Molecular mechanisms underlying carotenogenesis in the chromoplast: Multilevel regulation of carotenoid-associated genes

In nature, carotenoid function and mode of action are highly determined by the neighboring protein and lipid molecules. Therefore an understanding of the proteins' involvement in carotenoid sequestration would be of great help in elucidating carotenoid function in vivo. Based on a study of the expression of chromoplast-specific carotenoid-associated genes from cucumber corolla (CHRC and CHRD), a working model is presented wherein two major regulatory factors control carotenoid sequestration within the chromoplasts: (i) floral tissue-specific transcriptional regulators of chromoplastogenesis; and (ii) post-transcriptional regulators related to the amount/type of sequestered carotenoids. This model is supported by the major role transcriptional regulation was found to play in the temporal and spatial expression of the CHRC gene, and by the fact that phytohormones such as gibberellic acid (GA₃), abscisic acid and ethylene also acted as transcriptional regulators of CHRC expression. The primary response to GA₃ was localized within the CHRC promoter to a 290 bp fragment. Furthermore, we demonstrated strong down-regulation of CHRC expression at post-transcriptional and translational/post-translational levels resulting from inhibition of carotenoid biosynthesis, thus revealing a close link between carotenoid biosynthetic and sequestration machineries.