Gtgen3A, a novel plant GH3 β-glucosidase, modulates gentio-oligosaccharide metabolism in Gentiana

Gentiobiose, a β-1,6-linked glycosyl-disaccharide, accumulates abundantly in Gentianaceae and is involved in aspects of plant development, such as fruits ripening and release of bud dormancy. However, the mechanisms regulating the amount of gentio-oligosaccharide accumulation in plants remain obscure. The present study aimed to identify an enzyme that modulates gentio-oligosaccharide amount in gentian (Gentiana triflora). A protein responsible for gentiobiose hydrolysis, GtGen3A, was identified by partial purification and its peptide sequence analysis. The enzyme had a molecular mass of ∼67 kDa without a secretory signal peptide sequence. Sequence analysis revealed that GtGen3A could be a β-glucosidase member belonging to glycoside hydrolase family 3 (GH3). GtGen3A showed a homology to GH3 β-glucan exohydrolases, ExoI of Hordeum vulgare, and ExgI from Zea mays, which preferentially hydrolyzed β-1,3- and β-1,4-linked oligosaccharides. The purified recombinant GtGen3A (rGtGen3A) produced in Escherichia coli showed optimal reaction at pH 6.5 and 20°C. The rGtGen3A liberated glucose from β-1,2-, β-1,3-, β-1,4-, and β-1,6-linked oligosaccharides, and showed the highest activity toward gentiotriose among the substrates tested. Kinetic analysis also revealed that rGtGen3A preferentially hydrolyzed gentiotriose. Virus-induced gene silencing of Gtgen3A in gentian plantlets resulted in predominant accumulation of gentiotriose rather than gentiobiose. Furthermore, the expression level of Gtgen3A was almost similar to the amount of gentiobiose in field-grown gentians. These findings suggest that the main function of GtGen3A is the hydrolysis of gentiotriose to gentiobiose, and that GtGen3A plays a role in modulating gentiobiose amounts in gentian.