Abstract
Cyclic electron flux (CEF) around Photosystem I (PS I) is difficult to quantify. We obtained the linear electron flux (LEFO2) through both photosystems and the total electron flux through PS I (ETR1) in Arabidopsis in CO2-enriched air. ΔFlux = ETR1 – LEFO2 is an upper estimate of CEF, which consists of two components, an antimycin A-sensitive, PGR5 (proton gradient regulation 5 protein)-dependent component and an insensitive component facilitated by a chloroplastic nicotinamide adenine dinucleotide dehydrogenase-like complex (NDH). Using wild type as well as pgr5 and ndh mutants, we observed that (1) 40% of the absorbed light was partitioned to PS I; (2) at high irradiance a substantial antimycin A-sensitive CEF occurred in the wild type and the ndh mutant; (3) at low irradiance a sizable antimycin A-sensitive CEF occurred in the wild type but not in the ndh mutant, suggesting an enhancing effect of NDH in low light; and (4) in the pgr5 mutant, and the wild type and ndh mutant treated with antimycin A, a residual ΔFlux existed at high irradiance, attributable to charge recombination and/or pseudo-cyclic electron flow. Therefore, in low-light-acclimated plants exposed to high light, ΔFlux has contributions from various paths of electron flow through PS I.
Highlights
Arnon et al (1955) demonstrated photophosphorylation via a (CEF) around Photosystem I (PS I) by illuminating isolated thylakoids in the presence of vitamin K
The approach taken in this study has been to obtain the steadystate electron flux through PS I (ETR1) via the P700+ signal and the linear electron flux through both photosystems by oxygen evolution (LEFO2), at 25◦C, in broad-spectrum white incandescent light and CO2-enriched air
The total electron flux through PS I and the linear electron flux were compared under identical conditions, both being whole-tissue measurements
Summary
Arnon et al (1955) demonstrated photophosphorylation via a (CEF) around Photosystem I (PS I) by illuminating isolated thylakoids in the presence of vitamin K. Since CEF is essential for efficient photosynthesis (Munekage et al, 2004) and for photoprotection (Munekage et al, 2002; Abbreviations: ATP, adenosine triphosphate; CEF, cyclic electron flux around Photosystem I; Chl, chlorophyll; ETR1, the total electron transport rate through PS I; Flux, the difference between ETR1 and LEFO2; f I, the fraction of absorbed light partitioned to PS I; LEFO2, the linear electron flux through PS II estimated by gross O2 evolution; NDH, nicotinamide adenine dinucleotide dehydrogenase-like complex; P700, special chlorophyll dimer acting as the primary electron donor in PS I; PAM, pulse amplitude modulation fluorometer; PGR5, proton gradient regulation 5 protein; PGRL1, PGR5-like protein 1; PS I, Photosystem I; PQH2, plastoquinonol; PTOX, plastid terminal oxidase; Y(I), photochemical yield of PS I; Y(NA) and Y(ND), non-photochemical yield due to acceptor-side and donor-side limitation, respectively. Cyclic electron flow consists of two pathways: one dependent on a PGR5–PGRL1 protein interaction (i.e., interaction between the proton gradient regulation 5 protein and the PGR5-like protein 1) and the other on a chloroplastic nicotinamide adenine dinucleotide dehydrogenase-like complex (NDH) complex. The NDH complex improves CEF by adjusting the redox level of electron carriers in low light (Martin et al, 2015)
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