A cDNA coding for a 58-kD calcium-dependent calmodulin (CaM)-binding glutamate decarboxylase (CAD) previously isolated in our laboratory from petunia (Petunia Hybrida) (G. Baum, Y. Chen, T. Arazi, H. Takatsuji, H. Fromm  I Biol Chem 268: 19610-19617) was used to conduct molecular studies of GAD expression. GAD expression was studied during petunia organ development using the GAD cDNA as a probe to detect the GAD mRNA and by the anti-recombinant GAD serum to monitor the levels of GAD. GAD activity was studied in extracts of organs in the course of development. The 58-kD CaM-binding GAD is expressed in all petunia organs tested (flowers and all floral parts, leaves, stems, roots, and seeds). The highest expression levels were in petals of open flowers. Developmental changes in the abundance of GAD mRNA and the 58-kD GAD were observed in flowers and leaves and during germination. Moreover, developmental changes in GAD activity in plant extracts coincided in most cases with changes in the abundance of the 58-kD GAD. We conclude that the 58-kD CaM-binding GAD is a ubiquitous protein in petunia organs and that its expression is developmentally regulated by transcriptional and/or posttranscriptional processes. Thus, GAD gene expression is likely to play a role in controlling the rates of GABA synthesis during petunia seed germination and organ development.