX21 also distributed closely together on chromosome 9. Among these AdBBX genes, most of them were located in the chromosome arms, while only three genes AdBBX14, AdBBX17 and AdBBX18 distributed in the middle part of related chromosomes.

Protein sequence and classification analysis of AdBBX genes
The BBX proteins were classified into five subgroups on the basis of the conserved domains they contained, including the types and numbers of BBX and CCT domains [3,4]. We then analyzed the conserved domains in these AdBBX proteins, and found two distinct BBX domains (B-box1 and B-box2) and one CCT domain. To further investigate the conserved amino acid sequences of these conserved domains, the logos of Arachis duranensis B-box1 (CX 2 CX 8 CX 2 DXAXLCX 2 CDX 2 VHX 2 NXLX 3 H, X represents any amino acid), B-box2 (CX 2 CX 4 AX 3 CX 7 CX 2 CDX 3 HX 9 H) and CCT (RX 5 RX 3 KX 7 KX 2 RYX 2 RKX 2 AX 2 RXRXKGRFXK) were analyzed using Weblogo [31] (Fig. 2). In addition, the amino acid sequences of the B-box1, B-box2 and CCT domains were also aligned to analyze the correspondence positions of the conserved amino acid sequences (Fig. 3).
To further investigate the evolutionary relationship of these AdBBXs, we created a phylogenetic tree using their amino acid sequences (Fig. 4a). Conserved domain analysis revealed that 16 of the 24 AdBBX proteins had two BBX domains, B-box1 and B-box2, 11 members contained one CCT domain, and 7 genes had both BBX and CCT domains (Fig. 4b). The AdBBX proteins were also grouped into five subfamilies based on the diversity of the conserved domains (Fig. 4c). Group I and II, the difference 9 between which was the diverse of B-box2 domain, had a B-box1, a B-box2 and a CCT domain, and contained 3 and 4 members in Arachis duranensis, respectively. Class III subfamily had a B-box1 and a CCT domain and contained 4 members. The group IV subclass was considered to contain two B-box domains, a B-box1 and a B-box2, and had the most members, 9 AdBBX genes. Most of the AdBBX genes in the same clade were clustered together, except for AdBBX5 (AraduJ5IAH), which had a distinct relationship with other class III genes, but had a closer relationship with class V subfamily members (Fig.4a). To obtain some information from the well-studied BBX genes in other species, we analyzed the evolutionary relationship using BBX genes from Arabidopsis, rice and Arachis duranensis (Additional file 1: Figure S1). Phylogenetic analysis revealed that AdBBX8 and AdBBX24 were clustered together with the well-studied flowering time genes At5g15840 (CO) and Os06g16370 (Hd1) in Arabidopsis and rice, respectively [4], indicating both or either of these two genes may play important roles in flowering time regulation in Arachis duranensis.

Gene structures and conserved motifs of AdBBX genes
To investigate the exon-intron organizations, the genomic and CDS sequences of AdBBXs obtained from peanut genome database were analyzed using Gene Structure Display Server program [30]. The exon numbers of AdBBX genes changed from 1 (AdBBX19 and AdBBX22) to 5 (AdBBX4), and the intron numbers varied from 0 (AdBBX19 and AdBBX22) to 6 (AdBBX4 and AdBBX5). 9 AdBBX genes had both 5' and 3' UTRs, 4 members only contained 3' UTRs, 3 genes had only 5' UTRs and 8 members had no UTR (Fig. 5a). To further investigate the conservation and diversity of AdBBX protein structures, the putative motifs of these genes were predicted using MEME tools. A total of 15 distinct motifs were found in all the AdBBX proteins ( Fig. 5b and Additional file 2: Figure S2). Among these motifs, motif 1 and 5 were found in all the AdBBX proteins. The conservation of AdBBX protein structures was observed in genes clustered into the same clades, for example, all the members in subclass I shared 5 motifs, including motifs 1, 2, 3, 4 and 5, and subgroup II members shared 6 motifs, including motifs 1, 2, 4, 5, 14 and 15. In addition, structure diversity was also found among these AdBBX proteins. The motif numbers in AdBBX proteins varied from 2 (AdBBX19) to 7 (AdBBX7, AdBBX10 and AdBBX21), and some motifs were only found in specific AdBBX proteins, for example, motif 2 is specific to subgroups I, II and III members and was considered to be the CCT domain, and motif 15 was only found in subclass II members.

Duplication analysis of BBX genes in Arachis duranensis
Polyploidy is common in flowering plants during evolution and produces many duplicated gene pairs.
We then investigated the duplication events of AdBBX genes in Arachis duranensis and found 4 interchromosomal, but no tandem, duplicated gene pairs (AdBBX2/AdBBX16, AdBBX2/dBBX17, AdBBX3/AdBBX15 and AdBBX4/AdBBX9) (Fig. 6). Chromosome 3 had the most duplicated genes, with 3 members, followed by chromosome 7, with 2 duplicated members. In contrast, AdBBX genes located on chromosome 1, 4, 6, 9 and 10 have no duplication events. Moreover, all the duplicated gene pairs were found to belong to group IV, while no duplicated gene pairs were found in the other 4 groups.

Cis-acting elements in promoter regions display critical roles in regulating gene expressions in plants.
To further understand the expression responses of AdBBX genes, we analyzed the cis-acting elements in AdBBX promoter regions 2kb upstream of initiation codon using PlantCARE [36]. A total of 95 kinds of cis-acting elements were obtained and 53 types were predicted to have putative functions, including 7 development related elements, 5 environmental stress related elements, 4 site-binding related elements, 9 hormone-responsive elements, 4 promoter related elements and 24 lightresponsive elements (Additional file 3: Table S1). The binding sites related to development, including circadian control, metabolism regulation, stem expression, seed-specific regulation, differentiation of cells and cell cycle regulation (Fig. 7a), environmental stress, such as anaerobic, drought, low temperature, and defense and stress related cis-acting elements (Fig. 7b), and hormones, containing abscisic acid (ABA), gibberellic acid (GA), auxin and jasmonic acid (MeJA) related elements (Fig. 7c), were obtained in these promoters. In addition, the numbers and types of cis-acting elements varied in these AdBBX promoters, indicating their functional diversities in plant development regulation (Table   2). Among these putative functional elements, all the AdBBX genes contained light-responsive elements, which were represented to be the most abundant type in each of the AdBBX promoters, hormone-responsive elements and promoter related elements (Table 2 and Additional file 3: Table   S1), suggesting these genes shared some common pathways involving in plant development regulation. The promoter-related elements CAAT-box and TATA-box were found in all the AdBBX promoter regions, which might be the basic components for the promoters. Moreover, the lightresponsive element Box4 was obtained in 23 AdBBX promoters, except for AdBBX13, indicating that AdBBX genes play important roles in Box4-mediated light response regulation pathways.

Expression patterns of AdBBX genes in different tissues
To shed light on the potential functions of AdBBX genes during plant development, we investigated the expression levels of these 24 AdBBX genes in 22 different tissues (Fig. 8). AdBBX genes showed distinct transcription patterns in the tested tissues, indicating the functional diversity of these genes. In addition, expressions of duplicated gene pairs were also analyzed in these tissues. Some duplication events showed similar expression patterns in these tissues. For example, the duplicated gene pair AdBBX3/AdBBX15 was expressed almost the same abundance in all these tissues (Fig. 8), indicating the functional conservation of the duplicated genes. In contrast, some duplication events showed distinct expression levels in some tissues. For example, AdBBX9 showed high expression abundance in leaves and roots, while its duplicated gene pair AdBBX4 exhibited low expression level in these tissues (Fig. 8).

Discussion
In the past decades, the characterization of BBX genes, such as Arabidopsis CO and rice Hd1, from many species have greatly increased our knowledge about the molecular mechanisms involving in plant development. Peanut is an important crop in the world and provide essential oil for our daily life, thus the investigation of peanut BBX genes is of great help for understanding and improvement of peanut development. In this study, we identified and characterized 24 BBX proteins from the wild peanut Arachis duranensis and carried out comprehensive analysis of these genes.
BBX genes changed during plant evolution and the numbers and types of BBXs varied in different species [3][4][5][6]38]. For example, wild peanut Arachis duranensis, Arabidopsis, rice and pear contained 24, 32, 30 and 25 BBX members, respectively, and group IV contained the most BBX genes among these five subclasses in each of these species (Additional file 4: Table S2). The genome sizes of diploid wild peanut Arachis duranensis [24,39], Arabidopsis [40], rice [41] and pear [42] were 1.25 GB, 125Mb, 403Mb and 512 Mb, respectively. Thus the genome sizes have no directly relationship with the numbers of BBX members in these plants. In addition, the genes containing both BBX and CCT domains were designated as CO or CO-like (COL) proteins, and many CO-like genes (COL) were considered to be involved in circadian clock or flowering time regulation in Arabidopsis [4].
Approximately half of the BBX proteins were clustered to be CO or COL members (Group I, II and III members) in plants (Fig. 4), such as Arabidopsis (53.13%), rice (56.67%), pear (44%) and wild peanut Arachis duranensis (45.83%), indicating the evolution of CO and COL genes is conserved in these plants.
The cis-acting elements in promoter regions are responsible for the transcription of genes and the variety of the types and numbers of cis-acting elements in the promoter regions results in the difference of gene responses. AtBBX genes have been reported to participate in the regulation of many pathways, such as flowering time, circadian clock, abiotic stress and photomorphogenesis [3,4]. Different numbers and types of cis-acting elements were found in these AdBBX promoter regions, indicating the functional diversity of these genes. Many BBX genes in Arabidopsis were found to involve in light input signal pathways [4], and the light responsive elements were also found to be the 13 most abundance one in each of these AdBBX promoters, indicating these AdBBXs might work in response to light-dependent regulation pathways to contribute to plants' survival and adaption.
Moreover, many cis-acting elements were also obtained from promoter regions of the low expressed genes, including AdBBX3, AdBBX5, AdBBX15, AdBBX18 and AdBBX24 (Fig. 8). There are many factors, but not only cis-acting elements, affecting gene expression in plants. For example, the epigenetic modification and somatic genome variations were considered to change gene expression in many organisms [43]. Whether the low expressed genes were affected by these factors still need further investigation.
CO is an important factor involving in the regulation of flowering time in Arabidopsis and expressed highly at the apex of the seedlings and young leaves [44]. CO accelerates flowering time via activating the transcription of a RAF-kinase-inhibitor-like protein FT. AdBBX8 and AdBBX24 were the close homologous genes of CO in Arachis duranensis (Additional file 1: Figure S1). Soybean CO ortholog, GmCOL1a, GmCOL1b, GmCOL2a and GmCOL2b, were shown to be involved in flowering time regulation [45]. Genes evolved from the same origin might have similar functions, thus we investigated synteny relationships of CO orthologs/homologs from soybean (GmCOL1a, GmCOL1b, GmCOL2a and GmCOL2b) and Arachis duranensis (AdBBX8 and AdBBX24), respectively (Additional file 5: Figure S3). Synteny analysis revealed that AdBBX8 had closer relationships with soybean GmCOL1a and GmCOL1b than AdBBX24. In contrast, AdBBX8 and AdBBX24 showed similar close relationships with soybean GmCOL2a and GmCOL2b. Moreover, AdBBX8 was expressed highly in leaves, flowers, pistils and aerial gyn Ti, however, AdBBX24 exhibited extremely low expression levels in all the tissues (Fig. 8), indicating AdBBX8 might be the key factor acting a similar role as CO in flowering time regulation and AdBBX24 might be a redundant gene and lost functions during evolution. In addition, CO is regulated by circadian clock and its expression changed during the day [46], and AdBBX24 might be expressed at other time of the day rather than the tested time. Much work still need to do to investigate how AdBBX8 and AdBBX24 work in flowering time regulation.
Gene duplication produced new genes during evolution in many species. Some duplicated genes lost functions and some duplication events evolved new functions during gene duplication, compared to 14 their origin genes [47,48]. 4 duplicated gene pairs were found in Arachis duranensis and all these duplication events belonged to group IV subfamily, which contained only two BBX domains (Fig. 6), making group IV the largest subfamily in these groups. In addition, duplication events showed different exon-intron structures for each of these duplicated gene pairs (Fig. 5a) and cis-acting elements varied in the promoter regions of each of these duplicated gene pairs (Table 2), indicating the functional differentiation of these gene pairs during evolution. Moreover, the duplicated gene pairs AdBBX2/dBBX17, AdBBX3/AdBBX15 and AdBBX4/AdBBX9 contained similar motifs (Fig. 5b), and the expression of some duplication events showed similar levels in some tissues (Fig. 8), such as AdBBX3/AdBBX15, and thus they might remained some origin functions and participate in some common pathways.

Conclusions
In our present study, we identified and characterized 24 BBX genes from a wild peanut Arachis duranensis. The characteristics, such as conserved domains, gene structures, phylogenetic relationships, chromosomal distributions, gene duplications, synteny relationships, cis-acting elements and gene expressions, were investigated. AdBBX members distributed on 9 of the 10 chromosomes and were classified into five clades based on the diversity of the conserved BBX and CCT domains. 4 interchromosomal duplicated gene pairs were found, all of which belonged to group IV. Moreover, a total of 95 kinds of cis-acting elements were obtained and 53 types were predicted to have putative functions. The potential functions of AdBBX genes in various tissues were predicted based on their expressions and the results indicated AdBBX8 might be a key factor involving flowering time regulation. Our results will not only be useful for the understanding of AdBBX genes, but also provide essential information for further functional analysis of these members.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.

Availability of data and materials
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Figures, Tables And Supplementary Materials
The yellow, red, blue, purple and green colored gene names indicate group I, II, III, IV and V members, respectively.

Tables
Due to technical limitations, tables 1 and 2 are only available as downloads in the supplemental files section.