Rice is the major staple food for hundreds of millions of
people. It is generally consumed in its milled form with
outer layers (pericarp, tegmen and aleurone layers) removed.
The main reason for milling is to remove the
oil-rich aleurone layer, which turns rancid upon storage,
especially in tropical and subtropical areas. As a result, the
edible part of rice grains consists of the endosperm, filled
with starch granules and protein bodies, but it lacks several
essential nutrients for the maintenance of health, such as
carotenoids exhibiting provitamin A-activity. Thus, reliance
on rice as a primary food staple contributes to vitamin
A deficiency, a serious public health problem in at least 26
countries including highly populated areas of Asia, Africa
and Latin America (2).
A complementary intervention to existing strategies for
reducing vitamin A deficiencies in the highest-risk countries is
to fortify the major staple food, rice, with provitamin A
through plant breeding. This can only be achieved by recombinant
technologies rather than by conventional breeding, due
to the lack of any rice cultivars producing this provitamin in
the endosperm. Both because the transformation of rice is
well-established and because the entire carotenoid biosynthetic
pathway has been molecularly identified recently, it
seemed feasible to introduce the complete provitamin A (-
carotene) biosynthetic pathway into rice endosperm by genetic
engineering.
We have shown previously that immature rice endosperm
synthesizes the early intermediate geranylgeranyl diphosphate
in the provitamin A biosynthetic pathway. This compound is
not solely devoted to carotenogenesis but can be used as a
substrate to produce the uncolored carotene phytoene by expressing
the heterologous enzyme phytoene synthase in rice
endosperm (3). This result prompted further investigations to
install the entire pathway (1). Golden Rice, the resulting
prototype line, bears the potential—after further improvements
and testing—to contribute to the alleviation of vitamin
A deficiency, provided that access to the -carotene-rich seeds
by poor farmers in developing countries is possible at the same
cost as current popular cultivars. In a novel collaborative
agreement between the university-based inventors and the
private sector, an agreement has been signed guaranteeing this
circumstance. This is discussed further below.