Introduction

Cotton (Gossypium spp.) is an important crop that is used for making lint and cotton seed oil. Apart from the economic values of its fibers and seeds, cotton is also a model plant for the study of polyploids, cell elongation, and cell wall synthesis (Paterson et al. 2012). Owing to its importance, many studies on cotton have been carried out during the last decade (Li et al. 2015a; Chen et al. 2017; Gao et al. 2017; Lu et al. 2017; Zhang et al. 2017), particularly concerning cotton genomics. The genome sequences of two kinds of diploid cotton, Gossypium raimondii (G. raimondii) and Gossypium arboreum (G. arboreum), were released in 2012 and 2014, respectively (Paterson et al. 2012; Wang et al. 2012; Li et al. 2014). Genomes of two kinds of cultivated allotetraploid cotton, G. hirsutum and G. barbadense, were published in 2015 ( Li et al. 2015b; Yuan et al. 2015; Liu et al. 2015) and improved in 2018 (Wang et al. 2019). Based on the published sequence data, a comprehensive analysis of a particular gene family can be carried out to uncover its functions, evolution, and expression profiles. These analyses can help us to understand how genes in the gene family control traits at a genome-wide level.

BEL1-like homeobox (BLH) proteins are ubiquitous in plants and include four conserved domains: SKY, BELL, homeodomain, and VSLTLGL. This gene family is known to heterodimerize with KNOTTED1-like homeobox (KNOX) transcription factors (TFs) to regulate transcription of target genes (Müller et al. 2001; Chen et al. 2003; Smith and Hake 2003; Bhatt et al. 2004; Kanrar et al. 2008; Lal et al. 2011; Li et al. 2012). In plants, BLH genes form a small family and their functions are mostly repetitive and redundant. In the genomes of Arabidopsis thaliana, Oryza sativa, Solanum tuberosum, Zea mays, and Glycine max, there are 13, 14, 14, 15, and 34 BLH genes, respectively (Arnayd and Pautot 2014; Mukherjee et al. 2009; Sharma et al. 2014; Meng et al. 2018; Cao et al. 2015; Tao et al. 2018). Several members of this gene family were reported to contribute to developmental processes. Two BELL genes in Arabidopsis (SAW1 and SAW2) were reported to repress the BREVIPEDICELLUS (BR) gene, which is a class I KNOX protein involved in leaf patterning. Two other BELL genes in Arabidopsis, PNY and PNF, were also reported to act redundantly in the initiation, maintenance, and development of the shoot apical meristem (SAM) (Byrne et al. 2003; Rutjens et al. 2009; Ung et al. 2011), flowering initiation, and inflorescence architecture (Smith and Hake 2003; Ragni et al. 2008; Khan et al. 2015). In addition, BLH1 is described as a key regulator of ovule development in Arabidopsis (Brambilla et al. 2007; Bencivenga et al. 2012). BLH12 and BLH14 were reported to regulate internode patterning and vein anastomosis in maize (Katsutoshi et al. 2017). BEL11 was involved in chlorophyll synthesis and chloroplast development in tomato fruit (Meng et al. 2018). In potato, the BEL1-KNOX interaction was involved in the tuberization process and root growth (Chen et al. 2003; Lin et al. 2013); StBEL5 has been identified to function as an activator in cytokinin and auxin pathways and a repressor in gibberellic acid (GA) pathway, and also can induce tuber formation (Sharma et al. 2014). Similar to StBEL5, a recent study showed that StBEL11 and StBEL29 function as long-distance signals to regulate the growth of tubers in potato (Ghate et al. 2017). GmBLH4 in soybean might play a role in maintaining the SAM and flower organ development and was might also involve in the pre-harvest seed deterioration process (Tao et al. 2018). However, the BLH gene family has not been identified or characterized in cotton.

G. hirsutum is the most widely planted cotton species in the world, accounting for 90% of all cotton production (Soltis et al. 2004). In this study, 32, 16, and 18 BLH genes were identified from the G. hirsutum, G. arboreum, and G. raimondii genomes, respectively. The phylogenetic relationships, chromosomal location, gene structure, and conserved motifs of the BLH members were investigated to reveal the evolution of this family in cotton. Analysis of gene expression patterns revealed that genes in the same subfamily tend to have similar expression patterns. BLH1 subfamily were upregulated in  the fiber cells at 10 days post anthesis (DPA) and 15 DPA, indicating that these genes may play an important role in fiber cell elongation period. Besides, the expression levels of GhBLH1_3 and GhBLH1_4 genes were upregulated after GA and brassinosteroid (BR) treatment, suggesting that GA and BR regulate fiber cell development by enhancing the transcripts of GhBLH1_3 and GhBLH1_4 genes in cotton. Through these analyses, we have increased the knowledge concerning the evolution and function of BLH genes in G. hirsutum.

Materials and methods

Sequence retrieval of BLH and phylogenetic analysis

The 11 BLH CDS and protein sequences in Arabidopsis were downloaded from The Arabidopsis Information Resource (TAIR: http://www.arabidopsis.org/index.jsp). The BLH CDS sequences for three cotton species, G. hirsutum, G. arboreum, and G. raimondii, were acquired in CottonGen (https://www.cottongen.org/) by BLAST with the sequence of Arabidopsis thaliana as the queries. Protein sequences for three cotton species, G. arboreum, G. raimondii, and G. hirsutum, were also downloaded from the CottonGen website (https://www.cottongen.org). MEGA 6.0 (https://www.megasoftware.net) was employed to conduct the phylogenetic analysis. The protein sequences of the identified BLH gene family were imported into MEGA 6.0. After the multiple alignments performed with ClustalW with default parameters (Thompson et al. 1997), the unrooted phylogenetic tree was constructed using the neighbor-joining algorithm (Saitou and Nei 1987) and the bootstrap test was carried out with 1 000 iterations.

Chromosomal locations and gene duplication analysis

Gene annotation files downloaded from CottonGen (https://www.cottongen.org/) were employed to perform the location of BLH genes in G. hirsutum chromosomes. Mapchart 2.2 software (Voorrips 2002) was used to generate the chromosomal location image. The predicted BLH proteins were first aligned using ClustalW2 at EMBL-EBI (http://www.ebi.ac.uk/Tools/msa/clustalw2/) before a gene duplication analysis. Gene duplication events were defined according to the following conditions: the alignment region covered more than 80% of the longer gene and the identity of the aligned regions was over 80%.

Gene structure and conserved motif analysis

The exons and introns were explored by comparing the coding sequences with their genomic sequences using the online CSDS Program (http://gsds.cbi.pku.edu.cn/) (Hu et al. 2015). The conserved motifs were predicted with MEME website (http://meme-suite.org/tools/meme).

Expression profiles and promoter analysis

For expression analysis of GhBLH genes at different fiber developmental stages, publicly available gene expression files (0–15 DPA) were obtained from CottonGen (https://www.cottongen.org/). A heat map was created using MEV software (Eisen et al. 1998). The promoter sequence file was also downloaded from CottonGen (https://www.cottongen.org/). Cis-acting regulatory DNA elements within promoter regions (2 000 bp from the start codon) of selected GhBLH genes were scanned using the PLACE database (http://bioinformatics.psb.ugent.be/webtools/-plantcare/html/). The BR related cis-elements were identified manually.

Plant material preparation and quantitative real-time polymerase chain reaction (qRT-PCR)

The cotton cultivar Xuzhou 142 was grown in a climate-controlled greenhouse with a 16 h light and 8 h dark cycle at 30 °C. Developing cotton ovules were harvested at 1 DPA, soaked in 10% sodium hypochlorite for surface sterilization, and then rinsed eight times in distilled and deionized water. Ovules were excised from bolls, at 30 °C in darkness without agitation as previously described (Xiao et al. 2016). To study the responses of GhBLH genes to GA, BR, and 1-Naphthaleneacetic acid (NAA) application, the cultured ovules were treated with 1 μmol•L− 1 of GA, 5 μmol•L− 1 of BR, 5 μmol•L− 1 of NAA, respectively, treatments without chemical supplementation (CK) were used as the control. GA, BR and NAA were dissolved in ethanol, respectively. Ovules were then harvested for the qRT-PCR experiment after 6, 12, and 24 h treatments, respectively. Three biological replications were used for each time point.

Total RNA was extracted from these samples using PureLink™ RNA mini kit (Invitrogen, Lot no.1687455) with all procedures following the manufacturer’s protocol. For the synthesis of cDNA, 5 μg of total RNA was reverse transcribed with random hexamer primers using the SuperScript® III Reverse Transcriptase (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. In qRT-PCR experiments, each sample was performed with three replicates to ensure the accuracy of results. The reaction parameters were as follows: 95 °C for 10 min, followed by 40 cycles of 95 °C for 10 s and 56 °C for 30 s. A melting curve was generated from 65 °C to 95 °C. The one-way ANOVA analysis was performed using SigmaStat software. Primers for qRT-PCR analysis were listed (Supplementary Table 6). The cotton UBQ7 gene was served as the internal control for all related results.

Results

Genome-wide identification of the BLH gene family in cotton

To identify the BLH genes in G. hirsutum (tetraploid) and G. arboreum and G. raimondii (diploid) genomes, the BLH amino acid sequences from Arabidopsis were used as queries to perform a local BLAST search in cotton genome database. We identified 32, 16, and 18 BLH genes in G. hirsutum, G. arboreum, and G. raimondii genome, respectively (Table 1). In G. hirsutum, we identified a total of 34 BLH gene family members based on the sequence information from the genome assembled by NBI company (Table 1). Thirty-two homologous BLH genes were also found based on the assembled genome. Furthermore, another new genome version was used to analyze the BLH genes (Yang et al. 2019). As a result, a total of 39 BLH genes were identified in this version (Supplementary Table 1). These differences may be due to the assembly error in partial chromosomal regions. Among these genes, 14 GhBLH genes were originated from the At subgenome and 17 members were from the Dt subgroup. In addition, one gene (GhBLH3_2) was located in a scaffold (Supplementary Table 2). Thirty-one orthologs of GhBLH gene, except for GhBLH3_2, were identified in their two diploid ancestors, G. arboreum and G. raimondii. Interestingly, the genes orthologous of GaBLH7_1, GaBLH8_4 and GrBLH8_4 were lost in G. hirsutum (Supplementary Table 2). The lengths of the identified GhBLH proteins ranged from 459 amino acids (aa)(GhBLH11_2) to 832 aa (GhBLH2_3) with an average length of 654 aa. The GaBLH proteins ranged from 459 aa (GaBLH11_2) to 806 aa (GaBLH2_4) with an average length of 665 aa and the GrBLH proteins ranged from 459 aa (GrBLH11_1) to 799 aa (GrBLH8_4) with an average length of 642 aa. The coding sequences (CDS) and promoter sequences of 32 GhBLH genes were shown (Supplementary Table 3 and Supplementary Table 4).

Table 1 The BLH family members of G. hirsutum, G. arboreum, G. raimondii, and A. thaliana
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Phylogenetic analysis of the BLH gene family

To study the phylogenetic relationships of the BLH gene family, we performed a phylogenetic analysis of 77 BLH protein sequences, including 11 members from Arabidopsis, 16 from G. arboreum, 18 from G. raimondii, and 32 from G. hirsutum. As shown in Fig. 1, the BLH proteins were classified into 7 distinct groups: BLH1, BLH2/4, BLH3/10, BLH6/7, BLH8/9, BLH5, and BLH11, which were based on their sequence similarities with orthologous in Arabidopsis. Group BLH8/9 and BLH2/4 contained 16 and 14 BLH members, respectively, and constituted the two largest groups in the phylogeny. Group BLH6/7 and BLH1 each contained 13 members and group BLH5 contained 11 members. The smallest groups were BLH11 and BLH3/10, which only contained 5 members each. To validate the results from the neighbor-joining (NJ) method, we reconstructed the phylogenetic tree of BLH proteins with maximum likelihood method and found that BLH proteins were divided into 7 distinct groups (Supplementary Fig. 1), which was similar to the result of the NJ method.

Fig. 1
figure 1

Phylogenetic tree of the BLH family proteins in G. arboreum, G. hirsutum, G. raimondii, and Arabidopsis. The unrooted tree was generated using the MEGA 6.0 program with the neighbor-joining method. Bootstrap values from 1 000 replicates are indicated at each branch. The phylogenetic trees were constructed based on the full-length protein sequences of the BLHs

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Chromosomal distribution and duplication events of GhBLH genes

Based on the annotation of the G. hirsutum genome, 25 out of the 32 GhBLH genes were assigned to 15 chromosomes including seven were located on the At subgenome, eight on the Dt subgenome, while the remaining seven GhBLHs were located in unmapped scaffolds (Fig. 2). Eleven chromosomes in G. hirsutum had no GhBLH genes, whereas chromosome At_9 (with four genes) had the highest number of GhBLH genes. The remaining GhBLH genes were distributed evenly (one or two genes on each chromosome).

Fig. 2
figure 2

Distribution of BLH genes on G. hirsutum. The scale represents megabases (Mb). The chromosome numbers are indicated above each vertical bar

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To elucidate the expanded mechanism of the GhBLH gene family, we performed a gene duplication event analysis. Twenty-four GhBLH genes were produced by whole-genome duplication (WGD) and only one was produced by singleton duplication (Supplementary Table 5). The high proportion of GhBLH genes derived from WGD event indicates that this duplication event may play a crucial role in GhBLH gene expansion in the G. hirsutum genome.

Gene structure and conserved motifs of GhBLH genes

To better understand the similarity and diversity of the gene structures of GhBLHs, we constructed a phylogenetic tree with the deduced amino acids of GhBLHs (Fig. 3a) and investigated the exon/intron structures of GhBLH genes. GhBLH gene can be divided into three groups and all genes had multiple introns according to their exon/intron structures. As shown in Fig. 3b, the majority of GhBLH genes (84%) contained four exons in their DNA sequence. Only two genes, GhBLH1_1 and GhBLH1_2, had three exons in their DNA regions and another two genes, GhBLH2_1 and GhBLH9_2, each had five exons (Fig. 3b). The high similarities in GhBLH gene structure indicate that GhBLH is a conserved gene family. Conserved motifs in the 32 GhBLH proteins were further identified using the MEME online tool (Fig. 3c). Four conserved domains (motif 1, 2, 3 and 4) were found in all GhBLH proteins. Motif 5 accounted for 81.3% of the GhBLH members, suggesting that these features might have contributed to some specific functions in the GhBLH family. Most members had motifs 1, 2, 3, 4, and 5 indicating that they are highly conserved among GhBLH proteins. We also analyzed the similarity, gene structures and conserved motifs of GaBLHs and GrBLHs, respectively (Supplementary Fig. 2a and Supplementary Fig. 3a). All GaBLH contained four exons in their DNA sequences, except for GaBLH1_1, which had three exons (Supplementary Fig. 2b). Most GaBLH proteins possessed five conserved domains (motif 1, 2, 3, 4 and 5), except for GaBLH5_3, GaBLH8_1 and GaBLH8_4, in which motif 5 was lost (Supplementary Fig. 2c). All GrBLH contained four exons in their DNA sequences, except for GrBLH1_2 and GrBLH2_3, which had three exons, and GrBLH9_2, which had five exons (Supplementary Fig. 3b). Most GrBLH proteins possessed five conserved domains (motif 1, 2, 3, 4 and 5), except for GrBLH11_1, GrBLH8_4, GrBLH8_3 and GrBLH9_2, which lacked motif 5 (Supplementary Fig. 3c).

Fig. 3
figure 3

Phylogenetic tree, gene structure, and motif composition of the G. hirsutum BLH genes. a The phylogenetic tree was constructed using MEGA 6.0 with the neighbor-joining (NJ) method and 1 000 bootstrap replicates. b Exon and intron structure of BLH genes from G. hirsutum. The introns and exons are represented by black lines and red boxes, respectively. c Protein motifs. Each motif is represented using colored boxes

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Expression profiles of GhBLH genes in cotton fiber cells

Several reports demonstrated that BLH genes were involved in regulating plant development (Ragni et al. 2008; Ung et al. 2011; Bencivenga et al. 2012; Khan et al. 2015; Katsutoshi et al. 2017; Meng et al. 2018), which puts us to investigate the biological functions of GhBLH genes in fiber cells during different developmental stages. As shown in Fig. 4, the majority of GhBLH genes from the same subfamily had similar expression patterns. The expression levels of four genes, GhBLH1_3, GhBLH1_4, GhBLH1_5 and GhBLH1_6, which belonging to BLH1 subfamily, were upregulated in cotton fiber cells at 10 and 15 DPA(Fig. 4), indicating that these genes may play an important role in fiber cell elongation period. To better show the biological function of GhBLH genes, we analyzed the expression levels of GhBLH in other tissues. As a result, GhBLH1_2 was highly expressed in leaf and torus; GhBLH1_3 was upregulated in stamen; GhBLH4_2 showed increased transcripts in stem; both GhBLH5_3 and GhBLH5_4 were highly expressed in calycle, leaf and petal (Supplementary Fig. 4). In order to further study the function of GhBLH1, we investigated which genes can interact with GhBLH1. As a result, GhBLH1 could interact with several proteins, including KNAT7 and OFPs (Supplementary Fig. 5), which should be further validated with a yeast one-hybrid experiment.

Fig. 4
figure 4

Gene expression analysis of 32 BLH genes in G. hirsutum at 0, 3, 10, 15 DPA samples. 0 and 3 DPA samples were ovules. 10 and 15 DPA samples were fiber cells. For expression analysis of GhBLH genes at different fiber developmental stages, publicly available gene expression files (0–15 DPA) were obtained from CottonGen (https://www.cottongen.org/). A heat map was created using MEV software (Eisen et al. 1998). The expression levels are indicated by the color bar: red means high and green means low

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GhBLH1 genes is involved in phytohormone regulating cotton fiber development

The phytohormones GA, BR and auxin are known to contribute to fiber cell growth in cotton (Luo et al. 2007; Xiao et al. 2010; Zhang et al. 2011). GhBLH1_3, GhBLH1_4, GhBLH1_5, and GhBLH1_6 genes were highly expressed during cotton fiber cell development. In order to explore the effect of phytohormones on the expression of these genes, we not only analyzed the cis-acting regulatory DNA elements within promoter regions (2 000 bp from the start codon) of these four GhBLH genes, but also carried out the expression analyses of these GhBLH genes. Gibberellin-responsive cis-elements, GAREs, were found in the promoter regions of GhBLH1_3 and GhBLH1_4 (Fig. 5a). Another regulatory element, P-box, which is also a gibberellin signaling transduction related cis-element, was found in GhBLH1_5 and GhBLH1_6 promoter regions. Exogenous application of GA significantly activated the transcription of GhBLH1_3, GhBLH1_4 and GhBLH1_5 (Fig. 5b). BR-related cis-elements (CANNT) were found in GhBLH promoter region. For GhBLH1_3, GhBLH1_4, and GhBLH1_6, each had five BR-related cis-elements. GhBLH1_5 had eight BR-related cis-elements in its promoter region (Fig. 5c). The transcriptional level of GhBLH1_4 was significantly upregulated after BR treatment for 6 h (Fig. 5d). BR also enhanced GhBLH1_3 transcripts after it's applied for 12 h. Interestingly, auxin signaling transduction related cis-elements (TAG elements) were not found in any of these four GhBLH genes (Fig. 5e). Exogenous application of NAA did not promote the transcription of these GhBLH genes (Fig. 5f). These results suggested that expression levels of these genes were induced by GA and BR.

Fig. 5
figure 5

Expression patterns of several GhBLH genes within ovules under GA, BR, and NAA treatment, respectively. a Analysis of GhBLHs with GAREs and P-boxes presented in their promoter regions. b GA activated significant transcription of three GhBLH genes with GAREs and P-boxes in their promoter regions. c Analysis of GhBLHs with CANNT presented in their promoter regions. d BR promoted significant transcription activation of two GhBLH genes with CANNTG. e Analysis of GhBLHs with TAG elements presented in their promoter regions. f NAA failed to activate the transcription of four GhBLH genes with TAG elements. Relative expression levels were determined after normalizing all data to that of CK ovules, which was set to 1.0. Statistical significance was determined using one-way analysis of variance combined with Tukey’s test. *P < 0.05; **P < 0.01; ***P < 0.001. Note: The x-axis represents different hours (6, 12, and 24) after GA, BR, and IAA treatment, the y-axis indicates the relative expression levels. Error bars show the standard deviation of three biological replicates

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Discussion

The BLH gene family plays a very important role in many biological processes during plant growth and development. However, previous studies of the BLH family have been focused on Arabidopsis, with few reports in cotton. In this study, 32, 18, and 16 BLH genes were identified and characterized in three sequenced cotton species, G. hirsutum, G. raimondii, and G. arboreum, respectively. Compared with the BLH gene in Arabidopsis, more BLH genes were found in Gossypium. This may be because G. hirsutum is tetraploid cotton, which evolved from two kinds of diploid cotton containing an A-genome (G. arboreum) and D-genome (G. raimondii), repectively, and diverged about 1–2 million years ago (Paterson et al. 2012). Genome-wide analysis of the BLH gene family showed that the whole genome duplication contributed to the expansion of BLH members. Exon and intron structures and conserved domains varied modestly among different members in cotton, which may be because the expansion of BLH family genes was mostly shaped by whole genome duplication. Furthermore, combining the phylogeny of GhBLH proteins with their peptide length, we also found that GhBLH genes were highly conserved.

In Arabidopsis, AtBLH1 has been reported to be one of the major factors controlling ovule pattering, in particular, determining the identity and development of the integuments (Robinson Beers et al. 1992; Ray et al. 1994; Reiser et al. 1995; Brambilla et al. 2007). AtBLH1 switched one of the synergid cells into an egg cell in the embryo sac (Pagnussat et al. 2007). In the blh1 and spl double mutant, the ovules developed as finger-like structures without integuments (Balasubramanian and Schneitz 2002). In addition, AtBLH1 was involved in ovule development by interacting with MADS-box transcription factors and SPOROCYTELESS (Brambilla et al. 2007; Bencivenga et al. 2012). In this study, we found that four members of BLH1 subfamily, GhBLH1_3, GhBLH1_4, GhBLH1_5 and GhBLH1_6, were upregulated in cotton fiber cells at the elongation stage, implying that GhBLH1 genes may contribute to fiber development in cotton. As the key role of AtBLH1 gene in ovule development in Arabidopsis, we, therefore, speculate that GhBLH1_3, GhBLH1_4, GhBLH1_5, and GhBLH1_6 genes may play an important role in ovule growth in cotton, and more proofs were needed. GhBLH3_1 and GhBLH7_1 genes were also upregulated in cotton fiber cells at 10 or 15 DPA, indicating that GhBLH3_1 and GhBLH7_1 genes may be involved in fiber development in cotton.

The expression levels of GhBLH1_3 and GhBLH1_4 genes were upregulated after GA and BR treatments. To date, many studies have shown that BR and GA can significantly promote the elongation of cotton fiber cells (Luo et al. 2007; Xiao et al. 2010). Combining the high expression level of GhBLH1 genes in developing cotton fibers with the increased transcripts induced by both GA and BR treatments, we suggest that GA and BR regulate fiber cell development by enhancing the transcripts of GhBLH1_3, GhBLH1_4, and GhBLH1_5 genes in cotton. Overall, our study provides a basis to systematically elucidate the evolution and function of BLHs in cotton and to effectively characterize the precise biological roles of BLH1 and their utilization in future cotton breeding programs.

Conclusions

The BLH gene family plays a very important role in many biological processes during plant growth and development. This study deepens our understanding of the role of the GhBLH1 gene involved in fiber development and will help us in breeding better cotton varieties in the future.