Evidence for a respiratory chain in the chloroplast

Abstract

Evidence is given for the existence of an electron transport pathway to oxygen in the thylakoid membranes of chloroplasts (chlororespiration). Plastoquinone is shown to be a redox carrier common to both photosynthetic and chlororespiratory pathways. It is shown that, in dark-adapted chloroplasts, an electrochemical gradient is built up across the thylakoid membrane by transfer of electrons through the chlororespiratory chain as well as by reverse functioning of the chloroplast ATPases. It is proposed that these mechanisms ensure recycling of the ATP and NAD(P)H generated by the glycolytic pathway converting starch into triose phosphates. Chlororespiration is thus an 02-uptake process distinct from photorespiration and the Mehler reaction. The evolutionary significance of chlororespiration is discussed.Respiration is a process common to living cells in which reduced organic substrates are oxidized at the expense of molecular oxygen. The free energy recovered in this process is partly converted into heat and partly recovered by the coupled phosphorylation of ADP into ATP. The hydrolysis of ATP releases free energy available for cell functions. Conversely, photosynthesis generates strong reductants at the expense of the energy of light. This formation of strong reductants corresponds to the main energy-conversion process of photosynthesis. A minor part of the light energy is however used to phosphorylate ADP into ATP. The photosynthetic and respiratory apparatuses show strong similarities: (i) both implicate membrane-bound electron transfer chains, (ii) several membrane components are present in both systems (cytochromes, iron-sulfur proteins, quinones, ATPases), and (iii) both membrane systems form closed vesicles and generate ATP.Photosynthetic eukaryotic cells possess distinct organelles in which photosynthesis and respiration proceed-namely, chloroplasts and mitochondria. These organelles might be symbiotic prokaryotes that have transferred some of their genetic information to the host cell. In prokaryotic photosynthetic organisms, the photosynthetic and respiratory chains are located in the same membranes and share both electron transport components and coupling factors (1-3). One could thus question whether respiration occurs in the chloroplast as well. I show in this paper that, indeed, chlororespiration exists and is distinct from mitorespiration. MATERIALS AND METHODSChlamydomwnas reinhardtii wild-type 137C and mutant strains were grown in TAP medium (4) under an illumination of 200 lux. The mutants used in this work have been described (5-7). Chlorella pyrenoidosa was grown phototrophically as described (8).Open cell preparations were obtained as follows: 200 ml of cell suspension (2 X 106 cells/ml) was collected and suspended in 20 ml of 20 mM N-tris(hydroxymethyl)methylglycine(Tricine)/KOH, pH 7.8/10 mM NaCl/10 mM MgCl2/1 mM K2-HPO4/0.1 M sucrose/5% Ficoll. The cell suspension was passed through a Yeda press operated at 90 kg/cm2, diluted with 200 ml of Ficoll-lacking buffer, and centrifuged, a...

Evidence for an association of ndh B, ndh J gene products and ferredoxin‐NADP‐reductase as components of a chloroplastic NAD(P)H dehydrogenase complex

“…Alternatively, the NAD(P)H dehydrogenase complex may be involved in chlororespiration [8,26]. However, if chlororespiration has been extensively studied in unicellular green algae [7,8,27], its occurrence in higher plants chloroplasts is still controversial [27,28].…”

“…These chloroplastic genes, named ndh, have been shown to be transcribed [3] and the polypeptide products of three of them (NDH H, NDH I and NDH K) have been found in thylakoid membranes [4,5,6]. It has been suggested that ndh genes encode subunits of a chloroplastic NAD(P)H dehydrogenase complex [1] which might be involved in chlororespiration, a chloroplastic respiratory process mainly studied in the unicellular green alga Chlamydomonas [7,8]. Alternatively, the putative NAD(P)H dehydrogenase complex might participate in cyclic electron flow around photosystem 1 (PS1) by allowing electrons to enter the plastoquinone (PQ) pool from a stromal NAD(P)H pool [9,10].…”

The chloroplast Ndh complex mediates the dark reduction of the plastoquinone pool in response to heat stress in tobacco leaves

“…The physiological function of the plastid Ndh complex is unknown although it may contribute to cyclic electron transfer around the photosystem I (PSI) complex in the light [2,4,5] and is possibly involved in a respiratory electron transfer chain in the dark [6]. Although an intact Ndh complex is dispensible for growth of tobacco under`unstressed' growth conditions it is possible that the activity of the Ndh complex becomes more important under stress conditions that require enhanced ATP synthesis possibly driven by cyclic electron £ow around PSI [5].…”

A New Electrochemical Gradient Generator in Thylakoid Membranes of Green Algae

“…However, a permanent gradient was observed as well in the Chlamydomonas reinhardtii FUD50 mutant lacking this protein (2). The persistence of such a gradient in the absence of the chloroplast CF 0 CF 1 ATP synthase was a strong argument favoring the model of chlororespiration, which could also account for the pattern of dark oxidation and reduction of plastoquinone (2). In this model, the permanent gradient present in the absence of CF 0 CF 1 ATP synthase results from a coupled electron transfer through a chloroplast respiratory pathway, in which plastoquinone is reduced by a chloroplast NAD(P)H dehydrogenase and reoxidized at the expense of oxygen by a chloroplast oxidase.…”