*3.1. NAD*<sup>+</sup> *Transport*

As a coenzyme for redox processes, NAD<sup>+</sup> is playing important roles in the operation of a wide range of dehydrogenase activities, signaling pathways through their interaction with reactive oxygen species (ROS) and generation of NADH from oxidative phosphorylation. NAD<sup>+</sup> is essential for several metabolic pathways including glycolysis, TCA cycle, glycine decarboxylation, the Calvin–Benson cycle, and β-oxidation in peroxisomes [62]. Hence, the movement of NAD<sup>+</sup> from different subcellular compartments is mediated by different subcellular NAD<sup>+</sup> transporters. In plants, both de novo and salvage NAD<sup>+</sup> biosynthetic pathways culminate in the synthesis of nicotinate mononucleotide (NaMN) [63]. The salvage pathway starts with nicotinamide (NAM) or nicotinic acid (NA), while the de novo pathway starts in plastids using aspartate or tryptophan as precursors. Both metabolic fluxes converge in the formation of nicotinic acid mononucleotide (NAMN), which, in turn, gives rise to NAD+. Since the last step of NAD<sup>+</sup> synthesis takes place in the cytosol, NAD<sup>+</sup> must be imported into the mitochondria to allow TCA cycle metabolism and oxidative phosphorylation [64]. In Arabidopsis, there are three MCF members responsible for NAD<sup>+</sup> transport, AtNDT1 (AT2G47490) and AtNDT2 (AT1G25380), targeted to the IMM, and AtPXN (AT2G39970), located in the peroxisomal membrane [64,65]. Although previous research suggested that AtNDT1 is targeted to the inner membrane of chloroplasts [65], recent subcellular localization experiments and proteomics data revealed that AtNDT1 locates exclusively to the IMM [64]. Both AtNDT1 and AtNAD2 are able to complement the phenotype of a yeast mutant lacking NAD<sup>+</sup> transport [65]. Interestingly, both AtNDT1 and AtNDT2 have similar substrate specificity; importing NAD<sup>+</sup> against ADP or AMP; they do not accept NADH, nicotinamide, nicotinic acid, NADP+, or NADPH as transport substrates [65]. The AtPXN transporter has a more versatile transport behavior, able to accept NAD+, NADH, and CoA in vitro [66–68]. In addition, as a pyridine nucleotide, NAD<sup>+</sup> is involved in the transport of electrons within oxidation–reduction reactions as well as being a highly important component of cellular signaling [63]. Given that the redox status regulates the plant TCA cycle [69], NAD<sup>+</sup> import not only provides co-enzymes but might also act as a signal that regulates central metabolism.
