FIG. 4.
Proposed mechanisms by which proline metabolism mediates redox homeostasis and energy production via NADP+/NADPH. (1) NADP+ produced by proline biosynthesis may stimulate the pentose phosphate pathway (PPP), thereby supporting energy production and the biosynthesis of key molecules such as nucleotides. NADPH is utilized for reductive biosynthesis pathways and is critical for glutathione (GSH) and thioredoxin (Trx) antioxidant systems. (2) In plant chloroplasts, NADP+ produced from proline biosynthesis may replenish depleted NADP+ pools caused by inhibition of the Calvin cycle during stress. Maintaining adequate levels of NADP+ for electrons transfer to the ETC would help minimize ROS generation during stress. Dashed line indicates inhibition. GR, glutathione reductase; GPx, glutathione peroxidase; GSSG, oxidized glutathione; GSH, reduced glutathione; TrxR, thioredoxin reductase; Trx-(SH)2, reduced thioredoxin; Trx-S2, oxidized thioredoxin; NADP+, nicotinamide adenine dinucleotide phosphate; NADPH, nicotinamide adenine dinucleotide phosphate reduced form; Glucose-6-P, glucose-6-phosphate; G6PDH, glucose-6-phosphate dehydrogenase.