The cell wails of barley (Hordeum vulgare var. Himalaya) aleurone layers undergo extensive degradation during the tissue's response to gibbereilic acid. Previous work had shown that these cell wails consist almost entirely of arabinoxylan. In this study we show that gibbereilic acid stimulates endo-fi-1,4-xylanase activity in isolated aleurone layers. In addition, gibbereilic acid enhances the activity of two glycosidases: /8-xylopyranosidase and a-arabinofuranosidase. No gibbereilic acid-stimulated ceilulase activity was detected. Germination studies showed a similar pattern of enzyme development in intact seeds.Aleurone layers of barley and other cereal grains respond to gibberellic acid by synthesizing and releasing a select group of hydrolases, including a-amylase, protease, ribonuclease, f8-1,3-glucanase, and phosphatase (14, 25). These enzymes serve to solubilize the stored food reserves of the endosperm for use by the growing embryo. Hydrolase secretion is accompanied by the nearly complete autolysis of aleurone cell cytoplastic constitutents, as well as the thick cell walls (13,16,23 aleurone layers. We also extend previous cytological studies to include scanning electron microscopy in order to monitor the physical degradation of the wall. MATERIALS AND METHODSPlant Material. Barley seeds (Hordeum vulgare L. cv. Himalaya, 1972 harvest) were either sown directly into moist Vermieulite or were surface sterilized for 25 min in 20 % Clorox, rinsed with sterile distilled H20, and sown on moist, sterile sand in 250-ml Erlenmeyer flasks. The seeds were germinated in a growth chamber at a photoperiod of 14 hr and a constant temperature of 25 C.Isolated aleurone layers were prepared from imbibed halfseeds and treated with gibberellic acid by the procedure of Chrispeels and Varner (5), with the exception that succinate buffer (20 mm, pH 5.2) was substituted for acetate buffer (6, 12).Enzyme Assays. At the end of the incubation period, the medium from the GA-treated aleurone layers was decanted into 12-ml conical centrifuge tubes. The layers were rinsed once with 3 ml of succinate buffer (20 mm, pH 5.2) containing 10 mM CaCl2, and the combined medium and washings were centrifuged at 3000g for 5 min. Extractable enzyme was prepared by grinding the aleurone layers to a paste with a small amount of sand in a mortar which had been previously rinsed with succinate-CaCl2 buffer. Five ml of the buffer were added and the homogenate was poured into centrifuge tubes and centrifuged as above. Germinating seeds were extracted in the same manner after first removing the embryos. The supernatants obtained were either used directly or were further treated by precipitation with cold ethanol as described by Taiz and Jones (23) or with 75 % ammonium sulfate. The ammonium sulfate precipitate was redissolved in 1 ml of grinding buffer and dialyzed overnight at 4 C against the same buffer before use. Although initial experiments were carried out using the ethanol procedure it was subsequently determined that precipitation with ...