To “leaf” or not to “leaf” : Understanding the metabolic adjustments associated with leaf senescence

Sammanfattning: The adequate execution of the final developmental stage of a leaf, leaf senescence, is crucial to the long-term survival of the plant. During senescence cellular structures like membranes, proteins, lipids and macromolecules are degraded and released nutrients are relocated to developing parts of the plant, such as young leaves, stems, flowers, siliques and ultimately seeds that are dependent on this nutrient remobilization. The first visible sign of senescence is the yellowing of leaves indicating the degradation of chlorophyll and the dismantling of chloroplasts. As a consequence, senescing leaves cannot perform photosynthesis anymore and the delivery of energy from the chloroplast is compromised. As chloroplasts lose their function, the course of the senescence program requires a stable alternative energy sources that support nutrient remobilization while simultaneously ensuring a basic metabolism.To study leaf senescence I used the model plant Arabidopsis thaliana and applied different experimental approaches: Developmental Leaf Senescence (DLS), individual darkened leaves (IDL), completely darkened plants (DP) and a stay-green mutant which displays a delayed senescence phenotype during IDL. Using a combination of physiological, microscopic, transcriptomic and metabolomic analyses similarities and differences between these experimental setups were investigated with focus on the functions of mitochondria during leaf senescence.The catabolism of amino acids and the subsequent release of glutamate into the mitochondrial matrix seem to play an important role for nitrogen remobilization during DLS and IDL. Glutamate is then transported to the cytoplasm and transformed into glutamine, which can serve as long distance nitrogen export metabolite in the plant. Furthermore, senescing leaves in IDL are not only source tissues for nutrient remobilization in the plant, but we also detected labelled carbon in the darkened leaves, indicating a communication between the IDL and leaves in light. In contrary to the senescence inducing systems of DLS and IDL, in DP and the stay-green mutant investigated here, senescence is not induced by dark treatment. In both experimental setups we measured an accumulation of amino acids in the darkened leaves, in particular those with high N content. This could make reduced carbon available as alternative energy source during darkness. In this thesis we observed that mitochondria play an important role in nutrient reallocation processes during leaf senescence. The overall energy status of senescing tissues depends on mitochondria and especially amino acid metabolism seems to have a vital role during the senescence processes both for energy supply and nutrient reallocation.