KNOX (KNOTTED1-like homeobox) transcription factor family is a subfamily of the homeobox gene TALE (Three amino acid loop extension) protein superfamily, which is widely involved in the regulation of plant growth and development (Hamant and Pautot, 2010). Most KNOX proteins contain four typical conserved domains: KNOX1, KNOX2, ELK and HOX (Magnani and Hake, 2008; Gao et al., 2015).

KNOX gene family is a conservative gene family in the plant kingdom, which is widespread from lower plant phycophyte and bryophyte to higher plant spermatophyte (Gao et al. 2015). The first KNOX gene to be identified in plants was Knotted1 (Kn1) in maize (Vollbrecht et al., 1991). Following this discovery, the KNOX gene was identified in more and more plants (Gao et al., 2015), such as Arabidopsis, rice, poplar, cotton, apple and so on, and the functions of some KNOX genes have been deeply studied (Mukherjee et al., 2009; Xiong et al., 2018; Ma et al., 2019; Jia et al., 2020). However, little information is available on the KNOX gene family in the important food crop wheat (Triticum aestivum L.). At present, only a report about Takumi et al. (2000) cloned three KNOX genes from wheat with the help of maize Kn1 gene sequence.

Based on the structural characteristics, phylogenetic tree and expression pattern of KNOX genes, the KNOX gene family can be divided into three types: Class I, Class II, and Class KNATM, which is endemic to dicotyledons (Magnani and Hake, 2008; Gao et al., 2015; Xiong et al., 2018). The expression patterns and biological functions of KNOX genes of different classes have obvious differentiation. The expression of Class I genes is concentrated, mainly in SAM (Shoot apical meristem), and they play an important role in the differentiation and maintenance of meristem (Tsuda et al., 2011; Gao et al., 2015; Su et al., 2020). For example, the STM gene in Arabidopsis Class I plays an important role in the establishment of SAM in Arabidopsis embryos, and the seeds of Arabidopsis stm mutants can only produce cotyledons but not new leaves because of the lack of SAM. The KNOX gene in Class I is closely related to callus differentiation in the process of transgenic. The calli of the double deletion mutants of osh1 and osh15 genes in rice Class I can only form leaf-like structures, but no bud formation can be seen (Tsuda et al., 2011). When the KN1 gene of maize Class I was transferred into tobacco, the rate of seedling transformation was twice as high as that of the control (Xu et al., 2009). At the same time, Class I genes also play an important role in the control of leaf shape, internode elongation, hormone balance and the establishment of inflorescence structure, and can be used as transcriptional activators or suppressors (Hay and Tsiantis, 2010; Tsuda and Hake, 2015). The KNOX subfamily genes of Class II are generally widely expressed in various tissues and organs of plants, but there are relatively few reports about the function of Class II KNOX genes in plants, which are mainly related to the formation of the secondary cell wall. For example, the synergistic effect of KNAT7 and KNAT3 affects the deposition of the secondary cell wall and improves the mechanical support of the Arabidopsis stem (Wang et al., 2020). Class KNATM is a kind of KNOX gene subfamily peculiar to dicotyledons. It is found that KNATM gene is involved in regulating leaf polarity and leaf shape in Arabidopsis thaliana (Magnani and Hake, 2008).

As one of the most important food crops in the world, wheat is the staple food for about 35%~40% of the world's population (He et al., 2018). However, due to the complexity of wheat genome, there are few reports on the cloning and research of the TaKNOX gene.  The continuous improvement of wheat genome information (Iwgsc, 2018), it brings an opportunity to identify and analyze TaKNOX gene at the whole genome level. In the current study, TaKNOX gene was identified by using wheat genome information, combined with the characteristics of KNOX gene family, and the basic physical and chemical properties, chromosome distribution, gene replication, evolutionary relationship, gene structure, cis-acting elements and KNOX gene expression patterns in different tissues were analyzed, which laid a foundation for further study on the function and regulation mechanism of TaKNOX gene.

1 Results and Analysis

1.1 Identification and nomenclature of TaKNOX gene

Based on the amino acid sequence of Arabidopsis and rice KNOX protein, BLAST search, screening and verification were carried out in the wheat database, and a total of 36 TaKNOX genes were identified, which were named TaKNOX1~ TaKNOX36 (Table 1) according to the chromosome location of the gene. Among the proteins encoded by the TaKNOX gene family, the number of amino acids ranges from 153~389. TaKNOX protein MW range from 16.54~42.48 kDa, and the PI is 5.15~9.11. Except for TaKNOX13, TaKNOX15, TaKNOX18 and TaKNOX34, the PI of other TaKNOX proteins is less than 7, indicating that most TaKNOX proteins are acidic. Subcellular localization prediction analysis showed that most of KNOX genes were located in the nucleus, which was consistent with the transcription factor characteristics of KNOX gene.

Table 1 Physicochemical properties of TaKNOX gene family

1.2 Multiple alignments of TaKNOX proteins

Multiple sequence alignment showed that TaKNOX protein sequence contains four relatively conserved regions: KNOX1, KNOX2, ELK and HOX (Figure 1A). KNOX1 and KNOX2 domains are located at the N-terminal, while ELK and HOX are located at the C-terminal. Among them, HOX domain is the most conservative, showing the typical structural characteristics of the TALE homeobox protein superfamily, and there are three additional amino acid sequences between the first and second helix (P-Y-P). KNOX1, KNOX2 and ELK also showed effective conservatism. For example, KNOX2 contains a highly conserved E-L-D amino acid sequence, and the ELK contains a highly conserved E-L-K amino acid sequence (Figure 1B).

Figure 1 Multiple alignments of KNOX proteins in wheat Note: The black horizontal line in Figure A indicates the four conserved domains of KNOXI, KNOXII, ELK and HOX; Figure B shows the distribution and composition of protein characteristic motifs of each conserved domain; Letters stand for the amino acids; The height of the letters show the conservation level

1.3 Chromosome mapping and gene duplication analysis of TaKNOX gene

KNOX gene was distributed on all chromosomes except chromosome 3 in wheat, among which there was one KNOX gene on chromosomes 2, 6 and 7 (Figure 2). KNOX gene is the most common on chromosome 4A, with 6 KNOX genes. At the same time, most KNOX genes are distributed at the ends of chromosomes. Segmental duplication and tandem repeat are the main mechanisms of gene family expansion (Cannon et al., 2004). Collinear analysis of TaKNOX gene showed that it contained five pairs of tandemly repeated genes: TaKNOX1 and TaKNOX2, TaKNOX3 and TaKNOX4, TaKNOX6 and TaKNOX7, TaKNOX14 and TaKNOX15, TaKNOX22 and TaKNOX23 (In Figure 2, the red font is tandem repeated gene). In addition, 23 pairs of TaKNOX genes were found fragment repeat block, including most of the TaKNOX genes with partial homology (In Figure 2, the blue line represents the KNOX gene in the fragment repeat block). The results showed that there were tandem repeats and segmental duplication in the process of polyploidy in the expansion of TaKNOX gene.

Figure 2 Chromosome mapping and gene duplication of KNOX genes in the wheat genome Note: The blue line represents the TaKNOX gene pair in the fragment repeat block; The red font is tandem repeated TaKNOX gene

1.4 Phylogenetic analysis of TaKNOX gene

KNOX protein sequences of Arabidopsis, rice and the KNOX protein sequences of wheat were used to construct the KNOX phylogenetic tree using MEGA X software (Figure 3). Based on the phylogenetic tree, KNOX gene was divided into three categories: Class Ⅰ, Class II and Class KNATM. Class Ⅰ contains 38 genes, including 4 Arabidopsis genes, 9 rice genes, and 25 wheat genes. Class Ⅰ contains 19 genes, including 4 Arabidopsis genes, 4 rice genes, and 11 wheat genes. Class KNATM contains only one member (KNATM) of Arabidopsis, which is unique to dicotyledons (Magnani and Hake, 2008).

Figure 3 The phylogenetic tree of KNOX gene family in wheat, Arabidopsis and rice Note: KNOX genes from wheat, Arabidopsis and rice are represented in black, red and blue fonts, respectively

Class I was further divided into three subclasses: ClassⅠA, ClassⅠB, and ClassⅠC. ClassⅡ contains ClassⅡA and ClassⅡB (Figure 3). TaKNOX19, TaKNOX24 and TaKNOX26 of ClassⅠA were clustered with AtKNAT1 and AtSTM of Arabidopsis thaliana and LOC_Os03g51690(OSH1) of rice. TaKNOX1, TaKNOX2, TaKNOX3, TaKNOX4, TaKNOX6, and TaKNOX7 of ClassⅠB were clustered with AtKNAT2 and AtKNAT6 of Arabidopsis thaliana. TaKNOX20, TaKNOX25, and TaKNOX27 of ClassⅡA were clustered with AtKNAT7 of Arabidopsis thaliana (Figure 3). These TaKNOX genes may have similar biological functions to Arabidopsis and rice KNOX genes which are closely related to their phylogeny. At the same time, some of the KNOX homologous genes located in A, B and D subgenomes are closely related, and most of them are clustered together, such as TaKNOX1,  TaKNOX3,  TaKNOX6, TaKNOX2,  TaKNOX4,  TaKNOX7,  TaKNOX8,  TaKNOX9,  TaKNOX10,  TaKNOX12, 

TaKNOX19, TaKNOX24, TaKNOX26, TaKNOX28, TaKNOX30 etc.

1.5 The gene structure and protein conserved domain analysis of TaKNOX gene

To further explore the characteristics of TaKNOX gene family, based on an intraspecific phylogenetic tree (Figure 4A), we analyzed the gene structure and protein conserved domain distribution of TaKNOX gene (Figure 4). The results showed that the TaKNOX family was mainly composed of 3～6 exons and 4～5 introns. The number of exons in ClassⅠ ranges from 4～6, while almost all genes in ClassⅡ contain 5 exons (Figure 4B). It is worth noting that all ClassI genes, except TaKNOX11 and TaKNOX14, contain a long intron (Figure 4B), which is consistent with the structural characteristics of the ClassI KNOX gene family (Morimoto et al., 2005). Most TaKNOX proteins contain conserved protein domains of KNOX1, KNOX2, ELK and HOX, and are arranged in turn on the protein sequence. Some TaKNOX proteins lack two conserved domains, for example, TaKNOX11 and TaKNOX14 lack ELK and HOX conserved domains, TaKNOX29 and TaKNOX33 lack KNOX1 domain. But all TaKNOX proteins contain KNOX2 (Figure 4C). Meanwhile, based on the perspective of evolutionary relationships, the TaKNOX genes of the same subfamily are also conservative in gene structure and protein domain distribution.

Figure 4 The gene structure and protein conserved domain analysis of KNOX gene family in wheat Note: A: The phylogenetic tree of KNOX gene family in wheat; B: The gene structure composition of KNOX gene family in wheat; C: The conserved domain composition of KNOX gene family in wheat

1.6 Analysis of cis-acting elements on the promoter of TaKNOX gene

In order to analyze the potential expression regulation mechanism of TaKNOX gene family members, we identified the cis-acting elements of each member. We selected the cis-acting elements related to plant hormones, plant growth, stress response and light response for identifying (Figure 5). The results showed that the promoter regions of all TaKNOX genes contained elements related to plant hormones and stress response. For example, the cis-acting elements related to plant hormones are ABA response element ABRE, MeJA response element CGTCA-motif, TGACG-motif, GA response element TATC-box, GARE-motif, P-box and so on. The cis-acting elements related to stress response are low-temperature induction response element LTR, defense and stress related response element TC-rich repeats and so on. In addition, 28 TaKNOX genes contain cis-acting elements (O2-site, CAT-box, GCN4-motif, circadian) related to the regulation of plant growth and development. Moreover, 23 TaKNOX genes contain elements (CAT-box) related to the expression of plant meristem. These results showed that TaKNOX gene may play an important role in wheat growth and development, maintenance of meristem differentiation ability and response to stress (Figure 5).

Figure 5  Analysis cis-elements of the KNOX genes in wheat Note: A: Histograms of different colors are used to represent the sum of four kinds of cisacting elements in TaKNOX genes;  B: Heat map cisacting elements of each TaKNOX gene. Different colors indicate the number of cisacting elements, while white indicates that there are no cisacting elements

1.7 Expression patterns of TaKNOX genes in different tissues

In order to explore the tissue-specific expression patterns of KNOX gene in wheat at different growth and developmental stages, the expression data of 36 TaKNOX genes were analyzed, and the transcriptional levels of wheat roots, stems, leaves, spikes and seeds were studied. It was found that there were significant differences in the expression patterns of wheat TaKNOX gene in different tissues or organs (Figure 6). There is a certain correlation between the expression pattern of TaKNOX gene and its subclass. In Class I, except TaKNOX18 was mainly expressed in leaves, other members were mainly expressed in stems and panicles, and TaKNOX11, TaKNOX13, TaKNOX14 and TaKNOX15 were highly expressed in panicles. While in Class II, except TaKNOX34, TaKNOX35 and TaKNOX36 were highly expressed in leaf and other genes were widely expressed.

Figure 6  TaKNOX gene expression profiles in different tissues

2 Discussion

As an important regulatory factor of plant growth and development, KNOX gene lacks identification and functional research in wheat. At present, little information is available on the KNOX gene family in wheat (Triticum aestivum L.). Only Takumi et al. (2000) cloned three KNOX genes (TaKNOX12, TaKNOX19, and TaKNOX24 in this study) from wheat with the help of maize Kn1 gene sequence. Morimoto et al. (2005) analyzed the expression patterns of these three TaKNOX genes and the phenotypes of transgenic tobacco. With the continuous improvement of wheat genome information, it brings an opportunity to identify and analyze TaKNOX transcription factors at the whole genome level. In this study, the KNOX gene family of wheat was identified at the whole genome level for the first time, and the basic physical and chemical properties, gene structure, evolutionary relationship and expression pattern of TaKNOX gene were analyzed, which laid a foundation for future functional study of KNOX gene.

In this study, we identified 36 members of the KNOX gene family, more than Arabidopsis (9) and rice (13). Among these genes, there are 5 pairs of tandem repeats and 28 pairs of segmental duplication, indicating that the whole genome repetition plays an important role in the amplification of TaKNOX gene family, which leads to the larger size of TaKNOX gene family than Arabidopsis and rice. According to the protein sequence, evolutionary tree and gene structure characteristics, TaKNOX genes were divided into two categories: Class I and Class II. Among them, TaKNOX1~4, TaKNOX6~19, TaKNOX21~24, TaKNOX26, TaKNOX28 and TaKNOX30 exist in the same evolutionary branch with Class I genes in Arabidopsis and rice, and the remaining TaKNOX genes are clustered into one branch with Class II genes in Arabidopsis and rice. It is worth noting that TaKNOX12, TaKNOX19 and TaKNOX24 in Class I are most closely related to Arabidopsis AtSTM (AT1G6230), AtKNAT1 (AT4G08150) and rice OSH1 (LOC_Os03g51690). These three KNOX genes may have similar biological functions to AtSTM, AtKNAT1 and OSH1, that is, affecting the establishment of SAM in embryos and the formation of embryogenic calli (Hay and Tsiantis, 2010; Tsuda et al., 2011). Therefore, these three TaKNOX genes may be used to improve the efficiency of embryogenic callus formation in the process of wheat genetic transformation, to improve the genetic transformation efficiency of wheat.

The results of TaKNOX gene transcriptome data showed that the expression level of Class I genes (except for TaKNOX18 gene) were very low or non-expressed in other tissues, except in panicle and stem. However, Class II genes are expressed in almost all tissues of wheat, which is consistent with the expression patterns of KNOX I and KNOX II genes in other plants, indicating that plant KNOX proteins may be highly conserved in function (Gao et al., 2015). Moreover, the expression of TaKNOX8, TaKNOX9, TaKNOX10, TaKNOX12, TaKNOX19 and TaKNOX24 of Class I genes in wheat stem is the highest, and the expression is the highest in the early stage of stem development, and gradually decreases with the late development, which may indicate that these genes play an important role in the maintenance and regulation of wheat stem meristem. The function of these genes needs to be further investigated. Therefore, deeply understanding the function of TaKNOX gene will lay an important foundation for the study of wheat growth and development, morphogenesis and other processes.

3 Materials and Methods

3.1 Identification of TaKNOX gene family

To identify all KNOX genes in wheat genome database, the main steps are as follows: (1) Download the genomic data of Chinese Spring from Ensemble Plants database (http://plants.ensembl.org/Triticum_aestivum/Info/Index). (2) Download 9 KNOX protein sequences of Arabidopsis and 13 KNOX protein sequences of rice from TAIR website (https://www.arabidopsis.org/) and Rice Genome Annotation Project website (http://rice.plantbiology.msu.edu/index.shtml), respectively. (3) The KNOX protein sequences of Arabidopsis and rice were used as seed sequences, and the blast command of Tbtools (Chen et al. 2020) was used to search the wheat protein database, with the threshold of e-value < e^-10. (4) Submit the search results to the "PFAM" website (http://pfam.xfam.org/) for further verification of the Pfam number (PF03790 or PF03791) of KNOX conserved domain KNOX I or KNOX II (Finn et. al. 2016). Those with KNOX I or KNOX II domains are regarded as members of the KNOX gene family. The number of amino acids (AA), molecular weight (MW) and isoelectric point (PI) of each KNOX protein were predicted by ProtParam website (https://web.expasy.org/protparam/), and the subcellular localization of KNOX protein was predicted on WoLF PSORT website (https://wolfpsort.hgc.jp/).

3.2 Protein domain analysis and multiple sequence alignment of members of TaKNOX gene family

Analyzed the KNOX protein domain of wheat with the help of CDD website (https://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi) and Pfam website (http://pfam.xfam.org/). The multiple sequence alignment of KNOX protein sequence was carried out by using MUSCLE in MEGA X (Kumar et al., 2018), and the visualization was carried out by Jalview software.

3.3 Chromosome mapping and gene replication of members of TaKNOX gene family

The chromosome position information of the members of the KNOX gene family was obtained according to the wheat genome annotation file. MCScanX software (Wang et al., 2012) was used to calculate and obtain the tandem repeat information of collinear blocks and genes. Circos software (Krzywinski et al., 2009) was used to visualize the chromosome position information, collinear relationship, tandem repeat information of TaKNOX gene.

3.4 Phylogenetic analysis of KNOX gene family

The members of KNOX gene family of Arabidopsis, rice and wheat were used to construct a phylogenetic tree by MEGA X software. The multiple alignments of KNOX protein sequence were carried out by using the MUSCLE alignment method, and Neighbor-joining was selected to construct the phylogenetic tree. Specific parameters are as follows: Bootstrap method with parameters of 1 000, Poisson model.

3.5 Analysis of cis-acting elements in the promoter region of members of TaKNOX gene family

The promoter region of 2 000 bp upstream of the CDS sequence of each KNOX gene was obtained from the wheat genome database by TBtools software to analyze the cis-acting regulatory elements in the KNOX gene promoter. The obtained promoter region sequence was submitted to the PlantCARE database (http://bioinformatics.psb.ugent.be/webtools/plantcare/html/) to identify the type and number of cis-acting elements.

3.6 Analysis of tissue expression pattern of TaKNOX gene family

Download transcriptome data in different periods of 5 different tissues including root, stem, leaf, spike and seed from the wheat gene expression website (http://www.wheat-expression.com/) (Ramirez-Gonzalez et al., 2018). The tissue expression data of TaKNOX gene were transformed and visualized by TBtools software.

Authors’ contributions

ANN was the executor of the experimental design and experimental research in this study. SNN and LHH completed the data analysis and drafted the manuscript. AYW, BSL, MFF, ZZ, LH, ZY, and SCP participated in the design of the study and performed the analysis of the results. GGH conceived of the project, directed the design of the study, data analysis, draft and revision. All authors read and approved the final manuscript.

Acknowledgments

The study was supported by the National Natural Science Foundation of China (31901547), China Postdoctoral Foundation (2018M642741), and the Postdoctoral Research Project in Henan Province (001801013).

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