The Transport of Sugars to Developing Embryos Is Not via the Bulk Endosperm in Oilseed Rape Seeds 1[W][OA] Edward R. Morley-Smith, Marilyn J. Pike, Kim Findlay, Walter Ko¨ckenberger, Lionel M. Hill, Alison M. Smith*, and Stephen Rawsthorne Departments of Metabolic Biology (E.R.M.-S., M.J.P., L.M.H., A.M.S., S.R.) and Cell and Developmental Biology (K.F.), John Innes Centre, Norwich NR4 7UH, United Kingdom; and Sir Peter Mansfield Magnetic Resonance Centre, School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom (W.K.) The fate of sucrose (Suc) supplied via the phloem to developing oilseed rape (Brassica napus) seeds has been investigated by supplying [ 14 C]Suc to pedicels of detached, developing siliques. The method gives high, sustained rates of lipid synthesis in developing embryos within the silique comparable with those on the intact plant. At very early developmental stages (3 d after anthesis), the liquid fraction that occupies most of the interior of the seed has a very high hexose-to-Suc ratio and [ 14 C]Suc entering the seeds is rapidly converted to hexoses. Between 3 and 12 d after anthesis, the hexose-to-Suc ratio of the liquid fraction of the seed remains high, but the fraction of [ 14 C]Suc converted to hexose falls dramatically. Instead, most of the [ 14 C]Suc entering the seed is rapidly converted to products in the growing embryo. These data, together with light and nuclear magnetic resonance microscopy, reveal complex compartmentation of sugar metabolism and transport within the seed during development. The bulk of the sugar in the liquid fraction of the seed is probably contained within the central vacuole of the endosperm. This sugar is not in contact with the embryo and is not on the path taken by carbon from the phloem to the embryo. These findings have important implications for the sugar switch model of embryo development and for under- standing the relationship between the embryo and the surrounding endosperm. Most of the carbon for seed growth is supplied as Suc, imported from the maternal tissues of the plant. In oilseed rape seeds (Brassica napus) and legumes, two phases of Suc utilization by the growing seed can be recognized. During the first phase, much of the Suc entering the seed is converted to hexoses, which accumulate in the endosperm that occupies most of the internal volume of the seed. This coincides with a rapid increase in seed volume. The hydrolysis of Suc to hexoses probably contributes to this increase, by pro- viding a high level of osmoticum to drive water uptake by the seed. In this first phase, the embryo occupies a small fraction of the internal volume of the seed and its growth is primarily by cell division. During the second phase, Suc rather than hexose becomes the major sugar in the seed. This change coincides with a fall in acid invertase activity in the seed. Embryo cell division ceases and cell expansion accelerates. Storage product synthesis in the embryo becomes the major fate for Suc entering the seed (legumes [Weber et al., 1995; Borisjuk et al., 2003]; oilseeds [Baud et al., 2002; Hill et al., 2003]). The coincidence between the fall in the hexose-to-Suc ratio in the seed and the switch from cell division to expansion and storage product accumulation in the embryo has led to the suggestion that the two are causally related (Weber et al., 1996a, 1997, 2005; Borisjuk et al., 1998, 2002). Weber and colleagues propose that the high hexose-to-Suc ratio in the seed early in devel- opment promotes cell division and represses storage product accumulation in the embryo. As the hexose-to- Suc ratio falls, cell division ceases and the capacity for storage product synthesis increases. There is ample evidence that Suc and hexoses can indeed modulate expression of genes encoding enzymes involved in carbohydrate metabolism and in the biosynthesis of hormones implicated in seed development (Leo´n and Sheen, 2003; Koch, 2004; Rook et al., 2006). Further support for this sugar switch hypothesis for seeds comes from two sources. First, studies of bean seeds (Vicia spp.) show that the developmental switch from cell division to storage product accumulation in the embryo is tightly spatially correlated with hexose-to-Suc ratios through development (Borisjuk et al., 1998, 2002). Second, manipulation of the seed hexose-to-Suc ratio influences embryo development in transgenic plants and in tissue culture. In tissue culture, hexose-based 1 This work was supported by a Core Strategic Grant from the UK Biotechnology and Biological Sciences Research Council to the John Innes Centre and, at the University of Nottingham, by a University Research Fellowship of the Royal Society to W.K. * Corresponding author; e-mail alison.smith@bbsrc.ac.uk. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Alison M. Smith (alison.smith@bbsrc.ac.uk). [W] The online version of this article contains Web-only data. [OA] Open Access articles can be viewed online without a sub- scription. www.plantphysiol.org/cgi/doi/10.1104/pp.108.124644 Plant Physiology, August 2008, Vol. 147, pp. 2121–2130, www.plantphysiol.org Ó 2008 American Society of Plant Biologists 2121 Downloaded from https://academic.oup.com/plphys/article/147/4/2121/6107656 by guest on 11 June 2022