Nucellar projection transfer cells in the developing wheat grain

Summary Transfer cells in the nucellar projection of wheat grains at 25 ±3 days after anthesis have been examined using light and electron microscopy. Within the nucellar tissue, a sequential increase in non-polarized wall ingrowth differentiation and cytoplasmic density was evident. Cells located near the pigment strand were the least differentiated. The degree of differentiation increased progressively in cells further removed from the pigment strand and the cells bordering the endosperm cavity had degenerated. Four stages of transfer cell development were identified at the light microscope level. Wall ingrowth differentiation followed a sequence from a papillate form through increased branching (antler-shaped ingrowths) which ultimately anastomosed to form a complex labyrinth. The final stage of wall ingrowth differentiation was compression which resulted in massive ingrowths. In parallel with wall ingrowth deposition cytoplasmic density increased. During wall deposition, paramural and multivesicular bodies were prominent and were in close association with the wall ingrowths. The degeneration phase involved infilling of cytoplasmic islets within the wall ingrowths. This was accompanied by complete loss of the protoplast. The significance of this transfer cell development for sucrose efflux to the endosperm cavity was assessed by computing potential sucrose fluxes across the plasma membrane surface areas of the nucellar projection cells. Transfer cell development amplified the total plasma membrane surface area by 22 fold. The potential sucrose flux, when compared with maximal rates of facilitated membrane transport of sugars, indicated spare capacity for sucrose efflux to the endosperm cavity. Indeed, when the total flux was partitioned between the nucellar projection cells at the three stages of transfer cell development, the fully differentiated stage III cells located proximally to the endosperm cavity alone exhibited spare transport capacity. Stage II cells could accommodate the total rate of sucrose transfer, but stage I cells could not. It is concluded that the nucellar projection tissue of wheat provides a unique opportunity to study transfer cell development and the functional role of these cells in supporting sucrose transport.Abbreviations CSPMSA cross sectional plasma membrane surface area - LPMSA longitudinal plasma membrane surface area - PTS tri-sodium 3-hydroxy-5,8,10-pyrenetrisulfonate     

花椒球心胚及胚乳的发生和发育

对花椒珠心胚及胚乳的发生和发育过程进行了详细的细胞学及细胞学研究。主要研究结果如下;珠心胚发生前,有性胚囊发育过程中从大孢子发生到胚囊形成的各个阶段均可发生退化,退化频率５０％,未退化的胚囊发育成熟,成熟胚囊仅含卵器和两个极核。卵器最终退化,极核不经受精自发形成胚肥。当胚乳游离核达到１５或３２个时,最早的珠心胚原始细胞由靠近胚囊球孔端的珠心细胞分化形成。随着子房生长,多个原始细胞持续不断地从珠孔端     

Current opinions on endosperm transfer cells in maize

Endosperm transfer cells (ETC) mainly occur in the endosperm epithelial layer near the pedicel. They transport the nutrient unloaded by the maternal vascular tissue to filial tissues. Wall ingrowths of ETC can facilitate solute transportation. Sugar, especially glucose, is found to modulate the promoter activity of ZmMRP-1, a determinant of transfer cell-specific expression. The ZmMRP-1-encoded protein can transactivate the promoters of transfer cell-specific genes. Signalling and early events leading to wall ingrowth formation depend upon gene expression. Sucrose synthase and the cytoskeleton probably play a primary role in the wall ingrowth formation. The major solutes transferred by ETC are amino acids, sucrose, and monosaccharides, which is consistent with the expression of their transporters and transport-associated genes. In this paper, we review current opinions on the differentiation, wall ingrowth formation, and function of ETC in maize. According to the experimental materials provided by predecessors, we also give some speculations about the differentiation mechanisms of ETC and process of wall ingrowth formation.     

Relationship between development of endosperm transfer cells and grain mass in maize

The most basal endosperm cells of maize (Zea mays L.) began differentiating into transfer cells in 10 days after pollination (DAP). The thickening and ingrowths forming in the transfer cell wall were slow during 10 and 15 DAP. There were many vesicles, silky and string ball objects in cytoplasm, and the number of mitochondria and rough endoplasm reticulum increased. After 15 DAP, the wall thickening and ingrowths forming in the transfer cells sped up. By 20 DAP, the transfer cell zone had developed, there appeared 65 - 70 rows of cells in width and 3 - 4 layers of cell in depth, the obvious cell wall ingrowths presented strong positive reaction with periodic acid Schiff's reagent. After 20 DAP, no significant change appeared in the shape and structure of the transfer cells, and the transfer cells entered function stage. In the mature kernels (53 DAP), the most basal transfer cells were filled with ingrowths, however, dense cytoplasm was also found in these cells. The nuclei had quite irregular shapes in these cells. Some transfer cells contained black grains and crystals. A black layer formed in the pericarp tissue adjacent to the transfer cell zone. Full development of endosperm transfer cells was important for reduction of kernel abortion and increase of kernel mass. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the wheat endosperm transcriptome

Among the cereals, wheat is the most widely grown geographically and is part of the staple diet in much of the world. Understanding how the cereal endosperm develops and functions will help generate better tools to manipulate grain qualities important to end-users. We used a genomics approach to identify and characterize genes that are expressed in the wheat endosperm. We analyzed the 17,949 publicly available wheat endosperm EST sequences to identify genes involved in the biological processes that occur within this tissue. Clustering and assembly of the ESTs resulted in the identification of 6,187 tentative unique genes, 2,358 of which formed contigs and 3,829 remained as singletons. A BLAST similarity search against the NCBI non-redundant sequence database revealed abundant messages for storage proteins, putative defense proteins, and proteins involved in starch and sucrose metabolism. The level of abundance of the putatively identified genes reflects the physiology of the developing endosperm. Half of the identified genes have unknown functions. Approximately 61% of the endosperm ESTs has been tentatively mapped in the hexaploid wheat genome. Using microarrays for global RNA profiling, we identified endosperm genes that are specifically up regulated in the developing grain.     

Chromosome organization in wheat endosperm and embryo

We have analysed the chromosome organization in endosperm and embryo of bread wheat (Triticum aestivum L.), in order to compare these tissues with developing anthers, in which the centromeres associate, and the developing root xylem vessel cells, in which the chromosomes endoreduplicate to become polytene and associate via their centromeres. Both endosperm and embryo showed a typical Rabl configuration and a degree of non-homologous centromere association and the endosperm also showed extensive telomere association. Wheat endosperm is initially triploid and during its development a percentage of the nuclei increase their DNA content to 6C and 12C. 6C nuclei showed twice as many centromeres as 3C nuclei and the centromere number increased further in 12C nuclei. The higher the C-content of a nucleus the more the telomeres associated in endosperm. The vast majority of 12C nuclei showed six rye chromosome arms, although a few showed three associated groups of rye chromosome arms. This means that during endosperm development wheat nuclei show both polyploidization and polytenization.     

Characterisation of disproportionating enzyme from wheat endosperm

Disproportionating enzyme or D-enzyme (EC 2.4.1.25) is an α-1,4 glucanotransferase which catalyses cleavage and transfer reactions involving α-1,4 linked glucans altering (disproportionating) the chain length distribution of pools of oligosaccharides. While D-enzyme has been well characterised in some plants, e.g. potato and Arabidopsis, very little is known about its abundance and function in cereals which constitute the major source of starch worldwide. To address this we have investigated D-enzyme in wheat (Triticum aestivum). Two putative D-enzyme cDNA clones have been isolated from tissue-specific cDNA libraries. TaDPE1-e, from an endosperm cDNA library, encodes a putative polypeptide of 575 amino acid residues including a predicted transit peptide of 41 amino acids. The second cDNA clone, TaDPE1-l, from an Aegilops taushii leaf cDNA library, encodes a putative polypeptide of 579 amino acids including a predicted transit peptide of 45 amino acids. The mature polypeptides TaDPE1-e and TaDPE1-l were calculated to be 59 and 60 kDa, respectively, and had 96% identity. The putative polypeptides had significant identity with deduced D-enzyme sequences from corn and rice, and all the expected conserved residues were present. Protein analysis revealed that D-enzyme is present in the amyloplast of developing endosperm and in the germinating seeds. D-enzyme was partially purified from wheat endosperm and shown to exhibit disproportionating activity in vitro by cleaving maltotriose to produce glucose as well as being able to use maltoheptaose as the donor for the addition of glucans to the outer chains of glycogen and amylopectin.     

Genetic control of endosperm proteins in wheat

Summary Total endosperm proteins extracted from both several common wheat cultivars and some intervarietal substitution lines derived from them were fractionated according to their molecular weight in a high resolution one-dimensional gel electrophoresis. The four donor cultivars and the recipient one — Chinese Spring, possessed differentially migrating protein bands in the fractions of high molecular weight (HMW) glutenins and gliadins. Several of these bands were identified for the first time in this study. By utilizing intervarietal substitution lines the control of the HMW glutenins and gliadins by chromosomes of homoeologous group 1 was either reaffirmed or, for the new bands, established. Several HMW gliadin subunits showed a considerable variation in their staining intensity in the intervarietal substitution lines indicating that their expression was dependent on the genetic background.This paper is based on a portion of a dissertation to be submitted by G. Galili in partial fulfilment of the Ph.D. requirements of the Feinberg Graduate School, The Weizmann Institute of Science, RehovotThe Marshall and Edith Korshak Professor of Plant Cytogenetics     

Harnessing the developmental potential of nucellar cells: barriers and opportunities

Angiosperm nucellar cells can either use or avoid meiosis in vivo, depending on the developmental context. This unique ability contrasts with the conditions required in vitro, either for a reconstituted oocyte to avoid meiosis and produce clones by somatic cell nuclear transfer (SCNT), or for mammalian stem cells to undergo meiosis and produce synthetic sex cells (gametes). Current biotechnological initiatives to harness the potential of nucellar cells are based on the transfer of apomixis genes to sexual crop plants with the aim of producing clones through seeds. The elusive genetic basis of apomixis compels us to examine whether this process involves epigenetic factors. The elegant and versatile developmental platform available in nucellar cells should be explored as a genome-scale science and compared with mammalian stem cell biology for a holistic understanding of developmental programming and reprogramming in eukaryotes.     

Synthesis and degradation of proteins during wheat endosperm development

Synthesis of proteins rich in lysine declines progressively with endosperm development and these proteins appear to be degraded preferentially at later stages. The proteolytic enzymes in extracts of endosperms at a late stage of development release considerably more lysine radioactivity from labelled endosperm proteins as compared with the enzymes in endosperms at an early stage.     

Extraction of wheat endosperm proteins for proteome analysis

Total protein extracts of wheat endosperm are widely used for the analysis of the highly abundant gliadins and glutenins. In this review, the most popular total endosperm extraction methods are compared for their effectiveness in proteome coverage. A drawback of total endosperm extracts is that the enormous dynamic range of protein abundance limits the detection, quantification, and identification of low abundance proteins. Protein fractionation is invaluable for improving proteome coverage, because it reduces sample complexity while enriching for specific classes of less abundant proteins. A wide array of techniques is available for isolating protein subpopulations. Sequential extraction is a method particularly suited for subfractionation of wheat endosperm proteins, because it takes advantage of the specific solubility properties of the different classes of endosperm proteins. This method effectively separates the highly abundant gliadins and glutenins from the much less abundant albumins and globulins. Subcellular fractionation of tissue homogenates is a classical technique for isolating membranes and organelles for functional analysis. This approach is suitable for defining the biochemical processes associated with amyloplasts, specialized organelles in the endosperm that function in the synthesis and storage of starch. Subproteome fractionation, when combined with 2-DE and protein identification, provides a powerful approach for defining endosperm protein composition and providing new insights into cellular functions.     

Determinants of resistant starch accumulation in wheat endosperm

A part of the big three cereal crops in the world, wheat has become a major constituent of the everyday food chain and is grown at a massive scale to meet global demands. This makes it an important crop from an economic as well as food security perspective. Selection of high-quality cultivars and consistent trait enhancement for such cultivars is crucial, and in light of new challenges from climate change, this has become an absolute necessity of time. In this regard, we conducted a detailed qualitative and quantitative trait analysis for multiple commercially viable varieties of wheat, and corresponding results were subjected to a series of critical statistical analyses. Final results have shown that five cultivars including Uqaab-2000, Faisalabad- 85, Anmol-19, NARC-2009, and Pirsabak-2004 depicts higher levels of various essential qualitative and quantitative traits (including Starch content, grain weight, RS content, Protein content, etc.) and are most viable varieties for further growth and trait enhancements to meet regional and global food challenges.     

Analysis of programmed cell death in wheat endosperm reveals differences in endosperm development between cereals

Although maize endosperm undergoes programmed cell death during its development, it is not known whether this developmental feature is common to cereals or whether it arose inadvertently from the selection process that resulted in the enlarged endosperm of modern maize. Examination of wheat endosperm during its development revealed that this tissue undergoes a programmed cell death that shares features with the maize program but differs in some aspects of its execution. Cell death initiated and progressed stochastically in wheat endosperm in contrast to maize where cell death initiates within the upper central endosperm and expands outward. After a peak of ethylene production during early development, wheat endosperm DNA underwent internucleosomal fragmentation that was detectable from mid to late development. The developmental onset and progression of DNA degradation was regulated by the level of ethylene production and perception. These observations suggest that programmed cell death of the endosperm and regulation of this program by ethylene is not unique to maize but that differences in the execution of the program appear to exist among cereals.     

Chemical and physical properties of an arabinogalactan-peptide from wheat endosperm

1. An arabinogalactan-peptide from wheat endosperm was studied by using physicochemical techniques and some aspects of its chemical structure were determined. 2. The arabinogalactan-peptide is a non-associating, polydisperse macromolecule ([unk]=22000) which exhibits only minor non-ideal effects in aqueous solution. 3. Examination of the products of partial acid hydrolysis of the polysaccharide component showed that arabinose is present in the alpha-l-arabinofuranosyl configuration, and i.r.-absorption spectroscopy and optical-rotation studies suggest that the d-galactopyranose residues are linked by glycosidic linkages in the beta-anomeric configuration. 4. The arabinogalactan is linked to a peptide which represents 8% (w/w) of the arabinogalactan-peptide and which may be present as a molecular core. Partial degradation of the polymer by successive treatment with oxalic acid and NaOH showed that the linkage between polysaccharide and peptide involves galactose and hydroxyproline residues and is glycosidic in nature. A tentative model is proposed for the structure of the wheat endosperm arabinogalactan-peptide. 5. The subcellular location and function of the arabinogalactan-peptide is discussed in relation to previous work with related molecules.     

Purothionin from wheat endosperm reversibly blocks myogenic differentiation of chick embryonic muscle cells in culture

Purothionin from wheat endosperm is a cysteine-rich, basic polypeptide of about 5000 Da, which modifies membrane permeability of cultured mammalian cells. This peptide was found to block fusion of chick embryonic muscle cells in culture but allows proliferation and alignment. A purothionin concentration of 6 micrograms/ml (1.2 microM) was necessary for the complete prevention of myotube formation. Under similar conditions, incorporation of [35S]methionine occurred normally but the synthesis of muscle-specific proteins including creatine kinase and acetylcholine receptor was strongly inhibited. In addition, purothionin blocked the uptake of 86Rb+, immediately after its addition to the cultured myoblasts. No such effects were found with the purothionin chemically modified with acetic or succinic anhydride. Thus, the basic residues in purothionin appear to be associated with the inhibition of myogenic differentiation. These results suggest that purothionin exerts its regulatory effect on the transition from proliferative to differentiative myoblasts by interfering with membrane permeability or intercellular contact and recognition, which are necessary for the initiation of muscle differentiation.