The 58-Kilodalton Calmodulin-Binding Glutamate Decarboxylase Is a Ubiquitous Protein in Petunia Organs and Its Expression Is Developmentally Regulated

Abstract

A cDNA coding for a 58-kD calcium-dependent calmodulin (CaM)-binding glutamate decarboxylase (GAD) previously isolated in our laboratory from petunia (Petunia hybrida) (G. Baum, Y. Chen, T. Arazi, H. Takatsuji, H. Fromm [1993] J 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. l h e 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.

Characterization of Two Glutamate Decarboxylase cDNA Clones from Arabidopsis

“…These results suggest that other factors such as posttranslational events may regulate GAD activity in these organs. Chen et al (1994) made similar conclusions when comparing their results concerning developmental studies of petunia flowers with developmental studies of leaves; in their study a 58-kD peptide appeared to be more stable in flowers than in leaves. Also, in the siliques there was little or no GAD activity or transcript detected, but the 56-kD peptide was readily detected, suggesting that there could be other factors, such as posttranslational events, controlling GAD in these organs.…”

“…These data suggest that GAD activity may be controlled by transcriptional events or by RNA stability in leaves. A similar correlation among the accumulation of GAD transcript, GAD peptide, and in vitro GABA synthesis was observed in petunia (Chen et al, 1994). Combined, these data suggest that the transcriptional and posttranscriptional processes that control GAD activity in the leaves of Arabidopsis and petunia may be similar; however, it is too early to state whether these processes occur in the leaves of all plant species.…”

“…Chen et al (1994) demonstrated that a 58-kD CaM-binding GAD was expressed in various floral parts, seeds, stems, roots, and leaves of petunia. In addition, their data suggested that there was a correlation between in vitro GAD activity, abundance of the 58-kD peptide, and levels of GAD transcript in developing leaves and germinating seeds.…”

“…Recently, Ramputh and Bown (1996) demonstrated that elevated levels of GABA in the diet of oblique-banded leaf-roller larvae decreased their growth, development, and survival. In addition, Chen et al (1994) questioned whether GABA in plants was involved in the control of ion channels, as in animal neurons. Baum et al (1996) overexpressed a truncated version of a petunia GAD gene, which lacked the CaM-binding site, in transgenic tobacco plants and demonstrated that the CaMbinding domain was required for normal plant development and for the maintenance of GABA and Glu levels.…”

“…The authors suggested that posttranslational regulation in leaves and seeds may differ from that of flowers, and the process may play a role in controlling GAD activity in flowers. Chen et al (1994) used immunoblot analysis to monitor the accumulation of GAD in germinating petunia seeds. They identified three peptides of 48, 58, and 66 kD that cross-reacted with the antiserum to a recombinant petunia GAD (58 kD).…”