CHRC, ENCODING A CHROMOPLAST-SPECIFIC CAROTENOID-ASSOCIATED PROTEIN, IS AN EARLY GIBBERELLIC ACID-RESPONSIVE GENE

CHRC, a corolla-specific carotenoid-associated protein, is a major com ponent of carotenoid-lipoprotein complexes in Cucumis sativus chromopl asts. Using an in vitro flower bud culture system that mimics in vivo flower development, CHRC mRNA levels in corollas were shown to be spec ifically up regulated by gibberellic acid, The response to gibberellic acid was very rapid (within 20 min) and insensitive to protein synthe sis inhibition by cycloheximide. Abscisic acid, known to antagonize gi bberellin in many developmental systems, strongly down-regulated CHRC mRNA levels. The gibberellin synthesis inhibitor paclobutrazol exhibit ed a similar negative effect on CHRC expression. Inclusion of exogenou s gibberellic acid into the in. vitro bud culture system with the pacl obutrazol not only prevented the CHRC mRNA down-regulation, it up-regu lated transcript accumulation to the level of gibberellic acid-treated corollas, CHRC mRNA accumulation in response to gibberellic acid disp layed a dose-dependent increase up to 10(-4) M gibberellic acid, The u p-regulation could be detected with as little as 10(-7) M gibberellic acid. Based on these data, we suggest that CHRC is the first structura l gene identified to date whose expression is regulated by gibberellic acid in a primary fashion, The critical role of the rapid response of CHRC to gibberellic acid in aiding carotenoid sequestration while pre serving chromoplast structural organization is discussed.