Background

Alpha-amylase (α-D-1,4-glucan-4-glucanohydrolases, EC 3.2.1.1) is of critical importance to the breakdown of starch granules during seed germination [1,2,3,4,5]. It catalyzes the hydrolysis of internal α-D-1,4-glucosidic linkages in large polysaccharides to yield maltose and maltodextrin products [6]. In cereal crops, such as bread wheat and barley, two main isoforms of α-amylases have been extensively investigated during seed germination or in the gibberellic acid (GA3)-induced aleurone layers [1,2,3,4,5]. They were conserved in the biosynthesis and secretion from the scutellar epithelium and the aleurone layer to the starchy endosperm [7, 8]. Major differences in physicochemical and biochemical properties (i.e., sensitivity to Ca2+, stability at low pH and under heat treatment, and charge and serological characteristics) could be used to distinguish the two isoforms, which were eventually classified as high-pI and low-pI isoforms on the basis of isoelectric point (pI). The nomenclatures of high-pI and low-pI isoforms have not been consistent in the literature. In most of the literatures [9,10,11,12], researchers assigned high-pI and low-pI α-amylases as symbols of α-Amy-1 (Amy1) and α-Amy-2 (Amy2), respectively. Oppositely, some other literatures separately designated them as Amy2 and Amy1 [13,14,15]. Meanwhile, some authors [16, 17] named low-pI isoform as type A and high-pI as type B. In this work, we are prone to adopt high-pI α-amylase as Amy1 and low-pI as Amy2.

High-pI isoform (Amy1) was triggered by the commencement of seed germination and produced in higher concentration than that of low-pI α-amylase (Amy2), which was characterized by its synthesis both in kernel development and during seed germination [18]. Protein crystal structures of Amy1 and Amy2 were very similar, each consisting of three domains: a central conserved (β/α)8-barrel domain (domain A), an additional domain B nested between β3 and α3 of domain A, and a five-stranded C-terminal β-sheet domain (domain C) [14, 15, 19]. Substrate binding analysis revealed a starch granule binding surface site (SBS1) and a highly conserved active site in both of them among cereal grains, and when replaced by thio-maltotetraose as substrate analogues, a new Amy2-specific surface binding site at domain C had been discovered [15, 20]. Additionally, a novel wheat α-amylase (TaAMY3) was reported and considered as the most abundant isoform compared with the other known α-amylases throughout grain development [21, 22]. The newly described isoform HvAMY4 did not subject any predicted transit peptide and detected in various plant tissues [23]. Therefore, these four categories of α-amylases seem to accomplish starch degradations in different developmental stages or in various plant tissues, although their controlling and digesting mechanisms are still unclear. Accordingly, in cereal crops, genes encoding α-amylases have been divided into three or four subfamilies, i.e., TaAMY1 to TaAMY3 in bread wheat [12], HvAMY1 to HvAMY4 in barley [23] and OsAMY1 to OsAMY3 in rice [12]. Recently, AMY4 genes have been identified and added to wheat and barley [24, 25].

Because of functional importance to the transition from dormancy to germinating, and its association with high residual α-amylase activities in ripe wheat grains, Amy1 genes have been of interest to plant biologists for many years. This structural locus was located on the long arms of chromosomes 6A, 6B and 6D in bread wheat [10], and 6H in barley [26], respectively. It was thought to be complicated, compound and multigenetic, either consisting of tightly linked copies or involving some degree of gene duplications [4, 10]. As a self-pollination plant, bread wheat (AABBDD) is a product of at least two rounds of polyploidization, consisting three closely related diploid progenitors: T. urartu (A genome), Aegilops speltoides-related species (B genome) and Ae. tauschii (D genome) [27, 28]. The redundancy afforded by the hexaploid nature of bread wheat also increases difficulties to fully evaluate this structural locus. Based on simultaneous studies of polyacrylamide gel electrophoresis (PAGE), isoelectric focusing (IEF) and restriction fragment length polymorphism (RFLP), three Amy1 copies, located on the chromosome 6R, were determined in the diploid genome of rye [29]. Khursheed and Rogers cloned two types of barley α-amylase genes (Amy6–4 and Amy46) belonging to the high-pI multigene family, and confirmed their different mRNA levels in the GA3-induced aleurone cells [30]. In addition, functional analysis of the Amy6–4 promoter region indicated that the gibberellic acid response elements (GARE) was comprised of three conserved cis-acting boxes (‘pyrimidine’ box, ‘taacaaac’ box and ‘tatccac/t’ box) [31, 32]. Through binding to the central element (‘taacaaac’ box), the GA3-regulated transcription factor (GAMyb) activated transcription of Amy1 genes [33]. In rice, the RAmy1 subfamily (RAmy1A, RAmy1B and RAmy1C) consists of genes corresponding to the Amy1 classes of barley and wheat [12]. RAmy1A (AmyI-1) transcript was most abundant in germinating seeds and involved in the degradation of plastid starch granules [34, 35]. Recently, as reviewed by Mares and Mrva [36], pre-harvest sprouting (PHS) and late maturity α-amylase (LMA) were characterized by high levels of α-amylases in ripe wheat grains. Wheat lines with abnormal accumulation of α-amylases lowered whole meal falling numbers and resulted in reduced starch viscosity and poor flour quality [36,37,38]. Yang et al. [39] and Cheng et al. [40] isolated Amy1 genes involved in PHS-affected and LMA-affected lines, and detected high expression of Amy1 genes was strongly correlated with high levels of high-pI α-amylases.

Despite considerable progresses have been made in understanding the characteristics of Amy1 genes, to date, comprehensive analysis focusing on this multigene family still lacks. It has been well established that three major grass subfamilies (Pooideae, Ehrhartoideae and Panicoideae) evolve from a common ancestral cereal genome with a basic number of five chromosomes [41, 42]. Together with recently released genome datasets of hexaploid wheat and barley, this inner circle model facilitates access to investigate the conserved block carrying this structural locus with multiple high-homology gene members, and help us to better explore their structural organization and expansion dynamics. Therefore, elucidating functional diversification of duplicated copies of this multigene family is needed for further practices of agronomic traits improvement and molecular design breeding.

In this paper, we firstly estimate the exact copy number of Amy1 genes in grass. Then, we reconstruct the phylogeny and investigate structural organization. For further exploring evolutionary forces and understanding functional implications, we conduct potential natural selection tests, build three-dimensional (3D) protein homology structures, and quantify the expression profiles in depth from copy-specific levels.

Methods

Plant materials and tissue sampling

Chinese Spring (CS) (T. aestivum) and six nullisomic-tetrasomic (NT) lines for the homoeologous group 6, PI428191 (T. urartu), PI542268 (Ae. speltoides) and AS2404 (Ae. tauschii) were used for DNA extraction and cloning experiments; seeds were incubated at room temperature for 5 to 7 days under darkness condition. Based on preliminary phenotyping under three different field conditions (Sichuan shuangliu in 2014/2015, Sichuan shifang and Yunnan yuanmou in 2014/2015), three wheat cultivars (Guinong19, Mianmai43 and Jinan17) with high residual α-amylase activities in ripe grains and the absence of sprouting, and three landraces (Honghuamai, Siqiangxiaomai and Guangguangtou) with extreme low activities were selected for measurement of total α-amylase activities and preparation of mRNA in the developing grains. Spikes were tagged at anthesis and sampled at 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34 and 36 days post anthesis (DPA) in Sichuan shuangliu from late-March to early-May in 2016. For germination sampling, seeds of bread wheat (cv. CS) and barley (cv. Morex) were incubated on a moist filter paper in petri dishes under darkness condition (25 °C); similar seedlings were collected at 12, 24, 36, 48 h (h) after seed imbibition. During growth, field management followed normal field operations. The plant materials used in the study are held in our own lab.

Identification of Amy1 genes in grass

Two approaches (molecular cloning and in silico analysis) were used to identify Amy1 genes. Firstly, the public available Amy1 sequences were collected from databases of bread wheat and other Triticeae species (https://urgi.versailles.inra.fr/blast/). To detect the TaAmy1 copy number, primer pair TaAmy1-F/R was developed for cloning the full-length genomic sequences. Total genomic DNA was extracted following the cetyl trimethylammonium bromide (CTAB) method with minor modifications. PCR products were purified and sequenced using the same forward and reverse primers at Sangon Biotech (Shanghai, China). Due to limited Taq-polymerase fidelity, clones with sub-optimal quality were discarded for further analysis.

Two barley Amy1 genes, Amy6–4 (GenBank accession no. K02637) and Amy46 (GenBank accession no. J04202) [30], were employed as query sequences to blast against the genomes of bread wheat (IWGSC WGA v0.4, https://urgi.versailles.inra.fr/blast_iwgsc/), barley (http://webblast.ipk-gatersleben.de/), rice (http://rice.plantbiology.msu.edu/) and other grasses (https://phytozome.jgi.doe.gov/) with default setting parameters. BLAST hits with an expectancy value (E-value) of zero were subjected to the second round of BLAST searches within the genomes from which they were identified. All the retrieved and cloned Amy1 gene sequences were aligned with Clustal X 2.0 [43] and manually modified with BioEdit v7.2 [44]. Exons and introns were positioned by aligning full genomic sequences and their corresponding coding sequences, and visualized by the GSDS 2.0 server [45]. The pI values of putative amino acids were calculated using online computation tool (http://web.expasy.org/compute_pi/). Subcellular localization and cleavage site prediction were performed using the CBS TargetP software [46, 47].

Phylogenetic reconstruction and positive selection detection

Full-length coding sequences were used for phylogenetic analysis. Maximum likelihood trees were constructed by MEGA7 software [48] using the Tamura 3-parameter + GAMMA substitution model [49], the best fitting model as determined by the “Find Best DNA/Protein Models” function in MEGA7. All positions containing gaps and missing data were eliminated. Branch supports attached to each node were inferred from 1000 bootstrap replicates and values less than 75% were collapsed. Putative amino acids were used for estimating the genetic distance of Amy1 genes as described by Jones et al. [50]. Synonymous (Ks) and non-synonymous (Ka) substitution rates of paralogs and orthologs were calculated as described previously [51]. Codon-based substitution models, M0 (one-ratio) and Two-ratios [52, 53], M1a (Neutral) and M2a (Selection) [54, 55], M3 (discrete), M7 (beta) and M8 (beta & omega) [56], Model A and Model B [57, 58], were applied to detect branches or sites under positive selection, which were conducted using the codeml program [52] implemented in the PAML package [59]. Divergence time (T) was obtained using a synonymous rate of 6.5 × 10− 9 substitutions per site per year [60,61,62] as T = Ks / (2 × 6.5 × 10− 9).

Synteny investigation and repeats annotation

Chromosomes anchoring Amy1 loci were downloaded from online resources to construct a local genomic database. A total of 112 structural genes flanking the rice Amy1 locus (LOC_Os02g52700 and LOC_Os02g52710) were used as query markers to search against the local database using the basic tool NCBI-BLAST-2.4.0+ [63]. Reciprocal blastp [64] was carried out to confirm the orthologous relationships between pairs of corresponding structural markers. Genomic segments covering these markers were selected for detecting gene orders and synteny relationships. The identification of repetitive elements was analyzed by using a local BLASTN search against the non-redundant dataset of Triticeae Repeats (http://botserv2.uzh.ch/kelldata/trep-db/index.html).

Protein structure homology modeling

Two protein homologues, barley 1AMY [14] and rice 3WN6 [19], were extracted from the Protein Data Bank (PDB) archive (http://www.rcsb.org/pdb/home/home.do). Primary sequence alignment (not including the signal peptides) and secondary structure analysis were performed and displayed using the ESPript 3.0 server [65]. Annotation details of amino acids were inferred from the National Center for Biotechnology Information (NCBI) conserved domain database [66]. Protein structure homology modeling was built in the workplace of SWISS-MODEL [67,68,69] based on the molecular structure of a barley α-amylase-inhibitor complex (PDB ID: 1BG9) [70]. UCSF Chimera [71] was used for visualization and analysis of the resulting models.

Alpha-amylase assay

Developing grains were removed from the central part of the spikes. The wholemeal samples were dehydrated using VirTis freeze drying equipment and Lyophilizers (SP SCIENTIFIC). Alpha-amylase activity was determined following protocols of Whan et al. [22]. Data was expressed in ceralpha unit (CU) per g four or μg of protein as determined by Bradford assays [72] on the CERALPHA extracts.

RNA extraction and qRT-PCR

Total RNA from germinating seeds and developing grains was extracted using a TaKaRa RNAiso Plus kit (http://www.takara.com.cn/). RNA was quantified using NANODROP 2000c. Approximately 2 μg of total RNA was used for reverse transcription and cDNA synthesis using TaKaRa Reverse Transcriptase M-MLV (RNase H-) following the supplier recommendations. Real time PCR was performed using SYBR green PCR master mix (Bio-Rad) in a 20-μL reaction system on CFX Connect™ Real-Time PCR Detection System (Bio-Rad). RT-PCR data were extracted using CFX Manager 3.1 software (Bio-Rad) and analyzed by 2−ΔCT method. The expression of TaActin [22, 73] was used as an internal control for normalization. All the primers used in this work were listed in Additional file 1.

Results

Identification of Amy1 genes in grass

As summarized in Table 1, a total of 41 Amy1 genes were identified from the 13 investigated grass species. In bread wheat, there are three full-length copies in each of A and D genomes, and six copies (five full-length and one truncated TaAmy1-B6) in B genome. We also isolated these A- and D-genome copies in T. aestivum, T. urartu and Ae. tauschii, respectively. Multiple sequence alignment indicated that the genomic sequences of TaAmy1-D1, TaAmy1-D2 and TaAmy1-D3 were identical to AetAmy1-D1, AetAmy1-D2 and AetAmy1-D3, respectively. Pairwise identities were 98.7% between TaAmy1-A1 and TuAmy1-A1, 99.7% between TaAmy1-A2 and TuAmy1-A2, and 99.1% between TaAmy1-A3 and TuAmy1-A3. As for the B genome, we cloned three full-length copies (TaAmy1-B1, TaAmy1-B2 and TaAmy1-B3) in T. aestivum and six (AesAmy1-B1 to AesAmy1-B6) in Ae. speltoides, and pairwise identities (88.1 to 97.8%) were significantly lower than that of A and D genomes. All the isolated Amy1 sequences are clustered in Additional file 2. The existence of TaAmy1-B6 was verified by using primer pair TaAmy1-B6-F/R (Additional file 3A).

Table 1 Summary information of Amy1 genes identified in grass species
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In barley, we identified five copies located on the chromosome 6H, and three copies with unknown chromosomal locations. Because of existence of gapped sequences in the coding region, two of them were discarded for further analyses. Similar to TaAmy1-B6, HvAmy1–6 was also a truncated copy with approximate 682 bp gene sequences retained. Additionally, in silico analysis indicated that two copies were found in each of O. sativa, P. hallii and S. bicolor. Only a single copy was identified in each of B. distachyon, B. stacei, S. italica, S. viridis and Z. mays.

Exon and intron analysis revealed two types of exon-intron structures (Fig. 1). Pattern A contained three exons and two introns, whereas the second exon of pattern B was interrupted by a middle intron and separated into two exons. Furthermore, we calculated theoretical pI values, which range from 4.98 to 6.10. All the full-length Amy1 genes showed strong signals in secretory pathway (SP ≥ 0.986), and the length of signal peptides varied from 22 to 25 amino acids.

Fig. 1
figure 1

Schematic representation of two types of exon-intron structures. Pattern A was present in species of T. aestivum, T. urartu, Ae. speltoides, Ae. tauschii, H. vulgare, S. italica and S. viridis. Pattern B was found in B. distachyon, B. stacei, O. sativa, P. hallii, S. bicolor and Z. mays. Lengths of exons and introns were displayed proportionally. Because Amy1 genes shared high sequence identities with Amy2, we used the Amy2 exon-intron structure as a reference

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Phylogenetic and syntenic relationships

To determine evolutionary relationship of Amy1 family in grass, the phylogeny was reconstructed based on the maximum-likelihood method. The phylogenetic tree segregated Amy1 genes into three major clades with high bootstrap supports (99% or 100%), corresponding to the three subfamilies (Pooideae, Ehrhartoideae and Panicoideae) in grass (Fig. 2). In the Pooideae-Triticeae clade, 28 identified sequences from five Triticeae species were clustered into two groups: Group 1 (G1) and Group 2 (G2). G1 contained 14 members: two in each of T. urartu, Ae. tauschii, genomes A, B and D of wheat, three in Ae. speltoides, and one in H. vulgare. G2 contained one in each of T. urartu, Ae. tauschii, genomes A and D of wheat, three in each of Ae. speltoides and B genome, and four in H. vulgare. We further estimated the evolutionary distances. All the three clades (Pooideae-Brachypodium, Ehrhartoideae and Panicoideae) exhibited larger genetic distances with G1 than those with G2 (Additional file 4), suggesting that G2 might be the ancient archetype Amy1 genes in Triticeae.

Fig. 2
figure 2

Phylogenetic analysis of grass Amy1 genes. Forty-three protein-coding sequences from 13 species were involved. The tree is rooted on midpoint and drawn to scale, with branch lengths measured in the number of substitutions per site. A discrete Gamma distribution was used to model evolutionary rate differences among sites [5 categories (+G, parameter = 0.4987)]. Two low-pI Amy2 genes, FN179389 [23] and Amy32b (GenBank accession no. X05166) were used as outgroup

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To make insights into structural organization of Amy1 loci, the linear gene orders were analyzed across six grass genomes (Fig. 3A). The Amy1 regions in wheat and barley shared highly conserved gene orders. However, compared to Amy1 genes of rice located in the middle of segments, those of wheat and barley were dispersed on both ends of corresponding segments. The segments seemed to be separated into two parts, each of them was involved in an inversion event. A tandem gene duplication event was also found, which resulted in expansion of Amy1 family (Fig. 3A, B). The two Amy1 gene clusters apart from each other were corresponding to the phylogenetic groups G1 and G2, temporarily designated as Amy1λ and Amy1θ, respectively. Intervals between them, ranging from approximately 8.36 Mb on chromosome 6H to 21.42 Mb on chromosome 6B, were rich in repetitive elements such as long terminal repeat (LTR) retrotransposons and DNA transposons (Fig. 3A, Additional file 5).

Fig. 3
figure 3

(a) Synteny relationships of genomic segments covering Amy1 loci in rice, wheat, barley and B. distachyon. The Amy1 genes were red highlighted and scale bars were proportional to the length of chromosomal segments. (b) Schematic illustration of structural variations of Amy1 region

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Divergence time of Amy1λ from Amy1θ

As mentioned above, a tandem gene duplication event, followed by chromosomal rearrangements, led to the fixation of Amy1θ and Amy1λ in genomes of wheat and barley. Considering a single Amy1 copy in genera Brachypodium, this structural variation might occur prior to the divergence of wheat from barley (11.6 MYA) and after the divergence of wheat from Brachypodium (32–39 MYA) [42, 74]. To verify it, we calculated the pairwise mean synonymous substitution rates (dS) and estimated when this duplication event occurred. The divergence time of Amy1λ from Amy1θ occurred at approximately 36.3 ± 6.2 MYA in A genome (31.6 ± 5.7 MYA in T. urartu), 31.2 ± 5.6 MYA in B genome (39.4 ± 6.9 MYA in Ae. speltoides), 42.9 ± 7.7 MYA in D genome (42.9 ± 7.7 MYA in Ae. tauschii), and 36.0 ± 6.6 MYA in barley (Table 2). These results agreed with our hypothesis, and our estimates also suggested a higher substitution rate of Amy1 genes than the average 6.5 × 10− 9 in grass nuclear genes.

Table 2 Estimation of duplication and divergence time between groups
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Potential natural selection on Amy1λ genes

We applied nine different codon-substitution models to detect selection pressures at individual sites along specific lineages. Results obtained were presented in Table 3. The one-ratio model (M0) produced an estimated ω0 = 0.0519. The two-ratio model assigned two different ω ratios for the foreground branch G1 (ω1 = 0.182) and for all other background branches (ω0 = 0.0513). Site-specific models indicated variable selective pressures among these 424 codons. For example, the M3 model (K = 2) fit the data better than the one-ratio model, the Likelihood Ratio Test (LRT) statistics were 2∆ℓ = 420, with P < 0.001 and df = 2. Both the branch models and site-specific models failed to detect sites under positive selection, and most sites appeared to be under strong purifying selection. The branch-site model A fit the data significantly better than M1a, the test statistics were 2∆ℓ = 3.6, with P = 0.06 and df = 1. Model B did not fit the data significantly better than M3 (discrete with K = 2) (2∆ℓ = 3.68, P = 0.16, df = 2), but it suggested a proportion of sites (19.7%) were under positive selection along the G1 branch with ω2 = 1.197. Both in Model A and Model B, four sites (56 V, 119R, 189 K, 254 N) have been detected under selection at a less significant level (0.50 < P < 0.95*) in the Bayes Empirical Bayes analysis (BEB), and sites 119R (His97Arg) and 254 N (Asp233Asn) were up to the significant level (P > 0.95*) in the Naive Empirical Bayes analysis (NEB).

Table 3 Analysis of potential natural selection analysis among Amy1 genes
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Homology modeling

To check if there exists some functional divergence between proteins of Amy1θ and Amy1λ, initially, we analyzed the primary and secondary structures. We found two group-specific amino acid substitutions, His97Arg and Asp233Asn, located on the 4th β-strand in domain B and the 6th α-helix in domain A, respectively (Fig. 4). Then we built protein models, and found that when Asp233 was replaced by Asn233, Amy1λ proteins failed to form the α-amylase-acarbose inhibitor complex (Amy1-AF1) in the SBS1 region (Additional file 6).

Fig. 4
figure 4

Protein sequence alignment of 403 amino acid residues. Secondary structure prediction was based on structures of barley 1AMY (blue) and rice 3WN6 (red) with α-helices displayed as coils, β-strands as arrows, strict β-turns as TT letters, active sites as triangles, Ca2+ binding sites as stars and catalytic sites as circles. Domain A: a (β/α)8-barrel of 286 residues, domain B: 64 residues, connecting strand β3 and helix α4 of the barrel, domain C: 53 residues forming a five stranded anti-parallel β-sheet. Two amino acids (Arg97 and Asn233), specific to Amy1λ proteins, were highlighted in pink

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Expression profiles of Amy1 genes

We quantified transcript levels of Amy1θ and Amy1λ genes at germinating or early seedling stages. In bread wheat (cv. CS), the 11 full-length copies were divided into five subgroups, TaAmy1-A1/A2, TaAmy1-D1, TaAmy1-B1/B2/D2, TaAmy1-A3/D3 and TaAmy1-B3/B4/B5, based on sequence homology. TaAmy1-B1/B2/D2 transcripts were the most abundant, followed by TaAmy1-D1 and TaAmy1-A3/D3, and subgroups TaAmy1-A1/A2 and TaAmy1-B3/B4/B5 were less expressed under all the four sampling points (Fig. 5A). In barley (cv. Morex), HvAmy1θ was significantly expressed at higher levels than HvAmy1λ (Fig. 5B).

Fig. 5
figure 5

The relative expression levels of Amy1 genes in wheat (cv. CS, A) and barley (cv. Morex, B) during germinating or early seedling stages. Three technical replicates were performed in each expression analysis. Error bars represent SE

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We introduced three wheat lines with high levels of α-amylase activities in ripe grains and three extreme low-level landraces for α-amylase assay (Additional file 7). In the grain development, all the six lines retained high α-amylase activities until 24 DPA; and from 26 DPA through to the end, high-level lines declined with lower rates and resulted in higher levels compared with the landraces (Fig. 6A). These changes prompted us to further investigate whether the Amy1 copies were differentially expressed among these lines. Using an universal primer pair TaAmy1-RT-F/R, we observed an overall expression peak in Guinong19 at 28 DPA, which appeared slightly later in Mianmai43 and Jinan17. Their transcript levels were significantly higher than those of the other three landraces at 30 DPA (Fig. 6B). Further we performed copy-specific analysis at 28 DPA. It showed that the overall expression of TaAmy1 in developing grains was largely contributed by TaAmy1-A1/A2 and TaAmy1-D1, while TaAmy1-B1/B2/D2, TaAmy1-A3/D3 and TaAmy1-B3/B4/B5) were hardly detected (Fig. 6C).

Fig. 6
figure 6

Dynamic changes of total α-amylase activities and expression levels of TaAmy1 genes during grain development. (a) Changes of total α-amylase activities. (b) TaAmy1 relative expression during late stages of grain development. (c) Relative expression of five TaAmy1 subgroups at 28 DPA. Three technical replicates were performed in each expression analysis. Significance values were calculated using Duncan’s multiple range tests. Error bars represent SE

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Discussion

Copy number variation of Amy1 genes in bread wheat and barley

In this work, by molecular cloning and in silico analysis employing genomic resources generated recently, we isolated Amy1 genes in bread wheat and its diploid progenitors, T. urartu, Ae. speltoides and Ae. tauschii, representing the A, B and D genomes, respectively (Table 1), although the real progenitor of B genome has been in debate for years [75, 76]. Compared with their corresponding counterparts of bread wheat, the Amy1 genes of T. urartu and Ae. tauschii are highly conserved in terms of copy number and sequence identity, while those of Ae. speltoides are rather variable, which is consistent with previous reports [4]. Probably due to the amplification bias, we failed to experimentally isolate copies TaAmy1-A3, TaAmy1-B4 and TaAmy1-B5 (Table 1, Additional file 2) from CS, which could be retrieved from its whole genome sequence. However, we have confirmed their presence and the extremely high inter-cultivar conservation of each copy through sequencing more than 220 clones from 17 wheat lines.

In barley, eight Amy1 copies were detected, which was consistent with the results recently presented by Mascher et al. [77]. However, some nomenclature confusion has arisen as genes for high-pI isoforms have been alternatively called Amy1 [26] and Amy2 [78] for many years. Radchuk et al. [23] submitted a barley α-amylase sequence named as HvAMY3 (GenBank accession no. FN179391), which had an identity of 99% with Amy6–4 and 95% with Amy46. Actually, it should not represent a new gene family [22], but belongs to the Amy1 multigene family. Interestingly, in genomes of wheat and barley, we have found two interrupted Amy1 fragments: TaAmy1-B6 and HvAmy1–6, respectively. Analysis of their 4.1 kb upstream sequence indicates this truncating event might be caused by the insertion of a 1.2 kb Gypsy retrotransposon (Additional file 3B).

Structural variation of Amy1 loci in Pooideae

Syntenic analysis of Amy1 loci revealed apparent structural variations between Pooideae and the other grass species analyzed in this study. Despite of the conserved linear gene order extensively reported in previous genome wide analyses [42, 79, 80], we found a segmental inversion around the Amy1 loci occurring between 56.50 Mb - 59.11 Mb on chromosome 3 of B. distachyon comparing to corresponding region on chromosome 2 of rice (from 31.97 Mb to 33.40 Mb) (Fig. 3A). In corresponding regions on chromosomes 6 of barley and wheat, at least two segmental inversion events were observed (Fig. 3A, B). These structural alterations split the original Amy1 locus into two separated loci: Amy1θ and Amy1λ, which resulted in significantly altered organization of Amy1 comparing to those of some other grass species. The existence of these two Amy1 loci was supported by the results of Nishikawa et al. [81] and Cheung et al. [82], who described two separated TaAmy1 loci and five TaAmy1 copies dispersed on chromosome 6B. Three individual Rye Amy1 genes also provided some evidence of recombination and spanned a distance of 3 cM at the locus on chromosome 6RL [29]. The repeats invasion, especially LTR retrotransposon Copia (RLC) and Gypsy (RLG), and DNA transposon CACTA superfamily (DTC), also contributed to extension of intervals between Amy1θ and Amy1λ (Additional file 5).

Gene duplication is critical in supplying raw genetic materials to form gene families and producing new functions [83]. Copy number variation also reflects the dynamic genome evolutionary patterns. In wheat and barley, we have observed apparent evidence of tandem gene duplications, for example, four paralogous TaAmy1θ copies and two TaAmy1λ copies exist in chromosome 6B (Fig. 3A). These inter-group duplication events might occur prior to the intra-group duplications, as suggested by divergence time estimates (Table 2, Additional file 8). These results indicated that the Pooideae species underwent complex genome evolution.

Chromosomal distribution indicates that Amy1 loci are located on the conserved block 5 (ancestral chromosome A4), which only experiences a whole genome duplication (WGD), followed by chromosomal breakage and shuffling before divergence of these three subfamilies (Pooideae, Ehrhartoideae and Panicoideae) in grass [41, 42, 79]. On the knowledge of grass establishment and Amy1 structural variation, we proposed an evolutionary model of Amy1 genes. As illustrated in Fig. 7, the original single-copy structural locus in grass (Amy1) retains in species of B. distachyon, B. stacei, S. italica, S. viridis and Z. mays, respectively. Two-copy species of O. sativa, S. bicolor and P. hallii, each experiences a tandem gene duplication event. In Triticeae species, Amy1 is firstly duplicated into a pair of intermediate tandem repeats (Amy1–1 and Amy1–2). Ever since this duplication, followed by chromosomal rearrangement, Amy1–2 has evolved and diverged from Amy1–1 under limited adaptive selections (Table 3). Two distinct loci, Amy1θ (Amy1–1) and Amy1λ (Amy1–2), have been eventually fixed in genomes. Subsequently, several rounds of other recent tandem duplications within each locus continue to enlarge this multigene family. Nevertheless, Amy1 genes are absent in Dicot lineage, and should emerge after the branch of Dicots-Monocots, indicating the origin of the most recent common ancestor (MRCA) must have resulted from gene duplication.

Fig. 7
figure 7

A proposed evolutionary model for Amy1 locus from the most recent common ancestor (MRCA) among grass

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Divergence between Amy1λ and Amy1θ genes and functional importance

Not only the separation on physical positions, sequences of Amy1θ and Amy1λ genes are divergent. This is reflected by the phylogenetic analysis for that the Amy1θ and Amy1λ genes were clustered distinctly into two groups (Fig. 2), corresponding to groups G2 and G1, respectively. Additionally, length of the first intron varies between Amy1θ and Amy1λ genes of wheat and its progenitors. For example, the lengths of 370 bp or 380 bp were specifically appeared in Amy1θ, while this variation was not observed in Amy1λ genes (Additional file 9).

The fates (nonfunctionalization, neofunctionalization or subfunctionalization) of duplicated genes were strongly directed by adaptive selection [61, 84]. The nonsynonymous to synonymous substitution rate ratio (ω = dN / dS) is clearly recognized as a sensitive measure of positive selection at the nucleotide or protein levels. Using nine different codon-substitution models, we have identified two codon sites (Arg97 and Asn233) in Amy1λ genes under selection pressures (Table 3). The substitution of His97Arg was located on the 4th β-strand in domain B (Fig. 4), and did not share any functional evidence according to previous studies. The other codon site, Asp233Asn, was situated at the surface binding site (SBS1). SBS1 was involved in starch binding and substrate recognition [15, 20, 85]. This binding site owned two consecutive tryptophan residues (Additional file 6), which were characteristic of and functionally essential for cereal Amy1 or Amy2 isoforms [14, 15, 70]. Mutations on them strongly affected the ability of SBS1 binding to different starch types [85]. However, roles of these other surrounding residues (Asp233Asn, Gln226 and Val229 in Additional file 6) in starch binding or degradation have not yet been reported. Therefore, whether this substitution is functionally vital or not remains undiscovered. As the archetype Amy1θ genes commonly existing among grasses, the appearance of derivative Amy1λ genes indicates an unique gene duplication event in Triticeae species, and possibly implicated the potential subfunctionalization of Amy1 after divergence of the two groups.

Patterns of gene expression are usually associated with functional differentiation. In the present work, all the five subgroups of TaAmy1 and two subgroups of HvAmy1 are active in transcription during seed germination (Fig. 5A, B), indicating that the seed germination is triggered by the coordinated expressions of multiple Amy1θ and Amy1λ genes. In barley, HvAmy1θ (at least four copies) exhibited significantly higher abundance than that of HvAmy1λ (one copy) during germination (Fig. 5B). Considering great difference on copy number, the differential expression levels most likely owe to dosage effect. In wheat, both during germination and in the developmental stages, our study indicated that TaAmy1λ and TaAmy1θ genes were differentially expressed, and TaAmy1λ copies seemed to largely contribute to the total abundance of TaAmy1 genes. For example, TaAmy1-D1 and TaAmy1-B1/B2/D2 during germination, and TaAmy1-A1/A2 and TaAmy1-D1 in the developing grains were the main TaAmy1λ subgroups abundantly expressed (Fig. 5A and Fig. 6B, C).

As changes in regulatory sequences affect transcript levels and result in expression divergence in duplicated genes [86], we then compared the 18 promoter sequences (approximately 200 bp – 250 bp upstream of the ‘tata’ box) from bread wheat and barley. Previous reports indicated both Amy1 and Amy2 genes owned three GA-responsive elements [24, 31, 87]. Both the pyrimidine box and the ‘taacaaac’ box were conserved between Amy1θ and Amy1λ genes, while the ‘tatccac/t’ box showed a nucleotide substitution of C (Amy1θ) to T (Amy1λ) (Additional file 10). This substitution was also found in a highly conserved element (‘tatccatgcagtg’ box) of Amy32b, a representative of low-pI Amy2 gene family [87]. We also sequenced promoter (1.1 kb) and coding sequences of TaAmy1-A1, TaAmy1-A2 and TaAmy1-D1 from these six investigated wheat lines (Additional file 11), and failed to detect any informative variation associated with this apparent expression divergence. Thus, the regulation mechanisms of the elevated expression levels or activities, as suggested by Farrell et al. [88] and Yang et al. [39], needs to be further uncovered.

Conclusion

In this study, we present comprehensive analyses of Amy1 genes in wheat and barley. Copy number extension of Amy1 genes is evident. Under actions of tandem gene duplication and chromosome rearrangement, the original Amy1 locus was divided into two spatially structural loci (Amy1θ and Amy1λ). Potential functional divergence between them is clear according to their sequence mutations and expression differentiations. Genetically, the Amy1 multigene family originates from a single-copy structural locus, and its expansion pattern provides a divergent model during gene duplication and evolution. For agronomical practices, the observed expression differentiation of duplicated copies, and functional divergence between Amy1θ and Amy1λ genes will help to better understand the mechanism underlying the dynamic changes of α-amylase activity during germination or seed maturation, and provide clues or orientations for dissection of genetic factors impacting α-amylase activities in wheat and barley, which will be helpful to further identification of alleles favored for better yield and processing qualities.