Heat shock proteins do not provide thermoprotection to normal cellular protein synthesis, α‐amylase mRNA and endoplasmic reticulum lamellae in barley aleurone layers

Abstract

Heat shock in barley (Hordeum vulgare L. cv. Himalaya) aleurone layers induces the synthesis of heat shock proteins (hsps) and suppresses the synthesis and secretion of α‐amylase, the principal secretory protein. This is accompanied by the destabilization of α‐amylase mRNA and a concomitant dissociation of ER lamellae. In the absence of heat shock α‐amylase mRNA is extremely stable (Belanger et al. 1986. Proc. Natl. Acad. Sci. USA 83: 1354–1358). In most organisms there is a direct correlation between the synthesis of hsps and thermotolerance. The ability of hsps to provide thermoprotection to secretory protein synthesis, α‐amylase mRNA and ER lamellae was analyzed. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) of pulse‐chased, [35S]‐methionine‐labeled proteins revealed that the half‐life of hsps in barley aleurone cells recovering from heat shock was approximately 12 h. Within approximately 6 h, there was a recovery of α‐amylase mRNA and a reformation of ER lamellae. Heat shock protein synthesis was induced by either heat shock (40°C) or arsenite, the cells were allowed to recover for 8 h, then were re‐exposed to heat shock. Results from SDS‐PAGE showed that, despite the presence of hsps, α‐amylase synthesis was suppressed. Northern blot hybridizations showed that α‐amylase mRNA levels were reduced in heat‐shocked tissues. Transmission electron microscopy demonstrated that ER lamellar structures were dissociated. The synthesis of hsps did not enable barley aleurone cells to sustain the synthesis of any proteins at lethal temperature. In contrast, similar conditions established thermotolerance and provided thermoprotection to protein synthesis in germinating barley embryos. Our findings suggest that the aleurone layer does not become thermotolerant following the induction of hsp synthesis.