Partitioning of the proton motive force into delta phi and delta pH and its role in light regulation of photosynthesis

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Photosynthesis is driven by the conversion of light energy into chemical energy. The overall proton motive force (pmf = the sum of the transmembrane electric potential (delta phi) and pH gradient (delta pH)) which is built up across the thylakoid membrane along with photosynthetic electron transport represents the driving force for ATP synthesis. However, the importance of delta phi for other processes, that are regulated by the lumen pH (and hence also by the pH gradient), such as energy dissipation and zeaxanthin synthesis, is largely understudied. The partitioning of the pmf into its two components, delta phi and delta pH, can easily be studied in plants under in vivo conditions by measuring electrochromic absorption changes which are a direct measure for the transmembrane electric field. Our preliminary data indicated that the dynamics of the partitioning of the pmf do not correspond well with the dynamics of pH-regulated energy dissipation under in vivo conditions, supporting the hypothesis that also the transmembrane electric field may contribute to this photoprotective mechanism. The goal of the planned project is to elucidate the role of the transmembrane electric field (delta phi) in light activation, light acclimation, and regulation of photosynthesis. To this end we want to follow a spectroscopic approach (i) to perform a detailed characterization of the dynamics of pmf partitioning in response to different light conditions and in relation to pH-regulated photosynthetic processes and (ii) to study the impact of the artificial modification of the relative contributions of delta phi and delta pH to the overall pmf on pH-regulated processes.