Biophysics and Photosynthesis
The mission of Dr. Mark Aurel Schöttler's central infrastructure group “Biophysics and Photosynthesis” is to support photosynthesis-related measurements in the institute. To this end, the group offers gas exchange and chlorophyll-a fluorescence measurements as routine service techniques. Also, the set-up, programming and operation of growth chambers allowing the simulation of complex “field-like” changes in environmental parameters such as temperature, humidity, light intensity and light quality is supervised by the infrastructure group.
The “Biophysics and Photosynthesis” research group of Dr. Schöttler is part of Department 3, Prof. R. Bock, at the MPI-MP, in combination with a role as a science infrastructure group.
Service:
1) Gas exchange measurements are performed with four modern gas exchange systems equipped with several measuring heads for gas exchange analyses of leaves of different sizes, from single Arabidopsis leaves (1.5 – 3 cm2 area) to large leaves of up to 100 cm2 leaf area. Also, entire Arabidopsis rosettes or seedlings of other plant species can be measured. These systems allow a full control of environmental parameters such as temperature, humidity, CO2 and O2 concentration as well as actinic light intensity and light quality. Under these highly controlled conditions, it is possible to determine leaf respiration in darkness, leaf assimilation at different light intensities and CO2 concentrations, and transpiration (evaporation of water from the leaf). Furthermore, derived parameters such as stomatal conductance and ci (the CO2 concentration in the intercellular space) can be determined. In combination with chlorophyll-a fluorescence measurements (see below), also the role of other alternative electron sinks can be assessed.
2) Chlorophyll-a fluorescence measurements are mainly performed with Imaging-PAMs, which allow us to determine the spatial distribution of photosynthetic parameters such as linear electron flux over the leaf area. Technical options include high resolution images of small leaf areas, or overviews of photosynthesis in whole leaves or even entire plants. In this way, heterogeneous behavior of leaf photosynthesis due to developmental gradients in the leaf, different biotic and abiotic stresses, or due to the local or systemic application of chemical effectors can be resolved.
By coupling the Imaging-PAMs to gas exchange systems (see above), fluorescence imaging measurements can be performed under full control of environmental parameters such as temperature, humidity, CO2- and O2-concentration. Additionally, leaf absorptance measurements for the correct calculation of electron transport rates are performed as part of the service measurements.
3) As part of a scientific cooperation, more advanced spectroscopic techniques can be applied: These allow the determination of the relative stoichiometry of the photosynthetic complexes and of the thylakoid membrane energization and the activity of chloroplast ATP synthase in intact leaves. Also the precise absolute quantification of all redox-active components of the photosynthetic apparatus in isolated thylakoid membranes is possible.
Research:
The group of Dr. Mark Aurel Schöttler analyses the functional organization and regulation of the photosynthetic light reactions. We use spectroscopic techniques for the in vivo measurement of all major components of the photosynthetic apparatus. These techniques allow us to elucidate the response of the photosynthetic apparatus to changing metabolic ATP and NADPH demands, as caused, for example, by leaf development or abiotic stresses.
Usually, photosynthetic ATP and NADPH production is strictly adjusted to changes in their consumption by the Calvin-Benson cycle and downstream reactions of carbon assimilation, to avoid an over-reduction of the electron transport chain and electron transfer to alternative acceptors such as O2. Otherwise, the whole cellular redox poise might be disturbed, and an increased production of reactive oxygen species could result in the oxidative destruction of the photosynthetic apparatus and initiation of cell death pathways. We mainly focus on adaptive responses occurring on time scales of several hours to several weeks. We aim to obtain an integrated picture of the mechanisms, by which plants adjust photosynthetic ATP and NADPH production to the metabolic demand. For a better understanding especially of slower responses involving altered photosynthetic complex stoichiometry, mechanisms controlling photosynthetic complex accumulation and turnover are determined. With this knowledge, we ultimately will be able to engineer plants with improved photosynthetic performance, stress tolerance, and yield.
Long-term adjustment of photosynthetic electron transport to the metabolic ATP and NADPH demand
Long-term decreases in the metabolic consumption of ATP and NADPH mainly occur during leaf senescence or in response to drought and cold stress. This requires adjustments at the level of photosynthetic complex stoichiometry. Especially the contents of the cytochrome b6f complex, plastocyanin, and the chloroplast ATP synthase strictly correlate with assimilation capacity and, therefore, decrease in response to a reduced metabolic demand for ATP and NADPH. However, the actual metabolic and redox signals, which trigger these adjustments of the photosynthetic apparatus, are largely unknown. Potential candidate signals include reactive oxygen species, photoassimilate accumulation in the leaves (“sugar sensing”), phytohormones and the redox state of the stroma and the photosynthetic electron transport chain itself. To dissect the roles of the different signal classes, we use a wide range of mutants with defined changes in carbon metabolism and sugar sensing, detoxification of reactive oxygen species, and the redox state of the electron transport chain. We determine whether the different signal classes initiate specific adaptive responses of the photosynthetic apparatus. When specific adaptive responses are detected on the level of the light reactions, the underlying mechanisms, from gene expression and translation to photosynthetic complex assembly, maintenance and degradation, are elucidated.
Biogenesis, lifetime and turn-over of photosynthetic complexes
For a deeper understanding of the mechanisms adjusting photosynthetic complex content to the metabolic demand for ATP and NADPH, it is critical to know the lifetimes of the complexes. In case of a highly stable, long-lived complex, its content is mainly regulated on the level of controlled degradation, while in case of a short-lived complex with high protein turnover, its content is mostly regulated on the level of de novo biogenesis. While the biogenesis, lifetime and turnover of PSII have been thoroughly investigated since decades, much less is known about the other photosynthetic complexes. We mainly focus on the cytochrome b6f complex and PSI, using the inducible repression of essential nucleus-encoded structural subunits or of essential auxiliary proteins involved in complex assembly to stop the de novo biogenesis at different time points of leaf development and in different environmental conditions. Then, we follow the loss of the complexes, to determine their lifetimes, and the dependence of their turnover on different developmental states and abiotic stresses. Furthermore, novel auxiliary proteins involved in the biogenesis and turnover of these complexes are identified and functionally characterized.
