Investigating photosynthesis by proton motive force modulations via ATP synthase and cytochrome b6f complex

• Applicant: Dr. Felix Buchert
• Subject Area: Plant Biochemistry and Biophysics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461765884

Photosynthesis converts light energy to chemical energy. Tightly regulated processes enable light harvesting by two photosystems which is associated with linear electron transfer from water to CO2, and the generation of a proton motive force (pmf) that fuels the ATP synthase. Both photosystems are functionally coupled by mobile electron carriers and the cytochrome b6f complex, which is also part of a cyclic electron flow pathway around photosystem I in ATP-deprived conditions. The pmf has two components, an electric membrane potential (delta.psi) and an osmotic gradient of H+ ions (delta.pH). The delta.pH is crucial on two photoprotective levels: It triggers the onset of nonphotochemical chlorophyll fluorescence quenching to protect photosystem II, and it induces the slowdown of electron flow at the cytochrome b6f complex to protect photosystem I. On the other hand, an excessive delta.pH and delta.psi destabilise photosystem II. Therefore, the pmf is subject to qualitative and quantitative control for efficient photosynthesis. In this context, ion channels and antiporters but also the major proton-conductive modulator, the ATP synthase, are important.The ATP synthase is regulated on multiple levels and its ATP hydrolysis activity is elevating the pmf in darkness in the green alga Chlamydomonas reinhardtii. In contrast, the ATP synthase in the vascular plants shows very low ATPase activity in the dark. We have indications that this difference is linked to structural variations in a rotor subunit which will be tested in domain swapping mutants of Chlamydomonas and Arabidopsis thaliana. In this gain- vs. loss-of-function approach, we will examine and compare how the photosynthetic apparatus that is adapted to algal- and plant-specific photoprotection mechanisms is affected by the non-native ATP synthase behaviour. Moreover, we will explore an algal ATP synthase mutant that fails to throttle its activity in response to altered metabolic conditions, and plan to compare the phenotype to the situation in Arabidopsis. Since electron transfer via the cytochrome b6f complex is subjected to ATP synthase performance, we will use a b6f mutant which is hypersensitive to the lumen acidification. Thereby, we plan to obtain more insights into the established pmf quality in the context of regulatory ATP synthase adjustments. Furthermore, we explore the possibility of delta.psi sensing in the algal cytochrome b6f complex as a measure to keep the complex functional. Finally, we plan to investigate if a hybrid ATP synthase with a non-native rotor ring allows Chlamydomonas to grow at different light conditions by harbouring a modified pmf-to-ATP conversion efficiency.

• DFG Programme: Research Grants