Modulation of NADH Levels by Arabidopsis Nudix Hydrolases, AtNUDX6 and 7, and the Respective Proteins Themselves Play Distinct Roles in the Regulation of Various Cellular Responses Involved in Biotic/Abiotic Stresses

Arabidopsis Nudix hydrolases, AtNUDX6 and 7, exhibit pyrophosphohydrolase activities toward NADH and contribute to the modulation of various defense responses, such as the poly(ADP-ribosyl) ation (PAR) reaction and salicylic acid (SA)-induced Nonexpresser of Pathogenesis-Related genes 1 (NPR1)-dependent defense pathway, against biotic and abiotic stresses. However, the mechanisms by which these enzymes regulate such cellular responses remain unclear. To clarify the functional role(s) of AtNUDX6 and 7 and NADH metabolism, we examined the effects of the transient expression of the active and inactive forms of AtNUDX6 and 7 under the control of an estrogen (ES)-inducible system on various stress responses. The transient expression of active AtNUDX6 and 7 proteins suppressed NADH levels and induced PAR activity, whereas that of their inactive forms did not, indicating the involvement of NADH metabolism in the regulation of the PAR reaction. A transcriptome analysis using KO-nudx6, KO-nudx7 and double KO-nudx6/7 plants, in which intracellular NADH levels increased, identified genes (NADH-responsive genes, NRGs) whose expression levels positively and negatively correlated with NADH levels. Many NRGs did not overlap with the genes whose expression was reported to be responsive to various types of oxidants and reductants, suggesting a novel role for intracellular NADH levels as a redox signaling cue. The active and inactive AtNUDX6 proteins induced the expression of thioredoxin- h5, the activator of NPR1 and SA-induced NPR1-dependent defense genes, while the active and inactive AtNUDX7 proteins suppressed the accumulation of SA and subsequent gene expression, indicating that AtNUDX6 and 7 proteins themselves play distinct roles in stress responses.