Feedback limitation of photosynthesis at high CO₂ acts by modulating the activity of the chloroplast ATP synthase
2009
Kiirats, Olavi | Cruz, Jeffrey A. | Edwards, Gerald E. | Kramer, David M.
It was previously shown that photosynthetic electron transfer is controlled under low CO₂ via regulation of the chloroplast ATP synthase. In the current work, we studied the regulation of photosynthesis under feedback limiting conditions, where photosynthesis is limited by the capacity to utilise triose-phosphate for synthesis of end products (starch or sucrose), in a starch-deficient mutant of Nicotiana sylvestris Speg. & Comes. At high CO₂, we observed feedback control that was progressively reversed by increasing O₂ levels from 2 to 40%. The activity of the ATP synthase, probed in vivo by the dark-interval relaxation kinetics of the electrochromic shift, was proportional to the O₂-induced increases in O₂ evolution from PSII (JO₂), as well as the sum of Rubisco oxygenation (vo) and carboxylation (vc) rates. The altered ATP synthase activity led to changes in the light-driven proton motive force, resulting in regulation of the rate of plastoquinol oxidation at the cytochrome b₆f complex, quantitatively accounting for the observed control of photosynthetic electron transfer. The ATP content of the cell decreases under feedback limitation, suggesting that the ATP synthesis was downregulated to a larger extent than ATP consumption. This likely resulted in slowing of ribulose bisphosphate regeneration and JO₂). Overall, our results indicate that, just as at low CO₂, feedback limitations control the light reactions of photosynthesis via regulation of the ATP synthase, and can be reconciled with regulation via stromal Pi, or an unknown allosteric affector.
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