Metabolic engineering of tomato for high-yield production of astaxanthin

Dietary carotenoids have been shown to be beneficial to health by decreasing the risk of many diseases. Attempts to enhance carotenoids in food crops have been successful although higher plants appear to resist big changes of carotenoid biosynthesis by metabolic engineering. Here we report the generation of a more nutritious tomato by modifying the intrinsic carotenes to astaxanthin, a high-value ketocarotenoid rarely found in plants. This was achieved by co-expression of the algal β-carotene ketolase from Chlamydomonas reinhardtii and β-carotene hydroxylase from Haematococcus pluvialis, a unique pair of enzymes identified to co-operate perfectly in converting β-carotene to astaxanthin by functional complementation in Escherichia coli. Expression of the two enzymes in tomato up-regulated most intrinsic carotenogenic genes, and efficiently directed carbon flux into carotenoids, leading to massive accumulations of mostly free astaxanthin in leaves (3.12 mg/g) but esterified astaxanthin in fruits (16.1 mg/g) and a 16-fold increase of total carotenoid capacity therein without affecting the plant normal growth and development. This study opened up the possibility of employing crop plants as green factories for economical production of astaxanthin.