The Transport of Sugars to Developing Embryos Is Not via the Bulk Endosperm in Oilseed Rape Seeds 1(W)(OA)

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 understanding 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 providing 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