FIG. 1.

Overview of photosynthetic electron transport pathways in the chloroplast. Linear electron flow requires photosystems (PS) I and II working in series, leading to electron transfer from water to NADP⁺ to generate NADPH as reducing power. This involves electron transfer from PS II to PS I via plastoquinone (PQ), the cytochrome b₆f complex (Cyt b₆f), and plastocyanin (PC) as redox carriers. At the stromal side of PS I, electrons are subsequently donated to ferredoxin (Fdx), which functions as a mobile electron carrier distributing electrons to NADP⁺ via Fdx-NADP-reductase (FNR) to produce NADPH or directly to specific processes located in the stroma, such as S and N assimilation, biosynthetic pathways, and reactions involved in chloroplast redox regulation, catalyzed by Fdx-Trx-reductase (FTR) and thioredoxins (Trxs). NADPH produced by FNR is used by the carbon fixation cycle and various biosynthetic processes as a reductant and by NADPH-dependent Trx-reductase C (NTRC) for redox regulation. In the thyllakoid membrane, proton-gradient-regulation-like protein 1 (PGRL1) acts as an Fdx-PQ reductase in cyclic electron flow, reintroducing electrons from Fdx into the electron transport chain, as indicated by the dotted line (141). A second pathway for cyclic electron flow around PS I involving a NADH oxidase-like complex is not shown for clarity. Under conditions of acceptor limitation of PS I, electrons from PS I will photoreduce oxygen to reactive oxygen species (ROS). To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars