Gibberellic acid and the fine structure of barley aleurone cells : III. Vacuolation of the Aleurone cell during the phase of ribonuclease release

Abscisic acid activation of oleosin gene HvOle3 expression prevents the coalescence of protein storage vacuoles in barley aleurone cells

Protein storage vacuoles (PSVs) in aleurone cells coalesce during germination, and this process is highly coupled with mobilization of PSV reserves, allowing de novo synthesis of various hydrolases in aleurone cells for endosperm degradation. Here we show that in barley (Hordeum vulgare L.) oleosins, the major integral proteins of oleosomes are encoded by four genes (HvOle1 to 4), and the expression of HvOle1 and HvOle3 is strongly up-regulated by abscisic acid (ABA), which shows antagonism to gibberellic acid. In aleurone cells, all HvOLEs were subcellularly targeted to the tonoplast of PSVs. Gain-of-function analyses revealed that HvOLE3 effectively delayed PSV coalescence, whereas HvOLE1 only had a moderate effect, with no notable effect of HvOLE2 and 4. With regard to longevity, HvOLE3 chiefly outperformed other HvOLEs, followed by HvOLE1. Experiments swapping the N- and C-terminal domain between HvOLE3 and other HvOLEs showed that the N-terminal region of HvOLE3 is mainly responsible, with some positive effect by the C-terminal region, for mediating the specific preventive effect of HvOLE3 on PSV coalescence. Three ACGT-core elements and the RY-motif were responsible for ABA induction of HvOle3 promoter activity. Transient expression assays using aleurone protoplasts demonstrated that transcriptional activation of the HvOle3 promoter was mediated by transcription factors HvABI3 and HvABI5, which acted downstream of protein kinase HvPKABA1.

Aquaporin activity of barley tonoplast intrinsic proteins is involved in the delay of the coalescence of protein storage vacuoles in aleurone cells

The coalescence of protein storage vacuoles (PSVs) is one of the most prominent cellular changes occurring in cereal aleurone cells during germination. This structural change is highly coupled with the functional transition of this organelle from a storage compartment to a lytic section. Gibberellic acid (GA) promotes this process, whereas abscisic acid (ABA) prevents it. Previously, we demonstrated that ABA-inducible HvTIP3;1 plays a decisive role in ABA-mediated prevention of PSV fusion. In this follow-up study, we examined whether the aquaporin activity of tonoplast intrinsic protein (TIP) is related to its preventive effect on PSV fusion using various functional mutants. The defective forms of aquaporin (HvTIP3;1m and HvTIP3;1ΔNPA-GFPs for HvTIP3;1, and HvTIP1;2m for HvTIP1;2) were found to be less effective than the usual form in delaying the PSV fusion process occurring in GA-treated cells. In contrast, overexpression of HvTIP3;1m reduced the preventive effect of ABA on PSV fusion. Upon inhibition of aquaporin activity using mercury, PSV fusion occurred to a greater extent in ABA-treated barley protoplasts. These data suggest that the aquaporin activity of TIP is involved in the deterrent effect of TIP on PSV coalescence. TIP3-GFP barley transgenic seeds showed prolonged expression of the TIP3;1 transcript. Moreover, PSV fusion progressed at a much slower rate compared to wild type. Additionally, the degradation of storage proteins was not as efficient, suggesting that a metamorphic transition of PSVs to lytic organelles is closely correlated with the disappearance of HvTIPs and the PSV fusion process.

Abscisic acid prevents the coalescence of protein storage vacuoles by upregulating expression of a tonoplast intrinsic protein gene in barley aleurone

Tonoplast intrinsic proteins (TIPs) are integral membrane proteins that are known to function in plants as aquaporins. Here, we propose another role for TIPs during the fusion of protein storage vacuoles (PSVs) in aleurone cells, a process that is promoted by gibberellic acid (GA) and prevented by abscisic acid (ABA). Studies of the expression of barley (Hordeum vulgare) TIP genes (HvTIP) showed that GA specifically decreased the abundance of HvTIP1;2 and HvTIP3;1 transcripts, while ABA strongly increased expression of HvTIP3;1. Increased or decreased expression of HvTIP3;1 interfered with the hormonal effects on vacuolation in aleurone protoplasts. HvTIP3;1 gain-of-function experiments delayed GA-induced vacuolation, whereas HvTIP3;1 loss-of-function experiments promoted vacuolation in ABA-treated aleurone cells. These results indicate that TIP plays a key role in preventing the coalescence of small PSVs in aleurone cells. Hormonal regulation of the HvTIP3;1 promoter is similar to the regulation of the endogenous gene, indicating that induction of the transcription of HvTIP3;1 by ABA is a critical factor in the prevention of PSV coalescence in response to ABA. Promoter analysis using deletions and site-directed mutagenesis of sequences identified three cis-acting elements that are responsible for ABA responsiveness in the HvTIP3;1 promoter. Promoter analysis also showed that ABA responsiveness of the HvTIP3;1 promoter is likely to occur via a unique regulatory system distinct from that involving the ABA-response promoter complexes.

Analogous reserve distribution and tissue characteristics in quinoa and grass seeds suggest convergent evolution

Quinoa seeds are highly nutritious due to the quality of their proteins and lipids and the wide range of minerals and vitamins they store. Three compartments can be distinguished within the mature seed: embryo, endosperm, and perisperm. The distribution of main storage reserves is clearly different in those areas: the embryo and endosperm store proteins, lipids, and minerals, and the perisperm stores starch. Tissues equivalent (but not homologous) to those found in grasses can be identified in quinoa, suggesting the effectiveness of this seed reserve distribution strategy; as in cells of grass starchy endosperm, the cells of the quinoa perisperm endoreduplicate, increase in size, synthesize starch, and die during development. In addition, both systems present an extra-embryonic tissue that stores proteins, lipids and minerals: in gramineae, the aleurone layer(s) of the endosperm; in quinoa, the micropylar endosperm; in both cases, the tissues are living. Moreover, the quinoa micropylar endosperm and the coleorhiza in grasses play similar roles, protecting the root in the quiescent seed and controlling dormancy during germination. This investigation is just the beginning of a broader and comparative study of the development of quinoa and grass seeds. Several questions arise from this study, such as: how are synthesis and activation of seed proteins and enzymes regulated during development and germination, what are the genes involved in these processes, and lastly, what is the genetic foundation justifying the analogy to grasses.

An abscisic acid-induced protein, HVA22, inhibits gibberellin-mediated programmed cell death in cereal aleurone cells

Plant HVA22 is a unique abscisic acid (ABA)/stress-induced protein first isolated from barley (Hordeum vulgare) aleurone cells. Its yeast homolog, Yop1p, functions in vesicular trafficking and in the endoplasmic reticulum (ER) network in vivo. To examine the roles of plant HVA22, barley HVA22 was ectopically expressed in barley aleurone cells. Overexpression of HVA22 proteins inhibited gibberellin (GA)-induced formation of large digestive vacuoles, which is an important aspect of GA-induced programmed cell death in aleurone cells. The effect of HVA22 was specific, because overexpression of green fluorescent protein or another ABA-induced protein, HVA1, did not lead to the same effect. HVA22 acts downstream of the transcription factor GAMyb, which activates programmed cell death and other GA-mediated processes. Moreover, expression of HVA22:green fluorescent protein fusion proteins showed network and punctate fluorescence patterns, which were colocalized with an ER marker, BiP:RFP, and a Golgi marker, ST:mRFP, respectively. In particular, the transmembrane domain 2 was critical for protein localization and stability. Ectopic expression of the most phylogenetically similar Arabidopsis (Arabidopsis thaliana) homolog, AtHVA22D, also resulted in the inhibition of vacuolation to a similar level as HVA22, indicating function conservation between barley HVA22 and some Arabidopsis homologs. Taken together, we show that HVA22 is an ER- and Golgi-localized protein capable of negatively regulating GA-mediated vacuolation/programmed cell death in barley aleurone cells. We propose that ABA induces the accumulation of HVA22 proteins to inhibit vesicular trafficking involved in nutrient mobilization to delay coalescence of protein storage vacuoles as part of its role in regulating seed germination and seedling growth.

The Arabidopsis aleurone layer responds to nitric oxide, gibberellin, and abscisic acid and is sufficient and necessary for seed dormancy

Seed dormancy is a common phase of the plant life cycle, and several parts of the seed can contribute to dormancy. Whole seeds, seeds lacking the testa, embryos, and isolated aleurone layers of Arabidopsis (Arabidopsis thaliana) were used in experiments designed to identify components of the Arabidopsis seed that contribute to seed dormancy and to learn more about how dormancy and germination are regulated in this species. The aleurone layer was found to be the primary determinant of seed dormancy. Embryos from dormant seeds, however, had a lesser growth potential than those from nondormant seeds. Arabidopsis aleurone cells were examined by light and electron microscopy, and cell ultrastructure was similar to that of cereal aleurone cells. Arabidopsis aleurone cells responded to nitric oxide (NO), gibberellin (GA), and abscisic acid, with NO being upstream of GA in a signaling pathway that leads to vacuolation of protein storage vacuoles and abscisic acid inhibiting vacuolation. Molecular changes that occurred in embryos and aleurone layers prior to germination were measured, and these data show that both the aleurone layer and the embryo expressed the NO-associated gene AtNOS1, but only the embryo expressed genes for the GA biosynthetic enzyme GA3 oxidase.

cPrG-HCl a potential H+/Cl- symporter prevents acidification of storage vacuoles in aleurone cells and inhibits GA-dependent hydrolysis of storage protein and phytate

The putative H+/Cl- symporter cycloprodigiosin-HCl (cPrG-HCl) was used to investigate the role of vacuole acidification in cereal aleurone cell function. The protein storage vacuole (PSV) becomes acidified rapidly when aleurone cells are treated with gibberellic acid (GA) but not abscisic acid (ABA). We show that cPrG prevents PSV acidification in aleurone layers and prevents synthesis of secretory proteins such as alpha-amylase. Our data support the hypothesis that decreased hydrolase synthesis is a consequence of decreased hydrolysis of storage proteins in PSV. Support for this hypothesis comes from experiments showing that breakdown of barley 7S globulins and phytate is inhibited by cPrG in GA-treated aleurone layers. Decreased mobilization of PSV reserves is accompanied by reductions in the free amino acid pool size and in the amount of ions released from the aleurone layer. Vacuolation of the aleurone cell is a diagnostic feature of the response to GA, and vacuolation is also inhibited by cPrG. Evidence that cPrG acts as a potential H+/Cl- symporter in aleurone is presented. We show that cPrG does not inhibit the synthesis and secretion of alpha-amylase when Cl- ions are omitted from the incubation medium. Although cPrG blocks many GA-induced responses of aleurone layers, it does not affect early steps in GA signaling. The SLN1 protein, a negative regulator of GA signaling, is turned over in GA-treated cells in the presence and absence of cPrG. Similarly, synthesis of the transcriptional activator GAMYB is unaffected by the presence of cPrG in GA-treated cells.

The protein storage vacuole: a unique compound organelle

Storage proteins are deposited into protein storage vacuoles (PSVs) during plant seed development and maturation and stably accumulate to high levels; subsequently, during germination the storage proteins are rapidly degraded to provide nutrients for use by the embryo. Here, we show that a PSV has within it a membrane-bound compartment containing crystals of phytic acid and proteins that are characteristic of a lytic vacuole. This compound organization, a vacuole within a vacuole whereby storage functions are separated from lytic functions, has not been described previously for organelles within the secretory pathway of eukaryotic cells. The partitioning of storage and lytic functions within the same vacuole may reflect the need to keep the functions separate during seed development and maturation and yet provide a ready source of digestive enzymes to initiate degradative processes early in germination.

Signalling in the cereal aleurone: hormones, reactive oxygen and cell death

The cereal aleurone is widely used as a model system to study hormonal signalling. Abscisic acid (ABA) and gibberellins (GAs) elicit distinct responses in aleurone cells, ranging from those occurring within minutes of hormone addition to those that require several hours or days to complete. Programmed cell death is an example of a response in aleurone layers that is hormonally regulated. GAs promote cell death and cells in intact aleurone layers begin to die 24 h after GA treatment, whereas cell death of aleurone protoplasts begins 4 d after GA treatment. ABA prevents aleurone cell death and addition of ABA to cells pretreated with GA can delay cell death. Aleurone cells do not follow the apoptotic route of programmed cell death. Cells treated with GA, but not ABA, develop large, acidic vacuoles containing a spectrum of hydrolases typical of lytic compartments. Enzymes that metabolize reactive oxygen species are also present in aleurone cells, but ascorbate peroxidase, catalase and superoxide dismutase become less abundant after treatment with GA; activity of these enzymes increases or remains unchanged in ABA-treated cells. We propose a model whereby reactive oxygen species accumulate in GA-treated cells and lead to peroxidation of membrane lipids and plasma membrane rupture. ABBREVIATIONS: RO, reactive oxygen species; HR, hypersensitive response; PSV, protein storage vacuole; PCD, programmed cell death; CAT, catalase; SOD, superoxide dismutase; APX, ascorbate peroxidase.

The effect of monensin on intracellular transport and secretion of α-amylase isoenzymes in barley aleurone

The effect of monensin on the secretion of α-amylase and other enzymes from the aleurone layer of barley (Hordeum vulgare L. cv. Himalaya) was studied by electrophoresis followed by fluorography and by pulse-chase and organelle-isolation experiments. Monensin markedly inhibits the secretion, but not the synthesis, of α-amylase, acid phosphatase, and at least four other proteins from the aleurone layer. Monensin treatment causes α-amylase to accumulate within the protoplast, but its effect on the different α-amylase isoenzymes is not equal. The accumulation of isoenzyme 2 is not influenced by monensin while isoenzymes 1, 3 and 4 are not secreted but rather accumulate in the cell when monensin is included in the incubation medium. The α-amylase and acid-phosphatase activities which accumulate within the aleurone cells following treatment with monensin are localized in an organelle having a buoyant density greater than that of endoplasmic reticulum and less than that of mitochondria. In pulse-chase experiments with [(35)S]methionine, labelled proteins accumulate in this organelle in the presence of monensin and do not appear in the incubation medium. We conclude that monensin inhibits the secretion of proteins from the barley aleurone layer by influencing their intracellular transport.

Structural organization of ultrarapidly frozen barley aleurone cells actively involved in protein secretion

The ultrastructural organization of actively secreting barley (Hordeum vulgare L. cv. Himalaya) aleurone cells was examined using ultrarapid-freezing (<-10 000°C s(-1)) followed by freeze-fracture and freeze-substitution. Our analysis indicates that much of the evidence supporting a direct pathway from the endoplasmic reticulum (ER) to the plasma membrane (i.e. bypassing the Golgi apparatus) for the secretion of α-amylase (EC 3.2.1.1) may not be valid. Cryofixed ER cisternae show no sign of vesiculation during active α-amylase secretion in gibberellic acid (GA3)-treated cells. At the same time, Golgi complexes are abundant and numerous small vesicles are associated with the edges of the cisternae. Vesicles appear to be involved in the delivery of secretory products to the plasma membrane since depressions containing excess membrane material appear there. Treatment with GA3 also induces changes in the composition of Golgi membranes; most notably, the density of intramembrane particles increases from 2700 μm(-2) to 3800 μm(-2) because of an increase of particles in the 3-8.5-nm size range. A slight decrease in 9-11-nm particles also occurs. These changes in membrane structure appear to occur as the Golgi complex becomes committed to the processing and packaging of secretory proteins. We suggest that secretory proteins in this tissue are synthesized in the abundant rough ER, packaged in the Golgi apparatus, and transported to the plasma membrane via Golgi-derived secretory vesicles. Mobilization of reserves is also accompanied by dynamic membrane events. Our micrographs show that the surface monolayer of the lipid bodies fuses with the outer leaflet of the bilayer of protein-body membranes during the mobilization of lipid reserves. Following the breakdown of the protein reserves, the protein bodies assume a variety of configurations.

Gibberellic-acid-responsive protoplasts from mature aleurone of Himalaya barley

Gibberellic acid (GA3)-responsive protoplasts were prepared from mature aleurone layers of Himalaya barley. Protoplasts prepared in air (air-protoplasts) synthesized α-amylase (EC 3.2.1.1) in the presence of GA3 at a rate which was 4-5 times greater that in its absence. Protoplasts prepared in nitrogen (N2-protoplasts) took longer than air-protoplasts to respond to GA3 but α-amylase synthesis ultimately attained a rate which was similar to that for air-protoplasts and which was many times that occurring in the absence of the hormone. Many characteristics of the protoplast response were similar to those of intact aleurone layers. α-Amylase arose by new synthesis, its synthesis was inhibited by abscisic acid, it was isozymically similar to aleurone layer enzyme, most of it was secreted into the incubation medium and its synthesis was accompanied by accumulation of α-amylase mRNA. GA3-induced changes in protein synthesis and cell structure also resembled those of intact aleurone cells. We conclude that the response of the protoplasts to GA3 is normal and that they present a useful system for the study of GA3 action in barley aleurone.

The role of the endoplasmic reticulum in the synthesis and transport of α-amylase in barley aleurone layers

The subcellular site of α-amylase (EC 1.6.2.1) synthesis and transport was studied in barley aleurone layers incubated in the presence or absence of gibberellic acid (GA3). Using [(35)S]methionine as a marker, the site of amino-acid incorporation into organelles isolated from aleurone layers incubated with and without GA3 was determined following purification by isopycnic sucrose-density-gradient centrifugation. Incorporation of radioactivity into trichloroacetic-acid-insoluble proteins was greatest in those fractions exhibiting activity of an endoplasmic reticulum (ER) marker enzyme. Further fractionation of densitygradient fractions by sodium-dodecyl-sulfate polyacrylamide-gel electrophoresis showed that a major portion of the radioactivity in the ER fractions was present in a protein co-migrating with marker α-amylase. This protein was identified as authentic α-amylase by immunoadsorbent chromatography and affinity chromatography. The newly synthesized α-amylase associated with the ER was shown to be sequenstered within the lumen of the ER by experiments which showed that the enzyme was resistant to proteolytic degradation. The labelled α-amylase sequestered in the ER can be chased from this organelle when tissue is incubated in unlabelled methionine following a 1-h pulse of labelled methionine. The isoenzymic forms of α-amylase found in tissue homogenates and incubation media of aleurone layers incubated with and without GA3 were characterized after chromatography on diethylaminoethyl cellulose. In homogenates of GA3-treated aleurone layers, five peaks of α-amylase activity were detected, while in homogenates of aleurone layers incubated with-out GA3 only three peaks of activity were found. In incubation media, four isoenzymes were found after GA3 treatment and two were found after incubation without GA3. We conclude that at least five α-amylase isoenzymes are synthesized by the ER of barley aleurone layers and that this membrane system is involved in the sequestration and transport of four of these isoenzymes.

Protoplasts isolated from aleurone layers of wild oat (Avena fatua L.) exhibit the classic response to gibberellic acid

Viable, long-lived, gibberellic acid (GA3)-responsive protoplasts have, for the first time, been isolated from aleurone layers of mature wild oat (Avena fatua L.) grain. More than 90% of the cells of aleurone layers are recovered as protoplasts, and these respond to treatment with GA3 in essentially the same manner as the tissue from which they were derived. Protoplasts become vacuolate during incubation in vitro and, although not dependent upon GA3, vacuolation is markedly stimulated by the hormone. Amylase and ribonuclease (RNase) are produced and secreted only in the presence of GA3 and only after lag periods of 3 d and 4 d respectively. The amounts of amylase produced and secreted are proportional to GA3 concentrations as low as 1.61·10(-13) M. With increasing concentrations of mannitol in the culture medium both vacuolation and the GA3-induced production and secretion of enzymes are inhibited progressively, the latter being precluded by 0.6 M to 0.7 M mannitol.

Quantitative and qualitative changes in the endoplasmic reticulum of barley aleurone layers

Changes in the level of the endoplasmicreticulum (ER) marker enzyme cytochrome-c reductase (EC 1.6.2.1) were followed with time of imbibition of de-embryonated half-seeds of barley (Hordeum vulgare L.) and the subsequent incubation of their aleurone layers in gibberellic acid (GA3) and H2O. During imbibition there is an increase in the level of cytochrome-c-reductase activity and in the amount of 280-nm absorbance associated with this enzyme. When aleurone layers are incubated for a further 42 h in water, there is a doubling of the cytochrome-c-reductase activity. In GA3, the activity of cytochrome-c reductase reaches a maximum at 24 h of incubation and thereafter falls to below 70% of its level at the beginning of the incubation period. Changes in the cytochrome-c-reductase activity correlate with changes in the fine structure of the aleurone cell. The ER isolated in low Mg(2+) from aleurone layers incubated in buffer for up to 18 h has buoyant density of 1.13-1.14 g cc(-1) while that from layers incubated in GA3 for 7.5-18 h has a density of 1.11-1.12 g cc(-1). The α-amylase (EC3.2.1.1) isolated with the organelle fraction by Sepharose gel filtration is associated with the ER on isopycnic and rate-zonal density gradients, and its activity can be enhanced by Triton X-100. The soluble α-amylase fraction from Separose-4B columns, on the other hand, is not Triton-activated but is acid-labile. Acid phosphatase (EC3.1.3.2) is distributed in at least three peaks on isopycnic gradients. In low Mg(2+) the second peak of activity has a density of 1.12 g cc(-1) in GA3-treated tissue and 1.13-1.14 g cc(-1) in H2O-treated tissue. With high-Mg(2+) buffers, this peak of phosphatase activity disappears. Acid-phosphatase activity is not enhanced by Triton X-100 nor is it acid-labile.

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.

Purification and properties of two ribonucleases and a nuclease from barley seeds

Three enzymes possessing RNAase activity were isolated from barley seeds. These enzymes were further purified by ammonium sulphate precipitation DEAE-cellulose chromatography, gel filtration on Sephadex G-75 and DEAE-Sephadex A-50 chromatography. These enzymes have been characterized and classified as: 1. Plant RNAase I (EC 3.1.27.1). It has a pH optimum at 5.7 and molecular weight of 19 000. 2. Plant RNAase II (EC 3.1.27.1). It has a pH optimum at 6.35 and molecular weight of 19 000. 3. Plant nuclease I (EC 3.1.30.2). It has a pH optimum at 6.8 and molecular weight of 37 000. Two RNAases were purified to homogeneity by means of affinity chromatography on poly(G)-Sepharose 4B, as shown by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate.

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.

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.

Studies on the release of barley aleurone cell proteins: Autoradiography

Both uptake and incorporation of radioactivity from [(3)H]L-leucine into gibberellic-acid (GA3)-treated aleurone layers of barley (Hordeum vulgare L.) was enhanced by pretreatment with 5 mM potassium bromate. The effect of 5 mM KBrO3 on amino-acid incorporation was quantitative rather than qualitative and could be partly reversed by the addition of neutralized casein hydrolysate at 10 mg/ml. Autoradiographs of GA3-treated aleurone cells pulsed with [(3)H]leucine showed distribution of silver grains predominantly over the endoplasmic reticulum (ER) and aleurone grains. After chasing with carrier L-leucine for 60 min, fewer silver grains were associated with the ER and aleurone grains while nearly half of the silver was associated with the ground cytoplasm of the cell. Autoradiographs were prepared from aleurone cells previously stratified by ultracentrifugation. After a 10-min pulse of label, the silver grains were found over the central ER zone of centrifuged cells; however, with an increase in duration of the chase, label was found distributed throughout the aleurone grain and spherosome region of the cell. The silver grains which were located over the central zone of centrifuged cells at the end of the pulse were almost exclusively associated with the ER. There is no evidence for association of label with dictyosomes or with vesicles derived from dictyosomes. The experimental evidence indicates that labelled amino acids are incorporated into aleurone cells on the ER and are released from these cells without the participation of a membrane-bound vesicle.

Cytochemical localization of phosphatase in barley aleurone cells: The pathway of gibberellic-acid-induced enzyme release

Acid phosphatase has been localized by cytochemical techniques in aleurone layers of dry barley (Hordeum vulgare L.) grains, in imbibed half-grains and in isolated layers treated with and without gibberellic acid (GA3). A major fraction of the enzyme activity is located in the cell walls. During imbibition and incubation of layers without GA3 a steady increase of enzyme activity in the inner wall region indicates a continued release of enzyme into the walls, but there is no essential change in the distribution of wall-enzyme sites. On the other hand, when GA3 is present enzyme activity is found for the first time in regions of the wall that become digested during GA3 treatment. These results indicate that the digested wall channels act as preferential routes through which acid phosphatase is released from the aleurone layer. No digested wall channels are formed in the absence of GA3 and, there being no route for release of the enzyme, it accumulates in the inner regions of the wall around aleurone cells. Assays of enzyme activity in vitro support the conclusions based on the histochemical data. They indicate that release of acid phosphatase from aleurone layers is under strict GA3 control, but that some of the increase in acid phosphatase activity in the isolated during incubation is not GA3 dependent.Acid phosphatase is present in the protein matrix of aleurone grains in all stages except the dry grain. Enzyme activity persists in aleurone grains throughout GA3 treatment when enlargement of the grains and mobilization of reserves takes place. It is suggested that this phosphatase hydrolyses phosphate reserves within the aleurone grains.

The structure of the lettuce endosperm

The two-cell-layered endosperm of lettuce (Lactuca sativa L.) is characterized by thick cell walls and dense cytoplasm. The periodic-acid-Schiff's(PAS)-positive cell wall forms numerous peg-like projections which extend into the cytoplasm. The dense cytoplasm contains organelles of protein and lipid storage. The protein bodies are numerous and appear to be interconnected by narrow extensions of their envelopes. Spherosomes are also numerous; they occupy a peripheral position in the cytoplasm. Other organelles typical of plant cells (nuclei with prominent nucleoli, mitochondria, microbodies, dictyosomes and various vesicles) are also found in the ground cytoplasm of the endosperm cell. Germination of the seeds began after 14 h imbibition in light, and by 24 h 35-40% of the seeds had germinated. The cell walls of endosperm from seeds germinated in light for 12-15 h were extensively broken down as shown by the decrease in PAS staining of the wall. Cell-wall breakdown increased with the duration of imbibition, with the exception of the wall adjacent to the integument which showed no evidence of digestion. The structural complexity of the endosperm cell wall is correlated with the role this tissue plays in restricting embryo growth. Cell-wall breakdown is correlated with radicle protrusion, although a causal relationship between these two events is not proved.

Fractionation of the enzymes of the barley aleurone layer: Evidence for a soluble mode of enzyme release

Aleurone cells of barley (Hordeum vulgare L.) contain microbodies as determined by histochemical localization with diaminobenzidine. These microbodies can be isolated from both water and gibberellic acid (GA3) treated cells and identified on sucrose density gradients as glyoxysomes on the basis of their buoyant densities (1.25 g cm(-3)) and their enzyme complement. Fractionation of aleurone layer homogenates by differential centrifugation after varying times of exposure to GA3, however, does not indicate the presence of a discrete secretory vesicle containing either α-amylase or β-1,3-glucanase. Cytological evidence also suggests that at least β-1,3-glucanase is not released from these cells by means of a discrete secretory vesicle.

The function of the aleurone layer during galactomannan mobilisation in germinating seeds of fenugreek (Trigonella foenum-graecum L.), crimson clover (Trifolium incarnatum L.) and lucerne (Medicago sativa L.): A correlative biochemical and ultrastructural study

The reserve endosperm galactomannans of fenugreek (Trigonella foenum-graecum L.), crimson clover (Trifolium incarnatum L.) and lucerne (Medicago sativa L.) are broken down to free galactose and mannose in dry-isolated endosperms (devoid of embryo) incubated under germination conditions. Breakdown is prevented by inhibition of protein synthesis or of oxidative phosphorylation in the aleurone layer. Resting aleurone cells contain inter alia a large number of ribosomes more or less regularly distributed in the ground plasma. At the onset of germination, before galactomannan breakdown begins, polysomes are formed and seem, at least partly, to become associated with vesicles and flat cisternae both probably newly formed and derived from ER. Concurrently with galactomannan breakdown in the reserve cells, wall corrosion occurs in the aleurone layer, the contents of the aleurone grains disappear and the rough vesicles and cisternae proliferate. Later a large central vacuole is formed which incorporates smaller vacuoles emerging from the cytoplasm, and at the same time the rough ER vesicles and cisternae become highly distended.It is concluded that the cells of the aleurone layer are responsible for the synthesis and secretion into the storage cells of the enzymes necessary for galactomannan degradation. The physiology of galactomannan breakdown is compared and contrasted with that of starch mobilisation in the endosperm of germinating cereal grains.