Ralf Prändl

HSF3, a new heat shock factor from Arabidopsis thaliana, derepresses the heat shock response and confers thermotolerance when overexpressed in transgenic plants

Abstract Organisms synthesize heat shock proteins (HSPs) in response to sublethal heat stress and concomitantly acquire increased tolerance against a subsequent, otherwise lethal, heat shock. Heat shock factor (HSF) is essential for the transcription of many HSP genes. We report the isolation of two HSF genes, HSF3 and HSF4, from an Arabidopsis cDNA library. Transgenic Arabidopsis plants were generated containing constructs that allow expression of HSF3 and HSF4 or the respective translational β-glucuronidase (GUS) fusions. Overexpression of HSF3 or HSF3-GUS, but not of HSF4 or HSF4-GUS, causes HSP synthesis at the non-heat-shock temperature of 25° C in transgenic Arabidopsis. In transgenic plants bearing HSF3/HSF3-GUS, transcription of several heat shock genes is derepressed. Electrophoretic mobility shift assays suggest that derepression of the heat shock response is mediated by HSF3/HSF3-GUS functioning as transcription factor. HSF3/HSF3-GUS-overexpressing Arabidopsis plants show an increase in basal thermotolerance, indicating the importance of HSFs and HSF-regulated genes as determinants of thermoprotective processes. Plants transgenic for HSF3/HSF3-GUS exhibit no other obvious phenotypic alterations. Derepression of HSF activity upon overexpression suggests the titration of a negative regulator of HSF3 or an intrinsic constitutive activity of HSF3. We assume that stable overexpression of HSFs may be applied to other organisms as a means of derepressing the heat shock response.

Developmental regulation and tissue-specific differences of heat shock gene expression in transgenic tobacco and Arabidopsis plants

The heat shock (hs) response during plant growth and development was analyzed in tobacco and Arabidopsis using chimaeric β-glucuronidase reporter genes (hs-Gus) driven by a soybean hs promoter. Fluorimetric measurements and histochemical staining revealed high Gus activities in leaves, roots, and flowers exclusively after heat stress. The highest levels of heat-inducible expression were found in the vascular tissues. Without heat stress, a developmental induction of hs-Gus was indicated by the accumulation of high levels of Gus in transgenic tobacco seeds. There was no developmental induction of hs-Gus in Arabidopsis seeds. In situ hybridization to the RNA of the small heat shock protein gene Athsp17.6 in tissue sections revealed an expression in heat-shocked leaves but no expression in control leaves of Arabidopsis. However, a high level of constitutive expression of hs gene was detected in meristematic and provascular tissues of the Arabidopsis embryo. The developmental and tissue-specific regulation of the hs response is discussed.

Heat shock elements are involved in heat shock promoter activation during tobacco seed maturation

The soybean Gmhsp17.3-B heat shock promoter is developmentally regulated in transgenic tobacco, as indicated by the constitutive expression of a β-glucuronidase reporter in seeds [16]. In this paper, we show that both the heat shock promoter-driven β-glucuronidase activity and the mRNA of the endogenous Nthsp18P gene accumulate coincident with the onset of seed desiccation. Deletions of the soybean Gmhsp17.3-B promoter, encompassing the heat shock element (HSE)-containing regions, revealed a co-localization of sequences responsible for heat induction and developmental expression. Moreover, synthetic HSEs fused to a TATA box sequence had the potential to stimulate the developmental expression of a GUS reporter gene in seeds of transgenic plants.

Heat Stress-Dependent DNA Binding of Arabidopsis Heat Shock Transcription Factor HSF1 to Heat Shock Gene Promoters in Arabidopsis Suspension Culture Cells in vivo

Abstract Using UV laser cross-linking and immunoprecipitation we measured the in vivo binding of Arabidopsis heat shock transcription factor HSF1 to the promoters of target genes, Hsp18.2 and Hsp70. The amplification of promoter sequences, co-precipitated with HSF1-specific antibodies, indicated that HSF1 is not bound in the absence of heat stress. Binding to promoter sequences of target genes is rapidly induced by heat stress, continues throughout the heat treatment, and declines during subsequent recovery at room temperature. The molecular mechanisms underlying the differences between Hsp18.2 and Hsp70 in the kinetics of HSF1/promoter binding and corresponding mRNA expression profiles are discussed.

Molecular and applied aspects of the heat stress response and of common stress tolerance in plants

The heat shock (hs) response, the synthesis of heat shock proteins (HSPs) and the acquisition of stress tolerance, in plants is environmentally induced by heat stress and several other stresses including drought, heavy metal, and oxidative stresses. Thus HSPs, probably acting as molecular chaperones, are important determinants for developing tolerance against heat stress and probably also against other environmental stresses. Interestingly, HSPs are also synthesised in the absence of detectable environmental stress in the zygotic embryo of many plant species during seed maturation and during microspore embryogenesis. The HSPs therefore seem to play a key role in plant growth and development. The central regulator of the hs response is the hs transcription factor (HSF) whose activity for binding to the conserved hs promoter elements (HSE) and subsequent transcription of hs genes is induced upon hs. In plants multiple HSF-like genes have been isolated on the basis of HSE binding activity and conserved sequences. HSF genes are the targets for both, studying the regulation and manipulation of the hs response and hence stress tolerance.

The Role of HSF in Heat Shock Signal Transduction and Heat Shock Response in Plants

All organisms are exposed to different stress conditions in their natural environment. Particularly in plants, survival and yield are strongly influenced by environmental conditions. The response to heat stress is a model for acquisition of thermotolerance and it may also apply for the development of common stress tolerance of cells and organisms. The knowledge gained in recent years about stress signal transfer and regulation of gene expression led also to the first successes in changing stress tolerance traits in transgenic plants. This paper deals with the central role of the heat shock transcription factors in the signal transfer, the regulation of the heat shock response and the development of thermotolerance.