Two specific domains of the γ subunit of chloroplast F(o)F(1) provide redox regulation of the ATP synthesis through conformational changes

Participation of the stress-responsive CDSP32 thioredoxin in the modulation of chloroplast ATP-synthase activity in Solanum tuberosum

Plant thioredoxins (TRXs) are involved in numerous metabolic and signalling pathways, such as light-dependent regulation of photosynthesis. The atypical TRX CDSP32, chloroplastic drought-induced stress protein of 32 kDa, includes two TRX-fold domains and participates in responses to oxidative stress as an electron donor to other thiol reductases. Here, we further characterised potato lines modified for CDSP32 expression to clarify the physiological roles of the TRX. Upon high salt treatments, modified lines displayed changes in the abundance and redox status of CDSP32 antioxidant partners, and exhibited sensitivity to combined saline-alkaline stress. In non-stressed plants overexpressing CDSP32, a lower abundance of photosystem II subunits and ATP-synthase γ subunit was noticed. The CDSP32 co-suppressed line showed altered chlorophyll a fluorescence induction and impaired regulation of the transthylakoid membrane potential during dark/light and light/dark transitions. These data, in agreement with the previously reported interaction between CDSP32 and ATP-synthase γ subunit, suggest that CDSP32 affects the redox regulation of ATP-synthase activity. Consistently, modelling of protein complex 3-D structure indicates that CDSP32 could constitute a suitable partner of ATP-synthase γ subunit. We discuss the roles of the TRX in the regulation of both photosynthetic activity and enzymatic antioxidant network in relation with environmental conditions.

Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts

The proton motive force (pmf) generated across the thylakoid membrane rotates the Fo-ring of ATP synthase in chloroplasts. The pmf comprises two components: membrane potential (∆Ψ) and proton concentration gradient (∆pH). Acidification of the thylakoid lumen resulting from ∆pH downregulates electron transport in the cytochrome b6f complex. This process, known as photosynthetic control, is crucial for protecting photosystem I (PSI) from photodamage in response to fluctuating light. To optimize the balance between efficient photosynthesis and photoprotection, it is necessary to regulate pmf. Cyclic electron transport around PSI and pseudo-cyclic electron transport involving flavodiiron proteins contribute to the modulation of pmf magnitude. By manipulating the ratio between the two components of pmf, it is possible to modify the extent of photosynthetic control without affecting the pmf size. This adjustment can be achieved by regulating the movement of ions (such as K+ and Cl-) across the thylakoid membrane. Since ATP synthase is the primary consumer of pmf in chloroplasts, its activity must be precisely regulated to accommodate other mechanisms involved in pmf optimization. Although fragments of information about each regulatory process have been accumulated, a comprehensive understanding of their interactions is lacking. Here, I summarize current knowledge of the network for pmf regulation, mainly based on genetic studies.