For the developing world
The world-wide deficiency of vitamin A is being tackled both through
conventional plant breeding and by genetic manipulation. However, the use of
conventional plant breeding to deliver adequate intakes is dependent on
availability of carotenoid-rich staple foods. Often these are available for very
restricted times of the year in some societies. In those countries where rice is a
dietary staple the problem is particularly severe and the deficiency is likely to be
corrected only by the introduction of rice that has been genetically manipulated
to produce β
-carotene. However, yellow rice is produced and this may give rise
to problems of acceptability to consumers used to white rice.
Manipulation of the carotenoid pathway in rice
The nature of the challenges faced in manipulating plant secondary metabolites is
well illustrated through the attempts that have been made to produce carotenoids
in rice plants. A simplified version of the pathways leading to the synthesis of the
carotenoids principally found in food plants is shown in Fig. 7.2.
Immature rice endosperm is capable of synthesising the early pathway
intermediate geranylgeranyl diphosphate (GGDP). Four plant genes
corresponding to the enzymes phytoene synthase (psy) (1), phytoene desaturase
(2), zeta carotene desaturase (3) and lycopene cyclase (crt) (4) are required.
Enzyme (1) was obtained from the daffodil (Narcissus pseudonarcissus), (2)
from a bacterium Erwinia uredovora – which is capable of achieving steps (2)
and (3) from the single enzyme, and (4) from the daffodil.
The genes need to be expressed in a tissue-specific manner through the
insertion of specific promoters. This has been achieved in rice through the use of the daffodil psy gene . In rice the daffodil psy cDNA insertion is under the
control of an endosperm-specific promoter. The choice of promoter will very
much affect the timing and tissue-specific expression of a gene.
|Fig. 7.2 The carotenoid biosynthetic pathway
Surprisingly, seeds that expressed psy and crt did not accumulate lycopene.
Instead they contained β
-carotene and other xanthophylls. Thus it would seem
that the enzymes required to make these metabolites are either normally
expressed in rice endosperm or are induced if lycopene is formed. The
maximum level of carotenoids in the endosperm of plants that were
heterozygous for the transgenes was 1.6 mg kg-1
which is likely to help to
meet the nutritional needs of people consuming rice as a staple. Interestingly,
good progress is being made in adding a gene coding for ferritin – the iron
storageprotein found in mammalsand plants– to rice . It is likely that this
would also help improve the iron deficiency also seen in these communities if it
is shown to be bioavailable.
The controversy over the use of advanced technologies for producing
sustainable food in the developing world has been addressed by the developers
of modified rice. They have in effect waived all intellectual property rights for
exploitation of the technologies in the developing world, and are actively
involved in assisting the International Rice Research Institute to breed stable and
agronomically successful lines for use in vitamin A-deficient areas.