In-response to a phytohormone, gibberellic acid, the aleurone layers of barley seeds synthesize and secrete a-amylages, which are coded by a set of stable mRNAs. When aleurone layers are subjected to heat shock treatment, the synthesis of a-imylase is suppressed while heat shock proteins are induced. The suppression of a-amylase synthesis is not the result of translational control as reported in several other systems. Rather, the sequences of a-amylase mRNA are rapidly degraded during heat shock as shown by in vitro translation and dot blot hybridization with a cDNA probe. Upon recovery from heat shock, the tissue resumes the synthesis of a-amylase in 24 hr. However, in the presence of a transcription inhibitor, cordycepin, the resumption of synthesis of ao-amylase does not take place, indicating that new transcription of a-amylase genes is necessary for this recovery process. The degradation of a-amylase mRNAs correlates with the rapid destruction of endoplasmic reticulum as observed by electron microscopy, a phenomenon that has not been reported previously as a heat shock response. Since a-amylase mRNA is associated with the endoplasmic reticulum via membranebound polyribosomes, we suggest that the destruction of the endoplasmic reticulum during heat shock causes the destabilization and the eventual degradation of a-amylase mRNA.

No abstract

The aleurone layer of GA3-stimulated barley (Hordeum vulgare L., cv Himalaya) grains is normally devoted to the synthesis and secretion of hydrolytic enzymes. Heat shock, however, suppresses the synthesis of the main hydrolytic enzyme, a-amylase, by destabilizing its otherwise highly stable mRNA (FC Belanger, MR Brodl, T-hD Ho [1986] Proc Natd Acad Sci USA 83: 1354-1358. In this paper we document that heat shock causes the suppression of the synthesis of some normal cellular proteins, while the synthesis of other normal cellular proteins is unaffected by heat shock. There are two major isozymic forms of a-amylase encoded by distinct mRNAs. The mRNA levels for both isozymic forms and the mRNA levels of two other secretary proteins, a protease and an endochitinase, were markedly reduced during heat shock. However, the levels of actin and j-tubulin mRNAs, both nonsecretory proteins, were not diminished during heat shock. In addition, the levels of three other mRNA species detected by a set of unidentified cDNA clones (the sequence of one shows that it lacks a signal sequence) remained unchanged during heat shock. These data indicate that there are two classes of normal cellular protein mRNAs with regard to the effect of heat shock upon their persistence in the cell, and suggest that the distinction between them is whether or not they encode secretary proteins.Heat shock is, quite simply, a transient elevation in temperature, usually about 10 to 15oC above ambient temperature. This simple manipulation produces remarkable changes in gene expression. In every organism tested to date heat shock induces the synthesis of a characteristic set of proteins, the so-called hsps3 ( There is a translational bias established during heat shock that favors the translation of the newly transcribed hsp mRNAs (18). The mRNAs encoding normal cellular proteins persist in the cytoplasm; however, they are not translated during heat shock. When returned to normal temperatures, the synthesis of normal cellular proteins resumes even in the presence of actinomycin D, indicating that these mRNAs that had been quiescent during heat shock are reactivated during recovery (9).In yeast, the shift in gene expression is controlled at the transcriptional level. Heat shock induces the transcription of genes encoding hsps but represses the transcription of genes encoding normal cellular proteins. The normal cellular protein mRNAs already present in the cytoplasm continue to be translated (along with hsp mRNAs) until they are turned over at their normal, fairly rapid rates (13). In soybean a similar transcriptional regulation of normal cellular protein synthesis appears to be operating. Vierling and Key (20) have demonstrated that the mRNA for the small subunit of ribulose-1,5-phosphate carboxylase decreases during heat shock, presumably as it is turned over at its normal rate.We have previously investigated the heat-shock response in the cells of barley aleurone layers (2). a-Amylases are the principal enzymes synthesized and secreted by these ...

ABSTRACT

Exposure of plant cells to heat shock temperature results in the synthesis of a set of heat shock proteins and, in many cases, the interruption of normal cellular protein synthesis. In some plant secretory cells the interruption of normal cellular protein synthesis is accomplished by the destabilization of otherwise stable mRNAs, perhaps via the dissociation of the endoplasmic reticulum lamellae upon which these mRNAs are translated. Such a mechanism represents a novel means for the regulation of gene expression.