Ultimately, after phosphorylation of S230, it leads to transcriptional activation of Hsp genes ( Holmberg et al., 2001 Morimoto, 1998 Pirkkala, Nykanen & Sistonen, 2001).Īnother well investigated heat stress transcription system is that of yeast Hsf1 (ScHsf1) ( Sorger & Pelham, 1988 Sorger, 1991). During heat stress Hsf1 shuttles to the nucleus, assembles in heat stress granules, trimerizes and binds to heat stress elements. During non-stress conditions Hsf1 is a monomer and phosphorylated at S303, S307, S326 and S363, which results in repression of DNA-binding capacity and transcriptional activity. So far, 5 phosphorylation sites have been characterized for mammalian Hsf1: S230, S303, S307, S326 and S363 ( Chu et al., 1996). It was shown that the transcriptional activity of mammalian Hsf1 is regulated by phosphorylation ( Knauf et al., 1996 Kline & Morimoto, 1997 Holmberg et al., 2001). The central regulators of the expression of heat stress-responsive genes are the heat stress transcription factors (Hsfs) ( Baniwal et al., 2004 Kotak, Larkindale & Lee, 2007a Miller & Mittler, 2006 Morimoto, 1998 Nover & Scharf, 1997 Scharf, Hohfeld & Nover, 1998 Schöffl, Prandl & Reindl, 1998 von Koskull-Doring, Scharf & Nover, 2007). Accumulation of heat shock proteins (Hsps) are an essential part of the heat stress response and are assumed to play a central role in acquired thermotolerance in plants and other organisms ( Baniwal et al., 2004 Hartl & Hayer-Hartl, 2002 Haslbeck, 2002 Kotak, Vierling & Baumlein, 2007b Morimoto, 1998 Nakamoto & Vigh, 2007 Wang et al., 2004). In particular, temperatures above the optimum are sensed as heat stress, which disturbs cellular protein homeostasis. To survive under these conditions, complex signalling networks developed which allow terrestrial plants to efficiently sense and respond to stressful conditions. Due to their sessile lifestyle, plants are constantly subjected to a variety of biotic and abiotic stresses in their environment.