Extracellular beta-glucosidase activity in barley involved in the hydrolysis of ABA glucose conjugate in leaves

Abscisic acid conjugate concentrations increased in barley xylem sap under salinity, whereas it remained at a low level in the intercellular washing f luid (IWF) of barley primary leaves (Hordeum vulgare cv. Gerbel). Here it i s shown that IWF contains beta-glucosidase activity which releases abscisic acid (ABA) from the physiologically inactive ABA-glucose conjugate pool in the leaf apoplast, The following data support this conclusion and give the first biochemical and physiological characterization of the extracellular glucosidase activity in barley. Free ABA was released by the incubation of ABA glucose ester with IWF. The product exhibited the retention time of aut hentic ABA upon separation by thin layer chromatography and was identified by ABA-ELISA. p-Nitrophenol-beta-D-glucopyranoside (pNPG) was used as the s ubstrate for beta-glucosidases. The K-M (pNPG) was 1.8 mmol l(-1). The acti vity was affected by ABA glucopyranoside in a competitive type of inhibitio n with a K-I of 400 mu mol l(-1). Various hormone conjugates were compared with respect to their inhibitory effect on beta-glucosidase activity. Inhib ition was highest for the ABA glucopyranoside and the zeatin riboside, but insignificant for ABA methyl ester and zeatin-9-beta-D-glucoside. The speci fic activity of the beta-glucosidase was 16-fold greater in IWF as compared to crude leaf extracts confirming its extracellular compartmentation. The activity of beta-glucosidase was strongly increased after growth in hydropo nic medium supplemented with NaCl. The data support the hypothesis that the glucose conjugate is a long-distance transport form of ABA.