Agriculture Reference
In-Depth Information
at increasing the content of various nutrients (vitamins, essential amino acids, flavonoids,
lycopene, etc.) by genetic manipulation (Sevenier et al., 2002).
Vitamins are essential factors in the diet, and they must be obtained from the diet. In
addition, some vitamins are used as functional additives in food products. The edible part
of rice grains, the endosperm, lacks vitamin A, and a diet based mostly on rice consumption
may eventually cause vitamin A deficiency (Tucker, 2003). An outstanding achievement
has been the introduction of genes into rice that enabled the biosynthesis in the endosperm
of
-carotene, the precursor of vitamin A (Ye et al., 2000). The grain of the transgenic rice
had a yellow golden color and by itself contained sufficient
β
-carotene for human vitamin
A requirements. The authors of this work have waived all intellectual property rights for
exploitation of these technologies in the developing world, and are actively involved in
assisting the International Rice Research Institute to breed stable and agronomically suc-
cessful lines for use in vitamin A-deficient areas. Similar experiments have been performed
successfully in rapeseed, where introduction of a phytoene synthase gene also increased
the level of vitamin A precursor (Kishore and Shewmaker, 1999) and in tomato, where
introduction of a bacterial phytoene desaturase increased the
β
β
-carotene content in fruits
up to twofold (Romer et al., 2000).
Another lipid-soluble vitamin with an antioxidant role is vitamin E (
-tocopherol). Daily
intake of this vitamin in excess of a recommended minimum is associated with decreased
incidence of several diseases. Plant oils are the main source of dietary vitamin E, and they
generally have a high content of the vitamin E precursor
α
γ
-tocopherol. Overexpression
of
-tocopherol in
Arabidopsis
(Shintani and DellaPenna, 1998) and corn (Rocheford et al., 2002). Apart
from these examples, transgenic plants containing elevated levels of vitamin C have also
been produced (Herbers, 2003). These experiments have resulted in functional food with
enhanced health benefits, but there are now many laboratories in the public and private
sectors looking to achieve vitamin levels high enough in transgenic plants to merit extraction
from the plant (Herbers, 2003). Attempts to increase the content of carotenoids in fruit
by genetic manipulation are common (Romer and Fraser, 2005; Long et al., 2006), and
the reason is because they posses potent antioxidative, photoprotectant, and anticancer
properties (Fraser and Bramley, 2004; Hix et al., 2004).
Flavonoids are another group of secondary metabolites whose inclusion in the human
diet, in particular the flavonol group (e.g., quercetin and kaempferol), may give protection
against cancer and cardiovascular diseases (Chen et al., 1990; Hou, 2003; Ren et al., 2003).
The biosynthetic pathway leading to the synthesis of these compounds has been known
for a long time, and consequently, the design of strategies to increase the content of these
compounds has been possible. For example, transformation of tomato with a gene from
Petunia
, encoding a chalcone isomerase or with the maize transcription factor genes
LC
and
C
1, has resulted in fruits with an increased content of flavonoids (Muir et al., 2001; Bovy
et al., 2002). Interestingly, by suppression of an endogenous photomorphogenesis regulatory
gene,
DET1
, by RNA interference technology both carotenoid and flavonoid contents were
increased significantly, whereas other parameters of fruit quality were largely unchanged
(Davuluri et al., 2005).
When vegetables are the major components in the diet, there is a certain risk of iron
deficiency. Although some plants are rich in iron, availability is limited by the oxalic acid
and phytate-like substances present in the plant, which may complex this element. Oral
γ
-tocopherol methyl transferase greatly increased the seed level of
α
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