Biology subjects

Joliot, P.

Publications and source records attributed to Joliot, P..

3 recordsLinked to original sources

bioRxiv2020

ECS-based investigation of chloroplast ATP synthase regulation

The chloroplast ATP synthase (CF1Fo) contains a specific feature to the green lineage: a {gamma}-subunit redox domain which contains a cysteine couple and interacts with the torque-generating {beta}DELSEED-loop. Based on the recently solved structure of this domain, it was proposed to function as a chock. In vitro, {gamma}-disulfide formation slows down the activity of the CF1Fo at low transmembrane electrochemical proton gradient [Formula]. Here, we utilize in vivo absorption spectroscopy measurements for functional CF1Fo activity characterization in Arabidopsis leaves. The spectroscopic method allows us to measure the [Formula] present in dark-adapted leaves, and to identify its mitochondrial sources. Furthermore, we follow the fate of the extra [Formula] generated by an illumination, including its osmotic and electric components, and from there we estimate the lifetime of the light-generated ATP. In contrast with a previous report [Joliot and Joliot, Biochim. Biophys. Acta, 1777 (2008) 676-683], the CF1Fo {gamma}-subunit exists mostly in an oxidized form in the dark-adapted state. To study the redox regulation of the CF1Fo, we used thiol agent infiltration in WT and a mutant that does not form the {gamma}-disulfide. The obtained [Formula] -dependent CF1Fo activity profile in the two {gamma}-redox states in vivo reconciles with previous biochemical in vitro findings [Junesch and Graber, Biochim. Biophys. Acta, 893 (1987) 275-288]. The highest rates of ATP synthesis we measured in the two {gamma}-redox state were similar at high [Formula]. In the presence of the {gamma}-dithiol, similar rates were obtained at a ~45 mV lower [Formula] value compared to the oxidized state, which closely resembled the energetic gap of 0.7 {Delta}pH units reported in vitro.

plant biology↗

bioRxiv2017

Cyclic electron flow in Chlamydomonas reinhardtii

Cyclic electron flow (CEF), one of the major alternative electron transport pathways to the primary linear electron flow (LEF) in chloroplasts has been discovered in the middle of the last century. It is defined as a return of the reductants from the acceptor side of the Photosystem I (PSI) to the pool of its donors via the cytochrome b6f, and has proven essential for photosynthesis. However, despite many efforts aimed at its characterisation, the pathway and regulation of CEF remain equivocal, and its physiological significance remains to be properly defined. Here we use novel spectroscopic approaches to measure CEF in transitory conditions in the green alga Chlamydomonas reinhardtii. We show that CEF operates at the same maximal rate regardless of the oxygen concentration, and that the latter influences LEF, rather than CEF in vivo, which questions the recent hypotheses about the CEF supercomplex formation. We further reveal that the pathways proposed for CEF in the literature are inconsistent with the kinetic information provided by our measurements. We finally provide cues on the regulation of CEF by light.

plant biology↗

bioRxiv2017

Photosynthesis is heavily chlororespiration-sensitive under fluctuating light conditions

Photosynthesis needs to adjust to dynamically changing light intensities in order to maximize its efficiency, notably by the employment of alternative electron pathways. One of them is chlororespiration - initially described in Chlamydomonas reinhardtii. This electron transfer pathway, found in all photosynthetic lineages, consists of a reduction of plastoquinone (PQ) through an NAD(P)H:PQ oxidoreductase and quinol (PQH2) oxidation by Plastid Terminal Oxidase, PTOX. Hence, chlororespiration constitutes an electron pathway potentially antagonistic to the linear photosynthetic electron flow from H2O to CO2. However, the limited flow chlororespiratory enzymes can sustain suggests that their relative contribution, at least in the light and in steady-state conditions, is insubstantial. Here, we focused on the involvement of PTOX in Chlamydomonas reinhardtii during transitions from dark to light and vice versa. We show that the kinetics of redox relaxation of the chloroplast in the dark was greatly affected when PTOX2, the major plastoquinol oxidase in Chlamydomonas, is lacking. Importantly, we show that this has a direct physiological relevance, as the growth of a PTOX2-lacking mutant is markedly slower in intermittent light. The latter can be rationalized in terms of a decreased flux sustained by photosystem II due to a redox limitation at the acceptor side of the PSI during the illumination periods. We finally show that the long-term regulation of cyclic electron flow around PSI is strongly affected in the PTOX2 mutant, substantiating an important role of chlororespiration in the maintenance of chloroplast redox balance.

plant biology↗