Derepression of the Heat Shock Response and Genetic Engineering of Thermotolerance in Arabidopsis using Recombinant Heat Shock Factor

Abstract

The co-ordinate expression of HSPs is regulated by plant heat shock (HS) transcription factor (HSF) which recognizes the conserved HSE promoter sequences in a temperature-dependent fashion and subsequently stimulates transcription of HS genes. The activation of HSF is the key to an alteration of the HS response by genetic engineering, e.g. for constitutive synthesis of HSPs and increase in thermotolerance. Using Arabidopsis as a model for higher plants we have isolated Athsf1, the prime HSF gene responsible for the control of transcription of HS genes. This gene is constitutively expressed but its activation for trimerization, DNA-binding, and transcriptional stimulation of HS genes depends on heat stress. Ectopically over-expressed ATHSF1 is regulated in the same way in transgenic Arabidopsis. Derepression of HSF activity was achieved by transgenic expression of gene fusions between Athsf1 and a glucuronidase reporter gene. The recombinant HSF/GUS proteins are constitutively active at normal temperature as demonstrated by trimerization, DNA-binding and 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 provide evidence for a fundamental role of HSPs in plant stress tolerance.