Barley amy genes were initially mapped to chromosomes 6H and 7H with wheat–barley addition lines (Brown and Jacobsen, 1982; Muthukrishnan et al., 1984). Southern blot analysis of two different amy gene DNA probes detected at least six and three hybridization bands from addition lines containing barley chromosomes 6H and 7H, respectively (Muthukrishnan et al., 1984; Rogers and Milliman, 1984). There were multiple amy protein bands on SDS PAGE purified from the barley aleurone using cycloheptaamylose-sepharose affinity chromatography and at least four amy activity peaks separated by DEAE cellulose chromatography (Jacobsen and Higgins, 1982). These offered early experimental evidence of the amy multigene family. Isoelectric focusing (IEF) electrophoresis showed that purified amy proteins could be divided into two distinct groups, a low pI group with an isoelectric point of 4.5–5.1 and a high pI group with an isoelectric point of 5.0–6.6 (Jacobsen and Higgins, 1982). Due to its commercial and biological importance, a significant effort was made to clone amy genes. A genomic clone was identified as an amy gene (amy32b) and belongs to a low pI amy protein (Rogers and Milliman, 1983; Whittier et al., 1987). Two cDNA clones were also characterized as amy genes (amy6_4 and amy46) that belong to high pI enzymes (Khursheed and Rogers, 1988). Furthermore, 3D structures have been resolved for two barley amy proteins; one belonging to a low pI amy protein (1AMY) and the other to a high pI amy protein (1HT6) (Kadziola et al., 1994; Robert et al., 2003).

While experimental data has shown that amy proteins are coded by multigene families, the exact numbers of genes are unknown. Barley genome sequencing is a useful resource for identifying the number of amy genes and discovering their genomic features. The barley genome contained 12 amy genes (Mascher et al., 2017), which were grouped into four subfamilies (Table 1). Subfamily 1 consists of six members—four on chromosome 6H (533880485–542858990 bp) and two on the unsorted chromosome (195047130–196261798 bp, Table 1)—designated amy1_1a to amy1_1e and amy1_2. Four of which (amy1_1a to amy1_1d) have almost 100% sequence identity among members (Additional File 1: Supplementary Table S1A). One member (amy1_1e) is a truncated protein missing the carbohydrate-binding domain. Sequence identity analysis showed that the five amy1_1 proteins matched the cloned gene amy6_4 (Khursheed and Rogers, 1988), while the amy1_2 protein, with 95% sequence identity with amy1_1a, matched the cloned gene amy46 in both promoter and coding regions (Khursheed and Rogers, 1988). All amy1 members belong to high pI enzymes (Table 1) and have high sequence identity with a 3D structure-resolved protein 1AMY (Kadziola et al., 1994). Subfamily amy2 has three members on chromosome 7H (556169683–557427021 bp, Table 1), and are designated amy2_1 to amy2_3. They have >92% sequence identity among the members, and >72% when compared to amy1_1a. Amy2_3 had a high sequence identity with cloned gene amy32b (Rogers, 1985; Whittier et al., 1987) and 3D structure-resolved protein 1HT6 (Robert et al., 2003). They belong to genes coding for low pI enzymes (Table 1). Amy3 has one member localized on chromosome 5H (designated amy3), while amy4 has two members localized on chromosomes 2H and 3H (designated amy4_1 and amy4_2). The amy4 members have about 48% sequence identities compared between the members, or 43–46% sequence identity when compared to amy1_1a (Supplementary Table S1A).

Since amy1 and amy2 were located on chromosomes 6H and 7H, respectively, and many important malt quality QTLs were associated with these genetic loci, we believed that they were the most important members in relation to barley malt qualities (Hayes et al., 1993; Oziel et al., 1996; Zale et al., 2000; Gao et al., 2004; Li et al., 2010), we decided to investigate and compare gene and promoter structures for these two subfamilies in barley, rice, and wheat.

All amy proteins from amy1 and amy2 had the catalytic amino acid residues Asp₂₀₃, Glu₂₂₈, and Asp₃₁₀ (amy1_1a positions), apart from amy1_1e, which was a truncated protein missing Asp₃₁₀ (Figure 1). The near full-length proteins consisted of a central domain A forming (α/β)₈ barrel, a structural loop domain B and a carbohydrate-binding domain C (Figure 1). Domain C formed five anti-parallel sheets (Kadziola et al., 1994; Robert et al., 2003). Barley amy proteins from amy3 and amy4 also contain the catalytic amino acids and a carbohydrate-binding module as discovered in the Domain Database at NCBI². However, Asp₃₁₀ on the active site was replaced with Glu₃₁₀ for the two amy4 proteins (Figure 1).

Four barley amy1 genes (amy1_1a to amy1_1d) had the same genomic arrangements as two introns and three exons. The nucleotide numbers for the introns and exons were the same as amy1_1a to amy1_1d, being 23 and 107 bp for the introns and 87, 1002, and 252 bp for the exons (Supplementary Table S2). While amy1_2 had two introns and three exons, the nucleotide numbers differed from the amy1_1a to amy1_1d members, being 95 and 106 bp for the introns and 87, 945, and 252 bp for the exons. Amy2-1 to amy2_3 had three introns and four exons, but the nucleotide numbers for all introns and exons differed among the three amy genes (Supplementary Table S2). Amy3 had three introns and four exons like amy2, but the nucleotide numbers differed from amy2. Amy4 had more than five introns and six exons.

Promoters of all barley amy1 genes contained a conserved GA response complex (GARC) consisting of GARE (TAACAAA), pyrimidine (CCTTTT) and TATCCAC(T) boxes (Supplementary Table S3A and Figure 2) (Skriver et al., 1991; Gubler and Jacobsen, 1992; Rogers et al., 1994). There was also a cAMP-like response element (TGAGCTC) on amy1 promoters (Gubler and Jacobsen, 1992), which represses gibberellin action. Pyrimidine and TATCCAC boxes enhanced the expression of amy1 proteins. The conserved motifs on subfamily 2 members differed from those on subfamily 1 genes and also among subfamily 2 members (Supplementary Table S3B and Figure 3). All three amy2 genes had GARE (TAACAGAG) and pyrimidine (CCTTTT) boxes. The pyrimidine box was much closer to the translation start site (-17 bp) for amy2_2, but further away for amy2_1, and amy2_3 at -211 and -236 bp, respectively. The original pyrimidine box of the amy2_2 gene, at a similar position to amy2_1, and amy2_3, was mutated to CCATTT on amy2_2 (Figure 3). A TATCCAT box was found in two amy2 genes (amy2_1 and amy2_3), but it was replaced with TACCCAT in the amy2_2 gene. Furthermore, the amy2_2 promoter had a conserved O2S box (CTTGxxTCATC) and cAMP-like box (TGAGCTC). Genomic sequence analysis showed that both amy1 and amy2 genes had a GARC, where GARE was required for GA induction of amy expression with pyrimidine and TA(T/C)CCAT box controlling gene expression levels (Lanahan et al., 1992).

When promoter regions (-500 bp) of subfamilies 1 and 2 were analyzed, the sequence identities were high (>99%) among amy1_1a, amy1_1b, and amy1_1d (Supplementary Table S4A). However, the amy1_1e promoter region was truncated to -151 bp, despite being 100% identical to the promoter sequence of amy1_1a gene. The promoter region of amy1_1c had high sequence identity within -350 bp, but low sequence identity beyond -350 bp, compared to amy1_1a (Figure 2). The sequence of the amy1_2 promoter (-500 bp) was 64% identical to the amy1_1a promoter. The genomic locations of the conserved motifs in amy1 were the same for all amy1_1 members at -180, -199, -220, and -225 bp for TATCCA, GARE, cAMY-like and pyrimidine boxes, respectively (Supplementary Table S3A). However, the locations of these motifs from the amy1_2 gene was a nucleotide closer to the ATG translation start site compared to the locations of amy1_1 members (Supplementary Table S3A). The sequences of amy2 promoters had low sequence identity (59–68%) compared to their members (Supplementary Table S4B).

The rice genome contained 10 amy genes with three, one, four, and two members in subfamilies 1, 2, 3, and 4, respectively (Table 2). The number of amy1 and amy2 genes in rice was four, much less than the sum of amy1 and amy2 in barley. Rice amy3 had the most gene members. Two of the amy1 genes (LOC_Os02g52700 and LOC_OS02g52710) had three introns (Supplementary Table S2), unlike the barley amy1 genes, which all had two introns. The other amy1 gene (LOC_Os01g25510) had two introns. Alignment of the three amy1 protein sequences identified one protein (LOC_Os01g25510) with a very low sequence identity (24–30%) compared to the other two amy proteins (LOC_Os02g52700 and LOC_Os02g52710) (Supplementary Table S1B and Figure S1). The two amy1 proteins and one amy2 protein had similar secondary structures to the barley amy proteins. However, the amino acid compositions differed substantially on most of the β-strains and α-helices (Supplementary Figure S1). Rice had the same catalytic amino acids (Aps, Glu, and Asp) as barley. The promoters of the three rice amy1 genes had >65% sequence identities (Supplementary Table S4C). They contained the GARE (TAACAAA), pyrimidine (CCTTTT) and TATCCAT boxes (Supplementary Table S3C and Figure S2A) but not the cAMY-like box. The rice amy2 protein (LOC_Os06g49970) had 72% sequence identity compared to the two amy1 proteins (LOC_OS02g527100 and LOC_Os02g52710). However, the promoter of the rice amy2 gene only contained GARE (TAACAGAG), but not pyrimidine, TATCCAT or TATCCAC boxes (Supplementary Table S3C and Figure S2B).

The number of amy genes in the wheat A, B, and D genomes was 6, 10, and 8, respectively. The other four amy genes are located in unsorted chromosomes (Table 2). The wheat B genome had the most amy genes with four, three, one and two members in subfamilies amy1, amy2, amy3, and amy4, respectively. The number of amy1 and amy2 genes in each wheat genome did not exceed those in the barley genome (Table 2). All of the wheat amy2 genes had the same genomic arrangement as the barley amy2 genes with three introns and four exons (Supplementary Table S2). Most of the wheat amy1 genes had two introns and three exons except for 6BL4, Un1 and Un2, which had three or four introns. The protein sequence identities were high within the wheat amy1 or amy2, being >80% (Supplementary Tables S1C,D and Figures S3, S4). The sequence identities for promoters of amy1 genes were 50–100%, but much lower for amy2 gene promoters (Supplementary Tables S4D,E). The promoters of all wheat amy1 genes contained GARE (TAACAAA), pyrimidine, TATCCAT or TATCCAC boxes (Supplementary Figure S5). They also had a cAMP-like motif (TGAGCTC) box as per the barley amy1 gene promoters. Five of the wheat amy2 gene promoters contained a GARE (TAACAGAG) box, six contained pyrimidine and TATCCAT boxes, and seven had O2S motifs. The O2S motifs in the wheat genomes contained four variable nucleotides between the conserved CTTC and TCATC (Supplementary Figure S6), while the O2S in the barley amy2 promoters had two variable nucleotides (Figure 3).

Barley had the highest number of amy1 genes (six), while wheat had four in the B genome and rice had three (Table 2). Both barley and wheat had the same number of amy2 genes (B genome only), while rice had one. Rice contained the most amy3 genes (four), while barley and wheat had one each. All the barley, wheat and rice genomes contained two amy4 genes (Table 2). The intron numbers for amy2 genes were the same for barley, wheat and rice, but differed for the amy1 genes: barley had two, wheat had two or four, and rice had two or three (Supplementary Table S2). The intron numbers for amy3 and amy4 genes differed, ranging from two to nine (Supplementary Table S2). Barley amy1 genes had high sequence identities with wheat amy1 genes ranging from 83 to 97% (Supplementary Table S5). Barley amy2 genes also had high sequence identities with wheat amy2 genes (80–96%) (Supplementary Table S5). The barley and wheat amy1 and amy2 genes had similar promoter regions with sequence identities ranging from 50 to 76% (Supplementary Table S6). However, there was no similarity between barley and rice promoter sequences.

Barley amy proteins are grouped into four subfamilies according to their sequence properties (Mascher et al., 2017). The biological functions for each subfamily are unclear. Genetic mapping using molecular marker technologies showed that the regions associated with the genetic markers amy1 and amy2 on chromosomes 6 and 7 are important for malting qualities including amy enzyme activities and malt extracts (Hayes et al., 1993; Han and Ullrich, 1994; Oziel et al., 1996; Marquez-Cedillo et al., 2000; Emebiri et al., 2004). Thus, we conclude that amy1 and amy2 are the major genes responsible for starch degradation during seed germination. Other indirect evidence includes the induced expression of amy1 and amy2 genes during seed germination by GA (Khursheed and Rogers, 1988; Karrer et al., 1991). The abundance of mRNA levels ranks amy32b (amy2_3) > amy6-4 (amy1_1a to amy1_1e, possibly sum) > amy46 (amy1_2) about 24 h after GA induction (Khursheed and Rogers, 1988; Karrer et al., 1991). Barley amy1 have expanded members due to genome duplication, which may play a key role in barley becoming a malting commodity. Furthermore, many amy genomic and cDNA clones had been deposited on Genbank. Their relationships for some of the clones with amy1 and amy2 was shown by a phylogenetic tree at Supplementary Figure S7.

There was no direct evidence for the function of amy3 and amy4 proteins in barley. In wheat, amy3 was highly expressed in developing grains, which affected carbon partitioning and diacylglycerol accumulation (Whan et al., 2014), while the amy4 gene may be involved in starch degradation working in partnership with amy1 proteins (Mieog et al., 2017).

Barley amy enzymes are synthesized in barley aleurone layers induced by gibberellin (GA) (Chrispeels and Varner, 1967; Jacobsen et al., 1970). Two groups of proteins (A and B) were detected after GA induction. Their responses to GA induction differed in a time and GA concentration dependent manner (Jacobsen and Higgins, 1982). The group A proteins expressed earlier and required a low GA concentration, while the group B proteins were not detectable till 8 h after GA addition and required a higher GA concentration. However, the synthesis of group B proteins accelerated once expressed, and one of group B isoforms was most abundant at 24 h (Jacobsen and Higgins, 1982). The group A and B proteins were likely to be the products of amy1 and amy2 genes, respectively, as shown by changes in mRNA levels in response to GA (Khursheed and Rogers, 1988). Conserved motifs on amy1 and amy2 promoters played key roles in the induction of gene expression. A comparison of amy1 with amy2 promoters showed substantial differences in nucleotide composition of the conserved motifs. The GA-responsive element is TAACAAA on all amy1 promoters, but TAACAGAG on all amy2 promoters for all three crops. There is a cAMY-like responsive element close to the pyrimidine box on barley and wheat amy1 promoters, but not amy2 promoters. The difference in the conserved motifs may play a key role in GA-induced gene expression.

Gene expression induced by GA may have the same mechanism for all amy1 members, since they contain extract same number of motifs with same nucleotide sequences except Hvamy1_2 gene on which the TATCCAC box was replaced by TATCCAT. In contrast, barley amy2 members may be differentially regulated, particularly for the Hvamy2_2 gene, since the conserved motifs differed substantially from amy1, amy2_1, and amy2_3. Furthermore, no similar motifs were found in the promoters of amy3 and amy4.

Barley grains are often used for malting. Barley malts contain sufficient diastatic power (enzymatic hydrolytic activities) to completely convert starch to fermentable sugars. Extended numbers of amy genes and the presence of GA-regulatory motifs are important for barley to be used for malts (Table 1 and Figures 2, 3). However, wheat amy genes had similar genomic properties to barley amy genes with similar intron and exon structures (Supplementary Table S2). They also contained GA-regulatory motifs as in barley amy gene promoters (Supplementary Figures S5, S6). This could explain why wheat can also be used for malting (Fleming et al., 1960; Faltermaier et al., 2013). However, there are some problems with using wheat malts for brewing. Wheat grains lack husk, which is a problem for a brewing process called Lautering. Wheat also has higher protein (up to 20%). Wheat proteins promote foam formation, but also enhance haze issues (Faltermaier et al., 2015).