Acquisition of Thermotolerance in Soybean Seedlings : Synthesis and Accumulation of Heat Shock Proteins and their Cellular Localization.

When soybean Glycine max var Wayne seedlings are shifted from a normal growth temperature of 28 degrees C up to 40 degrees C (heat shock or HS), there is a dramatic change in protein synthesis. A new set of proteins known as heat shock proteins (HSPs) is produced and normal protein synthesis is greatly reduced. A brief 10-minute exposure to 45 degrees C followed by incubation at 28 degrees C also results in the synthesis of HSPs. Prolonged incubation (e.g. 1-2 hours) at 45 degrees C results in greatly impaired protein synthesis and seedling death. However, a pretreatment at 40 degrees C or a brief (10-minute) pulse treatment at 45 degrees C followed by a 28 degrees C incubation provide protection (thermal tolerance) to a subsequent exposure at 45 degrees C. Maximum thermoprotection is achieved by a 2-hour 40 degrees C pretreatment or after 2 hours at 28 degrees C with a prior 10-minute 45 degrees C exposure. Arsenite treatment (50 micromolar for 3 hours) also induces the synthesis of HSP-like proteins, and also provides thermoprotection to a 45 degrees C HS; thus, there is a strong positive correlation between the accumulation of HSPs and the acquisition of thermal tolerance under a range of conditions.During 40 degrees C HS, some HSPs become localized and stably associated with purified organelle fractions (e.g. nuclei, mitochondria, and ribosomes) while others do not. A chase at 28 degrees C results in the gradual loss over a 4-hour period of the HSPs from the organelle fractions, but the HSPs remain selectively localized during a 40 degrees C chase period. If the seedlings are subjected to a second HS after a 28 degrees C chase, the HSPs rapidly (complete within 15 minute) relocalize in the organelle fractions. The relative amount of the HSPs which relocalize during a second HS increases with higher temperatures from 40 degrees C to 45 degrees C. Proteins induced by arsenite treatment are not selectively localized with organelle fractions at 28 degrees C but become organelle-associated during a subsequent HS at 40 degrees C.