Synthesis of ketocarotenoids in the seed of Arabidopsis thaliana.

A cDNA coding for a gene necessary for synthesis of ketocarotenoids was cloned from the alga Haematococcus pluvialis and expressed in the seed of Arabidopsis thaliana. The expression of the algal beta-carotene-oxygenase gene was directed to the seed by use of the 2S, seed storage protein promoter napA. Extracts from seeds of the transgenic plants were clearly red because of accumulation of ketocarotenoids, and free and esterified forms of ketocarotenoids were found in addition to the normal carotenoid composition in the seed. The major ketocarotenoids in the transgenic plants were: 4-keto-lutein (3,3'-dihydroxy-beta-,epsilon-carotene-4-one), adonirubin (3-hydroxy-beta-,beta'-carotene-4,4'-dione) and canthaxanthin (beta-,beta'-carotene-4,4'-dione). 4-Keto-lutein differs from the more common adonixanthin only in the position of one double bond. To increase the substrate availability for the beta-carotene-oxygenase, these transformants were crossed with transgenic plants overexpressing a construct of an endogenous phytoene synthase gene, also under the control of the napA promoter. The resulting crossings gave rise to seeds with a 4.6-fold relative increase of the total pigment, and the three major ketocarotenoids were increased 13-fold compared to seeds of transgenic plants carrying only the beta-carotene-oxygenase construct.