A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

Developing plant embryos depend on nutrition from maternal tissues via the seed coat and endosperm, but the mechanisms that supply nutrients to plant embryos have remained elusive. Sucrose, the major transport form of carbohydrate in plants, is delivered via the phloem to the maternal seed coat and then secreted from the seed coat to feed the embryo. Here, we show that seed filling in Arabidopsis thaliana requires the three sucrose transporters SWEET11, 12, and 15. SWEET11, 12, and 15 exhibit specific spatiotemporal expression patterns in developing seeds, but only a sweet11;12;15 triple mutant showed severe seed defects, which include retarded embryo development, reduced seed weight, and reduced starch and lipid content, causing a “wrinkled” seed phenotype. In sweet11;12;15 triple mutants, starch accumulated in the seed coat but not the embryo, implicating SWEET-mediated sucrose efflux in the transfer of sugars from seed coat to embryo. This cascade of sequentially expressed SWEETs provides the feeding pathway for the plant embryo, an important feature for yield potential.     

沙葱种皮特性、种胚及种子浸提液与种子休眠的关系

本文以沙葱休眠种子为试材,检测其种皮透水性、透气性、超微结构、种胚休眠及种子浸提液。结果表明,酸蚀种子吸水12h达饱和,吸水率是完整种子的2倍;其呼吸强度最大值是完整种子的1.36倍。酸蚀处理使种子表面结构破损,角质层脱落,种孔露出。沙葱种子离体胚接种培养后2～3d,种胚开始萌发,萌发率达85％。沙葱完整种子与粉碎种子的浸提液对白菜种子发芽率、苗高、根长均有抑制作用,且随着浓度的提高而增强。     

哺乳动物毛色形成机制与影响因素

哺乳动物拥有丰富多彩的毛色,对于多种多样的动物毛色,影响其形成的可能原因较多,遗传物质与环境的相互作用不可忽视。本文针对调控黑色素细胞形成及褐黑色素、真黑色素合成的相关基因,分析了哺乳动物毛色形成的可能机制,并对研究不同毛色动物的同种动物遗传差异的方法进行了讨论。     

Light, Seed Coat and Gibberellic Acid in Relation to the Amylase Activity in Germinating Scots Pine Seeds (Pinus silvestris)

The amylolytic activity in homogenates from both embryos and endosperms of Scots pine seeds depends on the presence of α-amylase. The activity of this enzyme has been studied in relation to the light factor during the first 24 hours of the germination process. No positive correlations have been found between the observed activities and earlier observed enhancement of the starch degradation in light. The endosperm activities have been shown to be affected both by the presence of an intact coat and the presence of the embryo. The embryo influence could not be restored by gibberellic acid. The results have been discussed in relation to the physiology of gibberellin and ethylene. The author wishes to express his thanks to the Swedish Natural Science Research Council for financial support and to Miss Brigitte Thierbach for skilful, technical assistance.     

Wheat Seed Carboxypeptidase and Joint Action on Gliadin of Proteases from Dry and Germinating Seeds

A carboxypeptidase preparation, homogeneous according to polyacrylamidegel electrophoresis and ultracentrifugation, was obtained fromwheat seeds. The isolation procedure included (NH₄)₂SO₄ fractionation,gel-filtration on Sepharose-6B and affinity chromatography onCABS-Sepharose. Mr of the enzyme determined by gel-filtrationwas 126 000. The enzyme consisted of two non-identical subunitsof Mr 60 000 and 63 000. The pl of the carboxypeptidase was5.7. The inhibitory analysis revealed that the isolated enzymeis a serine carboxypeptidase. The carboxypeptidase preferentiallyhydrolysed N-substituted dipeptides with aromatic amino acidresidues at the C-terminus and showed weak hydrolysing activitytowards gliadin. The combined action of carboxypeptidase andaspartic proteinase from dry wheat seeds led to an increasein the degree of proteolysis of the storage protein comparedto that resulting from the sum of the action of individual enzymes.The cysteine proteinase from germinated wheat seeds caused completedegradation both of gliadin, which had first been treated withproteases of dry seeds (aspartic proteinase plus carboxypeptidase),and of untreated gliadin. However, when gliadin had first beenhydrolysed with dry seed proteases, the rate of its proteolysiswith cysteine proteinase increased 3–4 times comparedto the non-hydrolysed gliadin. The data indicate the importanceof preliminary modification of gliadin with dry wheat proteases,which, apparently, enhances the supply of nutritive substancesto the embryo in the course of seed germination. Key words: Wheat, gliadin, carboxypeptidase, proteinases, proteolysis     

A Soap Technique for Cell Separation to Study the Seed Coat of Sesbania punicea

A technique is described for separating plant cells used for morphological studies. The plant material is placed in a concentrated solution of olive oil castile soap for 1-2 days or more. The material is then thoroughly washed and placed between two glass slides. The upper glass slide is lifted from the lower one, then gently pressed down several times. Through this procedure Malpighian cells of the seed coat of Sesbania punicea. mesophyll cells of Euphorbia peplus and of Trifolium pratense and cortical cells of the aerial roots of Monstera deliciosa have been separated. Various shapes of the Malpighian cells of the Sesbania punicea seed coat can be observed along with intermediates.     

Rapid Elimination of Low-Copy DNA Sequences in Polyploid Wheat: A Possible Mechanism for Differentiation of Homoeologous Chromosomes

To study genome evolution in allopolyploid plants, we analyzed polyploid wheats and their diploid progenitors for the occurrence of 16 low-copy chromosome- or genome-specific sequences isolated from hexaploid wheat. Based on their occurrence in the diploid species, we classified the sequences into two groups: group I, found in only one of the three diploid progenitors of hexaploid wheat, and group II, found in all three diploid progenitors. The absence of group II sequences from one genome of tetraploid wheat and from two genomes of hexaploid wheat indicates their specific elimination from these genomes at the polyploid level. Analysis of a newly synthesized amphiploid, having a genomic constitution analogous to that of hexaploid wheat, revealed a pattern of sequence elimination similar to the one found in hexaploid wheat. Apparently, speciation through allopolyploidy is accompanied by a rapid, nonrandom elimination of specific, low-copy, probably noncoding DNA sequences at the early stages of allopolyploidization, resulting in further divergence of homoeologous chromosomes (partially homologous chromosomes of different genomes carrying the same order of gene loci). We suggest that such genomic changes may provide the physical basis for the diploid-like meiotic behavior of polyploid wheat.     

'Callose' in the Impermeable Seed Coat of Sesbania punicea

Dry legumes of Sesbania punicea (Cav.) Benth. contain impermeableseeds which are either beige-greenish or reddish. When keptin water for over one year they do not swell. Quick imbibitionwas induced by a deep cut, abrasion with coarse abrasive paper,immersion in concentrated sulphuric acid, in boiling water andin boiling KOH solutions. Neither lipid solvents (acetone, chloroform,cyclohexane, diethyl ether, ethanol, xylene), nor, at room temperature,KOH solutions up to 50% per cent, permeabilized the seeds. Astrong barrier to water entry appeared to be located in thepalisade layer which showed ‘light points’ in places,and which seemed to be transversal thickenings. In the testa,‘ callose’, which may play a role in the impermeabilityof the Sesbania punicea seed, was detected. Sesbania punicea, germination, seed dormancy, seed coat impermeability, ‘callose’     

Possible Roles for Cognin in Chick Embryo Neural Retina

In this review, we discuss current information about a cell-cellrecognition protein present in chick embryo neural retina. Thisprotein, retina cognin, has cell adhesion or aggregation promotingpropertiesin vitro. We discuss five questions. First, what isretina cognin (R-cognin)? Second, what do we know about cogninin chick retina? We discuss its histological distribution inretina and how that distribution changes during embryonic andearly post-hatching development. Third, where is cognin withincells? We review light microscopy evidence for its localizationin plasma membranes of somas and neurites of selected retinalneurons as an intrinsic membrane protein. Fourth, how is cognindistributed in membranes? We summarize evidence that cogninmight not be uniformly distributed over cellsurfaces and thatit might bind to specific proteins on the surfaces of otherretina cells. From the available information, we ask what wecan deduce about cognin's biological role in the neural retina.     

Enzymes of Intermediary Metabolism in the Aleurone Cells of Germinating Wheat

Several enzymes of the glycolysis-gluconeogenesis sequence,the pentose cycle, the tricarboxylic acid cycle, and the electrontransport chain, were studied in the aleurone tissue of germinatingTriticum vulgare. Since all the enzymes are located either inthe cytosol or in mitochondria, they acted as parameters ofthe development of metabolic capacity in these compartmentsduring germination. Each of the enzymes studied was presentin the aleurone tissue of the ungerminated grain. After 12 –18 h germination, the level of each enzyme began to increase,following a sigmoidal course which levelled off by the fourthday. This pattern of enzyme development in the aleurone tissuewas independent of the presence of the embryo, and, therefore,of hormonal control from that tissue (in contrast to the glyoxysomalenzymes). Studies using metabolic inhibitors indicated thatthe developments of the different cytosolic enzyme activitieswere dependent to different degrees upon RNA and protein synthesis.     

Activities of Hydrogen Peroxide-Scavenging Enzymes in Germinating Wheat Seeds

During imbibition and germination of wheat (Triticum aestivum)in the dark over 72 h, activities of the enzymes of the ascorbate(AsA)-dependent H₂O₂-scavenging pathway, AsA peroxidase, monodehydroascorbate(MDAsA) reductase, dehydroascorbate (DHAsA) reductase and glutathione(GSSG) reductase as well as superoxide dismutase (SOD), catalaseand guaiacol peroxidase were determined both in whole grainsand in isolated embryos and endosperm. With the exception of DHAsA reductase, activities of the otherenzymes assayed increased in germinating seeds, especially duringradicle emergence (between 24–48 h of imbibition). Theseincreases, particularly for AsA peroxidase, were much higherin the embryo than in the endosperm. Within 72 h of imbibition,activities per seed increased 116-fold for AsA peroxidase, 19-foldfor guaiacol peroxidase, 5-fold for catalase and only 1·4-foldfor SOD. In contrast to the decreases in DHAsA reductase, theother AsA recycling enzyme, MDAsA reductase, increased 5-foldwithin 72 h. The results indicate that, in wheat seeds, imbibition and germinationis associated with enhanced cellular capacity to detoxify H₂O₂.For this detoxification the operation of AsA peroxidase togetherwith the AsA-regenerating enzymes appears to be of particularimportance. Key words: Ascorbate peroxidase, germination, hydrogen peroxide detoxification, inhibition, wheat     

Histological Study of Seed Coat Development in Arabidopsis thaliana

Arabidopsis seed coat development using light and transmission electron microscopy revealed major morphological changes associated with the transition of the integuments into the mature seed coat. By the use of a metachromatic staining procedure, cytological events such as the production of phenolic compounds and acidic polysaccharides were followed. Immediately after fertilization, the cells of the inner epidermis of the inner integument became vacuolated and subsequently accumulated pigment within them. This pigment started to disappear from the cytoplasm at the torpedo stage of the embryo, as it became green. During the torpedo stage, mucilage began to accumulate in the cells of the external epidermis of the outer integument. Furthermore, starch grains accumulated against the central part of the inner periclinal wall of these cells, resulting in the formation of small pyramidal domes that persisted until seed maturity. At the maturation stage, when the embryo became dormant and colourless, a new pigment accumulation was observed in an amorphous layer derived from remnants of crushed integument layers. This second pigment layer was responsible for the brown seed colour. These results show that seed coat formation may proceed in a coordinated way with the developmental phases of embryogenesis. Received 25 May 1999/ Accepted in revised form 10 February 2000     

Autocatalytic Oxidation of Hemoglobin by Nitrite: A Possible Mechanism

Oxidation of oxyhemoglobin by nitrite ions to produce methemoglobin is one of the more employed procedures to oxidize the hemoprotein. The process takes place readily after a clear induction time. This behaviour is usually explained in terms of an autocatalytic reaction mechanism. However, the generally accepted mechanism is not autocatalytic and cannot explain the main features of the process. In the present work it is proposed that the characteristics of the process require the occurrence of a fast reaction between oxyhemoglobin and nitrogen dioxide. This process acts as a branching step, leading to the observed autocatalysis.     

甘蓝型黄籽油菜种皮色泽QTL作图

甘蓝型黄籽油菜具有低纤维、高蛋白及高含油量的优点,因而己成为广大油菜育种工作者研究的重点之一。利用甘蓝型黑籽品系油研2号作父本,计蓝型黄籽品系GH06为母本,获得132个单株的F2群体;以AFLP和SSR为主要分析方法,构建了包括164个标记的甘蓝型油菜遗传连锁图谱,其中包括125个AFLP标记、37个SSR标记及一个RAPD和一个SCAR标记,分布在19个连锁群上,覆盖油菜基因组2549．8cM,标记间平均距离15．55cM。利用多区间作图法,对种皮色泽QTL进行分析,在第5及第19连锁群上各检测到一个QTL位点,分别解释表型变异46％及30．9％。     

Increased Germination Rates of Baldcypress and Pondcypress Seed Following Treatments Affecting the Seed Coat

The effects of seed-coat removal, warm-water soaking, cold stratification, dry-cold storage and acid treatment on the germination of baldcypress (Taxodium distichum (L.) Rich.) and pondcypress (Taxodium distichum var. nutans (Ait.) Sweet) seed were studied. Removal of the hard seed coat resulted in a prompt and high percent germination, indicating that there was no physiological embryo dormancy. Cold stratification, warm-water soaking and acid treatment did not increase percent germination over the control after a 60-day germination period. They did promote the rate of germination however, having a significantly higher germination at 30 days. Dry-cold storage promoted the initial rate of germination, but resulted in a decreased percent germination below the control after 60 days. The above results suggest that moisture, and not cold, was the crucial component of stratification. The improved germination rate following acid treatment further suggests that any treatment which rendered the seed coat more permeable or softened it was more effective in increasing the rate of germination of baldcypress and pondcypress seed.     

Newly Synthesized mRNA Is Translated during the Initial Imbibition Phase of Germinating Maize Embryo

RNA synthesis is activated in the cells of the plant embryo very soon after the start of seed imbibition. We previously reported that mainly heterogeneous nuclear RNA is synthesized in the radicle of Zea mays embryo during the first hours of germination. The present study was undertaken in order to detect the time of appearance of the newly synthesized messenger RNA in the polysomes of germinating maize axes.

Free polysomes were prepared from embryonic axes rehydrated for 2 hours in the presence of radioactively labeled uridine. These polysomes were shown to be labeled and to contain labeled particles sedimenting, after dissociation with EDTA, in the 10S to 40S region of a sucrose gradient. The labeled polysomal RNA migrates heterogeneously in a gel with a mean size corresponding to about 16S, and 60% of these molecules are polyadenylated.

The data indicate that the newly synthesized RNA associated with the polysomes after 2 h of germination consists of messenger RNA molecules. Analysis of the polysomes prepared 0.5 and 1 h after the start of imbibition suggests that translation of the newly synthesized messenger RNA probably occurs within the 1st hour of imbibition of the isolated axis, thus well before the completion of the initial water uptake.

     

Seed Architecture Shapes Embryo Metabolism in Oilseed Rape

Constrained to develop within the seed, the plant embryo must adapt its shape and size to fit the space available. Here, we demonstrate how this adjustment shapes metabolism of photosynthetic embryo. Noninvasive NMR-based imaging of the developing oilseed rape (Brassica napus) seed illustrates that, following embryo bending, gradients in lipid concentration became established. These were correlated with the local photosynthetic electron transport rate and the accumulation of storage products. Experimentally induced changes in embryo morphology and/or light supply altered these gradients and were accompanied by alterations in both proteome and metabolome. Tissue-specific metabolic models predicted that the outer cotyledon and hypocotyl/radicle generate the bulk of plastidic reductant/ATP via photosynthesis, while the inner cotyledon, being enclosed by the outer cotyledon, is forced to grow essentially heterotrophically. Under field-relevant high-light conditions, major contribution of the ribulose-1,5-bisphosphate carboxylase/oxygenase–bypass to seed storage metabolism is predicted for the outer cotyledon and the hypocotyl/radicle only. Differences between in vitro– versus in planta–grown embryos suggest that metabolic heterogeneity of embryo is not observable by in vitro approaches. We conclude that in vivo metabolic fluxes are locally regulated and connected to seed architecture, driving the embryo toward an efficient use of available light and space.