Allocation of raffinose family oligosaccharides to transport and storage pools in Ajuga reptans: the roles of two distinct galactinol synthases

Raffinose family oligosaccharides (RFOs) are important phloem transport and storage carbohydrates for many plants. Ajuga reptans, a frost-hardy evergr een labiate, ideally combines these two physiological roles and served as o ur model plant to study the regulation and importance of RFO metabolism. Ga lactinol is the galactosyl donor for the synthesis of raffinose (RFO-trisac charide) and stachyose (RFO-tetrasaccharide), and its synthesis by galactin ol synthase (GolS) is the first committed step of the RFO biosynthetic path way. Two cDNAs encoding two distinct GolS were isolated from A. reptans sou rce and sink leaves, designated GolS-1 and GolS-2, respectively. Warm- and cold-grown sink and source leaves were compared, revealing both isoforms to be cold-inducible and GolS-1 to be source leaf-specific; GolS-1 expression correlated positively with GolS activity. Conversely, GolS-2 expression wa s comparatively much lower and its contribution to the total extractable Go lS activity is most probably only minor. These observations, together with results from phloem exudation and leaf shading experiments suggest that Gol S-1 is mainly involved in the synthesis of storage RFOs and GolS-2 in the s ynthesis of transport RFOs. Furthermore, in situ hybridization studies show ed GolS-1 to be primarily expressed in the mesophyll, the site of RFO stora ge, and GolS-2 in the phloem-associated intermediary cells known for their role in RFO phloem loading. A model depicting the spatial compartmentation of the two GolS isoforms is proposed.