Quantitative and qualitative changes in the endoplasmic reticulum of barley aleurone layers

Abstract

Changes in the level of the endoplasmicreticulum (ER) marker enzyme cytochrome-c reductase (EC 1.6.2.1) were followed with time of imbibition of de-embryonated half-seeds of barley (Hordeum vulgare L.) and the subsequent incubation of their aleurone layers in gibberellic acid (GA3) and H2O. During imbibition there is an increase in the level of cytochrome-c-reductase activity and in the amount of 280-nm absorbance associated with this enzyme. When aleurone layers are incubated for a further 42 h in water, there is a doubling of the cytochrome-c-reductase activity. In GA3, the activity of cytochrome-c reductase reaches a maximum at 24 h of incubation and thereafter falls to below 70% of its level at the beginning of the incubation period. Changes in the cytochrome-c-reductase activity correlate with changes in the fine structure of the aleurone cell. The ER isolated in low Mg(2+) from aleurone layers incubated in buffer for up to 18 h has buoyant density of 1.13-1.14 g cc(-1) while that from layers incubated in GA3 for 7.5-18 h has a density of 1.11-1.12 g cc(-1). The α-amylase (EC3.2.1.1) isolated with the organelle fraction by Sepharose gel filtration is associated with the ER on isopycnic and rate-zonal density gradients, and its activity can be enhanced by Triton X-100. The soluble α-amylase fraction from Separose-4B columns, on the other hand, is not Triton-activated but is acid-labile. Acid phosphatase (EC3.1.3.2) is distributed in at least three peaks on isopycnic gradients. In low Mg(2+) the second peak of activity has a density of 1.12 g cc(-1) in GA3-treated tissue and 1.13-1.14 g cc(-1) in H2O-treated tissue. With high-Mg(2+) buffers, this peak of phosphatase activity disappears. Acid-phosphatase activity is not enhanced by Triton X-100 nor is it acid-labile.