that result from alterations to carotenoid content in higher plants. In addition, protein in biosynthetic
complexes could be identified providingan insight into the mechanisms of metabolite
channelling, a regulatory process that appears to limit the effectiveness of metabolic engineering.
A common feature of many studies attemptingto manipulate carotenoid biosynthesis by
amplifying a single enzyme, judged to rate-limiting, has been the relative ineffectiveness of the
approach, in part due to the ability of other pathway components to compensate for the fluctuations.
These findings have led to suggestions for multiple gene manipulations or the use of
transcription factors to facilitate the co-ordinate expression of the pathway. Such an approach
has been used successfully with flavonoid formation. To date, no transcription factor influencing
carotenoid formation has been identified, but hopefully techniques such as activation tagging and
the Arabidopsis ORF knockout collection (SIGnal collection) will enable putative transcription
factors influencingcarotenoid formation to be identified. In addition, comparative bioinformatic
studies on promoter regions of carotenoid genes may also elucidate common binding motifs
involved in carotenoid formation.
Although this review has focused on carotenoid biosynthesis and manipulation in higher plants,
it is important not to lose sight of carotenoid formation in microorganisms and the metabolic
diversity that exists among bacteria and fungi. Gene shuffling and direct evolution with genes
from these sources could provide valuable products with improved catalytic activities and/or
altered regulatory properties. Finally, the consumer perception of GM crops means that it is
important to show conclusively that alterations to carotenoid content in plants have no adverse
effects and represent a substantial equivalent.
With regard to the perceived health benefits of dietary carotenoids, there are many epidemiological
and human supplementation studies underway that should clarify the rather confusing
evidence that has been published to date. Similarly, investigations on the relative benefits of pure
carotenoid supplements compared to high carotenoid foods are in progress. In all of these cases,
extrapolations from animal studies to humans need to be viewed cautiously. Finally, post
genomic technologies will allow studiers of human biomarkers that will help to elucidate the
precise modes of action of these compounds.