HORMONAL CONTROL OF ENZYME FORMATION IN BARLEY ALEURONE LAYERS
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The aleurone of RB‐3 shrunken ‐2 ( sh 2) maize kernels is deficient in α‐amylase activity during germination, but exogenous applications of gibberellic acid (GA 3 ) (0.001–10 μ m ) induced low levels of activity. The highest activity was measured in the aleurone of kernels treated with 10 μ m GA 3 (14,600 ± 945 units), but was lower than untreated Starchy ( Su ) aleurone tissues (35,280 ± 5,010 units). On isoelectric focusing gels, no α‐amylase isozymes were detected in the untreated sh 2 aleurone using starch zymograms or immunoblots, but the 1.0 and 10 μ m mm GA 3 treatments induced nearly all the isozymes (eight to ten) present in the Su aleurone. There was a very low level of α‐amylase mRNA in the untreated sh 2 aleurone, an intermediate level in the 1.0 μ m GA 3 ‐treated sh 2 aleurone, and the highest level in the untreated Su aleurone. On the confocal microscope, the 1.0 μ m GA 3 ‐treated aleurone cells had enhanced levels of cytoplasmic membranes and RNA compared to untreated sh 2 aleurone cells. The 1.0 μ m GA 3 treatment also induced shoot elongation in the sh 2 seedlings. The data demonstrate that the sh 2 aleurone is deficient in its function to produce α‐amylases, and exogenous GA 3 can partially restore cell function in the sh 2 kernels.
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The data from four F 2 progenies from a cross of a normal endosperm, multiple aleurone maize ( Zea mays L.) line with an o 2 /o 2 single aleurone line were heterogeneous but do not disprove the hypothesis that the multiple aleurone phenotype is conditioned by a single dominant factor. This factor segregated independently of opaque ‐2. In non‐opaque seeds, the effect of multiple aleurone layers on protein content of the endosperm or content of any amino acid was negligible. In o 2 /o 2 /o 2 endosperms, the presence of multiple aleurone layers as compared to single aleurone layers increased slightly the content of all the amino acids that are increased in o 2 /o 2 /o 2 endosperms as compared with O 2 /—/— endosperms. Most of the amino acids that were lower in o 2 /o 2 /o 2 endosperms relative to O 2 /—/— endosperms were decreased further by the presence of multiple aleurone layers. We conclude that the multiple aleurone layer factor should be useful in breeding programs designed to utilize the opaque ‐2 mutant.
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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. © 2014 Society of Chemical Industry
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The structural organization of the cell walls at the aleurone-sub-aleurone junction has been examined by fluorescence microscopy in ungerminated seeds of two varieties of Hordeum vulgare L. (cultivars Himalaya and Vanier). The sub-aleurone cell walls that are immediately adjacent to the aleurone layer are considerably thicker than the remainder of endosperm walls and contain extensive deposits of aniline blue-positive material. The latter was not significantly affected by periodate oxidation and was removed from tissue sections only by β-1,3-glucanases or hot dimethyl sulphoxide. These deposits may represent the primary substrate for endo-β-1,3-glucanases secreted by the aleurone layer during germination.
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Abstract The nature of the proteinases which are secreted by barley aleurone layers in response to gibberellic acid was studied by constructing pH vs. activity curves for the hydrolysis of gelatin by incubation media and aleurone layer extracts. The results indicate that the aleurone layers release several different proteinases. The main component is a labile sulphydryl enzyme with an optimum pH of 3.9. The other enzymes include two sulphydryl proteinases with pH optima between pH 5 and 6.5 and a metal‐activated enzyme active at pH 7.0. No differences could be demonstrated between the proteinases released by and retained in the aleurone layers.
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The expression of a-amylase genes in aleurone cells of a standard-height wheat Ramona 50 is known to be under the control of Gibberellic Acid (GA3) while the production of aamylase in aleurone layers of a dwarf wheat D6899 (carrying Rht3 gene) is known to be insensitive to GA3. Northern blot experiment was carried out in which RNA samples isolated from GA3-treated or untreated aleurone layers of both Ramona 50 and D6899 were subjected to electrophoresis and then hybridized with barley cDNA clones of high-pI and low-pI aamylase genes. It was shown that the blockage of GA3-induced expression of a-amylase genes in aleurone layers of D6899 was at the level of mRNA accumulation. Cold temperature treatment did not induce the GA3 sensitivity of a-amylase production in D6899. A 417 bp promoter sequence of aAmy2/54, one of the low pI a-amylase genes expressed in wheat aleurone cells, was synthesized using PCR and cloned into pUC19 plasmid. Gel retardation assays were performed to study DNA-protein interactions between this promoter sequence and percoll gradient purified aleurone proteins of Ramona 50 and D6899. Multiple aleurone proteins that bound to the promoter region were identified. Based on competitive binding studies, one DNA-protein interaction was defined as DNA-sequence-specific, while others appeared to be nonsequence-specific. Nuclear proteins from both GA3-treated and untreated aleurone cells of Ramona 50 or D6899 gave.
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Coroico corn ( Zea mays L.), a South American race of floury maize contained two to six layers of aleurone cells instead of the customary single aleurone cell layer found in ordinary yellow dent corn. In Coroico with an average of 3.7 aleurone cell layers, the aleurone made up 4.3% of the endosperm as against 2.1% in yellow dent corn. Total protein in Coroico aleurone is 35 to 38% compared with 22% in yellow dent corn. Lysine levels in aleurone proteins of all corns examined, both Coroico and yellow dent were comparable ranging from 4.0 to 4.4 g/100 g of protein. Because of the additive effect of the multiple aleurone and the high protein content of the aleurone, lysine in Coroico endosperm was higher than in that of yellow dent corn. Multiple aleurone is transmitted as a partial dominant character over the normal single aleurone condition. Improvement of protein nutritional quality using Coroico germ plasm appears worthwhile.
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Although the aleurone layer of ceral grain seeds has many advantages for the study of gibberellin action, it has the disadvantage that the hand-isolation of the aleurone layers is time-consuming. To overcome this disadvantage, a commercially available pasta machine was modified and used to remove aleurone layers from imbibed barley (Hordeum vulgare) seeds. This equipment allows isolation of a thousand layers in 5 minutes compared to the 3 to 4 hours required to hand-isolate them. The machine-made aleurone layers are gibberellic-acid responsive and the response is similar both qualitatively and quantitatively to that of hand-isolated layers.
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