Effect of temperature on the synthesis and secretion of alpha-amylase in barley aleurone layers

The effect of temperature on α-amylase synthesis and secretion from barley (c.v. Himalaya) half-seeds and aleurone layers is reported. Barley half-seeds incubated at 15 C in gibberellic acid (GA) concentrations of 0.5 and 5 micromolar for 16 hours do not release α-amylase. Similarly, isolated aleurone layers of barley do not release α-amylase when incubated for 2 or 4 hours at temperatures of 15 C or below following 12 hours incubation at 25 C at GA concentrations from 50 nanomolar to 50 micromolar. There is an interaction between temperature and GA concentration for the process of α-amylase release from aleurone layers; thus, with increasing GA concentration, there is an increase in the Q₁₀ of this process. A thermal gradient bar was used to resolve the temperature at which the rate of α-amylase release changes; thermal discontinuity was observed between 19 and 21 C. The time course of the response of aleurone tissue to temperature was determined using a continuous monitoring apparatus. Results show that the effect of low temperature is detectable within minutes, whereas recovery from exposure to low temperature is also rapid. Although temperature has a marked effect on the amount of α-amylase released from isolated aleurone layers, it does not significantly affect the accumulation of α-amylase within the tissue. At all GA concentrations above 0.5 nanomolar, the level of extractable α-amylase is unaffected by temperatures between 10 and 28 C. It is concluded that the effect of temperature on α-amylase production from barley aleurone layers is primarily on the process of enzyme secretion.     

A Correlation between a Ribonucleic Acid Fraction Selectively Labeled in the Presence of Gibberellic Acid and Amylase Synthesis in Barley Aleurone Layers

The effects of gibberellic acid on the incorporation of radio-active uridine and adenosine into RNA of barley aleurone layers were investigated using a double labeling method combined with acrylamide gel electrophoresis. After 16 hours of incubation, gibberellic acid stimulated the incorporation of label into all species of RNA, but the effects were very small (0-10%) for ribosomal and transfer RNA and comparatively large (up to 300%) for RNA sedimenting between 5S and 14S. This result was obtained for both isolated aleurone layers and for layers still attached to the endosperm. A similar but less marked pattern occurred in layers incubated for 8 hours, but the effect was not observed after 4 hours. The gibberellic acid-enhanced RNA labeling was not due to micro-organisms. The following evidence was obtained for an association between the gibberellic acid-enhanced RNA synthesis and α-amylase synthesis: (a) synthesis of α-amylase took place in parallel with incorporation of label into gibberellic acid-RNA; (b) actinomycin D inhibited amylase synthesis and gibberellic acid-RNA by similar percentages; (c) 5-fluorouracil halved incorporation of label into ribosomal RNA but had no effect on amylase synthesis and gibberellic acid-RNA; and (d) abscisic acid had little effect on synthesis of RNA in the absence of gibberellic acid, but when it was included with gibberellic acid the synthesis of both enzyme and gibberellic acid-RNA was eliminated. We conclude that large changes in the synthesis of the major RNA species are not necessary for α-amylase synthesis to occur but that α-amylase synthesis does not occur without the production of gibberrellic acid-RNA. Gibberellic acid-RNA is probably less than 1% of the total tissue RNA, is polydisperse on acrylamide gels, and could be messenger species for α-amylase and other hydrolytic enzymes whose synthesis is under gibberellic acid control.     

Response of barley aleurone layers to abscisic Acid

Cordycepin, an inhibitor of RNA synthesis in barley (Hordeum vulgare L.) aleurone cells, does not inhibit the gibberellic acid-enhanced α-amylase (EC 3.2.1.1.) synthesis in barley aleurone layers if it is added 12 hours or more after the addition of the hormone. However, the accumulation of α-amylase activity after 12 hours of gibberellic acid can be decreased by abscisic acid. The accumulation of α-amylase activity is sustained or quickly restored when cordycepin is added simultaneously or some time after abscisic acid, indicating that the response of aleurone layers to abscisic acid depends on the continuous synthesis of a short lived RNA. By analysis of the newly synthesized proteins by gel electrophoresis with sodium dodecylsulfate, we observed that the synthesis of α-amylase is decreased in the presence of abscisic acid while the synthesis of most of the other proteins remains unchanged. From the rate of resumption of α-amylase production in the presence of cordycepin and abscisic acid, it appears that abscisic acid does not have a measurable effect on the stability of α-amylase mRNA.     

Embryoless Wheat Grain: A Natural System for the Study of Gibberellin-induced Enzyme Formation

Yorkstar wheat, grown in New York State, has a high percentage (10-11) of grains without embryos. The embryoless grains have viable aleurone layers and show no sign of injury. These grains are able to support α-amylase synthesis only in the presence of gibberellin A₃ (GA₃). In the absence of GA₃ some protein synthesis occurs in embryoless grains during the early hours of soaking, indicating that such activity occurs prior to and independent of GA₃ induction of α-amylase. The level of β-amylase on a dry weight basis is the same in embryoless and normal grains and decreases with time of soaking. In the presence of GA₃, β-amylase decreases at a slower rate. Isoenzymes of α-amylase from GA₃-treated embryoless and normal grains show quantitative as well as qualitative differences. Cycloheximide (60 μg/ml) completely inhibits the synthesis of α-amylase by embryoless grains. Of the RNA synthesis inhibitors, actinomycin D (60 μg/ml) was ineffective while 6-methylpurine (60 μg/ml) gave 65% inhibition without decreasing the number of isoenzymes.     

Substrate induction of nitrate reductase in barley aleurone layers

Nitrate induces the formation of nitrate reductase activity in barley (Hordeum vulgare L. cv. Himalaya) aleurone layers. Previous work has demonstrated de novo synthesis of α-amylase by gibberellic acid in the same tissue. The increase in nitrate reductase activity is inhibited by cycloheximide and 6-methylpurine, but not by actinomycin D. Nitrate does not induce α-amylase synthesis, and it has no effect on the gibberellic acid-induced synthesis of α-amylase. Also, there is little or no direct effect of gibberellic acid (during the first 6 hr of induction) or of abscisic acid on the nitrate-induced formation of nitrate reductase. Gibberellic acid does interfere with nitrate reductase activity during long-term experiments (greater than 6 hr). However, the time course of this inhibition suggests that the inhibition may be a secondary one. Barley aleurone layers therefore provide a convenient tissue for the study of both substrate- and hormone-induced enzyme formation.     

In Vitro Synthesis and Processing of Wheat alpha-Amylase : TRANSLATION OF GIBBERELLIC ACID-INDUCED WHEAT ALEURONE LAYER RNA BY WHEAT GERM AND XENOPUS LAEVIS OOCYTE SYSTEMS

Wheat (Triticum aestivum) RNA was used to program synthesis of the α-amylase protein by Xenopus laevis oocytes. A 41,500-dalton protein was made which was identified as α-amylase by immunoprecipitation with rabbit anti-α-amylase antiserum raised against the purified wheat protein and by its co-migration with authentic α-amylase on sodium dodecyl sulfate polyacrylamide gels. Synthesis of α-amylase was dependent upon injection of RNA extracted from gibberellic acid-induced aleurone layers from wheat. The amount of α-amylase produced was proportional to the amount of RNA injected and reached a plateau within 4 hours after injection. When the same RNA was translated in a wheat germ cell-free translation system, a 43,000-dalton protein was produced. Addition of dog pancreas microsomal membranes to the wheat germ translation system resulted in processing of the α-amylase protein to a form which co-migrated with authentic α-amylase purified from malted wheat and with the protein synthesized in oocytes.     

Inhibition of Gibberellic Acid-induced alpha-Amylase Formation by Polyethylene Glycol and Mannitol

Both polyethylene glycol (PEG) and mannitol inhibit gibberellic acid-induced α-amylase production in barley aleurone layers. The effect of the osmotic solution is on enzyme synthesis rather than α-amylase secretion. The inhibition of α-amylase synthesis does not appear to be mediated via an indirect effect on respiration or protein synthesis. Rather it seems that the osmotic solutions reduce the extent of proteolysis of the stored aleurone grain protein thus making available less substrate for new protein synthesis.     

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 GA₃. B and C hordein polypeptides and the salt-soluble proteins β-amylase, protein Z, protein C, the chymotrypsin inhibitors (CI-1 and 2), the α-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, β-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 α-amylase (included as control) and of ASI showed antagonistic hormonal control: while GA promotes and ABA reduces accumulation of mRNA for α-amylase, these hormones have the opposite effect on ASI mRNA levels.     

Control of alpha-amylase mRNA accumulation by gibberellic Acid and calcium in barley aleurone layers

Pulse-labeling of barley (Hordeum vulgare L. cv Himalaya) aleurone layers incubated for 13 hours in 2.5 micromolar gibberellic acid (GA₃) with or without 5 millimolar CaCl₂ shows that α-amylase isozymes 3 and 4 are not synthesized in vivo in the absence of Ca²⁺. A cDNA clone for α-amylase was isolated and used to measure α-amylase mRNA levels in aleurone layers incubated in the presence and absence of Ca²⁺. No difference was observed in α-amylase mRNA levels between layers incubated for 12 hours in 2.5 micromolar GA₃ with 5 millimolar CaCl₂ and layers incubated in GA₃ alone. RNA isolated from layers incubated for 12 hours in GA₃ with and without Ca²⁺ was translated in vitro and was found to produce the same complement of translation products regardless of the presence of Ca²⁺ in the incubation medium. Immunoprecipitation of translation products showed that the RNA for α-amylase synthesized in Ca²⁺-deprived aleurone layers was translatable. Ca²⁺ is required for the synthesis of α-amylase isozymes 3 and 4 at a step after mRNA accumulation and processing.     

Characteristics of the process of enzyme release from secretory plant cells

Secretion—the outward movement of molecules across the plasmalemma—of α-amylase by barley (Hordeum vulgare L. cv. Himalaya) aleurone layers is an energy-dependent process that is not directly dependent upon protein synthesis or RNA synthesis and does not appear to be under the direct control of gibberellic acid or abscisic acid. Release—the movement of the secreted α-amylase molecules through the walls into the surrounding medium—is apparently diffusion limited and is markedly dependent upon the presence of ions.     

Water stress enhances expression of an alpha-amylase gene in barley leaves

The amylases of the second leaves of barley seedlings (Hordeum vulgare L. cv Betzes) were resolved into eight isozymes by isoelectric focusing, seven of which were β-amylase and the other, α-amylase. The α-amylase had the same isoelectric point as one of the gibberellin-induced α-amylase isozymes in the aleurone layer. This and other enzyme characteristics indicated that the leaf isozyme corresponded to the type A aleurone α-amylase (low pI group). Crossing experiments indicated that leaf and type A aleurone isozymes resulted from expression of the same genes.

In unwatered seedlings, leaf α-amylase increased as leaf water potential decreased and ABA increased. Water stress had no effect on β-amylase. α-Amylase occurred uniformly along the length of the leaf but β-amylase was concentrated in the basal half of the leaf. Cell fractionation studies indicated that none of the leaf α-amylase occurred inside chloroplasts.

Leaf radiolabeling experiments followed by extraction of α-amylase by affinity chromatography and immunoprecipitation showed that increase of α-amylase activity involved synthesis of the enzyme. However, water stress caused no major change in total protein synthesis. Hybridization of a radiolabeled α-amylase-related cDNA clone to size fractionated RNA showed that water-stressed leaves contained much more α-amylase mRNA than unstressed plants. The results of these and other studies indicate that regulation of gene expression may be a component in water-stress induced metabolic changes.

     

Alpha-amylase secretion by single barley aleurone layers

The secretion of α-amylase from single isolated (Hordeum vulgare L. cv Himalaya) aleurone layers was studied in an automated flow-through apparatus. The apparatus, consisting of a modified sample analyzer linked to a chart recorder, automatically samples the flow-through medium at 1 minute intervals and assays for the presence of α-amylase. The release of α-amylase from aleurone layers begins after 5 to 6 hours of exposure to gibberellic acid and reaches a maximum rate after 10 to 12 hours. The release of α-amylase shows a marked dependence on Ca²⁺, and in the absence of Ca²⁺ it is only 20% of that in the presence of 10 millimolar Ca²⁺. Withdrawal of Ca²⁺ from the flow-through medium results in the immediate cessation of enzyme release and addition of Ca²⁺ causes immediate resumption of the release process. The effect of Ca²⁺ is concentration-dependent, being half-maximal at 1 millimolar Ca²⁺ and saturated at 10 millimolar Ca²⁺. Ruthenium red, which blocks Ca²⁺ but not Mg²⁺ efflux from barley aleurone layers, renders α-amylase release insensitive to Ca²⁺ withdrawal. Inhibitors of respiratory metabolism cause a burst of α-amylase release which lasts for 0.5 to 5 hours. Following this phase of enhanced α-amylase release, the rate of release declines to zero. Pretreatment of aleurone layers with HCl prior to incubation in HCN also causes a burst of α-amylase release, indicating that the inhibitor is affecting the secretion of α-amylase and not its movement through the cell wall. The rapid inhibition of α-amylase release upon incubation of aleurone layers at low temperature (5°C) or in 0.5 molar mannitol also indicates that enzyme release is dependent on a metabolically linked process and is not diffusion-limited. This conclusion is supported by cytochemical observations which show that, although the cell wall matrix of aleurone layers undergoes extensive digestion after gibberellin treatment, the innermost part of the cell wall is not degraded and could influence enzyme release.     

Hormonal control of polyribosome formation in barley aleurone layers

The addition of abscisic acid to barley (Hordeum vulgare L. cv. Himalaya) aleurone layers at the same time as gibberellic acid completely prevents the gibberellin-induced increases in the percentage of polysomes, the formation of polyribosomes, and the synthesis of α-amylase, even when the molar concentration of gibberellic acid is four times greater than the concentration of abscisic acid. The addition of abscisic acid to aleurone cells producing α-amylase (midcourse addition) inhibits the further synthesis of α-amylase and decreases the percentage of polysomes but does not change the number of ribosomes per cell.     

Effect of Ethylene on the Release of alpha-Amylase through Cell Walls of Barley Aleurone Layers

A large portion of the gibberellic acid (GA₃)-induced α-amylase in isolated aleurone layers is transported into the incubation medium. In the presence of GA₃ and ethylene, an even larger portion of the enzyme is found in the medium. Employing an acid washing technique developed by Varner and Mense (Plant Physiol 1972 49:187-189), it was observed that ethylene significantly reduces the amount of α-amylase trapped by the thick cell walls of aleurone layers. However, the amount of enzyme remaining in the cell (within the boundary of plasma membrane) is not affected by ethylene. Ethylene has no observable effect on membrane formation as measured by the incorporation of [³²P]orthophosphate into phospholipids. Because of these observations it is suggested that ethylene enhances the release of α-amylase, i.e. transport of α-amylase across cell walls, but not the secretion of α-amylase, i.e. transport of α-amylase past the barrier of plasma membrane. The possible mechanism of this ethylene effect is discussed.     

Hypochlorite Disinfection Influences the GA(3)-Induced Synthesis of alpha-Amylase Isozymes by Isolated Barley Aleurone Layers

Aleurone layers isolated from half-seeds of Himalaya barley (Hordeum vulgare cv Himalaya) disinfected in hypochlorite solutions containing 1.0% available chlorine synthesized significantly less α-amylase in response to gibberellic acid than layers derived from half-seeds disinfected in 0.1% hypochlorite. This effect of hypochlorite involved neither a differential decrease in the synthesis of group A or B α-amylase isozymes nor a general decrease in α-amylase synthesis attributable to fewer viable aleurone cells in layers from half-seeds disinfected with 1% hypochlorite. Our results emphasize the need to evaluate the potential effects of routine disinfection procedures used in physiological and biochemical studies.     

Gibberellic Acid-enhanced synthesis and release of alpha-amylase and ribonuclease by isolated barley and aleurone layers

Gibberellic acid enhances the synthesis of α-amylase in isolated aleurone layers of barley-seeds (Hordeum vulgare var. Himalaya). In the presence of 20 mm calcium chloride the amount of enzyme obtained from isolated aleurone layers is quantitatively comparable to that of the half-seeds used in earlier studies. After a lag period of 6 to 8 hours enzyme is produced at a linear rate. Gibberellic acid does not merely trigger α-amylase synthesis, but it is continuously required during the period of enzyme formation. Enzyme synthesis is inhibited by inhibitors of protein and RNA synthesis. Small amounts of actinomycin D differentially inhibit enzyme release and enzyme synthesis suggesting 2 distinct processes. Gibberellic acid similarly enhances the formation of ribonuclease which increases linearly over a 48 hour period. During the first 24 hours the enzyme is retained by the aleurone cells and this is followed by a rapid release of ribonuclease during the next 24 hour period. The capacity to release the enzyme is generated between 20 and 28 hours after the addition of the hormone. Ribonuclease formation is inhibited by inhibitors of protein and RNA synthesis. These inhibitors also prevent the formation of the release mechanism if added at the appropriate moment.     

Regulation of the accumulation of mRNA for alpha-amylase in barley aleurone

The effect of gibberellic acid and Ca²⁺ on the accumulation of α-amylase mRNAs in aleurone layers of barley (Hordeum vulgare L. cv Himalaya) was studied using cDNA clones containing sequences of mRNAs for the high and low isoelectric point (pI) α-amylases. There is no significant hybridization between the two α-amylase cDNA clones under the hybridization and washing conditions employed. These clones were therefore used to monitor levels of mRNAs for high and low pI α-amylases. It is shown that although the synthesis of the high pI α-amylase proteins depends on the presence of Ca²⁺ in the incubation medium, the accumulation of mRNA for this group occurs to the same degree in the presence or the absence of Ca²⁺. The accumulation of low pI α-amylase mRNA is also not affected by the presence or absence of Ca²⁺ in the incubation medium. These results establish gibberellic acid, not Ca²⁺, as the principal regulator of α-amylase mRNA accumulation in barley aleurone, while Ca²⁺ controls high pI α-amylase synthesis at a later step in the biosynthetic pathway.