Gibberellic Acid Induces Vacuolar Acidification in Barley Aleurone

The roles of gibberellic acid (GA3) and abscisic acid (ABA) in the regulation of vacuolar pH (pHv) in aleurone cells of barley were investigated using the pH-sensitive fluorescent dye 2[prime],7[prime]-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). BCECF accumulated in vacuoles of aleurone cells, but sequestration of the dye did not affect its sensitivity to pH. BCECF-loaded aleurone cells retained their ability to respond to both GA3 and ABA. The pHv of freshly isolated aleurone cells is 6.6, but after incubation in GA3, the pHv fell to 5.8. The pHv of cells not incubated in hormones or in the presence of ABA showed little or no acidification. The aleurone tonoplast contains both vacuolar ATPase and vacuolar pyrophosphatase, but the levels of pump proteins were not affected by incubation in the presence or absence of hormones. We conclude that GA3 affects the pHv in aleurone cells by altering the activities of tonoplast H+ pumps but not the amounts of pump proteins.     

Characterization of the alpha-Amylases Synthesized by Aleurone Layers of Himalaya Barley in Response to Gibberellic Acid

The gibberellic acid (GA₃)-induced α-amylases from the aleurone layers of Himalaya barley (Hordeum vulgare L. cv Himalaya) have been purified by cycloheptaamylose-Sepharose affinity chromatography and fractionated by DEAE-cellulose chromatography. Four fractions (α-amylases 1-4) were obtained which fell into two groups (A and B) on the basis of a number of characteristics. Major differences in serological characteristics and in proteolytic fingerprints were found between group A (α-amylases 1 and 2) and group B (α-amylases 3 and 4). Also, the lag time for appearance of group B enzyme activity was longer than for group A, and the appearance of group B required higher GA₃ levels than group A. The components of each group behaved similarly, although differences in proteolytic fingerprints were detected.

These results together with those from other studies indicate that GA₃ differentially controls the expression of two α-amylase genes or groups of genes giving rise to two groups of α-amylases with many different properties.

     

Multiple Forms of Amylase Induced by Gibberellic Acid in Isolated Barley Aleurone Layers

The addition of gibberellic acid to isolated aleurone layers of barley (Hordeum vulgare L.) causes the production and secretion of four α-amylases. Two of these are stable at pH 3.7 and are not inactivated by ethylenediaminetetraacetate. The other two represent the classical barley α-amylases; i.e., they are inactivated at pH 3.7 and by reagents which from complexes with divalent metal ions. All four forms are synthesized de novo in response to the addition of gibberellic acid.     

Abscisic Acid Induces Mitogen-Activated Protein Kinase Activation in Barley Aleurone Protoplasts

Abscisic acid (ABA) induces a rapid and transient mitogen-activated protein (MAP) kinase activation in barley aleurone protoplasts. MAP kinase activity, measured as myelin basic protein phosphorylation by MAP kinase immunoprecipitates, increased after 1 min, peaked after 3 min, and decreased to basal levels after ~5 min of ABA treatment in vivo. Antibodies recognizing phosphorylated tyrosine residues precipitate with myelin basic protein kinase activity that has identical ABA activation characteristics and demonstrate that tyrosine phosphorylation of MAP kinase occurs during activation. The half-maximal concentration of ABA required for MAP kinase activation, 3 x 10-7 M, is very similar to that required for ABA-induced rab16 gene expression. The tyrosine phosphatase inhibitor phenylarsine oxide can completely block ABA-induced MAP kinase activation and rab16 gene expression. These results lead us to conclude that ABA activates MAP kinase via a tyrosine phosphatase and that these steps are a prerequisite for ABA induction of rab16 gene expression.     

Interactions between Gibberellic Acid, Ethylene, and Abscisic Acid in Control of Amylase Synthesis in Barley Aleurone Layers

Gibberellic acid-induced α-amylase synthesis in barley (Hordeum vulgare L.) aleurone layers was inhibited by abscisic acid, and the inhibition was partly removed by additional gibberellic acid alone and by ethylene alone. Together additional gibberellic acid and ethylene almost eliminated abscisic acid inhibition of amylase synthesis. Time course studies of these phenomena showed that the effect of abscisic acid, ethylene, and varying concentrations of gibberellic acid on the course of amylase synthesis were either to speed up or slow down the whole process and not to affect the lag phase or the linear phase separately. The data are discussed in relation to previous studies of abscisic acid-gibberellic acid interaction.     

cGMP Is Required for Gibberellic Acid-Induced Gene Expression in Barley Aleurone

The occurrence and roles of cGMP were investigated in aleurone layers and protoplasts isolated from barley (cv Himalaya) grain. Levels of cGMP in freshly isolated barley aleurone layers ranged from 0.065 to 0.08 pmol/g fresh weight of tissue, and cGMP levels increased transiently after incubation in gibberellic acid (GA). Abscisic acid (ABA) did not increase cGMP levels in aleurone layers. LY 83583 (LY), an inhibitor of guanylyl cyclase, prevented the GA-induced increase in cGMP and inhibited GA-induced [alpha]-amylase synthesis and secretion. The inhibitory effects of LY could be overcome by membrane-permeant analogs of cGMP. LY also prevented GA-induced accumulation of [alpha]-amylase and GAMYB mRNAs. cGMP alone was not sufficient to induce the accumulation of [alpha]-amylase or GAMYB mRNA. LY had a less dramatic effect on the accumulation of mRNAs encoding the ABA-responsive gene Rab21. We conclude that cGMP plays an important role in GA, but not ABA, signaling in the barley aleurone cell.     

Gibberellic Acid Regulates the Level of a BiP Cognate in the Endoplasmic Reticulum of Barley Aleurone Cells

The isolation of a 70-kilodalton protein from barley (Hordeum vulgare L.) aleurone layers that cross-reacts with an antibody against yeast binding protein (BiP) is reported. Endoplasmic reticulum isolated from aleurone layers treated with gibberellic acid contain much higher levels of the BiP cognate than do membranes isolated from layers treated with abscisic acid.     

Gibberellic Acid and Ion Release from Barley Aleurone Tissue: Evidence for Hormone-dependent Ion Transport Capacity

The release of potassium, magnesium, and phosphate ions from aleurone cells of barley (Hordeum vulgare L. cv. Himalaya) is a gibberellic acid-dependent process. The release of these ions is preceded by a lag period of 6 to 8 hours after gibberellic acid addition. The effect of gibberellic acid on the release of ions is not mediated through an effect on ion solubilization. Thus, gibberellic acid does not apreciably affect the sum of extracted and released ions relative to controls. Rather, the effect of the hormone is on the release process itself. Inhibitors of oxidative phosphorylation when added with gibberellic acid or at times up to 6 hours after gibberellic acid inhibition release. When these inhibitors are added after ion release has begun, however, rapid efflux of ions occurs. These results suggest a strong correlation between energy levels and ion transport capacity. Inhibitors of RNA and protein synthesis also inhibit gibberellic acid-stimulated ion release. Evidence suggests that RNA and protein synthesis are required to establish and maintain ion release capacity of aleurone cells.     

The Role of Membrane-Bound Enzymes in an Early Response of Aleurone Tissue to Gibberellic Acid

Treatment of aleurone layers of barley seed with gibberellicacid increases the observable phosphorylcholine glyceride transferaseactivity in a membrane fraction prepared from extracts of thealeurone cells. This gibberellic acid-dependent increase inglyceride transferase activity requires neither RNA synthesisnor protein synthesis. Membrane fractions prepared from mixturesof extracts of gibberellic acid-treated layers and control layershave a specific activity of glyceride transferase higher thanexpected on the basis of simple addition of the activities fromthe two sources. Therefore, some kind of activation is occurring.     

Fine Structural Changes in Barley Aleurone Cells During Gibberellic Acid-induced Enzyme Secretion

The fine structure of barley aleurone cells was studied in theenzyme secretion phase. An ultrastructural feature of this phaseis the formation of stacked rough endoplasmic reticulum (rER),for such a structure was never found in cells during the enzymesynthesis phase. Other structural features frequently observedin the secretion phase were amoeboid-shaped nuclei, the stackedrER wound round the nucleus and mitochondria, and a stream ofthe stacked rER directed to the plasmamembrane. Hordeum vulgare L, barley, aleurone cells, enzyme secretion, gibberellic acid     

Gibberellic Acid Controls the Secretion of Acid Phosphatase in Aleurone Layers and Isolated Aleurone Protoplasts of Avena fatua

The role of gibberellic acid (GA₃) in controlling the secretion(across the plasma membrane) and release (through the cell wall)of acid phosphatase (E.C. 3.1.3.2 [EC] .) from Avena aleurone layershas been investigated. Evidence from this comparative studywith intact aleurone layers and isolated aleurone protoplastsreveals that the secretion of acid phosphatase is under GA₃control. The mechanism underlying secretion and release of theenzyme from aleurone cells is discussed. Key words: Avena fatua, Acid phosphatase, Aleurone protoplasts, Gibberellic acid, Secretion     

Ergopeptine-Sensitive Calcium-Dependent Protein Phosphorylation System in the Brain

We studied a protein phosphorylation system that is regulated by the dopamine-mimetic ergot bromocriptine. Bromocriptine was found to inhibit selectively the endogenous phosphorylation of a threonine residue(s) in 50,000- and 60,000-dalton proteins in a synaptosome fraction. The bromocriptine-sensitive phosphorylation is stimulated by calcium and by calmodulin, and occurs predominantly in the brain. The inhibitory effect of bromocriptine was not mimicked by 3,4-dihydroxyphenylethylamine or by any of the neurotransmitters and related agents tested, but was mimicked, although less effectively, by other ergots that contain peptide moieties. In the hippocampus, the brain region with the highest content of the 50,000- and 60,000-dalton proteins, the ergopeptine-sensitive protein phosphorylation appears to be localized to interneurons or cell bodies whose axons synapse outside the hippocampus. The results raise the possibility that some of the bromocriptine- and ergopeptine-induced pharmacological effects in the CNS may be mediated by the inhibition of the calcium/calmodulin-dependent phosphorylation of these specific proteins.     

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.     

The Effect of Calmodulin Antagonists on Gibberellic Acid-induced Enzyme Secretion in Barley Aleurone Layers

Calmodulin activity was detected and assayed in barley aleuronecells. The effect of calmodulin antagonists on GA₃-induced enzymesynthesis and secretion in barley aleurone layers was also investigated.These calmodulin antagonists (chlorpromazine, haloperidol) inhibitedonly GA₂-induced -amylase secretion. This inhibitory effectwas intensified after 6 h of GA₃-incubation. This leads us tosuggest that some calmodulin-controlled mechanism is involvedin GA₂-induced -amylase secretion. Hordeum vulgare L., barley aleurone cells, gibberellic acid, -amylase secretion, calmodulin, calmodulin antagonist     

Abnormalities Induced in Barley Ears by Gibberellic Acid

Five varieties of spring barley were sprayed with gibberellicacid (GA) at various growth stages from the one-leaf to thefive-leaf stage. In short days a number of abnormalities ofthe ear were found. These abnormalities are described in themature ear, and in the case of the branched ear, the ontogenyof the abnormality is shown. The relationship of the abnormalitiesto the environment, to the stage at which the plant was treated,and to the variety is analysed. The role of CA in inducing these abnormal features is discussedand their resemblance to abnormalities induceed geneticallyand by enviromental manipulations is pointed out.     

Abscisic Acid and Gibberellic Acid Contents in Ripening Barley Seeds

The ABA and GA3 contents were investigated in barley seeds during maturation and after harvest. The highest amount of ABA was found in milk and wax ripeness – 13 ng and 11 ng per seed respectively. The level decreased during the later stages of maturation and during release of dormancy and was 1 ng per seed 6 weeks after harvest. The content of gibberellic acid decreased in a similar way but in an earlier stage of maturation. The determined amounts of GA₃ were: 0.4, 0.1, 0.03 and about 0.05 ng per seed respectively, in milk, wax and full ripeness and after harvest. The results of quantitative determination, obtained with the GLC method, corresponded to the growth effects in the test of the first leaf of oat.     

Direct Phosphorylation and Activation of a Mitogen-Activated Protein Kinase by a Calcium-Dependent Protein Kinase in Rice

The mitogen-activated protein kinase (MAPK) is a pivotal point of convergence for many signaling pathways in eukaryotes. In the classical MAPK cascade, a signal is transmitted via sequential phosphorylation and activation of MAPK kinase kinase, MAPK kinase (MKK), and MAPK. The activation of MAPK is dependent on dual phosphorylation of a TXY motif by an MKK, which is considered the sole kinase to phosphorylate and activate MAPK. Here, we report a novel regulatory mechanism of MAPK phosphorylation and activation besides the canonical MAPK cascade. A rice (Oryza sativa) calcium-dependent protein kinase (CDPK), CPK18, was identified as an upstream kinase of MAPK (MPK5) in vitro and in vivo. Curiously, CPK18 was shown to phosphorylate and activate MPK5 without affecting the phosphorylation of its TXY motif. Instead, CPK18 was found to predominantly phosphorylate two Thr residues (Thr-14 and Thr-32) that are widely conserved in MAPKs from land plants. Further analyses reveal that the newly identified CPK18-MPK5 pathway represses defense gene expression and negatively regulates rice blast resistance. Our results suggest that land plants have evolved an MKK-independent phosphorylation pathway that directly connects calcium signaling to the MAPK machinery.     

Modulation of Calmodulin mRNA and Protein Levels in Barley Aleurone

Changes in calmodulin (CaM) mRNA and protein were investigated in aleurone layers of barley (Hordeum vulgare L. cv Himalaya) incubated in the presence and absence of calcium, gibberellic acid (GA3), and abscisic acid (ABA). CaM mRNA levels increased rapidly and transiently following incubation of aleurone layers in H2O, CaCl2, or GA3. The increase in CaM mRNA was prevented by ABA. This increase in CaM mRNA was brought about by physical stimulation during removal of the starchy endosperm from the aleurone layer. CaM protein levels did not increase in response to physical stimulation. Only incubation in GA3 plus CaCl2 brought about a rapid increase in CaM protein levels in the aleurone cell. ABA reduced the level of CaM protein below that found at the beginning of the incubation period. The rise in CaM protein preceded increases in the synthesis and secretion of [alpha]-amylase. Immunocytochemistry with monoclonal antibodies to carrot and mung bean CaM was used to localize CaM in aleurone protoplasts. Monoclonal antibodies to tubulin and polyclonal antibodies to tonoplast intrinsic protein and malate synthase were used as controls. CaM was localized to the nucleus, the vacuolar membrane, and the cytosol, but was not associated with microtubules.