Cereals processing technology

70 Cereals processing technology
carboxylase (PEPC), which catalyses the initial fixation of atmospheric carbon
dioxide in maize, was introduced into japonica cultivars of rice, a C3 plant (Ku
et al. 1999). Transgenic plants exhibited reduced oxygen inhibition of
photosynthesis and photosynthetic rates comparable to those of non-transformed
plants. Such an approach for modifying one of the major physiological processes
in plants holds promise for the transformation of cultivars of the other subgroups
(indica, javanica) of rice and, indeed, for the transformation of other C3
crops. Other experiments have been reported which attempt to increase the
resistance of crop plants to environmental stresses such as ozone, high light,
drought, cold and heat. A common feature in stressed plants is the production of
free oxygen radicals which damage DNA, lipids and proteins. In this respect,
transformation procedures have been presented to increase the levels of
superoxide dismutase, ascorbate peroxidase and catalase in cells in order to
improve the tolerance of maize and rice to oxidative stress (Van Breusegem et
al. 1998).
4.8.4 Nutritional properties
Experiments have been directed towards modifying the nutritional quality of rice
grain. For example, introduction of a fatty acid desaturase gene from tobacco
into rice resulted in modification of the proportions of linoleic acid and linolenic
acid in fatty acids, with the former decreasing and the latter increasing,
respectively (Wakita et al. 1998). A significant recent advance has been the
transformation of rice to produce beta-carotene, a precursor of vitamin A. Thus,
the introduction of genes for phytoene synthase, phoetene desaturase, carotene
desaturase and lycopene cyclase from Narcissus, or a double-desaturase from
the fungus Erwinia uredovora resulted in transgenic plants with grain producing
yellow endosperm (Burkhardt et al. 1997). Some lines produced enough betacarotene
to supply the daily human requirements from 300 grams of cooked rice.
Since vitamin A deficiency affects affects about 7% of the world population
(mostly children), mainly in developing countries, this work represents a
significant advance in attempts to alleviate the problems of vitamin A
deficiency.
In the future, it may may be essential to engineer complex biochemical
pathways by the introduction of several transgenes into target species. In order
to provide a foundation for this technology, embryogenic tissues of rice have
been bombarded with a mixture of 14 different genes on pUC-based plasmids
(Chen et al. 1998). Eighty per cent of the regenerated plants contained more than
two and 17% more than nine of the transgenes. Importantly, plants with
transgenes were phenotypically normal and 63% set viable seed. Detailed
information collected over several seed generations from such plants will clarify
the interaction and expression of multiple transgenes in genetically engineered
plants, such as cereals.