b-carotene and led to the name “Golden Rice” (Ye et al. 2000). It turned out that plants
expressing just the phytoene synthase and the desaturase producedb-carotene, indicating
that rice grains already contained the metabolic activity to convert lycopene tob-carotene.
The gene for the desaturase originated from a bacterium,Erwiniaspp., whereas the other
genes originated from daffodil. The bacterial desaturase enzyme actually performs meta-
bolic steps normally carried out by two separate plant enzymes. Because the daffodil
gene products are normally found in plastids, they already contained sequences for a
plastid transit peptide to direct newly synthesized proteins to the proper cellular location.
The bacterial gene-encoding desaturase was modified by addition of a transit peptide to
direct it to plastids following translation, in much the same way the bacterial EPSPS
gene was modified for engineered RoundUp resistance.
Golden Rice produces carotene levels sufficient to impart a visible yellow color. One
concern with these plants, however, has been that the accumulation levels ofb-carotene
might not be sufficient to provide enough of the compound to be of nutritional benefit.
An improved version of transgenic rice referred to asGolden Rice 2, using a phytoene
synthase gene from corn rather than daffodil, was subsequently produced that accumulated
levels of carotenoids over 20 times higher than in the original Golden Rice (Paine et al.
2005). It is estimated that by eating modest amounts of Golden Rice 2, enoughb-carotene
can be provided to overcome vitamin A deficiency.
The large-scale dissemination of Golden Rice has been controversial (see Life Box 8.2).
Advocates maintain that this rice can provide provitamin A to millions of undernourished
children who need it. Rice is already widely grown and consumed in the target regions, and
so packaging the technology in this form takes advantage of an existing means to distribute
and administer the nutrient. Opponents of the technology counter that development of this
product is a tactic used by the biotechnology industry to drive acceptance of transgenic
foods worldwide. Many opponents also contend that vitamin supplements and food forti-
fication are superior methods for fighting the problem of vitamin A deficiency. Clearly,
this rice has the potential to help malnourished children, but contentious issues must be
resolved before it is accepted worldwide. At the very least, development of Golden Rice
demonstrates that it is possible to alter the natural abilities of plants to synthesize
complex chemicals, and to enhance their nutritional value.
8.4.2 Modified Plant Oils
The fatty acids produced by plants are the source of oils used in foods, and also have appli-
cations in cosmetics, detergents, and plastics. Oilseed rape (Brassica napus) has been used
as a plant oil source for many years. Canola is the common name for the cultivated form of
this plant, and has been bred through traditional means to contain low levels of harmful
glucosinolates and erucic acid. By engineering canola with a thioesterase gene that
originated in the California bay tree (Umbellularia californica), the oils that accumulate
contain much higher levels of beneficial fatty acids. The “bay leaf” thioesterase enzyme
expressed in canola causes premature chain termination of growing fatty acids, and
results in accumulation of 12-carbon lauric acid and 14-carbon myristic acid. The overall
level of lipids is not increased in these plants, as the increase in the short-chain molecules
is matched by a decrease in the amount of long-chain fatty acids such as the 18-carbon oleic
and linoleic acids. These short-chain fatty acids make the canola oil much more suitable as
replacement for palm and coconut oils in products such as margarine, shortenings, and
confectionaries.
8.4. TRAITS FOR IMPROVED PRODUCTS AND FOOD QUALITY 207