2.1 The stroma has two distinct thiol reduction systems (FTR-Trx
and FNR-NTRC)
The ferredoxin-thioredoxin reductase (FTR)-thioredoxin (Trx) system is
only active in light, as it requires electrons from photosynthesis. The
system works by transferring electrons from photosystem I (PSI) to
ferredoxin (Fd), which then transfers these electrons to
ferredoxin-thioredoxin reductase (FTR), which then transfers the
electrons to thioredoxin (Trx), and then finally transfers these
electrons to various target enzymes, such as FBPase and SBPase (Figure
1A)
(Schürmann
and Buchanan 2008; Michelet et al. 2013).
Trx contains cysteine residues that are redox-active and can reversibly
transfer the reducing potentials from light reactions to thiol-regulated
enzymes. Initially, two forms of Trx, -f and -m, were proposed to be
involved in this redox process
(Buchanan
1980) but later, more than 20 additional isoforms of Trx were found by
sequencing of the Arabidopsis thaliana genome. The newly
identified chloroplast Trxs were categorized into five classes, Trx-f,
m, x, y and z
(Collinet al. 2003; Yoshida, Matsuoka, Hara, Konno & Hisabori 2014;
Geigenberger, Thormählen, Daloso & Fernie 2017). Essentially, all Trxs
have the conserved sequence motif (WCGPC) which allows them to interact
more specifically with subsets of target enzymes
(Schürmann
& Jacquot 2000; Collin et al. 2003; Yoshida et al. 2014;
Geigenberger et al. 2017).
A second reduction system, NADPH-dependent thioredoxin reductase C
(NTRC), was discovered relatively recently as a chloroplast
thiol-regulatory system specifically in oxygenic organisms
(Serrato,
Pérez-Ruiz, Spínola & Cejudo 2004; Pulido et al. 2010; Carrillo,
Froehlich, Cruz, Savage & Kramer 2016) (Fig.1B). In contrast to the
FTR system, NTRC uses NADPH as a reducing power to deliver electrons to
target enzymes. NTRC appears to be critical under lower light
conditions, operating with even low LEF, or in the dark using NADPH
generated from the oxidative pentose phosphate pathway (OPPP)
(Neuhaus
& Emes 2000).