Journal Article

Extracellular β‐glucosidase activity in barley involved in the hydrolysis of ABA glucose conjugate in leaves

Karl‐Josef Dietz, Angela Sauter, Kathrin Wichert, David Messdaghi and Wolfram Hartung

in Journal of Experimental Botany

Volume 51, issue 346, pages 937-944
Published in print May 2000 | ISSN: 0022-0957
Published online May 2000 | e-ISSN: 1460-2431 | DOI:
Extracellular β‐glucosidase activity in barley involved in the hydrolysis of ABA glucose conjugate in leaves

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Abscisic acid conjugate concentrations increased in barley xylem sap under salinity, whereas it remained at a low level in the intercellular washing fluid (IWF) of barley primary leaves (Hordeum vulgare cv. Gerbel). Here it is shown that IWF contains β‐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 authentic ABA upon separation by thin layer chromatography and was identified by ABA‐ELISA. p‐Nitrophenol‐β‐d‐glucopyranoside (pNPG) was used as the substrate for β‐glucosidases. The KM(pNPG) was 1.8 mmol l−1. The activity was affected by ABA glucopyranoside in a competitive type of inhibition with a KI of 400 μmol l−1. Various hormone conjugates were compared with respect to their inhibitory effect on beta;‐glucosidase activity. Inhibition was highest for the ABA glucopyranoside and the zeatin riboside, but insignificant for ABA methyl ester and zeatin‐9‐β‐d‐ glucoside. The specific activity of the β‐glucosidase was 16‐fold greater in IWF as compared to crude leaf extracts confirming its extracellular compartmentation. The activity of β‐glucosidase was strongly increased after growth in hydroponic medium supplemented with NaCl. The data support the hypothesis that the glucose conjugate is a long‐distance transport form of ABA.

Keywords: Abscisic acid; abscisic acid glucose ester; apoplast; β‐glucosidase; barley; salt stress; root : shoot signalling.

Journal Article.  4746 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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