In higher plants formate dehydrogenase (FDH, EC 1.2.1.2.) is a mitochondrial, NAD-dependent enzyme. We previously reported that in potato (Solanum tuberosum L.) FDH expression is high in tubers but low in green leaves. Here we show that in isolated tuber mitochondria FDH is involved in formate-dependent O 2 uptake coupled to ATP synthesis. The effects of various environmental and chemical factors on FDH expression in leaves were tested using the mitochondrial serine hydroxymethyltransferase as a control. The abundance of FDH transcripts is strongly increased under various stresses, whereas serine hydroxymethyltransferase transcripts decline. The application of formate to leaves strongly enhances FDH expression, suggesting that it might be the signal for FDH induction. Our experiments using glycolytic products suggest that glycolysis may play an important role in formate synthesis in leaves in the dark and during hypoxia, and in tubers. Of particular interest is the dramatic accumulation of FDH transcripts after spraying methanol on leaves, as this compound is known to increase the yields of C 3 plants. In addition, although the steady-state levels of FDH transcript increase very quickly in response to stress, protein accumulation is much slower, but can eventually reach the same levels in leaves as in tubers.FDH catalyzes the oxidation of formate into CO 2 , and is a widespread enzyme in bacteria, yeasts, fungi, mammals, and plants. Several types of FDHs have been reported with differing cofactors (NAD or FAD), electron acceptors, substrates, and cellular locations. A wide diversity of FDH types is found in the bacteria, where they are involved in respiration (for review, see Sawers, 1994) and possibly in the maintenance of a reducing environment (Haynes et al., 1995). The NAD-dependent FDHs (EC 1.2.1.2) have been extensively studied in yeast and bacteria over the past decade because the favorable thermodynamics of the reaction, the easy removal of the product (CO 2 ), and the low cost of the substrate (formate) have made this enzyme a good candidate for industrial NADH regeneration (Allen and Holbrook, 1995). In methanol-utilizing yeasts such as Hansenula polymorpha, Candida boidinii, and Pichia pinus, NAD-dependent FDH plays a crucial role, together with formaldehyde dehydrogenase, in providing NADH to the respiratory chain (Van Dijken et al., 1976). Aside from formate, it is possible that S-formylglutathione might also be a substrate for FDH, as was suggested for yeasts (Van Dijken et al., 1976), pea (Uotila and Koivusalo, 1979), and Pseudomonas sp. 101 (Popov and Lamzin, 1994).In higher plants only NAD-dependent FDHs have been found, and these are localized in the mitochondrial matrix (Halliwell, 1974; Oliver, 1981; Colas des Francs-Small et al., 1993). Oliver (1981) showed that isolated leaf mitochondria from spinach (and, to a lesser extent, that from beet and tobacco) can oxidize formate with a tight coupling to oxidative phosphorylation. The first plant FDH cDNA to be published encodes a potato (So...

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