Effect of PGR5 Impairment on Photosynthesis and Growth in Arabidopsis thaliana

PGR5 has been reported as an important factor for the activity of the ferredoxin-dependent cyclic electron transport around PSI. To elucidate the role of PGR5 in C-3 photosynthesis, we characterized the photosynthetic electron transport rate (ETR), CO2 assimilation and growth in the Arabidopsis thaliana pgr5 mutant at various irradiances and with CO2 regimes. In low-light-grown pgr5, the CO2 assimilation rate and ETR were similar to the those of the wild type at low irradiance, but decreased at saturating irradiance under photorespiratory conditions as well as non-photorespiratory conditions. Although non-photochemical quenching of chlorophyll fluorescence (NPQ) was not induced in the pgr5 mutant under steady-state photosynthesis, we show that it was induced under dark to light transition at low CO2 concentration. Under low light conditions in air, pgr5 showed the same growth as the wild type, but a significant growth reduction compared with the wild type at 150 mol photons m(2) s(1). This growth impairment was largely suppressed under high CO2 concentrations. Based on the intercellular CO2 concentration dependency of CO2 assimilation, ETR and P700 oxidation measurements, we conclude that reduction of photosynthesis and growth result from (i) ATP deficiency and (ii) inactivation of PSI. We discuss these data in relation to the role of PGR5-dependent regulatory mechanisms in tuning the ATP/NADPH ratio and preventing inactivation of PSI, especially under conditions of high irradiance or enhanced photorespiration.