Spatio-temporal appearance of α-amylase and limit dextrinase in barley aleurone layer in response to gibberellic acid, abscisic acid and salicylic acid

BACKGROUND

Cereal seed germination involves mobilization of storage reserves in the starchy endosperm to support seedling growth. In response to gibberellin produced by the embryo the aleurone layer synthesizes hydrolases that are secreted to the endosperm for degradation of storage products. In this study analysis of intracellular protein accumulation and release from barley aleurone layers is presented for the important enzymes in starch degradation: α-amylase and limit dextrinase (LD).

RESULTS

Proteins were visualized by immunoblotting in aleurone layers and culture supernatants from dissected aleurone layers incubated up to 72 h with either gibberellic acid (GA), abscisic acid (ABA) or salicylic acid (SA). The results show that α-amylase is secreted from aleurone layer treated with GA soon after synthesis but the release of LD to culture supernatants was significantly delayed and coincided with a general loss of proteins from aleurone layers.

CONCLUSIONS

Release of LD was found to differ from that of amylase and was suggested to depend on programmed cell death (PCD). Despite detection of intracellular amylase in untreated aleurone layers or aleurone layers treated with ABA or SA, α-amylase was not released from these samples. Nevertheless, the release of α-amylase was observed from aleurone layers treated with GA+ABA or GA+SA.

Genetic architecture of limit dextrinase inhibitor (LDI) activity in Tibetan wild barley

BACKGROUND

Limit dextrinase inhibitor (LDI) inhibits starch degradation in barley grains during malting because it binds with limit dextrinase (LD). There is a wide genetic variation in LDI synthesis and inactivation during barley grain development and germination. However, the genetic control of LDI activity remains little understood.

RESULTS

In this study, association analysis was performed on 162 Tibetan wild accessions by using LDI activity, 835 Diversity Arrays Technology (DArT) markers and single nucleotide polymorphisms (SNPs) of the gene HvLDI encoding LDI. Two DArT markers, bpb-8347, bpb-0068, and 31 SNPs of HvLDI were significantly associated with LDI activity, explaining 10.0%, 6.6% and 13.4% of phenotypic variation, respectively. Bpb-8347 is located on chromosome 6H, near the locus of HvLDI, and bpb-0068 is located on 3H.

CONCLUSIONS

The current results confirmed the locus of the gene controlling LDI activity and identified a new DArT markers associated with LDI activity. The SNPs associated with LDI activity may provide a new insight into the genetic variation of LDI activity in barley grains.

Genotypic and environmental variation in barley limit dextrinase activity and its relation to malt quality

Variation in the limit dextrinase activity of barley malt, and the relationships between limit dextrinase activity and malt quality parameters were investigated using eight cultivars grown at seven diverse locations in China for two successive years. Limit dextrinase activity varied with genotype and location, with the levels ranging from 0.245 U/g to 0.980 U/g. The results showed that the variation in limit dextrinase activity was more attributable to the environment (location and year) than to the genotype. The response of limit dextrinase activity to the environment differed markedly among cultivars, and was reflected by large difference in coefficient of variation of cultivars across diverse locations. Regression analysis showed that limit dextrinase activity was negatively correlated with malt viscosity (r=-0.52, P<0.01), positively correlated with Kolbach index (r=0.38, P<0.01) and malt extract (r=0.30, P<0.05), but had no significant correlation with malt protein content and diastatic power.

Chemical synthesis of a dual branched malto-decaose: a potential substrate for alpha-amylases

A convergent block strategy for general use in efficient synthesis of complex alpha-(1-->4)- and alpha-(1-->6)-malto-oligosaccharides is demonstrated with the first chemical synthesis of a malto-oligosaccharide, the decasaccharide 6,6''''-bis(alpha-maltosyl)-maltohexaose, with two branch points. Using this chemically defined branched oligosaccharide as a substrate, the cleavage pattern of seven different alpha-amylases were investigated. Alpha-amylases from human saliva, porcine pancreas, barley alpha-amylase 2 and recombinant barley alpha-amylase 1 all hydrolysed the decasaccharide selectively. This resulted in a branched hexasaccharide and a branched tetrasaccharide. Alpha-amylases from Asperagillus oryzae, Bacillus licheniformis and Bacillus sp. cleaved the decasaccharide at two distinct sites, either producing two branched pentasaccharides, or a branched hexasaccharide and a branched tetrasaccharide. In addition, the enzymes were tested on the single-branched octasaccharide 6-alpha-maltosyl-maltohexaose, which was prepared from 6,6''''-bis(alpha-maltosyl)-maltohexaose by treatment with malt limit dextrinase. A similar cleavage pattern to that found for the corresponding linear malto-oligosaccharide substrate was observed.

Antisense downregulation of the barley limit dextrinase inhibitor modulates starch granule size distribution, starch composition and amylopectin structure

The barley protein limit dextrinase inhibitor (LDI), structurally related to the alpha-amylase/trypsin inhibitor family, is an inhibitor of the starch debranching enzyme limit dextrinase (LD). In order to investigate the function of LDI, and the consequences for starch metabolism of reduced LDI activity, transgenic barley plants designed to downregulate LDI by antisense were generated. Homozygous antisense lines with reduced LDI protein level and activity were analysed and found to have enhanced free LD activity in both developing and germinating grains. In addition the antisense lines showed unpredicted pleiotropic effects on numerous enzyme activities, for example, alpha- and beta-amylases and starch synthases. Analysis of the starch showed much reduced numbers of the small B-type starch granules, as well as reduced amylose relative to amylopectin levels and reduced total starch. The chain length distribution of the amylopectin was modified with less of the longer chains (>25 units) and enhanced number of medium chains (10-15 units). These results suggest an important role for LDI and LD during starch synthesis as well as during starch breakdown.

The proteinaceous inhibitor of limit dextrinase in barley and malt

Barley limit dextrinase catalyses hydrolysis of alpha-1,6-D-glucosidic bonds in branched poly- or oligosaccharides from starch. A specific inhibitor of this enzyme is found in mature barley kernels, but disappears after several days of germination. Two forms of this proteinaceous inhibitor, identical in amino acid sequence, have been isolated and characterized. They differ in attachment of cysteine or glutathione to a sulfhydryl group, possibly that of cysteine residue 59 of the inhibitor. They can form a 1:1 complex with limit dextrinase and are believed to interact specifically with the enzyme active site. The inhibitor present in mature barley can effectively reduce enzyme activity in barley germinated for a short time and in commercial malt.

Chemoenzymatic Synthesis of Branched Oligo- and Polysaccharides as Potential Substrates for Starch Active Enzymes

Oligo- and polysaccharides embodying the alpha-maltotriosyl-6(II)-maltotetraosyl structure were readily synthesized by transglycosylation of maltosyl fluoride onto panose and pullulan catalysed by the bacterial transglycosylase cyclodextrin glycosyltransferase (CGTase). The two products obtained proved useful for increasing the knowledge of substrate binding and processing at the active site of barley limit dextrinase that is involved in the metabolism of amylopectin by acting upon its branch points.

Modulation of activity and substrate binding modes by mutation of single and double subsites +1/+2 and -5/-6 of barley alpha-amylase 1

Enzymatic properties of barley alpha-amylase 1 (AMY1) are altered as a result of amino acid substitutions at subsites -5/-6 (Cys95-->Ala/Thr) and +1/+2 (Met298-->Ala/Asn/Ser) as well as in the double mutants, Cys95-->Ala/Met298-->Ala/Asn/Ser. Cys95-->Ala shows 176% activity towards insoluble Blue Starch compared to wild-type AMY1, kcat of 142 and 211% towards amylose DP17 and 2-chloro-4-nitrophenyl beta-d-maltoheptaoside (Cl-PNPG7), respectively, but fivefold to 20-fold higher Km. The Cys95-->Thr-AMY1 AMY2 isozyme mimic exhibits the intermediary behaviour of Cys95-->Ala and wild-type. Met298-->Ala/Asn/Ser have slightly higher to slightly lower activity for starch and amylose, whereas kcat and kcat/Km for Cl-PNPG7 are < or = 30% and < or = 10% of wild-type, respectively. The activity of Cys95-->Ala/Met298-->Ala/Asn/Ser is 100-180% towards starch, and the kcat/Km is 15-30%, and 0.4-1.1% towards amylose and Cl-PNPG7, respectively, emphasizing the strong impact of the Cys95-->Ala mutation on activity. The mutants therefore prefer the longer substrates and the specificity ratios of starch/Cl-PNPG7 and amylose/Cl-PNPG7 are 2.8- to 270-fold and 1.2- to 60-fold larger, respectively, than of wild-type. Bond cleavage analyses show that Cys95 and Met298 mutations weaken malto-oligosaccharide binding near subsites -5 and +2, respectively. In the crystal structure Met298 CE and SD (i.e., the side chain methyl group and sulfur atom) are near C(6) and O(6) of the rings of the inhibitor acarbose at subsites +1 and +2, respectively, and Met298 mutants prefer amylose for glycogen, which is hydrolysed with a slightly lower activity than by wild-type. Met298 AMY1 mutants and wild-type release glucose from the nonreducing end of the main-chain of 6"'-maltotriosyl-maltohexaose thus covering subsites -1 to +5, while productive binding of unbranched substrate involves subsites -3 to +3.

Purification, enzymatic characterization, and nucleotide sequence of a high-isoelectric-point alpha-glucosidase from barley malt

High-isoelectric-point (pI) alpha-glucosidase was purified 7, 300-fold from an extract of barley (Hordeum vulgare) malt by ammonium sulfate fractionation, ion-exchange, and butyl-Sepharose chromatography. The enzyme had high activity toward maltose (k(cat) = 25 s(-1)), with an optimum at pH 4.5, and catalyzed the hydrolysis by a retaining mechanism, as shown by nuclear magnetic resonance. Acarbose was a strong inhibitor (K(i) = 1.5 microM). Molecular recognition revealed that all OH-groups in the non-reducing ring and OH-3 in the reducing ring of maltose formed important hydrogen bonds to the enzyme in the transition state complex. Mass spectrometry of tryptic fragments assigned the 92-kD protein to a barley cDNA (GenBank accession no. U22450) that appears to encode an alpha-glucosidase. A corresponding sequence (HvAgl97; GenBank accession no. AF118226) was isolated from a genomic phage library using a cDNA fragment from a barley cDNA library. HvAgl97 encodes a putative 96.6-kD protein of 879 amino acids with 93.8% identity to the protein deduced from U22450. The sequence contains two active site motifs of glycoside hydrolase family 31. Three introns of 86 to 4,286 bp interrupt the coding region. The four exons vary from 218 to 1,529 bp. Gene expression analysis showed that transcription reached a maximum 48 h after the start of germination.

Isolation and characterization of the gene encoding the starch debranching enzyme limit dextrinase from germinating barley

The gene encoding the starch debranching enzyme limit dextrinase, LD, from barley (Hordeum vulgare), was isolated from a genomic phage library using a barley cDNA clone as probe. The gene encodes a protein of 904 amino acid residues with a calculated molecular mass of 98.6 kDa. This is in agreement with a value of 105 kDa estimated by SDS-PAGE. The coding sequence is interrupted by 26 introns varying in length from 93 bp to 825 bp. The 27 exons vary in length from 53 bp to 197 bp. Southern blot analysis shows that the limit dextrinase gene is present as a single copy in the barley genome. Gene expression is high during germination and the steady state transcription level reaches a maximum at day 5 of germination. The deduced amino acid sequence corresponds to the protein sequence of limit dextrinase purified from germinating malt, as determined by automated N-terminal sequencing of tryptic fragments coupled with matrix assisted laser desorption mass spectrometry. The sequenced peptide fragments cover 70% of the entire protein sequence, which shows 62% and 77% identity to that of starch debranching enzymes from spinach and rice and 37% identity to Klebsiella pullulanase. Sequence alignment supports the multidomain architecture and identifies both secondary structure elements of the catalytic (beta/alpha)8-barrel substrate, catalytic residues, and specificity associated motifs characteristic of members of the glycoside hydrolase family 13 which cleave alpha-1,6-glucosidic bonds. A remarkable distribution of the secondary structure elements to individual exons is observed.

A single limit dextrinase gene is expressed both in the developing endosperm and in germinated grains of barley

The single gene encoding limit dextrinase (pullulan 6-glucanohydrolase; EC 3.2.1.41) in barley (Hordeum vulgare) has 26 introns that range in size from 93 to 822 base pairs. The mature polypeptide encoded by the gene has 884 amino acid residues and a calculated molecular mass of 97,417 D. Limit dextrinase mRNA is abundant in gibberellic acid-treated aleurone layers and in germinated grain. Gibberellic acid response elements were found in the promoter region of the gene. These observations suggest that the enzyme participates in starch hydrolysis during endosperm mobilization in germinated grain. The mRNA encoding the enzyme is present at lower levels in the developing endosperm of immature grain, a location consistent with a role for limit dextrinase in starch synthesis. Enzyme activity was also detected in developing grain. The limit dextrinase has a presequence typical of transit peptides that target nascent polypeptides to amyloplasts, but this would not be expected to direct secretion of the mature enzyme from aleurone cells in germinated grain. It remains to be discovered how the enzyme is released from the aleurone and whether another enzyme, possibly of the isoamylase group, might be equally important for starch hydrolysis in germinated grain.