Heat stress proteins and transcription factors

In 1864, Julius Sachs [58] reported on an extended series of experiments defining the upper temperature limits of plant growth using a specially designed heat stress chamber for whole plants. The broad interest of plant physiologists in this topic has continued up to the present time and has provided insights into developmental, hormonal, circadian and seasonal influences on the intrinsic and inducible heat resistance of plants [59-65]. Another important root of hs research goes back to the first publication in 1866 by the German physician W. Busch [66] on the spontaneous regression of a skin tumour after local infection with Streptococcus erysipelatis. Following this, W. B. Coley [67] reported in 1893 on 47 cases of treatment of malignant surface tumours by Streptococcus infections or by injection of bacterial extracts (Coley's toxin). The curative effect is evidently due to hyperthermic damage to the tumour tissue and a local stimulation of the immune system. Hyperthermic treatment of cancer as well as investigations on the basis of cell death under heat stress conditions and survival of tumour cells due to induced thermotolerance became a major part of research in this field and was particularly stimulated after the discovery of induced HSP synthesis [68-72]. Heat stress-induced developmental defects were first described by F. M. Alsop in 1919 [73]. But it was Richard Goldschmidt who elaborated the basis for a developmental genetics in his report in 1935 [74] on the hs-induction of phenocopies of Drosophila developmental mutants. This enormous work was based on the analysis of about 500,000 individuals. His experimental techniques were later extended by N. Petersen and H. K. Mitchell [75] to Drosophila, and J. German [76], summarizing numerous observations in vertebrates, put forward a hypothesis of embryonic stress resulting in formation of abnormal organ anlagen. The brief outline of the early experimental results may help to understand the remarkable velocity and broad scope of scientific development initiated by the discovery of hs-inducible genes and the corresponding proteins [1, 2]. HSPs and the transcription factors regulating their expression (HSFs) will be the focus of this review.