NAD+ Biosynthesis and Signaling in Plants

Many metabolic processes that occur in living cells involve oxidation-reduction (redox) chemistry underpinned by compounds, such as glutathione, ascorbate, and/or pyridine nucleotides. Among these redox carriers, nicotinamide adenine dinucleotide (NAD(+)) represents one of the cornerstones of cellular oxidation and is essential for plant growth and development. In addition to its redox role, compelling evidence of a role for NAD(+) as a signal molecule in plants is emerging. In non-plant systems, NAD(+) biosynthesis can be the product of two independent pathwaysthe de novo and the salvage pathway. Despite the fact that homologous plant genes exist for the composite enzymes of most, if not all, of these pathways research into plant NAD(+) biosynthesis per se has lagged behind that of microbial and mammalian systems. Indeed, much plant work has been focused on additional roles of the composite enzymes in processes, such as DNA repair or post-translational modification of proteins. That said, in recent years, a number of studies, mostly in Arabidopsis, have been carried out in order to fill this knowledge gap. Therefore, in this review, we intend to present a synthesis of such molecular genetic studies as well as biochemical analyses that have begun to elucidate the regulatory hierarchies both of NAD(+) biosynthesis and the parallel activities of the enzymes that are involved in these pathways.