Heat Shock Proteins Are Not Required for the Degradation of α-Amylase mRNA and the Delamellation of Endoplasmic Reticulum in Heat-Stressed Barley Aleurone Cells

Abstract

When barley (Hordeum vulgare) aleurone layers are heat shocked, the synthesis and secretion of a-amylase and other secretory proteins is arrested and the synthesis of heat shock proteins (hsps) is induced. a-Amylase mRNA, normally a very stable mRNA, is actively degraded during heat shock. In addition, endoplasmic reticulum (ER) is delamellated during heat shock, possibly causing the destabilization of the mRNA for the secreted a-amylase. To ascertain whether or not hsps play any role in the destabilization of a-amylase mRNA or in the delamellation process of ER, heat shocked cells were treated with the transcription inhibitor cordycepin, which effectively inhibits the synthesis of hsps yet does not affect a-amylase synthesis after this enzyme has been fully induced by gibberellic acid (12 hours). In the absence of hsp expression, heat shock still causes the destabilization of a-amylase mRNA and the delamellation of ER. Alternatively, the synthesis of hsps may be induced in the absence of temperature increase by incubating cells in the presence of arsenite. Arsenite-induced expression of some hsps in the absence of increased temperature does not result in the destabilization of a-amylase mRNA or in the delamellation of ER. If cordycepin or cycloheximide are used to inhibit hsp synthesis during heat shock, the tissue recovers from heat shock with normal recovery kinetics. Although hsps have been implicated in the establishment of thermotolerance, our observations indicate that hsps do not play a role in the other heat shock-induced changes observable in aleurone cells. Furthermore, if the synthesis of hsp mRNA is inhibited during heat shock (by cordycepin) hsp mRNAs are synthesized later, during recovery, indicating that there is a stable inducer of hsp synthesis in aleurone tissues.

Heat Shock Inhibits Release of the Signal Recognition Particle from the Endoplasmic Reticulum in Barley Aleurone Layers

“…The same phenomenon has been observed for the mRNA levels for the secreted proteins endochitinase and protease, whereas the mRNA levels of the nonsecreted proteins actin and tubulin were not affected by heat shock (10). Through the use of RNA synthesis inhibitors we have determined that synthesis of the hsps is not required for this selective message degradation (11).…”