Downregulation of a Mitochondrial NAD+ Transporter (NDT2) Alters Seed Production and Germination in Arabidopsis.

Elmien Heyneke,Izabel De Souza Chaves,Jorge Alberto Condori Apfata,Andreas P M Weber,Alexandra Florian,Toshihiro Obata,Paula Da Fonseca-Pereira,Elias Feitosa-Araujo,H Ekkehard Neuhaus,Adriano Nunes-Nesi,Ferdinando Palmieri,Alisdair R Fernie,Nicole Linka,Tabea Mettler-Altmann,Marcel Viana Pires,David Barbosa Medeiros,Wagner L Ara�Jo

doi:10.1093/pcp/pcaa017

Abstract

Despite the fundamental importance of nicotinamide adenine dinucleotide (NAD+) for metabolism, the physiological roles of NAD+ carriers in plants remain unclear. We previously characterized the Arabidopsis thaliana gene (At1g25380), named AtNDT2, encoding a protein located in the mitochondrial inner membrane, which imports NAD+ from the cytosol using ADP and AMP as counter-exchange substrates for NAD+. Here, we further investigated the physiological roles of NDT2, by isolating a T-DNA insertion line, generating an antisense line and characterizing these genotypes in detail. Reduced NDT2 expression affected reproductive phase by reducing total seed yield. In addition, reduced seed germination and retardation in seedling establishment were observed in the mutant lines. Moreover, remarkable changes in primary metabolism were observed in dry and germinated seeds and an increase in fatty acid levels was verified during seedling establishment. Furthermore, flowers and seedlings of NDT2 mutants displayed upregulation of de novo and salvage pathway genes encoding NAD+ biosynthesis enzymes, demonstrating the transcriptional control mediated by NDT2 activity over these genes. Taken together, our results suggest that NDT2 expression is fundamental for maintaining NAD+ balance amongst organelles that modulate metabolism, physiology and developmental processes of heterotrophic tissues.

Highlights

Nicotinamide adenine dinucleotide (NADþ) is a central metabolite in cell metabolism, being involved in energy transactions and cellular signaling as well as acting as a coenzyme in various reactions (Rasmusson and Wallstr€om 2010, Gakiere et al 2018)
Like the yeast carrier ScNDT1 (Todisco et al 2006), AtNDT2 has an affinity of 0.15 mM for NADþ, which is similar to the NADþ affinity measured in isolated mitochondria of potato tubers (Tobin et al 1980)
This is in line with the significant reduction in the mitochondrial NADþ level MOSAIC DEATH 1 (MOD1) suppressor line (SOM592), identified as an ndt2 mutant by screening the ethyl methanesulfonate-mutagenized seeds of mod1 Arabidopsis plants (Luo et al 2019)

Summary

Introduction

Nicotinamide adenine dinucleotide (NADþ) is a central metabolite in cell metabolism, being involved in energy transactions and cellular signaling as well as acting as a coenzyme in various reactions (Rasmusson and Wallstr€om 2010, Gakiere et al 2018). Unbalanced NADþ metabolism results in changes in its levels and in the general redox state of the cell, thereby impacting plant growth and development (Noctor et al 2006, Hashida et al 2009). Changes in cellular redox state are intrinsic to plant metabolism and are determined by the oxidation and reduction of various redox-active substances, especially NAD(H) and NADP(H) (Geigenberger and Fernie 2014). These pyridine nucleotides act as cofactors of many enzymes and as such represent some of the most interconnected metabolites within the cellular network. To perform its functions inside the mitochondrion, plastid, endoplasmic reticulum, Golgi complex and peroxisome, it must be imported into the organelles in a carrier protein-mediated process (Palmieri et al 2009)

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