OSMOTIC REGULATION OF α-AMYLASE SYNTHESIS AND POLYRIBOSOME FORMATION IN ALEURONE CELLS OF BARLEY

Water stress inhibits the gibberellic acid (GA₃)-induced synthesis of α-amylase in aleurone layers of barley (Hordeum vulgare L.). Electron microscope evidence indicates that the effect of water stress induced by 0.6 M solutions of polyethylene glycol (PEG) is to reduce the binding of ribosomes to the endoplasmic reticulum. This was confirmed by sucrose density gradient centrifugation of polyribosome preparations from stressed cells. The reduction in polyribosome formation does not result from reduced ribosome activity as measured by [³H]peptidylpuromycin formation. Thus, calculation of percent active ribosomes shows that osmoticum has little effect on the ability of ribosomes to incorporate puromycin into nascent protein. Water stress does not cause a marked decrease in the total RNA level of aleurone cells. Estimates of total RNA in postmitochondrial supernatant fractions from stressed cells show only a reduction of 8–9% relative to the control. Membrane synthesis measured by [¹⁴C]choline incorporation is depressed by 15% in cells stressed with 0.6 M PEG for 2.5 hours.