dc.description.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. |
eng |