Conclusion
Although genetic engineering of the carotenoid pathway to
astaxanthin was successful, a specific side reaction occurred in rice
callus. The expressed bacterial ketolase not only converted b-carotene
and zeaxanthin to astaxanthin but also accepted the b-ionone
ring of a-carotene as substrate for ketolation. The resulting product
was identified as 4-keto-a-carotene. Its formation branches off
the desired b-carotene to astaxanthin route. This loss for the astaxanthin pathway may be avoided by further genetic engineering
of carotenogenesis by antisense or RNAi down-regulation of the
expression of rice specific lycopene e-cyclase depleting the formation
of a-carotene.