Aminoaldehyde dehydrogenases (AMADHs, EC 1.2.1.19) belong to the large aldehyde dehydrogenase (ALDH) superfamily, namely, the ALDH9 family. They oxidize polyamine-derived omega-aminoaldehydes to the corresponding omega-amino acids. Here, we report the first X-ray structures of plant AMADHs: two isoenzymes, PsAMADH1 and PsAMADH2, from Pisum sativum in complex with beta-nicotinamide adenine dinucleotide (NAD(+)) at 2.4 and 2.15 A resolution, respectively. Both recombinant proteins are dimeric and, similarly to other ALDHs, each monomer is composed of an oligomerization domain, a coenzyme binding domain and a catalytic domain. Each subunit binds NAD(+) as a coenzyme, contains a solvent-accessible C-terminal peroxisomal targeting signal (type 1) and a cation bound in the cavity close to the NAD(+) binding site. While the NAD(+) binding mode is classical for PsAMADH2, that for PsAMADH1 is unusual among ALDHs. A glycerol molecule occupies the substrate binding site and mimics a bound substrate. Structural analysis and substrate specificity study of both isoenzymes in combination with data published previously on other ALDH9 family members show that the established categorization of such enzymes into distinct groups based on substrate specificity is no more appropriate, because many of them seem capable of oxidizing a large spectrum of aminoaldehyde substrates. PsAMADH1 and PsAMADH2 can oxidize N,N,N-trimethyl-4-aminobutyraldehyde into gamma-butyrobetaine, which is the carnitine precursor in animal cells. This activity highly suggests that in addition to their contribution to the formation of compatible osmolytes such as glycine betaine, beta-alanine betaine and gamma-aminobutyric acid, AMADHs might participate in carnitine biosynthesis in plants.