ABSTRACT
In many organisms and also in plants there is correlative evidence that heat shock proteins (Hsps) protect cells from deleterious effects of heat stress (hs). Hsps are also implicated in common stress tolerance for many abiotic stresses. The co-ordinate expression of Hsps is regulated by plant heat shock transcription factors (HSFs) which recognize conserved promoter sequences in a temperature-dependent fashion and subsequently stimulate the transcription of hs genes. The activation of HSF is considered to be the key to an alteration of the hs response by genetic engineering, e.g. for generating mutants with a constitutive synthesis of Hsps and thus improved thermotoler-ance. Using Arabidopsis as a model for higher plants we have isolated Athsfl, a HSF gene responsible for the control of transcription of hs genes. This gene is constitutively expressed, and its activation for DNA-binding, and transcriptional stimulation depends on hs. Ectopically overexpressed ATHSF1 is regulated in the same way in transgenic Arabidopsis. However, derepression of HSF activity has been achieved by expression of gene fusions between Athsfl and a glucuronidase (GUS) reporter gene. The recombinant HSF/GUS proteins are constitutively active at normal temperature and result in a constitutive synthesis of Hsps. Transgenic plants exhibit a significantly higher level of basic thermotolerance. These data have implications for the mechanism of HSF regulation and demonstrate the fundamental role of Hsps in plant stress tolerance. The use of this strategy for genetic engineering of common stress tolerance in agriculturally important plants will be discussed.
