Polysaccharide hydrolases are released with mucilages after water hydration of flax seeds

Abstract

Water imbibition of flax seed induces secretion of mucilages whose physico-chemical properties vary according to genotype and environment. The viscosity and composition of mucilage have ecological implications and also affect the utility of the crop. Several types of enzymes are secreted along with the mucilage. Our objective was to study these enzymes in the context of the composition and physical properties of the mucilage. The kinetics of production by flax seeds (variety Eden) of i) mucilages, ii) glycosidases and iii) endo-hydrolases were followed over 48 h under sterile conditions. The impact of enzymatic activities on mucilage was investigated by SEC-MALLS, viscosimetry and sugar composition. The mucilages consisted mainly of rhamnogalacturonan-I (RG-I, 52-62%) and arabinoxylan (AX, 27-36%). RG-I related enzyme activities (rhamnogalacturonase and β,d-galactosidase) were quantified, together with AX related activity of α,l-arabinofuranosidase, β,d-xylosidase and β-xylanase. Maximal xylanase activity was reached after 4 h seed-hydration, when the minimal viscosity of the polysaccharides was observed, and the AX/RG-I ratio was the lowest. At that time, poly and oligosaccharides mainly contained pectic sugars. From 24 to 48 h water-hydration, when mucilages more tightly associated with cell walls were released, the glycosidase activities per g mucilage became maximal; the percentage, average molar-mass and viscosity of the polysaccharides decreased. Glucose, xylose and arabinose were the main sugars in the oligomer fraction. Our data confirmed the presence of β-d xylosidase and α-l-arabinofuranosidase activities and provided evidence for significant pectinase activities in flax mucilages. They also indicate an impact of enzymatic activities on the physicochemical properties of mucilages.