Redox chemistry of photosynthetic electron transport chain and associated redox regulators. The sketch displays thylakoid membrane compartments, intrinsic and extrinsic protein complexes of the photosynthetic electron transport chain, and associated redox mediators (green) and various redox transmitters (yellow; red contours mark novel ones described in the text in more detail) coupled to it. The yellow and red flashes indicate light-dependent charge separation in the reaction centers of PSII and PSI as well as their different absorption maxima (680 and 700 nm, respectively). Thick black arrows represent the main electron flow from water to NADP and the subsequent cellular processes (blue). Thin black arrows indicate the flow of a minor proportion of electrons used for regulatory redox reactions. Blue arrows mark the influence of regulators on distinct cellular processes as well as potential interactions. For detailed explanations, see the text.

Schematics of the dithiol-disulfide network of the chloroplast. The top layer depicts the supply pathways feeding electrons to the input elements Fd, NADPH, and GSH. In competition with other electron-draining reactions these input elements donate electrons to a set of redox transmitters. These adjust the redox state of a large set of target proteins, here represented by FBPase, malate dehydrogenase (MDH), FLN, and AGPase or donate electrons to thiol-dependent peroxidases (Prx, glutathione peroxidase [Gpx]). ROS, RNS, peroxides, and O₂ serve as final electron acceptors maintaining electron flux through the network by reoxidizing protein thiols. Some Grx function in deglutathionylation, FeS-cluster assembly/delivery, or as redox sensors and seem to lack disulfide reduction activity.

Redox regulation in plastid starch and lipid synthesis. AGPase as committed step of starch synthesis, and ACCase and MGD in plastid lipid synthesis are activated by reduction of regulatory thiols, either by Trx or in case of AGPase by NTRC as described in the text in more detail.

Redirection of leaf metabolism under highly reducing conditions. Leaf discs were illuminated in the absence or presence of the reductant DTT and supplied with ¹⁴C-Glc. Distribution of ¹⁴C label in different metabolite pools was investigated after 1 h (Kolbe et al., 2006). The diagrams compare the distribution patterns under control conditions (A) and after DTT treatment (B). The numbers give the relative distribution in percent of total. Green, Up-regulation; red, down-regulation.

Protein factors and signaling pathways involved in photosynthetic redox control of plastid transcription. The photosynthetic electron transport chain and its protein complexes are depicted as in Figure 1. The sketch refers to light conditions which preferentially excite PSII. Under these conditions the PQ pool becomes reduced, hence, activating the thylakoid-bound kinase STN7. This kinase is oppositely regulated at its lumenal side by a reduction signal from the Trx system that is not active under these conditions (Bräutigam et al., 2009). STN7 provides an input signal that presumably is integrated within a subsequent phosphorylation network targeted to the control of the PEP. PEP-associated proteins (PAPs) are given in yellow and described in text. One group of PAPs is mediating the phosphorylation signal, another one appears to integrate a second signal originating from the Trx system. Both signals together contribute to the transcriptional activation of the psaA/B operon (a mixed operon encoding a ribosomal subunit at its end [rps14, given in light pink]). The pathway by which Trx-z is reduced is unknown to date (question mark).