Heat-shock Response
When growing plantlets or tissues of plants are shifted to 42°C and above, the synthesis of normal proteins rapidly declines and instead a set of new proteins appears. Since these proteins are known to be induced by hike in temperature, they are often called heat-shock proteins (hsp). These proteins are known to be self-regulatory in that their synthesis is switched off after 6 to 8 hours at the elevated temperature while synthesis of the normal proteins resumes. The heat-shock proteins span over a wide range of molecular weight (15 to 102 kd). Several hsps are known to be induced also by heavy metals and arsenites.
The hsps are now known to occur in representatives of all the major groups of organisms. Curiously, a pre-treatment at elevated temperature (e.g., 2 hours at 45°C) eliminates the heat-shock response on subsequent exposure to similar conditions. It is believed that heat shock protein2 protect essential enzymes and nucleic acids from denaturation. Induction of synthesis of heat-shock proteins has also been observed under field conditions. In dry fields during summer when the leaf temperature reaches or exceeds the ambient temperature (>40"C), hsp's are synthesised as under experimental conditions.
Heat-shock response involves changes in transcriptional as well as translational control. The pre-existing transcripts for normal proteins remain intact for sometime while protein synthesis ceases. How do hsp's help in heat-shock avoidance? They probably help important cellular proteins to acquire conformations that would be safe and functional under high temperature and the protein will remain in soluble state in the cytoplasm.