Hormonal Control of Lecithin Synthesis in Barley Aleurone Cells: Regulation of the CDP-Choline Pathway by Gibberellin

The draft genomes of five agriculturally important African orphan crops

BACKGROUND

The expanding world population is expected to double the worldwide demand for food by 2050. Eighty-eight percent of countries currently face a serious burden of malnutrition, especially in Africa and south and southeast Asia. About 95% of the food energy needs of humans are fulfilled by just 30 species, of which wheat, maize, and rice provide the majority of calories. Therefore, to diversify and stabilize the global food supply, enhance agricultural productivity, and tackle malnutrition, greater use of neglected or underutilized local plants (so-called orphan crops, but also including a few plants of special significance to agriculture, agroforestry, and nutrition) could be a partial solution.

RESULTS

Here, we present draft genome information for five agriculturally, biologically, medicinally, and economically important underutilized plants native to Africa: Vigna subterranea, Lablab purpureus, Faidherbia albida, Sclerocarya birrea, and Moringa oleifera. Assembled genomes range in size from 217 to 654 Mb. In V. subterranea, L. purpureus, F. albida, S. birrea, and M. oleifera, we have predicted 31,707, 20,946, 28,979, 18,937, and 18,451 protein-coding genes, respectively. By further analyzing the expansion and contraction of selected gene families, we have characterized root nodule symbiosis genes, transcription factors, and starch biosynthesis-related genes in these genomes.

CONCLUSIONS

These genome data will be useful to identify and characterize agronomically important genes and understand their modes of action, enabling genomics-based, evolutionary studies, and breeding strategies to design faster, more focused, and predictable crop improvement programs.

Heat shock-induced changes in lipid and protein metabolism in the endoplasmic reticulum of barley aleurone layers

Heat shock in barley aleurone layers induces heat shock protein synthesis and suppresses secretory protein synthesis by selectively destabilizing their mRNAs. In addition, the endoplasmic reticulum (ER) membranes upon which secretory protein mRNAs are translated become vesiculated during heat shock, leading to the hypothesis that ER dissociation and targeted mRNA destabilization are linked mechanistically. Supporting this, ER can be heat adapted, and heat-adapted ER has higher levels of fatty acid saturation in membrane phospholipids which do not vesiculate upon heat shock. Secretory protein mRNAs are also more stable in heat-adapted cells. To understand better heat shock-induced changes in ER membranes, we examined ER membrane proteins and enzymes involved in phosphatidylcholine biosynthesis and phospholipid turnover in heat-shocked aleurone cells. Heat shock significantly increased the activity of phospholipases A2 and D, and shortly thereafter significant but gradual increases in choline kinase and phosphocholine glyceride transferase activities and a sharp increase in phosphorylcholine citidyl transferase activity were observed. Only minor changes were observed in SDS-PAGE analyses of proteins from sonicated ER membranes fractionated on continuous sucrose gradients. Overall, heat shock reduced total lipid in ER membranes relative to protein, and in intact, ultracentrifuged aleurone cells examined by light and electron microscopy the ER band appeared to increase in density. The changes in phospholipid metabolism coupled with the suppression of secretory protein synthesis indicate that in addition to inducing a classic heat shock response, high temperature also induces a classic unfolded protein response in the ER of this secretory cell.

Effect of Gibberellin and Heat Shock on the Lipid Composition of Endoplasmic Reticulum in Barley Aleurone Layers

The heat-shock responses of barley (Hordeum vulgare L. cv Hi- malaya) aleurone layers incubated with or without gibberellic acid (GA3) were compared. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that heat shock blocked the synthesis and secretion of secretory proteins from GA3-treated layers but not untreated layers. This suppression of secretory protein synthesis has been correlated with changes in endoplasmic reticulum (ER) membranes (F.C. Belanger, M. R. Brodl, T.-h.D. Ho [1986] Proc Natl Acad Sci USA 83: 1354-1358; L. Sticher, A.K. Biswas, D.S. Bush, R.L. Jones [1990] Plant Physiol 92: 506-513). Our secretion data suggested that the ER membranes of aleurone layers incubated without GA3 may be more heat shock tolerant. To investigate this, the lipid profiles of membrane extracts in aleurone layers labeled with [14C]glycerol were examined. Heat shock markedly increased [14C]glycerol incorporation into phosphatidylcholine (PC), and gas chromatography revealed an increase in the amount of saturated fatty acids associated with thin layer chromatography-purified PC in GA3-treated layers. In contrast, aleurone layers incubated without GA3 at normal temperature contained PC-associated fatty acids with a greater degree of saturation than GA3-treated layers. Heat shock modestly increased the degree of fatty acid saturation in untreated aleurone layers. This same trend was noted in fatty acids isolated from ER membranes purified by continuous sucrose density centrifugation. We propose that increased fatty acid saturation may help sustain ER membrane function in heat-shocked aleurone layers incubated in the absence of GA3.

Alternative pathways for phosphatidylcholine synthesis in olive (Olea europaea L.) callus cultures

Olive (Olea europaea L.) callus cultures were incubated with [2-14C]ethanolamine and [Me-14C]choline in order to study phospholipid synthesis. Radioactivity from [Me-14C]choline was shown to be incorporated into the phosphatidylcholine via the CDP-base pathway. [2-14C]Ethanolamine was primarily incorporated into phosphatidylethanolamine, but significant radio-activity was also detected in phosphatidylcholine, indicating the operation of a methylation route. Incubation with [2-14C]ethanolamine indicated that phosphatidylcholine and phosphatidylethanolamine incorporated radiolabel over a similar time course. This led us to investigate the possibility that phosphatidylcholine was being synthesized by a methylation pathway distinct from the direct methylation of phosphatidylethanolamine. There was extensive incorporation of [2-14C]ethanolamine into different components of the aqueous phase of the incubations, within which phospho-base derivatives of ethanolamine were prominent. These intermediates were identified and provided evidence for the operation of an alternative methylation pathway via phosphodimethylethanolamine for the biosynthesis of phosphatidylcholine in olives.

Warm growth temperatures decrease soybean cholinephosphotransferase activity

The activity of cytidine 5'-diphosphate (CDP) choline: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) in developing soybean (Glycine max L. var Williams 82) seeds was 3 to 5 times higher in cotyledons grown at 20 degrees C than in those grown at 35 degrees C. Some characteristics of the enzyme from cotyledons cultured at 20 and 35 degrees C were compared. In preparations from both growth temperatures, the enzyme showed a pH optimum of 7, K(m) of 7.0 micromolar for CDP-choline, and an optimum assay temperature of 45 degrees C. Both enzyme preparations were stimulated by increasing concentrations of Mg(2+) or Mn(2+), up to 10 millimolar and 50 micromolar, respectively, though Mn(2+) produced lower activities than Mg(2+). Enzymes from both 20 and 35 degrees C show the same specificity for exogenous diacylglycerol. No metabolic effectors were detected by addition of heat treated extracts to the assay mixture. The above findings suggest that the higher enzyme activity at 20 degrees C can be attributed to a higher level of the enzyme rather than to the involvement of isozymes or metabolic effectors. Enzyme activity decreased rapidly during culture at 35 degrees C, indicating a rapid turnover of the enzyme. The level of temperature modulation was found to be a function of seed developmental stage.

Hormonal regulation of alpha-amylase expression in barley aleurone layers : the effects of gibberellic Acid removal and abscisic Acid and phaseic Acid treatments

The expression of alpha-amylase isozymes in barley (Hordeum vulgare L.) aleurone layers is known to be maximally induced between 12 and 20 hours after addition of the phytohormone, gibberellic acid (GA(3)). Addition of another hormone, abscisic acid (ABA), or its metabolite, phaseic acid, during this time period resulted in reduced alpha-amylase expression. Expression of the high isoelectric point alpha-amylase isozyme group was affected much more by both of these treatments than was expression of the low isoelectric point alpha-amylase isozyme group. Addition of either the translation inhibitor cycloheximide or the transcription inhibitor cordycepin prevented the decrease in alpha-amylase mRNA levels after ABA treatment. Cordycepin also prevented the decreases in alpha-amylase expression that result from phaseic acid treatment. Midcourse GA(3) removal experiments were performed to determine whether ABA treatment and the removal of GA(3) have analogous effects on alpha-amylase expression. It was found that cordycepin treatment also prevented decreases in alpha-amylase mRNA levels resulting from GA(3) removal. We conclude that the suppression of alpha-amylase expression caused by ABA or midcourse GA(3) removal is dependent on continuous RNA and protein synthesis.

Does gibberellic Acid induce the transfer of lipase from protein bodies to lipid bodies in barley aleurone cells?

We have examined the effect of gibberellic acid (GA(3)) on the distribution of the enzyme responsible for mobilizing storage triacylglycerol in aleurone cells of Hordeum vulgare L. cv Himalaya. Using cellular fractionation techniques, we find that, in cells that have not been exposed to hormone, neutral lipase activity is principally associated with a pellet containing the membranes of protein bodies. If the cells are exposed to GA(3) for at least 1 hour, the majority of the lipase activity becomes associated with the lipid body fraction. The nature of the in vivo association between lipid bodies and protein bodies was examined using ultrarapid freezing followed by freeze-fracture electron microscopy. Our analysis indicates that the phospholipid monolayer surrounding the lipid body is directly continuous with the outer leaflet of the bilayer surrounding the protein body. Based on our data, we propose that lipase can be transferred from protein bodies (storage form) to lipid bodies (active form) by lateral diffusion within the plane of the fused phospholipid monolayer, and that the transfer can be controlled by gibberellic acid by an unknown mechanism.

Gibberellin-Induced Changes in the Populations of Translatable mRNAs and Accumulated Polypeptides in Dwarfs of Maize and Pea

Two-dimensional gel electrophoresis was used to characterize the molecular mechanism of gibberellin-induced stem elongation in maize and pea. Dwarf mutants of maize (d-5) and pea (Progress No. 9) lack endogenous gibberellin (GA(1)) but become phenotypically normal with exogenous applications of this hormone. Sections from either etiolated maize or green pea seedlings were incubated in the presence of [(35)S] methionine for 3 hours with or without gibberellin. Labeled proteins from soluble and particulate fractions were analyzed by two-dimensional gel electrophoresis and specific changes in the patterns of protein synthesis were observed upon treatment with gibberellin. Polyadenylated mRNAs from etiolated or green maize shoots and green pea epicotyls treated or not with gibberellin (a 0.5 to 16 hour time course) were assayed by translation in a rabbit reticulocyte extract and separation of products by two-dimensional gel electrophoresis. Both increases and decreases in the levels of specific polypeptides were seen for pea and corn, and these changes were observed within 30 minutes of treatment with gibberellin. Together, these data indicate that gibberellin induces changes in the expression of a subset of gene products within elongating dwarfs. This may be due to changes in transcription rate, mRNA stability, or increased efficiency of translation of certain mRNAs.

Hormonal regulation of phosphatidylcholine synthesis in plants. The inhibition of cytidylyltransferase activity by indol-3-ylacetic acid

Indol-3-ylacetic acid stimulated stem elongation within 1 h of treatment of Pisum sativum L. cv. Feltham First stem sections. This elongation was accompanied by an increase in the endogenous level of phosphocholine and a decrease in that of CDP-choline. Measurements in vitro of the CDP-base pathway enzymes showed an increase in choline phosphotransferase and a decrease in cytidylyltransferase activity on hormone treatment. These results indicate that the decrease in phosphatidylcholine labelling from [14C]choline that is observed on indol-3-ylacetic acid treatment of pea stem sections is caused by the decrease in cytidylyltransferase activity.

Gibberellic acid-stimulated alpha-amylase secretion and phospholipid metabolism in wheat aleurone tissue

1. Turnovers of [14C]glycerol-labelled phospholipids in wheat aleurone tissue have been measured by using a pulse-decay technique. The most metabolically active phospholipids were phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine and phosphatidylglycerol. 2. Gibberellic acid action on the tissue led to breakdown of phosphatidylcholine and stimulated turnover of the other phosphatides concomitant with the secretion of alpha-amylase from the tissue. After pulse-labelling of the aleurone tissue with [14C]glycerol, radioactivity was lost from the phospholipids and appeared quantitatively in triacylglycerols, suggesting a stoichiometric metabolism of the former into the latter. Although 1,2-diacylglycerol is an expected intermediate in such a conversion, the patterns of radioactivity in diacylglycerols gave no indication of this. 3. Several aspects of the response of aleurone tissue to gibberellic acid resemble the responses of exocytotic animal tissues to external agonists. In particular, our results and previous reports in the literature suggest that endomembrane flow, exocytosis, phosphatidylinositol turnover and a requirement of Ca2+ for enzyme secretion are common to both plant and animal systems. Although considerable differences also exist between the two, the similarities are sufficient to warrant further consideration that plants and animals might have conserved a similar hormone response-secretion mechanism.

Gibberellic acid and abscisic acid modulate protein synthesis and mRNA levels in barley aleurone layers

Using in vivo pulse labeling, changes in the pattern of protein synthesis were detected in isolated barley aleurone layers treated with fibberellic acid (GA3). GA3 greatly altered the relative rates of synthesis of many polypeptides, increasing some, notably α-amylase, and decreasing others. α-Amylase synthesis increased until it was the major product (over 60%) of protein synthesis after 24h. The pulse-labeled pattern of secreted polypeptides was also changed by GA3. There was the expected increase in α-amylase together with a number of other polypeptides but there was reduced secretion of several polypeptides also.Cell-free translation of RNA isolated from control and hormone-treated tissues was used to measure changes in mRNA levels. GA3 caused many changes, particularly in the level of mRNA for α-amylase. In vitro synthesized α-amylase, identified by immunoaffinity chromatography, had an Mr of 46 000. This polypeptide was partially processed to a polypeptide with Mr 44 000 by the addition of dog pancreas membranes to the in vivo translation mixture. The level of mRNA for α-amylase began to increase 2-4 h after GA3 was added and reached a maximum level of about 20% of total mRNA after 16 h. Thus after 16 h, the synthesis of α-amylase as a proportion of total protein synthesis, continued to increase while the level of its mRNA as a proportion of total mRNA remained constant. These results indicate that protein synthesis was modified more extensively than we can account for by changes in mRNA.Abscisic acid (ABA) reversed all of the effects of GA3 on protein synthesis and mRNA levels. It also promoted synthesis of a small number of new polypeptides and increased the level of some mRNAs. GA3 reversed the accumulation of ABA-promoted mRNAs. Although, ABA strongly suppressed the increase in the level of translatable mRNA for α-amylase, there was an even stronger inhibition of enzyme synthesis and accumulation.We conclude that both GA3 and ABA regulate protein synthesis both positively and negatively in aleurone cells largely by regulating levels of mRNA and in the case of α-amylase, possibly also by changing the efficiency of translation of its mRNA.

The action of exogenous gibberellic acid on protein and mRNA in germinating castor bean seeds

Gibberellic acid (GA3) stimulates water uptake in castor beans and increases the activity of certain enzymes associated with lipid mobilisation.The effect of the GA3 on the enzymes is possibly due to a general effect of the growth substance on protein synthesis. Gibberellic acid advanced the appearance of rRNA and poly (A(+))RNA in castor bean endosperms without specifically stimulating the synthesis of particular mRNA species. Thus these increased levels of mRNA and rRNA may act synergistically to affect the rate of a predetermined pattern of protein synthesis.

The action of exogenous gibberellic acid on isocitrate lyase -mRNA in germinating castor bean seeds

Gibberellic acid (GA3) stimulates isocitrate lyase activity of the endosperm during germination of castor bean seeds. Isocitrate lyase from castor bean was purified and an antibody to it was prepared from rabbit serum. This antibody was used to measure the amounts of isocitrate lyase-mRNA using an in vitro translation system. No specific stimulation of isocitrate lyase-mRNA by application of GA3 was detected. The stimulation of isocitrate lyase activity by exogenous GA3 may be accounted for by the action of the growth substance in advancing the overall production of rRNA and mRNA which accelerates the rate of total protein synthesis during germination. The application of Amo 1618 retards the production of isocitrate lyase activity but also retards protein synthesis in general. This suggests that endogenous gibberellins also act non-specifically in the regulation of protein synthesis during castor bean germination.

Endoplasmic Reticulum Formation during Germination of Wheat Seeds : A QUANTITATIVE ELECTRON MICROSCOPE STUDY

This study demonstrates germination-induced ultrastructural changes in wheat (Triticum aestivum L. cv Arthur) aleurone cells. Seeds imbided for 4 hours in water contained endoplasmic reticulum (ER) or ER-like membranes as vesicles or as short segments of membrane associated with the spherosomes on the periphery of aleurone grains. Aleurone cells incubated between 8 and 10 hours contained abundant ER membranes mainly associated with the nuclear envelope and, to a lesser extent, with the spherosomes surrounding the aleurone grain. The membranes located on the periphery of the nucleus occurred as regions of stacked cisternae. When aleurone cells were analyzed by morphometry, the increase in ER during incubation was found to be greater than 2-fold. During the same incubation period, other organelles did not change significantly. The early increase in ER was not affected by gibberellin incubation. Thus, the rapid proliferation of ER observed during the early stages of germination in aleurone cells of wheat is not likely to be controlled directly by gibberellin.

The isolation of endoplasmic reticulum from barley aleurone layers

Techniques for the isolation and purification of endoplasmic reticulum (ER) from aleurone layers of barley (Hordeum vulgare L.) were assessed. Neither differential centrifugation nor density gradient centrifugation of a homogenate separate the ER or other organelles of this tissue from the lipidcontaining spherosomes. Isopycnic sucrose gradient centrifugation of organelles first purified by molecular sieve chromatography on Sepharose 4B, however, results in separation of the organelles based on their differing buoyant densities. Manipulation of the magnesium concentration of the isolation media and density-gradient solutions affords isolation of ER at a density of 1.13-1.14 g cc(-1) and 1.17-1.18 g cc(-1). Electron microscopy shows that the membranes sedimenting at 1.13-1.14 g cc(-1) are devoid of ribosomes and are characteristic of smooth ER, while those sedimenting at 1.17-1.18 g cc(-1) are studded with ribosomes and have the features of rough ER. Endoplasmic reticulum isolated by isopycnic density gradient centrifugation can be further purified by rate-zonal centrifugation.

Endoplasmic Reticulum of Mung Bean Cotyledons: ACCUMULATION DURING SEED MATURATION AND CATABOLISM DURING SEEDLING GROWTH

Homogenates of mung bean cotyledons were subjected to equilibrium density centrifugation on linear sucrose gradients and the positions of the various organelles determined by assay of marker enzymes. Measurement of phospholipid distribution on such gradients showed that the major peak of phospholipid at a density of 1.11 to 1.13 grams per cubic centimeter coincided with the position of the endoplasmic reticulum (ER), confirming ultrastructural evidence that storage parenchyma cells are rich in ER. Germination and seedling growth were accompanied by a rapid decline in ER-associated phospholipid but a marked increase in the ER marker enzyme NADH cytochrome c reductase. Similar experiments with developing seeds indicated that the amount of ER-associated phospholipid increases during cotyledon expansion reaching a maximum during seed maturation. There was no subsequent decline during seed desiccation, instead ER-associated phospholipid levels were maintained in the dry seed until germination when catabolism was initiated 12 to 24 hours after the start of imbibition. This timing indicates that the observed ER breakdown is not an expression of the overall senescence of the cotyledons, but may represent the dismantling of the extensive rough ER used for reserve protein synthesis during cotyledon development.

Effects of gibberellic acid and of tunicamycin on glycosyl-transferase activities and on alpha-amylase secretion in barley

A crude membrane fraction was prepared from isolated aleurone layers, the secretory tissue of barley grains. The layers were pre-incubated in the presence or absence of the phytohormone gibberellic acid. The membranes catalyzed the transfer of [14C]mannose from GDP-[14C]mannose and of N-[14C]acetylglucosamine from UDP-N-[14C]acetylglucosamine to endogenous and exogenous dolichyl monophosphate. When gibberellic acid was present during the pretreatment the activity of the transferases was increased by a factor of two to three. A significantly increased activity was observable within four hours after the addition of gibberellic acid, whereas the gibberellic-acid-induced secretion of the glycoprotein alpha-amylase started only after 12 h. Tunicamycin inhibited the secretion of alpha-amylase by 60 to 80%. Intracellularly, however, no alpha-amylase was found to accumulate. On the other hand, tunicamycin did not inhibit the rate of total protein synthesis by more than 10%. The possibility is discussed that the synthesis of the protein portion of glycoproteins is specifically inhibited, when glycosylation is prevented.

Rapid degradation and limited synthesis of phospholipids in the cotyledons of mung bean seedlings

Seedling growth of mung bean is accompanied by the rapid catabolism of the three major phospholipids in the cotyledons (phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol). The decline starts 24 hours after the beginning of imbibition and by the 4th day of growth more than 50% of the phospholipids have been catabolized. Extracts of cotyledons of 24-hour-imbibed beans contain enzymes capable of degrading membrane-associated phospholipids in vitro. This degradation involves phospholipase D and phosphatase activity.Studies with radioactive acetate, glycerol, and orthophosphate indicate that the three major phospholipids are also synthesized in the cotyledons. Incorporation of glycerol and acetate into phospholipids of cotyledons is relatively constant throughout seedling growth, while the incorporation of [(32)P]orthophosphate steadily declines from a high value 24 hours after the start of imbibition. The newly synthesized phospholipids become associated with membranous organelles, especially the endoplasmic reticulum, and have an in situ half-life of 2 to 2.5 days.Determination of the activities of two enzymes involved in phospholipid biosynthesis (phosphorylcholine-glyceride transferase and CDP-diglyceride-inositol transferase) shows that the enzymes have their highest activities 12 hours after the start of imbibition. High activities for both enzymes were found in cotyledons of beans incubated at 1 C, indicating that the enzymes may preexist in the dry seeds.The experiments demonstrate that cotyledons start synthesizing new phospholipids immediately after imbibition, but that the rate of phospholipid catabolism far exceeds the rate of synthesis long before the cotyledons start to senesce.

The mode of secretion of α-amylase in barley aleurone layers

The involvement of the endomenbrane system of barley (Hordeum vulgare L.) aleurone cells in the secretion of gibberellin-induced hydrolases has been investigated at the biochemical level. Our results show that at least 40-60% of the α-amylase activity in homogenates of aleurone layers occurs in a membrane-bound, latent form. The latent α-amylase can be assayed quantitatively following disruption of membranes by treatment with Triton X-100, ethanol, sonication, or osmotic shock and shear. The association of α-amylase with the membrane is not an artifact arising from homogenization of the tissue, and acid protease is also enriched in the same subcellular fraction as the α-amylase. The membrane fraction with which the α-amylase is associated has many properties of the endoplasmic reticulum (ER). When membranebound α-amylase is prepared in buffers containing 3 mM MgCl2 two fractions from a sucrose step gradient contain most of the α-amylase activity. These fractions are enriched in the ER marker enzyme, NADH-dependent cytochrome-c reductase, and show densities characteristic of smooth and rough ER during subsequent purification on continuous gradients. In step gradients prepared with ethylenediaminete-traacetic-acid-treated membranes, α-amylase activity is contained primarily in one fraction having the density of smooth ER. Electron microscopy of the purified fractions is consistent with α-amylase being associated with smooth and rough ER. However, it has not been ruled out that the enzyme is also associated with plasma membrane, Golgi membranes, or tonoplast. Examination of the isoenzyme patterns of secreted, of total-homogenate and of membrane-associated α-amylases, as well as the results from pulsechase experiments using L-[(3)H]leucine for labeling of α-amylase, are all consistent with the hypothesis that membrane-associated α-amylase is an intermediate in the secretory process.

Development and intracellular localization of lipase activity in rapessed (Brassica napus L.) cotyledons

In homogenates of resting rapeseeds no lipase activity (glycerolester hydrolase, EC 3.1.1.3) could be detected using a titrimetric assay procedure. Following a 30-h lag-phase after imbibition, lipase activity increased sharply, reaching its maximum at day 4 after sowing. Simultaneously triglyceride content of the cotyledons decreased sharply. At any time during the 11-day period of seedling growth examined, only an alkaline lipase activity with a pH optimum around 9 was present. White light had essentially no effect on the development of lipase activity. However, the disappearance of lipase activity from the cotyledons after fat utilization was found to depend on nitrogen nutrition of the seedlings. The activities of the glyoxysomal enzymes catalase and malate synthetase showed the usual rise and fall patterns with peak activities at day 4 after sowing, independently of the mineral nutrition of the seedlings.About 90% of the lipase activity was associated with a microsomal membrane fraction. Resolution of this fraction by sucrose density gradient centrifugation (62,000 g for 14 h) yielded three distinct membrane fractions. Maximum activities of membrane marker enzymes were recovered from the gradients at following densities: The major portion of microsomal protein and lipase activity at 1.085 kg/l; microsomal malate synthetase and phosphorylcholineglyceride transferase at 1.116 kg/l; NADH-cytochrome c reductase and phosphorylcholinecytidyl transferase at 1.133 kg/l. Evidently in rapeseed cotyledons lipase activity is associated only with a discrete microsomal membrane fraction which sediments differently from membrane fractions of the endoplasmic reticulum.

Gibberellic Acid enhancement of DNA turnover in barley aleurone cells

When imbibed, deembryonated halfseeds from barley (Hordeum vulgare L., var. Himalaya) are incubated in buffer, the DNA content of the aleurone layer increases 25 to 40% over a 24-hour period. In contrast, the DNA of isolated aleurone layers declines by 20% over the same time period. Gibberellic acid (GA) causes a reduction in DNA levels in both halfseed aleurone layers and isolated aleurone layers. GA also increases the specific radioactivity of [(3)H]thymidine-labeled halfseed aleurone layer DNA during the first 12 hours of treatment. Pulse-chase studies demonstrated that the newly synthesized DNA is metabolically labile.The buoyant density on CsCl density gradients of hormone-treated aleurone DNA is identical with that of DNA extracted from whole seedlings. After density-labeling halfseed DNA with 5-bromodeoxyuridine, a bimodal absorption profile is obtained in neutral CsCl. The light band (1.70 g/ml) corresponds to unsubstituted DNA, while the heavy band (1.725-1.74 g/ml) corresponds to a hybrid density-labeled species. GA increases the relative amount of the heavy (hybrid) peak in halfseed aleurone layer DNA, further suggesting that the hormone enhances semiconservative replication in halfseeds.DNA methylation was also demonstrated. Over 60% of the radioactivity from [(3)H-Me]methionine is incorporated into 5-methylcytosine. GA has no effect on the percentage distribution of label among the bases.It was concluded that GA enhances the rate of DNA degradation and DNA synthesis (turnover) in halfseeds, but primarily DNA degradation in isolated aleurone layers. Incorporation by isolated aleurone layers is due to DNA repair. Semiconservative replication apparently plays no physiological role in the hormone response, since both isolated aleurone layers and gamma-irradiated halfseeds respond normally. The hypothesis was advanced that endoreduplication and DNA degradation are means by which the seed stores and mobilizes deoxyribonucleotides for the embryo during germination.

Dependence of Wound-induced Respiration in Potato Slices on the Time-restricted Actinomycin-sensitive Biosynthesis of Phospholipid

Actinomycin D prevents the full development in a 24-hour period of both wound respiration and cyanide resistance only when given in the first 10 to 12 hours following the cutting of potato tuber (Solanum tuberosum var. Russet) slices. The capacity for choline incorporation into phosphatidylcholine increases with slice aging and is inhibited by actinomycin D in the same time-restricted way. The time-restricted effectiveness of actinomycin D applies to the cutting-elicited enhanced synthesis of three critical enzymes of phosphatidylcholine synthesis, namely phosphorylcholine-glyceride transferase, phosphorylcholine-cytidyl transferase, and phosphatidylphosphatase. By contrast, actinomycim D given at any time is without effect on the measurable levels after 24 hours of a selection of glycolytic and mitochondrial respiratory enzymes. Neither succinic dehydrogenase nor cytochrome oxidase activity increases with time in aging potato slices in the presence or absence of chloramphenicol. The foregoing observations emphasize the central role of phospholipid, and ultimately membrane biosynthesis, in the development of wound-induced respiration.

Phospholipid-synthesizing Enzymes Associated with Golgi Dictyosomes from Pea Tissue

Golgi dictyosomal membranes isolated from pea (Pisum sativum) stem tissue, using a combination of rate zonal and isopycnic sucrose density centrifugation, were shown to bear cytidine diphosphate-choline:diglyceride phosphorylcholinetransferase, CDP-ethanolamine:diglyceride phosphorylethanolaminetransferase, and CTP:phosphorylcholine cytidyltransferase activities. Although the majority of the activity of the phospholipid-synthesizing enzymes was associated with the endoplasmic reticulum, the activity found in the Golgi system was about 25% of the total activity. These results suggest that Golgi dictyosomes probably synthesize at least part of the membrane phospholipids that they may need for their secretory function and for dictyosomal proliferation during cell growth, rather than importing this material entirely from the endoplasmic reticulum.

Early responses to gibberellic acid in a dwarf maize mutant (Zea mays L. d 1)

Early effects of gibberellic acid (GA3) (1-4 h treatment) on the ion ratios in a dwarf maize mutant (Zea mays L. d 1) showing normal growth after hormone treatment, have been investigated by electron microprobe analysis. GA3 exerts a different effect on the ion ratios in plastids, cytoplasm and vacuoles in short term experiments. The Cl content of chloroplasts and cytoplasm increases without a lag phase after GA3 treatment. The K content of plastids increases after a lag phase of 2 h, whereas in the cytoplasm an increase can be observed immediately after GA3 addition. The hormone has only little influence on the Ca content of the cell compartments investigated. Control experiments with water and the physiologically inactive GA3 methylester confirm the specifity of the short-term actions of GA3 on the ion ratios. The primary action of GA3 at the membrane level is discussed.

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 alpha-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 alpha-amylase activity after 12 hours of gibberellic acid can be decreased by abscisic acid. The accumulation of alpha-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 alpha-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 alpha-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 alpha-amylase mRNA.

Phospholipid synthesis and exchange in castor bean endosperm homogenates

Crude organelle preparations from castor bean (Ricinus communis L.) endosperm rapidly incorporate CDP-((14)C)choline and CDP-((14)C)-ethanolamine into phosphatidylcholine and phosphatidylethanolamine, respectively. Separation of organelles by sucrose density gradient centrifugation following incubation with these substrates demonstrated that most of the (14)C phospholipids thus formed were present in the endoplasmic reticulum membranes, although label was also found in mitochondria, proplastids, and glyoxysomes. The phospholipid-synthesizing enzymes, cholinephosphotransferase and ethanolaminephosphotransferase, are exclusively confined to the endoplasmic reticulum membrane fraction, suggesting that the appearance of (14)C-phospholipid in other organelles was due to phospholipid exchange. Phospholipid synthesis was inhibited by the cytoplasmic supernatant fraction. The active inhibitor in this fraction was not identified, but the inhibition was not significantly relieved by either dialyzing or boiling the supernatant. Phosphatidylcholine synthesis showed an absolute requirement for Mg(2+); the Michaelis constant was 1 mm. Ca(2+) was a potent inhibitor of Mg(2+)-stimulated phospholipid synthesis and enhanced the decay of (14)C-phospholipids from pre-labeled membranes, particularly when the membranes were resuspended in the cytoplasmic supernatant.The data are consistent with the concept that the endoplasmic reticulum is a major site of membrane proliferation where structural lipids, and possibly proteins, are inserted into, and thus expand, a pre-existing membrane fraction. Other organelle and cellular membranes could therefore originate from the proliferating endoplasmic reticulum by a process of membrane flow and differentiation.

Effects of abscisic acid on in vitro growth of cotton fiber

Abscisic acid (ABA) inhibits in vitro growth of cotton (Gossypium hirsutum L.) fiber and is effective only when applied during the first four days of culture started on the day of anthesis. Abscisic acid causes a small increase in potassium uptake by the ovules and also enhances leakage of potassium from them. During their period of rapid growth, fibers produced by ABA-treated ovules have a higher potassium content and a lower malate content as compared to fibers on untreated control ovules. Results are discussed in the light of earlier reports on the in vitro growth of cotton fiber and effects of abscisic acid on other plant tissues. It is suggested that ABA inhibits fiber growth, in part, by interfering with malate metabolism.

Occurrence of some enzymes in starchy endosperm and hormonal regulation of isoperoxidase in aleurone of wheat

Peroxidase, indoleacetic acid-oxidase, alkaline, and acid phosphatases were detected in dry starchy endosperm (minus aleurone) of wheat grain. The isoperoxidase pattern differed in different parts of the dry grain. Several new isoperoxidases were found in embryos after soaking. The intensity of isoperoxidases in aleurones was enhanced in the presence of embryo or 2 muM GA(3) after 24 hours of soaking, but decreased after 72 hours. Indoleacetic acid and kinetin had no effect on isoperoxidase of aleurone. Actinomycin D and cycloheximide had no effect on isoperoxidases of aleurones from embryonectomized or naturally occurring embryoless grains. However, these two inhibitors increased the intensity of isoperoxidases in aleurones of intact embryonated grains after soaking.

On the mechanism of light induction of plant microsomal cinnamic acid 4-hydroxylase

The specific activities of the enzymes cinnamic acid 4-hydroxylase and cytidine-5'-diphospho-choline: 1,2-diacylglycerol cholinephosphotransferase (E.C. 2.7.8.2) were determined in microsomal fractions from a) dark-grown and b) irradiated parsley (Petroselinum hortense Hoffm.) cell suspension cultures. Cinnamic acid 4-hydroxylase activity was increased 4-fold after light treatment, while cholinephosphotransferase activity was decreased by about one third of its original activity. It can be concluded that increased microsomal lecithin biosynthesis is not required for the light-induced increase of cinnamic acid 4-hydroxylase activity, although the latter enzyme has previously been found to require a lipid cofactor for activity.

Studies on Phospholipid-synthesizing Enzyme Activities during the Growth of Etiolated Cucumber Cotyledons

The enzymatic incorporation of sn-glycerol 3-phosphate into lipid by extracts of cucumber (Cucumis sativus) cotyledons showed an absolute requirement for ATP (saturation 2 mM). The incorporation was stimulated 4-fold by 0.2 mM oleate. Ethyldiaminetetraacetate stimulated the incorporation at concentrations below 1 mM and inhibited at higher concentrations. Mg(2+) did not affect the reaction. Triton X-100 and Cutscum inhibited the reaction, while a third detergent, Span 80, was stimulatory. p-Mercuribenzoate was inhibitory. The enzymatic reaction has a pH optimum in the range of 8.8 to 9.6. The Michaelis constant was 112 muM for sn-glycerol 3-phosphate. The major amount of product was phosphatidic acid, the remainder was diacylglycerol, monoacylglycerol, and an unknown phospholipid.The activity profiles for two glyoxysmal enzymes, malate synthetase and catalase, were compared to the activities of four enzymes involved in phospholipid synthesis. Phosphatidylcholine and phosphatidylethanolamine synthesis paralleled the activity profiles of catalase and malate synthetase, as well as the levels of endogenous diglycerides. sn-Glycerol 3-phosphate incorporation peaked at a later stage of cucumber cotyledon growth than the glyoxysomal enzymes and seemed to be the major pathway of phosphatidic acid synthesis. Diglyceride phosphokinase activity did not reach appreciable levels during the first 11 days of cucumber cotyledon growth.

Some effects of applied gibberellic Acid on the synthesis and degradation of lipids in isolated barley aleurone layers

An analysis of the lipids in isolated barley (Hordeum vulgare L.) aleurone layers after 12 hours incubation in the presence or absence of gibberellic acid showed no quantitative or qualitative changes. Longer incubation periods resulted in some lipid degradation which was greater in the presence of 1 mum gibberellic acid.Glycerolipid synthesis was measured in isolated barley aleurone layers during the first 12 hours of incubation in the presence or absence of gibberellic acid by following the incorporation of (3)H-glycerol. No significant effect of the hormone was found on either the incorporation of glycerol into lipids or on the types of lipid being synthesized.

An early response to gibberellic Acid not requiring protein synthesis

Cell-free extracts from gibberellic acid-treated barley (Hordeum vulgare L. cv. Himalaya) aleurone layers show phosphorylcholine glyceride transferase activity greater than that from control layers. The increase in activity is not prevented by a mixture of amino acid analogs nor by cordycepin under conditions in which it is demonstrated that the analogs and the cordycepin are entering the cells in effective concentrations. We conclude therefore that the GA(3)-dependent increase in phosphorylcholine glyceride transferase activity (which occurs within the first 4 hours of GA(3) treatment) does not require RNA synthesis or protein synthesis.

Hormonal control of messenger ribonucleic acid metabolism in barley aleurone layers

Ribonucleic acid containing poly(adenylic acid) [poly(A)-RNA] is present in barley aleurone layers. This poly(A)-RNA becomes labeled with radioactive precursors of RNA during the incubation of isolated aleurone layers with or without gibberellic acid. However, the rate of synthesis of poly(A)-RNA is enhanced by gibberellic acid. This enhancement begins within 3-4 hr of addition of the hormone and reaches a maximum, which is about 50-60% over the control, 10-12 hr after addition of the hormone. Cordycepin inhibits total RNA as well as poly(A)-RNA synthesis in barley aleurone layers. However, cordycepin inhibits the hormone-controlled synthesis of alpha-amylase (EC 3.2.1.1) only if it is added 12 hr or less after gibberellic acid. The insensitivity of alpha-amylase production to cordycepin after 12 hr of gibberellic acid treatment suggests that alpha-amylase is translated from stable messenger RNA.

Studies on the release of barley aleurone cell proteins: Kinetics of labelling

Protein release from gibberellic acid-treated aleurone cells of barley (Hordeum vulgare L.) was followed in pulse-chase experiments with radioactively labelled amino acids. After a 10-min pulse of [(3)H]leucine or [(3)H]tryptophan, label was incorporated into trichloroacetic-acid (TCA)-insoluble material; some of this was released into the incubation medium during a chase with carrier amino acid. This relase of TCA-insoluble material into the incubation medium had no appreciable lag period. Precipitation with rabbit-anti-α-amylase antibody of the radioactivity released from aleurone layers into the medium during chasing indicates that as much as 70% of the radioactivity present is α-amylase. Aleurone cell homogenates were fractionated by differential centrifugation after pulsing with labelled amino acids. Radioactivity in TCA-insoluble materials was distributed equally among all sediment fractions indicating that no specific accumulation of label occurred. Tissue was also fractionated after labelling with a pulse of [(3)H]tyrosine and [(14)C]-tryptophan, and the distribution of radioactivity in various fractions also showed that no preferential sedimentation of label occurred. Altogether, no experimental evidence could be found to support the hypothesis that proteins are released from aleurone cells via discrete secretory organelles.

Biosynthesis of phosphatidylethanolamine by enzyme preparations from plant tissues

The enzymic utilization of cytidine diphosphoethanolamine in the synthesis of phosphatidylethanolamine is localized in the microsomal fraction of spinach (Spinacia oleracea) leaves. The metal ion requirement can be satisfied by Mn(2+) (saturation approximately 0.6 mm) or Mg(2+) (saturation approximately 25 mm). The enzyme has a pH optimum of 8.0 in the presence of Mn(2+) and 7.5 in the presence of Mg(2+). A Michaelis constant of 20 mum was determined for cytidinediphos-phoethanolamine. Enzyme activity was stimulated by thiol compounds and inhibited by thiol reagents. No inhibition was obtained with cytidine monophosphate and Tween 80.The in vitro biosynthesis of phosphatidylethanolamine was inhibited by cytidine diphosphocholine and the biosynthesis of phosphatidylcholine was inhibited by cytidine diphosphoethanolamine. Activities of the two synthetic systems were indistinguishable on the basis of susceptibility to lyophilization and inhibition by thiol reagents.

Regulation of pentosan biosynthesis in barley aleurone tissue by gibberellic acid

Treatment of isolated barley aleurone layers with gibberellic acid (GA3) resulted in a progressive inhibition of cell-wall synthesis after a 4-h lag period. The incorporation of both [(14)C]arabinose and [(14)C]glucose into the cell wall was inhibited by GA3, but analysis of the labelled sugars in the polymerized product showed that the process most affected by the hormone treatment was pentosan biosynthesis. Labelling kinetics and pulse-chase analysis indicated that the pentosans were synthesized in the cytoplasm and subsequently transferred to the cell wall; GA3 did not significantly affect the latter step. The GA3-inhibited labelling of the cell-wall pentosans cannot be explained on the basis of an effect on uptake of radioactive cell-wall precursor, expansion of the free pentose pool, or degradation of newly-formed pentosan. GA3 inhibited the activity of a membrane-bound arabinosyl transferase present in the aleurone layers. This inhibition may explain the inhibition of cell-wall pentosan synthesis by GA3.

Osmotic regulation of alpha-amylase synthesis and polyribosome formation in aleurone cells of barley

Water stress inhibits the gibberellic acid (GA(3))-induced synthesis of alpha-amylase in aleurone layers of barley (Hordeum vulgare L.). Electron microscope evidence indicates that the effect of water stress induced by 0.6 M solutions of polyethylene glycol (PEG) is to reduce the binding of ribosomes to the endoplasmic reticulum. This was confirmed by sucrose density gradient centrifugation of polyribosome preparations from stressed cells. The reduction in polyribosome formation does not result from reduced ribosome activity as measured by [(3)H]peptidylpuromycin formation. Thus, calculation of percent active ribosomes shows that osmoticum has little effect on the ability of ribosomes to incorporate puromycin into nascent protein. Water stress does not cause a marked decrease in the total RNA level of aleurone cells. Estimates of total RNA in postmitochondrial supernatant fractions from stressed cells show only a reduction of 8-9% relative to the control. Membrane synthesis measured by [(14)C]choline incorporation is depressed by 15% in cells stressed with 0.6 M PEG for 2.5 hours.

Cytochemical localization and antigenicity of α-amylase in barley aleurone tissue

Gibberellic-acid(GA3)-induced α-amylase has been localised in barley aleurone layers using cytochemical methods and light microscopy. Evidence obtained from the use of a starch substrate film method as well as immunofluorescence indicated that the first amylase to appear in the cell was associated with aleurone grains, apparently with the outer membrane, and also with the peripheral cytoplasm. In GA3-treated tissue, the amylase distribution was much more diffuse, although patchy, throughout the cytoplasm and it tended to accumulate in the endosperm side of the cell. The possibility that the aleurone grain membrane is the site of gibberellin-induced enzyme synthesis and that it proliferates to become rough endoplasmic reticulum is considered. Immunological information was obtained which supports earlier indications that induced α-amylase consists of two different proteins, each with molecular heterogeneity.

Hormonal control of orthophosphate incorporation into phospholipids of barley aleurone layers

Gibberellic acid added to isolated barley aleurone layers enhances orthophosphate incorporation into chloroform-methanol-soluble compounds. The effect is measurable at 4 to 6 hours after the addition of gibberellic acid and reaches a maximum after 8 to 12 hours. The increase in the rate of orthophosphate incorporation is 3- to 5-fold over the rate in control layers incubated without gibberellic acid.The gibberellic acid enhancement of the rate of phospholipid labeling is inhibited within 1 to 2 hours by cycloheximide, 6-methylpurine, and abscisic acid.The increase in labeling of phospholipids occurs throughout the subcellular fractions rather than being restricted to a specific fraction or organelle. The increase in radioactivity in phospholipids as shown by thin layer chromatography is due to a proportional increase in all phospholipids.The enhancement by gibberellic acid of the rate of phospholipid labeling may be required for the subsequent production of gibberellic acid-induced hydrolases.

Enzymes of phospholipid metabolism in the endoplasmic reticulum of castor bean endosperm

The intracellular location of several enzymes concerned with phospholipid metabolism was investigated by examining their distribution in organelles separated on sucrose gradients from total homogenates of castor bean (Ricinus communis var. Hale) endosperm. The enzymes phosphatidic acid phosphatase, CDP-diglyceride-inositol transferase, and phosphatidyletha-nolamine-l-serine phosphatidyl transferase were all primarily or exclusively confined to membranes of the endoplasmic reticulum. These results and those reported previously on lecithin synthesis establish a major role of the endoplasmic reticulum in phospholipid and membrane synthesis in plant tissues.

Endoplasmic reticulum as the site of lecithin formation in castor bean endosperm

The properties of a discrete membranous fraction isolated on sucrose gradients from castor bean endosperm have been examined. This fraction was previously shown to be the exclusive site of phosphorylcholine-glyceride transferase. The distribution of NADPH-cytochrome c reductase and antimycin insensitive NADH-cytochrome c reductase across the gradient followed closely that of the phosphorylcholine-glyceride transferase. This fraction also had NADH diaphorase activity and contained cytochromes b(5) and P 450. On sucrose gradients containing 1 mM EDTA this fraction had a mean isopycnic density of 1.12 g/cm(3) and sedimented separately from the ribosomes; electron micrographs showed that it was comprised of smooth membranes. When magnesium was included in the gradients to prevent the dissociation of membrane-bound ribosomes, the isopycnic density of the membrane fraction with its associated enzymes was increased to 1.16 g/cm(3) and under these conditions the electron micrographs showed that the membranes had the typical appearance of rough endoplasmic reticulum. Together these data show that the endoplasmic reticulum is the exclusive site of lecithin formation in the castor bean endosperm and establish a central role for this cytoplasmic component in the biogenesis of cell membranes.

Regulation of beta-Glucan Synthetase Activity by Auxin in Pea Stem Tissue: I. Kinetic Aspects

Treatment of pea stem segments with indoleacetic acid (IAA) causes within 1 hour a 2- to 4-fold increase in activity of particulate uridine diphosphoglucose-dependent beta-glucan synthetase obtainable from the tissue. The IAA effect is observable in tissue from all parts of the elongation zone of the pea stem, and also in older tissue that is not capable of a cell enlargement response to IAA. A large increase in activity is caused by IAA only if synthetase activity in the isolated tissue has first been allowed to fall substantially below the intact plant level, and only if sucrose is supplied along with IAA. Treatment of tissue with sucrose alone after a period of sugar starvation causes a transient rise of synthetase activity. The decline in synthetase activity in absence of IAA, the rise caused by IAA, and the transient rise caused by sucrose are all strongly temperature-dependent. IAA and sucrose do not affect the activity of isolated synthetase particles. Synthetase activity in vivo is sensitive to as low as 0.1 mum IAA and is increased by IAA analogues that are active as auxins on elongation but not by nonauxin analogues. Activity begins to rise 10 to 15 minutes after exposure to IAA, which places this among the most rapid enzyme effects of a plant growth regulator heretofore demonstrated, and among the most rapid known metabolic effects of auxins. The effect is seen also with polysaccharide synthetase activity using uridine diphosphate-galactose or uridine diphosphate-xylose as substrates, and to a lesser extent with guanosine diphosphoglucose-dependent glucan synthetase activity. Glucan synthetase from IAA-treated tissue appears to have a higher affinity for uridine diphosphate-glucose than the control.

Regulation of beta-Glucan Synthetase Activity by Auxin in Pea Stem Tissue: II. Metabolic Requirements

The 2- to 4-fold rise in particle-bound beta-glucan synthetase (uridine diphosphate-glucose: beta-1, 4-glucan glucosyltransferase) activity that can be induced by indoleacetic acid in pea stem tissue is not prevented by concentrations of actinomycin D or cycloheximide that inhibit growth and macromolecule synthesis. The rise is concluded to be a hormonally induced activation of previously existing, reversibly deactivated enzyme. The activation is not a direct allosteric effect of indoleacetic acid or sugars. It is blocked by inhibitors of energy metabolism, by 2-deoxyglucose, and by high osmolarity, but not by Ca(2+) at concentrations that inhibit auxin-induced elongation and prevent promotion of sugar uptake by indoleacetic acid, and not by alpha, alpha'-dipyridyl at concentrations that inhibit formation of hydroxyproline. Regulation of the system could be due either to an ATP-dependent activating reaction affecting this enzyme, or to changes in levels of a primer or a lipid cofactor.

Effect of gibberellic Acid and actinomycin d on the formation and distribution of rough endoplasmic reticulum in barley aleurone cells

Analysis of structural changes in barley aleurone cells during germination or following incubation of isolated layers in gibberellic acid with or without actinomycin D revealed extensive development of rough endoplasmic reticulum. Following the assembly of stacked rough endoplasmic reticulum, vesiculation occurred mainly in basal regions of the cell, resulting in a polar distribution of rough endoplasmic reticulum vesicles. It is postulated that these vesicles are involved in protein secretion, because smooth vesicles, derived from the rough endoplasmic reticulum, apparently become appressed to the plasma membrane. The increased alpha-amylase in the ambient medium and in cell homogenates correlated directly with formation and subsequent vesiculation of the rough endoplasmic reticulum. Furthermore, when cells were treated with actinomycin D and gibberellic acid, alpha-amylase synthesis was inhibited by 45% and secretion by 63%. These cells were characterized cytologically by large areas of disarrayed segments of fragmented rough endoplasmic reticulum, corresponding to a high intracellular level of alpha-amylase. In addition, small lipid bodies common to the segmented regions of rough endoplasmic reticulum were surrounded by fine fibrous material, short segments of rough endoplasmic reticulum, and free ribosomes, suggesting that actinomycin D had interfered with development and organization of rough endoplasmic reticulum.