ABSTRACT
The acquisition of tolerance by the induction of cellular stress defense systems may be accomplished by different mechanisms. If cells or organisms are exposed to environmental stress, it is possible that they will develop or demonstrate tolerance. This enhances the chance of survival and extends longevity. Tolerance may also be acquired, however, by physiological adjustments in response to preexposure to sublethal concentrations of the stressor. In the case of heat stress, the cell has to defend itself against a physical stress, first causing denaturation of proteins. Little is known, however, of the mechanisms that lead to this type of tolerance, resulting in shorter recovery times and enhanced survival of heat-tolerant cells after heat stress. According to Krebs and Loeschcke, the costs of activating heat-shock proteins are not large, especially compared with the costs associated with stress in their absence, which makes the widespread presence of the heat-shock response across taxa not surprising.
