PGR5 is required to avoid photosynthetic oscillations during light transitions

Abstract

AbstractThe production of ATP and NADPH by the light reactions of photosynthesis and their consumption by the Calvin-Benson-Bassham (CBB) cycle and other downstream metabolic reactions requires careful regulation. Environmental shifts perturb this careful balance, leading to photo-oxidative stress and losses in CO2assimilation. Imbalances in the production and consumption of ATP and NADPH manifest themselves as transient instability in the chlorophyll fluorescence, P700, electrochromic shift and CO2uptake signals recorded on leaves. These oscillations can be induced in wild-type plants by sudden shifts in CO2concentration or light intensity, however mutants exhibiting increased oscillatory behaviour have yet to be reported. This has precluded an understanding of the regulatory mechanisms employed by plants to suppress oscillations. Here we show that the Arabidopsispgr5mutant, which is deficient in PGR5-dependent cyclic electron transfer (CET), exhibits increased oscillatory behaviour. In contrast, mutants lacking the NDH-dependent CET are largely unaffected. The absence of oscillations in thehope2mutant, which likepgr5, lacks photosynthetic control and exhibits high ATP synthase conductivity, ruled out loss of these photoprotective mechanisms as causes. Instead, we observed slower formation of proton motive force and by inference ATP synthesis inpgr5following environmental perturbation, leading to the transient reduction of the electron transfer chain and photosynthetic oscillations. PGR5-dependent CET therefore plays a major role in damping the effect of environmental perturbations on photosynthesis to avoid losses in CO2fixation.