Title : Analysing plant responses to extreme environments at different scales: from subcellular compartments to whole plants in soil.
Climatic factors and resource availability can constitute extreme environments, and among them, water availability, temperature and irradiation are most important. We have shown that accumulation of sugars in specific cellular compartments is a prerequisite of acclimation to low temperature, and based on mathematical simulations, we could show that establishment of freezing tolerance affords a sequence of transport events that re-allocate a large number of metabolites between plastids, cytosol and vacuole of a plant cell. A fast response to a temperature drop includes sequestration of sucrose into plastids, which is later replaced by raffinose, probably because of its minor importance in central metabolism. The capacity of sucrose synthesis in the cytosol correlates with freezing tolerance in the model plant Arabidopsis thaliana, most likely because accumulation of intermediates would inhibit secondary reactions of photosynthesis. Using chlorophyll fluorescence, it can be shown that cold sensitive accessions of Arabidopsis are incapable of maintaining linear electron flow at low temperature, ultimately resulting in the production of reactive oxygen species. A so far unanswered question is, why plants accumulate large amounts of hexoses in the cold. Using thermal imaging to control freezing processes in whole plants, we can show that hexoses reduce the amount of freezable water in leaf tissue, thus preventing dehydration of the protoplast, when ice forms in intercellular spaces. At high irradiance as well as low temperatures, electron transport to oxygen can lead to oxidative stress. Transfer of reducing equivalents from the plastids to mitochondria, where they can be safely oxidized, takes place via the transport of carboxylic and amino acids. Using the sugar sensor mutant gin2-1, we show that these processes are interlinked with sugar metabolism, resulting in high light sensitivity of a mutant plant that is not able to sense its sugar levels.