The heat-shock response of developing barley aleurone layers

Aleurone layers of mature germinating barley (Hordeum vulgare, cv. Himalaya) grains respond to heat shock by synthesizing heat-shock proteins (HSPs) and by selectively suppressing the synthesis of proteins normally translated by endoplasmic reticulum (ER)-bound ribosomes. To determine if this also was the case during seed development, we investigated the synthesis of proteins translated by ER-bound ribosomes in heat-shocked aleurone layers isolated from developing barley grains. The optimal induction temperature for the heat shock response in developing aleurone layers was 37.5°C, and temperatures above 42°C inhibited translation. HSPs with apparent molecular masses of 71.1, 66.2, 57.8, 19.1 and 18.8 kDa were induced. Other studies have shown that, in gibberellic acid (GA)-induced aleurone layers from mature barley grains, these temperatures were 40°C and 45°C, respectively. Furthermore, in developing aleurone layers, mRNAs encoding proteins translated by ER-bound ribosomes (mRNAs for a lipid transfer protein and a putative amylase/protease inhibitor) remained stable during heat shock. The ER membranes themselves remained in stacks, but the lumen became distended with electron-dense material. Heat shock prevented the movement of proteins from the ER into the rest of the endomembrane pathway. In contrast, other studies show that in mature, GA-induced aleurone layers, heat shock dissociates ER stacks and blocks translation, but the processing of secretory proteins in the endomembrane pathway is not inhibited. The observation that the same tissue at different developmental stages may respond differently to heat shock indicates that components of the heat-shock response are developmentally regulated. This system provides an opportunity to better understand the nuances of the heat-shock response, especially the post-transcriptional gene regulatory mechanisms that occur.