Abstract
Abiotic stress often negatively affects photosynthesis, which is the main source of NADPH. Reduced form of the coenzyme represents an indispensable compound for biosynthetic reactions, antioxidant systems, and for enzymes involved in regulation.
As the ratio NADP+/NADPH in plants influences redox reactions, the control of cell redox homeostasis is important for balancing metabolic processes and redox-dependent signalling. NADPH is produced mainly by enzymes from oxidative pentose phosphate pathway (glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase) but also other enzymes (e.g.
NADP-isocitrate dehydrogenase, NADP-malic enzyme, NADP-glutamate dehydrogenase, and non-phosphorylating NADP-glyceraldehyde-3-phosphate dehydrogenase) represent an alternative source of NADPH in stressed plants. NADP-glutamate dehydrogenase is involved in catabolism of proteins and amino acids and thus provides NADPH.
However, the role of this enzyme could be different under stress condition (e.g. in the case of increased protein synthesis). Reaction catalyzed with non-phosphorylating NADP-glyceraldehyde-3-phosphate dehydrogenase drives NADPH/NADP+ ratio to biosynthetic processes.
Enzymes of antioxidant system, glutathione reductase and monodehydroascorbate reductase, on the other hand, represent enzymes utilizing NADPH that are involved in stress. These enzymes participate in scavenging of reactive oxygen species that are produced in early stages of oxidative stress incidence by NADPH-oxidase.
Similarly, NADPH is essential for biosynthetic processes, e.g. for biosynthesis of osmotically active compounds that reduce effect of cold, drought, and salt stress (Δ1-pyrroline-5-carboxylate synthetase and Δ1-pyrroline-5-carboxylate reductase synthesizing proline, mannose-6-phosphate reductase forming mannitol).