Prospects of engineering thermotolerance in crops through modulation of heat stress transcription factor and heat shock protein networks

Cell survival under high temperature conditions involves the activation of heat stress response (HSR), which in principle is highly conserved among different organisms, but shows remarkable complexity and unique features in plant systems. The transcriptional reprogramming at higher temperatures is controlled by the activity of the heat stress transcription factors (Hsfs). Hsfs allow the transcriptional activation of HSR genes, among which heat shock proteins (Hsps) are best characterized. Hsps belong to multigene families encoding for molecular chaperones involved in various processes including maintenance of protein homeostasis as a requisite for optimal development and survival under stress conditions. Hsfs form complex networks to activate downstream responses, but are concomitantly subjected to cell-type-dependent feedback regulation through factor-specific physical and functional interactions with chaperones belonging to Hsp90, Hsp70 and small Hsp families. There is increasing evidence that the originally assumed specialized function of Hsf/chaperone networks in the HSR turns out to be a complex central stress response system that is involved in the regulation of a broad variety of other stress responses and may also have substantial impact on various developmental processes. Understanding in detail the function of such regulatory networks is prerequisite for sustained improvement of thermotolerance in important agricultural crops. The review compiles recent studies on model and crop plants which suggest that genetic engineering of Hsf-chaperone networks can improve thermotolerance. Although conserved in their basic functions, species-specific variations in the composition and interaction of the two central networks involved in regulation of stress response and maintenance of protein homeostasis under stressful but also normal growth conditions have been described. Hence, manipulations on the molecular level guided only by model plant-specific knowledge transfer might cause unexpected pleiotropic effects. Detailed knowledge of the multi-level regulatory mechanisms controlling the availability and activity of Hsf-chaperone networks undoubtedly is required to unravel promising targets for manipulation and selection of cultivars that can combine both high productivity and enhanced thermotolerance.