Effects of the embryonic axis and exogenous growth regulators on sunflower cotyledon storage protein mobilization

Cotyledons of sunflower seedlings ( Helianthus annuus L. cv. Giant gray stripe) expand and their protein content first rises then begins to decrease during the first three days of growth. Storage protein structures, which are visible with scanning electron microscopy, undergo modification that leads to storage protein disappearance by day 4 post-imbibition. Expansion of cotyledons detached from seeds prior to imbibition is greatly reduced, total protein levels remain high, and storage protein structures remain visible in cells of these cotyledons. Incubation of excised cotyledons in 1.0 μM benzyladenine or kinetin increases the rates of cotyledon expansion and storage protein loss to levels higher than in intact seedling cotyledons, Incubation in 10 μM indole-3-acetic acid inhibits cotyledon expansion and protein mobilization. More rapid hydrolysis of storage proteins in cotyledons of intact seedlings or detached cotyledons treated with cytokinin is further indicated in day 2 specimens by SDS-polyacrylamide gel electrophoresis. These results suggest a possible mechanism for regulation of cotyledon development by interactions of the promotive effects of cytokinin and inhibitory effects of auxin.     

DEVELOPMENTAL CHANGES IN THE COTYLEDONS OF LUPINUS LUTEUS L. DURING AND AFTER GERMINATION

Cotyledons of Lupinus luteus were sampled from 1 to 21 days after sowing and processed for light microscopy and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Length, width, and surface area of the cotyledons increased gradually until day 10. The thickness of the cotyledons increased from day 7 to day 12 and decreased thereafter. Morphometric analyses showed that the increase in length, width, and thickness of the cotyledon was due to cell expansion, and the decrease in thickness of the cotyledon was due to the decrease in the length of abaxial cells and in the total number of cells. Mesophyll development accompanied schizogenous and lysigenous air space formation. There were two structurally distinct types of protein bodies. Protein bodies in five to six layers of cells on the abaxial side did not contain globoids, while globoids were prominent in protein bodies in the center and adaxial side. Storage protein mobilization occurred first in the abaxial side of the cotyledon and proceeded toward the adaxial side. SDS-PAGE analysis showed that proteins ranged from 97 to 14 kD. High molecular weight α- and ß-conglutinins were more abundant in the abaxial region, whereas γ-conglutinin occurred in both abaxial and adaxial regions. In addition, there were five minor bands between 97 and 43 kD unique to abaxial region and five minor bands between 43 and 14 kD unique to adaxial region in the nonreduced protein profiles. The α- and ß-conglutinins began to decrease after imbibition and disappeared by day 7 after sowing. At this stage the subunits of ribulose-l,5-bisphosphate carboxylase/oxygenase and four new minor bands appeared.     

Differences in the activity and distribution of peroxidases from three different portions of germinating Brassica oleracea seeds

Peroxidase (POD, EC 1.11.1.7) activity, cellular localization and isozyme patterns were investigated in the seed integument, cotyledon and embryo axis of Brassica oleracea cv. Cappuccio during pregermination and seedling growth. Seeds started to germinate after 24 h of imbibition. POD activity was localized in the pigmented layer of the integument and in procambial strands of the cotyledon and embryo axis in the first 24 h of imbibition. It was localized in the integumental cells of palisade, pigmented and aleurone layers and in epidermal, meristematic, procambial cells and xylem elements of the root and hypocotyl after 48 h of imbibition. POD activity increased during germination and early seedling growth: in the integument, it reached a maximum value after 72 h of imbibition, in the embryo axis and cotyledons, it increased up to 144 h of imbibition. The increase in peroxidase activity was accompanied by the appearance of new isozymes correlated with the development of seedling tissues. The isozyme profile was characterized by nine peroxidases: isoperoxidase of 50 kDa peculiar to integuments, that of 150 kDa to cotyledons and that of 82 kDa to the embryo axis. During pregerminative phase isozymes of 84 kDa were detected in the integument and cotyledons, of 48.5 kDa in the embryo axis. After germination, peroxidase activity and the complexity of the isozyme pattern increased, suggesting that they play a relevant role after rupture of the integument.     

Proteolysis in the quiescent seed

The importance of the proteolytic system of the quiescent seed in regard to mobilization of storage protein was assessed by identification of proteases already present at this stage. Extracts of quiescent cotyledons of white lupin ( Lupinus albus ) submitted to 45 h autolysis in vitro displayed a higher degree of protein degradation than that observed in vivo after two days of seed imbibition. Differences in the susceptibility to proteolysis were verified by densitometric analysis of the polypeptides after electrophoretic separation. The pH dependence of the proteolytic activities and the responses to specific protease inhibitors showed that the proteolytic systems vary from quiescent to 1 - to 3-day-imbibed cotyledons. By labelling an endogenous globulin with ¹²⁵I a sensitive radiometric assay allowed the identification of both an acidic and a neutral proteolytic system in the quiescent cotyledon. Within the quiescent seed there already exists a high potential for initiating proteolysis, so that the requirement for proteolysis by specific endopeptidases synthesized de novo upon imbibition only applies to part of the reserve proteins.     

From embryo sac to oil and protein bodies: embryo development in the model legume Medicago truncatula

? The cell and developmental biology of zygotic embryogenesis in the model legume Medicago truncatula has received little attention. We studied M. truncatula embryogenesis from embryo sac until cotyledon maturation, including oil and protein body biogenesis. ? We characterized embryo development using light and electron microscopy, measurement of protein and lipid fatty acid accumulation and by profiling the expression of key seed storage genes. ? Embryo sac development in M. truncatula is of the Polygonum type. A distinctive multicellular hypophysis and suspensor develops before the globular stage and by the early cotyledon stage, the procambium connects the developing apical meristems. In the storage parenchyma of cotyledons, ovoid oil bodies surround protein bodies and the plasma membrane. Four major lipid fatty acids accumulate as cotyledons develop, paralleling the expression of OLEOSIN and the storage protein genes, VICILIN and LEGUMIN. ? Zygotic embryogenesis in M. truncatula features the development of a distinctive multicellular hypophysis and an endopolyploid suspensor with basal transfer cell. A clear procambial connection between the apical meristems is evident and there is a characteristic arrangement of oil bodies in the cotyledons and radicle. Our data help link embryogenesis to the genetic regulation of oil and protein body biogenesis in legume seed.     

Cotyledon cells of Vigna mungo seedlings use at least two distinct autophagic machineries for degradation of starch granules and cellular components

alpha-Amylase is expressed in cotyledons of germinated Vigna mungo seeds and is responsible for the degradation of starch that is stored in the starch granule (SG). Immunocytochemical analysis of the cotyledon cells with anti-alpha-amylase antibody showed that alpha-amylase is transported to protein storage vacuole (PSV) and lytic vacuole (LV), which is converted from PSV by hydrolysis of storage proteins. To observe the insertion/degradation processes of SG into/in the inside of vacuoles, ultrastructural analyses of the cotyledon cells were conducted. The results revealed that SG is inserted into LV through autophagic function of LV and subsequently degraded by vacuolar alpha-amylase. The autophagy for SG was structurally similar to micropexophagy detected in yeast cells. In addition to the autophagic process for SG, autophagosome-mediated autophagy for cytoplasm and mitochondria was detected in the cotyledon cells. When the embryo axes were removed from seeds and the detached cotyledons were incubated, the autophagosome-mediated autophagy was observed, but the autophagic process for the degradation of SG was not detected, suggesting that these two autophagic processes were mediated by different cellular mechanisms. The two distinct autophagic processes were thought to be involved in the breakdown of SG and cell components in the cells of germinated cotyledon.     

Different functions of vicilin and legumin are reflected in the histopattern of globulin mobilization during germination of vetch (Vicia sativa L.)

The temporal and spatial patterns of storage-globulin mobilization were immunohistochemically pursued in the embryonic axis and cotyledons of vetch seed (Vicia sativa L.) during germination and early seedling growth. Embryonic axes as well as cotyledons of mature seeds contain protein bodies with stored globulins. Prevascular strands of axes and cotyledons, the radicle and epidermal layers of axis organs were nearly exclusively stained by vicilin antibodies whereas the cotyledonous storage mesophyll gave similar staining for vicilin and legumin. Globulin breakdown started locally where growth and differentiation commenced in the axis. There, vicilin mobilization preceded legumin mobilization. Thus vicilin represents the initial source of amino acids for early growth and differentiation processes in vetch. Legumin presumably only serves as a bulk amino acid source for subsequent seedling growth during postgerminative globulin degradation. During the first 2–3 d after the start of imbibition the axis was depleted of globulins whereas no decrease in immunostainability was detected in the cotyledons except in their vascular strands where immunostainability was almost completely lost at this time. Continuous vascular strands were established at the third day when globulin breakdown was finished in the axis but had just started in the cotyledon mesophyll. Protein mobilization proceeded in a small zone from the epidermis towards the vascular strands in the center of the cotyledons. In this zone the storage cells, which initially appeared densely packed with starch grains and protein bodies, concomitantly transformed into cells with a large central vacuole and only a thin cytoplasmic layer attached to the cell wall. These results agree well with the hypothesis that during the first 2 d after imbibition the axis is autonomous in amino acid provision. After the endogenous reserves of the axis are depleted and the conductive tissue has differentiated, globulins are mobilized in the cotyledons, suggesting that then the amino acid supply is taken over by the cotyledons. For comparison with other degradation patterns we used garden bean (Phaseolus vulgaris L) and rape (Brassica napus L.) as reference plants. Received: 3 August 1999 / Accepted: 11 December 1999     

Immunoanalysis of dehydrins in Araucaria angustifolia embryos

The aim of this study was to describe the dehydrin content of mature Araucaria angustifolia embryos, a species of endangered and economically important conifers, native to southern Brazil, northeastern Argentina, and eastern Paraguay. The A. angustifolia seeds have been categorized as recalcitrant. Dehydrins were studied by western blot analysis and in situ immunolocalization microscopy using antibodies raised against the K segment, a highly conserved lysine-rich 15-amino acid sequence extensively used to recognize proteins immunologically related to the dehydrin family. Western blot analysis of the heat-stable protein fraction, as estimated by 15 % SDS-PAGE, revealed three main bands of approximately 20-, 26-, and 29-kDa; when 17.5 % SDS-PAGE was used, each band resolved into two other bands. Two thermosensitive dehydrin bands of around 16 and 35 kDa were common to the axis and cotyledons, and another thermosensitive band, with molecular mass of approximately 10 kDa, was present in the cotyledons only. Following alkaline phosphatase (AP) treatment, a gel mobility shift was detected for each one of the four main bands that can be due to phosphorylation. Dehydrins were detected in all axis and cotyledon tissues using in situ immunolocalization microscopy. At the subcellular level, dehydrins were immunolocalized in the nuclei, protein bodies, and microbodies. In the nucleus, dehydrins were found to be associated with chromatin. We concluded that the gel mobility shift for the four main bands (probably due to phosphorylation), the presence of thermosensitive bands, and the specific localizations in nuclei and protein bodies provide key starting points to understand the function of dehydrins in the embryo cells of this species.     

The endoplasmic reticulum of mung-bean cotyledons: Quantitative morphology of cisternal and tubular ER during seedling growth

The ultrastructure of the endoplasmic reticulum (ER) in storage parenchyma cells in the cotyledons of mung beans (Vigna radiata L.) was examined during germination and seedling growth. Two different methods were used to visualize the ER: thin (0.08 m) sections of tissue fixed in formaldehyde and glutaraldehyde and post-fixed with osmium tetroxide, and thick (1 m) sections of tissue fixed in buffered aldehyde and post-fixed with zinc iodide-osmium tetroxide (ZIO). Changes in relative amounts of ER were quantified by morphometry (stereology).The ER occurs in two forms: a cisternal form with associated ribosomes which can be seen at all stages from imbibition to cotyledon senescence, and a tubular form which initially has associated ribosomes. Stereoscopic images of thick sections of cotyledons of 2-day-old seedlings show that the tubular ER consists of a three-dimensional array of interconnecting tubules which have numerous connections with the cisternal ER. The network of tubules and cisternae extends throughout the cytoplasm enveloping the protein bodies. Germination and seedling growth are accompanied by a reduction in the total volume occupied by the ER. This reduction is the result of a preferential loss of tubular ER and occurs largely before protein mobilization. Cisternal ER decreases during the first 48 h of imbibition and seedling growth, but storage cells subsequently show an increase in cisternal ER just prior to and during the period of protein mobilization. Cisternal ER remains conspicuous during the last phase of reserve mobilization when starch is broken down and the cells are starting autophagy.Abbreviations ER endoplasmic reticulum - ZIO zinc iodide-osmium tetroxide This is the second in a series of papers on the endoplasmic reticulum of mung bean cotyledons. The first paper is referenced herein as Gilkes and Chrispeels (1980)     

DNA, RNA, protein and heterochromatin changes during embryo development and germination of soybean (Glycine max L.)

DNA, RNA, protein and heterochromatin were measured cytophotometrically in developing soybean (Glycine max) seeds. The average 2C DNA content for the soybean genome was 2.64 pg. The amounts of nuclear DNA in embryo axes showed no significant change during embryo development, whereas the DNA content in cotyledon nuclei increased significantly from 3.58 pg to 5.49 pg. The number of endopolyploid nuclei increased from 26% to 48% and the DNA content from 4.45 to 5.49 pg after cessation of cell division. The changes in RNA and protein content during embryo development were in general similar to those in DNA content. This can be interpreted that increased DNA levels in soybean cotyledons generated during embryogeny increase the protein synthesizing capacity. During the first 15 days of germination, the number of endopolyploid nuclei in cotyledons declined from 46% to 4%, and this decline is interpreted as DNA degradation providing a ready source of nucleosides and phosphates during early embryo growth. A later decline, however, between 15 and 20 days after germination, was age related similar to leaf senescence, because the percentage of endopolyploid nuclei remained unchanged while the number of non-viable cells increased. In senescing cotyledons, 73% and 80% of RNA and protein but only 20% of DNA were lost, as compared to dormant cotyledons. The heterochromatin (condensed chromatin) measurements indicated that nuclei of metabolically inactive dormant and senescent cotyledon nuclei contained an average of 33% more heterochromatin than nuclei from the green cotyledons of seedlings.     

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.     

The potential for predation induced somatic embryogenesis in storage cotyledons

Using seven tropical rainforest species of north Queensland, Australia, we simulated partial predation and compared the development of intact embryos and embryos from which various proportions had been removed, including in some cases the original root–shoot axis. Embryos of all species contained storage reserve cotyledons and germinate in a hypogeal manner. Species mean embryo masses ranged from 0.4–48.1 g. Partial embryo predation treatments were: ½ embryo treatment; embryos separated into component cotyledons, and ¼ embryo treatment; cotyledons separated as above and subsequently cut in half. Germination was scored as production of roots and shoots, or roots or shoots only. Embryos from all species produced seedlings in all treatments, even after the removal of up to 75 percent of the cotyledonary reserve. Moreover, the proportion germinating were not different between intact embryos and i) separated cotyledons, ii) cotyledon halves that maintained or were adjacent to the embryonic axis (except for the largest seeded species), and iii) cotyledon halves that did not include the root–shoot axis (four species). Thus, production of roots and shoots, or roots or shoots only was largely independent of the presence of the embryonic root–shoot axis – implying that somatic cells in storage cotyledons are capable of differentiating into the full range of cell types typical of shoots and roots in the absence of the root–shoot axis. The generality of this response across all seven species suggests somatic embryogenesis in storage cotyledons may be a more widespread phenomenon in tropical floras than currently considered.     

Effects of NaCl, Na2SO4, and mannitol on storage lipid mobilization in the cotyledons and roots of purple alfalfa seedlings

The effects of NaCl, Na₂SO₄, and mannitol on mobilization of storage lipids in the cotyledon mesophyll and root meristem cells during the germination of purple alfalfa (Medicago sativa L.) seeds were studied using the transmission electron microscopy technique. The ultrastructural analysis showed that the number and shape of specific organelles, oleosomes, depended on the osmotic effects of a salt used. In control seedlings grown on distilled water, lipid mobilization in roots terminated by the 4th day, and in cotyledons, only by the 8th day. The disruptions in the storage lipid mobilization in the treated seedlings were specific and depended on the stress agent used, the osmotic pressure induced by this agent, as well as on the tissue type. The presence of a great number of oleosomes in the cytoplasm of cells subjected to osmotic stress, in contrast to the control ones, demonstrated the inhibition of mobilization processes. All concentrations of Na₂SO₄ and mannitol suppressed lipid mobilization in the cotyledons and roots. At the same time, NaCl markedly suppressed this process in the root cells only at a higher osmotic pressure of the solution. The results obtained suggest that the cytological analysis of cotyledon storage-lipid mobilization can tentatively assess the resistance of dicotyledonous plants to abiotic stresses.     

Histochemical and biochemical observations on storage protein metabolism and protein body autolysis in cotyledons of germinating mung beans

Storage protein hydrolysis in the cotyledons of germinating mung beans (Phaseolus aureus Roxb.) was examined by histochemical techniques, and the autolytic capacity of isolated protein bodies was studied with biochemical methods. The localization of endopeptidase activity within the cotyledons was studied using an India ink-gelatin film technique. After 24 hours of imbibition, a low level of endopeptidase activity was found throughout the storage tissues of the cotyledons. A marked increase in activity was noted in cells farthest from the vascular bundles 48 to 60 hours after the start of imbibition. The decrease in storage protein followed the same spatial distribution starting in the cells farthest from the bundles. The cotyledons contain a population of cells in various stages of endopeptidase activity enhancement and storage protein degradation. A wave of endopeptidase activity moves progressively through the cotyledons towards the vascular bundles leaving behind areas devoid of stored reserves and low in endopeptidase activity. Observations on the morphology of protein bodies during germination indicate that the membrane surrounding them remains intact, while the reserves disappear. This result suggests that the protein bodies may be undergoing autolysis. To determine whether this may indeed be the case, protein bodies were isolated from the meal of mung bean seeds using an aqueous medium containing 80% glycerol. The protein body preparations and the cytoplasm were assayed for the presence of a number of enzymes which may be involved in the breakdown of the storage proteins. The protein bodies contained all, or nearly all, of the carboxypeptidase, α-mannosidase, N-acetyl-β-glucosaminidase, and caseolytic activity. The cytoplasm contained all, or most, of the leucine aminopeptidase and the trypsin-like activity (benzoyl arginine-p-nitroanalide as substrate). Incubation of the isolated protein bodies resulted in the release of amino acids. An analysis of the products of hydrolysis indicated that very little, if any, storage protein was being hydrolyzed during the incubation. Hydrolysis of the storage proteins present in the protein bodies was greatly accelerated by the addition of extracts from the cotyledons of 4-day-old seedlings. The results suggest that new enzymic activities not present in the protein bodies isolated from dry seeds must either be activated or synthesized and possibly added to the protein bodies before storage protein breakdown can begin.     

Proteins of cotyledons of mature horse chestnut seeds

This is the first characterization of proteins from storage parenchyma of cotyledons of mature dormant recalcitrant horse chestnut (Aesculus hippocastanum L.) seeds and evaluation the cell protein-synthesizing capacity. It was established that the content of protein in cotyledons did not exceed 0.5% of tissue fresh weight. Soluble proteins (the proteins of the postmitochondrial supernatant or cytosol) comprised the bulk (up to 90%) of total proteins. Protein of subcellular structures (20000 g-pellet) comprised 5–7% of total protein. Cotyledon proteins were heterogenous in their charges and molecular weights of subunits. Cotyledon protein was easily extracted with a salt (1 M NaCl); they comprised 90% of water-soluble albumin-like proteins. The proportion of globulins was insignificant; it did not exceed 5%. Most water-soluble proteins (more than 80%) were tolerant to heat denaturing. Among these heat-stable proteins, two major groups of polypeptides dominated: an electrophoretically homogeneous component with a mol wt of 24–25 kD and a complex group from three to five polypeptides with mol wts in the range between 6 and 12 kD. Native heat-stable proteins had disulfide bonds. Four fractions of heat-stable proteins were obtained by ammonium sulfate fractionation; three of them were alike in their polypeptide composition and contained major components with mol wts of 24–25 and 5–12 kD. It was established that the active translational machinery functioned in the cells of storage parenchyma in cotyledons of mature dormant horse chestnut seeds. During each stage of stratification, cotyledon fragments incorporated ³⁵S-methionine into TCA-insoluble material more actively than axial organs. We discuss cotyledon protein composition, their function as a storage organ, and a possible role of heat-stable proteins.     

The influence of the embryonic axis and cytokinins on reserve mobilization in germinating lupin seeds

The influence of the embryonic axis and cytokinins (CKs) onreserve mobilization has been examined in yellow lupin (Lupinusluteus L. cv. JSG 6167) seed during germination and during earlygrowth for up to 9 d in the dark. The study included determinationof starch, soluble sugars, proteins, and amino acid content.Amylolytic and proteolytic enzyme activity was also measuredin untreated cotyledons with intact embryo (attached) or detachedcotyledons (embryo removed), and in detached cotyledons followingtreatment with CKs namely, dihydrozeatin, (diH)Z, and 6-benzylaminopurine,BAP. Generally, the detached cotyledons showed reduced mobilizationand decreased enzymatic activity in comparison to attached cotyledons,indicating the importance of the embryonic axis in this process.However, a rise in protease activity and free amino acid contentwas detected in 9-d-old detached cotyledons suggesting thatthe end products do not necessarily inhibit enzyme activity.While (diH)Z was partially effective in inducing reserve mobilizationand enzymatic activity in detached cotyledons, the effect ofBAP was more pronounced and appeared to replace the embryonicaxis. The embryonic axis of this species has recently been shownto synthesize CKs which are transported to the cotyledons, arehighly stabe and induce cotyledon expansion and chlorophyllsynthesis. The results of the present investigation and previousstudies from this laboratory collectively indicate that theregulation of reserve mobilization in yellow lupin seeds appearsto be mediated, at least in part, by a stimulus, probably aCK, emanating from the embryonic axis. Key words: Lupinus luteus, cytokinins, benzylaminopurine, dihydrozeatin, embryonic axis, lupin seeds, reserve mobilization     

single cotyledon (sic) mutants of pea and their significance in understanding plant embryo development

Angiosperms are divided into two distinct classes—the dicotyledons (dicots) and monocotyledons (monocots)—based in part on the number of cotyledons in mature embryos. In this paper, we describe single‐cotyledon pea mutants, termed sic (single cotyledon), all of which show a degree of fusion between the cotyledons. The fusion in sic1 is along the margin of one cotyledon and is less complete than in sic2 embryos, but the effects of the mutations are additive in the double mutant. Occasionally sic2 mutants will show fusion of the two cotyledons into one cylindrical embryo in which the shoot apex becomes surrounded by the cotyledons. Both sic1 and sic2 mutants produce fertile plants. In the sic3 embryo, a single cotyledon is generated under the shoot apex that breaks the vascular connection between root and shoot, causing embryo lethality. The pattern of cotyledon development in all these mutants is identified by in situ mRNA hybridization and antibody labeling, using the storage protein vicilin as a cotyledon‐specific marker. These patterns indicate that the joining of the cotyledons was due to zonal growth. The results indicate that there are genes in pea that influence the positioning and the morphology of the cotyledon. A model for cotyledon development in pea is proposed that is based on the regulation of the positioning of cell clusters by the sic genes. Dev. Genet. 25:11–22, 1999. © 1999 Wiley‐Liss, Inc.     

Amino Acid Utilization in Seeds of Loblolly Pine during Germination and Early Seedling Growth (I. Arginine and Arginase Activity)

The mobilization and utilization of the major storage proteins in loblolly pine (Pinus taeda L.) seeds following imbibition were investigated. Most of the seed protein reserves were contained within the megagametophyte. Breakdown of these proteins occurred primarily following radicle emergence and correlated with a substantial increase in the free amino acid pool in the seedling; the majority of this increase appeared to be the result of export from the megagametophyte. The megagametophyte was able to break down storage proteins and export free amino acids in the absence of the seedling. Arginine (Arg) was the most abundant amino acid among the principal storage proteins of the megagametophyte and was a major component of the free amino acid pools in both the seedling and the megagametophyte. The increase in free Arg coincided with a marked increase in arginase activity, mainly localized within the cotyledons and epicotyl of the seedling. Arginase activity was negligible in isolated seedlings. Experiments with phenylphosphorodiamidate, a urease inhibitor, supported the hypothesis that arginase participates in Arg metabolism in the seedling. The results of this study indicate that Arg could play an important role in the nutrition of loblolly pine during early seedling growth.     

西瓜胚和胚乳的发育

应用显微技术对西瓜胚和胚乳的发育过程进行了观察并分析了西瓜胚珠败育的原因。西瓜胚发育属紫菀型。合子第一次分裂为不均等分裂 ,形成的基细胞体积明显较顶细胞大 ,两细胞均含有多个液泡。原胚发育过程中没有明显的胚柄。最外层的原胚细胞 ,与胚乳细胞相邻的壁上被胼胝质物质包围 ,且无外连丝存在 ;与胚囊壁相接的壁上无壁内突结构。胚的子叶体积增长的同时 ,子叶细胞内积累蛋白质和脂类物质 ,多糖物质的含量下降。胚乳发育属核型 ,在球形胚期开始自珠孔端向合点端细胞化 ,胚子叶分化出后开始自珠孔端向合点端退化。胚乳合点端在球形胚早期形成发达的胚乳吸器 ,开始呈游离核状态 ,后细胞化 ,在心型胚期之后退化。