Amino acid metabolism under drought stress in Arabidopsis thaliana

Abstract

Abstract: Due to climate change, drought periods will occur more frequently in the future. They will have a strong negative impact on crop yields. Drought stress leads to an osmotic imbalance and causes the closure of stomata to reduce water loss of transpiration. However, this reduces photosynthesis and ultimately leads to the formation of oxygen radicals, which may damage cell structure and function. If drought stress continues, a dramatic lack of energy is caused, which threatens plant life. To prevent irreversible damage, plants adapt their entire metabolism to resist drought stress at an early stage. This dissertation is dedicated to the adaptation of plants upon drought stress and the specific contribution of amino acid metabolism during this process. An in vitro experiment was performed to investigate the implications of a short but severe water deficit (Chapter 2.1). The Arabidopsis seedlings showed a strong decrease in protein content within 24h and at the same time a strong accumulation of the amino acids L-proline and GABA. Proteome analyses revealed that the aromatic amino acids were primarily used for the synthesis of stress mitigating secondary metabolites, such as flavonoids and anthocyanins, which are known to scavenge reactive oxygen species. Furthermore, a general induction of amino acid catabolism was observed, which provides sufficient amounts of L-glutamate for the synthesis of L-proline and GABA. Simultaneously, the catabolic pathways could represent an alternative source of reduction equivalents, which may fuel mitochondrial ATP production under carbon starvation conditions. In soil experiments were performed to investigate the plant drought stress response in a more physiological context (Chapter 2.2). In both, the in vitro and the in soil system, the plant stress response can be divided into distinct phases. The osmotically active amino acids, L-proline and GABA, are already produced in early phases of the water deficit and allow keeping the cellular water content constant for several days. Shortly before plants become irreversibly impaired by drought, a massive protein degradation takes place. This marks the beginning of the severe stress phase. Based on the proteome data and theoretical considerations, an experimental strategy was developed, which allows calculating absolute contents, concentrations and even copy numbers of individual proteins per leaf cell. As a result, the dynamic interconnection of protein homeostasis and amino acid homeostasis could be monitored and quantified on absolute scales. Our approach reveals the energy content of the released amino acids and indicates that their complete oxidation would cover the energy demand of the plant for several hours. In a review article, the regulatory properties of amino acids during the plant stress response were summarized and discussed (Chapter 2.3): Amino acids can be used as signal molecules, e.g. for inducing stomatal closure, as sensors of the nutrient content of cells or regulators for inducing their own catabolism. Our findings contribute to a general understanding of the effects of drought stress on the plant metabolism and shed light on the versatile and important roles of amino acids beyond their role in representing building blocks for protein biosynthesis.