Detailed characterization of heat shock protein synthesis and induced thermotolerance in seedlings ofSorghum bicolorL.

Abstract

Tolerance of both protein synthesis and seedling growth to a previously lethal high temperature can be induced by prior exposure to a sub-lethal temperature during which the synthesis of heat shock proteins (HSPs) occurs. In this study, a thermal gradient bar was used to measure the physiological effects of tem perature on seedlings of sorghum (Sorghum bicolor L.) in conjunction with studies of gene expression. The duration of HSP synthesis, both during continued high temperature treatment or on return to normal temper atures, was found to be very finely modulated and was dependent on the severity of the initial heat shock. The synthesis of heat shock proteins and the induc tion of thermotolerance were rapid, reversible and re-inducible phenomena. Maximal thermotolerance was obtained after treatments that induced the full complement of HSPs. Subsequent treatments that repressed HSP synthesis, also abolished thermotoler ance. The presence of HSPs, however, was not suffi cient for the tissue to be in a thermotolerant state and the results suggest that either their de novo synthesis, or some other factor, is required for the induction of thermotolerance. Pre-existing HSPs did not inhibit the synthesis of new HSPs. Although the kinetics of the synthesis of HSPs and the development of thermotoler ance show a tight correlation, the kinetics of the decay of thermotolerance and the degradation of HSPs were not linked. The functional state or distribution of HSPs may well change during the recovery process.