TROL-FNR interaction reveals alternative pathways of electron partitioning in photosynthesis.

Lea Vojta,Marina Kveder,Hrvoje Fulgosi,Jasenka Antunović Dunić,Vera Cesar,Hrvoje Lepeduš,Dejana Carić

doi:10.1038/srep10085

Abstract

In photosynthesis, final electron transfer from ferredoxin to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR). FNR is recruited to thylakoid membranes via integral membrane thylakoid rhodanase-like protein TROL. We address the fate of electrons downstream of photosystem I when TROL is absent. We have employed electron paramagnetic resonance (EPR) spectroscopy to study free radical formation and electron partitioning in TROL-depleted chloroplasts. DMPO was used to detect superoxide anion (O2.−) formation, while the generation of other free radicals was monitored by Tiron. Chloroplasts from trol plants pre-acclimated to different light conditions consistently exhibited diminished O2.− accumulation. Generation of other radical forms was elevated in trol chloroplasts in all tested conditions, except for the plants pre-acclimated to high-light. Remarkably, dark- and growth light-acclimated trol chloroplasts were resilient to O2.− generation induced by methyl-viologen. We propose that the dynamic binding and release of FNR from TROL can control the flow of photosynthetic electrons prior to activation of the pseudo-cyclic electron transfer pathway.

Highlights

In photosynthesis, final electron transfer from ferredoxin to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR)
We propose that other Fd-dependent pathways downstream of photosystem I (PSI) and different from the linear electron transfer (LET) become dominant by the dynamic detachment of ferredoxin NADP+ oxidoreductase (FNR) form TROL, suggesting novel mechanism of photosynthesis regulation
We have used O2.− specific spin trap DMPO (5,5-dimethyl-1-pyrroline-N-oxide) and general electron spin trap Tiron (4,5-dihydroxy-1,3-b enzenedisulfonic acid) to detect the production of ROS26,27 in conditions when FNR docking partner TROL is missing from the vicinity of PSI

Summary

Introduction

Final electron transfer from ferredoxin to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR). It is widely accepted that in conditions leading to over-reduction, pseudo-cyclic electron transfer enables electrons to flow from water to PSI, with molecular oxygen as alternative electron acceptor[5] This process, known as Mehler reaction, results in formation of H2O2 which is generated by the superoxide dismutase (SOD), and converted into water and oxygen by ascorbate peroxidase[9]. In the periods of changing light conditions, i.e. in the morning hours, FNR is released to stroma, where it acts as an efficient NADPH catalyst, allowing efficient LET Contrary to this scheme previous studies have reported that NADP+ photoreduction is very inefficient when the enzyme is not bound to the membranes[23] and that thylakoids devoid of FNR cannot reproduce WT rates of NADPH production[24]. Released FNR can act as efficient ROS scavenger, or Fd can partition electrons to other acceptors

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Conclusion