Messenger RNAs from the Scutellum and Aleurone of Germinating Barley Encode (1-->3,1-->4)-beta-d-Glucanase, alpha-Amylase and Carboxypeptidase

Substrate Specificities of Variants of Barley (1,3)- and (1,3;1,4)-β-d-Glucanases Resulting from Mutagenesis and Segment Hybridization

Barley (1,3;1,4)-β-d-glucanase is believed to have evolved from an ancestral monocotyledon (1,3)-β-d-glucanase, enabling the hydrolysis of (1,3;1,4)-β-d-glucans in the cell walls of leaves and germinating grains. In the present study, we investigated the substrate specificities of variants of the barley enzymes (1,3;1,4)-β-d-glucan endohydrolase [(1,3;1,4)-β-d-glucanase] isoenzyme EII (HvEII) and (1,3)-β-d-glucan endohydrolase [(1,3)-β-d-glucanase] isoenzyme GII (HvGII) obtained by protein segment hybridization and site-directed mutagenesis. Using protein segment hybridization, we obtained three variants of HvEII in which the substrate specificity was that of a (1,3)-β-d-glucanase and one variant that hydrolyzed both (1,3)-β-d-glucans and (1,3;1,4)-β-d-glucans; the wild-type enzyme hydrolyzed only (1,3;1,4)-β-d-glucans. Using substitutions of specific amino acid residues, we obtained one variant of HvEII that hydrolyzed both substrates. However, neither protein segment hybridization nor substitutions of specific amino acid residues gave variants of HvGII that could hydrolyze (1,3;1,4)-β-d-glucans; the wild-type enzyme hydrolyzed only (1,3)-β-d-glucans. Other HvEII and HvGII variants showed changes in specific activity and their ability to degrade the (1,3;1,4)-β-d-glucans or (1,3)-β-d-glucans to larger oligosaccharides. We also used molecular dynamics simulations to identify amino-acid residues or structural regions of wild-type HvEII and HvGII that interact with (1,3;1,4)-β-d-glucans and (1,3)-β-d-glucans, respectively, and may be responsible for the substrate specificities of the two enzymes.

Over-expression of (1,3;1,4)-β-D-glucanase isoenzyme EII gene results in decreased (1,3;1,4)-β-D-glucan content and increased starch level in barley grains

BACKGROUND

High content of (1,3;1,4)-β-d-glucan in barley grains is regarded as an undesirable factor affecting malting potential, brewing yield and feed utilization. Production of thermostable bacterial (1,3;1,4)-β-glucanase in transgenic barley grain or supplementation of exogenous bacterial (1,3;1,4)-β-glucanase has been used to improve malt and feed quality. The aim of the present study was to investigate the effect of over-expression of an endogenous (1,3;1,4)-β-glucanase on β-glucan content and grain composition in barley.

RESULTS

A construct containing full-length HvGlb2 cDNA encoding barley (1,3;1,4)-β-glucanase isoenzyme EII under the control of a promoter of barley D-Hordein gene Hor3-1 was introduced into barley cultivar Golden Promise via Agrobacterium-mediated transformation, and transgenic plants were regenerated after hygromycin selection. The T₂ generation of proHor3:HvGlb2 transgenic lines showed increased activity of (1,3;1,4)-β-glucanase in grains. Total β-glucan content was reduced by more than 95.73% in transgenic grains compared with the wild-type control. Meanwhile, over-expression of (1,3;1,4)-β-glucanase led to an increase in 1000-grain weight, which might be due to elevated amounts of starch in the grain.

CONCLUSION

Manipulating the expression of (1,3;1,4)-β-glucanase EII can control the β-glucan content in grain with no apparent harmful effects on grain quality of transgenic plants. © 2016 Society of Chemical Industry.

Barley aleurone cell development: molecular cloning of aleurone-specific cDNAs from immature grains

The cloning of 11 different homology groups of cDNAs representing genes expressed in aleurone, but not in starchy endosperm of 20-day-old barley grains is described. Among the cDNAs, four are aleurone-specific, while the remaining are also expressed in the embryo, but not in any other part of the plant.Sequence analysis of one of the aleurone-specific clones, B11E, reveals an open reading frame coding for an unidentified 10.4 kDa protein with a putative signal sequence and a possible metal-binding finger. The B11E gene has a high GC content in the 5' leader sequence (63%), as well as in the coding region (70%) compared to known cDNAs from the barley starchy endosperm. Northern analysis of B11E indicates maximum mRNA abundance around mid-phase of grain development.When isolated immature aleurone/pericarp is incubated in tissue culture medium (MS) the B11E message disappears, indicating a requirement for a diffusible factor from the intact grain for its continued presence.

Primary structure of the (1-->3,1-->4)-beta-D-glucan 4-glucohydrolase from barley aleurone

During germination of barley grains, the cell walls of the starchy endosperm are degraded by (1-->3,1-->4)-beta-glucanases (EC 3.2.1.73) secreted from the aleurone and scutellar tissues. The complete sequence of the aleurone (1-->3,1-->4)-beta-glucanase isoenzyme II comprises 306 amino acids and was determined by sequencing nine tryptic peptides (110 residues) and aligning them with the amino acid sequence deduced from a cDNA clone encoding the 291 NH(2)-terminal residues. Although no amino acid sequence homology with a bacterial (1-->3)(1-->4)-beta-glucanase is apparent, close to 50% homology is found with two large regions of a (1-->3)-beta-glucanase from tobacco pith tissue. The gene for barley (1-->3,1-->4)-beta-glucanase isoenzyme II shares with that for the alpha-amylase isoenzyme 1 a strongly preferred use of codons with G and C in the wobble position (94% and 90%, respectively). Both enzymes are secreted from the aleurone cells during germination. Such one-sided codon usage is not characteristic for the gene encoding the (1-->3)-beta-glucanase of tobacco pith tissue or the hor2-4 gene encoding the B(1) hordein storage protein in the endosperm.

Phytohormone-regulated beta-amylase gene expression in rice

The expression of beta-amylase genes in rice (Oryza sativa) and its regulation by phytohormones gibberellic acid (GA) and abscisic acid (ABA) were examined. Upon germination beta-amylase is synthesized de novo in aleurone cells and (GA) is not required. Exogenous addition of GA does not enhance the beta-amylase activity, while ABA inhibits the beta-amylase activity, mRNA accumulation, and the germination of rice seeds. GA can reverse ABA inhibition of beta-amylase expression, but not ABA inhibition of seed germination. Such regulation represents a new interaction of ABA and GA.

Alternative polyadenylation generates three low-pI alpha-amylase mRNAs with differential expression in barley

Specific low-pI alpha-amylase genes from barley (Hordeum vulgare L.) produced alternative mRNAs with a 17-base 3' extension (extension 1) or a 17-base extension beyond this (extension 2). The extended mRNAs do not arise from splicing of downstream sequences, and not all low-pI genes contain the extended sequences. All three mRNAs occur in aleurones and shoots, while extension 2 is missing from scutella. Also, the unextended mRNAs predominate in total mRNA, but the extended mRNAs predominate in membrane-bound polysomes. The extended sequences do not occur in previously characterized alpha-amylases, but 16 of 18 bases, mainly in extension 1, are identical with a sequence in the 3'-UTR of PAPI, a putative inhibitor of alpha-amylase. These observations suggest that the extended sequences could play a functional role in alpha-amylase expression.

Purification and characterization of (1-->3, 1-->4)-beta-glucan endohydrolases from germinated wheat (Triticum aestivum)

A (1-->3, 1-->4)-beta-glucan 4-glucanohydrolase [(1-->3, 1-->4)-beta-glucanase, EC 3.2.1.73] was purified to homogeneity from extracts of germinated wheat grain. The enzyme, which was identified as an endohydrolase on the basis of oligosaccharide products released from a (1-->3, 1-->4)-beta-glucan substrate, has an apparent pI of 8.2 and an apparent molecular mass of 30 kDa. Western blot analyses with specific monoclonal antibodies indicated that the enzyme is related to (1-->3, 1-->4)-beta-glucanase isoenzyme EI from barley. The complete primary structure of the wheat (1-->3, 1-->4)-beta-glucanase has been deduced from nucleotide sequence analysis of cDNAs isolated from a library prepared using poly(A)+ RNA from gibberellic acid-treated wheat aleurone layers. One cDNA, designated lambda LW2, is 1426 nucleotide pairs in length and encodes a 306 amino acid enzyme, together with a NH2-terminal signal peptide of 28 amino acid residues. The mature polypeptide encoded by this cDNA has a molecular mass of 32,085 and a predicted pI of 8.1. The other cDNA, designated lambda LW1, carries a 109 nucleotide pair sequence at its 5' end that is characteristic of plant introns and therefore appears to have been synthesized from an incompletely processed mRNA. Comparison of the coding and 3'-untranslated regions of the two cDNAs reveals 31 nucleotide substitutions, but none of these result in amino acid substitutions. Thus, the cDNAs encode enzymes with identical primary structures, but their corresponding mRNAs may have originated from homeologous chromosomes in the hexaploid wheat genome.

Barley (1----3,1----4)-beta-glucanase isoenzyme EI gene expression is mediated by auxin and gibberellic acid

Treatment of young barley leaves with indole acetic acid (IAA) or gibberellic acid (GA3) results in a dramatic increase in levels of (1----3,1----4)-beta-glucanase isoenzyme EI transcripts. In young roots of comparable age, levels of isoenzyme EI mRNA are high; IAA inhibits expression while GA3 has no effect on mRNA levels. The addition of both abscisic acid and GA3 to leaves, roots and aleurone layers leads to higher levels of (1----3,1----4)-beta-glucanase isoenzyme EI mRNA than is found with Ga3 alone. Little or no expression of (1----3,1----4)-beta-glucanase isoenzyme EII is detected in vegetative tissues, but in isolated aleurone layers GA3 enhances levels of isoenzyme EII transcripts, as does IAA. Thus, the two barley (1----3,1----4)-beta-glucanase genes respond quite differently to phytohormone treatment, depending on the tissue and its stage of development.

Structure of the genes encoding Hordeum vulgare (1----3,1----4)-beta-glucanase isoenzymes I and II and functional analysis of their promoters in barley aleurone protoplasts

Barley (1----3,1----4)-beta-glucanase isoenzyme II is synthesized in the aleurone cells during germination and secreted into the endosperm for hydrolysis of the cell walls. Its synthesis is stimulated by gibberellic acid (GA3) and repressed by abscisic acid. The gene for isoenzyme I is expressed in the aleurone, scutellum and prominently in young leaves. Close functional relatedness between the two enzymes is attested by 92% identity at the level of the amino acid sequence. The structural genes for the two enzymes each contain a large intron of 2505 bp and 2952 bp, respectively, in the codon for amino acid 25 of the 28-residue signal peptide. During evolution, homologous regions of the two introns have changed position and orientation. Furthermore, a large palindromic sequence of 327 bp in the 5' end of the intron is present only in the gene for isoenzyme II. In transient expression assays using barley aleurone protoplasts and chloramphenicol acetyl transferase as reporter the promoter of the isoenzyme I gene showed no response to GA3. However, removal of a unique 151 bp region extending from positions -402 to -552 upstream of the TATA box permitted low levels of GA3-induced expression of the reporter gene, suggesting a silencer function for this domain. High levels of GA3-responsive expression were obtained in aleurone protoplasts using the promoter of the gene encoding isoenzyme II. Truncation of this promoter revealed that sequences located within 253 bp upstream from the TATA box are sufficient to direct GA3-stimulated expression. Using the homologous barley aleurone protoplast transfection assay, it was possible to reproduce the in vivo expression characteristics of the genes for the barley (1----3,1----4)-beta-glucanase isoenzymes I and II with reporter gene constructs.

Expression of the gene encoding the PR-like protein PRms in germinating maize embryos

The PRms protein is a pathogenesis-related (PR)-like protein whose mRNA accumulates during germination of maize seeds. Expression of the PRms gene is induced after infection of maize seeds with the fungus Fusarium moniliforme. To further our investigations on the expression of the PRms gene we examined the accumulation of PRms mRNA in different tissues of maize seedlings infected with F. moniliforme and studied the effect of fungal elicitors, the mycotoxin moniliformin, the hormone gibberellic acid, and specific chemical agents. Our results indicate that fungal infection, and treatment either with fungal elicitors or with moniliformin, a mycotoxin produced by F. moniliforme, increase the steady-state level of PRms mRNA. PRms mRNA accumulation is also stimulated by the application of the hormone gibberellic acid or by treatment with silver nitrate, whereas acetylsalicylic acid has no effect. In situ RNA hybridization in isolated germinating embryo sections demonstrates that the PRms gene is expressed in the scutellum, particularly in a group of inner cells, and in the epithelium lying at the interface of the scutellum and the endosperm. The pattern of expression of the PRms gene closely resembles that found for hydrolytic enzymes, being confined to the scutellum and the aleurone layer of the germinating maize seed. Our results suggest that the PRms protein has a function during the normal process of seed germination that has become adapted to serve among the defence mechanisms induced in response to pathogens during maize seed germination.

Characteristics of alpha-Amylase during Germination of Two High-Sugar Sweet Corn Cultivars of Zea mays L

The role of the scutellum and the aleurone in alpha-amylase production in the high-sugar sweet corn cultivars Illini X-tra Sweet (shrunken-2, sh2) and Illinois 677a (sugary, sugary enhancer; su se) was compared to that in the starchy (Su) hybrid Funks G4646 with the use of alpha-amylase enzyme assays, isoelectric focusing, electron microscopy, and laser scanning confocal microscopy. The scutellum of Illinois 677a had low levels of alpha-amylase activity compared to that of Funks G4646 through 10 days after imbibition, and the aleurone of Illini X-tra Sweet had negligible activity. On the isoelectric focusing gels, the Illinois 677a scutellum had fewer alpha-amylase isozymes at 7 days compared to the Funks G4646 scutellum. The Illini X-tra Sweet aleurone had no alpha-amylase isozymes. Funks G4646 scutellar epithelial and aleurone cells contained abundant rough endoplasmic reticulum, polysomes, and dictyosomes at 5 and 7 days, respectively. The scutellar epithelial cells of Illinois 677a contained fewer of these structures by 5 days, and the Illini X-tra Sweet aleurone contained mostly lipid bodies through 7 days. Few cytoplasmic membranes and little RNA were detected with laser scanning confocal microscopy in the Illini X-tra Sweet aleurone compared to Funks G4646 at 7 days. These data suggest that the scutellum of Illinois 677a and the aleurone of Illini X-tra Sweet have impaired abilities to produce alpha-amylase.

A gene coding for a basic pathogenesis-related (PR-like) protein from Zea mays. Molecular cloning and induction by a fungus (Fusarium moniliforme) in germinating maize seeds

Pathogenesis-related proteins (PRs) are plant proteins produced in leaves in response to infection by pathogens including viruses, viroids, fungi and bacteria. Information on the presence and/or expression of PRs in monocotyledonous plants is scare. Here we report the identification of cDNA and genomic clones coding for a basic form of a protein from germinating maize seeds having a high homology with the group of PR-1 from tobacco. A cDNA library enriched in aleurone-specific sequences was prepared from maize seeds two days after germination. One clone was found to contain an open reading frame encoding a protein homologous to PR proteins from tomato (p14) and tobacco (PR-1 group). Sequence analysis of the corresponding genomic clone revealed that it was encoded by a single exon. Besides, DNA blot hybridization indicates that this PR-like protein is encoded by a single-copy gene in maize. The accumulation of its mRNA increases after rehydration of desiccated seeds. Furthermore, a relationship was found between its expression and infection by a natural pathogen of maize, the fungus Fusarium moniliforme. The possible role of this protein as a response mechanism following fungal infection in cereal seeds is discussed.

The isolation and characterization of a barley 1,3-1,4-beta-glucanase gene

The barley gene encoding isozyme I of 1,3-1,4-beta-glucanase was isolated and sequenced. The 6260-bp region sequenced included 1885 bp of the 5'-flanking region, the entire coding region, an intron of 2490 bp, and 792 bp of the 3'-flanking region. The 1,3-1,4-beta-glucanase mRNA was found to be regulated at the level of RNA accumulation by both gibberellins (positively) and abscisic acid (negatively) in barley aleurones. The mRNA for isozyme II preferentially accumulated (70%) relative to the mRNA for isozyme I (30%) in poly(A)-rich RNA isolated from material including both the aleurone and the scutellum tissues. The gene family encoding 1,3-1,4-beta-glucanase enzymes in barley was found to be comprised of two closely related genes, isozymes I and II, as well as several related sequences that could be identified by Southern blot analysis. The nucleotide sequence for the 5' untranslated leader and the coding region for the signal peptide of the isozyme II transcript were determined from a cDNA produced by the polymerase chain reaction. The structure of the protein encoded by the isozyme I gene is also discussed.

Localization of carboxypeptidase I in germinating barley grain

Activity measurements and Northern blot hybridizations were used to study the temporal and spatial expression of carboxypeptidase I in germinating grains of barley (Hordeum vulgare L. cv Himalaya). In the resting grain no carboxypeptidase I activity was found in the aleurone layer, scutellum, or starchy endosperm. During germination high levels of enzyme activity appeared in the scutellum and in the starchy endosperm but only low activity was found in the aleurone layer. No mRNA for carboxypeptidase I was observed in the resting grain. By day 1 of germination the mRNA appeared in the scutellum where its level remained high for several days. In contrast, little mRNA was observed in the aleurone layer. These results indicate that the scutellum plays an important role in the production of carboxypeptidase I in germinating barley grain.

Expression sites and developmental regulation of genes encoding (1→3,1→4)-β-glucanases in germinated barley

Expression sites of genes encoding (1→3,1→4)-β-glucan 4-glucanohydrolase (EC 3.2.1.73) have been mapped in germinated barley grains (Hordeum vulgare L.) by hybridization histochemistry. A(32)P-labelled cDNA (copy DNA) probe was hybridized to cryosections of intact barley grains to localize complementary mRNAs. No mRNA encoding (1→3,1→4)-β-glucanase is detected in ungerminated grain. Expression of (1→3,1→4)-β-glucanase genes is first detected in the scutellum after 1 d and is confined to the epithelial layer. At this stage, no expression is apparent in the aleurone. After 2 d, levels of (1→3,1→4)-β-glucanase mRNA decrease in the scutellar epithelium but increase in the aleurone. In the aleurone layer, induction of (1→3,1→4)-β-glucanase gene expression, as measured by mRNA accumulation, progresses from the proximal to distal end of the grain as a front moving away from, and parallel to, the face of the scutellum.

Differential synthesis in vitro of barley aleurone and starchy endosperm proteins

To widen the selection of proteins for gene expression studies in barley seeds, experiments were performed to identify proteins whose synthesis is differentially regulated in developing and germinating seed tissues. The in vitro synthesis of nine distinct barley proteins was compared using mRNAs from isolated endosperm and aleurone tissues (developing and mature grain) and from cultured (germinating) aleurone layers treated with abscisic acid (ABA) and GA(3). B and C hordein polypeptides and the salt-soluble proteins beta-amylase, protein Z, protein C, the chymotrypsin inhibitors (CI-1 and 2), the alpha-amylase/subtilisin inhibitor (ASI) and the inhibitor of animal cell-free protein synthesis systems (PSI) were synthesized with mRNA from developing starchy endosperm tissue. Of these proteins, beta-amylase, protein Z, and CI- 1 and 2 were also synthesized with mRNA from developing aleurone cells, but ASI, PSI, and protein C were not. CI-1 and also a probable amylase/protease inhibitor (PAPI) were synthesized at high levels with mRNAs from late developing and mature aleurone. These results show that mRNAs encoding PAPI and CI-1 survive seed dessication and are long-lived in aleurone cells. Thus, expression of genes encoding ASI, PSI, protein C, and PAPI is tissue and stage-specific during seed development. Only ASI, CI-1, and PAPI were synthesized in significant amounts with mRNA from cultured aleurone layers. The levels of synthesis of PAPI and CI-1 were independent of hormone treatment. In contrast, synthesis of alpha-amylase (included as control) and of ASI showed antagonistic hormonal control: while GA promotes and ABA reduces accumulation of mRNA for alpha-amylase, these hormones have the opposite effect on ASI mRNA levels.

Selective expression of a probable amylase/protease inhibitor in barley aleurone cells: Comparison to the barley amylase/subtilisin inhibitor

We have cloned and sequenced a 650-nucleotide cDNA from barley (Hordeum vulgare L.) aleurone layers encoding a protein that is closely related to a known α-amylase inhibitor from Indian finger millet (Eleusine coracana Gaertn.), and that has homologies to certain plant trypsin inhibitors. mRNA for this probable amylase/protease inhibitor (PAPI) is expressed primarily in aleurone tissue during late development of the grain, as compared to that for the amylase/subtilisin inhibitor, which is expressed in endosperm during the peak of storage-protein synthesis. PAPI mRNA is present at high levels in aleurone tissue of desiccated, mature grain, and in incubated aleurone layers prepared from rehydrated mature seeds. Its expression in those layers is not affected by either abscisic acid or gibberellic acid, hormones that, respectively, increase and decrease the abundance of mRNA for the amylase/subtilisin inhibitor. PAPI mRNA is almost as abundant in gibberellic acid-treated aleurone layers as that for α-amylase, and PAPI protein is synthesized in that tissue at levels that are comparable to α-amylase. PAPI protein is secreted from aleurone layers into the incubation medium.

Development of (1-->3,1-->4)-beta-d-Glucan Endohydrolase Isoenzymes in Isolated Scutella and Aleurone Layers of Barley (Hordeum vulgare)

An immunological assay has been used to investigate the synthesis of (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzymes from isolated barley aleurone layers and scutella. Enzyme release from both tissues is enhanced by 1 micromolar gibberellic acid and 10 millimolar Ca(2+), although increases induced by gibberellic acid are observed only in the presence of Ca(2+). Isoenzyme I is synthesized predominantly in the scutellum, while isoenzyme II is synthesized exclusively in the aleurone. A third, putative isoenzyme III has been detected in significant proportions in scutellar secretions and may also be secreted from aleurone layers. Both gibberellic acid and Ca(2+) appear to preferentially enhance isoenzyme II secretion from the aleurone and isoenzyme III secretion from scutella. The patterns of isoenzyme secretion are suggestive of tissue-specific differences in expression of the genes which code for (1-->3,1-->4)-beta-glucanase isoenzymes. Qualitatively similar results were obtained with barley cultivars harvested in Australia and North America.