An Abscisic Acid-Inducible Gene NICOTINAMIDASE 3 Is Regulated by REPRESSOR OF SILENCING 1 in Arabidopsis

Abstract

Nicotinamide adenine dinucleotide (NAD+) is a key electron-carrier during redox reactions in all living organisms. NAD+ also plays critical roles in ADP-ribosylation, protein deacetylation and calcium signaling. In eukaryotes, NAD+ can be synthesized via two metabolic pathways, the de novo and the salvage pathways. Here I report that NICOTINAMIDASE 3 (NIC3) which encodes a catalytic enzyme that converts nicotinamide into nicotinic acid in the NAD+ salvage pathway was controlled by DNA demethylase REPRESSOR OF SILENCING1 (ROS1) in response to abscisic acid (ABA). The ros1 mutants lost ABA-inducible expression of NIC3 due to DNA hypermethylation at the upstream region of its promoter. The nic3 mutants exhibited ABA hypersensitivity for seed germination and root elongation, and in addition, ectopic expression of NIC3 rescued ABA hypersensitive phenotypes of the ros1 mutant. This results suggest that NIC3 expression via ROS1-mediated DNA demethylation is crucial for ABA responses. I further investigated whether NIC3 conserves similar function of yeast homolog Pyrazinamidase/nicotinamidase 1 (Pnc1). Pnc1 has been known to promote rDNA silencing by increasing the activity of Silent information regulator 2 (Sir2), a NAD+-dependent histone deacetylase, associated with lifespan extension. Consistent with the amino acid alignment data that NIC3 contains catalytic residues responsible for Pnc1 activity, heterologous expression of NIC3 in yeast increased rDNA stability but failed to extend lifespan. Therefore, NIC3 is presumably a partially functional homolog of yeast Pnc1 and may have evolved to be involved in ABA responses with its expression regulated by active DNA demethylation in plants.