Evidence that the hexose-to-sucrose ratio does not control the switch to storage product accumulation in oilseeds: analysis of tobacco seed development and effects of overexpressing apoplastic invertase

Wild-type tobacco (Nicotiana tabacum L.) seed development was characterized with respect to architecture and carbohydrate metabolism. Tobacco seeds accumulate oil and protein in the embryo, cellular endosperm and inner layer of the seed coat. They have high cell wall invertase (INV) and hexoses in early development which is typical of seeds. INV and the ratio of hexose to sucrose decline during development, switching from high hex to high suc, but not until most oil and all protein accumulation has occurred. The oil synthesis which coincides with the switch is mostly within the embryo. INV activity is greater than sucrose synthase activity throughout development, and both activities exceed the demand for carbohydrate for dry matter accumulation. To investigate the role of INV-mediated suc metabolism in oilseeds, genes for yeast INV and/or hexokinase (HK) were expressed under a seed-specific napin promoter, targeting activity to the apoplast and cytosol, respectively. Manipulating the INV pathway in an oilseed could either increase oil accumulation and sink strength, or disrupt carbohydrate metabolism, possibly through sugar-sensing, and decrease the storage function. Neither effect was found: transgenics with INV and/or HK increased 30-fold and 10-fold above wild-type levels had normal seed size and composition. This contrasted with dramatic effects on sugar contents in the INV lines.