Group Leader

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Dr. Mark Aurel Schöttler
Phone:+ 49 331 567-8311

Department Bock

Biophysics and Photosynthesis Research

The group of Dr. Mark Aurel Schöttler analyses the functional organization and regulation of the photosynthetic light reactions. To understand this process more thoroughly, we currently investigate the role of small plastome-encoded subunits for photosynthetic complex assembly, stability and function, the role of PSI in photosynthetic flux control and the lateral differentiation of the thylakoid membrane and the integration of photosynthetic electron transport into the plant metabolism and its adjustment to the metabolic ATP and NADPH demands.

<p>To avoid an over-acidification of the thylakoid lumen and the detrimental production of reactive oxygen species, the photosynthetic ATP and NADPH production is closely adjusted to the ATP and NADPH consumption rates by the Calvin cycle and the subsequent reactions of dark metabolism. High rates of ATP and NADPH production in young leaves with high assimilation capacity are facilitated by high contents of the rate-limiting components cytochrome b6f complex, plastocyanin and ATP synthase. In response to a diminished assimilation capacity, ageing leaves as well as mutants with an impaired Calvin cycle repress photosynthetic electron transport via down-regulation of the cytochrome b6f complex, plastocyanin and ATP synthase. The contents of both photosystems remain largely unaltered, independent of the metabolic demands of the leaf.</p> Zoom Image

To avoid an over-acidification of the thylakoid lumen and the detrimental production of reactive oxygen species, the photosynthetic ATP and NADPH production is closely adjusted to the ATP and NADPH consumption rates by the Calvin cycle and the subsequent reactions of dark metabolism. High rates of ATP and NADPH production in young leaves with high assimilation capacity are facilitated by high contents of the rate-limiting components cytochrome b6f complex, plastocyanin and ATP synthase. In response to a diminished assimilation capacity, ageing leaves as well as mutants with an impaired Calvin cycle repress photosynthetic electron transport via down-regulation of the cytochrome b6f complex, plastocyanin and ATP synthase. The contents of both photosystems remain largely unaltered, independent of the metabolic demands of the leaf.

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For these projects, a wide array of methods such as chloroplast transformation, gene expression and translation analysis (microarrays, polysome isolation), biochemistry (proteomics, HPLC), biophysics (time-resolved difference absorption spectroscopy, time-resolved and 77K chlorophyll-a-fluorescence) and ultrastructure analysis (atomic force microscopy, electron microscopy) is applied. In the fields of chloroplast transformation and difference absorption spectroscopy, we are involved in method development and optimization.

The role of small plastome-encoded subunits for photosynthetic complex assembly, stability and function

In recent years, high resolution structures of the photosynthetic complexes have become available. However, the function of several small subunits, which often do not bind any of the redox-active cofactors of electron transfer, is not yet understood. We characterize knock-out mutants of these subunits with a special focus on complex assembly, stability and function. For essential subunits such as cytochrome-b559 of PSII, whose knock-outs result in complete loss of a complex, we generate point mutations for further analysis.

The role of Photosystem I

Photosystem I catalyses the reduction of ferredoxin and NADPH. Although this step is much faster than the rate-limiting reactions of linear electron flux at the cytochrome-bf-complex, PSI accumulates to far higher amounts than the cytochrome-bf-complex. This results in an up to tenfold potential PSI over-capacity relative to linear electron flux. One potential function of the “surplus” number of PSI might be optimization of plastocyanin oxidation, as plastocyanin has a low binding affinity to PSI. This could be compensated by increasing the number of interaction partners. On the other hand, indications do exist that PSI plays a key role in triggering the lateral differentiation of thylakoid membranes into grana stacks and stroma lamellae. The first step of grana stacking is a lateral segregation of PSI and PSII, due to electrostatic and steric effects. In this context, a minimum number of PSI per thylakoid membrane area might be essential to trigger these segregation processes.

To test these hypotheses, we construct mutants with reduced PSI accumulation. For this purpose, the rate of translation of the reaction center subunits is reduced by means of point mutations in both the translation initation codon and the Shine-Dalgarno ribosome binding sequence. These transformants are subjected to detailed functional and structural analyses involving PSI flux control, redox equilibration of the high potential chain, electron microscopy and atomic force microscopy.

The adjustment of photosynthetic electron transport to the metabolic ATP and NADPH demand

To avoid potentially deleterious side reactions, the photosynthetic production of ATP and NADPH has to be adjusted to the rate of their consumption by the Calvin cycle and the downstream reactions of carbon assimilation. Otherwise, the whole cellular redox poise might be disturbed, and an increased production of reactive oxygen species could result in destruction of the photosynthetic apparatus and initiation of cell death responses. The photosynthetic flux and therefore ATP and NADPH production are regulated predominantly at the level of the cytochrome-bf-complex and of plastocyanin, which decrease in response to a reduced metabolic demand for ATP and NADPH. However, the actual metabolic signals, which trigger these adjustments are unknown.

Potential candidate signals include reactive oxygen species, accumulating sugars (“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 will determine whether the different signal classes initiate specific adaptive responses of the photosynthetic electron transport chain, focussing on gene expression, translation, photosynthetic complex assembly and functional organization of the electron transport chain.

 
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