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).