The acquisition of desiccation tolerance in developing Vicia hirsuta seeds coincides with an increase in galactinol synthase expression and soluble α-d-galactosides accumulation

Abstract

Galactinol is the galactosyl donor for the biosynthesis of both the raffinose family oligosaccharides (RFOs) and galactosyl cyclitols (Gal-C). Its synthesis by galactinol synthase (GolS, EC 2.4.1.123) is the first committed step of the soluble α-d-galactosides biosynthetic pathway in orthodox seeds. The deposition of galactosides in seeds is suggested to be associated with desiccation tolerance (DT). In this work, for the first time, we cloned and characterized two Vicia hirsuta (L.) S.F. Gray galactinol synthase genes (VhGolS1, VhGolS2), analyzed galactinol synthase activity and measured the accumulation of galactosides of both sucrose and d-pinitol in relation to the acquisition of DT in developing seeds of this wild species.A developmentally induced increase of VhGolS1 expression preceded the rise of GolS activity in crude protein extract from maturing seeds, while the expression of the VhGolS2 gene remained low. GolS activity peaked just after the beginning of the maturation drying phase. The increase of GolS activity was not followed by galactinol accumulation, instead the high enzyme activity was related to high levels of galactose bound in soluble galactosides of the RFO and galactosyl pinitol series. Acquisition of DT coincided with an increase of VhGolS1 expression, high galactinol synthase activity and the accumulation of oligogalactosides in seeds. DT was positively correlated with the high content of soluble α-d-galactosides of both the RFO and galactosyl pinitol series as well as with the amount of galactose bound in these galactosides.