The thylakoid proton motive force in vivo. Quantitative, non-invasive probes, energetics, and regulatory consequences of light-induced pmf

Endogenous probes of light-induced transthylakoid proton motive force (pmf), membrane potential (Δψ) and ΔpH were used in vivo to assess in Arabidopsis the lumen pH responses of regulatory components of photosynthesis. The accumulation of zeaxanthin and protonation of PsbS were found to have similar pKa values, but quite distinct Hill coefficients, a feature allowing high antenna efficiency at low pmf and fine adjustment at higher pmf. The onset of “energy-dependent’ exciton quenching (qE) occurred at higher lumen pH than slowing of plastoquinol oxidation at the cytochrome b6f complex, presumably to prevent buildup of reduced electron carriers that can lead to photodamage. Quantitative comparison of intrinsic probes with the electrochromic shift signal in situ allowed quantitative estimates of pmf and lumen pH. Within a degree of uncertainly of ∼ 0.5 pH units, the lumen pH was estimated to range from ∼ 7.5 (under weak light at ambient CO2) to ∼ 5.7 (under 50 ppm CO2 and saturating light), consistent with a ‘moderate pH’ model, allowing antenna regulation but preventing acid-induced photodamage. The apparent pKa values for accumulation of zeaxanthin and PsbS protonation were found to be ∼ 6.8, with Hill coefficients of about 4 and 1 respectively. The apparent shift between in vitro violaxanthin deepoxidase protonation and zeaxanthin accumulation in vivo is explained by steady-state competition between zeaxanthin formation and its subsequent epoxidation by zeaxanthin epoxidase. In contrast to tobacco, Arabidopsis showed substantial variations in the fraction of pmf (0.1–0.7) stored as Δψ, allowing a more sensitive qE response, possible as an adaptation to life at lower light levels.