FdC1 and Leaf-Type Ferredoxins Channel Electrons From Photosystem I to Different Downstream Electron Acceptors.

Xiaoqian Guan,Mikko Tikkanen,Boon L Lim,Kam-Bo Wong,Chia Pao Voon,Shuai Chen

doi:10.3389/fpls.2018.00410

Abstract

Plant-type ferredoxins in Arabidopsis transfer electrons from the photosystem I to multiple redox-driven enzymes involved in the assimilation of carbon, nitrogen, and sulfur. Leaf-type ferredoxins also modulate the switch between the linear and cyclic electron routes of the photosystems. Recently, two novel ferredoxin homologs with extra C-termini were identified in the Arabidopsis genome (AtFdC1, AT4G14890; AtFdC2, AT1G32550). FdC1 was considered as an alternative electron acceptor of PSI under extreme ferredoxin-deficient conditions. Here, we showed that FdC1 could interact with some, but not all, electron acceptors of leaf-type Fds, including the ferredoxin-thioredoxin reductase (FTR), sulfite reductase (SiR), and nitrite reductase (NiR). Photoreduction assay on cytochrome c and enzyme assays confirmed its capability to receive electrons from PSI and donate electrons to the Fd-dependent SiR and NiR but not to the ferredoxin-NADP+ oxidoreductase (FNR). Hence, FdC1 and leaf-type Fds may play differential roles by channeling electrons from photosystem I to different downstream electron acceptors in photosynthetic tissues. In addition, the median redox potential of FdC1 may allow it to receive electrons from FNR in non-photosynthetic plastids.

Highlights

Located on the stromal side of the thylakoid membrane, plant-type ferredoxins (Fds) belong to a family of low molecular weight proteins with one simple [2Fe–2S] cluster bound to four conserved cysteine residues (Fukuyama, 2004)
The leaf-type Fds that are present in chloroplasts receive electrons derived from water during photosynthesis, while the root-type Fds that are present in non-photosynthetic plastids display a more positive redox potential and receive electrons from roottype ferredoxin-NADP+ oxidoreductase (FNR), which collect electrons from NADPH derived from the oxidative pentose phosphate (OPPP) pathway to maintain the efficiency of assimilation under a heterotrophic condition (Onda et al, 2000; Hanke et al, 2004a,b)
With differential evolutionary distances, FdC1 started to appear in green algae, while FdC2 could be found in cyanobacteria

Summary

Introduction

Located on the stromal side of the thylakoid membrane, plant-type ferredoxins (Fds) belong to a family of low molecular weight proteins with one simple [2Fe–2S] cluster (in the range of −200 to −450 mV) bound to four conserved cysteine residues (Fukuyama, 2004). Fds transfer electrons from PSI to a wide range of soluble enzymes, including FNR, FTR, NiR, glutamine-oxoglutarate aminotransferase (GOGAT), SiR, fatty acid desaturase, chlorophyll a oxygenase (CAO), etc. Fds play roles in the assimilation of carbon, nitrogen, and sulfur; the synthesis of amino acids, fatty acids, chlorophyll, and phytochromes; Abbreviations: Fd, ferredoxin; FdC, ferredoxin C; FNR, ferredoxin-NADP+ oxidoreductase; FTR, ferredoxin-thioredoxin reductase; NdhS, NADH dehydrogenase-like complex S; NiR, nitrite reductase; PGRL1, PGR5-like protein; PSI, photosystem I; and SiR, sulfite reductase. Multiple isoforms of Fds are found in cyanobacteria, algae, and higher plants (Bertini et al, 2002). They are divided into photosynthetic (leaf) and heterotrophic (root) types based on their localizations, sequences, and functional differences. Many novel interacting partners of ferredoxin have been detected by the largescale screening of potential candidates for electron acceptors, which has expanded our knowledge of the functionalities of ferredoxins (Hanke et al, 2011; Peden et al, 2013)

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Conclusion