Cell wall-associated isoflavonoids and β-glucosidase activity in Lupinus albus plants responding to environmental stimuli

Plants respond to various biotic or abiotic stimuli with different mechanisms and the mobilization of phenolic compounds is one of them. In white lupin (Lupinus albus L.) plants, isoflavones and their glucosides were localized in cell walls where the high constitutive activity of β‐glucosidase (EC 3·2·1·21) was also identified. The enzyme was partially purified from root cell walls. Its polymeric active form has 180 or 200 kDa as determined by non‐denaturing electrophoresis and gel filtration, respectively, and the isoelectric point is at pH 6·9. The enzyme is an exoglucosidase, preferentially hydrolysing conjugates of phenolic compounds with β anomers of glucose. It is not active against purified lupin cell walls. The specific β‐glucosidase activity varies in different tissues with the highest one in roots, and always higher in cell walls than in protoplast. The cell wall location of the enzyme was confirmed biochemically by its activity in intercellular washing fluids. Both aglycones and glycosides were also present in these fluids. The specific β‐glucosidase activity correlated well with the isoflavonoid aglycone/glycoside ratios in various tissues: the higher β‐glucosidase activity, the higher relative aglycone content. β‐glucosidase activity was found to be inducible under conditions of yeast elicitor treatment. Induction of the enzyme was accompanied by changes in the isoflavone secretion and accumulation, suggesting the regulatory role of β‐glucosidase activity in root exudation of isoflavonoids. No correlation however, was found between changes in β‐glucosidase activity and the presence of isoflavonoids in root exudates of plants grown under various nitrogen nutrition regimes.