Mutant pqr-216 from an Arabidopsis activation-tagged line showed a phenotype of increased tolerance to oxidative stress after treatment with 3 mum paraquat (PQ). Based on the phenotype of transgenic plants overexpressing the genes flanking the T-DNA insert, it was clear that enhanced expression of a Nudix (nucleoside diphosphates linked to some moiety X) hydrolase gene, AtNUDX2 (At5g47650), was responsible for the tolerance. It has been reported that the AtNUDX2 protein has pyrophosphatase activities towards both ADP-ribose and NADH (Ogawa et al., 2005). Interestingly, the pyrophosphatase activity toward ADP-ribose, but not NADH, was increased in pqr-216 and Pro(35S):AtNUDX2 plants compared with control plants. The amount of free ADP-ribose was lower in the Pro(35S):AtNUDX2 plants, while the level of NADH was similar to those in control plants under both normal conditions and oxidative stress. Depletion of NAD(+) and ATP resulting from activation of poly(ADP-ribosyl)ation under oxidative stress was observed in the control Arabidopsis plants. Such alterations in the levels of these molecules were significantly suppressed in the Pro(35S):AtNUDX2 plants. The results indicate that overexpression of AtNUDX2, encoding ADP-ribose pyrophosphatase, confers enhanced tolerance of oxidative stress on Arabidopsis plants, resulting from maintenance of NAD(+) and ATP levels by nucleotide recycling from free ADP-ribose molecules under stress conditions.