Induction of raffinose oligosaccharide biosynthesis by abscisic acid in somatic embryos of alfalfa (Medicago sativa L.)

Raffinose oligosaccharides (ROs) accumulate during late seed development in orthodox seeds, just after the levels of the phytohormone abscisic acid (ABA) rise during mid seed development. To elucidate a possible relationship between ABA and ROs, alfalfa somatic embryos (SE) were treated with exogenous ABA in a range of 50 nM to 50 μM. Only trace amounts of ROs are found in SE, providing an elegant experimental system to study the hormonal regulation of RO biosynthesis. Compared to controls without ABA, a significant accumulation of all ROs (galactinol, raffinose and stachyose) could be observed, even at the lowest ABA concentration used. At the highest ABA concentration, the amount of galactinol, raffinose and stachyose accumulated was three-, four- and seven-fold higher than that of controls, respectively. This accumulation was accompanied by a three-fold increase in galactinol synthase (GLS) activity, while the levels of raffinose synthase (RFS) and stachyose synthase (STS) activities remained almost constant. We therefore conclude that exogenous ABA induces the accumulation of ROs in alfalfa SE.