Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2004
Publication Date: 11/20/2004
Citation: Simkin, A.J., Underwood, B.A., Auldridge, M., Loucas, H.M., Shibuya, K., Schmelz, E.A., Clark, D.G., Klee, H.J. 2004. Circadian regulation of the phccd1 carotenoid cleavage dioxygenase controls emission of beta-ionone, a fragrance volatile of petunia flowers. Journal of Plant Physiology. 136:3504-3514. Interpretive Summary: Plant volatiles function in a large number of ecological roles including attraction of insects for pollination and as initial indicators of plant suitability for herbivores. Of particular interest to humans are plant volatiles that function as flavor and fragrance components in food. B-ionone is a low abundance terpenoid volatile that contributes significantly to tomato fruit flavor. Despite it's important role, very little is know about factors that regulate its production in plants. In collaboration with researchers at the University of Florida, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered a specific carotenoid cleavage enzyme in petunia flowers regulates the production of B-ionone. While produced only at trace levels, B-ionone emission was found to correlate with rhythmic circadian changes in transcript levels that encode the carotenoid cleavage enzyme. This research indicates that plants control B-ionone emission at the transcriptional level and that transgenic approaches may be used for the intentional modification of flavor and fragrance components in crops.
Technical Abstract: Carotenoids are thought to be the precursors of terpenoid volatile compounds that contribute to flavor and aroma. One such volatile, B-ionone, is important to fragrance in many flowers, including petunia (Petunia hybrida). However, little is known about the factors regulating its synthesis in vivo. The petunia genome contains a gene encoding a 9,10(9',10') carotenoid cleavage dioxygenase, PhCCD1. The PhCCD1 is 94% identical to LeCCD1A, an enzyme responsible for formation of B-ionone in tomato (Lycopersicon esculentum; Simkin AJ, Schwartz SH, Auldridge M, Taylor MG, Klee HJ  Plant J [in press]). Reduction of PhCCD1 transcript levels in transgenic plants led to a 58% to 76% decrease in B-ionone synthesis in the corollas of selected petunia lines, indicating a significant role for this enzyme in volatile synthesis. Quantitative reverse transcription-PCR analysis revealed that PhCCD1 is highly expressed in corollas and leaves, where it constitutes approximately 0.04% and 0.02% of total RNA, respectively. PhCCD1 is light-inducible and exhibits a circadian rhythm in both leaves and flowers. B-Ionone emission by flowers occurred principally during daylight hours, paralleling PhCCD1 expression in corollas. The results indicate that PhCCD1 activity and B-ionone emission are likely regulated at the level of transcript.