A Kinetic and Microautoradiographic Analysis of [14C]Sucrose Import by Developing Wheat Grains

Fisher, Donald B.; Wang, Ning

doi:10.1104/pp.101.2.391

Cited by 18 publications

(11 citation statements)

References 20 publications

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“…However, movement of assimilates into the endosperm continues, causing a steady decline of solutes in both the maternal tissues and the endosperm cavity (shown for Suc by Fisher and Gifford [1987] and by Fisher and Wang [1993]; and for total amino acids by N. Wang and D.B. Fisher [unpublished data]).…”

Section: Discussion Water Relations Are Substantially Altered In Detamentioning

confidence: 99%

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Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

Fisher

Cash-Clark

2000

Plant Physiology

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The water relations parameters involved in assimilate flow into developing wheat (Triticum aestivum L.) grains were measured at several points from the flag leaf to the endosperm cavity in normally watered (⌿ Ϸ Ϫ0.3 MPa) and water-stressed plants (⌿ Ϸ Ϫ2 MPa). These included direct measurement of sieve tube turgor and several independent approaches to the measurement or calculation of water potentials in the peduncle, grain pericarp, and endosperm cavity. Sieve tube turgor measurements, osmotic concentrations, and ⌿ measurements using dextran microdrops showed good internal consistency (i.e. ⌿ ϭ ⌿ s ϩ ⌿ p ) from 0 to Ϫ4 MPa. In normally watered plants, crease pericarp ⌿ and sieve tube turgor were almost 1 MPa lower than in the peduncle. This suggests a high hydraulic resistance in the sieve tubes connecting the two. However, observations concerning exudation rates indicated a low resistance. In water-stressed plants, peduncle ⌿ and crease pericarp ⌿ were similar. In both treatments, there was a variable, approximately 1-MPa drop in turgor pressure between the grain sieve tubes and vascular parenchyma cells. There was little between-treatment difference in endosperm cavity sucrose or osmotic concentrations or in the crease pericarp sucrose pool size. Our results re-emphasize the importance of the sieve tube unloading step in the control of assimilate import.One of the most striking aspects of fruit and seed physiology is the independence of their water relations from other parts of the plant. For seeds, stable water relations are required for normal embryo development (Walbot, 1978;Kermode, 1990; Bradford, 1994), for example, in preventing precocious germination. Embryo growth also requires a steady supply of nutrients. Since imported assimilates make up a large proportion of the embryo's osmotic environment, it is likely that these two requirements have some controls in common.In contrast to the embryonic environment, water relations and assimilate concentrations in the vegetative portion of the plant may vary considerably. Water potential especially may vary diurnally and/or decline progressively with the onset of drought conditions. Thus, reproductive structures, or at least their embryos, must be insulated from the variations in water potential experienced by the rest of the plant. At the same time, they typically maintain a constant growth rate (Barlow et al., 1980;Lang and Thorpe, 1989;Westgate et al., 1989;Lang, 1990) despite such variation.In part, this independence of seed and fruit water relations is often achieved by a functional discontinuity in the xylem at or near their point of attachment to the plant. Quite large ⌿ differences have been reported between seeds or fruit and vegetative portions of the plant, and even between the embryo and adjacent seed or fruit tissues (Bradford, 1994). However, as Bradford points out in his critical review of these issues, such differences have been reported even in instances where there seems to be no effective barrier to water movement (e.g. seeds in fleshy frui...

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Section: Discussion Water Relations Are Substantially Altered In Detamentioning

confidence: 99%

“…As for grains on normally watered plants (Fisher and Gifford, 1987;Fisher and Wang, 1993), these pools are rapidly depleted when the grains are detached from the ear (data not shown).…”

Section: Gradients Between the Grain Sieve Tubes And Surrounding Vascmentioning

confidence: 99%

Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

Fisher

Cash-Clark

2000

Plant Physiology

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“…After dissolving in distilled water, phloem sap was used for 14C counting and to determine the specific activity of solutes. Aliquots of the soluble tissue extract and the phloem sap were also separated into acidic, basic, and neutral fractions by passing through 0.1 cm3 of Sephadex ion-exchange columns (Fisher and Wang, 1993), and each fraction was counted for 14C activity. 14C-labeled metabolites in the neutral fraction of phloem sap were separated on highperformance thin-layer chromatographic plates at 75°C using acetonitri1e:water (3:1, v/v) as 'the mobile phase and then autoradiographed with x-ray film to identify labeled solutes.…”

Section: C 0 Labelingmentioning

confidence: 99%

Doubling the CO2 Concentration Enhanced the Activity of Carbohydrate-Metabolism Enzymes, Source Carbohydrate Production, Photoassimilate Transport, and Sink Strength for Opuntia ficus-indica

Wang

Nobel

1996

Plant Physiol.

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After exposure to a doubled CO, concentration of 750 pmol mol-' air for about 3 months, glucose and starch in the chlorenchyma of basal cladodes of Opuntia ficus-indica increased 175 and 57%, respectively, compared with the current CO, concentration of 370 pmol mol-', but sucrose content was virtually unaffected. Doubling the CO, concentration increased the nocturnal malate production in basal cladodes by 75%, inorganic phosphate (Pi) by 32%, soluble starch synthase activity by 30%, and sucrose-Pi synthase activity by 146%, but did not affect the activity of hexokinase. Doubling CO, accelerated phloem transport of sucrose out of the basal cladodes, resulting in a 73% higher dry weight for the daughter cladodes. Doubling CO, increased the glucose content in 14-dold daughter cladodes by 167%, increased nocturnal malate production by 22%, decreased total amino acid content by 61 %, and increased soluble starch synthase activity by 30% and sucrose synthase activity by 62%. No Plant Physiol 103: 51 9-524; P.S. J Exp Bot 45: 295-303), consistent with its higher source capacity and sink strength than under current CO,. These changes apparently do not result i n Pi limitation of photosynthesis or suppression of genes governing photosynthesis for this perennial Crassulacean acid metabolism species, as occur for some annual crops.Increases in the atmospheric CO, concentration generally enhance the rates of photosynthesis and growth for various C, plants, although the enhancement can diminish during long-term exposure to elevated CO, concentrations for annual plants with limited sinks (Sasek et al., 1985;Peet et al., 1986;Stitt, 1991). Such plants commonly respond to elevated CO, concentrations with a decrease of carboxylating enzyme activity and an increase of carbohydrate accumulation (Yelle et al., 1989a, 198913;Koner et al., 1995 vated CO, concentrations (Sage et al., 1989), the acclimation of photosynthesis may be a result of excess starch and sugar accumulation, which can cause feedback inhibition of photosynthesis (Sharkey and Vanderveer, 1989; Yelle et al., 1989a Yelle et al., , 1989bStitt, 1991). Based mainly on studies with annual crops and Arabidopsis, two hypotheses have been proposed to explain the feedback inhibition: (a) accumulation of starch and Suc can reduce the rates of their own synthesis, causing sugar phosphates to build up and deplete Pi pools in the chloroplasts and cytosol, thereby inhibiting photophosphorylation (Sharkey and Vanderveer, 1989;Stitt, 1991); and (b) buildup of sugars, especially Glc and SUC, can suppress (down-regulate) the expression of genes governing photosynthesis (Oosten et al., 1994;Sheen, 1994;Nie et al., 1995), resulting in a lowered photosynthetic rate.On the other hand, some C, tree species and perennial CAM plants do not show acclimation of photosynthesis during long-term exposure to elevated CO, concentrations (Idso and Kimball, 1994;Nobel and Israel, 1994;Ceulemans et al., 1995;Liu and Teskey, 1995). Doubling the atmospheric CO, concentration to 750 pmol mol-' a...

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“…The remaindix of the nucellus, at the embryo end where the cavity usually bends, was exposed by pulling off the overlying aleurone with forceps. The opened grains were incubated for another 3 to 4 h in perfusion medium to deplete the crease tissues of their Suc (Fisher and Wang, 1993), after which they were transferred to the uptake medium at room temperature (solution about 20°C). For good aeration, a11 incubations were performed on an oscillating shaker in solutions 5 mm or less in depth.…”

Section: Sugar Movement Lnto and Out Of The Crease Tissues Of Detachementioning

confidence: 99%

Sucrose Release into the Endosperm Cavity of Wheat Grains Apparently Occurs by Facilitated Diffusion across the Nucellar Cell Membranes

Wang

Fisher

1995

Plant Physiol.

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Nutrients required for the growth of the embryo and endosperm of developing wheat (Triticum aestivum L.) grains are released into the endosperm cavity from the maternal tissues across the nucellar cell plasma membranes. We followed the uptake and efflux of sugars into and out of the nucellus by slicing grains longitudinally through the endosperm cavity to expose the nucellar surface to experimental solutions. Sucrose uptake and efflux are passive processes. Neither was sensitive to metabolic inhibitors, pH, or potassium concentration. pChloromercuribenzene sulfonate, however, strongly inhibited both uptake and efflux, although not equally. Except for pchloromercuribenzene sensitivity, these characteristics of efflux and the insensitivity of Suc movement to turgor pressure are similar to those of sucrose release from maize pedicels, but they contrast with legume seed coats. Although the evidence is incomplete, movement appears to be carrier mediated rather than channel mediated. In vitro rates of sucrose efflux were similar to or somewhat less than in vivo rates, suggesting that transport across the nucellar cell membranes could be a factor in the control of assimilate import into the grain.Because symplastic connections are absent between the embryonic and maternal tissues of a developing seed, nutrients for embryo growth must take an apoplastic pathway to move between the two. The presence of this step, necessarily involving the release of solutes from the maternal symplast, has been used to considerable advantage in studies of phloem unloading and post-phloem transport in developing seeds (see reviews by Thorne, 1985;Patrick, 1990;Wolswinkel, 1992). The site of solute release in Vicia seeds (Offler and Patrick, 1993) and in wheat (Triticum aestivum L.) grains (Wang and Fisher, 1994b) is a layer of transfer cells in the maternal tissues facing the embryo or endosperm, respectively. Because plasma membranes are typically very effective barriers to solute movement, the transmembrane release of nutrients from these cells (the nucellus, in the case of wheat grains) deserves careful attention as a possible control site for the rate of assimilate import by the seed.

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A Kinetic and Microautoradiographic Analysis of [14C]Sucrose Import by Developing Wheat Grains

Cited by 18 publications

References 20 publications

Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

Doubling the CO2 Concentration Enhanced the Activity of Carbohydrate-Metabolism Enzymes, Source Carbohydrate Production, Photoassimilate Transport, and Sink Strength for Opuntia ficus-indica

Sucrose Release into the Endosperm Cavity of Wheat Grains Apparently Occurs by Facilitated Diffusion across the Nucellar Cell Membranes

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