Figure 2.

Chloroplast redox regulatory pathways in the light and the dark. The NTRC-2-Cys PRX (2CP) redox system constitutes a redox relay. During the day (LIGHT), the pool of TRXs is maintained reduced by photosynthetically reduced FDX. The redox state of 2-Cys PRXs is predominantly maintained by NTRC and, to a lesser extent, by TRXs (denoted by thick and thin arrows, respectively). Thus, the activity of NTRC avoids the drainage of electrons from TRXs, hence maintaining downstream targets reduced and active. Additionally, NTRC may directly interact with other chloroplast redox-regulated enzymes. During the night (DARK), input of reducing equivalents via reduced FDX ceases and TRXs mediate the transfer of reducing equivalents from reduced targets to 2-Cys PRXs and hydrogen peroxide, while targets consequently become oxidized and inactive. Reoxidation of redox regulated enzymes by molecular oxygen may also occur in the aerobic environment of the chloroplast.