| Photosynthetic electron transport and Calvin cycle reactions need to be co-regulated in order to provide optimal flux into end product and minimize the formation of reactive oxygen species leading to photo-inhibition. An important means of dissipating excess energy is mediated by an increased acidification in the lumen of thylakoid membranes of the chloroplast which has been proposed to occur through increased photochemistry through cyclic electron flow (CEF) via photosystem (PS) I, or by linear electron flow in the Mehler reaction. We have shown that decreases in the thylakoid membrane ATP synthase conductance to protons is an important component in this dissipation of excess energy and photoprotection. It is universal and it takes place in C3 plants, as well as in all three biochemical subtypes of C4 plants. C 4 plants showed a similar pattern of ATP synthase regulation to C 3 plants despite the differences in photosynthetic carbon metabolism. Down-regulation of ATP synthase proton conductivity at low CO2 and high light increases intrathylakoid H+ concentration which activates the energy dissipation mechanism, thus protecting PS II. Three mutants were tested which provided support for this hypothesis: one in photochemistry (related to CEF), one in CO2 fixation (Rubisco = ribulose 1,5-bisphosphate carboxylase oxygenase), and one in carbohydrate biosynthesis (starch-less). A possible mechanism of regulating ATP-synthase conductance to protons is through the levels of inorganic phosphate (Pi) in the chloroplast stroma, since this is a substrate for the enzyme. We tested this hypothesis using a starch-less mutant. This mutant is limited in utilizing the products of photosynthesis and is considered to cause a build-up of organic-P, a depletion of Pi and feedback inhibition of photosynthesis. ATP synthase conductivity closely followed the change in activity of carbon fixation reactions, which supports the hypothesis that Pi is a regulator of ATP synthase. The low Rubisco mutant of tobacco and a mutant of Arabidopsis which affects CEF also provided support for regulation of ATP synthase conductance having a key role in photoprotection and dissipation of excess energy. |
