ACTA BIOLOGICA CRACOVIENSIA

16 TH INTERNATIONAL SYMPOSIUM ON CAROTENOIDS
astaxanthin biosynthesis by engineering plants and plant cells
with microbial β-carotene ketolase genes. We had generated previously
a diverse population of maize plants in terms of
carotenoid profiles. A specific plant lineage was shown to accumulate
astaxanthin and other ketocarotenoids; however, astaxanthin
accumulation in this specific line was shown to be rather low
compared to β-carotene. It has been demonstrated experimentally
that the limited astaxanthin accumulation in plants is due to a
bottleneck in the conversion of adonixanthin to astaxanthin. This
is in agreement with the hypothesis that the low efficiency of βcarotene
ketolases in ketolating zeaxanthin to astaxanthin is the
major limitation for astaxanthin accumulation in engineered
plants.
SESSION 6
Our strategy to overcome this bottleneck is to screen for bacterial
ketolase genes encoding particular enzymes with preferential
substrate specificity for zeaxanthin and iontroduce these into
plants. We have identified two promising candidates, the ketolase
genes from Brevundimonas SD 212 and Chlamydomonas reinhardtii,
which mediate the formation of substantial amounts of
astaxanthin in engineered E. coli and transgenic rice callus. A further
elements of our strategy to enhance astaxanthin accumulation
in maize endosperm is to enhance ketolase versus hydroxylase
expression levels in the maize transformants by maize-specific
codon modifications of these two transgenes, use of strong
endosperm-specific promoters and of a 5'-UTR which is a translational
enhancer in monocotyledonous plants.
94 ACTA BIOLOGICA CRACOVIENSIA Series Botanica