Figure 1.

RFO synthesis and degradation. Reactions for synthesis (a) are represented with black arrows whereas those for degradation (b) are with gray arrows. The first step of RFOs biosynthesis starts with the formation of galactinol from UDP-galactose and inositol catalyzed by the plant enzyme galactinol synthase. Raffinose, stachyose, and verbascose are synthesized from sucrose by the subsequent addition of activated galactose moieties donated by galactinol using plant raffinose and stachyose synthases and galactan:galactan galactosyltransferase (GGT), respectively. Cleavage, shown by scissors, of raffinose by an α-galactosidase leads to the formation of either melibiose and fructose or sucrose and galactose. For RFOs of higher degree of polymerization (DP), an α-galactosidase activity results in RFOs of lower DP and galactose.