The mitochondrial NAD⁺ transporter (NDT1) plays important roles in cellular NAD⁺ homeostasis in Arabidopsis thaliana

Abstract

SummaryNicotinamide adenine dinucleotide (NAD +) is an essential coenzyme required for all living organisms. In eukaryotic cells, the final step of NAD + biosynthesis is exclusively cytosolic. Hence, NAD + must be imported into organelles to support their metabolic functions. Three NAD + transporters belonging to the mitochondrial carrier family (MCF) have been biochemically characterized in plants. AtNDT1 (At2g47490), focus of the current study, At NDT2 (At1g25380), targeted to the inner mitochondrial membrane, and At PXN (At2g39970), located in the peroxisomal membrane. Although At NDT1 was presumed to reside in the chloroplast membrane, subcellular localization experiments with green fluorescent protein (GFP) fusions revealed that At NDT1 locates exclusively in the mitochondrial membrane in stably transformed Arabidopsis plants. To understand the biological function of At NDT1 in Arabidopsis, three transgenic lines containing an antisense construct of AtNDT1 under the control of the 35S promoter alongside a T‐DNA insertional line were evaluated. Plants with reduced AtNDT1 expression displayed lower pollen viability, silique length, and higher rate of seed abortion. Furthermore, these plants also exhibited an increased leaf number and leaf area concomitant with higher photosynthetic rates and higher levels of sucrose and starch. Therefore, lower expression of AtNDT1 was associated with enhanced vegetative growth but severe impairment of the reproductive stage. These results are discussed in the context of the mitochondrial localization of At NDT1 and its important role in the cellular NAD + homeostasis for both metabolic and developmental processes in plants.

Plant Mitochondrial Carriers: Molecular Gatekeepers That Help to Regulate Plant Central Carbon Metabolism

“…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 + synthesis takes place in the cytosol, NAD + 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 + transport, AtNDT1 (AT2G47490) and AtNDT2 (AT1G25380), targeted to the IMM, and AtPXN (AT2G39970), located in the peroxisomal membrane [64,65].…”

“…Since the last step of NAD + synthesis takes place in the cytosol, NAD + 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 + 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].…”

“…In Arabidopsis, there are three MCF members responsible for NAD + 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 + transport [65].…”

Metabolic Roles of Plant Mitochondrial Carriers

“…Surprisingly, At NDT1 was found in mitochondrial membranes in proteome studies [ 69 ] and previously a GFP-tagging and immunolocalization study was not able to find At NDT1 targeted to chloroplast membranes [ 70 ]. Very recently, both At NDT1- and At NDT2-GFP fusion proteins were found exclusively located in the mitochondria, clearly indicating their mitochondrial localization [ 71 ].…”

Characterization of In Vivo Function(s) of Members of the Plant Mitochondrial Carrier Family

“…A more attractive proposition is thus that plants contain more MCF proteins due to the presence of the plastid organelle [ 24 ]. While arguments for this were initially made on comparison of the NAD transporters NDT1 and NDT2 [ 24 ], with the former thought to be a plastidial and the latter a mitochondrial localized transporter [ 26 ]; this needs revision now that, as detailed below, both have been reclassified as mitochondrial [ 27 , 28 ]. That said, there are several proteins that exhibit dual targeting and thus may fulfill different functions at the different locations.…”