Biomedical Engineering Reference
In-Depth Information
coli
mutant GP which was deficient in allosteric reg-
ulation. When this gene was inserted into potato
plants, the tubers accumulated higher levels of
starch but had normal starch composition and gran-
ule size. Since transgenic plants expressing the wild-
type
E. coli
GP had normal levels of starch, the
allosteric regulation of GP, and not its absolute level,
must influence starch levels. A different method of
modulating GP activity has been used by Muller-
Rober
et al.
(1992). They used an antisense con-
struct to reduce the level of plant GP to 2-5% of wild
type. These plants had very low levels of starch (also
2-5% of wild type) but had a very much higher
number of tubers. These tubers had six- to eightfold
more glucose and sucrose and much lower levels of
patatin and other storage proteins. Visser
et al.
(1991)
have modified the starch composition of potato plants.
They demonstrated that potato plants expressing an
antisense gene to the granule-bound starch synthase
have little or no amylose compared with wild-type
potato, where it can be as much as 20%.
Whereas some plants accumulate starch as a
carbon reserve, others accumulate high levels of
triacylglycerols in seeds or mesocarp tissues. Higher
plants produce over 200 kinds of fatty acids, some of
which are of food value. However, many are likely to
have industrial (non-food) uses of higher value
than edible fatty acids (Kishore & Somerville 1993,
Murphy 1999). Thus there is considerable interest
in using gene-manipulation techniques to modify
the fatty acid content of plants. As a first step towards
this goal, a number of genes encoding desaturases
have been cloned, e.g. the MS desaturase, which
converts linoleic acid to
these lipids are used in detergent synthesis. Some
plants, such as the California bay, accumulate MCT
because of the presence of a medium-chain specific
thioesterase. When the gene for this thioesterase
was expressed in rape-seed and
Arabidopsis
, the
transgenic plants accumulated high levels of MCT
(Voelker
et al.
1992).
Improving vitamin and mineral content
The vitamin and mineral content of plants varies
from species to species, and from tissue to tissue in a
particular plant. Milled cereal grains - basically the
endosperm component of the seed - are particularly
deficient in essential vitamins and minerals, and
yet they represent the staple food for much of the
world's population. In developed countries, where
diet is varied, this is seldom a problem. However, in
poorer nations, where cereal grains are often the
only food available, this can cause major health
problems. Iron deficiency is the most widespread
nutrient deficiency in the world, reflecting the com-
bination of low iron in cereal grains and the high
level of phytic acid, which reduces the efficiency of
iron absorption from the gut. Many cereal grains
also lack
-carotene, which is required in the diet for
vitamin A biosynthesis. Vitamin A deficiency in early
childhood leads to blindness, an avoidable conse-
quence of poor diet in many parts of Asia and Africa.
A number of attempts to increase the nutritional
value of cereals have been reported, and we discuss
two examples of genetically engineered rice below,
since this cereal is the staple diet of at least two-
thirds of the world's population. Lucca
et al.
(2001)
described three different routes to improving the
iron content of rice grain, which could be used singly
or in combination. The first route was to transform
rice with a gene encoding ferritin, an iron-storage
protein. The overexpression of bean ferritin in rice
endosperm led to the accumulation of iron in a
bioavailable form, resulting in transgenic rice grains
with twice the normal levels of iron. The second
route was to transform rice with a gene from the
fungus
Aspergillus fumigatus
encoding the enzyme
phytase, which metabolizes phytic acid. Since the
transgenic grain has lower levels of phytic acid, iron
absorption from the gut should be more efficient.
The third approach was to transform rice with a
β
α
-linolenic acid (Arondel
et al.
1992).
In most plants the
9 stearoyl acyl carrier protein
(ACP) desaturase catalyses the first desaturation
step in seed-oil biosynthesis, converting stearoyl-
ACP to oleoyl-ACP. When antisense constructs were
used to reduce the levels of the enzyme in developing
rape-seed embryos, the seeds had greatly increased
stearate levels (Knutzon
et al.
1992). The signi-
ficance of this result is that high stearate content is of
value in margarine and confectionery fats.
Most oil-seed crops accumulate triacylglycerols
with C
16
to C
22
acyl chains. However, plants that
accumulate medium-chain triacylglycerols (MCT),
with C
8
to C
12
acyl chains, would be very useful, for
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