Aldehyde dehydrogenases (ALDHs, EC 1.2.1) oxidize aldehydes into carboxylic acids, and thus are important in regulating the level of toxic aldehydes. ALDHs from eukaryotes were classified into more than 20 families. In mammals Family 2 ALDHs detoxify acetaldehyde and a similar function has been suggested in plants. Specifically, it has been hypothesized that plant Family 2 ALDHs oxidize acetaldehyde generated via ethanolic fermentation and produce acetate for acetyl-CoA biosynthesis through acetyl-CoA synthetase (ACS) in plastids, similar to the yeast pathway termed the "pyruvate dehydrogenase (PDH) bypass". Arabidopsis. thaliana contains three Family 2 ALDHs, two of which are mitochondrial and the other cytosolic. To test for the presence of the PDH bypass in plants, plants homozygous for T-DNA insertion alleles of the three encoding genes were fed with 14C-ethanol along with wild type controls. The comparisons between the mutant and wild type in their incorporation rates of 14C-ethanol into fatty acids provided direct evidence for the presence of PDH bypass in plants, which may only involve one of the mitochondrial Family 2 ALDHs rather than the other two. Although none of the single, double or triple mutants exhibited novel phenotypes when grown on soil, seedlings of the double mitochondrial aldh mutants grew shorter roots than wild type on MS media, suggesting a role for mitochondrial Family 2 ALDHs during root elongation.