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

Abstract

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.

Sucrose Concentration Gradients along the Post-Phloem Transport Pathway in the Maternal Tissues of Developing Wheat Grains

“…The Suc gradient in sorbitol-perfused grains was sharpest within the chalaza, in agreement with our observation of localized plasmolysis there (Wang and Fisher, 1994a). Movement in PCMBS-perfused grains was blocked in the nucellus, consistent with the view that nucellar cell membranes are the site of PCMBS sensitivity (Wang and Fisher, 1995). However, the failure of the crease Suc concentration to increase above 260 mM in PCMBS-perfused grains is curious, since the Suc concentration in the crease sieve tubes is substantially higher (about 450-600 mM; Fisher and Gifford, 1986;Fisher, 1990).…”

“…For comparison with two treatments shown to block post-phloem transport, the endosperm cavities of attached grains were perfused with 10 mM PCMBS (Wang and Fisher, 1995) and with 1660 mOsm sorbitol (Wang and Fisher, 1994a). Two grains were perfused for each treatment; both treatments were carried out on the same ear.…”

“…This property has been very useful in the interpretation of treatment effects, since changes in the size of the pool can be used as an indicator of which transport step was most affected by an experimental treatment. Thus, the pool decreases rapidly after grains are removed from the plant (Fisher and Wang, 1993;Wang and Fisher, 1994a) and increases when transport across the nucellar cell plasma membrane is blocked by PCMBS (Wang and Fisher, 1995), and when symplastic movement is blocked between the phloem and nucellus by sorbitol-induced plasmolysis of the chalazal cells (Wang and Fisher, 1994a). Each of these interpretations has distinct implications not only for the behavior of the total crease Suc pool but for the attendant alteration of the Suc distribution within the crease tissues.…”

Review: Nutrient loading of developing seeds

“…Evidence for facilitated diffusion, with comparable high apparent K m values (∼228 mM derived from fig. 5 of Wang and Fisher 1995) to those found for bean seed coats (∼400-500 mM sucrose , Ritchie et al 2003) has been reported as a suggested mechanism for sucrose release from the nucellus projection of developing wheat grains .…”

Phloem Import and Storage Metabolism Are Highly Coordinated by the Low Oxygen Concentrations within Developing Wheat Seeds

“…For pea, evidence exists that poorly selective pores are present in the plasma membrane of seed coat parenchyma cells to facilitate the unloading of assimilates (de Jong et al, 1996(de Jong et al, , 1997van Dongen et al, 2001). Also in wheat, nucellus unloading appears to be a passive process, thus driven by the concentration gradient (Wang and Fisher, 1995). Together with the experiments presented in this study, our study indicates that the metabolic activity of the endosperm acts on the nutrient unloading from the maternal tissue and thereby determines the proportion of assimilates transported by the phloem toward the seeds.…”