Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase, which sustain the so-called chlororespiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow, knocking out the plastid terminal oxidase (PTOX) in plants or microalgae leads to severe phenotypes that encompass developmental and growth defects together with increased photosensitivity. On the basis of a phylogenetic and structural analysis of the enzyme, we discuss its physiological contribution to chloroplast metabolism, with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes, such as, among others, the regulation of cyclic electron transfer and carotenoid biosynthesis, which have in common their dependence on the redox state of the plastoquinone pool, set largely by the activity of PTOX in darkness.

Sunlight drives photosynthesis but can also cause photodamage. To protect themselves, photosynthetic organisms dissipate the excess absorbed energy as heat, in a process known as nonphotochemical quenching (NPQ). In green algae, diatoms, and mosses, NPQ depends on the light-harvesting complex stress-related (LHCSR) proteins. Here we investigated NPQ inChlamydomonas reinhardtiiusing an approach that maintains the cells in a stable quenched state. We show that in the presence of LHCSR3, all of the photosystem (PS) II complexes are quenched and the LHCs are the site of quenching, which occurs at a rate of ∼150 ps−1and is not induced by LHCII aggregation. The effective light-harvesting capacity of PSII decreases upon NPQ, and the NPQ rate is independent of the redox state of the reaction center. Finally, we could measure the pH dependence of NPQ, showing that the luminal pH is always above 5.5 in vivo and highlighting the role of LHCSR3 as an ultrasensitive pH sensor.

In nature, plants experience large fluctuations in light intensity and they need to balance the absorption and utilization of this energy accordingly. Non-photochemical quenching (NPQ) is a rapidly-switchable mechanism which protects plants from photodamage caused by high light exposure by dissipating the energy absorbed in excess as heat. It is triggered by the ΔpH across the thylakoid membrane and requires the presence of the protein PsbS and the xanthophyll zeaxanthin. However, the site and mechanism of the quencher(s) remain equivocal. Here, we constructed a mutant of Arabidopsis thaliana which lacks LHCII, the main antenna complexes of plants, to verify its contribution to NPQ. The mutant has normally stacked thylakoid membranes, displays no upregulation of other LHCs but shows a relative decrease in PSI which compensates for the decrease of the PSII antenna. The mutant exhibits a ~60% reduction in NPQ, while the remaining NPQ resembles that of the Chl bless mutant, which lacks all PSII peripheral antenna complexes. We thus report that PsbS-dependent NPQ mainly occurs within LHCII, but there is an additional quenching site within the PSII core. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: