TRANSFER CELLS IN THE PLACENTAL PAD AND CARYOPSIS COAT OF PAPPOPHORUM SUBBULBOSUM ARECH. (POACEAE)

During caryopsis development the layers of the pericarp, integuments, and nucellus all contribute to the formation of the caryopsis coat. The coat consists of a layer of outer pericarp epidermal transfer cells, a collapsed and senescent layer of middle pericarp cells, and a discontinuous layer of inner pericarp epidermal transfer cells. The latter is not present across the placental pad. The integuments are present as a collapsed dense layer, the nucellus is discontinuous and cellular. The placental pad occurs at the ventral surface of the caryopsis, opposite the scutellum and coleorhiza. It consists of 15–20 collapsed cell layers, including the pigment strand and placental vascular bundle. From the inside several partially collapsed cell layers of the nucellar projection occur which contain transfer-cell walls. The middle dense layers, the pigment strand, consist of the middle pericarp remnant, plus the remains of the placental vascular bundle. The pericarp inner epidermis does not extend across the pad. The aleurone layer is a continuous uniseriate layer around the entire caryopsis except at the placental pad; here it is crushed and contains the remnant of a transfer-cell wall. The outer pericarp epidermis is a continuous layer of transfer cells across the pad. These cells contain membranous inclusions suggesting that they may be functional during germination.     

ULTRASTRUCTURE OF THE MATURE UNGERMINATED RICE (ORYZA SATIVA) CARYOPSIS. THE CARYOPSIS COAT AND THE ALEURONE CELLS

The results of a light and electron microscopic study of the caryopsis coat and aleurone cells in ungerminated, unimbibed rice (Oryza sativa) caryopses are presented. Surrounding the rice grain is the caryopsis coat composed of the pericarp, seed coat and nucellar layers. The outermost layer, the pericarp, consists of crushed cells and is about 10 μm thick. The seed coat, interior to the pericarp, is one cell thick and has a thick cuticle. Between the seed coat cuticle and endosperm are the remains of the nucellus. The nucellus is about 2.5 μm thick and has a thick cuticle adjacent to the seed coat cuticle. Interior to the caryopsis coat is the aleurone layer of the endosperm. The aleurone completely surrounds the rice grain and is composed of two cell types—aleurone cells that surround the starchy endosperm and modified aleurone cells that surround the germ. The aleurone cells of the starchy endosperm contain many aleurone grains and lipid bodies around a centrally located nucleus. The modified aleurone cells lack aleurone grains, have fewer lipid bodies than the other aleurone cells, and contain filament bundles (fibrils). Plastids of aleurone cells exhibit a unique morphology in which the outer membranes invaginate to form tubules and vesicles within the plastid. Transfer aleurone cells are not observed in the mature rice caryopsis.     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.     

利用原子吸收光谱法和电镜能谱仪法分析重金属Pb、Cd在水稻颖果不同部位中的分配

比较研究了传统原子吸收光谱法与电镜能谱仪法分析重金属Pb、Cd在水稻籽粒不同部位中的分配情况,研究结果表明:两种方法均显示,Pb、Cd在水稻颖果中的分配为胚>糊粉层>果皮>近糊粉层处胚乳>颖果中心,其中,胚、糊粉层和果皮Pb、Cd含量差异较小,整个胚乳Pb、Cd含量变化也不大,但胚、糊粉层和果皮Pb、Cd含量显著高于胚乳.扫描能谱仪能够准确反映重金属在植物不同部位分布的差异,结果与原子吸收光谱法无显著差异.扫描能谱仪法的优点在于操作方便快捷,但是只能用于定性,不能定量测定出某一部位重金属的准确含量,因此,不能代替传统的原子吸收光谱法.     

Changes in cell wall polysaccharides of germinating barley grains

Decorticated barley grains were germinated at 25° for 6 days, until the endosperm reserves were nearly exhausted. The neutral monosaccharide components of the hydrolysates of the cell walls and gums from the embryo, aleurone layer and starchy endosperm and the endospermic starch were determined at daily intervals. The amount of embryo cell wall polysaccharide increased 40 times and glucose became the major component, followed in abundance by xylose and arabinose. The cell wall and gum polysaccharides of the aleurone layer (plus testa) and the starchy endosperm declined during germination and their compositions altered. The endospermic starch also decreased. In the early stages of germination the apparent composition of the cell walls of the aleurone layer and starchy endosperm depended upon how they had been prepared. After 6 days the cell walls and gums had provided a significant carbohydrate supply to the living tissues, equivalent to 18.5% of the endospermic polysaccharide degraded during growth, starch having provided the remaining 81.5%.     

Sink anatomy in relation to solute movement in rice (Oryza sativa L.): A summary of findings

The results of studies on assimilate and water transport in the developing caryopsis of rice are summarised. Evidence is presented for a symplastic movement of solutes as far as the aleurone layer. However, transport into the apoplast at the nucellus/aleurone interface appears to be a necessary step due to the absence of plasmodesmata at this site. It is suggested that water leaves the caryopsis during grain filling by the isolated cell walls of the pigment strand, the suberised walls of these cells functioning to isolate the apoplast from the symplast and thereby allowing opposing fluxes of water and assimilates to occur in the dorsal region of the grain.     

Studies on the Germination of Cereals: 3. The Effect of the Covering Layers on the Uptake of Water by the Embryo of the Wheat Grain

The uptake of water by imbibition, vacuolation, and growth ofthe embryo, has been determined when wheat grains were placedunder germination conditions before maturation was completed;and the effect of the covering layers has been investigatedin a red and a white variety at three stages during ripening. The water lost during ripening was replace by imbibition withoutcausing the embryo to expand sufficiently to rupture the coveringlayers. The cells of the coleorhiza and epiblast absorbed waterby vacuolation as soon as they were imbibed; but afurther periodelapsed before the scutellum became imbibed and water uptakewas associated with an increase in embryo dry weight. In both varieties, uptake opf water by the imbibed embryo wasprevented by the inner layer of the pericarp and the tesa inunripe grains, and by the epidermal layer of the pericarp duringthe early staes of ripening. But after the final stage of rapiddesiccation the covering layers of the white grains were rupturedwhen the embryo absorbed water by vacuolation; whereas the coveringlayers of the red grains remained intact and the uptake of waterwas delayed until a sufficient period had elapsed for nutrientsfrom the endosperm to be available for growth ofr the embryo. The varietal difference in germinationj is therefore attributedto the greater strength of the covering layers of the red grains,which are not disrupted during ripening, and prevent expansionof the embryo until the water absorbing capacity has been increasedby the transfer of reserves from the endosperm.     

Water uptake and distribution in non-dormant and dormant wild oat (Avena fatua L.) caryopses

The influence of seed coat modification and light quality onwater uptake and distribution in caryopses of dormant and non-dormantlines of wild oat (Avena fatua L.) was determined using NMRmicroimaging. Non-dormant seeds absorbed water more rapidlythan dormant seeds during imbibition on distilled water. Thiseffect was detected first in the embryo-scutellar region (8h) and later in the proximal endosperm (12 h). Cutting the testaand pericarp close to the embryo or scarification with KOH promotedrapid embryo/scutellum hydration and germination. Cutting atthe middle part of the caryopsis did not enhance embryo hydrationnor did it greatly improve germination. The sensitivity of waterdistribution to the phytochrome germination effect was examined.Significant differences in imbibitional water uptake by embryos-scutellumtissue were detected by 18 h following red-light (germinationpromoter) compared with far-red (germination inhibitor) treatment.The results indicated that both the rate and the sequence ofembryo/scutellum hydration were important in initiating germinationin dormant seeds. A refinement of the model that describes waterimbibition in wild oat seeds during the early stages of germinationis discussed. Key words: Water uptake, water distribution, Avena fatua, seed coat modification, light quality, dormant and non-dormant seeds     

The structure and function of the scutellum of the Grarnineae

NEGBI, M., 1984. The structure and function of the scutellum of the Gramineae. Four kinds of scutella, of which only the first is universally known, can be distinguished in the Gramineae. (1) The scutellum sew stricto , the kind most commonly described in textbooks. In this scutellum the only growth activity during germination is the development of every epithelial cell into a separate elongated papilla. These papillae are involved in secretion of hydrolases, gibberellins and other hormonal factors which in their turn activate the aleurone layer; and in absorption of the mobilized endosperm reserves. (2) The kind characteristic of Auma is found in several genera. In this the scutellar tip elongates during germination, reaches the distal end of the endosperm sac and develops papillae over its whole surface. (3) The kind found in Cizuniu in which the scutellar tip elongates and extends to the far end of the caryopsis during embryo development, but not during germination. In this scutellum only the abaxial surface faces the bulk of the storage endosperm and probably only this surface becomes papillate. Several bamboo genera have the kind of scutellum characterized by Melocannu . This scutellum has evolved as a storage organ and in mature caryopses the endosperm is reduced. This kind is associated with vivipary and with the presence of storage tissue in the pericarp.
The vascularization and the structure of the scutellar epithelium, as studied mainly in a limited number of species belonging to the first kind, are related to the functions of the scutellum. The scutellum has a prime role in controlling the mobilization of endosperm reserves.     

Transfer aleurone cells inSetaria lutescens (Gramineae)

Summary Typical aleurone cells occur around the periphery of the caryopsis. These cells are tabular with moderately thick walls and lack cell wall ingrowths. Transfer aleurone cells only occur adjacent to the placental vascular bundle, which supplies the developing embryo and endosperm. These specialized aleurone cells are approximately columnar, with thick walls bearing ingrowths on the outer radial and outer tangential walls. The wall ingrowths of transfer aleurone cells appear similar to those of transfer cells previously described and quite likely also function in short-distance transport of substances.Journal paper No. J-6737 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1685.     

Tissue-specific localization of aspartic proteinase in developing and germinating barley grains

Resting seeds of several plant species, including barley grains, have been reported to contain aspartic proteinase (EC 3.4.23) activity. Here, the expression of the Hordeum vulgare L. aspartic proteinase (HvAP) was studied in developing and germinating grains by activity measurements as well as by immunocytochemical and in-situ hybridization techniques. Southern blotting suggests the presence of one to two HvAP-encoding genes in the barley genome, while Northern analysis reveals a single 2.1-kb mRNA in grains and vegetative tissues. Western blotting with antibodies to HvAP shows the same subunit structure in different grain parts. In developing grains, HvAP is produced in the embryo, aleurone layer, testa and pericarp, but in the starchy endosperm HvAP is present only in the crushed and depleted area adjacent to the scutellum. During seed maturation, HvAP-encoding mRNA remains in the aleurone layer and in the embryo, but the enzyme disappears from the aleurone cells. The enzyme, however, remains in the degenerating tissues of the testa and pericarp as well as in resting embryo and scutellum. During the first three days of germination, the enzyme reappears in the aleurone layer cells but is not secreted into the starchy endosperm. The HvAP is also expressed in the flowers, stem, leaves, and roots of barley. The wide localization of HvAP in diverse tissues suggests that it may have several functions appropriate to the needs of different tissues.Abbreviations DAA days after anthesis - DTT dithiothreitol - HvAP Hordeum vulgare aspartic proteinase Both authors have contributed equally to this workWe thank Mart Saarma, Pia Runeberg-Roos, Alan Schulman and Yrjö Helariutta for helpful discussions during the study, Tiina Arna and Sari Makkonen for their help in proteinase activity experiments as well as Jaana Korhonen (Department of Pathology, University of Helsinki), Salla Marttila and Ilkka Porali (Department of Biology, University of Jyväskylä, Jyväskylä, Finland) for their advice on microscopical techniques. We also thank Liisa Pyhälä and Leena Liesirova for the production of the antibodies to HvAP at the National Public Health Institute, Helsinki. This study was supported by grants from the Ministry of Agriculture and Forestry and the Academy of Finland.     

ANATOMY OF THE CARYOPSIS OF BRIZA MAXIMA (POACEAE)

Briza maxima (quaking grass) is a cosmopolitan grass common to Europe and North and South America. It grows in disturbed soils and on roadsides. The hemispherical caryopsis is embedded between a leaflike lemma and flattened palea. The embryo is of the festucoid type. The scutellum shows two surrounding ridges at the edge of the scutellum/endosperm boundary, and has lateral lobes. A broad epiblast extends toward the embryo apex and is continuous with the dorsal surface of the coleorhiza. The single-layered aleurone surrounds the starchy endosperm and is discontinuous around the embryo. The caryopsis coat is thin, except at the placental pad where it is thickened by the pigment strand and the nucellar projection.     

Spatial and temporal regulation of DNA fragmentation in the aleurone of germinating barley

During germination of barley grains, the appearance of DNA fragmentation started in aleurone cells near the embryo and extended to the distal end in a time-dependent manner. DNA fragmentation was demonstrated to occur only after the expression of -amylase mRNA in the aleurone layer. In addition, cell wall degradation started in cells near the embryo on the sides facing the endosperm. Subsequently cell wall degradation extended to the lateral cell walls and to cells more to the distal end of the grain. A typical alteration of the nucleus was observed by electron microscopy and an almost complete degradation of DNA was found in the nucleus while the nuclear envelope remained intact. The results indicate that programmed cell death occurred in aleurone cells during germination. A model is proposed for the regulation of programmed cell death in aleurone cells during germination involving ABA levels and cell wall degradation.     

A structural study of germination in celery (Apium graveolens L.) seed with emphasis on endosperm breakdown

Germination of celery seed occurred after 6 d of imbibition in light. During this time the embryo enlarged at the expense of the adjacent endosperm cells and at the time of germination was 2–3 times as long as in the dry seed. Breakdown of the endosperm cells near the root cap preceeded radicle emergence. None of these changes occurred in darkness.Endosperm digestion began adjacent to the embryo and spread radially. In degrading cells, the aleurone grains often became larger and fewer in number. The cell walls were modified and appeared to undergo partial degradation. Ultimately the cells seemed to lose their contents. In cells adjacent to the root cap, similar changes occurred except there was a transient appearance of starch grains. Radial progression of endosperm breakdown also occurred in isolated endosperm treated with gibberellin A₄₊₇.The results indicate that (1) the stimulus for breakdown of celery endosperm emanates from the embryo in response to light; (2) the stimulus may be a gibberellin because changes in endosperm cells and the sequence of endosperm digestion during germination resemble the responses of isolated endosperm to gibberellin; and (3) the radial progression of endosperm breakdown during germination may be the result of a sequential response of cells to a uniformly applied stimulus rather than the result of gradual embryo expansion.     

Development of aleurone and sub-aleurone layers in maize

Electron-microscope studies indicate that the aleurone tissue of maize (Zea mays L.) starts developing approximately 10–15 days after pollination in stocks that take ca. 40 days for the aleurone to mature completely. Development commences when specialized endosperm cells adjacent to the maternal nucellar layer start to differentiate. Differentiation is characterized by the formation of aleurone protein bodies and spherosomes. The protein bodies of the aleurone layer have a vacuolar origin whereas the protein bodies of the immediate underlying endosperm cells appear to develop from protrusions of the rough endoplasmic reticulum. Thus, two morphologically and developmentally distinct types of protein bodies are present in these adjacent tissues. The spherosomes of the aleurone layer form early in the development of this tissue and increase in number as the tissue matures. During the final stages of maturation, these spherosomes become closely apposed to the aleurone grains and the plasma membrane. No further changes are apparent in the structure of the aleurone cells after 40 days from pollination when the caryopsis begins to desiccate.     

Proteomes of the barley aleurone layer: A model system for plant signalling and protein secretion

The cereal aleurone layer is of major importance due to its nutritional properties as well as its central role in seed germination and industrial malting. Cereal seed germination involves mobilisation of storage reserves in the starchy endosperm to support seedling growth. In response to gibberellic acid produced by the embryo, the aleurone layer synthesises hydrolases that are secreted to the endosperm for the degradation of storage products. The barley aleurone layer can be separated from the other seed tissues and maintained in culture, allowing the study of the effect of added signalling molecules in an isolated system. These properties have led to its use as a model system for the study of plant signalling and germination. More recently, proteome analysis of the aleurone layer has provided new insight into this unique tissue including identification of plasma membrane proteins and targeted analysis of germination-related changes and the thioredoxin system. Here, analysis of intracellular and secreted proteomes reveals features of the aleurone layer system that makes it promising for investigations of plant protein secretion mechanisms.     

Models for gibberellic acid transport and enzyme production and transport in the aleurone layer of barley

Gibberellins are growth hormones produced in the embryo of grain released during germination. They promote growth through the production of enzymes in the aleurone layer surrounding the endosperm. These enzymes then diffuse into the endosperm and produce the sugars required by the growing acrospire. Here we model the transport of gibberellins into and along the aleurone layer, the consequent production of enzymes, and their transport into the endosperm. Simple approximate solutions of the governing equations are obtained which suggest that the enzymes are released immediately behind a gibberellin front which travels with almost constant speed along the aleurone layer. The model also suggests that this propagation speed is determined primarily by conditions near the scutellum-aleurone junction, which may enable the embryo to actively control the germination process.     

Posttranslational regulation of pyruvate, orthophosphate dikinase in developing rice (Oryza sativa) seeds

Pyruvate, orthophosphate dikinase (PPDK; E.C.2.7.9.1) is most well known as a photosynthetic enzyme in C₄ plants. The enzyme is also ubiquitous in C₃ plant tissues, although a precise non-photosynthetic C₃ function(s) is yet to be validated, owing largely to its low abundance in most C₃ organs. The single C₃ organ type where PPDK is in high abundance, and, therefore, where its function is most amenable to elucidation, are the developing seeds of graminaceous cereals. In this report, we suggest a non-photosynthetic function for C₃ PPDK by characterizing its abundance and posttranslational regulation in developing Oryza sativa (rice) seeds. Using primarily an immunoblot-based approach, we show that PPDK is a massively expressed protein during the early syncitial-endosperm/-cellularization stage of seed development. As seed development progresses from this early stage, the enzyme undergoes a rapid, posttranslational down-regulation in activity and amount via regulatory threonyl-phosphorylation (PPDK inactivation) and protein degradation. Immunoblot analysis of separated seed tissue fractions (pericarp, embryo + aleurone, seed embryo) revealed that regulatory phosphorylation of PPDK occurs in the non-green seed embryo and green outer pericarp layer, but not in the endosperm + aleurone layer. The modestly abundant pool of inactive PPDK (phosphorylated + dephosphorylated) that was found to persist in mature rice seeds was shown to remain largely unchanged (inactive) upon seed germination, suggesting that PPDK in rice seeds function in developmental rather than in post-developmental processes. These and related observations lead us to postulate a putative function for the enzyme that aligns its PEP to pyruvate-forming reaction with biosynthetic processes that are specific to early cereal seed development.     

Water uptake and distribution in germinating tobacco seeds investigated in vivo by nuclear magnetic resonance imaging

The regulation of water uptake of germinating tobacco (Nicotiana tabacum) seeds was studied spatially and temporally by in vivo (1)H-nuclear magnetic resonance (NMR) microimaging and (1)H-magic angle spinning NMR spectroscopy. These nondestructive state-of-the-art methods showed that water distribution in the water uptake phases II and III is inhomogeneous. The micropylar seed end is the major entry point of water. The micropylar endosperm and the radicle show the highest hydration. Germination of tobacco follows a distinct pattern of events: rupture of the testa is followed by rupture of the endosperm. Abscisic acid (ABA) specifically inhibits endosperm rupture and phase III water uptake, but does not alter the spatial and temporal pattern of phase I and II water uptake. Testa rupture was associated with an increase in water uptake due to initial embryo elongation, which was not inhibited by ABA. Overexpression of beta-1,3-glucanase in the seed-covering layers of transgenic tobacco seeds did not alter the moisture sorption isotherms or the spatial pattern of water uptake during imbibition, but partially reverted the ABA inhibition of phase III water uptake and of endosperm rupture. In vivo (13)C-magic angle spinning NMR spectroscopy showed that seed oil mobilization is not inhibited by ABA. ABA therefore does not inhibit germination by preventing oil mobilization or by decreasing the water-holding capacity of the micropylar endosperm and the radicle. Our results support the proposal that different seed tissues and organs hydrate at different extents and that the micropylar endosperm region of tobacco acts as a water reservoir for the embryo.