Biology subjects

Degen, G. E.

Publications and source records attributed to Degen, G. E..

5 recordsLinked to original sources

bioRxiv2026

Loss of PSAE redirects PGRL1 to photosystem I and enhances PGR5-dependent cyclic electron transfer in Arabidopsis

Electrons energised by light energy at photosystem I (PSI) primarily enter the linear photosynthetic electron transfer (LET) or cyclic electron transfer (CET) pathway. The balance between LET and CET activity is a crucial factor in the regulation of photosynthesis, since CET increases the stoichiometry of proton to electron transfer. The additional transmembrane proton gradient ({Delta}pH) triggers feedback control of light harvesting and electron transfer photoprotection via non-photochemical quenching (NPQ) and photosynthetic control (PCON), maintaining the balance between the output of the light reactions and the downstream metabolism. Previously, it was found that Arabidopsis mutants lacking the stromal-facing membrane-extrinsic PSAE subunit of PSI (psae1-3) show enhanced CET activity, decreased LET and lower PSI oxidation in excess light. Here we show that high CET activity in psae1-3 primarily depends on the Proton Gradient Regulation 5 (PGR5)-dependent CET pathway rather than the NDH-dependent pathway. High CET is abolished in the psae1-3 pgr5CAS double mutant. In the psae1-3 ndho double mutant the elevated proton flux and CET are largely maintained; however, CO2 fixation and growth are significantly worsened, indicating that NDH still makes a physiologically meaningful contribution in the psae1-3 background. Biochemical analysis revealed that PGRL1 is redistributed from its normal mixed membrane distribution to become predominantly PSI-associated in psae1-3, providing a physical basis for the enhanced PGR5-dependent CET. These results underscore the primary importance of the PGR5-dependent CET pathway for optimal photosynthesis and CO2 fixation in Arabidopsis and establish the organisation of the PSI acceptor side as a key regulatory determinant of the CET/LET balance. HighlightLoss of the PSI acceptor-side subunit PSAE enhances predominantly PGR5-dependent cyclic electron transfer and redirects PGRL1 to PSI in Arabidopsis, while NDH contributes to maintaining CO2 fixation when the PSI stromal side is disrupted.

plant biology↗

bioRxiv2025

Disequilibrium between chloroplast proton motive force and ATP levels in Arabidopsis

Current dogma holds that CO2 fixation by photosynthesis requires additional ATP production via PGR5-dependent cyclic electron transfer (PGR5-CET) to augment the NADPH and ATP produced by linear electron transfer (LET). Here we use the MgATP2- fluorescent biosensor ATeam1.03-nD/nA and find that while proton motive force (pmf) in the Arabidopsis pgr5CAS mutant is only 50-75% of that of the wild-type, MgATP2- levels are unchanged. These data demonstrate that disequilibrium exists between pmf and MgATP2- concentration in vivo, indicating that PGR5-CET is not required for ATP augmentation.

plant biology↗

bioRxiv2024

Tethering ferredoxin-NADP+ reductase to photosystem I promotes photosynthetic cyclic electron transfer

Fixing CO2 via photosynthesis requires ATP and NADPH. Linear electron transfer (LET) supplies both metabolites, yet depending on environmental conditions, additional ATP is required which can be generated by cyclic electron transfer (CET). How the balance between LET and CET is set remains largely unknown. Ferredoxin(FD)-NADP+ reductase (FNR) has been suggested to act as the switch, channelling photosynthetic electrons to LET when it is bound to photosystem I (PSI) or CET when bound to cytochrome b6f. The essential role of FNR in LET precludes the use of a direct gene knock-out to test this hypothesis. We circumvented this problem by using CRISPR-Cas9 gene editing in Chlamydomonas reinhardtii to create a chimeric form of FNR tethered to PSI via PSAF. Chimeric FNR mutants exhibited impaired photosynthetic growth relative to the wild type, and decreased LET and non-photochemical quenching (NPQ) under high CO2 conditions. However, higher rates of CET and increased NPQ were found in the mutants under low CO2 or low O2 conditions where CET is normally up-regulated. Thus, rather than PSI-FNR interactions promoting LET, tethering FNR to PSI promotes CET at the expense of LET and CO2 fixation.

biochemistry↗

bioRxiv2023

PGR5 is required to avoid photosynthetic oscillations during light transitions

The production of ATP and NADPH by the light reactions of photosynthesis and their consumption by the Calvin-Benson-Bassham (CBB) cycle and other downstream metabolic reactions requires careful regulation. Environmental shifts perturb this careful balance, leading to photo-oxidative stress and losses in CO2 assimilation. Imbalances in the production and consumption of ATP and NADPH manifest themselves as transient instability in the chlorophyll fluorescence, P700, electrochromic shift and CO2 uptake signals recorded on leaves. These oscillations can be induced in wild-type plants by sudden shifts in CO2 concentration or light intensity, however mutants exhibiting increased oscillatory behaviour have yet to be reported. This has precluded an understanding of the regulatory mechanisms employed by plants to suppress oscillations. Here we show that the Arabidopsis pgr5 mutant, which is deficient in PGR5-dependent cyclic electron transfer (CET), exhibits increased oscillatory behaviour. In contrast, mutants lacking the NDH-dependent CET are largely unaffected. The absence of oscillations in the hope2 mutant, which like pgr5, lacks photosynthetic control and exhibits high ATP synthase conductivity, ruled out loss of these photoprotective mechanisms as causes. Instead, we observed slower formation of proton motive force and by inference ATP synthesis in pgr5 following environmental perturbation, leading to the transient reduction of the electron transfer chain and photosynthetic oscillations. PGR5-dependent CET therefore plays a major role in damping the effect of environmental perturbations on photosynthesis to avoid losses in CO2 fixation.

plant biology↗

bioRxiv2022

Supercharged PGR5-dependent cyclic electron transfer compensates for mis-regulated chloroplast ATP synthase

The light reactions of photosynthesis couple electron and proton transfers across the thylakoid membrane, generating NADPH, and proton motive force (pmf) that powers the endergonic synthesis of ATP by ATP synthase. ATP and NADPH are required for CO2 fixation into carbohydrates by the Calvin-Benson-Bassham cycle (CBBC). The dominant {Delta}pH component of the pmf also plays a photoprotective role in regulating photosystem II (PSII) light harvesting efficiency, through non-photochemical quenching (NPQ), and cytochrome b6f (cytb6f) to photosystem I (PSI) electron transfer, via photosynthetic control. {Delta}pH can be adjusted by increasing the proton influx into the thylakoid lumen via upregulation of cyclic electron transfer (CET) or decreasing proton efflux via downregulation of ATP synthase conductivity (gH+). The interplay and relative contributions of these two elements of {Delta}pH control to photoprotection are not well understood. Here, we show that an Arabidopsis ATP synthase mutant (hope2) with 40% higher proton efflux, has supercharged CET. Double crosses of hope2 with the CET-deficient pgr5 and ndho lines reveal that PGR5-dependent CET is the major pathway contributing to higher proton influx. PGR5-dependent CET allows hope2 to maintain wild-type levels of {Delta}pH, CO2 fixation and NPQ, however photosynthetic control remains absent, and PSI is acceptor-side limited. Therefore, high CET in the absence of ATP synthase regulation is insufficient for PSI photoprotection.

plant biology↗