Transport and metabolism of raffinose family oligosaccharides in transgenic potato

Raffinose family oligosaccharides (RFOs) are involved in the storage and transport of carbon and serve as compatible solutes for protection against abiotic stresses like drought or cold. RFOs are usually transported in plant species that load sugars symplastically into the phloem. Loading probably occurs by a polymer trapping mechanism which establishes a concentration gradient of assimilates between the mesophyll and the vasculature. Transgenic approaches have demonstrated phloem transport of small molecules produced in the companion cells of apoplastic loading species, but these molecules have been non-native transport substances to plants. In this study, transgenic potato plants with constitutive or companion cell specific overexpression of galactinol synthase (GS) or GS plus raffinose synthase (RS) are characterized, which together provide new insights into the metabolism and transport of RFOs in plants. It is demonstrated that raffinose and galactinol are both transported in the phloem and that, whilst the effect of GS overexpression is promoter-independent, that of RS is dependent on the promoter used. The presence of significant amounts of galactinol in the phloem is shown and also that transgenic potato is unable to transport large amounts of raffinose despite high RS expression and substrate concentrations. These data indicate that there may be additional features of intermediary cells, the specialized companion cells of RFO transporting plants, required for significant RFO synthesis and transport that are currently not well-understood.