Upon exposure to heat or cold, Arabidopsis thaliana seedlings undergo rapid transcriptional reprogramming of several hundreds of genes that promote stress tolerance. Despite extensive characterisation of the transcriptional responses to these stimuli, however, relatively little is known about the mechanisms by which temperature signals are perceived and transduced in plant cells. High or low seasonal temperatures have large impacts on crop productivity and are expected to intensify given current global climatic projections. It is therefore of agricultural importance to better understand temperature signalling pathways in plants in order to find solutions to this problem. In this thesis, a chemical genomics screen for molecules activating or repressing heat-inducible genes in A. thaliana was performed in collaboration with Syngenta and the biological targets of these chemicals were predicted based on structural similarities to compounds with known modes of action. Many molecules that affect the function of chloroplasts or mitochondria either activate or repress heat-responsive genes, thus implicating these organelles in the regulation of plant temperature responses. In addition, the translation inhibitor cycloheximide was identified as a repressor of heat-inducible genes and an activator of early cold-inducible genes. Diverse translation inhibitors trigger a cytosolic influx of calcium ions and several inhibitors of translation elongation were found to strongly activate cold-inducible gene expression in a calcium-dependent manner. Furthermore, it was demonstrated that cold shock causes rapid translation repression in A. thaliana seedlings and that the elongation factor LOS1 is involved in cold- or cycloheximide-induced gene expression, thus implicating translational machinery in the regulation of temperature signalling in plants. Finally, one of the chemicals identified in the screen, S01A463859Y, was found to improve heat resilience in A. thaliana and may therefore be of potential use in enhancing crop productivity during thermal stress.

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