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Throughout their life cycle, plants adjust growth in response to their developmental and environmental situation within the limits of their energetic capacities. This capacity is defined by the local sugar availability, which is constantly modulated through synthesis, transport and consumption of sugar. The monitoring of sugar presence is carried out by a complex signalling network in which simple sugars (e.g. glucose, fructose and sucrose) act as metabolic signals for the modulation of physiological processes. However, often it remains unclear whether the regulation is induced by the simple sugars themselves or by their derivatives generated during sugar metabolism. This thesis focuses on the dissection of distinct sugar signals, their generation, perception and impact on the modulation of gene expression and cell division both in cell culture and young seedlings.

Based on a stem-cell-like A. thaliana cell culture, which could be sustained in a hormone-free media, a new biological system, supplied with Xyl as the only carbon source was developed. The performance of a variety of sugar and sugar analogue treatments in this novel system allowed for the identification of sugar-responsive candidate genes, which were specifically regulated by glucose, fructose and sucrose. For several genes (e.g. bZIP63, AT5g22920, TPS9, MGD2 and BT2), this regulation required both sugar transport into the cytosol and metabolisation for the generation of the signal. Furthermore, gene expression analyses in young A. thaliana seedlings indicated the requirement for the catalytic activity of hexokinase 1 in the regulation of bZIP63, Atg22920 and BT2 under conditions of a perturbed carbohydrate balance. These findings have been combined in a proposed model for the transcriptional regulation of bZIP63, AT5g22920, TPS9, MGD2 and BT2, which further proposes a function of those genes in the regulation of cell division.

The optimisation of a protocol for long-term real-time live-cell imaging provided a valuable tool to show that, similar to gene expression, the progression of cell division depended on a sugar-type-specific regulation at the single-cell level; this regulation was most likely caused by prolongation of the interphase. Together with the observation of cell death and growth arrest of the primary root in intact seedlings in response to the glucose analogue 2dog, this led to the conclusion that sugar signals themselves were sufficient to induce cell division. However, the continuation of cell cycle progression and consequently organ growth over long-time required the availability of the energy contained in the sugar.