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Biomedical subjects

Ulrich Wobus

Publications and source records attributed to Ulrich Wobus.

8 recordsLinked to original sources
PubMed2003

The role of invertases and hexose transporters in controlling sugar ratios in maternal and filial tissues of barley caryopses during early development.

To analyse carbohydrate metabolism and its role during early seed development of barley we characterised genes encoding two cell wall-bound invertases (HvCWINV1 and HvCWINV2) and two putative hexose transporter-like genes (HvSTP1 and HvSTP2). No typical vacuolar invertase gene could be identified. Instead, a gene encoding sucrose:fructan 6-fructosyltransferase (HvSF6FT1), an enzyme with soluble acid invertase activity, was isolated and characterised. Furthermore, enzyme activities and sugar levels were measured. HvSF6FT1-mRNA levels and acid soluble invertase activity are highest in the maternal pericarp 1-2 days after flowering (DAF). HvSF6FT1 is strongly expressed in regions flanking the main vascular bundle and to a lower extent in filial endospermal transfer cells, which persist until maturity and never accumulate starch. In contrast, cell wall-bound invertase HvCWINV2 is expressed early in development mainly in the style region and later on in pericarp areas which transiently accumulate starch and undergo degradation later in development. The hexose transporter HvSTP2 shows a temporal and spatial expression pattern similar to HvCWINV2. Transcripts of HvCWINV1 have been localised within the first row of endospermal cells and in the outermost area of the nucellar projection as well as in endospermal transfer cells before starch filling; the same regions of the endosperm are labelled with a hexose transporter HvSTP1-probe. HvSTP1 is expressed at very low levels within the pericarp but much higher in the syncytial endosperm at 3 DAF and in endospermal transfer cells 7 DAF. The temporal and spatial association of HvCWINV1 and HvSTP1 expression indicates that hexoses liberated by the invertase within the endospermal cavity are taken up by the transporter to be delivered into the central uncellularised space of the endosperm to supply mitotically active endosperm cells with hexoses. The results are discussed and compared with published data on the role of soluble sugars as signal molecules in seed developmental processes.

Carbohydrate Metabolism↗
PubMed2003

Differentiation of legume cotyledons as related to metabolic gradients and assimilate transport into seeds.

Legume seed development is closely related to metabolism and nutrient transport. To analyse this relationship, a combination of biochemical, histological and transgenic approaches was used. Sugars within tissue sections have been quantitatively measured by metabolic imaging. During cotyledon differentiation glucose gradients emerge related to a particular cell type, with higher concentrations in non-differentiated premature regions. Sucrose in creases at the beginning of maturation in a layer underneath the outer epidermis expressing a sucrose transporter. Sucrose distribution is initially controlled by uptake activity and the permeability within the parenchyma and, later on, also by differences in growth and starch accumulation. Increased sucrose levels are accompanied by increased levels of sucrose synthase and ADP-Glc pyrophosphorylase mRNAs, but carbon flux into starch is initially still low. Rates increase at a stage when hexose concentrations become low, allowing increased flux through the sucrose synthase pathway. Transfer cell formation represents a regional specification of the cotyledonary epidermis for embryo nutrition characterized by increased transport-active cell surfaces and up-regulated expression of transport-related genes. The E2748 pea seed mutation blocks epidermal differentiation into transfer cells and leads to the loss of epidermal cell identity. Embryos with impaired epidermis cannot tolerate elevated levels of sucrose and respond with disorganized growth. The E2748 gene product is required for transfer cell formation in developing cotyledons with no other function during plant growth. Seed coat permeability provides a hypoxic environment for embryo development. However, at maturity, seed energy supply is not limited indicating fundamental developmental and metabolic adaptations. Results from transgenic seeds show that altered expression of single genes induces complex and unexpected changes. In AGP-antisense seeds the block in starch synthesis leads to pleiotropic effects of water and nitrogen content and induces temporal changes in seed development.

Biological Transport↗
PubMed2003

Peptide and amino acid transporters are differentially regulated during seed development and germination in faba bean.

Two peptide transporter (PTR) homologs have been isolated from developing seeds of faba bean (Vicia faba). VfPTR1 was shown to be a functional peptide transporter through complementation of a yeast mutant. Expression patterns of VfPTR1 and VfPTR2 as well as of the amino acid permease VfAAP1 (Miranda et al., 2001) were compared throughout seed development and germination. In developing seeds, the highest levels of VfPTR1 transcripts were reached during midcotyledon development, whereas VfAAP1 transcripts were most abundant during early cotyledon development, before the appearance of storage protein gene transcripts, and were detectable until late cotyledon development. During early germination, VfPTR1 mRNA appeared first in cotyledons and later, during seedling growth, also in axes and roots. Expression of VfPTR2 and VfAAP1 was delayed compared with VfPTR1, and was restricted to the nascent organs of the seedlings. Localization of VfPTR1 transcripts showed that this PTR is temporally and spatially regulated during cotyledon development. In germinating seeds, VfPTR1 mRNA was localized in root hairs and root epidermal cells, suggesting a role in nutrient uptake from the soil. In seedling roots, VfPTR1 was repressed by a dipeptide and by an amino acid, whereas nitrate was without influence.

Amino Acid Transport Systems↗
PubMed2002

Antisense-inhibition of ADP-glucose pyrophosphorylase in Vicia narbonensis seeds increases soluble sugars and leads to higher water and nitrogen uptake.

We previously reported on Vicia narbonensis seeds with largely decreased alpha- D-glucose-1-phosphate adenyltransferase (AGP; EC 2.7.7.27) due to antisense inhibition [H. Weber et al. (2000) Plant J 24:33-43]. In an extended biochemical analysis we show here that in transgenic seeds both AGP activity and ADP-glucose levels were strongly decreased but starch was only moderately reduced and contained less amylose. The flux control coefficient of AGP to starch accumulation was as low as 0.08, i.e. AGP exerts low control on starch biosynthesis in Vicia seeds. Mature cotyledons of antisense seeds had increased contents of lipids, nitrogen and sulfur. The protein content was higher due, in particular, to increased sulfur-rich albumins. Globulin fractions of storage proteins had a lower ratio of legumin to vicilin. Isolated cotyledons partitioned less [14C]sucrose into starch and more into soluble sugars with no change in the protein fraction. Respiration of isolated cotyledons and activities of the major glycolytic and carbohydrate-metabolizing enzymes were not affected. Sucrose and the hexose-phosphate pool were increased but UDP-glucose, 3-phosphoglyceric acid, phospho enolpyruvate, pyruvate, ATP and ADP were unchanged or even lower, indicating that carbon partitioning changed from starch to sucrose without affecting the glycolytic and respiratory pathways. Soluble compounds were increased but osmolality remained unchanged, indicating compensatory water influx resulting in higher water contents. Developmental patterns of water and nitrogen accumulation suggest a coupled uptake of amino acids and water into cotyledons. We conclude that, due to higher water uptake, transgenic cotyledons take up more amino acids, which become available for protein biosynthesis leading to a higher protein content. Obviously, a substantial part of amino acid uptake into Vicia seeds occurs passively and is osmotically controlled and driven by water influx.

Amino Acids↗
PubMed2002

Spatial analysis of plant metabolism: sucrose imaging within Vicia faba cotyledons reveals specific developmental patterns.

During legume embryogenesis the differentiation of the cotyledons proceeds gradually in a wave-like manner. The process is metabolically and genetically controlled and regulated by sugars. In order to perform a spatial and temporal analysis of the sugar distribution pattern a new method was developed to specifically measure sucrose directly in tissues via bioluminescence and single photon counting. This enabled a quantitative sucrose imaging with a resolution close to the single cell level. The procedure was applied on sections of Vicia faba cotyledons covering the main stages of histodifferentiation. Young embryos before the storage phase contained moderate levels of sucrose, which were evenly distributed. At the onset of maturation high concentrations were present within a tissue layer covering the outward half of the coytledons. This layer was directly underneath the epidermis expressing a sucrose transporter gene indicating that epidermal transporters caused the high sucrose accumulation in the underlying tissue. At that stage the sucrose gradient was inversely oriented compared with cell size and the starch content. Cells within the interior were larger, contained starch but low sucrose. Thus, the sucrose pattern is controlled by uptake activity and permeability within the parenchyma. However, during the main storage phase actively elongating and starch accumulating cells contain highest sucrose concentrations indicating that differences in growth and starch accumulation also affect intracotyledonary sugar distribution. High sucrose concentrations were correlated with transcript levels of sucrose synthase and ADP-Glc pyrophosphorylase indicating a signaling function for sucrose to induce starch biosynthesis on the gene expression level. Carbon flux through the sucrose synthase pathway towards starch increased when hexoses levels decreased.

Carbohydrate Metabolism↗
PubMed2002

Legume embryos develop in a hypoxic environment.

Specific morphological and biochemical characteristics of seeds can cause oxygen deficiency within maternal and embryonic tissues. In this study, optical sensors were used to measure O(2) profiles across developing seeds of Vicia faba and Pisum sativum and developmental and environmental modulations of internal O(2) levels were studied. In addition, the metabolic state of developing embryos was analysed by monitoring adenylate energy charge, adenylate nucleotides and the levels of nucleotide sugars. Within the seed coat O(2) concentration decreased sharply to approximately 3% towards the inner border. Lowest O(2) levels were detected within the endospermal cavity between the seed coat and embryo. It is probable that low seed coat permeability provides an hypoxic environment for legume embryo development. The O(2) concentration in embryonic tissue changed during development with the lowest levels in the early stages. Measured in darkness, the levels were below 3%, but increased upon illumination indicating that photosynthesis significantly contributes to internal O(2) levels. Only in very young embryos were ATP levels and energy charge low. Otherwise they were maintained at a constant higher value. ADP-glucose and UDP-glucose did not show large fluctuations. Throughout embryo development fermentative activity did not play a major role. Obviously, specific mechanisms prevent seed tissues from becoming anoxic during development. The possible role of low oxygen on seed metabolism and on the control of seed development in legumes is discussed.

Adenine Nucleotides↗
PubMed2002

A pea seed mutant affected in the differentiation of the embryonic epidermis is impaired in embryo growth and seed maturation.

During legume seed development the epidermis of the embryos differentiates into a transfer cell layer which mediates nutrient uptake during the storage phase. This specific function of the epidermal cells is acquired at the onset of embryo maturation. We investigated this process in the pea seed mutant E2748. The epidermal cells of the mutant embryo, instead of turning into transfer cells, enlarge considerably and become vacuolated and tightly associated with adjacent seed tissues. Expression of a sucrose transporter gene that is upregulated in wild-type transfer cells decreases in the mutant and changes its spatial pattern. This indicates that the outermost cell layer of mutant cotyledons cannot acquire transfer cell morphology but loses epidermal cell identity and does not function as a sucrose uptake system. Seed coat growth as well as composition, concentration and dynamics of sugars within the endospermal vacuole are unchanged. The loss of epidermal identity has severe consequences for further embryo development and is followed by disruption of the symplast within the parenchyma, the breach of the developmental gradient, lower sucrose and starch levels and initiation of callus-like growth. It is concluded that the E2748 gene controls differentiation of the cotyledonary epidermis into transfer cells and thus is required for the regional specialisation with a function in embryo nutrition.

Cell Differentiation↗